Dinosaurs - Platypterygius longmani

Introduction

Platypterygius longmani was an ichthyosaur, a dolphin-like marine reptile that roamed the Eromanga Sea of inland Australia during the Early Cretaceous. This large ichthyosaur was a marine predator, feeding on fish and cephalopods. Ichthyosaurs had the largest eyes of any vertebrate, and keen eyesight would have helped Platypterygius to spot prey in dark, cold waters.

Identification

Ichthyosaurs were air-breathing, dolphin-like marine reptiles with spindle-shaped bodies, long, toothed snouts with up to 200 conical teeth, front and back limbs that formed two sets of paddles, and shark-like dorsal and tail fins. Unlike dolphins or whales that have horizontal tails, the tails of ichthyosaurs were vertically aligned.

Platypterygius was a large-bodied ophthalmosaurid with a low-crowned skull, long snout and long postorbital region. Its tooth roots are quadrangular in cross section. Platypterygius longmani differs from other Platypterygius species in characters of the external nares, humerus, forefin and vertebrae.

Habitat

Platypterygius lived in the cool, high-latitude, epicontinental Eromanga Sea - an inland sea that repeatedly covered vast areas of inland Australia from 125-100 million years ago (Aptian-Albian-Cenomanian). Glendonites and boulders that may have been rafted by ice are evidence of cold to near-freezing conditions during the winter.

Distribution

Platypterygius longmani has been found across Australia in areas once submerged under the Eromanga Sea. Specifically, it has been found in Queensland (the Wallumbilla Formation, Toolebuc Formations and Allaru Mudstone), New South Wales (Griman Creek Formation), South Australia (Bulldog Shale), Northern Territory (Darwin Formation), and Western Australia (Alinga Formation, Molecap Greensand, and Birdrong Sandstone).

Feeding and diet

Platypterygius, like other ichthyosaurs, would have eaten fish, squid and other marine invertebrates. One specimen from South Australia was found with belemnites in its stomach region that appear to have been swallowed whole.

Life history cycle

Ichthyosaurs gave birth to live young (in fact, several specimens have been fossilized in the process of giving birth). Young Platypterygius (2m long) have been found in South Australia and Queensland.

Fossils description

Platypterygius longmani is well represented by fossil material. The holotype is from Dunluce Station near Hughenden, Queensland and Platypterygius is common in the Toolebuc Formation of the northeastern Eromanga Basin. It is less common in the southwestern part, where plesiosaur fossils are more often found.

Fossil material includes partial skeletons with skull, forefin elements, shoulder girdle and much of the spine. The SANTOS Ichthyosaur, discovered at White Cliffs, NSW in 1987 and owned by the South Australian Museum (where it is currently on display), is one of the most complete specimens of Platypterygius known. Fossil snails and bivalves were found in association with the skeleton, perhaps living on its decaying bones. Belemnites were found in its ribcage, evidence that these squid-like cephalopods were part of its diet.

Notes: Material referred to Ichthyosaurus australis (=Myopterygius australis) and Ichthyosaurus marathonensis (=M. marathonensis) most likely belongs to Platypterygius. The type material of Platypterygius australis was apparently non-diagnostic and has since been lost, and all Australian Platypyterygius fossils have been assigned to a new species, P. longmani (Wade 1990, but see McGowan and Motani 2003 for a contrary opinion).

Evolutionary relationships

Ichthyosaurs evolved during the Triassic and survived nearly to the end of the Mesozoic (they are not known beyond the Cenomanian, about 90 million years ago). Ichthyosaurs were diapsid reptiles, but they have no close living relatives and their direct ancestors are unknown.

Platypterygius was a member of the Ophthamosauridae, a long-snouted, cosmopolitan family of ichthyosaurs distinguished by their large size. The evolutionary relationships of ophthalmosaurs are debated, but they are generally considered a basal group within Ichthyosauria.

References

• Kear, B. P. 2003. Cretaceous marine reptiles of Australia: a review of taxonomy and distribution. Cretaceous Research 24: 277-303.
• Kear, B. P. 2005. Cranial morphology of Platypterygius longmani Wade, 1990 (Reptilia: Ichthyosauria) from the Lower Cretaceous of Australia. Zoological Journal of the Linnean Society 145 (4): 583-622.
• McCoy, F., 1867. On the occurrence of Ichthyosaurus and Plesiosaurus in Australia. Annual Magazine of Natural History 19: 355-356.
• McGowan, C. and Motani, R. 2003. Ichthyopterygia. Handbuch der Paläoherpetologie Part 8, Verlag Dr. Friedrich Pfiel, München.
• Wade, M. 1984. Platypterygius australis, an Australian Cretaceous ichthyosaur. Lethaia 17: 99-113.
• Wade, M. 1990. A review of the Australian Cretaceous longipinnate ichthyosaur Platypterygius, (Ichthyosauria, Ichthyopterygia). Memoirs of the Queensland Museum 28:115-137.

Further reading

• Long, J. A. et al. 2002. Dinosaurs of Australia and New Zealand and Other Animals of the Mesozoic Era. New South Wales University Press, Sydney.
• Wallace, D. R. 2007. Neptune's Ark: from Ichthyosaurs to Orcas. University of California Press, Berkeley.