Plesiosaur Day, 18 November 2004 - Some notes on the meeting
Franziska Grossman
reviewed her recent work on the Toarcian plesiosaurs of the German Posidonia Shale. Though some of the data presented here was also spoken about at SVPCA 2004 (also held in Leicester), there was much new information. 14 plesiosaur specimens are known from this unit, only five of which have been described. Using skull morphology, tooth ornamentation and limb proportions, Franziska was able to show that Plesiosaurus brachypterygius and P. guilelmiimperatoris could be distinguished on numerous features. The former is heterodont with symmetrical tooth ornamentation, has a snout that appears constricted in dorsal view, and exhibits proportionally shorter limbs compared to body length than P. guilelmiimperatoris. P. guilelmiimperatoris lacks a constricted snout, exhibits asymmetrical tooth ornamentation, and has limbs that are c. 25% of body length (compared to c. 20% inP. brachypterygius). Inclusion of these two taxa within a cladistic analysis shows that neither is close to P. dolichodeirus, and thus neither really belongs to the genus Plesiosaurus. A new generic name is needed for P. brachypterygius, and it appears to be close to (thought not congeneric with) Occitanosaurus and Microcleidus. Meanwhile, Seeleyosaurus holzmadensisWhite 1940 seems to be a junior synonym of P. guilelmiimperatoris meaning that Seeleyosaurus is the correct generic name for this taxon.
The recent description of a third ostensible Posidonia Shale taxon, Plesiopterys wildii O'Keefe 2004 for specimen SMNS 16812, has added a new facet to Franziska's research. Franziska argued that some of O'Keefe's conclusions on the morphology of this specimen were debatable, and in fact the animal does not appear to be a distinct taxon but a juvenile specimen of P. guilelmiimperatoris. What does appear to be a third taxon is present and is represented by a single near-complete juvenile specimen (SMNS 51945). With a total length of about 3.5 m this taxon would clearly have reached a larger adult size than either of the other Posidonia species. Its proportions are also quite different from those of P. guilelmiimperatoris and P. brachypterygius but is mashed skull meant that Franziska wasn't able to accurately determine its affinities. Given that the specimen consists of an otherwise complete skeleton this assertion is debatable to say the least! Other areas that Franziska hopes to look at in future concern palaeoecology and the reconstruction of plesiosaur musculature.
Adam Smith
discussed controversy and consensus in plesiosaur phylogeny, covering the affinities of polycotylids, the topology of elasmosaurids and the affinities of the 'southern weirdos' (viz, Aristonectes, Kaiwhekea and kin). By correcting and recoding parts of O'Keefe's (2001) analysis Adam produced a modified, though largely similar, tree. However, the main focus of his research was producing a novel analysis of plesiosauroids based on characters coded from the literature. In view of one of Franziska's assertions it was interesting that Plesiopterys grouped as the sister-taxon to P. guilelmiimperatoris. Occitanosaurus, Microcleidus and Brancasaurus formed successive outgroups to an elasmosaurid-polycotylid-cryptoclidid clade, and polycotylids were found to be closer to cryptoclidids than to elasmosaurids. Adam also looked at the rate of skeletal character acquisition in plesiosauroids across time. The rate of evolution appears to be slow and constant for most of the Jurassic, but with marked conservatism and notably slow rates of innovation during the Cretaceous among some lineages.
Adam is to start work soon on Rhomaleosaurus cramptoni at Dublin.