P. dolichodeirus, or Plesiosaurus, was not a dinosaur, but a plesiosaur—a marine reptile (Greek—near to lizard) from the Mesozoic Era of the Early Jurassic period. Plesiosaurus was the first species of its kind to be discovered. Though much has been learned about its anatomy and evolution, there are still many topics of debate over the species. Some of the debates surrounding the Plesiosaurus include the flexibility of its neck, its specific means of locomotion, its ability to walk on land, and whether it laid eggs or gave birth to live young. Since to date no evidence has been found of Plesiosaurus bearing live young in the water, it is believed that the marine reptile came ashore- as turtles do- in order to nest in the sand.
|Prehistoric Era||Early Jurassic
|Weight||200 lbs (90 kg)|
|Length||16 feet (5 meters)|
|Height||3.3 feet (1 meters)
|Territory||Europe, North America
Plesiosaurus thrived in the shallow waters apparently, with fossils being primarily found in shallow sea beds and also freshwater lakes. It’s theorized that the long neck of Plesiosaurus would have served its existence in the shallow waters by allowing it to remain at the optimum depth levels while its head could be either submerged for foraging and feeding, or up above the water for breathing. Being reptilian, Plesiosaurus did take air from the atmosphere rather than the ocean.
The marine reptile is another apparent example of a species which seems to exhibit many characteristics of convergent evolution. Classification has had its challenges, with prior type species P. brachypterygius, P. guilielmiiperatoris, and P. tournemirensis having since been reclassified to other genera.
What Did Plesiosaurus Look Like?
The physical appearance of Plesiosaurus was characterized by a distinctly turtle-like form, with a flat pectoral arch, a broad body, long neck, large paddle-like flippers with five digits each, and a short tail.
Plesiosaurus sported a rib-cage from its neck to its tail, consisting of many variations in shape and service, with hatchet-shaped cervical ribs featuring two articular heads. The dorsal ribs are thick still, with one articular head. Sacral ribs are short and rounded on each end; caudal ribs vary according to their location along the tail portion of Plesiosaurus. Between its shoulder and pelvis, Plesiosaurus sports belly ribs- otherwise known as gastralia. These gastralia are much smaller rib-like formations that have a bone on the midline that is flanked on each side by three other bones.
Plesiosaurus’ finnage, paddles, or limbs consisted of five digits on the hand paddle, six bones in the wrist, distinctive curvature to the humerus and a strong pillar-like radius. Six bones in the ankle also, with five digits in the foot paddles. Hind limbs were long and narrow, and in adults were smaller in comparison to the front limbs.
What Did Plesiosaurus Eat?
Plesiosaurus was one of the largest water-dwelling creatures in the ocean during its time. Its jaw was very powerful and some scientists speculate it may have had the most crushing power of any animal. This characteristic, along with its razor sharp teeth, probably made Plesiosaurus one of the deadliest predators in the ocean- at least to the fish and mollusks. These teeth were not suitable for chewing, nor were the neck and cranial structures able to withstand the wild ripping and tearing of bringing down larger prey. The diet of Plesiosaurus therefore was probably limited to small aquatic life that it could easily swallow in a bite or two.
Plesiosaurus was a carnivore, more specifically a piscivore that ate primarily small marine life such as fish, and mollusks also. A diet practiced by early amphibians, piscivory involved evolution to insectivory and then ultimately herbivory, added by reptilian species. Plesiosaurus had sharp teeth that jutted forward and interlocked, a common evolution of piscivores, helping them to grab and keep the quick and slippery fish. Eyes towards the top of its head suggest that Plesiosaurus hunted from below which goes to further disprove the swan-like technique of hunting.
It was originally thought that the Plesiosaurus kept its head above water in a swan-like fashion and quickly dove to catch its prey; however, some scientists postulate that the neck of Plesiosaurus was not that flexible.
How Did Plesiosaurus Move?
Being a water-dwelling reptile, Plesiosaurus used its four large flippers to move through the water. These flippers were very stiff and resemble those of modern day turtles. It has been argued whether the flippers moved in a rowing fashion or a more flying fashion, but some scientists believe that the most probable movement was a modified combination of these two styles of locomotion. The Plesiosaurus did have a tail, but it was very short and was probably not used much in the way of mobility.
It has also been debated whether Plesiosaurus could walk on land. Early theories suggested it was possible, but more recent findings have found that the flippers of Plesiosaurus would probably not have been able to support its weight for movement on land, but that they could possibly push the creature through shallow waters. It makes sense then that Plesiosaurus would move to the shallower beach areas and lay eggs that would be land-bound except during tide. The theory that shallow waters were the primary place of brooding and feeding residence is enforced by the geological indicators of fossil recovery.
It is thought that Plesiosaur utilized its strong and well-developed paddle limbs as its major means of propulsion while not only using a row-boat motion for control, but also advanced enough to use its paddles more as wings and fly through the water as a modern day penguin does. In the shallowest regions, Plesiosaurus would have glided along the surface almost floating, yet guiding itself with its paddles which likely also did serve as some sort of land appendage- although not necessarily providing the increased land-bound access of a lighter crocodile of today, which can support its own weight on dry land.
Where Did Plesiosaurus Live?
The majority of Plesiosaurus fossils have been found around England and Germany, but fossils have also been found on every continent, including North America. Being sea-bound creatures in addition to the varied locations of recoveries indicates that Plesiosaurus moved about expansively in the ocean, as many other sea creatures will, and as birds migrate the world over, by sky.
When Plesiosaurus was not traveling, it preferred to move in the shallow waters near land and feed, likely breed, and nest on land covered by shallow waters. A marine reptile, Plesiosaurus was thought for many years to have laid eggs in the sand. Recent fossil discoveries indicate that Plesiosaurus was a water-bearer and gave birth to live young. A fossilized Plesiosaurus skeleton was found with a same-species skeleton inside, in the form of a fetus.
The Discovery of Plesiosaurus
The genus Plesiosaurus was discovered and subsequently named in 1821, in reference to the view that plesiosaurs were closer to reptiles than were the more fish-like ichthyosaurs. Plesiosaurus observations were derived from the scattered remnant fossil finds from the Bristol region, Dorset and Lyme Regis areas by William Conybeare and Henry De la Beche. The genus type species, P. dolichodeirus, was named in 1824 by Conybeare based on a complete skeleton discovered by the collector Mary Anning in the Early Jurassic rocks of the lower Lias Group in 1823.
Recent fossil discoveries of Plesiosaurus have clued scientists into the likely possibility that it was a live-bearer of its young, directly contradicting the popular presumption from prior discoveries. Plesiosaurus has since been discovered in areas all over the world, including North America and seas of Australia.
Unique Characteristics of Plesiosaurus
Plesiosaurus was another prehistoric animal that somehow utilized gastroliths, or stomach stones. Gastroliths were recovered from the Bearpaw Shale of Montana by Darby and Ojakangas in 1980. The Bearpaw Shale is a sedimentary rock formation of the Phanerozoic-Mesozoic-Late Cretaceous period that consists of gray and brown dark clay shale and thick beds of bentonite. While the stomach stones of Plesiosaurus are present in significant number, it remains unclear to some exactly why they were used, or if in fact they were an accidental ingestion. It appears that, since Plesiosaurus was a carnivore and wouldn’t have had much control over how it dealt with its food with no chewing ability beyond the first bite, Plesiosaurus would have utilized the stones for harder portions of ingested items such as mollusk shells.
The Significance of Plesiosaurus
Plesiosaurus was the first Mesozoic reptile of its kind to be discovered. As a result, it has since become a virtual waste bin, or go-to, taxon and many other water-dwelling reptiles, such as Plesiosaurus guilielmiiperatoris, Plesiosaurus brachypterygius, and Plesiosaurus tournemirensis, were thrown in to this category. They have since been moved to different generas. Plesiosaurus discovery has significantly contributed to scientists’ modern understanding of sea-bound evolution and daily life of evolving Plesiosauroidea of the period. The Plesiosaurus has also become a part of popular culture as some believe that the mythical Loch Ness Monster is a modern evolution of Plesiosaurus.