Many people, usually un-informed talking heads that appear on populist nature documentaries who want to make claims that will grab your attention, will say that sharks have remained unchanged since the time of the dinosaurs. It’s wrong. The Mesozoic Era, the age of the dinosaurs, was a time of great transition for sharks. Sharks had existed on Earth for millions of years before the first dinosaurs appeared, ever since the Devonian Period when creatures like Cladoselache swam in the oceans around 370 million years ago. However, these were very primitive sharks that bore only a slight resemblance to most of the sharks that are found in the oceans today. The closest visual comparisons that we have for many prehistoric species are those that are found in very deep water, even though these sharks are still thoroughly modern in their genetics and evolutionary history. Sharks that are described as “modern” by biologists and paleontologists appeared towards the end of the Mesozoic Era during the late Cretaceous Period. Examples of prehistoric “modern sharks” are Cretoxyrhina and Squalicorax, both of which look like many shark species that are alive today.
During the Mesozoic Era, new shark forms emerged that could be described as transitional, a sort of in-between stage between the primitive sharks of the Paleozoic Era and the modern sharks of the very late Mesozoic. The most recognizable of these transitional species were a group of sharks called the “hybodonts”. Part of the reason why the hybodonts are regarded by many as THE shark group of the dinosaur age is because they lasted for such a long time. The hybodonts first emerged during the Carboniferous Period and they stuck around until the very end of the Cretaceous – that’s a LONG time! The hybodonts, therefore, existed throughout the entire duration of the Mesozoic Era. No wonder that they are considered the archetypal Mesozoic shark. Another reason for their status as the sharks that most people think of as dinosaur-age sharks is that they were widespread. Hybodont sharks existed all over the world during the Mesozoic, and they appeared to have existed in every aquatic niche: freshwater, brackish, and saltwater. Perhaps, like modern-day Bull Sharks, they had the ability to migrate in and out of water with different salinity levels without suffering adverse effects.
The most well-known of all of the hybodont sharks is its eponymous member Hybodus, a genus composed of several species which survived and thrived during the dinosaur age. It measured 6 feet long, and it occupied marine habitats around the world, although it is especially known from fossils found in Europe. Below is a drawing that I made of it based upon numerous fossils and scientific articles that I found. I noticed that the profile of the creature looked remarkably similar to a modern-day Blunt-Nosed Six-Gill Shark (Hexanchus griseus), with its rounded blunt nose and the characteristic humped back. Hybodus’ pectoral fins were surprisingly small and convex along the posterior edge, looking similar to the pectoral fins on numerous species of bottom-dwelling sharks. I get the impression that Hybodus was somewhat lethargic and spent much of its time cruising near the sea bottom, but that’s just my guess. The drawing was made with No. 2 pencil.
Keep your pencils sharp.