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A re-evaluation of Secernosaurus koerneri and Kritosaurus australis(Dinosauria, Hadrosauridae) from the Late Cretaceous of ArgentinaAlbert Prieto-Marquez a;Guillermo C. Salinas b

a Division of Paleontology, American Museum of Natural History, New York, New York, U.S.A. b

Ciudad Universitaria, Nuñez-Capital Federal-Pabellón II, Universidad de Buenos Aires, Buenos Aires,Argentina

Online publication date: 19 May 2010

To cite this Article Prieto-Marquez, Albert andSalinas, Guillermo C.(2010) 'A re-evaluation of Secernosaurus koerneri andKritosaurus australis (Dinosauria, Hadrosauridae) from the Late Cretaceous of Argentina', Journal of VertebratePaleontology, 30: 3, 813 — 837To link to this Article: DOI: 10.1080/02724631003763508URL: http://dx.doi.org/10.1080/02724631003763508

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Journal of Vertebrate Paleontology 30(3):813–837, May 2010© 2010 by the Society of Vertebrate Paleontology

ARTICLE

A RE-EVALUATION OF SECERNOSAURUS KOERNERI AND KRITOSAURUS AUSTRALIS(DINOSAURIA, HADROSAURIDAE) FROM THE LATE CRETACEOUS OF ARGENTINA

ALBERT PRIETO-MARQUEZ*,1 and GUILLERMO C. SALINAS2

1Division of Paleontology, American Museum of Natural History, Central Park W. and 79th Street, New York, New York 10024,U.S.A., redshore@gmail.com;

2Ciudad Universitaria, Nunez–Capital Federal–Pabellon II, Universidad de Buenos Aires 1428, Buenos Aires, Argentina,hadropirata@gmail.com

ABSTRACT—Hadrosaurids form the most diverse and derived clade of ornithopod dinosaurs. Although well representedin Asia and North America, its presence in South America is known only from rare and fragmentary remains that arepoorly documented and mostly unstudied. As a result, the impact of these animals on the phylogenetics and biogeographyof hadrosaurids as a whole is poorly known. Here, we provide a revised and complete osteology of the type specimens andhypodigms for the only two taxa known from South America, Secernosaurus koerneri and Kritosaurus australis. Likewise, weinfer the phylogenetic position and historical biogeography of South American hadrosaurids using a nearly complete taxo-nomic sampling of hadrosaurid species. Parsimony methods were used to infer phylogenetic relationships, whereas Fitch par-simony and Dispersal-Vicariance analyses were implemented to reconstruct ancestral areas. Kritosaurus australis is regardedas a junior synonym of Secernosaurus koerneri, based on a combination of iliac and pubic characters unique to these two taxa.Inclusion of S. koerneri within the genus Kritosaurus is not supported by the phylogenetic analysis. S. koerneri is inferred tobe a member of the Kritosaurus-Gryposaurus clade within Saurolophinae, as the sister taxon to the Argentinean unnamedhadrosaurid from Salitral Moreno. Another unnamed hadrosaurid from Big Bend National Park, Texas, is positioned as theclosest outgroup to the South American clade. The results of this biogeographical analysis supports the hypothesis that theSecernosaurus clade originated in South America during the late Campanian after a dispersal event (probably followed byvicariance) from southern North America before the end of that geologic stage.

INTRODUCTION

Hadrosauridae is a diverse clade of herbivore dinosaurs fromthe Late Cretaceous of Europe, Asia, the Americas, and Antarc-tica (Weishampel et al., 1990; Forster, 1997; Case et al., 2000).In North America and Asia numerous hadrosaurid specieshave been erected based on abundant and well-preserved ma-terial, including complete articulated skeletons, multi-individualaggregations, embryonic material, and soft tissue impressions(Rozhdestvensky, 1957; Dodson, 1971; Varricchio and Horner,1993; Godefroit et al., 2004). In contrast, our knowledge ofhadrosaurid diversity and evolution in South America is compar-atively poor, with very few taxa erected to date (Brett-Surman,1979; Bonaparte et al., 1984).

Although hadrosaurids have been recorded in South Amer-ica since early in the 20th century (Brett-Surman, 1975), most oftheir remains are too fragmentary to diagnose at generic or spe-cific levels (Casamiquela, 1964, 1980; Alonso, 1980; Alonso andMarquillas, 1986; Bonaparte, 1996; Coria, 1999; Gonzalez Rigaand Casadio, 2000a, 2000b). Consequently, of the approximately50 valid species of hadrosaurids, only 2 come from South Amer-ica. Nearly all the discoveries of South American hadrosauroidshave occurred in Argentina, with the exception of a posteriorvertebra from the Aptian-Albian of Brazil and two teeth fromthe Aptian of Uruguay (Salinas et al., 2005). The most com-plete and best-preserved hadrosauroid material corresponds tomembers of Hadrosauridae (Fig. 1). These consist of the type ofSecernosaurus koerneri (Brett-Surman, 1979), the type and hy-podigm of Kritosaurus australis (Bonaparte et al., 1984), and the

*Corresponding author.

extensive, mostly undescribed collection of the Salitral Morenobonebed (late Campanian-early Maastrichtian Allen Formation,Rıo Negro province, southern Argentina; Powell, 1987). Thepresent study focuses on the material of S. koerneri and K.australis.

The remains of Secernosaurus koerneri, the first hadrosauridnamed from South America, were collected in 1923 by an ex-pedition from The Field Museum (Chicago, IL) led by J. B.Abbott (Brett-Surman, 1975). The team recovered the partialskeleton of a juvenile hadrosaurid from Upper Cretaceous stratanear the head of the Chico River, east of Lake Colhue Huapi(southern Chubut province, Argentina) (Fig. 1). This materialremained unstudied until the 1970s at which time Brett-Surmanprovided a cursory diagnosis (1975, 1979). He regarded it asthe type of a new genus and species of hadrosaurid, Secer-nosaurus koerneri. This diagnosis was based on characters of thesupraacetabular and postacetabular processes of the ilium (Brett-Surman, 1979). Other materials, such as braincase fragmentsand additional partial postcranial bones, were never described.Brett-Surman (1979) first considered Secernosaurus koerneri asa primitive form outside Saurolophidae ( = Lambeosaurinae+ Saurolophinae; Prieto-Marquez, 2008), although more closelyrelated to Saurolophinae than to Lambeosaurinae. Based onhis interpretation of iliac primitive characters in Secernosauruskoerneri, he entertained the possibility of a South Americanorigin for hadrosaurids, an alternative hypothesis to the morewidely accepted paradigm of an Asian origin that he had alsocontemplated (Brett-Surman, 1979). Later, Brett-Surman (1989)regarded S. koerneri as Hadrosaurinae (Saurolophinae of thepresent study) incertae sedis due to the lack of diagnostic cranialdata needed for a more precise classification.

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FIGURE 1. Geographical location of themost complete and abundant remains ofhadrosaurid dinosaurs found in South Amer-ica. The map shows two provinces of southernArgentina, Rıo Negro and Chubut. Redrawnafter Bonaparte et al. (1984) and Salgado et al.(2007).

From 1982 to 1986, numerous cranial and postcranial elementscorresponding to several hadrosaurid specimens were collectedin the late Campanian–early Maastrichtian Los Alamitos For-mation, in the Arroyo Verde region of southeastern Rio Negroprovince (Bonaparte et al., 1984) near the border with Chubutprovince in southern Argentina (Fig. 1). Bonaparte et al. (1984)referred these materials to the genus Kritosaurus on the basisof pelvic and dentary characters (see discussion below) anderected the new species K. australis. The presence of a speciesof Kritosaurus, a genus otherwise known only from the westerninterior of the United States, in South America led Bonaparteet al. (1984) and Bonaparte and Rougier (1987) to entertain thepossibility that hadrosaurids from North America had occupiedPatagonia.

When comparing Kritosaurus australis with Secernosaurus ko-erneri, Bonaparte et al. (1984) and Bonaparte (1996) found prac-tically no meaningful differences between the pubis and scapulaof these taxa. However, these authors maintained their validityon the grounds that the iliac postacetabular process of S. koerneriis more triangular in lateral view than in K. australis. In addition,Bonaparte et al. (1984), Bonaparte (1996), and Wagner (2001)regarded the morphology of the supraacetabular and postacetab-ular processes of the ilium of S. koerneri as affected by post-mortem distortion. Wagner (2001) also found K. australis indis-tinct from S. koerneri and referred the former to the genus Secer-nosaurus as S. australis. He maintained the specific distinction be-tween the Chubut and Rio Negro specimens because of their tem-poral and geographical separation. More recently, Salinas et al.(2006) found K. australis to be closely related to non-hadrosauridhadrosaurs from Europe, such as Telmatosaurus transsylvanicus,and not cogeneric with Kritosaurus. The authors based this con-clusion on what they felt were distinctive triangular predentary

denticles, a short dentary edentulous portion, and the shape ofthe scapular blade of K. australis.

Only Horner et al. (2004) have included Kritosaurus aus-tralis in a cladistic analysis of hadrosaurid relationships. Intheir phylogeny, this species formed a polytomy with Saurolo-phus and Naashoibitosaurus ( = Kritosaurus, in agreement withWilliamson [2000]) within Hadrosaurinae (Saurolophinae of thisstudy). That polytomy was supported by one synapomorphy: apointed and symmetrically triangular anterior process of the ju-gal. Horner and colleagues considered Saurolophus to be moreclosely related to K. australis due to the presence of low zy-gapophyseal peduncles on cervical vertebrae. Likewise, these au-thors found no characters supporting the inclusion of K. australiswithin the genus Kritosaurus.

Because the aforementioned anatomical studies were incom-plete and the phylogenetic position and biogeography of SouthAmerican hadrosaurids have not been scrutinized using analyti-cal techniques within a more comprehensive context encompass-ing all known hadrosaurids, we chose to reanalyze these fos-sils and establish their true import. To achieve this goal, we re-vised and documented the anatomy of the only two taxa knownin South America, Secernosaurus koerneri and Kritosaurus aus-tralis. We focused our description mostly on those charactersthat are diagnostic or potentially informative phylogenetically,as well as on those skeletal elements that have never been de-scribed before. In doing so, we addressed a number of ques-tions regarding the taxonomy and evolutionary history of SouthAmerican hadrosaurids. Is the material of S. koerneri diagnostic?Could K. australis be cogeneric and conspecific with S. koerneri?What is the phylogenetic position of South American taxa withinHadrosauroidea and Hadrosauridae? More specifically, what isthe relationship (if any) of K. australis with Gryposaurus and

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Kritosaurus? Where did the most recent common ancestorof South American hadrosaurids evolve? How and when didhadrosaurids occupy South America?

In this paper, we accept the osteological data and conclusionspresented by Horner (1992) and Williamson (2000) in regardingKritosaurus navajovius as a valid taxon that is distinct from Gry-posaurus spp. Therefore, throughout the text we will refer K. no-tabilis (discussed in Bonaparte et al. [1984]) to the genus Gry-posaurus as G. notabilis.

Institutional Abbreviations—AMNH, American Museum ofNatural History, New York, New York, USA; CPC, ColeccionPaleontologica de Coahuila, Saltillo, Coahuila, Mexico; DMNH,Denver Museum of Natural History, Denver, Colorado, USA;FMNH, The Field Museum, Chicago, USA; MACN, Museo Ar-gentino de Ciencias Naturales Bernardino Rivadavia, BuenosAires, Argentina; MOR, Museum of the Rockies, Bozeman,Montana, USA; MSNM, Museo Civico di Storia Naturale di Mi-lano, Milano, Italy; ROM, Royal Ontario Museum, Toronto, On-tario, Canada; UTEP, Centennial Museum at the University ofTexas at El Paso, Texas, USA.

MATERIALS AND METHODS

We examined the type and all material referable to Secer-nosaurus koerneri housed at the FMNH, as well as the holotypeand hypodigm of Kritosaurus australis housed at the MACN.

The phylogenetic position of Secernosaurus koerneri wasreassessed using a nearly complete taxonomic sample ofhadrosaurid species. Unless otherwise indicated, all the anatomi-cal comparisons between the aforementioned materials and thoseof other taxa were based on direct examinations of specimens.This analysis was part of a much broader study to elucidatethe interrelationships of all Hadrosauridae by Prieto-Marquez(2008). For the present analysis, we included 12 outgroup and39 ingroup (hadrosaurid) taxa. Outgroup taxa consisted of 2non-hadrosauroid iguanodontoideans and 10 non-hadrosauridhadrosaurs. Species of Iguanodon and Ouranosaurus nigerien-sis that are iguanodontoideans distantly related to hadrosauridswere used to root the tree by outgroup comparison (Wiley,1981; Maddison et al., 1984). Character data consisted of 299discrete observations of osteological morphology. Of these, 196were binary, 99 were multistate unordered, and 4 were or-dered multistate characters (Supplementary Data 1 through 4,www.vertpaleo.org/jvp/JVPcontents.html). The searches for theoptimal tree(s) using parsimony were carried out in PAUP ver-sion 4.0b10 (Swofford, 2002). Two methodologies were imple-mented. In the first one, parsimony was applied with all of thecharacters having the same weight. In the second one, we ap-plied the weighted parsimony of Goloboff (1993), with a k fitparameter of 3. We performed heuristic searches using a ran-dom additional sequence of 10,000 replicates for the unweightedparsimony and 5000 replicates for the weighted parsimony anal-ysis. We used the option of branch-swapping by tree-bisection-reconnection (TBR) in both parsimony analyses (Swofford et al.,1996a). Bremer support (Bremer, 1988) was assessed by comput-ing decay indices (Donoghue et al., 1992) with MacClade ver-sion 4.0 (Maddison and Maddison, 2003) and PAUP. Bootstrapproportions (Felsenstein, 1985) were computed in PAUP, con-sisting of 10,000 replicates using heuristic searches; each searchwas performed using random additional sequences with branch-swapping by TBR and 1000 replicates. Ancestral states were re-constructed using parsimony in MacClade version 4.0 (Maddisonand Maddison, 2003).

Inference of the historical biogeography of Secernosaurus ko-erneri in the context of all other hadrosauroids required the re-construction of ancestral areas. These were reconstructed on theweighted parsimony phylogeny (see Prieto-Marquez, 2008, for a

discussion on this choice). Fitch parsimony (Fitch, 1971) and theDispersal-Vicariance (Ronquist, 1996; 1997) method were imple-mented to reconstruct ancestral areas for all hadrosauroid clades.

Phylogenetic nomenclature in this paper partially follows def-initions and usage in Wagner (2001), Wagner and Lehman(2009), and Prieto-Marquez (2008; Supplementary Data 5 and 6,www.vertpaleo.org/jvp/JVPcontents.html).

SYSTEMATIC PALEONTOLOGY

DINOSAURIA Owen, 1842ORNITHISCHIA Seeley, 1888

HADROSAUROIDEA Sereno, 1986HADROSAURIDAE Cope, 1870SAUROLOPHINAE Brown, 1914

SECERNOSAURUS Brett-Surman, 1979SECERNOSAURUS KOERNERI Brett-Surman, 1979

Synonymy—Kritosaurus australis Bonaparte et al., 1984; Secer-nosaurus australis Wagner, 2001.

Holotype—FMNH PP13423: two dental battery fragments,partial basioccipital and basisphenoid, two cervical centra, twoneural arches of anterior dorsal vertebrae, one nearly completeanterior dorsal vertebra, four centra (one with a neural arch)from middle dorsal vertebrae, two centra and one fragment withpostzygapophyses from posterior dorsal vertebrae, a sacral rib,neural spine of posterior dorsal or sacral vertebra, five anteriorand one posterior posterior centra, three fragmentary ribs, nearlycomplete right scapula, distal end of the right humerus, proxi-mal ulnar fragment, right metacarpal III, nearly complete rightilium, partial left ilium, right pubis, left and right ischial shafts,distal femoral condyle, and multiple small fragments of unknownanatomical origin. FMNH PP13423 is probably a small subadultspecimen, as noted by Brett-Surman (1989) based on small bodysize (total length [TL] ∼4–5 m), unfused neural arches, and dis-articulated basisphenoid-basioccipital elements.

Hypodigm—MACN-RN 2 (originally the type of Kritosaurusaustralis): right prefrontal, left postorbital, a partial sacrum in-cluding four fused centra, several disarticulated sacral vertebraeand ribs and various middle caudal vertebrae, left and right ster-nals, right coracoid and scapula, left and right ilia, left and rightischia lacking the distal half of each shaft, left and right pubes, leftfemur, and proximal right femur. MACN-RN 142: fragmentarybraincase with parietal, frontal, and postorbital, a right frontal,a fragment of predentary, right maxilla, anterior regions of thedentaries, subadult left dentary, and a left scapula. MACN-RN143: fragmentary braincase including parts of the basioccipital,prootic, and basisphenoid. MACN-RN 144: partial skull roof andbraincase including supraoccipital, parietal, frontals, postorbitals,and a fragment of squamosal. MACN-RN 145: left radius andulna, distal end of a femur, left metatarsals II and III, and var-ious middle dorsal vertebrae. MACN-RN 146: right scapula, leftilium lacking the preacetabular process, fragmentary preacetab-ular process, proximal region of a left ischium. MACN-RN 826:right sternal, proximal half of a right scapula, left ilium lackingthe pubic and ischial peduncles, and various cervical and anteriorand middle dorsal vertebrae. MACN-RN 987: articulated seriesof distal caudal vertebrae. MACN-RN 990: pedal phalange III1.MACN-RN 991: fragment of left squamosal, fragment of rightsurangular. MACN-RN 997: right tibia. MACN-RN 998: rightdentary (heavily reconstructed with plaster). Based on the rel-ative size between these specimens and the mean size of largerhadrosaurid specimens from around the world, it is likely that theLos Alamitos fossils represent individuals that are larger than thetype but are have not reached full adult size.

Occurrence—The holotype of Secernosaurus koerneri wasmost likely collected from late Campanian–early Maastrichtianstrata of the Upper Bajo Barreal Formation (see discussionbelow). The label that accompanies the fossil material in the

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collections of FMNH states that this material was found 2 milesto the east of the head of the Chico River. This location is nearthe Lake Colhue Huapi and to the west of the Gulf of SanJorge (southeastern Chubut province, Patagonia, southern Ar-gentina) (Casal et al., 2007). The MACN specimens (holotypeand hypodigm of Kritosaurus australis) were collected from lateCampanian–early Maastrichtian strata from the middle section ofthe Los Alamitos Formation, near the Arroyo Verde, southeast-ern Rıo Negro province, near the border with Chubut province(Patagonia, southern Argentina) (Bonaparte et al., 1984).

Revised Diagnosis—Hadrosaurid dinosaur that differs fromall other members of the clade on the basis of the followingautapomorphies and unique combination of appendicular char-acters: V-shaped nasofrontal suture where the anterior marginof each frontal forms a median triangular process that is later-ally deep, anteriorly excavated, and has an anteriorly offset ven-tral margin. Pubis with a distal blade that differs from that ofall other hadrosauroids (except that of species of Gryposaurus,Prosaurolophus, and probably Saurolophus) in having a subrect-angular lateral geometry with parallel dorsal and ventral margins;the prepubic process differs from that of species of Gryposaurus,Prosaurolophus, and Saurolophus in having a ventral margin thatforms a wider arch, showing a more gentle ventral deflection dis-tally. Ilium with the unique combination of the following fourcharacters: dorsomedially twisted postacetabular process forminga ventromedially oriented brevis shelf-like surface; median longi-tudinal ridge on the dorsal surface of the postacetabular process;postacetabular process nearly as long as the iliac central plateand oriented posterodorsally; and asymmetrical and anteropos-teriorly extensive supraacetabular process.

OSTEOLOGICAL DESCRIPTION

Cranial Elements

Mandibular Complex—The predentary of Secernosaurus ko-erneri bone is represented by a fragment of the anterior re-gion (Fig. 2A–B). It is labiolingually compressed and shows fournearly complete denticles. The anterior surface is flat. Medially,the bone becomes slightly thicker labiolingually. Two additionaldenticles, located medially to the four preserved ones, were bro-ken at their bases. The preserved denticles are relatively tall,slightly recurved lingually, labiolingually compressed, and sub-rectangular. The medial-most denticle appears triangular andlonger, but both of these features result from erosion of its mar-gins. The denticles are closely spaced with a separation of up to25% basal length. The lingual side of the predentary preservespart of the transverse lingual groove. The ventral region of thefragment is missing due to breakage, more so laterally than me-dially.

The dentary of Secernosaurus koerneri is represented by threespecimens. The most complete dentary, MACN-RN 142B (Fig.2C), measures 25 cm in length. It is heavily abraded, missingall the dentition. The ventral and dorsal (including the coronoidprocess) portions of its posterior region have been eroded away.Likewise, the dorsal alveolar margin is also abraded, particularlyanteriorly. A larger individual, MACN-RN 142 (Fig. 2D–F), isrepresented only by the left and right symphyseal regions. Thethird specimen, MACN-RN 998, is 48 cm in length. However,this exemplar is so poorly preserved and heavily reconstructed(with no discernible structures other than the overall lateral pro-file of the strongly deflected edentulous region, main mandibularramus, and unusually long coronoid process) that it is uncertainhow much of the dentary is actually composed of fossil material.Therefore, this description is based on MACN-RN 142 and 142B.The proximal edentulous margin of the dentary is very short inboth specimens, with a length equivalent to 10–15% of the lengthof the dental battery. The proximal edentulous margin forms anangle of approximately 140◦ with the occlusal plane. The ratio

between the lingual projection of the symphyseal process and thelabiolingual width of the dentary is approximately 2:1. The angleof deflection of the ventral margin of dentary end is 20◦ relativeto the occlusal plane. The symphyseal end gently curves linguallyforming a wide arch. Although heavily abraded, in MACN-RN142B there are 10 recognizable alveolar grooves. Each alveolargroove is narrow and vertically oriented. Based on the individ-ual width and space occupied by these 10 alveolar grooves, weestimate a minimum number of 25–30 tooth positions for the en-tire dental battery. As seen in the larger MACN-RN 142, thereare at least four teeth arranged dorsoventrally in each alveolus.The height/width ratio of dentary tooth crowns is approximately2.8. There is a single ridge on the enameled side of tooth crowns.This ridge is straight and occupies a median position in the crown.Marginal denticles are small but are too poorly preserved to de-duce their specific morphology. Both the mesial and distal mar-gins of the tooth have the same density and size of denticles.

The surangular is represented by a fragment containing the ar-ticulation surface for the quadrate, part of the medial articularsurface for the splenial and angular, and the proximal region ofthe posterior process (Fig. 2H–I). Its morphology is consistentwith that of a typical hadrosaurid surangular. The lateral flangethat bounds the articulation surface for the quadrate cotylus isconvex and expands laterally forming a wide semicircle. The ar-ticular surface for the quadrate is concave and its medial wallfaces laterodorsally. The ventral region of the surangular is me-dially offset and contains the articular surface for the angular.This surface is vertically inclined and dorsoventrally shallow. Theventral surface of the surangular is anteroposteriorly convex andsmooth.

Facial Complex—The maxilla of Secernosaurus koerneri is rep-resented by a single exemplar, MACN-RN 142, measuring 23 cmin length. As is typical of hadrosaurs, the maxilla is roughly tri-angular in lateral profile and mediolaterally compressed. Its ex-ternal surface shows signs of intense weathering, so that the bonestructure is exposed in several areas. Likewise, the lateral surfacehas been removed from the anterior region (exposing seven natu-ral molds of the alveolar grooves) and ventral to the jugal-maxillajoint surface (showing portions of the dental battery) (Fig. 3A).The dorsal premaxillary articular surface of the anteroventralprocess is also missing. In Secernosaurus koerneri, the region an-terior to the jugal-maxilla joint is very deep (Fig. 3A–B), a con-dition shared with species of Gryposaurus. The anteroventral re-gion of the maxilla is steeply inclined ventrally and forms an anglegreater than 40◦ with the tooth row. Only the base of the dorsalprocess is present, with a poorly and partially preserved articularsurface for the lacrimal. The anterolateral promontory (the re-gion anteroventral to the dorsal process) is located anterior to themid-length of the maxilla, as in Gryposaurus. Adjacent and an-teroventral to the jugal-maxilla joint surface there is a large ellip-soidal foramen. The lateroventral margin of the jugal articulationsurface forms a prominent ridge. Posteriorly, this ridge is continu-ous with the lateral margin of the ectopterygoid shelf, a conditiontypically observed in nearly all other hadrosaurids. On the oppo-site (medial) side, relative to the lateral margin of the maxilla-jugal joint, there is a flange for the palatine articulation. Thisflange is prominent and expands dorsally and slightly medially.The ectopterygoid shelf comprises 40–45% of the length of themaxilla. In Secernosaurus koerneri this shelf is ventrally inclined16◦ relative to the tooth row. An even more inclined ectoptery-goid shelf is found primitively in non-hadrosaurid hadrosaurssuch as Bactrosaurus johnsoni and Protohadros byrdi. The lat-eral margin of the ectopterygoid shelf is dorsoventrally thick andprominent. At the posterior end of the maxilla, the pterygoidprocess is not preserved. On the medial side of the maxilla, therow of alveolar foramina is located within the dorsal half of thedorsoventral depth of the element. The exact number of alve-olar positions in the dental battery is uncertain but a minimum

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FIGURE 2. Mandibular elements of Secernosaurus koerneri. A and B, predentary, MACN-RN 142, in anterior and posterior views, respectively. C,dentary, MACN-RN 142B, in medial view. D, detail of tooth crowns, MACN-RN 142. E and F, anterior dentary fragment, MACN-RN 142, in medialand anterior views. G, fragment of dentary dental battery, MACN (uncatalogued), in medial view. H and I, surangular, MACN-RN 991, in dorsal andlateral views, respectively.

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FIGURE 3. Facial elements of Secernosaurus koerneri. A–C, right maxilla, MACN-RN 142, in lateral, medial and ventral views, respectively. D andE, anterior fragment of right jugal, MACN (uncatalogued), in medial and dorsal views, respectively. F, prefrontal, MACN (uncatalogued), in lateralview. G, squamosal fragment MACN-RN 991, in lateral view.

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number of 40 teeth can be observed. The occlusal plane containsa maximum of two teeth arranged labiolingually (Fig. 3C), as inall other hadrosaurids. No morphological details are appreciatedin the teeth, except for the presence of a single median ridge thatappears to be symmetrically positioned in the tooth crown.

The jugal is represented by a fragment that includes the or-bital margin and the proximal region of the anterior process (Fig.3D–E). The apex of the anterior process of the jugal is missing.However, judging from the lateral outline of the articulation sur-face for the jugal in the maxilla (Fig. 3A), the anterior process ofthe jugal was probably triangular. The medial surface of the ante-rior process is bounded posteriorly by a sharp ridge. The orbitalmargin forms a wide arch.

The prefrontal is represented by two partial elements. Themost complete one preserves most of the dorsal region, alongthe orbital margin, and the proximal portion of the anteroven-tral segment of the bone (Fig. 3F). The lateral orbital margin isthicker posteriorly near the articulation with the postorbital. Thedorsal surface of the prefrontal is slightly concave. Anteriorly, theprefrontal curves anteroventrally and its dorsal surface graduallyfaces more laterally.

The postorbital is exemplified by a partial left disarticu-lated element (MACN-RN 2) and both articulated postorbitalsof the skull roof MACN-RN 144. The postorbital of Secer-nosaurus koerneri is remarkable in having an anteroposteriorlyvery broad central region (Fig. 5B), broader than in most otherhadrosaurids, such as for example Gryposaurus spp., Brachy-lophosaurus canadensis, Prosaurolophus maximus, Saurolophusspp., and lambeosaurines. The anterior margin (forming the pos-terior border of orbit) is nearly vertical. The posterior margin ofthe postorbital, which ventrally becomes part of the jugal pro-cess, is directed anteroventrally and forms an angle of 40◦ withthe nearly vertical anterior margin. As is common in hadrosaurs,the anterodorsal margin of the postorbital, which forms the pos-terodorsal corner of the orbit, is rugose due to the presence ofshort vertical indentations. The articulation with the frontal oc-curs anteromedially and the sutural border between these two el-ements is oriented diagonally and facing anterolaterally. The pos-terior squamosal process is dorsoventrally compressed and rel-atively broad mediolaterally. Proximally, its medial and lateralmargins are parallel to each other. Posteriorly, the dorsal sur-face of this process gradually faces more laterally approachingthe articulation with the squamosal. However, the latter is notpreserved. The ventral segment of the jugal process is missing inall the specimens, leaving a triangular cross-section proximally.Ventrally, the medial apex of this triangular cross-section con-nects with a well-defined ridge that extends medially into the lat-erosphenoid. This ridge bounds posteriorly the orbital cavity.

The squamosal is known from an isolated fragment (MACN-RN 991) composed of the quadrate cotylus and part of the pos-terodorsal surface of the element (Fig. 3G), and by part of thepostorbital and parietal processes of the left squamosal in theskull roof MACN-RN 144. The dorsal surface of the squamosalis gently convex anteroposteriorly. The quadrate cotylus, as pre-served with its eroded lateral surface, is deep and broad antero-posteriorly. On the posterior side of MACN-RN 991, the surfacecomprised between the parietal and posterolateral processes ismediolaterally concave and rugose, as is common in hadrosaurs,and bounded mediodorsally by a ridge. On the ventral side ofthe squamosal, a diagonal, posteromedioally oriented and shal-low ridge delimits the region containing the quadrate cotylus andthe posterior surface from a smooth and concave area that ex-tends medially.

Neurocranial Complex—The frontal of Secernosaurus koerneridiffers from that of other hadrosaurid specimens in the morphol-ogy of the nasal articular surface. In S. koerneri, the anterior and

medial margins of each frontal form a triangular process with theapex directed anteriorly (Figs. 4, 5A, and 6). The anterior bonesurface is deeply recessed, whereas the ventral margin of eachprocess is anteriorly offset relative to the dorsal border. Later-ally, a vertical ridge separates the nasal articular surface fromthe prefontal joint surface. When both frontals are articulated,the two triangular processes form a V-shaped articular surfacefor the nasal. We have also observed a bifid nasofrontal jointin other saurolophines such as Gryposaurus spp. and Edmon-tosaurus spp., and in hadrosaurid outgroup taxa such as Bac-trosaurus johnsoni, Lophorhothon atopus, and Tanius sinensis.However, the specific morphology of these triangular processesin S. koerneri, with its deep recessed surface and anteriorly off-set ventral margin, is autapomorphic for this species. The nasalarticular surface of the frontal becomes progressively shallowertowards the midline of the skull roof. Posterior to the nasofrontalarticulation, the dorsal surface of the frontal, between the midlineof the skull and the joint with the postorbital, is slightly concave.Posteriorly and medially, the frontal dorsal surface is slightly ele-vated to meet the parietal. The ectocranial surface of the frontalis relatively elongated anteroposteriorly, with a length/width ra-tio greater than 0.8. On the ventral side of the frontal, the cere-bral fossa is moderately deep and bounded laterally by a thickborder that contains the articular surfaces for the presphenoidand orbitosphenoid (Fig. 8C). The annular ridge is low and gen-tly rounded. The olfactory depression is only partially preservedbut it appears to have been as wide or wider mediolaterally thanthe cerebral fossa (Fig. 8B–C). In MACN-RN 144, the anteriorend of the postorbital appears to have prevented the anterior tipof the frontal from reaching the orbital margin.

As observed in the only preserved parietal, in MACN-RN144, this element is very elongated anteroposteriorly, with alength/width ratio greater than 2.35. This condition is also foundin other saurolophines, such as Saurolophus spp., Prosaurolophusmaximus, Gryposaurus notabilis, and Edmontosaurus spp. Theparasagittal crest is nearly straight, with a slight posteroventralinclination. The median frontal process is anteroposteriorly shortand subtriangular.

The basioccipital (Figs. 5C, 6, and 7) is slightly longer antero-posteriorly than it is wide mediolaterally. The transversal suturewith the basisphenoid is shown on the ventral side of FMNHP13423, anterior to the spheno-occipital tubercles (Fig. 7D). Theposterior two-fifths of the basioccipital, which include the occip-ital condyle, are shallow posteriorly and contain a ventral re-cess. It is also distinctly narrower mediolaterally. Anterior to thisarea, the basioccipital is wider and contains two eroded spheno-occipital tubercles. The long axis of each of these tubercles is an-teromedially directed. The spheno-occipital tubercles are sepa-rated by a median triangular excavation that widens posteriorly(Fig. 7D). Laterally, the dorsal margin preserves the articulationsurface for the exoccipital and the ventral border of the vestibu-locochlear foramen (Fig. 7A–C).

Anterior to the basioccipital is the basisphenoid (Figs. 5C, 6,and 7). This bone is incompletely preserved in all collected speci-mens, lacking the anterior region that supports the pterygoid pro-cesses. Anterior to the sphenooccipital tubercles, the basisphe-noid of S. koerneri lacks the distinct mediolateral constrictionthat precedes the pterygoid processes in all other hadrosauroids.Instead, the bone is mediolaterally wide and has a small and shortknob-like lateral expansion at mid-length of the anteroposteriorextension of the basisphenoid (Fig. 7D).

The laterosphenoid is partially preserved in the skull roofMACN-RN 144 (Fig. 6) and an isolated braincase fragment,MACN-RN 143 (Fig. 8D). The laterally projected postorbitalprocess is short and thick. The basisphenoid process formsthe anterior boundary of the trigeminal foramen, but does not

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FIGURE 4. Skull roof and braincase of Secernosaurus koerneri, MACN-RN 142, in dorsal view. Darkest grey areas represent plaster reconstruction.

enclose the ophthalmic sulcus (V1 in Fig. 8D). The prootic andthe postorbital processes form an extensive concave and postero-laterally facing surface, located adjacent and posterior to the pos-terior limit of the orbital cavity.

At the anterior end of the neurocranium, the paired presphe-noids form a cup-like structure (V-shaped in anterior view andsubtriangular in ventral aspect) with the apex pointing anteriorly(Fig. 6). The presphenoid contacts the frontal dorsally and the or-bitosphenoid posteriorly. The dorsal surface of the presphenoidforms the ventral margin of the anterior opening of a large andsubellipsoidal olfactory canal. The latter is about 30% as wide asthe combined breadth of the ectocranial surface of the frontals.

Articulating with the posterior margin of the presphenoid isthe orbitosphenoid (Fig. 6). This element is oval in shape andmediolaterally compressed, contributing to form the lateroven-tral wall of the anterior region of the braincase. Anteriorly, theorbitosphenoid articulates with the laterosphenoid and, dorsally,with the frontal. The lateral, exposed surface of the orbitosphe-noid is nearly flat.

In lateral view, the supraoccipital has the characteristicwedge-like morphology seen in hadrosaurids (Fig. 8E–F), be-ing dorsoventrally compressed and posterodorsally oriented. Inventral view, the element is fan-shaped, expanding posteriorly inmediolateral breadth. The ventral surface is rugose due to the

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FIGURE 5. Skull roof and braincase of Secernosaurus koerneri, MACN-RN 142. A, anterior, B, lateral, and C, posterior views. Darkest grey areasrepresent plaster reconstruction.

presence of longitudinal striations. The dorsal surface shows twomedian longitudinal and shallow grooves for reception of theoverlapping parietal.

The exoccipital-opisthotic complex (Figs. 4, 5B–C, and 6)is only poorly and partially preserved, being heavily recon-structed with plaster (particularly the right side, MACN-RN144). Still, it allows recognition of a relatively long shelf dor-sal and posterior to the foramen magnum, as in Gryposaurusspp. and Edmontosaurus spp. The proximal, posterolateral re-gion of the left paroccipital process is better preserved, show-ing that this process projects laterally a short distance fromthe ventral half of the neurocranium before curving ventrally.Ventral and anterior to the exoccipital roof, there are twolarge oval depressions, immediately dorsal and lateral to theforamen magnum, as in other hadrosaurids (Prieto-Marquez,2005).

Postcranial Elements

Vertebral Column—Vertebral elements are indistinct fromthose of all other hadrosaurids. The cervical centra are opistho-coelous and mediolaterally broad (Fig. 9A–D). Each parapoph-ysis is large and oval, with its long axis oriented anteroposteriorly.Postzygapophyses are supported by large and massive postzy-gapophyseal processes (Fig. 9C–D). These processes arch pos-terodorsally and diverge laterally from the neural arch.

The neural arches of the anterior dorsal vertebrae (Fig. 9E–I)are posteriorly tilted and the bases of the neural spines are broadanteroposteriorly. Ventral and medial to the postzygapophysesthere are two large and deep excavations. The centra are an-teroposteriorly longer than they are deep and wide. The mid-dle dorsals (Fig. 9J–K) have slightly opisthocoelous centra withheart-shaped and equidimensional anterior and ventral surfaces.The lateral surfaces are anteroposteriorly concave and converge

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FIGURE 6. Skull roof and braincase of Secernosaurus koerneri, MACN-RN 142, in ventral view. Darkest grey areas represent plaster reconstruction.

ventrally into a keel-like structure. The transverse processesare dorsolaterally and posterolaterally oriented. The centra ofthe more posterior dorsals are anteroposteriorly narrower (Fig.9L–M). Otherwise they are morphologically similar to those ofthe middle dorsals.

The sacrum is partially represented by several sacral vertebrae(Fig. 10C) and disarticulated neural arches with ribs and partiallycomplete neural spines. The preserved portion of the sacrum con-tains four fused centra. Each transverse process fuses ventrallywith the iliac bar. This union forms a thin wing of bone thatstretches from the transverse processes ventrally and laterally tothe iliac blade. Sacral ribs are wing-like laminae that extend lat-erally between the transverse processes and the centra.

Caudal centra have anterior and posterior surfaces with hexag-onal contours (Fig. 10D–G) as is typical in hadrosaurids. Thesecentra are anteroposteriorly compressed and have concave lat-eral surfaces. The postzygapophyses are small circular facets lo-cated on the posterior sides of the bases of the neural spines.Transverse processes are narrow, rod-like, and horizontal. Neural

spines are mediolaterally thick and project posterodorsally. Theneural canals are circular and relatively small.

As in other hadrosaurids, the sternal bone is formed by apaddle-like anteromedial plate located at the end of a long andelongated ‘handle-like’ posterolateral process (Fig. 10H). Themedial side of the sternal is slightly concave, particularly on theanteromedial plate. The posterolateral process is longer thanthe anterior plate. The process is mediolaterally compressedand ellipsoidal in cross-section. The posteroventral process isvery prominent. In the more complete specimen (MACN-RN2), its length is greater than the width of the posterolateralprocess.

Pectoral Girdle—The coracoid (Fig. 11C) has a long andcurved ventral process, with a length/width ratio of 0.78. Thehumeral and scapular facets form an angle of 126◦. The lateralmargin of the scapular facet is 17% longer than the lateral mar-gin of the humeral facet. The anteromedial margin of the cora-coid is concave and ends medially into a prominent bicipitalknob.

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FIGURE 7. Secernosaurus koerneri, frag-ment of basioccipital and basisphenoid, holo-type FMNH P13423, in A, dorsal, B, posterior,C, lateral, and D, ventral views.

The pseudoacromion process of the scapula (Fig. 11A, B, andD) is relatively large and projects laterally. Dorsal to the pseu-doacromion process, the surface of the bone merges anteriorlyinto a slightly concave surface that connects to the coracoid facet.Posterior to the pseudoacromion process, the lateral surface ofthe scapula has a pronounced and robust deltoid ridge. The nar-rowest region of the scapula (‘scapular neck’) amounts to 60% ofthe dorsoventral depth of the proximal end of the element. Thelateral surface of the proximal region of the scapular blade, pos-terior to deltoid ridge, is dorsoventrally convex. The medial sideis flat. The distal region of the scapular blade reaches 90–95% ofthe depth of the proximal region of the scapula.

Forelimb—Forelimb elements in Secernosaurus koerneri areindistinct from those of other hadrosaurids. The humerus isrepresented by a right distal fragment in FMNH P13243 (Fig.12A–B). The ulnar condyle is mediolaterally and anteroposteri-orly broader than the radial one. The condyles are separated bywell-incised notches. The mediolateral diameter of the shaft in

the this fragment is 60% of the width across the distal condyles. Asmall isolated juvenile humerus was also found in the Los Alami-tos collection (Fig. 12C) and might be referable to Secernosauruskoerneri in the absence of evidence to the contrary. In this ex-emplar, which conforms to the typical morphology of a hadrosurhumerus, the only remarkable attribute is that the deltopectoralcrest is shorter than half the length of the humerus. However, theanteromedial region of the crest has been eroded away.

The lateral and medial flanges of the proximal region of theulna (Fig. 12D–F) enclose a wide radial notch. The lateral flangeis less prominent and has a convex surface. Proximally andthroughout most of its length, the shaft of the ulna has a triangu-lar cross-section. The olecranon process is subconical and promi-nent. The ulna becomes gradually thinner distally, but it is slightlyexpanded (more mediolaterally than dorsoventrally) at the distalend.

The radius (Fig. 12G) is subcylindrical in overall shape. It is aslong as the ulna. The proximal end is cup-like and has a flattened

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FIGURE 8. Neurocranial elements of Secer-nosaurus koerneri. A and B, frontal and pari-etal, MACN-RN 142, in dorsal and ventralviews, respectively. C, right frontal, MACN(uncatalogued), in ventral view. D, brain-case fragment, MACN-RN 143, in left lat-eral view. E and F, supraoccipital, MACN-RN144, in right lateral and posterodorsal views,respectively.

proximal surface. Distally the radius is also expanded but moregradually and to a lesser extent than it is proximally.

Metacarpal III is a long as slender element, approximately ninetimes longer than it is wide at mid-shaft (Fig. 12H–I). The lateralmargin of the proximal surface is slightly longer and less verti-cally oriented than the medial margin. The dorsal surface of themetacarpal is nearly flat. The lateral and medial sides along theshaft contain longitudinal striations indicating where metacarpalII and IV would articulate.

Pelvic Girdle—Of all postcranial elements, those from thepelvic region are the most diagnostic and phylogenetically infor-mative. In the ilium (Figs. 13, 14, and 15A–B), the preacetabu-lar process forms an angle of 145◦ with the plane containing theventral margins of the pubic and ischial peduncles. The proxi-mal region of the preacetabular process is as deep as half thedepth of the central plate of the ilium. As in all hadrosaurids,the ventral apex of the supraacetabular process is located ante-rior to the lateral ridge of the posterior protuberance of the is-chial peduncle. The supraacetabular process projects ventrally toa level that is between half and 75% of the depth of the iliacplate. The process is anteroposteriorly broad, comprising 75%of the length of the central plate. The supraacetabular processis asymmetrical with a posteriorly skewed and V-shaped lateralprofile. The posterodorsal margin of the process forms a well-defined ridge that is continuous with the proximodorsal marginof the postacetabular process. The pubic peduncle is triangularas in all other Hadrosauridae. The ischial peduncle is formed bytwo protrusions. The posterior one is more dorsally located, asin all other hadrosaurids. The postacetabular process is nearlyas long as the central plate. The most notable attribute of thisprocess is that it is dorsomedially twisted and forms a medioven-tral surface (Figs. 13, 15B, 17C, and 18B) similar to the brevisshelf of various hadrosaurid outgroup taxa, such as Bactrosaurusjohnsoni and Tanius sinensis. Outside South America, withinHadrosauridae this shelf is only seen in those lambeosaurines

belonging to the Hypacrosaurus altispinus clade (e.g., H. altispi-nus, Fig. 17D and 18A, and ilia probably referable to Velafronscoahuilensis) and in the unnamed form from Big Bend Na-tional Park (UTEP P37.7.222) (Fig. 18C) and the Salitral Morenohadrosaurids. The laterodorsal surface of the postacetabular pro-cess of S. koerneri has a median longitudinal ridge (Fig. 18B) thatis only shared by the Big Bend (Fig. 18C) and the Salitral Morenohadrosaurids.

In the ischium (Fig. 15C–D), the iliac peduncle is long, with alength/width ratio of 2.3. The posterodorsal margin of the distalregion of the iliac peduncle is slightly rounded. Its distal articu-lar facet forms an angle of 128◦ with the acetabular margin of thepeduncle. The ischial shaft is posterodorsally directed, formingan angle greater than 180◦ with the long axis of the pubic pe-duncle. The ischial shaft is only represented in the type speci-men, FMNH P13423 (Fig. 15D). The ischial shaft is anteropos-teriorly elongated and relatively thin. Proximally, the bone ismediolaterally expanded and mediolaterally convex and is trian-gular in cross-section. This convexity is produced by a smoothdorsal ridge, which gradually disappears distally as the shaftbecomes mediolaterally compressed and rod-like. The medialside has longitudinal striations indicating the articulation surfacewith the other ischial shaft. The preserved distal end is oval incross-section.

The pubis of Secernosaurus koerneri is characterized by along and rectangular prepubic process (Fig. 16). A similar mor-phology is found in Gryposaurus spp., Prosaurolophus max-imus, and Saurolophus spp. The proximal constriction (‘neck’)is as long as the distal expanded region of the prepubic pro-cess. The point of maximum concavity of the dorsal marginof the proximal constriction of the prepubic process is locatednearly above that of the ventral margin. The ventral margin ofthe prepubic process forms a very wide arch. The iliac pedun-cle is large and massive. As in other hadrosaurids, it is tetrahe-dral in shape, with a concave posterolateral surface and slightly

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FIGURE 9. Axial elements of Secernosaurus koerneri. A and B, cervical centrum, FMNH P13423, in right lateral and ventral views, respectively.C and D, cervical vertebra, MACN-RN 826, in lateral and dorsal views, respectively. E and F, anterior dorsal vertebra, FMNH P13423, in posteriorand lateral views, respectively. G–I, anterior dorsal vertebra, FMNH P13423, in right lateral, anterior and dorsal views, respectively. J and K, dorsalvertebra, MACN-RN 826, in anterior and lateral views. L and M, centrum of posterior dorsal vertebra, FMNH P13423, in anterior and right lateralviews, respectively.

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FIGURE 10. Axial elements of Secer-nosaurus koerneri. A and B, neural spine ofposterior dorsal vertebra, FMNH P13423,in posterior and lateral views, respectively.C, partial sacrum, MACN-RN 2, in lateralview. D and E, anterior caudal centrum,FMNH P13423, in anterior and right lat-eral views, respectively. F and G, caudalvertebra, MACN-RN 2, in right lateral andposterior views, respectively. H, sternal bone,MACN-RN 826, in ventrolateral view.

FIGURE 11. Pectoral girdle of Secer-nosaurus koerneri. A, left scapula, MACN-RN 142, in lateral view. B, right scapula,MACN-RN 146, in lateral view. C, coracoid,MACN-RN 2, in lateral view. D, right scapula,FMNH P13423, in lateral view.

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FIGURE 12. Forelimb elements of Secer-nosaurus koerneri. A and B, distal end ofright humerus, FMNH P13423, in posteriorand lateral views, respectively. C, juvenile righthumerus, MACN-RN 980, in anteromedialview. D and E, proximal region of a left ulnain posterior and medial views, respectively. F,left ulna, MACN-RN 145, in lateral view. G,left radius, MACN-RN 145, in dorsal view. Hand I, metacarpal III, FMNH P13423, in dorsaland proximal views, respectively.

convex anterior and medial sides. The ischial peduncle is verylong and slender, with a length/width ratio of nearly 4.5 in theLos Alamitos specimens (Fig. 16A–B) and 4.0 in the smallertype exemplar (Fig. 16C). Such a long and slender ischial pedun-cle is shared by Prosaurolophus maximus (e.g., TMP 84.1.1 andMOR 454).

Hindlimb—Like the forelimb and the vertebral column, theseelements in Secernosaurus koerneri are indistinct from those of allother hadrosaurids. Accordingly, the femur is straight and colum-nar (Fig. 19A–B). A lateral indentation separates the femoralhead from the medial surface of the greater trochanter that formsthe lateral side of the proximal end of the femur. The fourthtrochanter comprises nearly 30% of the total length of the fe-mur. The distal condyles are mediolaterally compressed and aremore posteriorly than anteriorly expanded. The medial condyleis larger and mediolaterally thicker.

In the tibia (Fig. 19C–D), the cnemial crest extends distally intothe anterior surface of the proximal half of the element. The ex-panded proximal third of the tibia is cup-like in medial outlineand more than twice the diameter of the central shaft. The cne-mial crest expands anterolaterally forming the anterolateral sideof the proximal end of the tibia. The two posteromedial condylesare eroded away. The shaft of the tibia is slightly compressedmediolaterally.

Metatarsal II (Fig. 19E and G) is mediolaterally com-pressed. Distally, the element becomes mediolaterally wider.This metatarsal is dorsoventrally expanded proximally and, toa lesser degree, distally. The proximal surface is trapezoidal,whereas the distal face is strongly compressed mediolaterally anddorsoventrally elongated with a lenticular outline. The distal sur-face is anteriorly inclined. The ventral side forms a ridge alongthe proximal third of the metatarsal.

Metatarsal III (Fig. 19F and H) is longer and more robust thanmetatarsal II. It is dorsoventrally compressed and subrectangu-lar at mid-length. The proximal end is mediolaterally expandedwhereas the distal one expands dorsoventrally. The medial sideis concave proximally to articulate with the lateral, convex profileof metatarsal II. Proximally there is a flange that protrudes ven-trally. The proximal surface is crescentic and holds the convexlateral side of the proximal surface of metatarsal II. The distalsurface is subrectangular, mediolaterally concave, and dorsoven-trally convex.

PHYLOGENETIC POSITION OF SECERNOSAURUSKOERNERI

Both parsimony analyses inferred that Secernosaurus ko-erneri is a member of the Kritosaurus-Gryposaurus clade

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FIGURE 13. Right ilium of the holotype ofSecernosaurus koerneri, FMNH P13423, in A,lateral, B, dorsal, C, posterior, and D, medialviews.

within Saurolophinae (Fig. 20 and Supplementary Data 7,www.vertpaleo.org/jvp/JVPcontents.html). Unambiguoussynapomorphies in support of the inclusion of Secernosauruskoerneri within Saurolophinae are dentary marginal denticlesabsent or very reduced to small papillae along the apical halfof the dorsal half of the crown (character 10–2); relatively longparietal, length/width ratio greater than 2.35 (character 155–2);relatively long exoccipital-supraoccipital shelf above the foramenmagnum, which is substantially longer (often twice or more) thanthe diameter of the foramen magnum (character 171–2); longiliac peduncle of the ischium with a length/width ratio greaterthan 2 (character 277–2); and pubic peduncle of the ischiumoriented parallel or nearly parallel to the ischial shaft (character279–2).

Inclusion of Secernosaurus koerneri within the Kritosaurus-Gryposaurus is supported by the following unambiguous synapo-morphies: anterior end of the anterodorsal process of the nasalnot reaching the anterior margin of the narial foramen (character87–0); anteroventral process dipping steeply ventrally and form-ing an angle of 40◦ or greater with the tooth row: anterior regionof the maxilla appearing dorsally ‘inflated’ and anteroposteriorlycompressed (character 95–2); posterior end of the postorbitalposterior ramus extending to a point anterior to the quadratecotylus without overlapping the latter (character 141–0); and dor-sal margin of the infratemporal fenestra located substantially

more dorsally than the dorsal margin of the orbit with the pos-terior region of the skull roof inclined anteroventrally relative tothe frontal plane (character 201–1).

Likewise, all the analyses recovered a clade composed of Se-cernosaurus koerneri and the Salitral Moreno hadrosaurid. This issupported by the following unambiguous synapomorphies (char-acters and character states numbers, separated by a hyphen, re-fer to the documentation shown in Supplementary Data 1 and5, www.vertpaleo.org/jvp/JVPcontents.html): angle between thelateral margins of the facet for scapular articulation and theglenoid greater than 115◦ (character 216–0); asymmetrical iliacsupraacetabular process with a posteriorly skewed lateral pro-file (character 250–0; convergent in Brachylophosaurus canaden-sis, Maiasaura peeblesorum, and Tsintaosaurus spinorhinus); pos-terodorsal margin of the lateroventral rim of the supraacetabu-lar process forming a well-defined ridge that is continuous withthe dorsal margin of the proximal region of the postacetabu-lar process (character 252–0; convergent in Gryposaurus latidens,Prosaurolophus maximus, and all other iguanodontoideans, ex-cept remaining saurolophines); and thick ischial shaft, thicknessgreater than 7.5% its length (character 282–2; convergent in Bac-trosaurus johnsoni, Hypacrosaurus altispinus, Lambeosaurus lat-icaudus, Corythosaurus, and Parasaurolophus cyrtocristatus).

In addition, the parsimony analyses inferred that the clos-est outgroup to the Secernosaurus-Salitral Moreno clade is the

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FIGURE 14. Evidence of deformation in the right ilium of the holo-type of Secernosaurus koerneri, FMNH P13423. A, postacetabular pro-cess in lateral view, showing medial collapse of the bone (white arrow). B,posteacetabular process in posteroventral view, showing bone breakage(white arrow). C, supraacetabular process in lateroventral view, showingbreakage marks at the ventral region of the process (white arrow).

unnamed Big Bend UTEP hadrosaurid. This relationship is sup-ported by the following unambiguous synapomorphies: coracoidwith slightly longer scapular facet: ratio between the length ofthe lateral margin of the facet for the scapular articulation andthe length of the lateral margin of the glenoid greater than 1and up to 1.30 (character 215–1; convergent in Probactrosaurus,Lophorhothon atopus, and the Sabinas saurolophine); brevisshelf at the base of the postacetabular process of the ilium (char-acter 256–1; convergent in the iguanodontoidean outgroup taxato clade 9, Tsintaosaurus spinorhinus, and the Hypacrosaurus al-tispinus clade); well-developed medioventral ridge on the medialside of the postacetabular process that forms an oblique and ex-panded flange forming the medial margin of an extensive brevisshelf that faces more ventrally than medially (character 257–3);and presence of a anteroposteriorly oriented median ridge onthe laterodorsal surface of the postacetabular process (character258–1).

HISTORICAL BIOGEOGRAPHY OF SECERNOSAURUSKOERNERI

The reconstruction of ancestral areas using Fitch parsimony(Supplementary Data 8, www.vertpaleo.org/jvp/JVPcontents.html) and DIVA (Supplementary Data 9, www.vertpaleo.org/jvp/JVPcontents.html) suggests that hadrosaurids occupiedSouth America after a dispersal event from within the Kri-tosaurus-Gryposaurus clade. This dispersal would have occurredno later than the late Campanian and is in agreement with a simi-lar conclusion reached by Weishampel and Weishampel (1983)and Bonaparte et al. (1984). Kritosaurus navajovius and Gry-posaurus spp., outgroup taxa to the Secernosaurus clade (com-posed of the two South American taxa and the Big Bend UTEPsaurolophine), lived in western North America during the earlyCampanian–early Maastrichtian. According to the parsimonyanalysis, the ancestral area of the Secernosaurus clade was prob-

ably located in North America. However, the DIVA analysis in-ferred a widespread distribution for the ancestor of this cladein North and South America after which time the two Argen-tinean taxa would have diverged after a vicariant event. Both ofthese scenarios are congruent with the distribution of the out-group taxon to the S. koerneri-Salitral Moreno clade in the lateCampanian of Big Bend National Park, Texas. The presence ofthis taxon in southern North America provides an intermediatestage in the hypothetical dispersal of members of the Kritosaurus-Gryposaurus clade from western North America to South Amer-ica during the late Campanian.

The existence of a land connection between both Americancontinents during the late Cretaceous has recently been sup-ported by several studies (Pough et al., 2004). In particular, sub-duction of the southern part of the North American plate un-der the eastward-moving Caribbean plate may have given riseto a Proto-Antillean volcanic arc (Duellman, 2001; Pindell andKennan, 2002; Hedges, 2006). This chain of islands would havebeen located between North and South America in the posi-tion currently occupied by Central America, thereby providinga dispersal route for hadrosaurids and other vertebrates duringat least the Late Campanian (Hedges, 2006). The Antilles arebelieved to have subsequently moved eastward during the mi-gration of the Caribbean plate (Duellman, 2001). By the Maas-trichtian, this connection would have been interrupted (Pindelland Kennan, 2002). This scenario is congruent with vicariancebeing involved in the evolution of the Secernosaurus-SalitralMoreno clade in South America. A similar vicariance scenario,whereby the Caribbean plate carried ancient Mesozoic biota east-wards, has previously been posited to explain the evolution ofvarious endemic terrestrial vertebrates in the Antilles (Hedges,2006).

DISCUSSION

Provenance of the Holotype of Secernosaurus koerneri

In the original publication on Secernosaurus koerneri, Brett-Surman (1979) reported that the type specimen of thishadrosaurid was collected from the San Jorge Formation, nearthe head of the Chico River. However, Windhausen (1918) usedthe name San Jorge Formation to refer to a marine sequenceof strata of late Paleocene–early Eocene age. More recently,Gonzalez Riga and Casadio (2000) indicated that the only SanJorge Formation currently known in Argentina does not containfossils and is of early Paleozoic age. Thus, it seems unlikely thatthe type of S. koerneri came from the San Jorge Formation in sofar as it is currently defined.

Weishampel and Horner (1990) and Casal et al. (2007) indi-cated that Secernosaurus koerneri came from the Bajo BarrealFormation, whereas Bonaparte (1996), Novas (1997), and Case etal. (2000) pointed out that it came from the Laguna Palacios For-mation. The late Cretaceous strata of the San Jorge Basin, whichcomprises the region where S. koerneri was originally found, iscomposed of the upper two formations of the Chubut Group,the Bajo Barreal and the Laguna Palacios formations. The La-guna Palacios Formation overlies the Bajo Barreal Formation inthe western region of the basin (Umazano et al., 2008). In theeastern side, the Laguna Palacios Formation grades laterally intothe Bajo Barreal Formation (Umazano et al., 2008). Genise etal. (2002) pointed out that no dinosaur remains have been foundin the Laguna Palacios Formation and that those purportedly re-covered from that unit were found in areas where the latter isnot exposed. That leaves the Upper Member of the Bajo BarrealFormation as the most plausible provenance of the fossils. Thisview is supported by the finding of an isolated hadrosaurid ilumin the Upper Member of the Bajo Barreal Formation, near thearea where the S. koerneri material was found (Luna et al., 2003).

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FIGURE 15. Pelvic elements of Secer-nosaurus koerneri. A, partial left ilium inlateral view, FMNH P13423. B, left ilium,MACN-RN 2, in lateral view. C, proximal halfof right ischium, MACN-RN 2, in lateral view.D, lateral view of the ischial shaft of FMNHP13423.

The age of this section of the Bajo Barreal Formation has beenestimated as Campanian-Maastrichtian (Casal et al., 2007).

Synonymy of Kritosaurus australis and Secernosaurus koerneri

Brett-Surman (1979) diagnosed Secernosaurus koerneri on thebasis of the following iliac characters: postacetabular process thatis greatly deflected dorsomedially and elongate, unlike that ofany hadrosaur or iguanodontian; preacetabular process deflectedventrally at an angle greater than in Hadrosaurus (ANSP 10005,that he synonimized with Kritosaurus); and supraacetabular pro-cess relatively smaller than in any hadrosaur of the same size.However, these characters are not autapomorphic for S. koerneri.

We agree with Bonaparte et al. (1984), Bonaparte (1996),and Wagner (2001) that the right ilium of FMNH P13423is distorted. At first glance, the ilium of S. koerneri appearsto be unique in having a long, tapering, and dorsoventrallycompressed postacetabular process (Fig. 14). However, thetapering shape of the postacetabular process is most likely theresult of deformation. Several signs of bone breakage supportthis possibility. The proximal half of the lateral surface of thepostacetabular process has collapsed inwards (medially) as if the

process had been subjected to dorsoventral compression (Fig.14A). The distal half of the postacetabular process shows signsof medially and ventrally directed twisting. This is evidencedby a longitudinal fracture on the ventral surface that producesa concave ventral relief (Fig. 14B). This deformation couldhave produced the tapering morphology. Unfortunately, thelack of postacetabular process in the left ilium prevents anaccurate estimation of the extent of the deformation in thisprocess. Still, the dorsomedial twisting and the anteroposteriorelongation of the postacetabular process are probably natural.However, we do not consider the elongation of the postac-etabular process as autapomorphic for S. koerneri. A very longpostacetabular process (i.e., more than 90% the length of thecentral body of the ilium) is also found in Prosaurolophusmaximus, Saurolophus osborni, and Gryposaurus spp.(character 255, Supplementary Data 1, www.vertpaleo.org/jvp/JVPcontents.html). Although the dorsomedial twisting ofthe postacetabular process is probably natural, it may have beenfurther accentuated by post-depositional distortion in FMNHP13423.

The small size of the supraacetabular process is hereattributed to the same post-depositional deformation. In

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FIGURE 16. Pubes of Secernosaurus ko-erneri. A, right pubis, MACN-RN 2, in lateralview. B, left pubis, MACN-RN 2, in lateralview. C and D, right pubis, FMNH P13423, inlateral and ventral views, respectively.

particular, the lateral surface of the central plate of the rightilium of FMNH P13423 ventral to the supraacetabular pro-cess shows breakage marks suggestive of the process being lat-erodorsaly displaced (Fig. 14C, white arrows). The supraacetab-ular process in the non-deformed left ilium of FMNH P13423(Fig. 15A), although missing the lateral edge, appears to havebeen as large and ventrally projected as in any member ofHadrosauridae. The ventral deflection of the preacetabular pro-cess is within the range of state one of character 243 (angle lessthan 150◦), a condition present in all hadrosaurids and variousnon-hadrosaurid hadrosaurs (Prieto-Marquez, 2008; Supplemen-tary Data 1, www.vertpaleo.org/jvp/JVPcontents.html).

Originally, Bonaparte and colleagues diagnosed Kritosaurusaustralis based on the “cranial features observable in the denti-tion, pectoral and pelvic girdles that are coincident with those ofthe different species of Gryposaurus, except for the more promi-nent denticles in the predentary and a slight difference in the dor-sal margin of the ilium, which is convex up to the middle of theiliac plate and concave beyond that point, while in Gryposaurus

the inflexion point between the convex and concave margins oc-curs more posteriorly” (Bonaparte et al., 1984:292). Characterscoincident with Gryposaurus notabilis were loosely explained asfollows: the curvature, cross-section, and length of the preacetab-ular process; the proportions and morphology of the central plateof the ilium; the morphology and extension of the iliac postac-etabular process; the extension of the edentulous region of thedentary; and the morphology of the symphyseal region (Bona-parte et al., 1984). The two characters that led Bonaparte andcolleagues to erect a new species for Los Alamitos specimenswere the cited prominence of the predentary denticles and theinflexion point between the convexity and concavity of the dorsalmargin of the ilium being above the mid-length of the acetabu-lum. However, these two characters are not autapomorphic forKritosaurus australis.

Predentary denticles being particularly developed and promi-nent in Kritosaurus australis are not unique to this species.For example, we have observed similarly large denticles inthe specimen MSNM V345 of Gryposaurus notabilis. Likewise,

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FIGURE 17. Presence and absence of bre-vis shelf in the postacetabular process of fourspecies of hadrosaurs; images shown in ven-tral view. A, Bactrosaurus johnsoni, cast ofSBDE 95E. B, Brachylophosaurus canaden-sis, MOR 1071–8-2–98-469. C, Secernosauruskoerneri, MACN-RN 2. D, Hypacrosaurus al-tispinus, AMNH 5204. Abbreviation: BS, bre-vis shelf.

predentary denticles are usually eroded and incompletely pre-served in the majority of hadrosaurid specimens (including thetype of Kritosaurus australis, MACN-RN 2; see descriptionabove), especially in those including complete articulated skele-tons. This probably represents a preservation bias against findinglarge predentary denticles in other taxa.

As for the location of the inflexion point between the con-cave and convex profiles of the dorsal margin of the ilium, itis also found above the mid-length of the acetabular margin inother hadrosaurids. Examples are Brachylophosaurus canaden-

sis (MOR 794), Hypacrosaurus altispinus (AMNH 5204), Ed-montosaurus annectens (DMNH 1943), Maiasaura peeblesorum(ROM 44770), and Gryposaurus notabilis (ROM 764).

Various morphological attributes of the ilium and pubis, whichare both unique to and shared by Secernosaurus koerneri and Kri-tosaurus australis, led us to regard the latter as a junior synonymof the Chico River species. The FMNH P13423 specimen (thetype material of S. koerneri) is indistinct from the MACN ma-terials used by Bonaparte et al. (1984) as type and hypodigm ofK. australis. In particular, the pubis of FMNH P13423 is nearly

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PRIETO-MARQUEZ AND SALINAS—LATE CRETACEOUS HADROSAURIDS FROM ARGENTINA 833

FIGURE 18. Absence and presence (rectangles) of the dorsal ridge ofthe postacetabular process of the ilium in three species of hadrosaurids.A, Hypacrosaurus altispinus, AMNH 5204. B, Secernosaurus koerneri,MACN-RN 2. C, Unnamed Big Bend hadrosaurid (UTEP P37.7.222).

identical in morphology to that of MACN-RN 2. Although thedistal region of the prepubic process is incompletely preservedin FMNH P13423, the morphology of what is left of the pro-cess in this specimen conforms to that of MACN-RN 2. Specif-ically, the same gentle curvature of the dorsal and ventral mar-gins of the proximal constriction (‘neck’) of the prepubic pro-cess is found in both MACN-RN 2 (Fig. 16A–B) and FMNHP13423 (Fig. 16C). Likewise, this proximal constriction is verylong and slender in both specimens. A less obvious similarity isin the geometry of the lateral outline of distal blade of the pre-pubic process. In MACN-RN 2, the distal blade is subrectangu-lar and elongate, as in Gryposaurus notabilis (e.g., ROM 764).At the point, along the dorsal margin, where the distal publicblade becomes ventrally deflected, the dorsal borders of the pu-bic ‘neck’ and the distal blade form an angle of 140◦ in bothMACN-RN 2 and FMNH P13423. In the Los Alamitos speci-men, this point of the dorsal margin of the pubis shows signsof abrasion in the left and right pubes (Fig. 16A–B), so that inlife this point may have been angular as in FMNH P13423. Thedorsal and ventral margins of the pubic blade of FMNH P13423are parallel, as in MACN-RN 2, suggesting that in the formerthe complete geometry of the distal blade would probably besubrectangular as in the Los Alamitos exemplar. All these sim-ilarities between both exemplars are important because the ge-ometry of the prepubic process is the only postcranial characterthat is known to be diagnostic to generic or specific levels amonghadrosaurids (e.g., Edmontosaurus annectens; see character 265,Supplementary Data 1, www.vertpaleo.org/jvp/JVPcontents.htmlin). In addition, the ischial peduncle of the pubes of MACN-RN2 and FMNH P13423 is proportionally longer (length/width ratioof 3 or more) than in other hadrosaurids, except Prosaurolophusmaximus (e.g., MOR 454; character 272, Supplementary Data1, www.vertpaleo.org/jvp/JVPcontents.html). Although, consid-ered in isolation, none the aforementioned characters is di-agnostic of either Kritosaurus australis or Secernosaurus ko-erneri, the combination of all of them is nowhere found inHadrosauria.

The ilium of Secernosaurus koerneri is also indistinct fromthat of the type and hypodigm of Kritosaurus australis. Bothtaxa share a unique combination of three characters. The firstof these characters is an anteroposteriorly extensive and asym-metrical supraacetabular process. The second character dealswith the dorsomedial rotation of the postacetabular process de-scribed by Brett-Surman (1979). This rotation causes the me-dial surface of this process to face ventromedially. This ventro-medial surface forms a structure comparable to the brevis shelfof some hadrosaurid outgroup taxa, such as Bactrosaurus john-soni, Tanius sinensis, and Iguanodon spp. Our ancestral statereconstruction indicated that the brevis shelf in these Argen-tinean ilia is not homologous to the primitive condition seenin hadrosaurid outgroups. A similar and also independently de-rived structure, resulting also from dorsomedial rotation of thepostacetabular process, was observed in the lambeosaurines Hy-pacrosaurus altispinus (e.g., AMNH 5204) and possibly Velafronscahuilensis (CPC unnumbered specimen). Finally, the ilia of Se-cernosaurus koerneri and Kritosaurus australis show a medianlongitudinal ridge on the dorsal surface of the postacetabularprocess. The combination of these three iliac characters wasonly observed in these two taxa and in the undescribed ilia col-lected from the Salitral Moreno bonebed (Rıo Negro province,southern Argentina). The only other hadrosaurid with an il-ium having a median ridge on the dorsal surface of the postac-etabular process and a brevis shelf-like structure originatingfrom dorsomedial rotation of this process is UTEP P37.7.222,a large ilium corresponding to an unnamed hadrosaurid fromBig Bend National Park, Texas (Davis, 1983). However, thelatter has an anteroposteriorly shorter supraacetabular processand the long axis of the postacetabular process is oriented

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FIGURE 19. Hindlimb elements of Secernosaurus koerneri. A and B, left femur, MACN-RN 2, in anterior and medial views, respectively. C and D,tibia, MACN-RN 997, in lateral and posterior views, respectively. E and G, left metatarsal II, MACN-RN 145, in medial and dorsal views, respectively.F and H, left metatarsal III, MACN-RN 145, in medial and dorsal views, respectively.

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PRIETO-MARQUEZ AND SALINAS—LATE CRETACEOUS HADROSAURIDS FROM ARGENTINA 835

FIGURE 20. Single most parsimonious tree resulting from the weighted parsimony analysis of 49 hadrosaur taxa, showing the phylogenetic positionof Secernosaurus koerneri ( = Kritosaurus australis). At each node, the pair of numbers above a branch represents, from left to right, a decay indexand a bootstrap proportion. Bootstrap proportions lower than 20 are indicted by a hyphen. OTU stands for Operational Taxonomic Unit.

horizontally rather than posterodorsally as in Secernosauruskoerneri.

CONCLUSIONS

Osteological re-evaluation of the types and hypodigms of Kri-tosaurus australis and Secernosaurus koerneri led us to concludethat the former is most likely a junior synonym of the latter. The

results of a phylogenetic analysis of S. koerneri with a nearlycomplete taxonomic sample of hadrosaurid species did not sup-port inclusion of this species within the genus Kritosaurus. How-ever, S. koerneri was inferred to be a member of the Kritosaurus-Gryposaurus clade within Saurolophinae as the sister taxon ofthe Argentinean hadrosaurid from Salitral Moreno. An unnamedhadrosaurid from Big Bend National Park, Texas, was positionedas the closest outgroup taxon to the South American clade.

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The reconstruction of the ancestral area for the South Amer-ican hadrosaurid clade using Fitch parsimony and Dispersal-Vicariance analysis supported a previous hypothesis that positeda dispersal of Gryposaurus-related hadrosaurids from North toSouth America no later than the late Campanian.

ACKNOWLEDGMENTS

For access to specimens under their care, we are most gratefulto P. Barrett, B. Battail, M. Brett-Surman, D. Brinkman, K. Car-penter, S. Chapman, J. Cheng, L. Chiappe, Z. Csiki, R. Culbert-son, T. Culver, P. Currie, T. Daeschler, R. Dante, C. Delgado, J.Desojo, B. Espinosa, D. Evans, Z. Fang, M. Feuerback, R. Gaete,A. Galobart, E. Garcıa, J. Gardner, T. Gates, M. Getty, P. Gode-froit, M. del R. Gomez, M. Goodwin, D. Goujet, D. Grigorescu,A. Heckert, A. Henrici, R. Hernandez, P. Holroyd, J. Horner,B. Iwama, B. Jacobs, L. Jacobs, Y. Jun, J. Kobalynski, A. Kra-marz, M. Lamanna, J. Lamb, W. Langston, T. Lehman, C. deLeon, L. Liping, J. Li-Young, S. Lucas, E. Lund, K. Madalena,S. Maganuco, P. Makovicky, B. McLeod, C. Mehling, M. Mon-tellano, I. Morrison, C. Munoz, L. Murray, M. Norell, H. Osmol-ska, P. Owen, J. Padilla, J. Peel, M. del C. Perrillat, M. Pierce,G. Plodowski, Z. Qin, R. Reisz, L. Rinehart, T. Rowe, K. Sabath,G. Salinas, C. Dal Sasso, C. Serrano, K. Seymour, K. Shepherd,W. Simpson, E. Steurbaut, S. Stuenes, Li Tao, P. Taquet, D. Vine-yard, O. Vogel, D. Weishampel, C. Weißbrod, D. Winkler, Xu X.,L. Zanno, R. Zapata, K. Zhang, L. Zhong, and R. Zuniga. Thiswork was completed in partial fulfillment of the requirements forthe senior author’s Ph.D. degree in the Department of BiologicalScience at Florida State University, and he is especially grate-ful to his advisor G. M. Erickson, and committee members W.Parker, D. Swofford, S. Steppan, and F. Ronquist. This study wassupported by the Charlotte and Walter Kohler Charitable Trust,the Department of Biological Science at Florida State Univer-sity, the National Science Foundation (EAR 0207744 and DBI0446224 grants presented to G. M. Erickson), The Field Museum,and a grant (CGL2005–07878-C02–01) from the Ministry of Ed-ucation and Science of Spain presented to A. Galobart.

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Submitted November 12, 2008; accepted September 07, 2009.

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