But are they really older than trees? Experts told Live Science that the answer depends on what we classify as a shark.
The modern sharks we see today first emerged around 195 million years ago, during the Jurassic period. Fossils of ancient sharks from this time can be traced to living lineages, namely Hexanchiformes, an order of sharks identified by their six or seven gills and single dorsal fin.
For instance, the first true tree in the evolutionary record is considered to be Archaeopteris, an extinct genus discovered in a 385 million-year-old fossil forest near Cairo, New York. Looking at these fossils, Archaeopteris has many characteristics of trees we have today, including a tall, woody trunk; leaves; and a deep root system. Other early trees — such as Eospermatopteris found in ancient forests near Gilboa, New York, and Calamophyton in southwest England — also appeared in the fossil record 390 million and 393 million years ago, respectively.
A microscopic image of skin cells from the dogfish shark. (Image credit: Mohammed Al Ali via Getty Images)
"It’s called tessellated calcified cartilage, and that's a very important hallmark of most of the shark-like fishes and their ancestors," said Maisey, who has studied a spiny fish fossil called Doliodus problematicus from the early Devonian. If this definition is used, these sharks first appeared around 400 million years ago, before trees emerged.
These transition fossils, where fish were evolving into sharks and gaining shark-like traits, introduce a gray area in the definition of sharks. "When you get back into the ancient past, when you're not dealing with things that we have living examples to compare with anymore, then it becomes a bit more tricky [to figure out what they are]," Maisey said.
Related mysteriesFossils also present a very limited picture. Many contain only fragments of an animal, like a fallen tooth, so scientists are still piecing together the clues of exactly how and when sharks arose. That means every fish fossil discovery often leads to more questions about what other transition fossils could be out there, Maisey said.
"That's why I think the fossil record is so intriguing," Maisey said. "Because it may not give us all the answers, but it certainly raises questions."
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