In this episode we discuss sharks and their evolutionary history. Sharks are well know today as the top predators in many of the world’s ocean environments, but this wasn’t always the case. We’re covering the features that define a sharks and taking a look at some of the most interesting cases along the 400 million year story of shark evolution.
Today there are two dominant groups of fish in the ocean; the Osteichthyes (bony fish) and Chondrichthyes (cartilaginous fish). As both names suggest, the distinction between the groups mainly has to do with what their skeletons are made from. Bony fish have skeletons of bone like our own, and these days they are the most common group of fish. Cartilaginous fish instead have a skeleton made almost completely out of cartilage, though their teeth are dentine. They include a wide variety of members, ranging from filter feeders to bottom dwellers to large predators.
Chondrichthyes are broken into two sub-classes, the Elasmobranchii and Holocephali. Holocephali contains the group known as the chimeras (also called ghost, rat, and spook fish). These are medium to small predators that swim slowly in deeper waters. Today they are fairly rare, though at one time they were quite dominant in the oceans. The Elasmobranchs include the more recognizable groups of Selachimorpha (sharks) and Batoidea (skates, rays, and sawfish). Among sharks there are 13 orders, nine extant and 4 extinct.
This is actually an extinct group of chimera that likely gave rise to modern shark ancestors. They were some of the dominant predators during the Devonian and Carboniferous. Many of their members appeared very “shark-like” and were notably weird.
History of Sharks
As you will often hear, shark have a long history. It start with the acanthodians, or spiny sharks. While not true sharks it was likely from this group that the first shark ancestors evolve. They were common from the Silurian to Permian.
The first shark-like fossil we see are actually only a scale from Silurian deposits in Siberia and a teeth from the Devonian. It is here in the Devonian, 419 to 358 million years ago, that we see the first ancestors to the elasmobranch lineage.
It’s not until the Jurassic that we see more modern features in sharks start to appear. More efficient tail fins and a more mobile set of jaws allowed sharks to take a more dominant role in the oceans. By the mid-Cretaceous, we finally have modern-looking sharks and many of the modern lineages swimming the oceans. After the Cretaceous sharks become the top predators in the ocean and in the Miocene the largest shark known evolves, C. megalodon.
Sharks today are still among the top predators in many ocean ecosystems, but they’ve started having to deal with a new apex predator. Human hunting and side-effects are causing shark numbers in many areas to plummet. Certain groups are being pushed dangerously close to extinction.
It would be a shame to lose such a long-lasting group now only due to misunderstanding and fear.