One of the most infuriating things about being a paleontologist is being able to study some of the coolest organisms that have ever inhabited the Earth, yet never being able to see one in life. We’ll never know with complete surety what color they were, what they sounded like, and how they moved. Thankfully, new technology has allowed paleontologists to test hypotheses about these ancient animals, allowing us to get closer and closer to fully understanding the biota of a time long past.
One such group of organisms that has puzzled paleontologists for over 200 years are the plesiosaurs, a group of Mesozoic marine reptiles that first showed up in the Jurassic and persisted as apex predators until their demise in the Late Cretaceous. Paleontologists have debated for decades about how they swam, as no such living analogous animal has a body plan identical to these long-necked, four-flipper critters.
And as much as people have tried, no one seems to get a good video of the Loch Ness Monster in mid-swim! I just don’t understand why!*
*(Just kidding, Nessie isn’t real, folks. You heard it here first, from a real paleontologist, not an actor playing a paleontologist on [insert pseudoscience cable channel here])
Many modern tetrapods have evolved their own ways of swimming in the ocean: penguins, sea turtles, sea lions, and whales, for example. Previous studies have compared the possible swimming stroke and gait of plesiosaurs to the flight stroke seen in penguins and turtles, or even compared the movement to a rowing stroke like oars of a boat. And while all of these hypotheses are plausible, based on the musculature and anatomy of plesiosaurs, they still can’t fully explain how the four flippers of a plesiosaur moved in relation to each other. Did they move together in synchrony, or alternate between front and hind limbs. Likewise, did the propulsion come from the forelimbs or the hindlimbs? Or the steering? So many unanswered questions!
The closest thing to four-flipper swimming would assumably be sea turtles, but even then they are constrained by their shells and would unlikely move in a way similar to plesiosaurs. So the debate has continued, without an entirely satisfactory answer for most interested parties.
A new study published this week in PLOS Computational Biology by researchers at the Georgia Institute of Technology led by Greg Turk, in collaboration with paleontologist Adam Smith at Wollaston Hall, Nottingham Natural History Museum, has taken a stab at solving the decades-old question of how plesiosaurs swam.
This study uses computer simulation to build virtual three-dimensional models of a plesiosaur within a liquid, to provide a way to test different modes of swimming. They built a virtual life-sized plesiosaur model based on Meyerasaurus victor, a small (3.35 m) plesiosaur form the Lower Jurassic of Germany. Meyerasaurus presents a generalized morphotype for plesiosaurs, with a moderately long neck (as plesiosaurs have both long- and short-neck varieties). The skeleton was built using fossil data, and muscles, cartilage, skin, etc., were built upon it using evidence from other taxa.
Outlines of Meyerasaurus model showing the available ranges of motion in each simulation. (A) shows anteroposterior range, (B) shows transverse section of pectoral region showing the dorsoventral ranges of forelimbs, (C) shows degree of rotation available in all simulations, (D) shows a transverse section through pelvic region showing dorsoventral ranges of the hindlimb. The colors show the contrasting ranges of motion: green (narrow), pink (medium), and blue (wide). From Liu et al (2015)
The team had to address a seemingly endless multitude of variables and parameters, in order to assure that the model is moving under realistic and biologically plausible conditions. For example, each limb on a plesiosaur has one single mobile joint: the glenohumeral joint in the forelimbs, and the acetabulum-femoral joint in the hindlimbs. The team had to take into account the maximum and minimum allowable up/down and forward/backward movement of each fin before the joint would dislocate (which helps no one, really, especially the plesiosaur), as well as the possible amount of pronation/supination of each limb. Other factors had to be considered as well, such as how the neck and tail might move and influence the movement of the organism. In this instance, the team remained conservative and prevented movement in the model of the neck and tail, focusing more on the movement of the limbs.
The team ran multiple simulations under different conditions: all four flippers moving synchronously, flippers alternating, or each set moving while the other remains immobile, for example. They even conducted tests with different weights of the plesiosaur, in hopes of testing how that might influence the motion. In total, the team ran thousands of different simulations, each with changes to the variables and parameters, which are all outlined in the paper in PLOS Computational Biology.
The study found that the most effective motion for plesiosaur swimming is an underwater flight motion, not dissimilar to modern-day penguins.
“Our results show that the front limbs provide the powerhouse for plesiosaur propulsion, while the hindlimb are more passive,” said Smith.
Their study concludes that the rear flippers really played little role in the propulsion of the plesiosaur, but may have more likely been used for steering and stability.
The team will likely continue this study, with future computer simulations testing the degree of agility plesiosaurs gain from their rear flippers. The method can also be applied to understand the swimming motion of other prehistoric animals.
“Plesiosaur swimming has remained a mystery for almost 200 years, so it was exciting to see the plesiosaur come alive on the computer screen,” said Smith.
So while the very thing that all paleontologists want, to see their organisms “come alive,” may not be attainable through the outlandish methods seen in movies such as “Jurassic Park”, they just may very well be attainable through the digital world.
See a video of the Meyerasaurus model in motion here: https://www.plos.org/wp-content/uploads/2015/12/plesiosaur_medium_range.mp4
Read the paper in its entirety here: Liu S, Smith AS, Gu Y, Tan J, Liu CK, Turk G (2015) Computer Simulations Imply Forelimb-Dominate Underwater Flight in Plesiosaurs. PLoS Computat Biol 11(12): e1004605. dos:10.1371/journal.pcbi.1004605
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