Egertonodus

Name Meaning:  Egerton Tooth

Geologic Era:  Permian though Cretaceous (this specimen from the Triassic era)

Location Found:  England

Estimated Range:  Worldwide

Size:  2 meters long

Extinction:  66 million years ago

This is part of a spine of a hybodontid shark, specifically Egertonodus.  This spine would have been anterior to the dorsal fin, giving the appearance of a spike coming slightly forward from the dorsal fin proper.  This was a very successful group of sharks, existing from the Permian to the end of the dinosaur era.  Fossils of hybodontid sharks are found worldwide, indicating a successful group of animals. 

The function of the spine was probably defensive.  At only about 2 meters long, Egertonodus was by no means the largest predator in its ecosystem.  Potential threats included marine crocodiles, mosasaurs, and other, larger sharks.  Having a spine sticking out of one’s back would at the very least have been something of a visual deterrent for predators, as few animals are willing to have a sharp spine rammed dangerously close to the brain.  Due to its relatively small size, the spine might not have stopped very large predators, though it would probably make some of the smaller ones think twice.  Spines aside, Egertonodus’ body plan was very similar to many modern sharks, a group of animals that has not changed greatly for two hundred million years.

Another interesting feature of Egertonodus is that it had two different kinds of teeth.  Some were sharp, intended for dealing with slippery prey such as fish.  Others toward the back of the mouth were more rounded, probably intended to deal with shelled organisms such as ammonites.  Such a generalist diet is advantageous, as Egertonodus would not be restricted to one type of prey, which in turn meant that it was not restricted in where it could live.  Hybodontid sharks such as Egertonodus went extinct very near the time the dinosaurs became extinct as well. 

Along with hybodontid sharks, the world also lost most of the marine reptiles and other organisms such as ammonites.  If Egertonodus was even partially reliant on ammonites or other shelled creatures for food, the loss of this prey would have been devastating for these animals.  Another possibility is that, in addition to ammonites, Egertonodus may have depended on other animals that went extinct around the same time, such as mosasaurs, particularly juveniles.  Even if Egertonodus had the teeth to catch slippery prey like fish, it’s body plan suggests that it was not a terribly fast swimmer.  In this case, even being a generalist predator was not enough to save Egertonodus .

Image Credits:
Skeleton:  By User:Haplochromis – Self-photographed, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2975860
Life Restoration:  By Nobu Tamura (http://spinops.blogspot.com) – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=19459716