A CYNODONT FROM THE UPPER TRIASSIC OF EAST GREENLAND: TOOTH REPLACEMENT AND DOUBLE-ROOTEDNESS MICHAEL D. SHAPIRO’ AND FARISH A. JENKINS, JR.' ABSTRACT, a new genus and species of cynodont royal des Sciences naturelles de Belgique, from the Upper Triassic Fleming Fjord Formation of Brussels; MCZ, Museum of Comparative East Greenland possesses double-rooted postcanine teeth and a nonaltemate pattern of tooth replace­ Zoology, Harvard University, Cambridge, ment. The specimen represents an addition to the Massachusetts; MGUH, Geological Mu­ known diversity of Karly Mesozoic taxa with multi­ seum, University of Copenhagen; and rooted dentitions (tritylodontids, Sinoconodon sp., MNHP, Institut de Paleontologie, Muse­ haramiyids, morganucodontids, Meurthodon galli- um National d’Histoire Naturelle, Paris. cus), and casts doubt on traditional interpretations of die Interdependency of reduced tooth replacement patterns and teeth with multiple roots. SYSTEMATIC PALEONTOLOGY Order Therapsida Broom, 1905 INTRODUCTION Infraorder Cynodontia Owen, 1861 The Upper Triassic Fleming Fjord For­ mation of Jameson Land, East Greenland, Family incertae sedis preserves a diverse fossil vertebrate fauna Mitredon cromptoni new genus and that includes mammals, theropod and pro- species sauropod dinosaurs, plagiosaurid and cy- clotosaurid amphibians, turtles, aetosaurs, Etym ology. The generic term refers to phytosaurs, and pterosaurs (Jenkins et al., the highly peaked primary cusps, a com­ 1994, 2001). Mammals are represented bination of English mitre, the high-peaked primarily from the upper Tait Bjerg Beds ecclesiastical headdress, from Greek m itra, and include Kuehneotlierium, ct. Brachy- turban, and Greek odous (odon ), tooth. zostrodon, and the haramiyid H aram yavia The specific name honors A. W. Crompton clemmenseni (Jenkins et al., 1994, 1997). for his important contributions to our un­ We describe here an additional compo­ derstanding of the paleobiology and evo­ nent of the fauna, a cynodont that bears lution of eynodonts. double-rooted teeth, the only known spec­ H olotype. MGUH VP 3392, MCZ field imen of this taxon. A comparable form of number 11/G95 (Figs. IA, B), a partial left Late Triassic age is Meurthodon gallicus dentary bearing an incomplete alveolus (Russell et al., 1976; Sigogneau-Russell mesially, three unerupted postcanine and Hahn, 1994; Godefroit and Battail, teeth, roots of four other (erupted) po­ 1997), represented by isolated teeth from stcanines, and an empty tooth crypt dis­ Rhaetic deposits in France, but this taxon tal ly. differs in significant details. Horizon. Uppermost dolostone o f Tait The following abbreviations of institu­ Bjerg Beds, 0rsted Dal Member of the tional names are used: IRSNB, Institut Fleming Fjord Formation. L o ca lity. 71°32.929'N, 22°55.450'W, 1 Department of Organismic and Evolutionary Bi­ north o f /Erenprisdal at its confluence olog)', and Museum of Comparative Zoology, Harvard with Pingel Dal, Jameson Land, East University, Cambridge, Massachusetts 02138. Greenland. Bull. Mus. Comp. Tool., 156(1): 49-58, October, 2001 49 50 Bulletin Museum of Comparative Zoology, Vol. 156, No. I A 2 m m Figure 1. Left dentary of Mitredon cromptoni. MGUH VP 3392, in (A) lingual view and (B) occlusolabial view. In occlusolabial view, the mandibular canal is visible distally as a filled cast through a window cut in the labial aspect of the jaw; the canal obscures the basal crown of Pc0. Age. Late Triassic (PNorian—Rhaetic; with laterally compressed cusps arranged Jenkins et al., 1994). in a longitudinal row and clearly separated Diagnosis. Cingular cusps on postcanine from each other (Bonaparte and Barber­ teeth absent or lost, a derived character ena, 1975; Sigogneau-Russell and Hahn, shared with galesaurid cynodonts, C ynog- 1994). A feature shared with M eurthodon nathus, and Probelesodon (Hopson and gallicus (Russell et al., 1976; Sigogneau- Kitching, 1972; Hopson and Bargliusen, Russell and Hahn, 1994) and some “chi- 1986). Differs from Therioherpetidae niquodonts” (Kemp, 1982) is a recurved (Bonaparte and Barberena, 1975) and oth­ cusp a (following the nomenclature of er nonmammalian cynodonts in possessing Crompton and Jenkins, 1968); that is, the bifurcate postcanine tooth roots (likely mesial crest is longer and more horizontal convergent with multirooted tritylodon- than the distal crest, which is shorter and tids) and lacking alternate tooth replace­ more vertically oriented. A derived feature ment. Characters shared with “chiniquo- shared with Meurthodon, Sinoconodon, donts” are crowns of lower postcanines and Mammaliaformes (sensti Wible, 1991) Tooth Replacement and Double-Rootedness • Shapiro and Jenkins 51 D & & MGUH VP 3392 MNHP SNP1W MNHP SNP210W IRSNB R163 aligned at "gumline" aligned at cusps Figure 2. Comparison between (A) Pc., of Milredon cromptoni, MGUH VP 3392, (B) the type specimen of Meurthodon gallic us, and (C. D) two isolated teeth referred to Meurthodon gallicus. Specimens MNHP SNP210W and IRSNB R163 were selected for comparison in addition to the type specimen for their gross similarity to MGUH VP 3392. In each column (B, C, and D), a Meurthodon tooth (shaded) is compared to Pcs of Mitredon (outline) in two ways to minimize the effects of size and completeness on morphologic comparison between specimens. First, the middle row depicts comparisons based on alignment of two teeth at the inferred gumline. Second, in the bottom row. cusps are aligned as closely as possible. Although the alignment of MGUH VP 3392 at the gumline" with IRSNP R163 and with the cusps of MNHP SNP1W reveal overall similarities in shape, all specimens referred to Meurthodon lack a cusp e. Cusp b of Pc5 in Mitredon is also substantially smaller than the corresponding structure in Meurthodon. Cusp designations in (A) after Crompton and Jenkins (1968). MNHP SNP1W redrawn from Sigogneau-Russell and Hahn (1994); MNHP SNP210W and IRSNB R163 redrawn from Godefroit and Battail (1997). For top row, scale bar = 1 mm. is roots of lower postcanine teeth bifur­ in the latter, and only 40% in the former; cate, a character once considered diagnos­ Fig. 2); and the absolute mesiodistal tic of Mammalia (e.g., Crompton and Jen­ length of Pc5 is 20% smaller than that of kins, 1979). Distinguished from M. gallicus the type specimen of M. gallicus (3.5 mm by the presence of a cuspule mesial to versus 4.2 111111). None of the three lower cusp I) (on Pc3); a inesiodistally longer cusp teeth of MGUH VP 3392 closely resem­ a (if the type specimen o f M. gallicus and bles the single isolated tooth of the type Pc5 of Mitredon cromptoni, new genus and specimen of M. gallicus (Russell et al., species, are scaled to the same mesiodistal 1976; Signoneau-Russell and Ilahn, 1994) length at the crown-cervical junction, the nor other isolated teeth that Godefroit and mesiodistal length of cusp a is approxi­ Battail (1997) subsequently referred to mately 60% total mesiodistal crown length that taxon (Fig. 2). 52 Bulletin Museum of Comparative Zoology, Vol. 156, No. 1 (PC,) Pc6 Pc5 Pc4 Pc3 Pc2 Pc1 m.ca B 1 cm Figure 3. Mitredon cromptoni and contemporaneous Greeniandic mammals. (A) Schematic reconstruction of the left lower jaw of Mitredon cromptoni, MGUH VP 3392. Lower jaws of the Late Triassic mammals (B) Haramiyavia (redrawn from Jenkins et al. [1997]) and (C) Morganucodon (redrawn from Hopson [1994], in Bonaparte and Crompton [1994]). Abbreviations: Pc, post­ canine tooth; i.gr, internal dentary groove; f.me, mental foramen; m.ca, mandibular canal. DESCRIPTION bial to Pc2. An internal groove extends lon­ gitudinally along the inferior, lingual as­ Lower Jaw pect of the dentary, indicating the pres­ The specimen is a partial left dentary, ence of postdentary bones. The mandibu­ slightly convex along its ventral margin, lar canal is exposed through breakage on and missing the anterior and posterior the lingual aspect of the jaw between Pc2 ends of the ramus; erupted teeth had bro­ and Pc4 (Fig. IA). The canal passes to the ken off postmortem, but several unerupt­ labial side of Pc5 and is preserved in cross ed teeth are preserved within the ramus. section at the break across the posterior Although the total number of postcanine end of the specimen (Fig. IB). teeth is uncertain, the seven tooth posi­ tions are here referred to as Pct (most me­ Teeth sial) through Pc7 (most distal) (Fig. 3). The Lower postcanine 1 is indicated by a mental foramen is situated ventral and la­ partially preserved alveolus. In the next Tooth Replacement and Double-Rootedness • Shapiro and Jenkins 53 tooth position, Pc,, is an nnerupted tooth bears an elongate, tapered cusp a, the apex of which the apex of cusp a and most of of which is directed between the roots of the lingual half of the crown are preserved; the predecessor tooth (Fig. 3). Cusp c is the apex of cusp a abuts a root fragment rounded and without a pointed apex, un­ of tlie erupted tooth that was in the pro­ like the other c cusps preserved in this cess of being replaced. Cusp a is laterally specimen. Much of this cusp lies on the compressed and, unlike the recurved, lingual side of the mandibular canal and is asymmetrical cusp a of the unerupted best observed occlusolabially. Lower post­ tooth at Pcs, appears to be nearly sym­ canine 6 is smaller than Pc5 (Fig. 3), sug­ metrical. Two successively smaller cusps gesting that PeK may be the? ultimate tooth lie distal to cusp a; their apices are direct­ in the lower dentition; however, a cryptlike ed slightly distally, comparable to the distal depression distal to Pc6, as well as an anal­ cusps of Pc5 but unlike the strictly dorsally ysis of tooth replacement, appears to in­ directed orientation of cusp a. The mesial dicate that a more distal tooth position end of the tooth is not preserved and (Pe7) may have been present. therefore the presence or absence of me­ sial cusps is not possible to determine.