SilesianTriassic

Eosauropterygia indet. cf. Nothosaurus

Middle Triassic, Gogolin Beds, Upper Silesia, Poland

The specimen is a well-preserved isolated caudal rib belonging to a member of the clade Eosauropterygia, likely Nothosauridae (cf. Nothosaurus).

The rib is 22 mm long. The width of the proximal end is 4.5 mm, and the distal end is 3.4 mm. The head of the proximal portion is partially damaged, revealing an internal cancellous bone structure. The articular surface is straight and oriented almost perpendicular to the rib shaft. The complete distal portion is also oriented almost (at a very slight angle) perpendicular to the rib shaft.

The rib shaft is flattened and exhibits a compressed blade-like cross-section. One margin is bent into a slight but distinct arc whereas opposite margin is slightly S-shaped. The shaft is tapered on one side near the articular head and tapers gradually towards the distal end. The surface shows subtle longitudinal striations running parallel to the long axis.

Apart from the minor abrasion of the proximal head, the rib is complete. The presence of a distinct articular head rules out the identification as gastralium. The shaft is too short and too flattened to be classified as a typical dorsal rib. Morphology is not consistent with Placodontia ribs (including caudal elements), which are significantly more robust and pachyostotic. Representatives of Archosauria are likewise excluded based on their typically rounded or oval rib cross-sections.

Considering all the morphological features mentioned above and the stratigraphic context of the Lower Muschelkalk (Gogolin Beds), the specimen is most consistent with Eosauropterygia indet. The flattened blade-like morphology and overall proportions closely resemble caudal ribs of Nothosaurus marchicus illustrated by Klein et al. (2015). The fact that the specimen is an isolated element allows for conservative taxonomic assignment only.

References:

• Klein, N., Voeten, D. F. A. E., Lankamp, J., Bleeker, R., Sichelschmidt, O. J., Liebrand, M., Nieweg, D. C., & Martin Sander, P. (2015). Postcranial material of Nothosaurus marchicus from the Lower Muschelkalk (Anisian) of Winterswijk, The Netherlands, with remarks on swimming styles and taphonomy. Paläontologische Zeitschrift, 89(4), 961–981. https://doi.org/10.1007/s12542-015-0273-5
• Klein, N., & Scheyer, T. (2013). A new placodont sauropterygian from the Middle Triassic of the Netherlands. Acta Palaeontologica Polonica. https://doi.org/10.4202/app.2012.0147
• Diedrich, C. (2014). Marine Lariosaurus (Sauropterygia) records from the Middle Triassic Germanic Basin – stratigraphic and palaeobiogeographic importance for Tethyan and Germanic Basin correlation. Albertiana, 42, 31-42.