The Shark Family Line Is Older Than Trees, Modern Sharks Are Not

Sharks are older than trees. That is the claim, and the shark experts asked about it say it holds up, as long as "shark" means the whole family line and not just the animals recognised today.

The first trees were late arrivals

Trees are not an ancient feature of the planet. Cindy Looy, a paleobotanist at the University of California, Berkeley, told Live Science that the timing of the first trees is not very controversial: they evolved in the Middle Devonian epoch, roughly 380 million to 390 million years ago. A research paper in the journal Nature places the origin of trees within the mid-Devonian, a window it gives as 398 million to 385 million years ago, so the exact edges of that period are still being argued over.

What the fossils show is more concrete. The first true tree in the record is usually taken to be Archaeopteris, found in a 385-million-year-old fossil forest near Cairo, New York. It had the parts expected of a tree: a tall woody trunk, leaves and a deep root system. Other early trees appear even earlier. Eospermatopteris, from the ancient forest at Gilboa, New York, dates to about 390 million years ago, and Calamophyton, found in southwest England, to about 393 million years ago.

Modern sharks came much later

If "shark" means the sharks alive today, the picture flips. The modern sharks seen now first emerged around 195 million years ago, during the Jurassic period, long after forests had spread across the land. Fossils from that time can be traced to living lineages, including Hexanchiformes, an order of sharks recognised by six or seven gills and a single dorsal fin. On that definition, sharks are not older than trees at all.

The extinct relatives that settle it

The older claim rests on sharks' extinct relatives. John Maisey, the curator-in-charge emeritus of fossil fish at the American Museum of Natural History in New York, said the answer is all in the fine print, and that the shark lineage within the fishes generally does go back further than trees. Fossils from the sister groups of modern sharks reach back through the Permian period and into the Devonian, a stretch of time from about 359 million to 420 million years ago.

The most abundant of these were the hybodonts, an extinct group of spiny fish whose cartilage, the flexible material that makes up a shark's skeleton, hardened in a pattern similar to that of modern sharks. Maisey described that pattern, called tessellated calcified cartilage, as an important hallmark. It is the kind of detail that decides whether a fossil counts as a shark, and therefore whether the shark line is older than the first tree.