When did the first sharks appear on Earth?

Our universe is thought to be about 15 billion years old while the Earth is generally considered to have been around for the last four or five billion years. The first sharks appeared on Earth about 450 million years ago and are represented by only a few fossil scales dating back to the Ordovician period some 455 million years BP; cartilaginous skeletons rarely survive the traumas of fossilization. The oldest fossil shark teeth date back some 400 million years and belong to a shark called Leonodus, although we know little about how this shark looked. Many prehistoric sharks were adorned with weird and wonderful spines and armour, which many palaeontologists consider served as a defensive mechanism.

Even today, nobody is sure from what modern sharks evolved. Some suggest that they are derived from a 2m (6ft) bullhead-like shark that lived during the early Jurassic, about 180 million years ago. The late, great shark biologist Aidan Martin wrote an informative article discussing possible modern shark ancestors on the Origin of Modern Sharks page at his ReefQuest site. Whatever the ancestor of modern sharks was, the sharks we see today (sometimes referred to as neoselachians) were a feature of our oceans by the mid-Cretaceous, about 100 million years ago. Modern day sharks survived the mass extinction at the end of the Cretaceous, about 65 million years ago, superseding the dinosaurs.

Approximately 200 million years after the first sharks swam the oceans, rays arrived. Conventional wisdom had, until recently, suggested that skates and rays (collectively termed batoids) originated from sharks that gradually became adapted to living on the seabed – an idea referred to as the hypnosqualean hypothesis. Research by a team led by Christophe Douday at the Dalhousie University in Canada, however, found molecular evidence to refute the idea that batoids are derived from sharks. In their 2003 paper to the Molecular Phylogenetics and Evolution, the geneticists presented data supporting the concept of shark and batoid monophyly and apparently rejecting the hypnosqualean hypothesis. Instead, it appears that the seven or so putative synapomorpies used to define this Hypnosqualean superorder are either sympleisiomorphic, or a consequence of convergent evolution. In other words, the skates and rays don't share a recent common ancestor with the sharks and the features that suggest they're closely related were either inherited from a more distant common ancestor (older than the most recent - i.e. sympleisiomorphic) or evolved independently in the batoids because it’s best suited to the environment in which they live (i.e. convergence).