Mammal-like tooth from the Upper Triassic of Germany

Abstract

Staatliches Museum fu¨r Naturkunde, Rosenstein 1, D-70191, Stuttgart, GermanyFIGURE 1. Stratigraphic section of the Lehrbergschichten at Geiss-gurgelbach showing location of horizon that yielded SMNS 80766 andinset map showing the locality at Geissgurgelbach.Recent discoveries have extended the antiquity of the Mammalia(sensu Lucas and Luo, 1993) back to the late Carnian, about 228 Ma,and reveal a greater diversity of Late Triassic mammals and advanced,non-mammalian cynodonts than was known as little as two decades ago(compare reviews of Clemens et al., 1976 and Clemens, 1986, withthose of Lucas and Hunt, 1994 and Godefroit and Battail, 1997). Wenow add to this Late Triassic diversity a tooth from the German Keuperthat is significant because it potentially fills a morphological gap in theevolutionary transition from the postcanine tooth of a dromatheriid cy-nodont to that of a morganucodontid mammal. SMNS 5 StaatlichesMuseum fu¨r Naturkunde, Stuttgart, Germany.PROVENANCE AND AGESeegis (1997:pl. 27, fig. 10) illustrated SMNS 80766, a tiny tooth ina jaw fragment, identifying it as ‘‘Cynodontia indet.’’ This fossil wascollected at Geissgurgelbach, northeast of Stuttgart in Baden-Wu¨rttem-berg (Gauss-Kruger-Coordinates: R 35 41 690/H 54 15 740) (Fig. 1).The specimen came from the lower, dolomitic part of the Obere Lehr-bergbank, which is in the upper part of the Lehrbergschichten (5upperpart of Untere Bunte Mergel; Fig. 1). These strata are of Otischalkian(late Carnian) age based on the presence of the primitive phytosaurPaleorhinus (in the Kieselsandstein) and the primitive metoposauridMetoposaurus (Lucas, 1998). Palynology also is consistent with a lateCarnian age (Seegis, 1997).DESCRIPTIONSMNS 80766 (Fig. 2) is a single tooth in a bone fragment. Antero-posterior length of the tooth crown is 1.12 mm, and maximum crownwidth is 0.58 mm. The principal crown cusp is inclined, presumablyposteriorly, and one side of the tooth is convex, presumably labially.The bone rises posterior to the tooth, which is apparently the base ofthe ascending ramus of the dentary. If we have oriented these landmarkscorrectly, then SMNS 80766 is the lower right, posteriormost postcaninetooth in a dentary fragment.The tooth crown is dominated by a single, tall, conical cusp (cuspa). This cusp has a blunt apex with a small, circular, abapical wearfacet. Cusp a has a nearly flat (very slightly convex) surface that com-prises most of the lingual aspect of the tooth. The labial surface of cuspa is more convex with several striae that originate near the crown apexand diverge toward the crown base.Cusp b is much smaller than and lower than cusp a. Cusp b is at theanterior edge of the tooth with a small cleft separating it from cusp a.Thus, viewed lingually, cusp b is a very small, low, blunt triangle.A chip missing from the postero-labial edge of the tooth probablyincluded a low cusp c. The postero-labial part of the crown is a broad,slightly concave cingular shelf. The anterior edge of this cingulid beginsnear the postero-labial slope base of cusp a and widens posteriorly sothat the widest part of the crown is posterior to cusp a. The labial edgeof the cingulid is a low ridge with three tiny bumps that may be calledcuspids (or stylids). The crown then narrows somewhat posteriorly tobe gently rounded. Thus, the occlusal shape of the crown is a veryacute triangle with its steep apex pointed anteriorly.The crown base below the cusps is columnar, and the root base isdivided into two, highly divergent roots. This root division begins wellbelow the crown base.DISCUSSIONSMNS 80766 is a unique dental morphotype. It probably representsa new taxon, but we believe it is not a sufficiently complete specimenupon which to base a new name. We thus identify SMNS 80766 as anindeterminate cynodont (as did Seegis, 1997), and note that it has mor-phological features that distinguish it from both dentally advanced cy-nodonts (dromatheriids) and primitive mammals (morganucodontids).SMNS 80766 has more divided roots than any dromatheriid. Itscrown is not as trenchant as the typical dromatheriid tooth, and nodromatheriid tooth has a cingulid. However, unlike most mammals (Sin-oconodon is an exception: Crompton and Luo, 1993; Luo, 1994), thereis no evidence that SMNS 80766 interlocked with the opposing uppertooth; i.e., it lacks a system of wear facets. Furthermore, in contrast toearly mammals such as Morganucodon (e.g., Mills, 1971; Kermack etal., 1973; Clemens, 1980), Brachyzostrodon (Sigogneau-Russell, 1983;Hahn et al., 1991) and Megazostrodon (Crompton, 1974; Gow, 1986),SMNS 80766 lacks cuspids other than a-b-c, has no antero-lingual cin-gulid and does not have root division up to the crown base. Note,though, that in the Early Jurassic mammal Sinoconodon not all post-canine roots are fully divided (Luo, 1994; Zhang et al., 1998). In SMNS