The Traditional View
Wild (1978) established the concept that the small, multi-cusp toothed specimens attributed to Tanystropheus were juveniles of the larger forms without multi-cusped teeth. This would involve morphological changes during ontogenetic growth.
The question is, why wasn’t phylogenetic evolution (speciation) offered as an alternative?
The Heretical View
Earlier I blogged that the differences were too many to support an ontogenetic growth series between the smaller Tanystropheus specimens and the larger ones. Today we’ll look at the feet and you can decide whether or not such changes can be attributed to ontogeny or are better explained as a phylogenetic change attributed to evolution. Of course the split would have occurred earlier and the two species would have been adapted to distinct niches based on size and tooth morphology.
As in Pterosaurs
Peters (2011) was able to speciate various specimens attributed to Pterodactylus, Pteranodon, Rhamphorhynchus, Germanodactylus and other pterosaurs by looking only at the variation in the foot morphologies. The same process is present here. If these two Tanystropheus pedes represent younger and older variations of the same species, why do the distinct changes that appear in the jaws (distinct teeth, for instance) extend to the feet?
This is especially important in a clade that otherwise demonstrates isometric growth patterns. (They don’t change much as they mature, contra traditional studies.)
Perhaps a more parsimonious solution is to place the smaller specimen in the bushy lineage of the larger specimen. Different sizes = different diets = different teeth.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Bassani F 1886. Sui Fossili e sull’ età degli schisti bituminosi triasici di Besano in Lombardia. Atti della Società Italiana di Scienze Naturali 19:15–72.
Li C 2007. A juvenile Tanystropheus sp.(Protoro sauria: Tanystropheidae) from the Middle Triassic of Guizhou, China. Vertebrata PalAsiatica 45(1): 37-42.
Meyer H von 1847–55. Die saurier des Muschelkalkes mit rücksicht auf die saurier aus Buntem Sanstein und Keuper; pp. 1-167 in Zur fauna der Vorwelt, zweite Abteilung. Frankfurt.
Nosotti S 2007. Tanystropheus longobardicus (Reptilia, Protorosauria: Reinterpretations of the anatomy based on new specimens from the Middle Triassic of Besano (Lombardy, Northern Italy). Memorie della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano, Vol. XXXV – Fascicolo III, pp. 1-88
Peyer B 1931. Tanystropheus longobardicus Bass sp. Die Triasfauna der Tessiner Kalkalpen. Abhandlungen Schweizerische Paläontologie Gesellschaft 50:5-110.
Renesto S 2005. A new specimen of Tanystropheus (Reptilia Protorosauria) from the Middle Triassic of Switzerland and the ecology of the genus. Rivista Italiana di Paleontologia e Stratigrafia vol. 111, no. 3, 377–394. online pdf
Wild R 1973. Die Triasfauna der Tessiner Kalkalpen XXIII. Tanystropheus longobardicus (Bassani) (Neue Ergebnisse). – Schweizerische Paläontologische Abhandlungen 95: 1-16.