Placodonts were a group of extinct marine reptiles from the Middle to Late Triassic (Anisian-Rhaetian). They originated in the western Tethys and expanded east, keeping to near shore environments and, for the most part, chomping away at shelled invertebrate fauna. Because of this diet, more-or-less a hallmark of the group, their teeth are instantly recognizable as being good for crushing.
As a group the were pretty diverse, but divide easily into two main groups: the unarmored placodonts (Placodontoidea) and the shelled placodonts (Cyamodontoidea). We know that placodont tooth morphology changes from group to group, getting flatter the further removed species are from their common ancestor with other Sauropterygians.
How does this change in tooth morphology affect tooth function, and do later occurring placodonts achieve an ‘optimal’ tooth for crushing hard prey items? To answer these questions, we measured the curvature of teeth across the placodont phylogeny and related these curvatures model teeth whose ability to break shells and to withstand applied forces have been quantified. In this way we were able to predict an ‘optimal’ tooth, based on functional trade-offs.
While some groups of placodonts do seem to achieve this ‘optimal’ tooth shape, the latest occurring group of placodonts, the Placochelyids do something completely different. Placochelyids are easily picked out of a placodont line-up, as they have lost their premaxillary teeth and developed an elongate rostrum.
It had been proposed that the Placochelyids were doing something different from other placodonts, possibly filling the same niche that eagle rays fill today, unearthing and eating buried bivalves or even pelagic crustaceans. This is supported by their tooth morphology, which is more complicated than the flat surfaces of other cyamodontoids, having a crescent shaped concavity on one side of the tooth surface and a small cusp on the other side. This tooth morphology may be another approach to eating hard prey items, using the concavity to either disperse force or hold prey in place, while the small cusp concentrates the force applied to the prey.
For more information check out the paper