Plesiosaur Day, 18 November 2004 - Some notes on the meeting
Additional Contributions
From Adam Morrell
Richmond Marine Fossil Museum, Australia (Email 9/12/2004)
Something that I am really interested in is the origin of the Cretaceous clades - especially the Elasmosauridae. My focus is on the Early Cretaceous (Aptian/Albian) elasmosaurids from Australia. Only one skeleton has been described previously (Persson, 1960) and it was described as a "relatively" short-necked form, with a "primitive" Jurassic-grade pectoral girdle (without the intercoracoid foramen typical of the later Cretaceous elasmosaurids). As such, it formed a good evolutionary intermediate between Jurassic forms such as Muraenosaurus and Cretaceous forms such as Hydrotherosaurus.
However, examination of the type material and comparison with other, much more complete material from the same horizon shows the above description to be inaccurate. The Australian form did have a well-developed intercoracoid vacuity and long neck (~60 vertebrae), as well as many other characters of later Cretaceous elasmosaurids, such as broad epipodials and an anterodistal "knee" in the humerus. All of the characters that are typical of the Late Cretaceous forms had already evolved by the late Aptian. This is not really a surprise if earlier Cretaceous forms such as Brancasaurus and Callawayasaurus are taken into account.
This brings me to the debate about the phylogeny of the elasmosaurids. Traditionally, Jurassic long-necked forms such as Microcleidus and Muraenosaurus have been viewed as early elasmosaurids, near the base of a clade leading to the North American and New Zealand Late Cretaceous forms such as Elasmosaurus and Mauisaurus. Brown's (1981; 1993) familial diagnoses for the Elasmosauridae define the synapomorphies for the group as having elongate and numerous cervical vertebrae, with platycoelous articulations, lateral longitudinal keel, etc.
Then there's the alternative view that seems to have first been raised by Bakker in 1993 (although never acknowledged in the literature, seemingly for various reasons). In this argument, there has been much convergence between plesiosaur clades and the "elasmosaur" bodyplan has evolved independently multiple times, with the Cretaceous long- necked forms representing a distinct clade from the Jurassic one(s). This was further developed and tested by Carpenter, O'Keefe, etc. based on mainly on more "reliable" cranial characters. If this is the case, then Brown's familial diagnosis is a list of homoplasies.
This is an interesting debate, and one that I hope my research will be able to offer insight into. Although my specimens were already highly derived by the end of the Early Cretaceous, I'm hoping to identify some plesiomorphic characters that will be able to link the Cretaceous group with other plesiosaur families, whether these are the Jurassic long-necked taxa or otherwise.
What I don't understand is why this debate doesn't seem to crop up in much of the literature. Many workers in Europe (and even Australia) continue to describe new "elasmosaurid" specimens from the Jurassic, or describe Cretaceous taxa based on comparison to forms such as Muraenosaurus and Brown's diagnosis. These articles never even discuss the alternative view, despite mounting evidence for it.
Do you have any idea why this is? Is there some fundamental disagreement in the palaeontological community with the argument for a polyphyletic Elasmosauridae? Certainly, there have never been any definitive arguments to support separate origins of Jurassic and Cretaceous long-necked clades. O'Keefe's 2001 phylogeny left the Cretaceous elasmosaurs hanging off an unresolved trichotomy. And even if the Cretaceous elasmosaurs did share a close relationship with other Cretaceous clades such as the polycotylids (as suggested by Carpenter), then this would not preclude forms such as Muraenosaurus or cryptoclidids from being ancestral to the Cretaceous group.
Or are people just continuing to describe new specimens and not worrying about higher-level taxonomy while the phylogeny of the Plesiosauria is in a state of flux?
For my part, I'm focusing on the phylogeny of the Cretaceous Elasmosauridae. This was certainly an incredibly conservative group, with very little change in the last 50 million years of the Cretaceous. There have been attempts at cladistic analyses of the Elasmosauridae in the past but they weren't comprehensive. When some initial analyses placed taxa such as Muraenosaurus outside of the Elasmosauridae, characters such as the presence of a long neck were actually weighted to put them back inside the clade.
My work is focusing on postcranial characters, of which there are very few to differentiate between the Cretaceous specimens. I'm playing around with morphometric techniques and multivariate stats to try and identify new characters in the proportions of elements. Despite what appear to be major proportional differences between specimens (which are often noted in the literature), my preliminary results are showing very little differentiation between specimens. This was a very morphologically conservative group. This has interesting functional implications, which can be further investigated through character correlations in the phylogeny. If the shared features of Jurassic and Cretaceous long-necked forms are the result of convergence, then the similarities in proportions are remarkable, including the proportions of the limbs, etc. This indicates strong functional constraints for this bodyplan. Whatever the purpose of the elongated neck, it must have been very useful to have lasted the length of the Jurassic and Cretaceous, despite the obvious physical disadvantages it must have conveyed to swimming ability and overall fitness.
But then there are subtle differences in the form of the neck and some other elements, so there probably was some specialisation within the overall bodyplan. I've also been trying to develop some multivariate stats to compare changes in the proportions of vertebral centra along the vertebral column. As you probably well know, there are certainly some distinct patterns and differences between taxa. I guess it shows up so well in elasmosaurs because there's so much differentiation along the vertebral column, even within the dorsal series, especially when compared to other plesiosaurs.
Anyway, I have a lot of material to get through here. Over 100 elasmosaurid specimens have been collected from Australia, mostly labeled as "Kronosaurus" in museum collections. Colin McHenry realized what they were but has never had a chance to do anything with them. There are about 10 reasonably complete postcranial skeletons to work with, as well as a couple of skulls. A huge part of my thesis will be just raw specimen descriptions.