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Thursday, 18 August 2011

Euryapsids: Return of the Enaliosauria

After much allusion to classifying reptiles by the structure of their skulls, and particularly mentioning Euryapsids, I thought it would be a good time to put up a refresher course on the topic.

The following is a current classification of representative fossils thought to be Euryapsids. they reange from some very early and very small lizard-like forms at the top the the rather larger and better adapted (to marine life) Thalattosaurs and Ichthyosaurs after that in the darker green section, Then Placodonts in the lighter green and finally Nothosaurs and Plesiosaurs in the yellow-green at the bottom of the chart.

[The family Tree of the Euryapsida or Enaliosauria after Owen.]

And the whole family tree of Reptiles in this new version:

[Older chart, Sea Reptiles of the Mesozoic.]

The classification of Reptiles into subclasses was once a straightforewatd thing: There were Anapsids without an opening in the skull behind the eye, Diapsids with an opening (fenestration) in both the high position and low position, two on each side: and then the Euryapsida with the fenestration in the high position as indicated by the skull bones whereas the Synapsids had one opening in the low position. Several of these distinctions are crucial, in that the Anapsids are the most basic type the others derived from (and which are now represented by turtles and tortoises) while the Synapsids and none other were the ancestors of the Mammals.

But the Diapsids were a large and diverse lot, including snakes, lizards, crocodiles, dinosaurs and birds. Some scientist saw it as more useful to divde the Diapsids into the more primitive Lepidosaurs, including lizards and snakes, and the Archosaurs or Ruling Reptiles, which included the dinosaurs and birds. And while they were working on that notion, some researchers began to suspect that Euryapsids were only a specialized type of Diapsids that lost the lower fenestration and now expressed the upper one preferentially. It turned out that this was probably true and that Plesiosaurs and Ichthyosaurs were closely related to each other after all, and the both of them (as Euryapsids) were most closely related to the dinosaurs. In fact, the Euryapsids fall between the Synapsid ancestors of the Mammals and the dinosaurian ancestors of the Birds, which is an independant clue some of them might have tended to develop homeothermy.
Here are an assortment of well-known Euryapsids arranged to correspond roughly to the Family Tree chart given above, and the photos all get larger if you click on them, at which time you may find them on the family tree as given.

Finally, it is worth pointing out with all of this talk about Plesiosaur's "Inflexible" necks that several of the well-preserved original fossils often show a good degree of curving and twisting in the neck-enough so that there would be a good reason to suspect the living animals were capable of assuming "Nessie" poses, assuming that the pose did not require a good deal of bending at the base of the neck.

The sinusoid curve toward the end of this neck is especially noteworthy. All of these fossils belong to the Plesiosaurus type and were originally referred to that genus, but some of them are now assorted into different genera.

Best Wishes, Dale D.

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