Thursday, 31 December 2009

Ophiacodon, a close relative to the Gorgons: Just what was it getting up to in the Permian swamps?

Ophiacodon was a Pelycosaurian mammal like reptile, a more primitive relative of the Gorgonopsids, living about 290 million years ago. It has often been portrayed as an aquatic organism, sculling through lakes and rivers, swallowing up fish and amphibians.

The shape of the skull is perplexing for a river animal. Modern crocodiles have flat heads to reduce resistance when the head is swung sideways in an attack, but Ophiacodon has the opposite condition; with a deep narrow snout. This created extraordinairy drag when the head of Ophicodon was swung sideways in an underwater attack.

Another anatomical feature that defies an aquatic existence is the tail. The tail is relatively short, thin and narrow. In all modern swimming reptiles the tail is deep and powerful, so that it can propel these animals efficiently through the water. Ophiacodon was clearly incapable of doing this with such a meak tail, and this was clearly not a swimming organ.

Even more confusing is the contradicting array of aquatic specialisations. The wrist and ankle joints are weak, inhibiting terrestrial locomotion, the head massive making it awkward and ungainly on land, with eyes placed on the top to allow Ophiacodon to see above water whilst keeping the rest of its head submerged.

The teeth are many; the jaws may contain up to 166 teeth. These teeth are however relatively small for a fish eating animal, unable to stab through slippery mucous covered amphibians, these were useless for catching large prey.

It seems likely that Ophiacodon was aquatic after all. With such weak ankle joints it was hardly able to move freely on land. I hypothesise that this animal was living in very shallow water, possibly only 1 foot deep. This was enough to support its weight without putting strain on its ankles. The curved lower jaws could have been used to spoon up small fish and amphibians burrowing in the sediment or hiding in the weeds. The teeth would only have allowed small animals to be consumed and the narrow, but tall head suggests rather than sideways movements to catch prey, Ophiacodon was dipping its head vertically into the shallow water to scoop out prey.

An explanation for the small tail: In such shallow water the tail would have been useless to get around. Although 1 foot deep water could help relieve the weight of that massive head it was unable to completely submerge the animal, preventing it from swimming away. Instead Ophiacodon slowly shunted itself about the shallows with its weak legs ,the thin tail dragging uselessly behind. Skin traces show that this animal had no scales on its skin, possibly to prevent drag when moving through the water, however there are bony scutes on the underbelley, suggesting that it was often dragged along the beds of shallow rivers and needed to be protected.

The eyes are placed on top of the deep head, allowing Ophiacodon to keep its eyes above the water even when feeding on the shallow riverbed. This permitted it to watch out for predators that may be stalking it from the riverbank. In water only 30 cm deep a 3 meter long reptile is completely visible and vulnerable. By spotting predators early on Ophiacodon could retreat into deeper water.

The nostrils open low down on the snout close to the tips of the jaws. Perhaps a blocking mechanism allowed the reptile to smell under water without drowning. The eyes were stationed above the water and may have been an unreliable means of finding prey. Ophiacodon may have smelt out the tunnels of burrowing amphibians, running its low set nostrils along the riverbed, then travelling upstream to the burrow and scooping out the inhabitant.

Ridges above the eyes prevented the sun from interfering with its vision. Perhaps the deep head had the use of creating an area of shade below the eyes, preventing reflections that may have otherwise distorted the movements of its prey.

I hope this is a convincing enough reconstruction of Ophiacodon's unusual lifestyle based on as much evedence as I could find. Next time I will be exploring more relatives of the Gorgons, possibly the poisonous Therocephalians.

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