New skulls of the basal sauropodomorph Plateosaurus trossingensis from Frick, Switzerland: is there more than one species?

Abstract

The Triassic basal sauropodomorph Plateosaurus trossingensis is well-known from mass accumulations at the German localities of Trossingen and Halberstadt and the Swiss locality of Frick, and is significant especially regarding its taphonomy and proposed developmental plasticity. These implications, however, rely on the assumption that this material derives from a single species, which has been questioned. Here we describe new skull material from Frick including eight complete and six partial skulls, more than doubling the number of known skulls of P. trossingensis. This exceptional sample size allows for gaining a deeper understanding of variability that may occur in a single species. The new material includes the first known juvenile skulls of Plateosaurus, allowing for detecting ontogenetic changes. An attempt is made to distinguish between variability caused by taphonomic plastic deformation and intraspecific variability. Plastic deformation may shorten, but not widen bones. A number of characters commonly included in phylogenetic analyses of basal sauropodomorphs are shown to be variable within P. trossingensis, and possibly require re-evaluation. Although P. trossingensis skulls are highly variable, many of the variable characters include intermediate character states and therefore are continuous. No groupings based on skull features, locality, or stratigraphy are apparent. Consequently, the analyzed skull material from the bonebeds of Frick, Trossingen, and Halberstadt bears no evidence for the presence of more than one species.

Extreme growth plasticity in the early branching sauropodomorphMassospondylus carinatus

“…Ascertaining growth plasticity in dinosaurs is therefore an area where there is great potential for palaeobiological advances, but one that is hindered by the rarity of fossil sample sets spanning the full ontogenetic sequence of a single taxon. EBSMs present potential solutions to these limiting factors, as several genera are known from large sample sizes containing multiple growth stages [9,[22][23][24][25][26][27][28][29].…”

Issi saaneq gen. et sp. nov.—A New Sauropodomorph Dinosaur from the Late Triassic (Norian) of Jameson Land, Central East Greenland

“…In Pl. trossingensis MSF 12.3 this divergence is 60 • and possibly closer to the "in vivo" state [53]. This process marginates the posterior margin of the supratemporal fenestra laterally and contacts the posterior process of the parietal medially.…”

“…This foramen is not observed on the disarticulated or exposed medial surface of the premaxillae of the plateosaurids U. tolentinoi (UFSM11069) [18] and Pl. trossingensis (AMNH FARB 6810 [52], MSF 16.1 [53] and MSF 15.8.935 [53]), nor the derived massospondylid Mas. carinatus (BP/1/5241) [16].…”

“…An anteriorly located first premaxillary tooth is similar to that of the juvenile Pl. trossingensis (MSF 15.8B) and is thought to be related to ontogeny [53], as in mature specimens of Pl. trossingensis (i.e., MSF 11.4, MSF 15.8, and 16.1) there is usually a gap between the anterior tip of the premaxilla and the first premaxillary tooth.…”

“…The nasal is relatively shorter in NHMD 164741 and NHMD 164758 than in Plateosaurus. In Plateosaurus, the nasal is longer than half the skull roof length, a distinctive feature for the genus [52,53,63]. The main body of the nasal is dorsally convex and overlaps laterally the apex of the maxillary dorsal process.…”