#seeleyosaurus

[ildarotyrannus]

I apologize for such a long delay. I couldn't find the motivation to keep posting here. But, as I promised in my last post, here are three more drawings made for last year's scientific conference. :)

Front half of Dakosaurus andiniensis, massive metriorhynchid from Late Jurassic of Argentina. As with Cricosaurus, I based this reconstruction on metriorhynchid specimens with skin. Note the presence of speculative skin parasites, whose marks were founds in fossil skin. Also, there are salt glands in antorbital fenestra. This feature is based on this study: https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.24678 BTW, some recons portray Dakosaurus with crocodile-like facial integument and some recons even have "lips"! I decided to diversify from these both options and did it with smooth streamlined skin.

Prognathodon sp. reconstruction based on purportedly juvenile specimen with soft tissue outlines from Late Cretaceous of Jordan. This small individual was described in this paper: https://www.nature.com/articles/ncomms3423 Preserved skin impressions include silhouettes of the skin fringes of the flippers and the caudal fin. The caudal fin is quite small, but in adults it could be proportionally larger. This finding confirmed that mosasaurs converged with ichthyosaurs and metriorhynchids to develop a fish-like caudal fin.

And finally, Seeleyosaurus guilelmiimperatoris (formerly species of Plesiosaurus), a microcleidid plesiosaur from Early Jurassic of Germany. Holotype of this species is almost complete specimens with some skin impressions including rhomboidal skin flap. Perhaps all plesiosaurs had caudal fins of one form or another. However, their orientation is controversial: many paleontologists are of the opinion that the fin was vertical, but others think it was horizontal.The arguments in favor of the second point of view are the rigidity of the chest, the flat and wide shape of the tail in the proximal part and the absence of bending in the distal part of the tail: https://www.researchgate.net/publication/339423536_Peculiarities_of_the_Structure_and_Locomotor_Function_of_the_Tail_in_Sauropterygia I decided to choose this version for reconstruction. Which of the scientists is closer to the truth, new findings will show. All of these drawings were done with black gel ink pen and black colored pencil with small additions of white gouache.

[alphynix]

Seeleyosaurus guilelmiimperatoris here was a smallish plesiosaur (about 3.5m long / 11'6") found in Germany during the early Jurassic, about 182 million years ago.

And back in the 1890s, a specimen of this species was discovered with soft tissue impressions showing a diamond-shaped tail fin.

But despite us knowing about plesiosaur tail flukes for such a long time, they're surprisingly under-represented in reconstructions, never seeming to have become associated with the popular image of these animals in the same way that early pterosaur's tail vanes did. It doesn't help that no other direct impressions of plesiosaur tail fins have ever been found, or that the Seeleyosaurus specimen's soft tissue got painted over at some point in the mid-1900s, making it incredibly difficult to study without causing further damage. 

(Perhaps modern non-invasive scanning techniques could be able to see under the paintjob, but as far as I'm aware nobody's tried that yet.)

These tail fins are usually assumed to have been vertically oriented like those of other aquatic reptiles, moving side-to-side and acting like a rudder. However, there's also a hypothesis that their fins might have actually been horizontal more like those of modern cetaceans and sirenians, based on several anatomical quirks – such as their tail regions being very wide and flat at the base, and the vertebrae at the tip being unusually pygostyle-like, very different from the way the tail bones of vertically-finned reptiles look.

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