Tuesday, June 27, 2006

Ichthyosaur wars and marvellous mixosaurs

Another significant hurdle: today I finished making the required changes to my thesis (should have dealt with it sooner, but you know how it is). So right now I’m feeling pretty fired up about Eotyrannus and Yaverlandia, and I really should work hard on getting the manuscripts done and submitted.

Other interesting things have been happening. My good friend Bernie Dempsey phoned me on Saturday. He’s been out filming Honey buzzards Pernis apivorus and looking for European nightjars Caprimulgus europaeus recently, but the most interesting thing is that he’s been discovering lots of stag beetles, all of them with their abdomens eaten out by some large predatory tetrapod. If this sounds familiar it’s because this formed the subject of a recent blog post (go here). We’re going to compare corpses to see if all the animals were killed the same way. The reason he was phoning was to see if I knew what was killing the beetles (I didn’t answer, but merely directed him to this blog).

I also caught up recently with my good artist/writer friend Steve White (website here). After talking about British big cats, the Sultan’s elephant and all manner of other things with him I felt especially keen to complete my post on the Cupar roe deer carcass (first mentioned in the British big cats post). It’ll follow soon. But it’s something that happened during the week (on Tuesday 20th) that’s most inspired me lately, and ironically it was a talk that I gave. How arrogant is that? The talk was titled ‘Ichthyosaurs: the Mesozoic ‘fish lizards’’.

Followers of my posts might have noted that I do quite a bit of talk-giving, usually to local natural history and geology groups. I don’t know if I do more public speaking than other academics, let alone Ph.D. students, but it sure feels like it sometimes. Then again, I don’t know any academics that are in the same situation as me (devoid of all personal finance and funding). Anyway, while this stuff is fresh in my mind I feel I may as well do a post on some of the highlights.

As I’m sure I’ve said before, in talks I like to cover things that are genuinely new to the majority of the audience. And for any group of tetrapods there are always more than enough new, exciting things to cover. So in talking about ichthyosaurs I covered the basics: stuff such as, while they hung on until as late as the early part of the Late Cretaceous, they should best be regarded as animals of the Triassic and Early Jurassic as this is when their diversity was at its peak. Stuff such as the hyperphalangy and polydactyly that evolved in the limbs of some lineages, the well-known story of how soft-tissue-bearing specimens were first discovered, and all that data from Holzmaden (and other places) on ichthyosaur birth and babies.

Exploding whales, breech babies and toxic shock

On birth and babies, I contend that not all females ‘preserved in the act of giving birth’ really were giving birth when they died. Instead these individuals may have died while pregnant, with decomposition gases later pushing unborn babies out of the cloaca. Exactly this occurs in the dead bodies of beached whales today: pregnant females may have babies protruding from the birth canal, and males often have a distended penis that, similarly, has been extruded from the body cavity by gases building up inside. Of course this leads us on to the subject of exploding whales, but we won’t go there for now. I have some nice anecdotes.

Some ichthyosaurs had breech births, as their babies are preserved protruding head-first. Here again we have an analogy with cetaceans. Baby whales and dolphins ordinarily emerge tail-first, and are thus only ‘triggered’ to take their first breath when the head emerges. But if the head emerges first, the baby drowns, and its little corpse is then lodged in the mother’s birth canal. The mother then becomes slowly poisoned as the baby decomposes wedged inside her, and she dies of toxic shock. It’s not nice, but it happens, and it’s a reasonable (albeit untestable!) speculation to think that breech-birth ichthyosaur mothers sometimes died the same way too. If I remember correctly this idea was first proposed by Deeming et al. (1996), and I’ve a feeling that Naish (1997) picked up on it.

Marvellous mixosaurs

Some of the neatest new data on ichthyosaurs comes from newly appreciated taxonomic diversity. Mixosaurs are a fairly well studied and long-known group of basal Triassic ichthyosaurs, best known for little Mixosaurus (total length c. 1.5 m) named in 1887 for specimens from Middle Triassic Europe. Mixosaurs have always been depicted as rather dull and conventional (above is Zdenek Burian's famous, but very dated, life restoration of Mixosaurus). But it now seems that at least some of them were bizarre. Really really bizarre.

Middle Triassic Europe, North America and Spitsbergen was home to the mixosaur Phalarodon, named by John Campbell Merriam in 1910. At the back of its jaws are massive, rounded crushing teeth (properly known as tribodont teeth): proportionally huge, and in fact proportionally among the biggest of any ichthyosaur. The teeth at the jaw tips were slender and subconical, so Phalarodon seems to have been a generalist, perhaps picking up small soft-bodied prey with the rostral teeth, and crushing big hard-shelled prey with the tribodont teeth further back. Incidentally, a huge percentage of Triassic marine reptiles had tribodont crushing teeth like Phalarodon, and it’s a good question as to why this was so common at the time, and so much rarer afterwards. I might cover this when I produce a post on placodonts.

What also makes Phalarodon interesting is the presence of a proportionally large sagittal crest on the back of its head. Strongly compressed laterally and projecting dorsally from the skull roof to a height similar to that of the cranium itself, it must have had an important function, but we aren’t too sure what that was. A site for muscle attachment is the most popular explanation.

Like Mixosaurus, Phalarodon wasn’t particularly big, with P. major from Germany getting to perhaps 3.5 m. But the best is yet to come. The weirdest mixosaur – and, in my opinion, the weirdest ichthyosaur – is the freakish Contectopalatus atavus. Only known from the Middle Triassic of Germany, it was a giant compared to other mixosaurs, with some incomplete specimens indicating complete lengths of 5 m. Its skull was slender-jawed and, while its many subconical teeth were blunt-tipped, it lacked the huge tribodont teeth of Phalarodon. It seems not to have gone around crushing molluscs or prey like that, therefore. It also has a sagittal crest, but it’s even more prominent than that of Phalarodon. Sticking from the top of the skull like a piece of card, the sagittal crest seems to have been flanked by shallow concavities on the skull roof. Again, all of this may have been for muscle attachment, but nobody’s really sure.

A big, mysterious and bizarre ichthyosaur, Contectopalatus was originally recognised as a new species in the 1850s, but not until 1998 did Michael Maisch and Andreas Matzke name it as a new genus (Maisch & Matzke 1998). For additional data on it, see Maisch & Matzke (2000a, b, 2001). Their reconstruction of its skull is shown above. It’s at this point that I should note that not all ichthyosaur experts agree that Phalarodon and Contectopalatus are truly distinct from boring little Mixosaurus. Ryosuke Motani has strongly disagreed with this classification, and argues that all three forms should be synonymised (Motani 1999). Indeed Motani and Maisch & Matzke differ in their opinions on so many matters of ichthyosaur taxonomy and phylogeny that we talk of the ‘Ichthyosaur wars’, though it’s not as if the workers involved would ever get physically aggressive with one another (I assume). Motani is a student of Chris McGowan, or ‘god’ as those in the ichthyosaur research community sometimes call him.

Whatever its taxonomic status, there’s no denying that Contectopalatus was unusual and interesting. This begs the question as to why it’s not better known: I have yet to see a single artistic restoration of it, for example. Back when the BBC were still deciding which animals they were going to include in the Sea Monsters series (fronted by Nigel Marven) they screened in 2003, I (via Dave Martill, one of their technical consultants) strongly recommended use of Contectopalatus. But they didn’t go with it. Shame. So there it sits, in the literature, unexploited and largely unknown.

At 5 m in length, Contectopalatus is reasonable in size for a Triassic ichthyosaur, but it’s not exceptional. The more derived cymbospondylids and shastasaurs grew to larger sizes and were also far more formidable, with their stout, keeled teeth and robust jaws indicating that they were macropredators that perhaps filled the role that pliosaurs and mosasaurs did later on in the Mesozoic. And it’s among shastasaurs that we find the biggest of all ichthyosaurs, and indeed the biggest of all marine reptiles. I was going to talk about them here, but now I can’t. I was also going to talk about the swordfish that speared Alvin the DSRV and about Excalibosaurus and Eurhinosaurus and about so much else, but it will have to wait to another time.

For the latest news on Tetrapod Zoology do go here.

Refs - -

Deeming, D. S., Halstead, L. B., Manabe, M. & Unwin, D. M. 1995. An ichthyosaur embryo from the Lower Lias (Jurassic: Hettangian) of Somerset, England, with comments on the reproductive biology of ichthyosaurs. In Sarjeant, W. A. S. (ed) Vertebrate Fossils and the Evolution of Scientific Concepts. Gordon and Breach Publishers, pp. 463-482.

Maisch, M. W. & Matzke, A. T. 1998. Observations on Triassic ichthyosaurs. Part III: A crested, predatory mixosaurid from the Middle Triassic of the Germanic Basin. Neues Jahrbuch fur Geologie und Palaontologie, Abhandlungen 209, 105-134.

- . & Matzke, A. T. 2000a. The Ichthyosauria. Stuttgarter Beiträge zur Naturkunde Serie B (Geologie und Paläontologie) 298, 1-159.

- . & Matzke, A. T. 2000b. The mixosaurid ichthyosaur Contectopalatus from the Middle Triassic of the German Basin. Lethaia 33, 71-74.

- . & Matzke, A. T. 2001. The cranial osteology of the Middle Triassic ichthyosaur Contectopalatus from Germany. Palaeontology 44, 1127-1156.

Motani, R. 1999. The skull and taxonomy of Mixosaurus (Ichthyopterygia). Journal of Paleontology 73, 917-928.

Naish, D. 1997. Aspects of Ichthyosaur Evolution and Ecology With Comments on Cross-Taxon Convergence Seen Throughout Marine Tetrapods. Research Project Report 1997/97, Department of Geology, University of Southampton, pp. 80.

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Blogger Unknown said...

very very interesting, Darren... it's been a while since I visited your blog (trying to finish off my own thesis, and current work is pressing too) - but always a very good read...

9:54 AM  
Blogger Unknown said...

How is that blogpost about Alvin vs. the swordfish coming along?
We having been waiting so long, oh so long!

10:33 AM  

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