From the abstract:
Neenan 2013 reported, “Placodonts are a clade of durophagous sauropterygians that inhabited the eastern and western margins of the Tethys Ocean from the lower Middle to the end Triassic. The group consists of two morphotypes: the plesiomorphic, paraphyletic, and unarmored ‘placodontoids’, which are known only from the Middle Triassic, and the heavily armored Cyamodontoidea, which span the entire Middle and Late Triassic. However, the evolutionary relationships and origins of the Placodontia have remained unclear until now, particularly in the light of new taxa described from China, the majority of which are yet to be included in a phylogeny and described in detail. In order to resolve the systematic relationships of placodonts, micro-computed tomography was used on several crania from both European and Chinese taxa. This method not only allows accurate reconstruction of external osteology, but also of obscured structures such as the braincase and inner ear. For the first time, a comprehensive phylogeny including all eastern and western placodont taxa is thus presented. Among the Chinese forms the basal placodont Placodus inexpectatus clusters with European ‘placodontoid’ taxa, while Glyphoderma and Psephochelys form a clade with the highly nested Placochelyidae, thus pulling this node back into the Late Middle Triassic. This indicates that all placodont clades originated during a period of intense speciation during the Middle Triassic, with cyamodontoid taxa diversifying into the Late Triassic on both sides of the Tethys. Additionally, a new, exquisitely preserved skull of a juvenile placodontiform from Winterswijk, the Netherlands has provided a wealth of evidence concerning both the paleogeographic and evolutionary origins of crown group Placodontia. Characters such as a single row of teeth on the palatine place the new taxon on the stem to Placodontia, indicating an origin of the clade in the western Tethys, which then radiated eastwards. Furthermore, the dentition is not adapted for durophagy, indicating the unusual dental arrangement of palatine teeth in placodonts did not initially evolve as a result of consuming hard-shelled prey. As the most plesiomorphic clade of the most successful and diverse marine reptile radiation known, the placodonts are essential for understanding the origins and diversification of Sauropterygia. The new data are therefore of great significance, providing insight into the paleobiogeographic and paleoecological changes that occurred on the stem leading to the more derived sauropterygians.”
Neenan 2013 reports origins and relationships have been unclear in the Placodontia. Not sure where this came from. Placodonts have always been allied with basal Sauropterygia (stated so in Wikipedia), despite their obvious morphological differences. Neenan neatly divided placodonts into shelled and unshelled plesiomorphic forms, but in the large reptile tree, one of the armored placodonts, Henodus, actually nests with Placodus, an unarmored placodont to the exclusion of the other armored placodont. (The broad muzzle gives them away as related).
The “juvenile” mentioned by Neenanis Palatodonta, which may just have a small skull, like Claudiosaurus on a much larger body, given phylogenetic bracketing. Rather than a juvenile, this taxon is simply primitive on the large reptile tree.
Neenan 2013 does not spell out the new relationships mentioned in the abstract, but the large reptile tree does, if anyone is interested…
Neenan, J 2013. Origins, systematics and paleoecology of placodont marine reptiles. Journal of Vertebrate Paleontology abstracts 2013.