Jesairosaurus and the origin of drepanosaurs

Updated November 29, 2015 with new data and a new nesting for Jesairosaurus and the drepanosaurs. 

The authors of Wikipedia
have no idea what drepanosaurs are other than reptiles. In the large reptile tree drepanosaurs were descended from Jesairosaurus and, more primitively, the basal tritosaur lepidosauriform, Palaegama. Here we’ll start with Palaegama (Fig. 1)the father of all drepanosaurs and kuehneosaurs.

Figure 3. Drepanosaurs and their ancestor sisters, Jesairosaurus and Palaegama to scale.

Figure 1 Drepanosaurs and their ancestor sisters, Jesairosaurus and Palaegama to scale.

Palaegama is a basal lizard-like lepidosauriform that shared few obvious traits with the highly derived drepanosaurs. It does not take a transitional taxon, like Jesairosaurus (Fig. 1) to make the case for a relationship here because deletion does nothing to tree topology.

Now we narrow our focus
to Jesairosaurus, the father of all drepanosaurs (Fig. 2). Essentially Jesairosaurus was a long torso palaegamid lepidosauriform. The skulls are readily comparable. The limbs are shorter in the derived taxon.

Figure 1. New reconstruction of the basal lepidosauriform, Jesairosaurus (Jalil 1993).

Figure 2. New reconstruction of the basal lepidosauriform, Jesairosaurus (Jalil 1993).

Jesairosaurus lehmani (Jalil 1997) Early to Middle Triassic ~240 mya was originally described as a prolacertiform and thus related to Prolacerta. All shared traits are by convergence here. By virtue of its nesting, Jesairosaurus is also basal to the kuiehneosaurs, like Coelurosauravus (Fig.  3).

 

Figure 4. Jesarosaurus to scale with sisters Palaegama and Coelurosauravus.

Figure 3. Jesarosaurus to scale with sisters Palaegama and Coelurosauravus.

With its bigger torso and smaller limbs,
Jesairosaurus was not a speedster. This was the first step in the evolution of the slow-moving, arboreal drepanosaurs. The high scapula of drepanosaurs finds an origin in Jesairosaurus.

It is unfortunate
that the hind feet and tail are not known for Jesairosaurus, because both of these body parts underwent a great transformation in early drepanosaurs.

As in Palaegama, the pelvis of Jesairosaurus was relatively tiny and included an anterior process of the ilium that developed further in drepanosaurs. While  Palaegama employed its expanded ilium to sprint, drepanosaurs were not sprinters, but arboreal climbers and clingers, like modern day chamaeleons.

 

References
Jalil N-E 1997. A new prolacertiform diapsid from the Triassic of North Africa and the interrelationships of the Prolacertiformes. Journal of Vertebrate Paleontology 17(3), 506-525.

wiki/Prolacertiformes

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8 thoughts on “Jesairosaurus and the origin of drepanosaurs

  1. Is the scale bar in Fig 1 supposed to be 5cm? I think the relative sizes and scale to 10cm is maintained correctly in Figure 3, but the scales in Figs 1 and 2 suggest that Huehue. and Jesair. were about equal in head+body length.

  2. The “olecranon sesamoid” is the olecranon. The identifications of Renesto haven’t been countered by any reasonable source, and indeed the mere positions of the bones imply the homology as huge semi-circular ulna, with a distal, rod-like ulnare. The functional mechanics of this arrangement are not problematic save they should restrict elbow flexion. This can be further predicted by examining the relationships of the extremely elongated ulnare in other megalancosaurids compared to the radiale, and the extremely short ulna in those taxa in comparison. There is no reason to suggest that a sesamoid is a likelier explanation, save historical doubt over the identification of the structure as a ulna, rather than simply calling it an ulna. The simpler explanation, without contravening data, is that the bone is an ulna; this is the null hypothesis.

    • I understand what you are saying, Jaime. You are following Renesto’s original interpretation that blew everybody’s mind. Silvio and I worked this out earlier when I blogged on the elbow itself and he is now on board with the new interpretation having seen an olecranon sesamoid or olecranon bone, what have you, on all related taxa going back to Sphenodon. In particular the olecranon is largest on the closest sister, Megalancosaurus. Both the olecranon and the ulna have a very similar morphology as the same bones in Drepanosaurus other than one is gracile, the other robust. That is the contravening data you seek. And we have several Megalancosaurus that display the same, previously overlooked, morphology. Here is the url for the elbow blog: https://pterosaurheresies.wordpress.com/2011/12/26/where-is-the-ulna-on-drepanosaurus/ I would be happy to send you the additional evidence on request.

      • The problem is is that you’re arguing that the “sesamoid” is a replacement for the ulna, ignoring the natural morphology indicated by other megalancosaurs, and are doing this on NO morphological ground whatsoever. Especially since sesamoids have a peculiar histology from typical endochondral bones … like the ulna … and for which no test has been done. I would suggest considering this when suggesting something so extreme as a giant sesamoid overtaking and replacing the ulna/humerus contact, which is far-fetched. I have also looked at the detailed photos in Renesto’s possession, and there is no evidence, aside from your “tracing”, that Megalancosaurus preonensis’s olecranon isn’t an actual olecranon, rather than some isolatable sesamoid, regardless of whether a sesamoid is present or not. The issue isn’t even that Megalancosaurus preonensis HAS a sesamoid in the olecranal region, but that the large semi-lunate bone IS a sesamoid. Position is not the only thing that counts. Remember, Extraordinary claims needs extraordinary evidence!

  3. I’m actually making the conservative interpretation, Jaime. What looks like the ulna IS the ulna. It has the same morphology as the Megalancosaurus ulna. What looks like a crescentic extra bone IS a crescentic extra bone as shown by Megalancosaurus. The claim otherwise, as you and the earlier Renesto paper have, needs more than extraordinary evidence. It needs a miracle. Renesto has changed his mind. I urge you to do so also.

    • This is not the conservative opinion: such a one would require agreement with typical arrangement and morphology FIRST. No sesamoid bone forms an intermediate placement between two bones in this manner; they are instead formed within tendons or muscles to serve as reducers of straight around joints or around regions where muscles or tendons would be compressed, as beneath phalangeal-phalangeal or metacarpal/metatarsal-phalangeal joints, where they are best known.

      A good example would, in fact, be your identification of the ulnar sesamoid in OTHER taxa, which is often small or forms the olecranon in some, but not all, ulnae, where it is otherwise interpolated within the elbow-extending tendons of the humerus. Despite this, there is no apparent sesamoid in the location of the Megalancosaurus preonensis limb you illustrate in either post; you should be the most wary, but aren’t, about biased treatment of cracks or “features” on a slab such as the holotype or referred forelimb illustrated by Renesto and Pinna, especially when you ignore a more distal feature of the slab where the bedding matrix is visible “within” the ulna, but this is not “shown” as a feature. Without clear evidence, even the semi-lunate condylar form of the elbow joint, the outlever portion of which you treat as a whole sesamoid despite it bearing a portion of the articulation with the humerus, is a natural feature, as noted by Renesto in his anatomical comparions. It wouldn’t even matter if the very tip of that structure were a sesamoid, it is unlikely to involve the joint itself. This goes doubly for trying to exapt such a bone into a functional “ulna”, forcing separation of the ulna and humerus without good reason or data. Claiming the structure is a sesamoid, and calling that the conservative argument, without backing this up doesn’t help.

      • I see your logic, but by following your logic you toss out the olecranon bone, which is present in all ancestral taxa, and you greatly expand the ulnare, which is tiny in all ancestral taxa. In both hypotheses the elbow goes through some bizarre changes, so that’s a wash. In your hypothesis the ulnare not only grows to the size of the ulna, but it adopts its unique morphology (with all Z-dimension elements crushed in Megalancosaurus). That’s beyond the realm IMHO, when you have a more parsimonious A=A and B=B hypothesis.

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