Muraenosaurus

 

Muraenosaurus was a medium-sized plesiosaur, with the largest specimen measuring up to 5.2 metres (17 ft) long.^[5] The plesiosaur had 66 pre-sacral vertebrae; 44 of which were cervical. Starting from the base of the skull and moving posteriorly the cervical vertebrae elongate and the neural spines broaden. This may be indicative of increased muscle attachment towards the base of the neck.^[6] There is also a gradual flattening and reduction of the zygapohesis until about the 30th vertebra where all traces of the platform are gone. Posterior to the 44 cervical vertebrae, Muraenosaurus had 3 pectoral vertebrae and 19 thoracic vertebrae.^[1][6] The pectoral vertebrae resemble the terminal cervical vertebrae but display a broadened face of the centrum and a slight posterior incline in the neural spines. In the dorsal vertebrae the rib has been raised onto the neural arch. The neural spines have elongated anteroposteriorly and compressed vertically.^[1]

The pectoral girdle in Muraenosaurus is broader than in most plesiosaurs and helped to situate the animal as a member of Cryptoclididae. The coracoids reach widths of nearly 14 inches. The forelimb is compressed mediolaterally when compared to the hind limb as well as limbs of other plesiosaurs.^[1][7] This compression is represented in the aspect ratio of Muraenosaur limbs. The aspect ratio of the hind limbs is much greater than that of the fore limbs, representing a longer and more slender form.^[7] The high aspect ratio in the forelimbs may have been used for increasing maneuverability at some cost to the animal's endurance.

When Harry Seeley first described Muraenosaurus it was thought to belong to Elasmosauridae instead of Cryptoclididae. Muraenosaurs and elasmosaurs share a plesiosauromorph body type typified by having a small head at the end of a long neck.^[8] Like elasmosaurids, Muraenosaurus has a long neck. Of the 66 presacral vertebrae in Muraenosaurus, 44 of them are cervical. It was initially believed that this innovation leads to a largely flexible neck and a relatively short and sturdy body. More recent research has shown that while mobile, the neck of plesiosauromorphs was not as flexible as previously thought.^[9] The head of Muraenosaurus is also very small compared to both its neck and body length, measuring only about 37 centimetres (15 in) long.^[1][10][11] Both of these traits are common in elasmosaurids which led to the initial diagnosis of muraenosaurs in the family Elasmosauridae. However, this is actually a case of convergent evolution between cryptoclidids and elasmosaurs.^[8][12] The clade of cryptoclidids creates a unique tree in relation to the paleomorphology of discovered specimens. For example, within cryptocleidoidae there are short necked plesiosaurs such as Kimmerosaurus as well as the long necked Muraenosaurus. The defining features of Cryptocleidoidia include a low fin aspect ratio, a wide rounded interpterygoid vacuity, and extreme specialization of the cheek region.^[8] The interpterygoid vacuity is completely absent in elasmosaur species but well represented in cryptoclidids, including Muraenosaurus.^[8] The aspect ratio of both the fore and hind limbs of Muraenosaurus is far lower than the aspect ratio of elasmosaur specimens. The fore limbs especially resemble the flippers of Pliosauridae, an extinct group of less elongate marine reptiles that shared the seas with plesiosaurs, more than they resemble the typical plesiosaur morphology. Cryptoclidus, another cryptoclidid plesiosaurimorph, shares this trait with Muraenosaurus.^[7]

Muraenosaurus was initially discovered in the Oxford Clay which represents an ancient sea that was both shallow, with an average depth less than 50 meters, and warm (20 °C).^[13][14] The Oxford Clay deposits are found in southern England and parts of France. The sea was abundant with nutrients and light filtered easily through the shallow water to create a highly productive ecosystem. The sea floor was littered with bivalves, arthropods, gastropods and foraminifera while the pelagic zone was home to a wide variety of species from marine reptiles to teleosts. The Oxford Clay was so productive that over 100 genera have been recovered from the sediment.^[13][14]

 

Muraenosaurus’ role in the ecosystem is probably more comparable to elasmosaurs than other cryptoclidids due to the plesiosauromorph body plans shared between elasmosaurs and muraenosaurs. Cryptoclidids have varying morphology and it is difficult to assess their ecological role as a collective unit. Long necked plesiosaurs have been discovered with varying contents lithified within their stomachs which give some indication of what Muraenosaurus may have been eating.^[15] Preferred prey types seem to be teleost fish and cephalopods.^[15] In order to exploit such different prey species, plesiosaurs would need multiple different feeding strategies.^[9][15]

 

Initially, long necked plesiosaurs were thought to be strictly fish eaters due to their conical teeth, a shared trait with modern piscivores. It was proposed that plesiosaurimophs would swim with a straight neck and attack pelagic fishes.^[9] The straight neck would have been used in order to avoid creating drag by arching its neck upward into the water column. Additionally, some researchers have proposed that by swimming with its head directly in front of its body, plesiosaurs would be able to reach pelagic fishes before they felt the change in water pressure caused by the large body of the plesiosaur. Essentially, the head would precede the pressure difference.^[9] It is also proposed that species like Muraenosaurus fed upon benthic fishes by floating above them and reaching its head down into the benthos.^[15] Plesiosauromorphs may also have employed a strategy called benthic grazing where they would harvest relatively immobile species such as cephalopods from the sea floor. Gastroliths have often been found within the stomachs of extinct marine reptiles and have been associated with the plesiosauromorph body type. One of the proposed uses for gastroliths is the grinding of tough, shelled foods like cephalopods.^[15][16]

The food chain did not stop at the plesiosaurimorphs. The oceans in the Mesozoic were dominated by pliosaurs, apex predators that would have preyed upon species like Muraenosaurus.^[13] Pliosaurs were not the only animals that would have fed on Muraenosaurus though. Evidence of this comes from several cases of shark teeth being found with plesiosaurimorph specimens.^[17] However it is not certain if the specimens were alive when attacked or whether sharks were feeding on carcasses of deceased plesiosaurimorphs. The shredding teeth of lamniform sharks indicates that perhaps they were scavengers or detrivores and that they only fed upon dead plesiosaurs.^[17] Muraenosaurus was an important piece of the ecosystem both as a carnivore and as a source of food to other species.

 

Gastroliths have been a common find among the stomach contents of extinct marine reptiles. Their occurrence has led to two main hypotheses regarding the significance of the rocks.^[16] The first proposed usage, as described above, was to crush hard shelled food engulfed by the animal.^[16] The second hypothesis is that gastroliths were swallowed in order to help maintain controllable buoyancy within the water column. Modeling of Cryptoclidus body types has indicated that the use of gastroliths as ballast is unrealistic.^[18] In order to effect the animal's buoyancy, a Muraenosaur would have to consume over 10% of its body mass in stones.^[18] Observed masses of stone collected from plesiosaur stomachs are far lower than the modeled mass required to effect buoyancy. However, from the modeling a new possibility emerged and that is that gastroliths may have helped prevent rolling in animals like Muraenosaurus. Models have indicated that not only do stones reduce pivoting at depth but it is also possible they dampened the ossilations in a plesiosaur's neck, helping provide stability to counter underwater currents.^[18]

• List of plesiosaur genera
• Timeline of plesiosaur research