Muraenosaurus

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Muraenosaurus
Temporal range:
Ma
↓
M. leedsi fossils
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: †Sauropterygia
Order: †Plesiosauria
Family: †Cryptoclididae
Subfamily: †
Muraenosaurinae
Genus: †Muraenosaurus
Seeley, 1874
Type species
†Muraenosaurus leedsii
Seeley, 1874

Muraenosaurus (from the Latin "

Charles Leeds’ collection. The first muraenosaur was recovered with pieces missing from the skull and many of the caudal vertebrae absent. Because the animal was described from Charles Leeds’ collection it was given the name Muraenosaurus Leedsi. M. leedsi is the most complete specimen belonging to the genus Muraenosaurus and also the only species that is undoubtedly a member of the genus.[1] Two other species have been tentatively referred to as members of the genus Muraenosaurus: M. reedii and Muraenosaurus beloclis Seeley 1892,[2][3] which in 1909 became the separate genus Picrocleidus
.

Muraenosaurus reedii was discovered in

junior synonym of Pantosaurus.[4] Picrocleidus beloclis is another plesiosaur originating in the middle Jurassic and found in the Oxford clay formation. Picrocleidus has previously been considered synonymous with Muraenosaurus although there are doubts in the relationship between the two groupings.[2]

Description

M. leedsi with a human to scale

Muraenosaurus was a medium-sized plesiosaur, with the largest specimen measuring up to 5.2 metres (17 ft) long.

neural arch. The neural spines have elongated anteroposteriorly and compressed vertically.[1]

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.

Classification

M. beloclis fossils

When

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]

Paleobiology

Habitat

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).

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]

Diet and predation

Restoration of two M. leedsi

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.

cephalopods.[15] In order to exploit such different prey species, plesiosaurs would need multiple different feeding strategies.[9][15]

Side view of M. leedsi

Initially, long necked plesiosaurs were thought to be strictly fish eaters due to their conical teeth, a shared trait with modern

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

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.

Plesiosaur gastroliths

Gastroliths and Buoyancy

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]

See also

References

  1. ^ a b c d e f Seeley, HG. 1874. On Murænosaurus Leedsii, a Plesiosaurian from the Oxford Clay. Part I. Quarterly Journal of the Geological Society 30: 197-208.
  2. ^ a b Wilhelm BC. 2010. A New Partial Skeleton of a Cryptocleidoid Plesiosaur from the Upper Jurassic Sundance Formation of Wyoming. Journal of Vertebrate Paleontology , 30, 6, 1736-1742.
  3. ^ a b Muraenosaurus? Reedii, Sp. Nov. and Tricleidus? Laramiensis Knight, American Jurassic Plesiosaurs. The Journal of Geology, 20, 4, 344-352.
  4. ^ O'Keefe FR, and Wahl W. (2003). "Current taxonomic status of the plesiosaur Pantosaurus striatus from the Upper Jurassic Sundance Formation, Wyoming". Paludicola. 4 (2): 37–46.
  5. ^ Brown, D. S. (1981). "The English Upper Jurassic Plesiosauridea (Reptilia) and a review of the phylogeny and classification of the Plesiosauria". Bulletin of the British Museum of Natural History. 35 (4): 253–347.
  6. ^ a b Richards, CD. 2011. Pleisiosaur body shape and its impact on hydrodynamic properties. Huntington, WV: Marshal University Libraries
  7. ^ a b c O’Keefe FR. 2001. Ecomorphology of plesiosaur flipper geometry. Journal of Evolutionary Biology, 14, 6, 987-991
  8. ^ a b c d O’Keefe, FR. 2001. A cladistics analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia). Helsinki: Finnish Zoological and Botanical Pub. Board
  9. ^ a b c d Zammit M. 2008. Elasmosaur (Reptilia: Sauropterygia) neck flexibility: Implications for feeding strategies. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology , 150, 2, 124-130.
  10. ISBN 1-84028-152-9
    .
  11. ^ Andrews, CW. 1913. A descriptive catalogue of the Marine Reptiles of the Oxford Clay, Part II. British Museum (Natural History). pp.205pp.
  12. ^ O’Keefe FR. 2002. The evolution of plesiosaur and pliosaur morphotypes in the Plesiosauria (Reptilia: Sauropterygia). Paleobiology, 28, 1, 101-112
  13. ^ a b c Martill DM. 1994. The trophic structure of the biota of the Peterborough Member, Oxford Clay Formation (Jurassic), UK. Journal of the Geological Society , 151, 1, 173-194.
  14. ^ a b Martill DM. 1991. Fossils of the Oxford Clay. Palaeontological Association, 1991.
  15. ^ a b c d e McHenry CR. 2005. Bottom-feeding plesiosaurs. Science, 310, 5745.
  16. ^ a b c Cicimurri DJ. 2001. An Elasmosaur with Stomach Contents and Gastroliths from the Pierre Shale (Late Cretaceous) of Kansas. Transactions of the Kansas Academy of Science , 104, 3, 129-142.
  17. ^ a b Shimanda K. 2010. A remarkable case of a shark-bitten elasmosaurid plesiosaur. Vertebrate Paleontology, 30, 2, 592-597
  18. ^ a b c Henderson DM. 2006. Floating point: a computational study of buoyancy, equilibrium, and gastroliths in plesiosaurs. Lethaia, 39, 3, 227-244.
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