All sharks are carnivores. From the sunny surface waters to the darkest depths, every selachian species lives by feeding on other animals. Of course, the great whites, tigers, and the ones that get lots of basic cable screen time – the macropredators – are the most famous, but the largest sharks of all feed on some of the smallest organisms in the ocean. These sharks are planktivores, and paleontologists have rediscovered two ancient sharks that pioneered a diet based on the very very tiny.
The Cretaceous sharks took a circuitous course to discovery. Back in 2007, Kenshu Shimada – a professor at DePaul University and research associate at Kansas’ Sternberg Museum of Natural History – described the plankton-feeding, megamouth shark Megachasma comanchensis from teeth found in Colorado. Other researchers disagreed with Shimada’s interpretation. The teeth were not those of a megamouth, they countered, but were the damaged and abraded teeth of an already-named, fish-eating shark.
Shortly after Shimada described another fossil megamouth, however, paleontologist Bruce Welton approached him with a curious shark tooth from the 100 million year old rock of Texas. “That fossil tooth was beautifully preserved with virtually no signs of damage, and yet, it was practically identical to Megachasma comanchensis teeth I described in 2007,” Shimada says. That was enough to send Shimada, Welton, and their coauthors back to have another look at the controversial Cretaceous megamouths.
It turned out that shark teeth from Russia went through a similar back-and-forth. Teeth named Eorhincodon casei and thought to be those of a filter-feeder were later reinterpreted as those of a slice-and-dice sort of shark. Yet, as Shimada and colleagues found, the teeth from Russia were extraordinarily similar to those from the United States and were from about the same geologic age. Despite their geographic range, all the teeth could be attributed to the same genus. Shimada and coauthors have therefore dubbed the shark Pseudomegachasma, the “false megamouth”.
“Because Pseudomegachasma is based solely on isolated teeth, its exact mode of lifestyle is inferential,” Shimada says, but he notes that ” The overall size and shape of Pseudomegachasma teeth are nearly identical to teeth of Megachasma“, the modern megamouth shark. Since the living megamouth has specialized teeth adapted to straining plankton from the water, it’s likely that Pseudomegachasma fed the same way.
Strangely, though, Shimada and his colleagues found that “today’s megamouth shark has no direct evolutionary link to the Cretaceous Pseudomegachasma.” The fossil sharks were more closely related to the snaggletoothed “sand tigers” than the modern megamouth. The two shark lineages are a new case of convergent evolution – fish-eating sharks gradually being adapted to have teeth better-suited to sieving little invertebrates and other morsels from the water column. And, at about 100 million years old, “Pseudomegachasma represents the oldest known plankton-feeding shark in the fossil record that evolved independent of the four known lineages of modern-day planktivorous cartilaginous fishes: the megamouth sharks, basking sharks, whale sharks, and manta rays,” Shimada says.
So why did all these fish make the switch? “Exactly what triggered the evolution of planktivory in each lineage is still uncertain,” Shimada says, “but the discovery of Pseudomegachasma does tell us that plankton were abundant enough to support the fossil shark in warm shallow oceans during the mid-Cretaceous.” Even if we can’t directly sample Cretaceous seas, the evolution of plankton predators are still indicators of how the oceans were changing. Or, in other words, looking to some of the marine realm’s ancient “gentle giants” may provide critical information about what was going on with some of the smallest. “Together with the recent recognition of some gigantic planktivorous bony fishes that also lived during the Mesozoic,” Shimada says, “I believe we have just barely begun to scratch the surface of the elusive plankton-feeding diet regime that existed in ancient marine ecosystems.”
Shimada, K., Popov, E., Siversson, M., Welton, B., Long, D. 2015. A new clade of putative plankton-feeding sharks from the Upper Cretaceous of Russia and the United States. Journal of Vertebrate Paleontology. doi: 10.1080/02724634.2015.981335