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Cretaceous Research 35 (2012) 118e123

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Cretaceous Research

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New remains of Priohybodus arambourgi (Hybodontiformes: Hybodontidae)from Late Jurassice?earliest Cretaceous deposits in Uruguay

Matías Soto*, Daniel Perea, Pablo ToriñoDepartamento de Evolución de Cuencas, Facultad de Ciencias, Iguá 4225, 11400 Montevideo, Uruguay

a r t i c l e i n f o

Article history:Received 7 July 2011Accepted in revised form 2 December 2011Available online 9 December 2011

Keywords:PriohybodusHybodontidaeCutting dentitionLate JurassicWestern Gondwana

* Corresponding author.E-mail address: matiassoto1@gmail.com (M. Soto)

0195-6671/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.cretres.2011.12.001

a b s t r a c t

We describe new multicuspid, laterally compressed, and serrated tooth crowns from the Batoví Member(Late Jurassice?earliest Cretaceous) of the Tacuarembó Formation, Uruguay. These specialized toothcrowns are referable to Priohybodus arambourgi (Hybodontidae). Among them, there is a pathologicalcrown with the second lateral cusplet larger than the first, a morphological feature not described before.The total number of lateral cusplets and the angle between the main cusp and the first pair of lateralcusplets reaches higher values than reported previously. We also provide measurements of the crownsand the first published scanning electron micrographs of the denticles. Finally, we review the temporaland geographical distribution of the species. The former appears to be restricted to the KimmeridgianeHauterivian/Barremian interval, which agrees with the age proposed for the Tacuarembó Formation onthe basis of other evidence.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Priohybodus arambourgi d’Erasmo, 1960 is a non-marinehybodontid shark with specialized dentition. The species, theonly known remains of which are tooth crowns and dorsal finspines, has been recorded from Late Jurassic deposits of Somalia,North Yemen, Ethiopia and Uruguay and Early Cretaceous depositsof Tunisia and Libya (d’Erasmo, 1960; Tabaste, 1963; Cappetta,1987; Goodwin et al., 1999; Duffin, 2001; Cuny et al., 2004; LeLoeuff et al., 2010). Reported from the Tacuarembó Formation ofUruguay by Perea et al. (2001), it was the first taxon to providea biostratigraphic criterion on which to date the unit as LateJurassiceEarly Cretaceous. Since then, the sample size has signif-icantly increased, allowing a more detailed study to be carried out.The aim of this contribution is to shed more light on Gondwanancontinental Late JurassiceEarly Cretaceous fish faunas, whichaccording to López-Arbarello (2004) and López-Arbarello et al.(2008) are poorly known, and particularly on P. arambourgi. Fordetailed information concerning the geological setting see Pereaet al. (2001, 2009).

Institutional abbreviation. FC-DPV, Colección de Vertebrados Fósiles,Facultad de Ciencias, Montevideo, Uruguay.

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2. Material and methods

More than one hundred crowns (and hundreds of fragments)have been collected from the Tacuarembó Formation (for localityinformation, see Perea et al., 2001). The measurements of the mostcomplete specimens are listed in Table 1. Two variables weremeasured on tooth crowns employing a digital calliper: crownheight (CH), which is the basoapical maximum height (from thebase of the crown to the tip of the central cusp), and crown baselength (CBL), which is the mesiodistal maximum length (measuredat the base of the crown). The ratio CH/CBL was then calculated,which is a measure of the elongation or squatness of the toothcrown. The aim is to construct a growing database of Priohybodustooth measurements that may eventually serve as a basis forstatistical comparisons with African specimens. Histograms of CHand CBL were constructed using PAST 2.02 software (Hammer et al.,2001).

3. Systematic palaeontology

Class Chondrichthyes Huxley, 1880Subclass Elasmobranchii Bonaparte, 1838Order Hybodontiformes Maisey, 1987Family Hybodontidae Owen, 1846Unnamed subfamilyGenus Priohybodus d’Erasmo, 1960

Table 1Measurements of 62 complete or almost complete tooth crowns of Priohybodusarambourgi from the Tacuarembó Formation, ordered by increasing crown height.

Catalogue number Crown height(CH), in mm

Crown base length(CBL), in mm

CH/CBL

FC-DPV 1014 3.30 6.25 0.53FC-DPV 1660 3.90 5.55 0.70FC-DPV 1409 3.93 9.05 0.43FC-DPV 1387 3.98 8.98 0.44FC-DPV 1388 4.00 7.00 0.57FC-DPV 1390 4.00 6.15 0.65FC-DPV 1617 4.20 7.20 0.58FC-DPV 1620 4.30 6.75 0.64FC-DPV 1619 4.35 6.35 0.69FC-DPV 1621 4.65 6.80 0.68FC-DPV 1004 4.95 8.80 0.56FC-DPV 1005 5.10 6.93 0.74FC-DPV 1618 5.13 8.40 0.61FC-DPV 1015 5.23 8.95 0.58FC-DPV 1012 5.25 8.20 0.64FC-DPV 1623 5.25 8.65 0.61FC-DPV 1389 5.25 6.85 0.77FC-DPV 1011 5.40 8.85 0.61FC-DPV 1382 5.53 10.75 0.51FC-DPV 1397 5.58 9.53 0.59FC-DPV 1622 5.60 11.20 0.50FC-DPV 1018 5.63 6.43 0.88FC-DPV 1010 5.80 9.40 0.62FC-DPV 1383 5.85 10.65 0.55FC-DPV 1384 5.88 9.10 0.65FC-DPV 1002 5.88 10.80 0.54FC-DPV 1616 5.95 7.55 0.79FC-DPV 1016 6.15 8.65 0.71FC-DPV 1396 6.18 9.40 0.66FC-DPV 1386 6.40 10.20 0.63FC-DPV 1398 6.43 9.58 0.67FC-DPV 1009 6.55 9.30 0.70FC-DPV 1613 6.58 9.10 0.72FC-DPV 1003 6.60 8.85 0.75FC-DPV 1903 6.60 8.38 0.79FC-DPV 1399 6.80 13.75 0.49FC-DPV 1402 6.85 9.45 0.72FC-DPV 1007 7.00 11.78 0.59FC-DPV 1615 7.05 9.45 0.75FC-DPV 1624 7.10 8.65 0.82FC-DPV 1381 7.13 11.35 0.63FC-DPV 1657 7.38 11.10 0.66FC-DPV 909 7.40 10.00 0.74FC-DPV 906 7.45 15.00 0.50FC-DPV 1612 7.55 12.25 0.62FC-DPV 1395 7.70 12.63 0.61FC-DPV 1008 8.00 15.60 0.51FC-DPV 1400 8.05 13.00 0.62FC-DPV 1380 8.35 16.90 0.49FC-DPV 1391 8.75 15.63 0.56FC-DPV 1392 9.20 12.20 0.75FC-DPV 1378 9.20 14.60 0.63FC-DPV 1399 10.30 14.90 0.69FC-DPV 1605 10.45 15.55 0.67FC-DPV 903 10.50 16.63 0.63FC-DPV 1385 10.88 17.35 0.63FC-DPV 1401 11.10 11.70 0.95FC-DPV 1144 11.50 14.58 0.79FC-DPV 907 11.60 15.60 0.74FC-DPV 1143 12.95 17.75 0.73FC-DPV 1604 13.75 18.60 0.74FC-DPV 1379 15.05 16.90 0.89

M. Soto et al. / Cretaceous Research 35 (2012) 118e123 119

Priohybodus arambourgi d’Erasmo, 1960Figs. 1, 2

Material. 62 complete or almost complete tooth crowns (see Table 1for catalogue numbers). FC-DPV 2507, partial tooth crown.

Locality. Martinote, 10 km east of Batoví Hills, Tacuarembó province.Coordinates: 62�110W, 35�410S. Stratigraphic position: base of Batoví

Member, Tacuarembó Formation (see also Perea et al., 2001), LateJurassice?earliest Cretaceous.

Description. All tooth crowns are labio-lingually compressed andmost are symmetrical (Fig. 1A, note that wear gives a falseappearance of asymmetry; see also Perea et al., 2001, pl.1, figs. 1e6,2009, fig. 3), although some others show slight to moderateasymmetry, with the central cusp being distally inclined (Pereaet al., 2001, pl.1, figs. 7, 8). Almost all crowns show three pairs oflateral cusplets, although a few bear one or two additional pairs ofminute cusplets (Fig. 1A, B), thus giving a total number of 4e5 pairsof lateral cusplets (higher than reported by Duffin, 2001 and Cunyet al., 2004). Lateral cusplets diverge from the central cusp, insome cases very strongly (Fig. 1E).

The labial face of the crown is rather flat, while the lingual face isconvex. The enameloid is smooth on both faces, folds or ridgesalways being absent. Most crowns showapical wear on their centralcusps, aswell as faint vertical cracks. There is a conspicuous serratedocclusal crest over all cusps (Fig. 1 and Perea et al., 2001, pl.1, figs1e8). Denticle (¼ serration) density ranges from 2.7 to 5.35 denti-cles/mm, with the density being reduced in larger teeth. Denticlesize can be variable even among adjacent denticles (Fig. 1F).

Crown height ranges from 3.30 to 15.05 mm (mean 7.01), whilecrown base length ranges from 5.55 to 18.60 (mean 10.77).Although larger crowns do exist, they are usually fragmentary, withfew exceptions (see Perea et al., 2009, fig. 3). Crown height valuesvary from less than half of CBL (CH/CBL ¼ 0.43) to subequal to CBL(CH/CBL ¼ 0.95). Histograms of CH and CBL (giving a distributionwhich is skewed towards small teeth) and a bivariate plot of thedata have been constructed (Fig. 2).

FC-DPV 2507 (Fig. 1B) is a partial crownwith a broad, triangular,high central cusp, flanked by four lateral cusplets. Strikingly, thesecond lateral cusplet is larger than the first (Fig. 1B), a feature notobserved before among the specimens from the TacuarembóFormation, nor reported for P. arambourgi elsewhere (normally thelateral cusplets decrease in size away from the main cusp, the firstone being the largest of all lateral cusplets; Fig. 1A). Also the firstcusplet shows an irregular apical profile. FC-DPV 1604, the largestcrown in the sample considering CBL, has a slender central cuspwith the apex being lingually recurved, and a high CH/CBL ratio(0.74). FC-DPV 1401 and 1609 (Fig. 1C, D) among others preservethe root, which is tabular and pierced with small (and occasionallylarger) foramina, and slightly lingually angled (Fig. 1D).

4. Discussion

4.1. New material

All teeth recovered from the Tacuarembó Formation representthe same taxon (Perea et al., 2001, 2009), and agree with the diag-nosis of P. arambourgi (e.g., Goodwin et al., 1999; Duffin, 2001). Thedentition is rather homodont, the more asymmetric teeth beingprobably of lateral position (Duffin, 2001; Perea et al., 2001), and theCH/CBL ratiodecreasesdistally. Itmustbenoted thateven the largestUruguayan specimens are considerably smaller than some Africanspecimens. Indeed, overlap among Uruguayan and Libyan teeth issmall, the latter (withCBL¼10e39mm)attaininga largermaximumsize (Duffin, 2001; Perea et al., 2001). Duffin and Cuny (2008) re-ported that the largest crowns measure up to 40 mm in height and35mm inwidth. (Alternatively, the bias towards small crowns in theUruguayan sample can be explained if most teeth belong to eitherjuvenile individuals or distal positions.) In addition, some Tunisianspecimensare striated (Cunyet al., 2004),whereas all theUruguayanspecimens show smooth enameloid, as the remaining Africanspecimens. Finally, the angle between the main cusp and the apical

Fig. 1. A, typical Priohybodus arambourgi tooth crown from the Tacuarembó Formation. B, FC-DPV 2507, aberrant partial tooth crown; note that the second lateral cusplet is thelargest; lateral cuspslets are numbered. C, D, FC-DPV 1609, partial crown and root. C, lingual view. D, mesial/distal view. E, FC-DPV 1144, almost complete tooth crown in lingualview; note the slightly obtuse angle between the central cusp and the apical edge of the first pair of lateral cusplets. F, SEM micrograph showing close-up of central cusp denticles ofa P. arambourgi tooth crown from the Tacuarembó Formation. Scale bar represents 5 mm.

M. Soto et al. / Cretaceous Research 35 (2012) 118e123120

edge of the first pair of lateral cusplets is greater in the Uruguayanspecimens (reachingorevensurpassing90�; Fig.1E),while it is acuteat least in someTunisian specimens (see Cunyet al., 2004, pl.1,figs.1,2). We believe these differences may eventually prove to be signif-icant enough to consider the erection of a new Priohybodus speciesfor at least part of the Tunisianmaterial, unless they are regarded asthe product of geographical or stratigraphical variation. It must benoted that the holotype of P. arambourgi comes from the UpperJurassic of Somalia (d’Erasmo, 1960).

FC-DPV 2507 is also clearly referable to P. arambourgi. Its raremorphology, not reported before, is interpreted as the result ofa pathological malformation, probably caused by an injury at thelevel of the dental lamina. FC-DPV 1604 is probably from a mesialposition, owing to the slenderness of the central cusp and its

lingually recurved apex. The characters of the root in FC-DPV 1609and 1401 agree with those mentioned by Duffin (2001) and Cunyet al. (2004), and are present in most hybodontids (Rees, 2008).

4.2. Palaeoecology

Priohybodus belongs in the family Hybodontidae (Duffin, 2001;Cuny et al., 2003, 2004), particularly to a subclade whosemembers bear specialized high-crowned, multicuspid teeth(Hybodontinae sensu Maisey, 1989), although according to recentresearch the Hybodontinae is restricted to Hybodus and Egertono-dus, with Priohybodus being placed in an unnamed subfamily alongwith Planohybodus and Secarodus (Rees, 2008; see below). Thescarcity of high-crowned hybodont teeth may be at least in part

Fig. 2. A, B, histograms of the measurements of P. arambourgi tooth crowns listed in Table 1. A, histogram of CH. B, histogram of CBL. C, bivariate plot of the measurements listedin Table 1.

M. Soto et al. / Cretaceous Research 35 (2012) 118e123 121

correlated with the absence of a triple layered enameloid (Duffinand Cuny, 2008).

The development of cutting dentition among hybodonts (Fig. 3)occurred several times independently (Cuny et al., 2008).Besides Priohybodus it is known in Planohybodus from theBathonianeBarremian ofwesternEurope and “Neocomian”of Brazil(Pinheiro et al., 2008; Rees and Underwood, 2008; Bermúdez-Rochas, 2009), Secarodus from the Bathonian of England (Rees andUnderwood, 2008), Pororhiza from the Albian of the DemocraticRepublic of Congo (Casier,1969), Thaiodus fromtheAptianeAlbianofThailand and Tibet (Cappetta et al., 1990; Cuny et al., 2003, 2008),and the recently described Mukdahanodus from the pre-AptianLower Cretaceous of Thailand (Cuny et al., 2009), the first threetaxa being high-crowned and the other three being low-crowned.

It should be noted that forms with fully serrated dentitions(Priohybodus, Pororhiza, Thaiodus and Mukdahanodus) wererestricted in time from the Late Jurassic to the Early Cretaceous.Their disappearance was probably linked to a transgressive peakrecorded in the Late Albian and a commensurate reduction incoastal plain habitats (Cuny et al., 2009). The compact single crys-tallite enameloid of Priohybodus and Thaiodus appears to becorrelated with the development of a serrated dentition. This alsooccurs in the Palaeozoic Carcharopsis (Duffin and Cuny, 2008),which is considered to be a cladodont/ctenacanthiform (Ginteret al., 2010).

Priohybodus arambourgi shares with the hybodontids Planohy-bodus ensis and Secarodus polyprion the presence of labio-linguallycompressed, serrated crowns (although strictly speaking only largePlanohybodus ensis teeth show denticles; Underwood and Rees,2002). Secarodus polyprion also shares with P. arambourgi thepresence of divergent cusplets (Rees and Underwood, 2008). These

similarities, some recognized over 50 years ago by d’Erasmo (1960),have led a few authors (Cuny et al., 2004; Soto, 2005; Rees andUnderwood, 2008) to hypothetize that these taxa were related.Indeed, Rees (2008) grouped Priohybodus, Planohybodus andSecarodus in an unnamed subfamily within the Hybodontidae,herein termed the “priohybodontines”.

Hybodonts became less significant in marine environmentscoincident with neoselachian radiation, which has led to theconclusion that most hybodonts were gradually displaced frommarine environments by neoselachians in the Jurassic, beingconfined to specialized niches (with cutting dentition in someforms) in freshwater environments (Reif, 1988; Rees, 1998; Cunyet al., 2003, 2004, 2009). Perhaps the best example of the successof these freshwater hybodonts is in Early Cretaceous deposits ofThailand (Cuny et al., 2003, 2008, 2009). In the TacuarembóFormation, a freshwater environment is strongly suggested by thepresence of conchostracans (spinicaudatans), unionoid bivalves,ceratodontiform dipnoans, and mawsoniid coelacanths (Martínezand Figueiras, 1991; Martínez et al., 1993; Shen et al., 2004; Pereaet al., 2009; Soto et al., 2010).

Serrated teeth developed convergently in several chon-drichthyan lineages (Cappetta, 1986; Duffin, 2001; Duffin and Cuny,2008), including hybodontiforms (see above), neoselachians (e.g.,carcharhiniforms and lamniforms) and forms of uncertain phylo-genetic relationships (e.g., Carcharopsis). The high-crowned andrelatively homodont dentition of Priohybodus, characters that musthave some biomechanical significance during feeding, recall bothextinct and extant neoselachians of the cutting-dentition type(Cappetta, 1986; Duffin, 2001).

Estimations of maximum body size for Priohybodus range from1.5 m (Perea et al., 2001) to more than 2 m (Cuny et al., 2004). We

Fig. 3. Late Jurassic palaeogeographic restoration, depicting the known occurrences ofhybodontiforms with cutting dentition: grey, Late Jurassic occurrences; black, EarlyCretaceous occurrences. AeF, Priohybodus arambourgi. A, KimmeridgianeHauterivian,Uruguay. B, Late Jurassic, Somalia. C, Tithoniane?Early Cretaceous, Ethiopia. D, Kimmer-idgianeTithonian, North Yemen. E, Hauterivian/Barremian (or older), Libya. F, Hau-terivian/Barremian (or older), Tunisia. GeI, Planohybodus. G, Neocomian, Brazil (P. sp.). H,CallovianeOxfordian, England (P. peterboroughensis); Bathonian, England, Scotland andFrance (P. grossiconus); BerriasianeBarremian, England; HauterivianeBarremian, Spain(P. ensis); Berriasian, Denmark (P. sp.). I, Pororhiza molimbaensis: Albian, DemocraticRepublic of Congo. J, Secarodus polyprion: Bathonian, England. K, Mukdahanodus trisiva-kulii: pre-Aptian Early Cretaceous,Thailand. L,M, Thaiodus ruchae. L, AptianeAlbian, Tibet.N, AptianeAlbian, Thailand. Ages taken fromCappetta et al. (1990), Goodwin et al. (1999),Duffin (2001), Pinheiro et al. (2008), Rees and Underwood (2008), Bermúdez-Rochas(2009), Cuny et al. (2009) and Le Loeuff et al. (2010). Map based on palaeogeographicalrestoration by Ronald Blakey (http://jan.ucc.nau.edu).

M. Soto et al. / Cretaceous Research 35 (2012) 118e123122

agree with Cuny et al. (2004) that the taxon was one of the toppredators in its environment and probably needed a significantwater depth to be able to survive. Deposits of large lakes and riversystems have been recorded in the Tacuarembó Formation (Pereaet al., 2001, 2009), and remains of other large aquatic organisms,such as semionotids (Perea et al., 2001), coelacanths (Soto et al.,2010), and a giant clam (Martínez et al., 1993) have been found init. The high profile of the teeth suggests they were not very resis-tant to tensile stress. Thus, as suggested by Perea et al. (2001), it isunlikely that Priohybodus preyed upon the heavily armouredsemionotiform fish. It seems more plausible to consider that it fedon large, soft prey as proposed by Duffin (2001) and Cuny et al.(2008). Thus, among the fish from the Tacuarembó Formation,coelacanths (Soto et al., 2010), at least juvenile individuals, were themost probable prey of Priohybodus.

4.3. Biostratigraphic and palaeobiogeographic implications

As stated above, Priohybodus was the first biostratigraphicallyrelevant taxon to be discovered in the Tacuarembó Formation,

allowing Perea et al. (2001) to propose a Late JurassiceEarlyCretaceous age for the unit. The radiometric dating of the overlyingbasaltic flows of the Arapey Formation (132 Ma; Féraud et al., 1999)implies that the unit is not younger than Hauterivian. An age notolder than Late Jurassic is implied by radiometric datings (165 Ma;de Santa Ana and Veroslavsky, 2003) of the basaltic flows of theGaspar Formation, which underly the probable Middle JurassicItacumbú Formation (de Santa Ana and Veroslavsky, 2003), whichin turn underlies the Tacuarembó Formation.

The Late JurassiceEarly Cretaceous age agrees with other recentevidence, such as records of the conchostracan Orthestheria(Migransia), the dipnoan “Ceratodus” tiguidiensis, Ceratosaurus-liketheropod teeth and the coelacanth Mawsonia (Shen et al., 2004;Soto and Perea, 2008, 2010; Soto et al., 2010).

The Priohybodus remains have come from the base of the lowermember (Batoví Member) of the Tacuarembó Formation, of LateJurassice?Early Cretaceous age (Shen et al., 2004; Soto and Perea,2008; Perea et al., 2009). Scherer and Goldberg (2007) consideredon the basis of independent evidence that a latest Jurassic age is themost probable for the onset of the aeolian sedimentation in theBotucatu Formation, a Brazilian unit which is lithologically corre-lated with the non-fossiliferous upper member of the TacuarembóFormation (Rivera Member; Perea et al., 2009). A Late JurassiceEarly Cretaceous age is also suggested by palaeomagnetic data(Tamrat and Ernesto, 2006). The stratigraphic range of Priohybodushad been thought to reach the Aptian/Albian (Duffin, 2001; Cunyet al., 2004), although recently it has been suggested that theTunisian Douiret Formation and its Libyan equivalent are Hau-terivian/Barremian in age, if not older (Cuny et al., 2010; Le Loeuffet al., 2010). Thus, the biochron of the genus now being Kimmer-idgianeHauterivian/Barremian further constrains the age proposedfor the Tacuarembó Formation.

Freshwater taxa may contribute to reconstructing ancientdrainage systems (Patterson, 1975). Priohybodus is regarded asa freshwater form (Cuny et al., 2004; Anderson et al., 2007)restricted to Western Gondwana (Fig. 3). It must be noted thatNorth African occurrences are younger (Early Cretaceous ofTunisia and Libya; Fig. 3), which suggests that the species rangednorthwards from the south (which is at odds with the purportedEuropean origin of “priohybodontines”), and that Eastern Africanspecimens are rare (in the Upper Jurassic of Somalia, Ethiopia andNorth Yemen; Fig. 3), which may reflect either an expansion ofbrackish coastal environments, already proposed for the MugherMudstone Formation (Goodwin et al., 1999) or a collectingartifact.

Priohybodus remains are abundant only in Uruguay, Tunisia andLibya (Duffin, 2001; Cuny et al., 2004; Le Loeuff et al., 2010; thiscontribution). Taking into account the palaeogeographic distribu-tion we believe it is just a matter of time before the taxon isdiscovered in Brazil and West Africa. Indeed, in Uruguay as well asin several African rock units it is associated with other freshwatertaxa such as ceratodontid and arganodontid/asiatoceratodontiddipnoans and/or mawsoniid coelacanths (e.g., Maisey, 2000;López-Arbarello et al., 2008; Soto and Perea, 2010 and referencestherein; Soto et al., 2010), these taxa being simultaneously alsopresent in “middle” Cretaceous deposits in Brazil (e.g., Dutra andMalabarba, 2001; Medeiros and Schultz, 2001; Castro et al.,2004; Medeiros et al., 2007; Santos and Carvalho, 2009; Sotoand Perea, 2010).

Acknowledgements

We are indebted to G. Arratia, D.D. Bermúdez-Rochas, H. Botella,H. Cappetta, G. Cuny, M.S.S. de Carvalho, C.A. Duffin, J. Le Loeuff andJ. Rees, who provided important bibliographic references. Valuable

M. Soto et al. / Cretaceous Research 35 (2012) 118e123 123

comments by C.A. Duffin, G. Cuny and C.J. Underwood greatlyimproved an early version of this manuscript. This is a contributionto Project ANII-FCE-2007-110.

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