Sunday, March 25, 2007

Right! The lizard-fish conversion. Very easy.

We have good clues about how fish evolved into land-dwelling animals, but how about those great aquatic reptiles of the Mesozoic, the ichthyosaurs, mososaurs and plesiosaurs? How did they evolve from landlubbers to seafarers?

When I saw a story a few days ago of a fossil primitive crocodilian, of the species Thalattosuchia, found in Oregon, I noted how much the artist's rendering looked like an ichthyosaur, despite the lack of a dorsal fin. This made me wonder: Did the ichthyosaurs evolve from primitive crocodilians? Evidently not, since the earliest thalattosuchian fossils found so far are from the Jurassic, while ichthyosaurs appeared as early as the Triassic, though they are believed to have evolved from more primitive diapsids that may also have been ancestral to crocodilians. That raised another question: Could crocodilians have evolved from ichthyosaurs, thus providing an example of evolution taking one group of creatures from land to sea, then back to a semi-terrestrial existence? Again, the answer seems to be no, as fossil crocodilians (but not thalattosuchians) have also been found in Triassic strata.

So, the superficial resemblance between thalattosuchians and ichthyosaurs (long beaks; vertical, fish-like tails) may be an example of convergent evolution.

Update: On the subject of Triassic crocs, Brooklyn Heights Blog colleague (if a newbie to the team may be so bold) qfwfq kindly gives me a link to a piece about an early croc, Effigia okeeffeae, "discovered" in a collection of unclassified fossils at the American Museum of Natural History. This critter ran on two legs and bore a strong resemblance to ornithomimid dinosaurs that didn't appear until the late Jurassic, about eighty million years after Effigia. Ornithomimids are themselves cited as examples of convergent evolution with ratite birds (for example, ostriches), so adding Effigia to the mix gives us an example of triple convergence.