Palatodonta (Fig. 1), the basal placodont, is known from a skull only (or perhaps that is all that was published by Neenan et al. 2013). If we want to see what the rest of Palatodonta looks like, we have to guess based on the shapes of sister taxa, like Claudiosaurus, Paraplacodus and Pachypleurosaurus. This is known as phylogenetic bracketing. These sister taxa differ in neck/torso and limb proportions (Fig. 1), along with several other skull and limb ratios. There is no guarantee that the estimate will be right, but it provides a best guess as to the transition from Cladiosaurus to Paraplacodus.
If this is at all close to reality…
We already know that Palatodonta had a larger and taller skull than the ancestral Claudiosaurus. Likely the neck was shorter and the torso longer with taller neural spines based on the morphology of Paraplacodus. The scapula likely separated from the coracoid in Palatodonta. The ilium likely lost its posterior process because it is lost in sauropterygians.
The palate is interesting
Claudiosaurus has a shagreen of tiny teeth over most of the palate. Pachypleurosaurus lost these palatal teeth. Palatodonta reduced the number but increased the size of the palatal teeth. In Paraplacodus the enlargement of the maxillary teeth expanded the maxilla medially, reducing the internal nares and pushing them toward the midline. The suborbital fenestra disappeared as the ectopterygoid narrowly bordered the toothless pterygoid in Paraplacodus.
What does it mean that the internal nares are so reduced in placodonts? Of course the overall size of Paraplacodus is many times larger than Palatodonta. With that increased size and likely more lethargic lifestyle perhaps rapid exchange of large amounts of air was not so important.
Neenan JM, Klein N and Scheyer TM 2013. European origin of placodont marine reptiles and the evolution of crushing dentition in Placodontia. Nature Communications 4:1621. – DOI: 10.1038/ncomms2633 |www.nature.com/naturecommunications. wiki/Palatodonta