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Western Australian Pliosaurid

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I am currently doing some volunteer work at the Museum of Western Australia, cleaning fossil fragments from Cretaceous marine reptiles. These fossils were collected from the Muderong Shale, near Kalbarri, Western Australia and come from the lower Aptian stage of the early to middle Cretaceous period (approximately 120 mya). Identified marine reptiles from the shale include a medium-sized Ophthalmosaurid Ichthyosaur, a medium-sized Leptocleidid Plesiosauroid, an Elasmosaurid and a large Pliosaurid. So far only the Leptocleidid remains have been formally described, given the name Leptocleidus clemai (Cruikshank & Long, 1997).

The Pliosaurid remains were tentatively assigned to the genus Kronosaurus, because this is the only large Pliosaurid genera known from the Aptian stage of Australia. The genus Kronosaurus currently contains two recognised species, K. queenslandicus (Longman, 1924) and K. boyacensis (Hampe 1992). Both species reached lengths in excess of 10 metres (McHenry, 2009). The Muderong Shale Pliosaurid is estimated at approximately 6 metres in length, smaller than its eastern relatives but still a very impressive reptile and most likely the apex predator of its ecosystem. The currently identified material consists only of fragments from the pectoral and pelvic girdles, as well as of the humeri and femur.

There are unfortunately no reliably skeletal reconstructions for Kronosaurus, despite the genus being represented by several reasonably complete specimens. The proportions of this illustration were based on the dimensions given by McHenry (2009) for the specimen, QM F10113. This specimen includes a partial skull associated with a mostly articulated vertebral column and parts of the limb girdles. The skull measured 187.6 cm in basal length, compared to an estimated total length of 851.6 cm. This meant the head occupied more than 22 percent of the total length. Unlike the similarly large Pliosaurus funkei (Knutsen et al., 2012) the hindlimbs of Kronosaurus were actually proportionally longer than the forelimbs.

Although the Plesiosauroid, Mauriciosaurus fernandezi (Frey et al., 2017) preserved evidence of small rectangular scale-like structures, the Pliosaurid, Attenborosaurus conybeari (Bakker, 1993) allegedly preserved evidence of smooth scaleless skin (Sollas, 1881). It can be inferred that Pliosaurids were largely devoid of scaly integument, similar to Ichthyosaurs. I have chosen to reflect this in my illustration. The tail fluke is entirely speculative but based on the shape of the tail fluke of the Plesiosauroid, Seeleysaurus guilelmiimperatoris (Dames, 1895).


References

Bakker, R. T. (1993). “Plesiosaur extinction cycles - events that mark the beginning, middle and end of the Cretaceous.” In: Caldwell, W. G. E., Kauffman, E. G. (eds) “Evolution of the Western Interior Basin: Geological Association of Canada. Special Paper, 39: 641-664.

Cruikshank, A. R. I., Long, J. (1997). “A new species of pliosaurid reptile from the Early Cretaceous Birdrong Sandstone of Western Australia.” Records of the Western Australian Museum, 18: 263-276.

Dames, W. B. (1895). “Die plesiosaurier der süddeutschen Liasformation.” Königliche Akademie der Wissenschaften.

Frey, E., Mulder, E., Stinnesbeck, W., Rivera-Sylva, H., Padilla-Gutiérrez, J., González-González, A. (2017). “A new polycotylid plesiosaur from the early Late Cretaceous of northeast Mexico.” Boletín de la Sociedad Geológica Mexicana, 69(1): 87-134.

Hampe O. (1992). “Ein großwüchsiger Pliosauride (Reptilia: Plesiosauria) aus der Unterkreide (oberes Aptium) von Kolumbien.” Courier Forschungsinstitut Senckenberg145: 1-32.

Knutsen, E. M., Druckenmiller, P. S., Hurum, J. H. (2012). “A new species of Pliosaurus (Sauropterygia: Plesiosauria) from the Middle Volgian of central Spitsbergen, Norway.” Norwegian Journal of Geology, 92(2-3): 235-258.

Longman H. A. (1924). “A new gigantic marine reptile from the Queensland Cretaceous, Kronosaurus queenslandicus new genus and species.” Memoirs of the Queensland Museum8: 26-28.

McHenry, C. R. (2009). “Devourer of Gods: The palaeoecology of the Cretaceous pliosaur Kronosaurus queenslandicus.” The University of Newcastle.

Sollas, J. W. (1881). “On a new species of Plesiosaurus (P. Conybeari) from the Lower Lias of Charmouth; with observations on P megacephalus, Stutchbury, and P. brachycephalus, Owen.” Quarterly Journal of the Geological Society of London, 37: 440-480.

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