One of the earliest vertebrate apex predators may not have been a giant after all.
Dunkleosteus terrelli 鈥� often portrayed as a 9-metre-long, armoured, shark-like predatory fish with bladed jaws instead of teeth 鈥� may have been a much smaller animal, with odd, chunky, shortened proportions.
The nearly dozen known species of Dunkleosteus are arthrodires, a variety of early fishes that reached their heyday in the Devonian period, more than 360 million years ago. They had rugged armour plating covering their heads and the front half of their bodies. D. terrelli was thought to be the largest arthrodire, exceeding the size of today鈥檚 largest predatory sharks and possibly wielding .
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But little study has been devoted to it, particularly in regards to just how big the knife-jawed beast truly was, says at Case Western Reserve University in Ohio. 鈥淰irtually all the previous estimates of Dunkleosteus were based on no data. They were all approximate guesstimates,鈥� he says.
To better calculate D. terrelli鈥檚 probable size, Engelman studied the relationship between head and total body length in various fish groups, both contemporary and long extinct. To his surprise, the relationship was remarkably universal across very distantly related fish groups, even in the extinct arthrodires. Head length could predict total length within 17.5 per cent, a boon for estimating arthrodire sizes, since the back half of the fishes鈥� cartilaginous skeletons lack armour and rarely fossilise.
Engelman applied this approach to D. terrelli and found that the largest individuals were no more than 4 metres long and roughly 1700 kilograms. The fish also had a squat, deep body akin to a tuna.
The body shape is closest to what we see in open-water fishes that are adapted for fast swimming, says Engelman. This contrasts with traditional depictions of arthrodires as slow and sluggish.
鈥淢arine vertebrates in the late Devonian may have already begun experimenting with the specialised body plans we consider normal in modern oceans,鈥� says Engelman.
The findings also suggest that Devonian vertebrates were much smaller than researchers thought, says Engelman. The shrunken Dunkleosteus and its relatives were still the giants of their time.
鈥淭his means that the Devonian oceans were devoid of creatures comparable to the largest marine megafauna alive today,鈥� says Engelman. 鈥淟arge body size didn鈥檛 evolve until much later than usually thought.鈥�
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