Plesiosaur Bites

I’ve been rather quiet again recently, however, as coauthor of an article just published in PLOS ONE, I’ve good reason to come out of my shell today. The new paper describes and names the Weymouth Bay Pliosaur, a spectacular almost complete skull over 2m long from. As discussed in the open access paper (do take a look), the specimen is sufficiently different from all other pliosaurs to warrant a scientific name of its own, Pliosaurus kevani.

Pliosaurus kevani was named in honour of Kevan Sheehan, the Osmington Mills café owner who collected most of the skull, piece by piece, over a period of eight years during daily walks along the foreshore. Kevan collected chunks up to 60 kg in mass as they weathered out from the Jurassic aged Kimmeridge Clay Formation sea-cliff. The specimen was purchased with funding secured by Dorset County Council’s museum service from the Heritage Lottery Fund Collecting Cultures programme and Dorset and Devon county councils. It was prepared between 2010 and 2011 by Scott Moore-Fay and went on public display in Dorchester County Museum in July 2011.

Richard Forrest, who was involved with the project from the beginning, first had the idea of putting together a ‘dream team’ of British plesiosaur specialists to study and describe the skull. This is the first collaboration of its kind among plesiosaur researchers (as far as I know), and I feel lucky to have had the opportunity to contribute to it under the driving force of our lead author, Roger Benson.

Roger Benson (left) and Richard Forrest (right) collecting data from the Weymouth Bay Pliosaur – Pliosaurus kevani

The massive skull has a long snout, circular orbits, huge temporal openings for the jaw musculature, and a deep mandible. Large portions of the skull have been crushed flat during fossilisation, so one of my tasks was to reconstruct the skull to show how it might have appeared before it was flattened. After several versions and much input from Mark Evans, I’m pleased with how it turned out, and I think we have a pretty accurate reconstruction of Pliosaurus. On the basis of this reconstruction I’ve also had a go at restoring the life appearance of the head of P. kevani in profile. Despite its large size and massive teeth, the head is rather gracile.

Pliosaurus belongs to a group of plesiosaurians known as thalassophonean pliosaurs. If you haven’t heard of them before, that’s because the name Thalassophonea, or “sea slayers”, was proposed just this year (Benson & Druckenmiller, 2013) for a natural group of derived giant pliosaurids including Pliosaurus, Liopleurodon, and Kronosaurus. Thalassophoneans were macropredators, that is, giant predators doing the sort of dirty work in the Middle-Late Jurassic and Cretaceous that rhomaleosaurids had done in the Early Jurassic. The paper also discusses the evolution of pliosaurids. The earliest thalattophoneans have a long mandibular symphysis, but in later member it becomes shorter. This trend is related to a shift in the dietary habits of pliosaurs from primarily fish-eaters to macropredators. In conjunction with this trend, we demonstrate that pliosaurids tend to follow Cope’s Rule – they get larger throughout their evolutionary history.

We also name two other new species of Pliosaurus in the paper, P. westburyensis and P. carpenteri, based on material in the Bristol Museum & Art Gallery from Westbury, Wiltshire. Again, there are aspects of the morphology in these specimens that distinguish them from one another, but don’t justify new genus names. So, add these new species to the existing list of valid Pliosaurus species (P. funkei, also known as Predator X, P. brachydeirus, P. rossicus, there might be one or two more, pending thorough description of the material) and we find ourselves with a rather large number of species within a single genus (although some invalid species of Pliosaurus are sunk too). Future research might show greater generic diversity among these species, but that’s really dependent on the discovery of more satisfactory fossil material.

There was a time when I’d leap into tippy-tappy action at the first sniff of a newly named plesiosaur. Unfortunately, I haven’t been keeping Plesiosaur Bites up to date and a few new taxa have passed me by. Of course, when I say “a few”, what I really mean is we are swamped by the things. Little wonder I haven’t been able to keep up.

A few years ago I plotted a graph in my PhD thesis (Smith, 2007, Figure 2.2.) to show the number of valid plesiosaur species and genera named in successive 20-year time intervals since 1821 (when the first plesiosaur was named [Plesiosaurus]). The data ended in 2007, the year I submitted my thesis, but showed that new taxa were being erected at a relatively steady rate throughout the 19th and 20th century (Figure 1). The rate started to pick up during the 1990s and I extrapolated the data into 2008-2020 based on the first seven years of the 21st century. I predicted 30 new genera in the period 2001-2020, which would represent a huge post-2001 leap in the number of new valid plesiosaurs. Well, so much for my crude calculations. It’s only 2012 and my ‘huge’ prediction has already been surpassed.

Figure 1. Tally of the number of new plesiosaur taxa per 20-year interval (from Smith, 2007, Y-axis adjusted for direct comparison with Figure 2 below). 2001-2020 predicted based on 2001-2007 data.

An exciting new paper published this week in the journal Science (Vol. 333, p.870-873) provides the first direct evidence for live birth in plesiosaurs, and may have implications for plesiosaur behaviour (O’Keefe & Chiappe, 2011).

The plesiosaur Polycotylus giving birth to a single large baby. Based on new fossil evidence. Image by S. Abramowicz/NHM

Whether plesiosaurs laid eggs or gave birth to live young has been a topic of speculation for nearly 200 years. They have sometimes been portrayed crawling out of the water to lay eggs in the manner of sea turtles, and while palaeontologists have long suspected that plesiosaur anatomy is incompatible with movement on land, empirical evidence either way has been lacking.

The new evidence comes in the form of a fossil plesiosaur skeleton with a fetus preserved in the body cavity. Both individuals have diagnostic characteristics indicating they are the same species, the small individual displays embryonic features and is in the correct position to be a fetus, and there are no signs of it being eaten (bite marks or acid wear). These numerous lines of evidence confirm that this fossil represents a mother and her unborn fetus. This demonstrates that plesiosaurs did not lay eggs and were therefore able to lose their ties with land and spend their entire lives in the ocean.

The evolution of live birth in plesiosaurs would have allowed them to lose all ties with land. Depictions like the one above of a Jurassic plesiosaur (Hydrorion) are therefore highly unlikely. Painting by Burian.

The newly described fossil plesiosaur is a polycotylid (Polycotylus), one of the last types of plesiosaurs to evolve. It was discovered in Late Cretaceous rocks in Kansas, USA. Polycotylids were highly derived plesiosaurs with torpedo-shaped body outlines and wing-like flippers, a relatively short neck (as far as plesiosaurs go) and a very short tail. They were almost penguin-like in general appearance and also similar to penguins, they would have been fast and agile swimmers.

An unusual aspect of this fossil is the size of the fetus. Most viviparous reptiles give birth to a brood of several small individuals. In contrast, this new fossil shows that at least some plesiosaurs gave birth to a single very large individual, much like whales do today. Many other marine reptiles including ichthyosaurs and mosasaurs gave birth to live young, but this study suggests that plesiosaurs differed in that they invested energy and time into a single individual. This sort of reproductive strategy is often associated with gregarious behaviour and parental care, so the authors of the paper suggest that maybe plesiosaurs were excellent parents too. This hypothesis is fascinating although it would be quite unusual for reptiles.

Illustration showing the relative size of a mother Polycotylus and newborn baby. From O'Keefe & Chiappe, 2011)

Fossils of basal sauropterygians (pachypleurosaurs and nothosaurs), close relatives of plesiosaurs, also show that they gave birth to broods of several small live babies, so it is unclear when the evolutionary shift in reproductive strategy occurred in the sauropterygian lineage. It is certainly possible that the first plesiosaurs were more like their ancestors in terms of reproductive behavior. More fossils will ultimately be required to fill in the bigger picture, but for now, it is wonderful to be able to say with certainly that plesiosaurs gave birth to live young.

Reference

O’Keefe, F. R. & Chiappe, L.M. 2011. Viviparity and K-selected life history in a Mesozoic marine reptile. Science, 333, 870-873.

So far, 2008 has seen a healthy number of new papers on plesiosaurs and a few new taxa too. Way back in February, Druckenmiller and Russell (2008a) introduced Nichollsia borealis, a plesiosaur of uncertain affinity, based on a beautifully preserved specimen from Alberta, Canada. More recently, Druckenmiller and Russell (2008b) published a large scale cladistic analysis of plesiosauria to try and make sense of plesiosaur relationships, especially the affinities of Leptocleidus and Nichollsia – this is a substantial piece of work. Both papers stem directly from Druckenmiller’s PhD thesis.

Sato and Wu (2008) erected a new taxon Borealonectes russelli, a pliosaur they identify as a rhomaleosaurid, based on a skull and partial postcranium from the Canadian Arctic Archipelago. rhomaleosaurids also recieved treatment from Smith and Dyke (2008) who described the skull of Rhomaleosaurus cramptoni - the holotype of the family. They also present a full body reconstruction of the 7m long genus, and a cladistic analysis dedicated to pliosaurs.

Rhomaleosaurus skeleton – Figure 2 from Smith and Dyke (2008)

Long necks received attention from Zammit et al. (2008) who investigated the flexibility of an elasmosaurid cervical column, confirming the common presumption that swan-like postures were impossible in beasts such as Elasmosaurus. Bardet et al. (2008) described a partial plesiosaur skeleton from Asturias, helping to elucidate plesiosaur diversity in the Pliensbachian and presenting a rare specimen from Spain.

Finally (for now, I may have overlooked one or two papers), Smith (2008) presented an overview of plesiosaurs aimed at a popular audience. It covers basic aspects of the anatomy and biology of plesiosaurs. I hope this article will fill the void present between technical papers and children’s books and help people ‘get into’ the scientific literature, which can be quite daunting otherwise.

Plesiosaur anatomy – Figure 1 from Smith (2008)

References -

Bardet, N., M.., Fernández, J. C. García-Ramos, Z. P. Suberbiola, L. Piñuela, J. I. Ruiz-Omeñaca, and P. Vincent. 2008. A juvenile plesiosaur from the Pliensbachian (Lower Jurassic) of Asturias, Spain. Journal of Vertebrate Paleontology, 28, 258-263.

Druckenmiller, P. S. and Russel, A. P. 2008a. Skeletal anatomy of an exceptionally complete specimen of a new genus of plesiosaur from the Early Cretaceous (Early Albian) of Northeastern Alberta, Canada. Palaeontolgraphica, 283, 1-33.

Druckenmiller, P. S. and Russel, A. P. 2008b. A phylogeny of Plesiosauria (Sauropterygia) and its bearing on the systematic status of Leptocleidus Andrews, 1922. Zootaxa, 1863, 120pp.

Sato, T. and Wu, X-C. 2008. A new Jurassic pliosaur from Melville Island, Canadian Arctic Archipelago. Canadian Journal of Earth Science, 45, 303-320.

Smith, A. S. 2008. Fossils explained 54: plesiosaurs. Geology Today. 24, (2), 71-75.

Smith, A.S. and Dyke, G.J. 2008. The skull of the giant predatory pliosaur Rhomaleosaurus cramptoni: implications for plesiosaur phylogenetics. Naturwissenschaften, 95, 975-980.

Zammit, M.; Daniels, C. B. and Kear, B. P. 2008. Elasmosaur (Reptilia: Sauropterygia) neck flexibility: Implications for feeding strategies. Comparative Biochemistry and Physiology, Part A 150, 124–130