Hypothetical Tanystropheus Egg

Here’s how you paint yourself into a corner…
Huehuecuetzpalli and several pterosaur juveniles and embryos demonstrate that the tritosaurian lizards matured isometrically. In other words, just before hatching, tritosaurs had the proportions of adults. Among pterosaurs we’ve seen this in a juvenile Pteranodon, a juvenile Zhejiangopterus, a juvenile Tapejara, a juvenile Tupuxuara, the IVPP embryo, the JZMP embryo and Pterodaustro.

Now the question is…
If present in the giant, hyper-long-necked Tanystropheus, how in heck would you be able to fit that long neck inside an eggshell?

Juvenile?
Wild (1973) suggested that the smaller 4′ Tanystropheus with several tooth cusps was a juvenile of the 20′ giant with giant conical teeth, but at the time isometric growth had not been established for the Tritosauria. Now these two appear to be distinct species. Even if the smaller specimen is a juvenile, it still has an incredibly long neck, still difficult to fit inside an eggshell.

So, here we put the hypothesis to the test
(Prior to discovering Tanystropheus egg fossils, of course) let’s stuff Tanystropheus into an elliptical egg shape (Fig. 1).

Other than a few very angled neck vertebrae, Tanystropheus fits okay. Are these angles too extreme? Did Tanystropheus embryos have shorter cervicals? We don’t know.

The Eggshell
As in pterosaurs the eggshell was likely an extremely thin leathery surface, more like a pillow case, not a hard ellipse, like a chicken egg. As in other tetrapod embryos, the bones and their articulations with each other were likely much softer and more flexible than in the adult, enabling this strange configuration, adopted only during the last few weeks prior to hatching as the embryo reached hatchling size.

By Land or By Sea?
Earlier we looked at a new hypothesis for vertical feeding in Tanystropheus based on stomach contents, its long neck and the crinoid-filled environment of Triassic seas. Whether Tanystropheus laid its eggs on land or in water may be immaterial if they, like pterosaurs and many other lizards, retained eggs within the mother until just prior to hatching. In that case the egg shell was more like a placenta, essentially producing a live birth. In that case, let’s look for babies near the pelvic region in future fossils.

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.

References
Wild R 1973. Die Triasfauna der Tessiner Kalkalpen XXIII. Tanystropheus longobardicus(Bassani) (Neue Ergebnisse). – Schweizerische Paläontologische Abhandlungen 95: 1-16.

wiki/Tanystropheus