Many members of the prehistoric menagerie are familiar. Creatures like Triceratops, Dimetrodon, and Smilodon reach far beyond stacks of academic papers to become celebrities in their own right. But just because an ancient taxon is recognizable doesn’t mean that we know as much as we’d like about how such animals behaved. Plesiosaurs are a perfect example.
These marine reptiles have been confounding paleontologists since the time of their discovery. There’s no modern equivalent to look to for guidance, no quad-paddled, long-necked reptile to guide us as to how Elasmosaurus and its relatives moved and acted. What we know has been painstaking extracted from rock and bone to spur debates that have been going on for nearly two centuries.
How long-necked plesiosaurs used those stupendous appendages of theirs has been a source of persistent puzzlement. Classic restorations made them noodle-necked, playing off the old anatomical shorthand that plesiosaurs were basically snakes threaded through the bodies of turtles. More recently, however, paleontologists have come to know plesiosaur necks as relatively inflexible. The necks of these marine reptiles couldn’t tie themselves in knots, but were relatively restricted by the mobility between individual bones. A new study by researchers Ramon Nagesan, Donald Henderson, and Jason Anderson underscores this point.
Paleontologists have uncovered dozens of plesiosaur species from rocks spanning 135 million years of plesiosaur history, but one of the most beautiful is that of Nichollssaura borealis. This is a relatively new plesiosaur, found in Alberta’s oilsands in 1994 and named in 2009, and comes to us through Cretaceous time as an articulated and virtually complete skeleton. The snaking curve in the skeleton’s neck and the outstretched paddles gives the plesiosaur a resemblance to old 19th century depictions of these marine reptiles sunning themselves on ancient beaches.
Such a spectacular specimen offers plenty of opportunities to better understand plesiosaur anatomy and biology, of course, and in the case of the new study Nagesan and colleagues used 3D scans of the skeleton to get a better idea of neck flexibility in Nichollssaura. In short, the researchers made a virtual model to run the plesiosaur through its paces that’s impossible to do with fossilized bone without damaging or destroying the skeleton.
Naturally, there are caveats to studies like these. Soft tissues - muscles, tendons, and more - had their own roles to play in neck mobility, and what a prehistoric animal could do versus what it actually did are not necessarily the same. Nevertheless, Nagesan and colleagues found that Nichollssaura had the most flexibility in the side-to-side range of motion. Based on the bare bones, this plesiosaur could have angled its neck into a near complete loop. The plesiosaur was also able to angle its neck upwards and downwards, but not nearly with the same flexibility. This might indicate that side-to-side neck motions were more important to its behavior than movements in the vertical plane.
So what this does this mean for plesiosaurs? First, it tells us that not all the long-necked species were the same. Previous studies found some plesiosaurs that had similar side-to-side neck mobility and others that were more flexible in the up-and-down range. This could be related to feeding. Some plesiosaurs have been found with crustaceans and other foods likely scraped from the bottom - perhaps from more vertically-oriented species - while plesiosaurs like Nichollssaura might have been better at nabbing fish and squid from the water column. Matching neck mobility to gut contents - if such lucky fossils could be found - would act as a check on the idea. Nichollssaura is only one expression of a group that thrived for well over a hundred million years. Surely there was more than one way to be a plesiosaur.