If you’ve ever flipped through a book of prehistoric creatures or ambled through a major museum’s fossil halls, you’ve probably seen a plesiosaur.
These were the four-flippered marine reptiles that patrolled the seas for almost the entire Mesozoic era, some 250 to 66 million years ago. Some plesiosaurs were big-headed apex predators. Others had ludicrously long necks and snatched up fish and crustaceans with their little jaws.
Now, Marshall University paleontologist F. Robin O’Keefe has discovered that some of them filled their bellies in a way thought to be impossible for the aquatic reptiles: filter feeding.
The findings, presented last month at the annual Society of Vertebrate Paleontology meeting in Dallas, centered on a plesiosaur that has puzzled paleontologists for over 25 years. Named Mortuneria, this plesiosaur was found in the 66-million-year-old rock of Seymour Island, Antarctica.
Along with a closely related animal called Artistonectes found in Chile, Mortuneria was informally called one of “the hoopy jaws” for its large, hoop-shaped mouth that made it stand out from other known plesiosaurs, O’Keefe says.
Paleontologists Sankar Chatterjee and Bryan Small, who initially described Mortuneria in 1989, mooted the idea that the marine reptile’s odd jaw and needle-like teeth were adaptations for trapping small prey. So O’Keefe went back to re-describe the odd animal and seek out alternate feeding strategies.
“I look at it, and I was baffled”, O’Keefe says. But after weeks of hard work, he found that Mortuneria possessed an array of anatomical features that meant it must have been filter feeding. First of all, the teeth of Mortuneria don’t interlock like they do in other plesiosaurs.
“The teeth of the upper jaw stick down, and the teeth of the bottom jaw stick out and down”, O’Keefe says. This arrangement makes “a good sieving battery.” More than that, the relatively slender jaws of Mortuneria would have contacted each other during a bite, indicating that there’s no force to their bite. If the plesiosaur tried to chomp struggling prey, O'Keefe expects its jaws would have shattered.
The evidence all suggests that the marine animals must have been doing something other than trying to nab struggling prey. Along with a deep palate, which would have helped push water out of the mouth to strain out tasty morsels, these features instead indicate that Mortuneria was targeting much smaller prey than most of its relatives.
O’Keefe envisions Mortuneria feeding in much the same way that gray whales do today. The plesiosaur probably dipped its head down to the bottom, O'Keefe speculates, “scooping up mud and sifting out all the critters.”
Now that he’s identified these filter-feeding features in Mortuneria, O’Keefe suspects that some earlier plesiosaurs may have pioneered the lifestyle. Kimmerosaurus from England and Tatenectes from Wyoming, both dating to the Late Jurassic, have a similar needle-toothed, big-mouthed look to them.
“Filter feeding in plesiosaurs, from the material that I know, evolved at the end of the Jurassic and went extinct, and at the end of the Cretaceous and went extinct,” O’Keefe says. It seems the plesiosaurs didn’t just inhabit the seas, they truly ruled them, from monstrous macropredators to mud-grubbing bottom feeders.