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Triconodont mammals from the medial Cretaceous ofUtahRichard L. Cifelli a & Scott K. Madsen ba Oklahoma Museum of Natural History and Department of Zoology , University ofOklahoma , Norman, Oklahoma, 73019b Dinosaur National Monument , P. O. Box 128, Jensen, Utah, 84035Published online: 24 Aug 2010.

To cite this article: Richard L. Cifelli & Scott K. Madsen (1998) Triconodont mammals from the medial Cretaceous of Utah,Journal of Vertebrate Paleontology, 18:2, 403-411, DOI: 10.1080/02724634.1998.10011068

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Journal of Vertebrate Paleontology 18(2):403-411 . June 1998© 1998 by the Society of Vertebrate Paleontology

TRICONODONT MAMMALS FROM THE MEDIAL CRETACEOUS OF UTAH

RICHARD L. CIFELLJI and SCOTT K. MADSENZ'Oklahoma Museum of Natural History and Department of Zoology, University of Oklahoma, Norman, Oklahoma 73019;

2Dinosaur National Monument, P. O. Box 128, Jensen, Utah 84035

ABSTRACT-Herein we de scribe triconodontid mammals from the upper part of the Cedar Mountain Formation(placed approximately at the Albian-Cenomanian. or Lower-Upper Cretaceous, boundary), Emery County, Utah.Three new species. based on dentulou s jaw fragments and isolated teeth. are included; one is referred to Astrocon ­odo n , one to Corviconodon (both previously rec orded from the Aptian-Albian). and one to a new genus . Thisdiversity is comparable to that seen in the Aptian-Albian C loverly Formation. and differs from succeeding faunasof the North American Cretaceous, in which triconodontids are rare or lacking altogether. The species differ greatlyin size; the largest, at an estimated body weight of 750 g, is the largest known triconodontid and pre sumablyincorporated vertebrate prey into its diet. Triconodont relationships are problematic . Craniodental evidence supportsa monophyletic Triconodontidae, and there is weak support for a monophyletic grouping of the North AmericanCretaceous taxa within the family. However, an exclusive common ancestry for North American Cretaceous tricon­odontids cannot be readily identified among the morphological diversity known for Late Jurassi c taxa. Instead,limited evidence at hand sugges ts that Late Jurass ic Triconodontidae (Triconodon , Priacodon , Tri ora codon) form amonophyletic group.

INTRODUCTION

Triconodonts are of general interest because of their relative­ly simple, serially tricuspate molars, a pattern widely acceptedas being primitive for mammals and proximate relatives ofMammalia (e.g., Jenkins and Crompton, 1979; Rowe, 1993 ;Crompton and Luo, 1993). The integrity of a taxonomic group­ing (Order Triconodonta) including these taxa has been chal­lenged in recent years, however, because a growing body ofevidence supports relationship of Triconodontidae within Mam­malia, to the exclusion of Early Jurassic taxa, suggesting thatTriconodonta may be polyphyletic (see summary by Rougier etaI. , 1996).

As recognized by Jenkins and Crompton (1979), the Tricon­odontidae ranges from the Late Jurassic well into the Late Cre­taceous, and includes six genera (Cifelli, Wible, and Jenkins,1998). In North America, two genera, collectively including sixspecies, are known from the Upper Jurassic Morrison Forma­tion (Simpson, 1929 ; Rasmussen and Callison, 1981). A singlebut evidently extremely abundant species, Astroconodon deni­soni, has been described from the Aptian-Albian Trinity Groupof northern Texas (Patterson, 1951, 1956; Slaughter, 1969 );three species, Corviconodon monranensis and two unidentifiedtaxa, have been reported from the approximately equivalentCloverly Formation (see Ostrom, 1970; Jacobs et aI. , 1991 ) ofMontana and Wyoming (Clemens et aI., 1979 ; Jenkins andCrompton, 1979 ; Cifelli et al ., 1998 ). The youngest record ofTriconodontidae in North America is that of Alticonodon lin­doei , known by two specimens from the lower Campanian up­per Milk River Formation, Alberta (Fox, 1969. 1976).

Herein we describe Triconodontidae from the medial Creta­ceous Cedar Mountain Formation of Emery County, Utah (Fig.1). The Cedar Mountain Formation, named by Stokes (1944,1952 ) for a series of terrigenous sedimentary rocks overlyingthe Jurassic Morrison Formation and underlying the CretaceousDakota Formation, crops out broadly in western Colorado andeastern Utah. Two units are generally recognized within theunit, an underlying Buckhorn Conglomerate and an unnamed,upper "shale" member (Stokes, 1952) . Dinosaur faunas of sus­pected Barremian and Aptian-Albian age have been reportedfrom the Buckhorn Conglomerate and lower part of the "shale"

member, respectively (Kirkland. 1996). The upper part of the" shale" member has produced a palynoflora alternatively con­sidered to be of Albian (Tschudy et al ., 1984) or Cenomanian(Nichols and Sweet, 1993 ) age . Concentrated efforts, includingmicrovertebrate quarrying, screenwashing, and associated tech­niques (Cifelli et aI., 1996), have yielded a diverse vertebratefauna from a narrow stratgraphic interval, 10-20 m below thecontact with the Dakota Formation, in the upper part of the" shale" member of the Cedar Mountain Formation. Multiple,concordant radiometric dates (40ArfJ9 Ar, single-crystal laser fu­sion) obtained for sanidine crystals from a volcanic ash strati­graphically interposed among several of the most prolific sitesgive an age determination of 98 .39 ± 0.07 Ma for the fauna,which includes 72 taxa based on 4,882 specimens from 31 sites(Cifelli et aI., 1997). The only mammals hitherto described fromthe Cedar Mountain Formation are Multituberculata (four spe­cies referred to the Late Cretaceous genus Paracimexomys and,perhaps, two additional, unidentified taxa; Eaton and Nelson,1991) and the " marsupial-like" tribosphenidan Kokopellia jud­di (see Cifelli, 1993).

The triconodontid specimens described herein were collectedat eight sites (Fig. 1) in upper parts of the Cedar MountainFormation, Emery County, Utah. The stratigraphic position ofthe se sites is presented elsewhere (Cifelli et aI., in press); de­tailed locality data are on file at the Oklahoma Museum ofNatural History and are available to qualified investigators uponrequest. Most of the fossil materials consists of isolated teeth,many of which are fragmentary. Tooth positions and variabilitywere established on the basis of comparison with an exceptionalseries of a morphologically similar, unnamed taxon from theCloverly Formation (Jenkins and Crompton, 1979) and withoriginal materials of Triconodontidae from the Morrison For­mation (Simpson, 1925a, b; 1929). Cusp terminology for cheekteeth follows that of Crompton and Jenkins (1968); measure­ments, in millimeters, were taken with a Reflex microscope(MacLarnon, 1989). Abbreviations for institutions cited in thetext: FHSM, Sternberg Museum of Natural History, Fort HaysState University; FMNH, Field Museum of Natural History;OMNH, Oklahoma Museum of Natural History; SMU, ShulerMuseum of Paleontology, Southern Methodist University;YPM, Yale Peabody Museum.

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Ca tI eDale •

FIGURE I. Distribution of triconodontid-bearing microvertebrate lo­calities in the Cedar Mountain Formation, Emery County, Utah. Num­bers correspond to OMNH sites; OMNH V80 I is the same as RoughRoad Quarry of Nelson and Crooks (1987) and RRQ of Eaton andNel son (1991). Distribution of the Ced ar Mountain Formation (shadedpattern), which follows the flank s of the San Rafael Swell (an anticline),is after Nel son and Crooks (1987).

SYSTEMATIC PALEONTOLOGY

TRICONODONTIDAE Marsh, 1887Genus ASTROCONODON Patterson, 1951

ASTRODONODON DELICATUS, sp. nov.(Figure 2, Table I)

Etymology-L., delicate, dainty, tender, in allusion to thesize and gracile appearance of teeth referred to the species.

Holotype-OMNH 3290 I, anterior part of right lower molarwith cusps a and b.

Hypodigm-The holotype, and OMNH 29598, posterior partof left ultimate upper premolar, perhaps P4; OMNH 25783,right upper molar; 29656, posterior part of right upper molarwith cusps A and C; 33018, anterior part of right upper molarwith cusps A and B; FHSM VP10397, right penultimate lowerpremolar, perhaps p3; and OMNH 29668, left ultimate lowerpremolar, perhaps p4.

Localities and Horizon-OMNH localities V239 (OMNH25783, 3290 I), V695 (OMNH 29598), V696 (OMNH 29668),V868 (OMNH 33018), and V801 (OMNH 29656, FHSMVP10397; OMNH locality V801 is the same as RRQ or RoughRoad Quarry of Nelson and Crooks, 1987 ; Eaton and Nelson,1991), 10-20 m below upper contact of upper member, CedarMountain Formation, Emery County, Utah (Fig . I) .

Diagnosis-Smallest species of the genus: dental measure­ments about 80 % of the size of A. den isoni. Differs from thisspecies, and other known North America Cretaceous Tricono­dontidae, in lacking a lingual cingulid on lower molars and ,

where known, by lacking cingulids on the ultimate lower pre­molar.

Description-The holotype (OMNH 32901 ; Fig . 2A-C), anincomplete, unworn lower molar, is remarkable among NorthAmerican Cretaceous triconodontids for its small size and inlacking a lingual cingulid. Cusps a and b are relatively tall ,slender, acute (when viewed from the side), and only slightlyrecumbent; judged from comparison with other triconodontids(A . denisoni, Priacodon, Trioracodoni , cusp height (particularlythat of cusp b) suggests that the tooth is a posterior molar.Nonetheless, the cusps are remarkably tall , falling in the upperpart of the range observed for A. denisoni. A deep groove ispresent at the anterior margin of the tooth.

The upper molars (Fig. 2D-G) are highly similar to those ofA. den isoni (e.g., FMNH PM 885; upper molars of A. denisonihave not been described), differing chiefly in size and in lackingfine labial sulci delimiting the main cusps. The upper molarsof both species lack the pronounced lingual cingulum, labialcingulum, distinct separation of cusps, and bilobed appearanceof the lingual part of the crown seen in Late Jurassic taxa tPria­codon, Trioracodon). Three principal and subequal cusps, A,B, and C, can be discerned on the basis of indentations on thelingual surface of the crown; labially, the positions of cusps Band C are indicated by faint ribs rather than relief at the occlusalsurface and, in the unworn to lightly worn condition, at least,by slight relief at the occlusal surface. With increased wear(e.g., OMNH 25783; Fig . 2E) , deep, V-shaped notches, pro­duced by occlusion with the lower molars, develop betweencusps A-B and B-C; beveled wear surfaces are also presentanterior to cusp B and posterior to cusp C. Presumably, thenotch between A and B was produced by lower molar cusp a,the notch between A and C was produced by lower molar cuspc, and the shear surfaces anterior to B and posterior to C (ofadjacent molars) were produced by lower molar cusp b, as inJurassic Trioracodon (see Cromptom, 1974) . Late in wear, theselast two shearing surfaces probably occluded with both cusp band the adjacent surface of cusp d on the next anterior lowermolar, as in Cretaceous Corviconodon (Cifelli et al ., 1998). Theanterior face of OMNH 25783 bears a deep, V-shaped groovethat extends all of the way to the base of the root. A sharpridge, present on the posterior face of the crown, pre sumablyextended down the root (which is abraded); these counterpartfeatures produce a tongue-and-groove interlocking mechanismbetween adjacent teeth, as in the lower molars (Patterson,1951) . A ridge is lacking from the posterior face of OMNH29656, suggesting that it is a last upper molar.

Premolars have not been described previously for Astrocon­odon . OMNH 29598 (Fig. 2H-I), the posterior part of an ulti­mate upper premolar (perhaps P4) bearing cusps C and D, hasa squared heel defined by two minor, basal cuspules. Cusp D,which is strongly developed in comparison to the condition inJurassic Triconodontidae (Triora codon, Priacodoni , is muchlower than cusp C, and a distinct notch lies in the crest con­necting them; a well-defined lingual cingulum is present. Pos­terior lower premolars are more distinctly quadricuspate than inJurassic taxa, owing to stronger development of cusps band d,and, like the lower molar, lack both labial and lingual cingulids.The penultimate premolar (Fig . 2J-K), presumably p3, is an­teroposteriorly elongate, with a lower cusp a and with cusp cplaced lower on the posterior flank of cusp a than in the ulti­mate premolar. The ultimate lower premolar (OMNH 29668;Fig . 2L-M) is comparatively taller and shorter anteroposteri­orly . The heel lacks accessory cuspules and is much higher thanon the preceding tooth , with cusp d being placed nearly halfwayup the crown-a further point of contrast with Jurassic Tricon­odontidae, in which cusp d is much lower. Cusps c and dareclosely juxtaposed; cusp a, the principal cusp of the tooth, isslender, and is slightly recumbent posteriorly. Faint keels de-

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CIFELLI AND MADSEN-CRETACEOUS TRICONODONTS FROM UTAH 405

FIGURE 2. Astroconodon delicatu s, sp . nov . A-C, OMNH 32901 (holotype), right mX in occlu sal (A) , labial (B), and lingual (C) views. D­G , OMNH 25783, right MX in occlusal (D), lingual (E), anterior (F), and labial (G) views. H-I, OMNH 29598, posterior part of left ultim ateupper prem olar (P4 ?) in labial (H) and lingu al (I) view s. J-K, FHSM VP10397, right penultimate lower premolar (p3 ?) in lingual (J) and labial(K) views. L-M, OMNH 29668, left ultimate lower premolar (p4 ?) in lingu al (L) and labial (M) view s. Scale bar = I mm .

TABLE I. Dental measurements (mm) of Triconodontidae from theCedar Mountain Formation, Emery County, Utah . AP, greatest ante ro­posterior length; W, greatest width.

scend the anterior and posterior faces of cusp a, but these arenot developed into sharp crests. Cusp b (which is missing itstip) is strong and is placed at the base of the crown.

Tooth Specimen no. AP W Genus CORV/CONODON Cifelli, Wible, and Jenkins, 1998

Astroconodon delicatus, sp . nov. CORVlCONODON UTAHENS/S, sp . nov .

mX OMNH 32901 (type) 0.68 Figure 3, Table 1MX OMNH 25783 1.96 0.96 Etymology-Named for the State of Utah, where the typeMX OMNH 29656 1.01MX OMNH 33018 0.66 and referred specimens of the species were collected.p3? FHSM VPI0397 2.09 0.69 Holotype-OMNH 26362, right lower molar lacking the tip4 ') OMNH 29668 1.65 0.74 of cusp b.p .

Corviconodon utahensis, sp. nov . Hypodigm-The holotype, and OMNH 27423, left lower

mX OMNH 26362 (type) 3.36 1.13 molar with cusps a, c, and d (the latter missing its tip); 25608,mX OMNH 27423 1.11 posterior part of left lower molar with cusps c and d; 25623,mX OMNH 25623 3.52 0.95 fragment of mandible with right lower molar missing cusp api-m5 OMNH 25786 1.31 0.83 ces; 25786, ultimate right lower molar; 25788, anterior part ofP2? OMNH 32913 3.27 1.22 right lower molar with broken cusps a and b; 27633, posteriorP3? OMNH 25782 3.23 0.95 part of right lower molar with cusps c and d; 32913, left anteriorP4? OMNH 25784 1.27

upper premolar, perhaps P2; 25782 and 25827, right upperJugulator ampliss imus, gen . et sp. nov . ?penultimate premolars, perhaps P3 ; 25784, left ?ultimate upper

mX OMNH 33850 (type) 4.64 1.55 premolar, perhaps P4 ; 34015, posterior part of left upper molarmX OMNH 25785 5.02 1.51 with cusps C and D; and 29763, posterior part of right lowermX OMNH 26716 4.39 1.71PI ? OMNH 25624 0.91 anterior premolar, perhaps p2, with half of cusp a and with

cusps c-d present.

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FIGURE 3. Corviconodon utahensis, sp . nov . A-C, OMNH 26362 (holotype), right mX in occlu sal (A) , lingual (B), and labial (C) views. D­F, OMNH 257 86 , ultimate right mX in occlusal (D), labial (E), and lingual (F) views. G-I, OMNH 25623, fragm ent of right mandible with mXin occlusal (G), posterior (H), and anterior (I) views. J-K, OMNH 32913, left anterior upper premolar (P2 ?) in lingu al (J) and labial (K) views.L-M, OMNH 25782, right penultimate (P3 ?) upp er premolar in lingu al (L) and labial (M) views. N-O, OMNH 25784, left ultimate (P4 ?) upperpremolar in lingual (N) and labial (0) views. P-Q, OMNH 29763, posterior part of right lower anterior (p2 ?) premolar in labial (P) and lingual(Q) views. Scale bar = 2 mm .

Localities and Horizon-OMNH localities V234 (OMNH25623), V235 (OMNH 25608, 29763), V239 (OMNH 25782,25784, 25788, 29709, 25786), V240 (OMNH 25827, 26362),V695 (OMNH 27423), and V794 (OMNH 27633, 32913); 10­20 m below upper contact, upper member, Cedar Mountain For­mation, Emery County, Utah (Fig . 1).

Diagnosis-Similar to C. montanensis, and differing fromother, comparable North American Cretaceous triconodontids(Astroconodon, new taxon described below) in having relativelylower-crowned lower molars and an extremely small last lowermolar; differs from the former in having more posteriorly re­cumbent, less acute cusps, and from the latter in having moreacute molar cusps and lower molar cusp d consistently devel­oped as a fingerlike projection, not an anteroposteriorly devel­oped ridge. Differs from most closely similar species, C. mon­tanensis , in having a relatively shorter ultimate lower molarbearing only cusps a and b, rather than a, b, and c.

Description-The holotype, OMNH 26362 (Fig. 3A-C), ta­pers posteriorly, as does OMNH 27423, whereas OMNH 25623

is somewhat longer, narrower anteriorly, and broader posteri­orly; presumably these differences in shape and proportions aredue to different positions (which cannot be determined to locus)in the molar series. The lingual cingulum is distinct except onthe last molar (OMNH 25786; Fig . 3D-F), in which it fadesout posteriorly. Molars bear a strong anterior groove and a sa­lient posterolabial ridge, each of which extends to the base ofits respective tooth root, which serve to interlock adjacent teeth.The cusps are relatively lower, more posteriorly recumbent, andform more obtuse angles in profile than do those of Astrocon­odon spp . The cusp bases are not so deeply separated by con­cavities labially as they are lingually, a point of contrast withLate Jurassic triconodontids (e.g., Trioracodon, Priacodon) .Cusps a-c are subequal in height; carnassial notches are presentbetween cusps a-b and c-d. Cusp b is complete only in OMNH25786, an ultimate lower molar, in which it is somewhat broad­er and slightly shorter than cusp a. Cusp b is also broader-basedthan cusp a in the holotype (OMNH 26362), in which the tipof the former is missing. Cusp d, which is slightly shorter than

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CIFELLI AND MADSEN-CRETACEOUS TRICONODONTS FROM UTAH 407

cusp c and is closely appressed to that cusp, forms a gracile,fingerlike projection at the back of the tooth, as it does in As ­troconodon and in contrast to the condition in the new taxondescribed below. The last lower molar is notable for its smallsize and the presence of only two cusps , a and b; in this respect,it differs from the ultimate molar of C. montanensis, which isalso small but retains three cusps, and from both Astroconodonand Alticonodon, which have quadrituberculate last lower mo­lars (Fox , 1969; Turnbull, 1995). Insofar as is known, the frag­ment of the left upper molar (OMNH 34015, not figured) issimilar to that of Astroconodon delicatus, except for its muchlarger size.

Shearing surfaces, best seen on the holotype, are deeplyincised, flat, shiny, and marked by parallel, posterodorsallyoriented striae (Fig. 3C) . A well-developed facet is present onthe posterior flank of cusp a (presumably for the opposingsur face of cusp A on the corresponding upper molar) and pos­terior face of cusp b (presumably for the opposing surface ofupper molar cusp B) . These wear surfaces show that devel­opment of sharp cutting surfaces on the crests supported bycusps occurred as the incising facets wore sufficiently to reachthose crests. Thus, precise molar occlusion was achievedthrough dental wear rather than predetermined morphology­a point of contrast between triconodontids and therian mam­mals (Jenkins and Crompton, 1979; Crompton, 1995). Presum­ably, the two wear facets extended onto adjacent surfaces, theanterior flank of cusp c and the anterior flank of cusp a, re­spectively. Ultimately, they presumably were joined by a thirdfacet (for upper molar cusp C)-which develops first on theanterolabial face of cusp b and later extends onto the poster­olabial surface of cusp d of the preceding molar-as they arein the closely related species Corviconodon montanensis (Ci­felli et al ., 1998).

Upper premolars representing the last three loci, perhaps P2­4, are included in the hypodigm (Fig. 3J-0). Where it can bedetermined, the lingual cingulum is complete, the labial cin­gulum is present on the posterior third of the tooth, and cuspsA-D are present. The premolars are each of about the samelength, increasing in width and inflation posteriorly; the lasttwo, which are subequal in height, are noticeably taller than thepreceding tooth . Cusps Band D are better developed than inJurassic Triconodontidae. Cusp B, which lies in a basal position,projects more anteriorly on the anteriormost tooth represented(presumed P2) ; on the succeeding tooth, it is placed somewhatmore lingually and is more closely appressed to the base ofcusp A. Faint keels ascend the anterior and posterior faces ofcusp A. Cusp C, placed near the base of the tooth on the an­teriormost of the three known premolars, progressively achievesa higher position on the posterior slope of cusp A in succeedingpremolars; a weak carnassial notch between the two cusps pro­gressively becomes better developed, as well . Cusp D, whichis continuous with the lingual and labial cingula, is basal inposition; on presumed P2, it lies near the midline of the tooth,whereas it is somewhat more lingually placed on the succeedingtooth (it is not represented on the presumed last premolar),where it is more poorly defined and supports a crest connectingto cusp C.

OMNH 29763 is the posterior part of an anterior lower pre­molar, perhaps p2, with part of cusp a and with cusps c-d pres­ent (Fig . 3P-Q). The lingual cingulid is present ; a labial cin­gulid is lacking. A weak keel descends the posterior flank ofcusp a, and similar keels are present on both the anterior andposterior faces of cusp c. The latter cusp is placed low on thecrown of the tooth, which is characteristic of triconodontid an­terior premolars, and cusp d forms a small projection at theposterior base of the crown.

JUGULA TOR, gen. nov .

Etymology-L., cutthroat, murderer (Brown, 1954); in al­lusion to the carnivorous lifestyle suggested by size and toothmorphology, particularly the vicious appearance of the lowerincisor.

Type and Only Species-Jugulator amplissimus, sp . nov.Distribution-Upper member of the Cedar Mountain For­

mation (Albian-Cenomanian), Emery County, Utah .Diagnosis-As for the type and only species.

J UG ULA TOR AMPLISSIMUS, sp. nov.(Figure 4, Table I)

Etymology-L., largest, in reference to the fact that the spe­cies is the largest known member of the Triconodontidae.

Holotype-OMNH 33850, cracked but complete left lowermolar.

Hypodigm-The holotype, and OMNH 25785 , left lowermolar, probably the last in the series, with cusp apices broken;25791 , anterior part of right lower molar; 26415 , fragment ofleft lower molar with cusps c and d; 26716, right lower molar,missing the tip of cusp b; 27556, anterior part of right lowermolar, perhaps the first in the series, with a and b cusps; 29710,posterior part of left lower molar with cusp d and broken cuspsa and c; 25624, left anterior upper premolar, probably the firstin the series (perhaps PI); 25793 , fragment of maxilla withposterior part of left anterior premolar, perhaps P2, and rootsof the succeeding tooth, perhaps P3; 26416 and 27507, posteriorparts of posterior upper premolars, probably the last in the se­ries (perhaps P4) ; 26715, posterior part of left anterior lowerpremolar, perhaps p2; and 27636, left lower incisor.

Localities and Horizon-QMNH localities V234 (OMNH25624), V235 (OMNH 26415), V239 (OMNH 25785 , 25791,25793, 29710), and V695 (OMNH 26715 , 26716, 27507,27556, and 33850); 10-20 m below contact with overlying unit,upper member, Cedar Mountain Formation, Emery County,Utah (Fig . 1).

Diagnosis-Largest known member of the Triconodontidae.Lower molars differ from those of most closely similar taxa(Astroconodon , Corviconodon) in being relatively wider and inhaving cusps that are more obtuse and posteriorly recumbent inprofile view , in having more broadly separated apices of a, b,and c, and in having more evenly curved labial faces, with onlyminor concavities separating cusp bases; cusp d developed asan anteroposteriorly oriented ridge rather than a fingerlike pro­jection on presumed posterior molars.

Description-Lower molars of Jugulator amplissimus (Fig .4A-K) are striking because of their great size . As in the othertaxa described herein, anterior and posterolingual faces aremarked by deep grooves and prominent ridges, respectively,that extend to the bases of the tooth roots (Fig . 4G-H). Thelingual cingulid, which is complete, is slightly crenulated; alabial cingulum is absent. The labial face of the crown forms arelatively smoothly curving surface, with only minor interrup­tions formed by the concavities at cusp crests and bases, unlikethe condition in Jurassic Triconodontidae, in which the lingualand labial faces of the cusps are more nearly symmetrical.Cusps a, b, and c have broadly separated apices, are noticeablyrecumbent posteriorly, and form rather obtuse angles in profileview compared, for instance, to those of Astroconodon. Themolar cusps are subequal in height; cusp a is, by a slight mar­gin , the talIest, and cusp d is slightly shorter than the othercusps. Cusp d on presumed posterior molars (OMNH 25785 ,26716) is enlarged and, where known, bears an anteroposteri­orly oriented crest (Fig . 4D-H); the last molar is unreduced.

Wear surfaces are best seen on OMNH 26716 (Fig . 4D-H),where there is a concave, posterodorsalIy oriented facet (pre­sumably for cusp A) between cusps a and c; this is continuous

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FIGURE 4. Jugulator amplissimus, gen . et sp. nov. A-C, OMNH 33850 (holotype), left mX in occlusal (A) , lingual (B) , and labial (C ) views.D-H, OMNH 26716, right mX (last molar?) in occlusal (D) , lingual (E) , labial (F) , posterior (G), and anterior (H) view s. I-K, OMNH 25791 ,anterior part of right mX in occlusal (I), lingual (J), and labial (K) views. L, M, OMNH 27507, posterior part of ultimate right uppe r premolar(P4 ?) in labial (L) and lingual (M) views. N-O, OMNH 25624, left anterior upper premolar (PI) in lab ial (N) and lingu al (0) views. P-Q,OMNH 26715, posterior part of left anterior lower prem olar (p2 ?) in labial (P) and lingual (Q) views. R-S, OMNH 27636, left lower incisor inlabial (R) and lingual (S) view s. Scale bar = 2 mm.

with but distinct from a fainter wear surface, perhaps for theopposing posterior face of cusp A, that extends anteroinferiorlyfrom the apex of cusp c. Another major facet, presumably forupper molar cusp B, extends anteroinferiorly from the apex ofcusp a, incorporating the face of that cusp down to a pointbelow the notch between cusps band a.

Teeth or tooth fragments are known for the first three upperpremolar loci . Presumed PI (OMNH 25624; Fig. 4N-Q) has afaint lingual cingulum; cusp B, basally placed, is well marked.Cusp A is relat ively short and bears a sharper posterior thananterior keel; the back of the tooth is missing. All that can besaid of presumed P2 is that it had a robust heel with a small Dcusp. The succeeding tooth, presumed P3, had a much largeranterior than posterior root, as is typical of triconodontid pos­terior premolars. Both fragments of the ultimate upper premolar(presumed P4) preserve only cusps C and D; labial and lingualcingula are present, and on both specimens the posterolingualpart of the cingulum bears a small but distinct wear facet (Fig .4M), presumably for the tip or anterior face of cusp b of thefirst lower molar. This is the first evidence we are aware of forocclusal facets on triconodontid ultimate upper premolars; a

small, analogous facet may be present on P3 of Trioracodonbisulcus, YPM 10344 . Although the ultimate upper premolar is,in part, functionally linked to the molar series, it does not havethe interlocking mechanism seen in the molars (see Patterson,1951). Cusp C is much higher than cusp D and is placed onthe posterior slope of cusp A, as shown by the position of thenotch between the two cusps. Only a part of one anterior lowerpremolar (presumed p2) is known; it (OMNH 26715; Fig . 4P­Q) is similar to, but larger than, that described for Corvicono­don utahensis, differing in having a small heel projecting pos­teriorly from cusp d.

The lower incisor (OMNH 27636; Fig . 4R-S) is remarkable.It greatly resembles the single lower incisor (Jenkins andCrompton, 1979 :78) of an unnamed triconodontid from theCloverly Formation, but is much larger. The main part of thecrown lies mesially, forming a bluntly pointed cusp with flank­ing ridges mesially and distally, and somewhat resembles acupped hand. Using the same simile, a strong, thumblike heel ,bearing two cusps that form a sharp ridge, is present distally.A small wear facet, perhaps produced by occlusion with anupper incisor, is present on the dorsodistal part of the heel.

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There is also apical wear on the more mesial of the two heelcusps, and on the adjacent distal ridge of the main part of thecrown.

DISCUSSION

Taxa from the Cedar Mountain Formation add substantiallyto the known morphology and diversity of North American Tri­conodontidae, but unfortunately present little new evidencebearing on relationships of (or within) the family. Triconodon­tidae may be recognized as a monophyletic unit based on therelative size of molar cusps (Jenkins and Crompton, 1979), re­duction of molar accessory cusps (Cifelli et aI., 1998) and, pos­sibly, basicranial anatomy (Rougier et al., 1996) . With in thefamily, Late Jurassic (Priacodon , Trioracodon, Triconodon)and Cretaceous (Astro conodon, Alticonodon , Corviconodon,Jugulatory genera form morphologically distinct groups. Vari­ations within each temporal cluster are evident, but in mostcases the significance of these differences is uncertain becausethe distribution or polarity of characters (or both) is poorly un­derstood. The least ambiguous character group that we believeto unite all North America Cretaceous species as a monophy­letic group, to the exclusion of Jurassic taxa, is the extensivelydeveloped molar interlocking system, in which a groove ex­tending the entire anterior face of a molar houses a ridge de­veloped on the posterior face of its predecessor. This feature,together with the strong vertical development of lower molarcusp d, is found in all of the taxa from the Cedar MountainFormation, as it is in Astroconodon (see Patterson, 1951) andAlticonodon (see Fox, 1969, 1976) . Of these taxa, tooth occlu­sion and wear has only been studied in Corviconodon (Cifelliet al., 1998), in which upper molar cusp B progressively (withincreasing wear) comes to occlude with both lower molar cuspb and the adjacent area of cusp d on the preceding tooth. Webelieve that these features are functionally related, and that thiswear pattern probably holds true for other taxa .

In Astroconodon, Corviconodon, and to a much greater ex­tent, Jugulator, lower molars tend to have a somewhat asym­metrical appearance: the labial face s form gently rounded sur­faces, and concavities are not nearly so deep as they are lin­gually. This is another character that we suspect is derived incomparison to the condition seen in Jurassic taxa (Triora codon,Priacodon) , in which the labial and lingual faces of lower mo­lars are more similar. Most advanced in this respect is the LateCretaceous Alticonodon, and it is tempting to suggest a specialrelationship to Jugulator, at least, on this basis. However, Al­ticonodon is so obviously advanced that the significance of thissuperficial similarity is uncertain: a great morphological hiatusseparates the taxa from the Early-medial and Late Cretaceous.

The conditions of several other notable characters are knownonly for one or a few taxa ; their significance cannot be properlyjudged at present, but it is worthwhile to point them out. OfCretaceous Triconodontidae, complete upper molars are knownonly for Astroconodon. Upper molars differ from those typicalof Late Jurassic taxa (Priacodon, Trioracodon) in lacking thelingual cingulum, which gives upper molars a distinctive, bi­lobate appearance, and in the fact that there is negligible cusprelief in labial view : development of notches is largely restrict­ed to the lingual side of the tooth. By comparison to outgrouptaxa such as Morganucodon (see Crompton and Jenkins, 1968:fig. 2), Dinnetherium (see Jenkins et al ., 1983 :fig. 1), and Mega­zostrodon (see Crompton, 1974 :figs . 5, 6), the reduction of theupper molar cingulum is probably an advanced characteristic ofAstroconodon. Judged by similarity of lower molars, it is likelythat the upper molar cingulum was similarly reduced in otherEarly and medial Cretaceous taxa. However, we note that thelingual cingulum on upper molars of Late Jurassic taxa appearsto be advanced in its own right: unlike the condition in outgroup

taxa, it bears wear surfaces that appear to shear en echelon(Hiiemae and Kay, 1973) with those developed on upper molarcusps, and suggests greater lateral translation of the mandibleat the termination of jaw closing than in Cretaceous taxa . Henceit is not clear that the Cretaceous taxa were derived from acommon ancestor with upper molar construction and functionrepresented by known taxa of the Late Jurassic.

Interpreting postcanine dental formulae is similarly problem­atic . In Corviconodon, Astroconodon denisoni, and a morpho­logically similar species from the Cloverly Formation (Jacobset al., 1991), a count of four premolars and five molars is found(Jenkins and Crompton, 1979; Turnbull, 1995 ; Cifelli et aI.,1998) . The formula for Jugulator is unknown, but its closesimilarity to the unnamed species from the Cloverly Formation,as well as representation by available specimens, suggests thatfour premolars were probably present in this taxon as well. Asshown by Simpson (1928, 1929), the lower postcanine formulaefor Triconodon, Triora codon, and Priacodon are p4 m4, p4 m3,and p3 m4, respectively. Most of the obvious outgroup taxa arenot of much help in this regard (Cifelli et aI., 1998) . However,tooth positions among Mammalia are more commonly lost thangained. Hence, in terms of both dental formula and aspects ofupper molar structure, it appears that known Jurassic taxa donot represent the primitive pattern from which these Cretaceoustaxa arose: the possibility must be entertained that their ancestrymore plausibly may be found elsewhere. Simpson (1928) re­garded Amphilestidae (considered by him to be a subfamily ofTriconodontidae) to represent the primitive dental pattern fortriconodonts, both in terms of coronal morphology and dentalformula. Amphilestes has a postcanine formula of p4 m5, thatof Phascolotherium is p2 m5, and the formula for Phascolodonis not known but it is thought to have had more than four lowermolars (Simpson, 1928, 1929) . The gobiconodontid Gobicon­odon ostromi has two premolarifonns and five molariforms inthe lower postcanine dentition (Jenkins and Schaff, 1988) . Inthis context, it is interesting to note that Corviconodon monta­nensis has an extremely small last molar, differing from that ofAstroconodon in both relative size and in the fact that it hasonly three cusps. C. utahensis, which is younger than the spe­cies from the Cloverly Formation, perhaps by more than 10 Ma,has an even smaller last molar that bears only two cusps­hence there is temporal evidence, at least, for suggesting thatthis tooth was undergoing progressive reduction. Summing upthe limited and somewhat equivocal evidence now available,we concur with Simpson (1928) in believing that a postcanineformula of p4 m5 is primitive for Triconodontidae. Under thisinterpretation, Jurassic members of the family have lost at leastone premolar and one molar.

Jugulator is characterized by a large, complex lower incisor,and in this regard it is similar to the unnamed species from theCloverly Formation, in which only one incisor is present (Jen­kins and Crompton, 1979) . Incisors of triconodonts are poorlyknown: Trioracodon mordax and Priacodon fruitaensis eachhave two lower incisors; in the former; the lateral incisor issmall and spatulate (Simpson, 1928; Rasmussen and Callison,1981). Among amphilestids, four stout, subspatulate lower in­cisors are known for Phascolotherium bucklandi, and three orfour, the last of which is columnar, are present in Amphilestesbroderipii (see Simpson, 1928) . Clearly, the incisor of Jugu­lator is advanced by comparison, and it is likely that incisormorphology will become useful for phylogenetic reconstructionin triconodontids when anterior teeth become better represent­ed . Regardless, the morphology in Jugulator is suggestive ofconsiderable grasping, puncturing, and perhaps cutting capabil­ity, and further suggests that the lower incisor played an integralrole in a predaceous lifestyle.

The three species from the Cedar Mountain Formation differconsiderably in size and, presumably, ecology. Recognizing that

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there are no strictly comparable mammals living today, and thatestimates based on living models are thus likely to be crude,we nevertheless estimated body mass for each of the three spe­cies, using dental measurements (mean length of lower molar;estimated for Astroconodon delicatus based on tooth propor­tions in a sample [N = 8] of A. denisoni housed at SMU andFMNH) and the regression equations of Van Valkenburgh(1990). We obtained estimates of 317, 523, and 749 g for A.delicatus, C. utahen sis, and J. amplissimus , respectively. Livingmammals in this size range with functionally analogous molarsare opportunistic feeders, those at the lower size of the rangerelying heavily on insects and other invertebrates, and the largertaxa feeding more extensively on vertebrates (Nowak, 1991) .We know of no way to judge if these or other triconodontidswere piscivorous, as has been proposed on the basis of an anal­ogous molar cusp pattern seen in archaeocete whales and pho­cid pinnipeds (Slaughter, 1969) . However, we point out thattriconodontids differ fundamentally from these taxa in havingtightly occluding molar teeth with well-developed shearing sur­faces (as emphasized by the molar interlocking mechanism; seealso Jenkins and Crompton, 1979) ; perhaps study of microwearfeatures (see , e.g., Strait, 1991) will provide the basis for refinedinterpretation of diet. Slaughter (1969) also cited co-occurrencewith certain types of fish as a line of evidence supporting aquat­ic preference for Astroconodon deni son i- an argument that hasbeen proposed for two Cretaceous triconodonts from Morrocoas well (Sigogneau-Russell, 1995) . Like fossils of most otherMesozoic mammals, triconodontid specimens from Utah wereobtained from overbank and other strata of fluvial origin (seeNelson and Crooks, 1987) : they were deposited in water, andfor this reason the associated vertebrate fauna includes a mix­ture of terre strial and aquatic taxa (Cifelli et aI., in press) .Hence, we know of no evidence that speaks one way or anotherregarding habitat preference of these and other triconodont taxa(see also Jenkins and Crompton, 1979) .

ACKNOWLEDGMENTS

We thank F. A. Jenkins, C. R. Schaff, J. Gauthier, R. H.Tedford, M. C. McKenna, R. J. Emry, and M. E. Nelson foraccess to specimens in their care , and to W. D. Turnbull, D. A.Winkler, and L. L. Jacobs for providing us with cast s of Astro­conodon. We are grateful to R. L. Nydam, K. S. Smith, E. M.Larson, S. T. Judd, J . Judd, D. F. Schmidt, E. Miller, and C.Miller for various help in the field and lab, and to T. Rasmussen,San Rafael Resource Area, for the cooperation and assistanceof the U.S . Bureau of Land Man agement. Special thanks aredue to W. D. Turnbull for supplying us with published andunpublished data on Astroconodon denisoni, and to DennisHuffman, former Superintendent of Dinosaur National Monu­ment, for his continuing support and encouragement. This re­search was supported by NGS grants 4762-91 and 5021-92, andNSF grant DEB 9401094 to RLC.

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