Field of Science

The First Morphological Evidence for Turtles as Archosauromorphs

Bhullar, B.S., and G.S. Bever. 2009. An archosaur-like laterosphenoid in early turtles (Reptilia:Pantestudines). Breviora 518:1-11. doi: 10.3099/0006-9698-518.1.1

Abstract: Turtles are placed with increasing consistency by molecular phylogenetic studies within Diapsida as sister to Archosauria, but published gross morphology–based phylogenetic analyses do not recover this position. Here, we present a previously unrecognized unique morphological character offering support for this hypothesis: the presence in stem turtles of a laterosphenoid ossification identical to that in Archosauriformes. The laterosphenoid is a tripartite chondrocranial ossification, consisting of an ossified pila antotica, pila metoptica, and taenia medialis + planum supraseptale. It forms the anterior border of the exit for the trigeminal nerve (V) and partially encloses the exits for cranial nerves III, IV, and II. This ossification is unique to turtles and Archosauriformes within Vertebrata. It has been mistakenly dismissed as anatomically dissimilar in these two groups in the past, so we provide a complete description and detailed analysis of correspondence between turtles and Archosauriformes in each of its embryologically distinct components. A preliminary phylogenetic analysis suggests other potential synapomorphies of turtles and archosaurs, including a row or rows of mid-dorsal dermal ossifications.

Thanks to Randall Irmis for bringing this to my attention.

6 comments:

  1. Don't snakes have a laterosphenoid bone? Could it be that this is actually a primitive feature for amniotes that is lost in some lepidosaurs rather than a synapomorphy of archosaurs + turtles?

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  2. Yeah, I'm not real sure that a laterosphenoid bone is the smoking gun everyone's looking for in this case.

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  3. Its probably worth reading the paper before jumping on their conclusions...

    1. The authors point out that this is not just about the presence of a laterosphenoid, its that basal turtles share several morphological features of the laterosphenoid only found in archosauriforms.

    2. The authors never claim that this is a "smoking gun"! In fact, they are quite conservative in the implications of this feature - only pointing out that it is an intruiging character, and that if you assume turtles are diapsids, there are several other characters that optimize as synapomorphies shared by turtles and archosauromorphs. But, the authors are the first to point out in the discussion that this is based on very limited character evidence, should be interpreted with caution, and is certainly not incontrovertible proof of an archosauromorph ancestry for turtles.

    That being said, I think this feature is very intriguing given that molecular evidence consistently places turtles within or sister to Archosauria.

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  5. Correcting a typo in the last version; hi Paul -- this is where being primarily a squamate morphologist actually comes in handy. Snakes are highly nested within Squamata no matter the phylogeny one uses, and their 'laterosphenoid' is neither topologically nor developmentally similar to the archosaur laterosphenoid. The archosaur laterosphenoid closes off the trigeminal foramen from the front (as the ossified pila antotica), whereas the snake laterosphenoid is _not_ an ossified pila antotica and appears in the middle of the trigeminal foramen to divide it into a couple of openings. Further, uncontroversial stem-group snakes (Dinilysia, Najash) do not have a laterosphenoid. No other squamate has a laterosphenoid. Moreover, we have fairly well-preserved interorbital regions for several early parts of the two major amniote lineages, notably captorhinomorph panreptiles and 'Eothyris-grade' panmammals. Those which I have personally seen bear a sphenethmoid ossification anteriorly but no laterosphenoid ossification with an ossified pila antiotica closing off the trigeminal foramen, among other structures.

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  6. Having said that, Jacques Gauthier points out, quite correctly, that one need not take too many steps outside of Amniota before the interorbital region becomes a pretty solid tube of bone all the way back. So... according to our best current evidence, no, a laterosphenoid was not present in the amniote ancestor. However, that evidence is not exactly multitudinous. Case not closed.

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