Platypterygius australis - credit: Kronosaurus Korner.
Etymology
The genus name Platypterygius, means ‘flat fin’ in Greek, and the species name australis means ‘southern’ in Latin.
Relationships
Platypterygius australis is a member of the Ichthyosauria, a group of extinct marine reptiles commonly called ichthyosaurs (meaning ‘fish lizards’). The origins of ichthyosaurs are linked to forms like Cartorhynchus from the Early Triassic 250 million years ago. This small aquatic animal had a short snout, lizard-like body and flipper-like limbs that were capable of seal-like movements on land.
True ichthyosaurs continued to evolve throughout the Middle to Late Triassic, diversifying into a range of fully-aquatic forms including eel-like and enormous deep-bodied genera. Most ichthyosaurs that survived through the Jurassic and Early Cretaceous had streamlined bodies comparable in shape to tuna, sharks and dolphins. This generalised body plan, displayed in Platypterygius, includes a long snout, stiffened body, dorsal fin, large front flippers, small hind flippers and vertical tail fluke.
Within the Ichthyosauria, Platypterygius belongs to the Platypterygiinae in the Ophthalmosauridae. Recent research suggests that Platypterygius australis is more closely related to the Russian platypterygiines Pervushovisaurus and Simbirskiasaurus than other species of Platypterygius. This may result in Platypterygius australis being placed in a new genus in future studies.
Reconstruction of the skull of Platypterygius australis (modified from Kear 2005).
Discoveries
Platypterygius australis is relatively common from fossil sites around Richmond, Hughenden, Julia Creek, Barcaldine and Boulia in Queensland. Material has also been found in the Northern Territory, South Australia and New South Wales.
The first fossils of Platypterygius australis were discovered by Mr James Sutherland in 1865 on Marathon Station near Richmond. This material, a series of vertebrae, was described by Professor Frederick McCoy in 1867 as belonging to the European genus Ichthyosaurus, the best known ichthyosaur at the time. In addition to ‘Ichthyosaurus’ australis, a purported second species ‘Ichthyosaurus marathonensis’ was described from an incomplete snout in the same area by Mr Robert Etheridge in 1888. These species were placed in the genus Myopterygius in the 1920s and 1940s until research by Dr Christopher McGowan in 1972 synonymised both forms as Platypterygius australis.
Perhaps the best preserved specimen of Platypterygius australis was collected by Mr J. Edgar Young in 1935 from Telemon Station near Hughenden. This specimen consists of an articulated skull, partial vertebral column and front flippers. It was described in detail by Dr Mary Wade in 1990 as Platypterygius longmani. Subsequent research by Dr Maria Zammit synonymised this name with Platypterygius australis.
Kronosaurus Korner displays several significant specimens of Platypterygius from Richmond. A juvenile specimen ‘Minnee’ was discovered by Kronosaurus Korner ambassadors Mr Gary and Mrs Barb Flewelling in 2011 at Free Fossil Hunting Site 1. It represents the only known articulated ichthyosaur skeleton in Australia in which the complete vertebral column (up to the end of the tail) and hind flippers are preserved. Such morphology has not been scientifically described for Platypterygius australis.
In 2014, the Wilson family discovered another significant Platypterygius specimen several metres away from ‘Minnee’ site. At the behest of seven year old Amber Wilson, father Tony revealed a hockey puck-shaped vertebra hidden underneath a large rock. With the help of mother Lisa and brother Darcy, the family uncovered additional vertebrae and a complete skull. The specimen was nicknamed ‘Wilson’ in their honour.
The Platypterygius australis specimen 'Wilson'. Note the teeth that have fallen out of the jaws. Credit Patricia Woodgate.
Geology
Platypterygius australis is known from 100 million year old deposits from Outback Queensland. This includes the Late Albian Toolebuc Formation and Allaru Mudstone from the Rolling Downs Group.
Description
The skull of Platypterygius australis contains large eyes, although they are relatively smaller than those of other ophthalmosaurid ichthyosaurs. The robust jaws contained over 100 interlocking conical (cone-shaped) teeth covered in fine longitudinal ridges. They are loosely attached by roots within long grooves inside the jaws. The relatively large square-shaped roots are marked with notches permitting space for new teeth to grow underneath. The jaws display small channels and grooves that possibly contained an electrosensory system.
While the neck of Platypterygius australis is short, the rigid body is relatively elongate. The body contains short disc-shaped vertebrae which are hourglass-like in transverse cross-section. These vertebrae lack interlocking processes present on the vertebrae of terrestrial animals, as their bodies needed less support underwater.
The large front flippers are relatively broader than those of any other ichthyosaur. Both front and smaller hind flippers display polydactyly (having many digits). Each flipper contains over ten rows of variably-sized phalanges. Over 100 of these cube-shaped phalanges are present in each flipper.
The powerful tail ended in a crescent-shaped vertical fluke. While the bottom lobe of this fluke contained a series of small downward-slanted vertebrae, the top lobe was composed of strong connective tissue.
Fossils of Platypterygius australis, top - left to right: tooth, vertebra and phalanx; bottom: the tail of 'Minnee'.
Palaeobiology
It is possible that Platypterygius used its good sense of vision for hunting. Similar to sharks, it also may have hunted by detecting electric fields emitted by its prey. In contrast to these senses, research by Dr Benjamin Kear suggests that the small inner ear bones of the skull were too thick to receive sound vibrations, rendering Platypterygius deaf.
The stomach content of some Platypterygius australis specimens contains the remains of fish, turtle hatchlings and small birds. It is possible that Platypterygius dispatched such small-bodied prey by shaking them side to side within its jaws.
Some Platypterygius australis fossils show signs of intraspecific fighting (combat between members of the same species). This includes jaw bones which have possible bite marks from other Platypterygius. Such injuries might have been sustained over fights for mates or territory.
Platypterygius would have swum underwater by moving its strong tail side-to-side. The broad front and hind flippers probably aided manoeuvrability.
The presence of several mother and baby Platypterygius specimens from Richmond suggests that the shallow waters of the Eromanga Sea were an ideal place for calving. Like modern dolphins, Platypterygius gave birth to live young. A large female specimen collected by Mr Jim Sheahan in 2004 from Boremba Station contains the fossilised vertebrae of an embryo. This specimen is currently on display at Kronosaurus Korner.
Based on ichthyosaur fossils from overseas, pregnant Platypterygius carried multiple offspring at once. These young were birthed head-first to prevent possible drowning. At one metre in length, baby Platypterygius were relatively large and well-formed when born.
Ichthyosaurs like Platypterygius were probably warm blooded and had active lifestyles. As air breathers, they would have come to the surface take in oxygen. In contrast to dolphins, their nostrils were not placed on their backs like blowholes.
References
Fischer, V., Arkhangelsky, M. S., Naish, D., Stenshin, I. M., Uspensky, G. N. and Godefroit, P. (2014). Simbirskiasaurus and Pervushovisaurus reassessed: implications for the taxonomy and cranial osteology of Cretaceous platypterygiine ichthyosaurs. Zoological Journal of the Linnean Society, 171: 822-841.
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: 583-622.
Kear, B. P. and Zammit, M. 2013. In utero foetal remains of the Cretaceous ichthyosaurian Platypterygius: ontogenetic implications for character state efficacy. Geological Magazine 151: 71-86.
Kear, B. P., Boles, W. E. and Smith, E. T. 2003. Unusual gut contents in a Cretaceous ichthyosaur. Proceedings of the Royal Society of London Biology Letters Supplement. 270, 206-208.
Motani, R., Jiang, D. Y., Chen, G. B., Tintori, A., Rieppel, O., Ji, C. and Huang, J. D. 2014. A basal ichthyosauriform with a short snout from the Lower Triassic of China. Nature 517: 485-488.
Zammit, M. 2010. A review of Australasian ichthyosaurs. Alcheringa 34: 281-292.
Zammit, M. and Kear, B. P. 2011. Healed Bite Marks on a Cretaceous Ichthyosaur. Acta Palaeontologica Polonica 56: 859-863.
Zammit, M., Kear, B. P. and Norris, R. M. 2013. Locomotory capabilities in the Early Cretaceous ichthyosaur Platypterygius australis based on osteological comparisons with extant marine mammals. Geological Magazine 151: 87-99.