|The Early Days of a Better Nation|
Sunday, May 22, 2005
The Scientist's Apprentice
The Jurassic marine crocodile Metriorhynchus was a lithe and elegant beast. We know a fair bit about it, including that it sometimes suffered from arthritis. You can see the fossilised femora, one of which has a rough knob at the end, where there should be a smooth one. I've held these stone bones, or a pair very like them, and grimaced at that ancient agony myself. For a few months in 1976 I knew almost all there is to know about Metriorhynchus; I had read the textbook references, looked up the articles on which they were based, and looked at many of the specimens on which the articles were based; I saw the original display-drawer of laid-out bones of the beast's hind foot in the very arrangement that I'd seen drawn in a dozen places. And I found, or thought I found, that the standard drawing was wrong, and with it much that we think we know about the Jurassic marine crocodile.
The question I was trying to answer, for a final year Zoology undergraduate dissertation, was this. Crocodiles spend most of their time in water but can walk, indeed run, on land. Turtles have a laborious trek up the beach. Most marine reptiles couldn't manage even that. A Mososaur or an Ichthyosaur is as marine-adapted as a dolphin. And like a dolphin, they gave birth to live young. They still laid eggs, but the eggs hatched inside. One famous fossil is of an ichthyosaur at moment of giving birth.
Now, your Jurassic marine crocodile, right, is sort of betwixt and between. Its legs look like flimsy flippers, very unlike the sturdy hind leg of a modern croc. On the other hand, or leg, their foot bones aren't the almost undifferentiated platter of tarsals that you see in the old ichthyosaur. You can tell them apart and fit them together. They articulate, but (you might think) a bit pointlessly, because the whole palm or foot was completely flat. And, and, there is no evidence at all that Metriorhynchus laid its eggs anywhere but on land. But looking at that floppy foot, you fancy Metriorhychus mums-to-be had a hard time of it up on the mud-flat.
So, with the telling vagueness that's the dead give-away of a bad extinction story, a just-was story you might say, this amphibious condition was hand-waved to as their fatal flaw. Perhaps because my tutor had a doubt about this, and certainly because the Hunterian Museum contained a good few specimens, I chose to investigate just how far the marine adaptations of Metriorhynchus had actually gone. In particular, I looked at whether there might be more to the articulation of the foot than met the eye.
The Hunterian Museum is quiet, with the sort of hum that might be an aural hallucination. The smell is of locusts and wild honey, like John the Baptist's menu. The windows are like in a church. There is armour and parchment. There are vases and mummies. Every length and lath of wood is polished to a force-field sheen. Around the hall are galleries where minerals and fossils lie under sloping glass. And under these displays are drawers that glide out, in memory, as if on wheels. They are full of detritus and shards labelled in india ink and held together with varnish and Sellotape.
In a corner of one of these galleries I had a table and a chair, and on that table I laid out bones taken from the drawers, and looked at them and puzzled over them, and doodled them, and fiddled with suspending them from bits of thread, and read all about Metriorhynchus when I wasn't skiving off and reading about something more exciting, like the Portuguese Revolution or The Outcasts of Foolgarah (by Frank Hardy. It's a great book.) I took more than one girlfriend to see that table. Come up and see my fossils. It wasn't much, but it hardened them for the experience of seeing my bedsit. (Mouse footprints in the frying pan lard. Trace fossils! No, they weren't impressed either.)
Anyway, I checked all the specimens I could find, including in the basement of the Natural History Museum where they keep the stuff not on public view: the dragon's egg, the Woking Martian, the Piltdown skull; and, more excitingly, the above mentioned bones of the arthritic crocodile and the original reconstruction of the hind foot, in a little tray lined with indented baize. I drew it and made notes. All the bones were flat, and the foot was a flat paddle.
Then, back at the Hunterian, I started pulling out the drawers and rummaging through the bits. Ribs mostly, teeth, bits of jaw. In among all the rubbish I found a calcaneus - a heel-bone. It wasn't flat, like every other Metriorhynchus calcaneus. It was the same shape as the calcaneus of a modern crocodile. I think I may have found an astragalus as well - the next bone down - but that doesn't matter, because ...
The heel-bone is connected to the foot bone, and these bones lived. Because they weren't flattened, you could see the planes where they articulated, like facets. And when I looked again at the other bones, I could see that they were all flatter than they should have been, and they all had lots of tiny cracks, just as if ... just as if ... they'd all been crushed under tons and tons of mud.
The Jurassic marine crocodile hadn't had a flat foot after all. It's just that the bones of the standard specimens had all been flattened.
So, with black thread, black cards, and Blu-Tac, I and the Museum supervisor (who was keen, and helpful, and a fine photographer) I put together a new reconstruction of the hind foot and photographed it. The new view of the foot was of a proper foot, not a paddle. It was a foot that could push, not just flap. And in the nick of time I typed the whole thing up and got it to the office on the dot of five on the final day. And my dissertation passed, and was filed in the vaults of the Zoology Department, where it probably remains.
Every picture of Metriorhynchus is still wrong.