Friday, October 26, 2007

Xenacanthus: Permian Oddball

Xenacanthus was an oddball shark, even for a prehistoric species. It was shaped like an eel, with a nasty poisonous spine sticking straight out of the top of it's head. Since it lacked the well-known speed; large, fearsome teeth; and seemingly endless jaw capacity of modern sharks, it's comforting to know the poor guy had at least this one way to defend itself from the big, bad Dimetrodon, the Godzilla of the Permian world.

(c) Robert T. Bakker

Xenacanthus was found exclusively in freshwater environments - like the swampy shallows of North Texas 290 mya, but also all over the world, in places like Europe, Australia, South America and North America - such as the Museum's site near Seymour. It would have slithered through these ancient waters like an underwater snake, using a fin that ran along its back from head to tail.

This Permian oddball was part of an evolutionary line of sharks that first appeared in the Devonian, proliferated like mad through the Carboniferous and Lower Permian periods before taking a final bow towards the end of the Triassic. 220 million years in all - not bad.

Field sketch of Xenacanthus with Diplocaulus - possible prey? (c) Robert T. Bakker

By nature sharks don't make good fossils - most of their bodies are soft cartilage, which decays before the necessary processes can take place. However, Xenacanthus jaws and braincases were hard bone, meaning that we can recover more of this species that most other sharks. In Seymour, the team has found pieces of these, along with lots of Xenacanthus teeth and poison spines. Also in the area - shed teeth and coprolite from Dimetrodon, meaning this wasn't a site Xenacanthus came to relax.


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Tarrington and Matt said...

How could have the dimetradon fed on the Xenacanthus

Houston Museum of Natural Science said...

Hi Tarrington and Matt,

It seems like all the Xenacanthus would have to do is drag the Dimetrodon out to sea, right? Except that the water around here was very shallow at the time. So the Dimetrodon could see the Xenacanthus as it swam and pick a time to attack; there wasn't really any place for the Xenacanthus to escape. They also had such a slow metabolism that they only had to eat three or four sharks for the whole year.

Thanks for your question!


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