Wednesday, August 11, 2010

A Mosasaur's Tail

If you're ever on the game show Jeopardy and your Daily Double requires you to identify an anguilliform swimmer, you would do well to answer, "What is an eel, Alex?". Eels, with their broad, ribbon-like tails, get around by undulating most of their body. During the Mesozoic, a very different group of animals navigated the seas in a similar fashion: many of the aquatic lizards called mosasaurs.

A new study of a remarkably complete specimen of the mosasaur Platecarpus tympaniticus has found that it employed a different form of locomotion. An international team of researchers led by Johan Lindgren of Lund University has published a paper at PLoS ONE about the specimen called LACM 128319, which preserves much of its skeleton, scale impressions, and even traces of its internal organs and retina.

By comparing the measurements of its caudal vertebrae, Lindgren et al. determined that the tail of Platecarpus was hypocercal, meaning that it was divided in two lobes with a longer lower half. Platecarpus' tail identifies it as a carangiform swimmer, meaning that much less of its body would have flexed to provide propulsion. This has important implications for the kind of hunter it would have been: anguilliform swimmers like eels are very flexible, but it comes at the expense of speed. The fastest swimmers, like tuna, employ what's called thunniform locomotion: their bodies are very stiff, with only their big, crescent-shaped tail fins moving to provide propulsion. Platecarpus existed in the middle of this spectrum, so it was probably a relatively fast swimmer, the better to catch the fish it fed on. Also adding to its speed, its scales were smaller than the most primitive mosasaurs, a feature which stiffened its body and decreased drag.


New restoration of Platecarpus by Dimitri Bodanov, showing hypocercal tail fin. From wikimedia commons.

The member of the mosasaur family that exhibits the most refined traits for living in the water, the 40 foot giant Plotosaurus, was also a carangiform swimmer. Its scales, also known from fossil impressions, were even smaller than those of Platecarpus. Living in the middle of the mosasaurs' span on Earth and tens of millions of years before Plotosaurus, Platecarpus can be considered yet another "transitional" form to tuck in your back pocket for those times when you're called upon to defend the honor of evolution.

Without applying the outdated "ladder of progress" model to evolution, it is fair to say that evolutionary lines can have a progressive "arc" as time goes on. It makes sense that when competition for resources is stiff, bodies respond by adapting for greater efficiency. Using what they observed in Platecarpus, Lindberg et al. compared mosasaurs to other lines of secondarily aquatic animals, including cetacean mammals, marine crocodiles, and icthyosaurs. They found that the basic streamlined body plan, flipper-like limbs, and carangiform locomotion evolved consistently within the first ten million years after these groups' first appearance in the fossil record. Living in water places unique constraints on vertebrate's bodies, and it makes a lot of sense that similar body plans would evolve independently in different animals. There's just no better way to make a living as a big marine predator

If you're interested in the mesozoic sea monsters, one of the most comprehensive resources is Mike Everhart's Oceans of Kansas website. It includes an 1899 description of a Platecarpus fossil written by Charles Sternberg, a collector who got his start working for Edward Drinker Cope.

One last thing: If Sue, Leonardo, and Lyuba deserve cute nicknames, LACM 128319 is equally deserving. But I haven't come across one. Any brilliant ideas?

2 comments:

  1. Pete. Pete the Platecarpus.

    I'd be remiss in failing to mention that only one group of tetrapods were thunniform swimmers: derived ichthyosaurs. What interesting to me is that mosasaurs and ichthyosaurs never got rid of their hindlimbs, even though they were probably of little importance to swimming, especially in ichthyosaurs, yet cetaceans chucked 'em pretty quickly.

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  2. Zach, I'd be remiss in failing to mention that some lizards and snakes use their back legs (or vestigial spurs in the case of snakes) to stimulate the female to mate with them, while mammals tend to do lots of other mushy stuff... I wonder if the retention of the back legs in Mesozoic aquatic reptiles was because of some reproductive function...?

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