Sharks are paleontological paradoxes. They have an extensive fossil record going back 409 million years, yet, except in cases of exceptional preservation, little more than their teeth remain. They are everywhere yet are nearly invisible, their identity and appearance often contingent upon what we know about their living relatives.
But what do you do when you’ve got a shark tooth that doesn’t resemble the dentition of any species known to swim the modern seas? That’s the puzzle the late paleontologist Shelton Applegate faced while studying the different fossil shark teeth of Pyramid Hill. Among the teeth found at this 23 million year old site in the San Joaquin Valley were small, hooked teeth with little nubs of side cusps sitting on an A-shaped root. Teeth like this had never been found before, and, in a 1973 report, Applegate surmised that they belonged to a new species.
Despite their novelty, Applegate didn’t formally name the curious teeth. They entered the rank and file of known fossils without a name, even as they began to turn up at other sites of similar age in California and Oregon. Then a chance discovery of a mysterious modern shark started to bring the fossil fish into focus.
On November 15, 1976 the U.S. Navy ship AFB-14 hauled up a large shark that had been accidentally ensnared by the vessel’s sea anchor. This was a totally new species of blunt-headed, filter-feeding shark – officially named Megachasma pelagios in 1983. Fossil shark experts immediately took notice. The newly-found shark had teeth very much like those Applegate reported from Pyramid Hill a decade earlier. There was little doubt that those teeth, and similar specimens excavated elsewhere, represented an ancient megamouth shark.
Even then, though, the shark languished in obscurity. No one gave the fossil shark an official name. The shark was all but forgotten. Now that has finally changed.
In the latest issue of the Journal of Vertebrate Paleontology, DePaul University paleoichthyologist Kenshu Shimada and colleagues have named the shark Megachasma applegatei to honor its initial discoverer. The prehistoric shark’s teeth differ in some key ways from the modern species – such as shorter crowns and having a pair of side cusplets – but the dental remains are similar enough that the 23 million year old shark currently sits as the sister species to the living one.
Exactly what Megachasma applegatei looked like is difficult to tell beyond teeth. Modern megamouths have about 85 teeth in their upper jaws and 125 in their lower jaws, but whether the fossil species had a similar count is impossible to know on the basis of isolated teeth alone. The same problem obscures how large the shark actually was, but, using the relationship between tooth and body size in the modern species as a template, Shimada and colleagues estimate that most Megachasma applegatei would have been about 12 feet long, with the largest reaching an impressive 27 feet in length – about ten feet longer than the longest known specimen of the extant shark.
How the fossil shark made a living is just as enigmatic. While most similar to those in the maw of the modern megamouth, the prehistoric teeth also retain some characters reminiscent of sand tiger sharks. Instead of being a dedicated filter-feeder, Shimada and coauthors tentatively suggest, maybe Megachasma applegatei was snaffling up small fish as well as zooplankton. And even though the geological context of the teeth suggest that the shark slurped small prey at a variety of depths, it’s unclear whether the fossil megamouth was at home in disparate habitats or was making daily journeys through the water column like its modern relative. The present was the key to identifying this extinct shark, but the past still holds many selachian secrets.
Shimada, K., Welton, B., Long, D. 2014. A new fossil megamouth shark (Lamniformes, Megachasmidae) from the Oligocene-Miocene of the Western United States. Journal of Vertebrate Paleontology. 34, 2: 281-290. doi: 10.1080/02724634.2013.803975