Weird Fossil Sharks!

Some time ago, Darren Naish posted to the Shark mailing list on two new bizarre fossil species of shark. One just looked weird, but the other was truly weird; several anatomical features suggest it could have shocked you with an electrical field! I'll let Darren explain (with a few additions of mine in brackets):

I've just come from a wonderful lecture on fossil sharks and the lecturer, a British paleontologist, Dr. Chris Duffin, told us all about two incredibly bizarre new forms he is describing. Both are very elongate and possess unusual features. One is a neoselachian [a shark] with a long, u-shaped skull, I can't remember much more about it. The other is from the Jurassic Holzmaden locality and has the characteristic cranial vert shield-thing of a ray, the teeth of an orectolobiform [order Orectolobiformes: carpet sharks, e.g. nurse sharks, wobbegongs etc.] and a long, pointed rostrum [stiff cartilage that makes up the snout in sharks] with lateral bumps that look like they should support teeth but don't. Its body and tail are long and low with a single knife-fish-style fin running on the ventral surface: these features and the large blocky myotomes [muscle] *suggest* that it was capable of GENERATING AN ELECTRIC FIELD.

Electric ability is not unknown in elasmobranchs, as the torpedo and electric rays can create electric current, and all elasmobranchs, of course, have an extraordinary electrosensory system. The production of electricity is usually for one of three purposes: predation (to shock prey, as electric rays do), sensing the environment (as in mormyrid elephant nose fish and a few other teleosts that inhabit dark, murky tropical rivers), or social communication (including finding mates; again this has been found in mormyrid fish and some other tropical river fishes). If indeed this ancient orectolobiform possessed electric powers, we may be tempted to ask: why don't carpet sharks still have the ability to create an electric field? The answer to this question may lie in the fact that being electric involves many physiological specializations that may not be worth the ability. Modern orectolobiforms live in clear, tropical waters where sensing the environment is easily done with eyesight and electroreception that all elasmobranchs possess; likewise, they are successful predators of crustaceans and small fish, and don't appear to need any "help" in that regard. Thus, the ability to produce electric current may simply not be adaptively significant--in other words, it would cost more than its benefit to possess this ability.

Spiny-Headed Sharks

This possibly electric fossil sharks brings to mind the cases of two other fossil sharks that lived in the seas of the Carboniferous, around 300 million years ago. They had strange projections from their heads!

The first was Stethacanthus, a 1 m long shark (belonging to the extinct stethacanthid family) with a thick, bony spine projecting from its shoulder region, on the top of which resided a platform bearing rows of sharp tubercle-like teeth (overall, the set-up gave the appearance of a radar turret on top of the shark). There were also tubercle-like teeth on the top of the head, underneath the projection's platform. The presence of teeth led Rainer Zangerl, who discovered the animal in Montana, to suggest that the projection atop the shoulders was a method of scaring away predators; the large "radar turret", along with the "teeth", would have made for a striking resemblance to some larger fishes' gaping jaws! This would have been especially convincing when Stethacanthus was half-buried in the seafloor sand, with just its thick spine sticking out. Of course, this idea has never been confirmed and remains speculation.

The next shark in question, named Falcatus, is another stethacanthid and like Stethacanthus had a strange projection from its shoulder region--this time, a thick L-shaped spine that pointed forward like the barrel of a tank. This "barrel" also had "teeth" on it (rows of them), and the top of the head had the same sort of teeth, like Stethacanthus. However, in Falcatus this spine was only present in sexually mature males. This suggests that in Falcatus the bony spine was used as a pre-mating courtship display, much like deer and many other hooved mammals tussle with their antlers and horns. In fact, a fossil exists in which a female and a male Falcatus are locked to each other by way of the male's spine. However, in this specimen, the female was above the male, and the male was facing away from the female; evidently they needed some practice!