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Sharks of Sedimental Value: Shark Fossils and Why They’re Hard to Solve - by Intern Shane White

If you’re a shark enthusiast, you probably know all sharks and rays are chondrichthyans, meaning their skeleton is composed of a soft tissue called cartilage. With sharks first evolving over 410 million years ago, it’s understandable that the soft skeletons of sharks would deteriorate much quicker than other animals, meaning it’s much harder to guess what they were like unless their skeletons were preserved in special sediment. So how are shark fossils found at all?

Although they have cartilaginous skeletons, the teeth of sharks are composed of dentin, a material tough enough to be preserved as a distinct, hardened fossil. Also, with sharks continuously losing their teeth and regrowing them throughout their life (up to 40,000 teeth in a lifetime), shark teeth are relatively abundant compared to the fossils of other animals. And so, scientists try to estimate the body plan and life history of an entire animal just from teeth, which can be very difficult and continuously leads to scientific discussions.

Photo: Chelle Blais

With shark species being exposed to different diets in different areas, their teeth can reveal what they likely feed on and how. Thus, scientists often estimate the evolutionary history of extinct sharks by comparing their fossilized teeth to the teeth of living species. However, many species evolve the same traits without being related, and so this method isn’t incredibly reliable. This would be like trying to solve a jigsaw puzzle where several pieces fit into the same spot, but only one piece matches the picture on the box.

A common example of a puzzle in shark paleontology is megalodon (Otodus megalodon), the largest shark ever discovered, growing up to around 18 metres (59 feet) long. Many people believed the megalodon was simply a larger ancestor of the great white shark (Carcharadon carcharius). When comparing the teeth of these two species side-by side, many would agree their size is the most notable difference and that the teeth shapes are fairly similar (Figure 1). However, through continuous studies on megalodon fossils, most recent work leads scientists to think megalodons are actually much more closely related to mako sharks, such as the shortfin mako (Isurus oxyrinchus), the fastest swimming shark alive today. Fossil records have shown that great whites and megalodon shared the oceans at the same time, and likely even competed over the same food sources. Therefore, the megalodon didn’t evolve into the white shark, but rather, they shared a common ancestor and evolved independently.

Figure 1: Side-by-side comparison of a tooth from the extinct megalodon shark (Otodus megalodon) on the left next to a tooth of the great white shark (Carcharadon carcharius) on the right. (Source: The Trustees of the Natural History Museum, London)

Although the case of the megalodon is the most popular example of a mysterious prehistoric chondrichtyan, some other very interesting examples are the falcatus sharks and the helicoprion. The falcatus sharks (Falcatus falcatus) were as little as a foot long, and lived around 325 million years ago. Luckily, some of their fossils were found in lime stone, a sediment allowing most of their soft skeleton to be preserved well. What’s so intriguing about falcatus sharks are the sword-like appendages found on the heads of the males (Figure 2), which many believe may have assisted in mating.

Figure 2: A female (top) and male (bottom) set of falcatus shark (Falcatus falcatus) fossils, preserved in limestone found in the Bear Gulch formation of Montana, USA. (Source: Richard Lund and Eileen D. Grogan, Smithsonian Ocean)

The helicoprion, living over 270 million years ago, is my personal favorite example. What makes this animal so interest ing is it’s “tooth whorl”, a spiral of teeth that has often been mistaken as the spirals of mollusk shells (Figure 3). Centuries of research and debate have led scientists to believe this buzzsaw-like jaw was used as the lower jaw of the animal (Figure 4), likely to help it eat octopus and other soft-bodied underwater prey. It wasn’t until 2013 that it was decided helicoprion wasn’t a shark at all, but rather a chimaera or “ratfish”, a branch of chondrichthyans separate from sharks and rays. This example really illustrates the creativity of chondrichthyan evolution, and also how hard it can be to accurately represent an animal when it leaves so little behind.

Figure 3: Several ammonite shells (left) compared to the fossil of an extinct helicoprion tooth whorl (right). (Sources: The Trustees of the Natural History Museum, London; OCEANA USA)

Figure 4: An artist’s rendition of how modern estimates describe the head of the extinct helicoprion. (Source: Ray Troll, National Geographic)

In conclusion, it can be very difficult to accurately understand how a shark lived and how it came to be, especially when you’re millions of years too late to see it in the wild. With sharks and their relatives surviving five massive extinction events, I think it’s incredible we can find remnants of them today and still somehow scientifically guess what they were like, even if we’re not right the first time around. New discoveries are always being made as technology continues to progress and as people continue to stay passionate in the worlds of fossils and sharks. I’m happy to be one of these passionate people, and I hope this article has grown your fascination in the expanding world of the fossils of sharks and their relatives.


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