fossils

Plesiosaurs and their absurdly long necks

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Tristan
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Guest Writer
Since their discovery and subsequent naming almost 200 years ago plesiosaurs have captured the imagination of people around the world with their dramatic appearance and incredible anatomy. An extremely successful group, plesiosaurs ruled the Mesozoic oceans for millions of years, exploring a wide range of habitats and niches. Despite their iconic nature we still lack a solid understanding about many aspects of their lifestyle. At the center of this discussion is the lingering question of what exactly they used their necks for.

It was the Spring of 1867, 23 kilometers northeast of Fort Wallace, Kansas, where American army surgeon Theophilus Turner made one of the most important fossil discoveries of the decade. While he was only able to recover a few vertebrae, he recognized that they must come from some kind of extinct animal, and it was not long before the fossils found themselves in the possession of up-and-coming American Paleontologist Edward Drinker Cope. Cope went on to contact Turner and request his help in order to obtain the rest of the specimen, and in March 1868 Cope gave the creature its name: Elasmosaurus platyurus.


Cope’s publishing of Elasmosaurus is infamous among those familiar with his participation in the Bone Wars, a time of prolific species naming in paleontology between Cope and his rival Othniel Charles Marsh. This is because Cope’s original reconstruction of Elasmosaurus was restored backward, with the head on the end of what was actually its tail. This obvious anatomical error was a source of great embarrassment for Cope, and he was quick to react by recalling as many copies of the article as he could, a mistake that Marsh never let him forget. Despite this early hiccup, the animal that Cope described has remained interesting to scientists.


Elasmosaurus was a plesiosaur, part of an extraordinarily diverse and successful group of marine reptiles that ruled the Earth’s oceans for 160 million years. They lived in a wide range of environments from near-shore estuaries to the open ocean, and some even lived in freshwater habitats. Plesiosaurs came in all shapes and sizes, ranging from as big as a whale to smaller than your average porpoise, and had equally diverse diets. Their iconic nature has even netted them roles in popular media, including Jules Verne’s Journey to the Center of the Earth and Cooper and Schoedsack’s King Kong.


Plesiosaurs, known scientifically as the Plesiosauria, are separated into two morphological groups based on body form, the pliosauromorphs and the plesiosauromorphs. Pliosauromorphs are characterized by their short necks and big heads, while plesiosauromorphs are famous for their tiny heads and incredibly long necks. Cope’s Elasmosaurus belongs to this latter group, and when reconstructed correctly had a neck almost 7 meters (22 feet) long. Other closely related elasmosaurs, like Styxosaurus from the Dakotas and Albertonectes from Alberta had similarly dramatic neck lengths. Albertonectes in particular has 72 vertebrae in its 7-meter long neck, making it the owner of the most neck vertebrae of any known animal.


But why have such a tremendously long neck? In Cope’s defense, his original version of Elamosaurus with a short neck and long tail seems to be the more plausible animal. The short neck of his backwards elasmosaur would reduce drag as the animal propels itself through the water, while the long tail could be used for propulsion via undulations. This kind of body plan is seen in many living and extinct reptiles from crocodiles to sharks, and even other extinct marine reptiles like mosasaurs. This body form seems to work very well given its prevalence, so what would cause a group of aquatic reptiles to evolve a body that is the exact opposite?


Scientists have debated the function of plesiosaur necks for decades, alongside a myriad of other quirks in their anatomy. For a long time, paleontologists assumed that plesiosaurs used their necks like the heads of snakes, snapping up prey with quick, rapid strikes of the head and neck. However, this view became complicated as people studied plesiosaur necks more closely. The consensus is now that many long-necked plesiosaurs actually had quite stiff necks with limited motion.


The vertebrae of most plesiosaurs are big and bulky, with interlocking projections that keep the neck held fairly straight. This is quite unlike most reconstructions which portray them with snake-like flexible necks that dart through the water, something that populated early art. The neck vertebrae also show signs of being covered by extensive muscles which stiffened and strengthened the neck and would have made it very heavy. While the necks were probably not stiff to the point of being rod-like, plesiosaurs probably required all of these stiffening structures in order to swim properly. This is because any flexibility in the neck would cause plesiosaurs to tumble while moving quickly, as viscous drag would try to push the neck to either side of the animal.


Biomechanical models show that plesiosaurs had more problems to overcome. Long necks slow animals down in aquatic habitats, as the neck creates an extra surface that needs to be pushed through the water, and the pressure of the water itself is always trying to push the neck to one side as the animal moves. To make things worse, the extra long neck of elasmosaurs would also make them slower to turn due to inertia and make them less stable overall. Add to these disadvantages other major biological issues to overcome, such as breathing and swallowing down a massive neck and pumping blood from the heart to the head, and you have a very difficult adaptation to manage, let alone wield successfully.


With all these challenges, one must wonder what would make such a long neck worth it? While long necks have many drawbacks when it comes to maneuverability in water, they make up for it in other ways. For example, some research into diving birds like gannets suggests that their long necks actually help the birds act like a spear in the water, slicing through waves while traveling at high speeds. While long necks can cause a lot of drag at low speeds and during turns, they also help modern birds reduce drag on the body at high speed and when traveling in a line. It is possible that plesiosaurs could use their necks in a similar way.


Many long-necked plesiosaurs also evolved lengthened wing-like flippers, which acted much like the wings of soaring birds. This would allow for continuous underwater movement over long distances. Some researchers have also suggested that the long necks of elasmosaurs might have been used to generate lift in a similar way. Their long necks have a slight bend around the middle of their length which could be used like the curved wing of a plane. This would further help with long-distance cruising.


Put all this together and we can infer that the longest-necked plesiosaurs were probably doing lots of relatively straight, long-distance swimming in open water. This would make them a lot like today’s long-flippered migrating whales, such as humpbacks. This is further supported by fossils, as many of the longest-necked plesiosaurs seemed to prefer open water environments, while shorter-necked and shorter-flippered plesiosaur remains are often found closer to shore.


So now that we have a general idea about the anatomy and biology of some of these long-necked plesiosaurs, how did they use their incredible necks? A popular hypothesis is that the neck was used in some sort of stealth predation, keeping the small and unassuming head away from the massive body. Keeping the head and body so far apart would allow plesiosaurs to sneak up on shoals of fish or squid without alerting them to their presence. Some biomechanical models have even suggested that the shape of the neck might minimize turbulence in the water, which would make it difficult for fish to sense lurking plesiosaur predators.


Another hypothesis is that, like long-necked dinosaurs, the necks of many plesiosaurs could have been used to increase feeding range. A slow-moving or resting plesiosaur could scan its neck across a large area of water and scoop up any small animals that it finds. This way the neck could save energy by minimizing movement, allowing plesiosaurs to act like predatory oceanic “grazers.” The neck could even be used to extend into areas out-of-reach of other large marine predators, like inside tight crevices or along narrow reefs.


The extreme necks of many elasmosaurs also brings to mind the incredible display structures of many living and extinct creatures. Current research suggests that the horns, frills, spikes, plates, and sails of many extinct reptiles were likely used as visual display or in sex-driven competitions. Perhaps just like how giraffes and elephant seals use their necks to compete with each other, elasmosaurs could use their powerful and heavily-muscled necks in competitions with others of their species. Imagine the twisting and turning of twenty-foot necks in a deadly underwater ballet.


But some plesiosaurs do not fit any of these models. Many of the feeding-related explanations for plesiosaur necks assume that long-necked plesiosaurs were mostly feeding on small fish and squid. However, we know from stomach contents that long-necked plesiosaurs had a wide range of diets. Large and small crustaceans, sharks, small pterosaurs, and even partially-digested ichthyosaur embryos have been found in plesiosaur stomachs. Some species like Mortuneria and Artistonectes even show extensive filter-feeding adaptations. This suggests these species fed on small animals buried in sediment, similar to gray whales.


It is hard to imagine how a long-necked, small-headed body plan would benefit these animals. Most aquatic predators have a tendency to evolve larger skulls and shorter necks to help them deal with armored prey like crustaceans or larger vertebrates. Filter-feeding animals try to develop large jaws to maximize the amount of food they can filter in their mouth. While Mortuneria and Artistonectes have larger and more hoop-like jaws than other plesiosaurs, their heads are still very small when compared to many modern vertebrate filter-feeders.


There are even a few plesiosaurs have necks that defy any known analogs. Some species of rhomaleosaurids and leptocleidids have asymmetrical neck vertebrae, with the tops of every other vertebrae in the neck bulging out to the right. This bizarre adaptation is seen in all specimens of the same species and still has no proper explanation. Did the vertebrae shape support some kind of odd muscle configuration? Or could it help the neck move more easily in one direction? Maybe these plesiosaurs had a weird soft-tissue organ inside of their neck that did not preserve, but left imprints behind? Scientists puzzle over these anatomical quirks, but it is interesting that rhomaleosaurid and leptocleidid plesiosaurs are on opposite sides of the plesiosaur family tree. This suggests that their asymmetric necks evolved multiple times independently.


With a group as successful and diverse as plesiosaurs, it is very likely that they had many uses for their necks. Like many adaptations in nature, the body parts of animals serve a number of different purposes. Giraffe necks help to increase their feeding range, provide them a means to fight over mates, and even help support thermoregulation by giving them a larger surface area to shed heat. Living things have a knack for learning how to make parts of their bodies into multi-purpose tools and there is no reason why plesiosaurs would be any different. Plesiosaur necks could have been used for all sorts of behaviors depending upon the circumstances the animal is under or the specific species it is.


Turner did not live to see his Elasmosaurus captivate the imagination of the world. He died unexpectedly in Fort Wallace in the summer of 1889, a few months before Cope properly published Elasmosaurus and a whole year before Joseph Leidy pointed out Cope’s mistake. Nonetheless, his original discovery of just a few vertebrae kicked off a scientific pursuit to better understand the mysteries of the ancient world. Understanding how Elasmosaurus and other long-necked plesiosaurs lived and evolved not only teaches us what they were like when alive. It shows how animals can take such a mundane anatomical feature like the neck and push it to biological extremes through natural selection.


References
Benson, R. B. J., Evans, M. & Druckenmiller, P. S. 2012b. High diversity, low disparity and small body size in plesiosaurs (Reptilia, Sauropterygia) from the Triassic–Jurassic boundary. PLoS ONE 7, e31838.
F. Robin O'Keefe, Hallie P. Street, Jean Pierre Cavigelli, John J. Socha and R. Dennis O'Keefe A Plesiosaur Containing an Ichthyosaur Embryo as Stomach Contents from the Sundance Formation of the Bighorn Basin, Wyoming. Journal of Vertebrate Paleontology Vol. 29, No. 4 (Dec. 12, 2009), pp. 1306-1310
Noè, L.F., Taylor, M.A., and Gómez-Pérez, M. 2017. An integrated approach to understanding the role of the long neck in plesiosaurs. Acta Palaeontologica Polonica 62 (1): 137–162.

Image Credit: Joschua Knüppe

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Tristan
Stock

Guest Writer


http://sulc.us/p9rsc
http://www.eartharchives.org/articles/plesiosaurs-and-their-absurdly-long-necks/