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A New Specimen of Eurylambda Aequicrurius and Considerations on ``Symmetrodont'' Dentition and Relationships

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PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 3398, 15 pp., 4 ®gures March 27, 2003 A New Specimen of Eurylambda aequicrurius and Considerations on ``Symmetrodont'' Dentition and Relationships GUILLERMO W. ROUGIER,1 BARTON K. SPURLIN,2 AND PETER K. KIK3 ABSTRACT A new specimen of the tinodontid ``symmetrodont'' Eurylambda (Simpson, 1925a, 1929) from the Late Jurassic Como Bluff Quarry, Morrison Formation, is described. The specimen, a complete upper left molariform, is probably an M1. The major crown cusps of Eurylambda show similarities to those of triconodontids on the one hand and to spalacotherioids on the other. Cusp B of basal mammaliaforms is tentatively proposed as homologous with the cusp traditionally described as a stylocone in Eurylambda and with cusp B9 of Peralestes. These homologies imply that the stylocone is ancestrally a small cusp in the lineage leading to Theria and that the development of a parastylar lobe or ``hook'' is a derived feature of post-tinodontid mammals. If accepted, this scenario results in a more complex origin for the therian upper molar than previously recognized. Wear facet 1 (Crompton, 1971) of holotherians would not be homologous between Kuehneotherium±Eu- rylambda±Zhangheotherium, on the one hand, and the therians, on the other. INTRODUCTION in a bias favoring the preservation of lower teeth and jaws. Therefore, our knowledge of Information on the upper molars of early dental evolution during the Mesozoic relies Mesozoic mammals is sparse, due largely to heavily on mandibular and lower molar mor- the mechanical fragility of the maxilla when phology (Crompton, 1971; Prothero, 1981). compared with the mandible, which results Discovery of upper molariforms is always a 1 Research Associate, Division of Paleontology, American Museum of Natural History. Assistant Professor, Department of Anatomical Sciences and Neurobiology, School of Medicine, University of Louisville, Louisville, KY 40292. e-mail: [email protected] 2 Department of Neurology, School of Medicine, University of Louisville, Louisville, KY 40292. e-mail: [email protected] 3 Department of Anatomical Sciences and Neurobiology, School of Medicine, University of Louisville, Louis- ville, KY 40292. e-mail: [email protected] Copyright q American Museum of Natural History 2003 ISSN 0003-0082 2 AMERICAN MUSEUM NOVITATES NO. 3398 welcome addition because it allows the eval- sessing the bases of both roots. The crown uation of models of evolutionary transfor- was broken into halves at the base of the mation that have been based primarily on paracone and has been previously repaired. lower teeth, with occluding upper teeth being The crown of the new specimen is more buc- based on hypothetical reconstructions (Mills, colingually compressed than the type speci- 1964; Kermack et al., 1964; Crompton, men and is slightly longer mesiodistally, with 1971). the major cusps forming an obtuse angle of We report here on an isolated upper left about 1718 (with the type specimen forming molariform of Eurylambda aequicrurius an angle of about 1688). These features sug- (Simpson, 1925a, 1929). This is only the sec- gest that the new specimen is a more anterior ond specimen known for this taxon originally tooth than is the type. If tooth variance in described as Amphidon aequicrurius (Simp- other symmetrodonts is used as a model (Ci- son, 1925a) from the Late Jurassic Morrison felli, 1999; Cifelli and Gordon, 1999; Cifelli Formation in Wyoming. The new specimen and Madsen, 1999; Hu et al., 1997, 1998; was collected at Como Bluff Quarry in 1883 undescribed specimen in prep.), the new by members of the O.C. Marsh expedition tooth probably represents the position direct- and catalogued at the National Museum of ly mesial to the type, which was originally Natural History as an undetermined Meso- regarded as M1 (Simpson, 1925a, 1929) but zoic mammal tooth with the number USNM is here interpreted as a probable M2. How- 2846. Como Bluff is also the locality where ever, USNM 2846 has a slightly more pro- the Yale University party collected the type nounced ecto¯exus than does the type. This specimen of Eurylambda. becomes more pronounced in distal elements Discovery of a second specimen of Eury- of other ``symmetrodonts'', that is zhang- lambda is important because the type, a frag- heotheriids (Hu et al., 1997, 1998; unde- mentary maxilla with one tooth, has previ- scribed specimen in prep.), spalacotheriids ously been the only model for the upper den- (Cifelli and Gordon, 1999; Cifelli and Mad- tition of basal or obtuse angled ``symmetro- sen, 1999) and Peralestes (Owen, 1871; donts'' (Crompton and Jenkins, 1967, 1968; Simpson, 1928; Clemens, 1963). This latter Crompton, 1971; Ensom and Sigogneau- consideration introduces some doubts about Russell, 2000). Furthermore, the incomplete the more mesial position of USNM 2846 type is missing the parastylar region of the proposed here. tooth. Eurylambda has been suggested to be The crown has the shape of an elongated a junior synonym of Tinodon (Prothero, ``D'', with the straight side oriented buccally 1981; Fox, 1985; McKenna and Bell, 1997; and slightly excavated by the shallow ecto- Sigogneau-Russell and Ensom, 1998; Ensom ¯exus. Six cusps are fairly regularly placed and Sigogneau-Russell, 2000) and we tenta- on the outline of the ``D''. The largest of tively follow this interpretation here (see be- them is the centrally placed paracone (cusp low), although for convenience we continue A in terminology of Crompton and Jenkins, to use the name Eurylambda for the upper 1968). Three cusps are distal to the paracone: molars. the metacone (cusp C), cusp D, and a small metastyle (following the terminology of SYSTEMATIC PALEONTOLOGY Crompton and Jenkins (1967) and subse- quent modi®cations) forming the extreme TINODONTIDAE FOX, 1985 distolabial corner of the crown. Mesial to the paracone is the fairly lingual cusp B9 (as in- Tinodon bellus Marsh, 1879 (Amphidon aequi- terpreted by Hu et al. (1997, 1998) or ``cusp crurius Simpson, 1925a; Eurylambda Simpson, 1929). on anterior crest'' of Patterson (1956), and a small stylocone in the mesiobuccal corner of the crown (see discussion below). The styl- DESCRIPTION ocone is joined to cusp B9 by a sharp crest (Figures 1, 2) that descends abruptly from the mesial slope MATERIAL: The specimen (USNM 2846) is of B9. The small stylocone extends distally an isolated complete left upper molar pos- as a fairly robust crest that reaches the mid- 2003 ROUGIER ET AL.: EURYLAMBDA AEQUICRURIUS 3 dle of the buccal surface of B9. The area buc- gard to the main axis of the tooth, and sub- comesial to cusp B9 is raised and forms a equal in size. They differ markedly from the small platform which sets off a small thick- condition in Spalacotheroides (Cifelli and ening of the mesial slope of the stylocone. Madsen, 1999), whose roots are mesiodistal- Cusp B9 is a main cusp of the crown and ly compressed and parallel to each other. is not connected to a cingulum of any kind. It has a concave mesial face and a teardrop- DISCUSSION shaped wear facet that truncates its apex. Dis- tal to its apex, cusp B9 is connected to the The importance of Eurylambda in our un- massive face of the paracone by a short, near- derstanding of early ``symmetrodont'' rela- ly horizontal ridge. A shallow notch is formed tionships has recently been stressed by En- at the intersection of the distal crest of cusp som and Sigogneau-Russell (2000). Eury- B9 and the mesial edge of the paracone. lambda, putatively a junior synonym of Tin- The paracone is the largest and tallest odon, represents one of the few examples of cusp, being twice as high as the next highest upper tooth morphology among ``symmetro- cusp in the crown, the metacone. The para- donts'' and it has played a crucial role in the cone is slightly asymmetrical mesiodistally, study of dental occlusion (Crompton and with a blunter, more vertical mesial edge and Jenkins, 1967, 1968; Crompton, 1971). Eur- a sharper, longer, and more gently sloping ylambda represents the archetype for basal distal crest. The lingual side of the paracone nontribosphenic holotheres (Hopson, 1994; is almost ¯at while the buccal side is strongly Wible et al., 1995). convex. These differences give the paracone Ensom and Sigogneau-Russell (2000) re- a conspicuous hooklike shape. cently described a new species of Tinodon, The metacone (cusp C) is separated by Tinodon micron, from the Purbeck Lime- deep notches from both the paracone and stone Group, Lower Cretaceous of England. cusp D. It is a fairly symmetrical cusp, al- The type specimen is a lower molar that though the distal ridge is somewhat longer agrees quite closely with the American spe- and less vertical. Its lingual face is grooved cies of Tinodon. In addition to lower molars, by a distinctive wear facet, which covers not Ensom and Sigogneau-Russell (2000) attri- only cusp D, but extends to the metastyle. buted an isolated upper molar to the same Cusp D (which may be interpreted as the species and compared it to Eurylambda.We larger of two metastylar cusps) is separated concur with their analysis in noting the close by a relatively shallow embrasure from the similarities between the two North American metacone (C) and by a deep notch from the specimens of Eurylambda and the English metastyle. The wear facet mentioned above specimen, DORCM GS 694, referred to T. ¯attens its lingual surface while its labial as- micron. The main difference between the pect is strongly convex. The distal metastyle American specimens of Eurylambda and that is a poorly differentiated small cuspule that from the Purbeck is the presence of a well- forms a small, sharp posterior edge for inter- developed cusp B9 in the former, which is locking with the following molar.
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    Early Cretaceous amphilestid ('triconodont') mammals from Mongolia ZOFIAKIELAN-JAWOROWSKA and DEMBERLYIN DASHZEVEG Kielan-Jaworowską Z. &Daslueveg, D. 1998. Early Cretaceous amphilestid (.tricono- dont') mammals from Mongotia. - Acta Pal.aeontol.ogicaPolonica,43,3, 413438. Asmall collection of ?Aptianor ?Albian amphilestid('triconodont') mammals consisting of incomplete dentaries and maxillae with teeth, from the Khoboor localiĘ Guchin Us counĘ in Mongolia, is described. Grchinodon Troftmov' 1978 is regarded a junior subjective synonym of GobiconodonTroftmov, 1978. Heavier wear of the molariforms M3 andM4than of themore anteriorone-M2 in Gobiconodonborissiaki gives indirect evidence formolariformreplacement in this taxon. The interlocking mechanismbetween lower molariforms n Gobiconodon is of the pattern seen in Kuchneotherium and Ttnodon. The ińterlocking mechanism and the type of occlusion ally Amphilestidae with Kuehneotheriidae, from which they differ in having lower molariforms with main cusps aligned and the dentary-squamosal jaw joint (double jaw joint in Kuehneotheńdae). The main cusps in upper molariforms M3-M5 of Gobiconodon, however, show incipient tńangular arrangement. The paper gives some support to Mills' idea on the therian affinities of the Amphilestidae, although it cannot be excluded that the characters that unite the two groups developed in parallel. Because of scanty material and arnbiguĘ we assign the Amphilestidae to order incertae sedis. Key words : Mammali4 .triconodonts', Amphilestidae, Kuehneotheriidae, Early Cretaceous, Mongolia. Zofia Kiel,an-Jaworowska [zkielnn@twarda,pan.pl], InsĘtut Paleobiologii PAN, ul. Twarda 5 I /5 5, PL-00-8 I 8 Warszawa, Poland. DemberĘin Dash7eveg, Geological Institute, Mongolian Academy of Sciences, Ulan Bator, Mongolia. Introduction Beliajeva et al. (1974) reportedthe discovery of Early Cretaceous mammals at the Khoboor locality (referred to also sometimes as Khovboor), in the Guchin Us Soinon (County), Gobi Desert, Mongolia.
  • Paleontological Discoveries in the Chorrillo Formation (Upper Campanian-Lower Maastrichtian, Upper Cretaceous), Santa Cruz Province, Patagonia, Argentina

    Paleontological Discoveries in the Chorrillo Formation (Upper Campanian-Lower Maastrichtian, Upper Cretaceous), Santa Cruz Province, Patagonia, Argentina

    Rev. Mus. Argentino Cienc. Nat., n.s. 21(2): 217-293, 2019 ISSN 1514-5158 (impresa) ISSN 1853-0400 (en línea) Paleontological discoveries in the Chorrillo Formation (upper Campanian-lower Maastrichtian, Upper Cretaceous), Santa Cruz Province, Patagonia, Argentina Fernando. E. NOVAS1,2, Federico. L. AGNOLIN1,2,3, Sebastián ROZADILLA1,2, Alexis M. ARANCIAGA-ROLANDO1,2, Federico BRISSON-EGLI1,2, Matias J. MOTTA1,2, Mauricio CERRONI1,2, Martín D. EZCURRA2,5, Agustín G. MARTINELLI2,5, Julia S. D´ANGELO1,2, Gerardo ALVAREZ-HERRERA1, Adriel R. GENTIL1,2, Sergio BOGAN3, Nicolás R. CHIMENTO1,2, Jordi A. GARCÍA-MARSÀ1,2, Gastón LO COCO1,2, Sergio E. MIQUEL2,4, Fátima F. BRITO4, Ezequiel I. VERA2,6, 7, Valeria S. PEREZ LOINAZE2,6 , Mariela S. FERNÁNDEZ8 & Leonardo SALGADO2,9 1 Laboratorio de Anatomía Comparada y Evolución de los Vertebrados. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Avenida Ángel Gallardo 470, Buenos Aires C1405DJR, Argentina - fernovas@yahoo. com.ar. 2 Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina. 3 Fundación de Historia Natural “Felix de Azara”, Universidad Maimonides, Hidalgo 775, C1405BDB Buenos Aires, Argentina. 4 Laboratorio de Malacología terrestre. División Invertebrados Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Avenida Ángel Gallardo 470, Buenos Aires C1405DJR, Argentina. 5 Sección Paleontología de Vertebrados. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Avenida Ángel Gallardo 470, Buenos Aires C1405DJR, Argentina. 6 División Paleobotánica. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Avenida Ángel Gallardo 470, Buenos Aires C1405DJR, Argentina. 7 Área de Paleontología. Departamento de Geología, Universidad de Buenos Aires, Pabellón 2, Ciudad Universitaria (C1428EGA) Buenos Aires, Argentina. 8 Instituto de Investigaciones en Biodiversidad y Medioambiente (CONICET-INIBIOMA), Quintral 1250, 8400 San Carlos de Bariloche, Río Negro, Argentina.