Ancient Seas #5

Dunkleosteus

Dunkleosteus skull. Photo from Earth Touch

Kingdom: Animalia

Class: Placodermi

Conservation Status: Extinct (EX)

Found: Northern Hemisphere (based on fossil locations)

Era: Late Devonian

Lived: 358-382 million years ago

Diet: Carnivorous, most likely other armoured fish

Weight: 1 tonne*

Size: 6m (19.7ft)*

Habitat: Deep water

*Based on the largest of the ten known species.

Dunkleosteus were ancient, armoured fish which inhabited the Earth over 350 million years ago, first discovered in 1873. They belong to the Placoderm family, which derives its meaning from "plate" in Greek (plax-) and "derm" in reference to skin. They were fish with articulated armour plates around their heads and thorax. The ocean used to be full of these types of fish and the Dunkleosteus were among the biggest. Well, some of them were.

There are ten known species of Dunkleosteus, the largest being D.terrelli. This was the one tonne, 6m (19.7ft) beast with an excessively strong bite force. In comparison, the smallest was D.newberryi which only grew to 28cm (11in) long and had a much narrower snout. The remaining eight species were a range of different sizes with some rivaling D.terelli and others nearly as small as D.newberryi. All of them possessed the same armoured head filled with bony, plate teeth.

We know the Dunkleosteus was plated because plates are part of the fish which gets fossilised. This has led to some speculation as to what the back half looks like. Until recently, their tail end has been based on smaller arthrodire, another placoderm with thin tails. However, a recent and better preserved specimen was discovered suggests that placoderms were affected much more by regional variance. This has led to the current theory that Dunkleosteus has a much more shark-like figure, even down to the larger anterior lobe on its tail.

Their armoured exterior made Dunkleosteus a slow but powerful swimmer. Most of its power was in its jaws. The biggest of the Dunkleosteus had a bite force of up to 6,000N (612kg, 1,349lbs) at the tip of its jaw, and 7,400N (715kg, 1,664lbs) along the blades of its teeth. That is some power! (I'll have to get Dylan to tell me what that translates to in PSI.) This power was coupled with the fact it could open and close its jaws very quickly which has led scientists to believe their hunting style was much like that of a modern suction feeder. (Think Goblin Sharks with their extending mouth.) Their bite is insanely quick. It took them only 50-60 milliseconds to open and close their jaws, presumably around some unsuspecting prey.

So how do they get their beak-shaped mouth to move so quickly? Their jaw is formed in a four-bar linkage mechanism. Their skull, thoracis shield, lower jaw, and jaw muscles were all joined in a loop to create that deadly speed and power. Why did they need this powerful jaw? My guess from the research that I've done is because there were a lot of placoderms about in those days and there was only one way to eat them. To bite through the armour. 

Picture from Prehistoric Wildlife

Fun Facts!

• The name Dunkleosteus is derived from the Greek osteus (bone) and in honour of David Dunkle.
• He was the vertebrate paleontologist at the Cleveland Natural History Museum in 1956.
• The Dunkleosteus gained popularity thanks to the Walking With Dinosaurs series.
• Fossils have been found in North America, Morocco and Europe.
• Fossil records suggest that they would spit out the bones of their prey rather than digest them.
• They were among the first marine creatures to internalise egg fertilisation, like modern sharks.