'Penny' - credit: Kronosaurus Korner.
Etymology
The polycotylid specimen QM F18041 has not been given a scientific name. It has several nicknames including ‘Penny the Plesiosaur’ and the ‘Richmond Pliosaur’.
Relationships
While it has not been formally described, it is clear that QM F18041 belongs within the Plesiosauria, an extinct group of marine reptiles known for their variably-sized necks, broad bodies, flipper-like limbs and short tails. Plesiosaurs evolved during the Triassic and diversified into several different groups that survived through the Jurassic and Cretaceous.
Within the Plesiosauria, QM F18041 is a member of the Polycotylidae, a family of Cretaceous plesiosaurs that have relatively long skulls and short necks. The general appearance of polycotylids has previously drawn comparisons to members of the Pliosauridae such as Kronosaurus (hence the ‘Richmond Pliosaur’ moniker). However, recent phylogenetic analyses of the Plesiosauria indicate that polycotylids were not closely related to pliosaurids but belong to the Leptocleididae in the Plesiosauroidea. This position confirms that polycotylids were much more closely related to long-necked elasmosaurids than pliosaurids and that long skulls and short necks evolved independently several times within the Plesiosauria.
Phylogenetic studies of the Polycotylidae suggest that QM F18041 was a primitive member of the group. It may have been most closely related to Plesiopleurodon and Edgarosaurus from the United States of America.
Along with slightly older polycotylid remains from the Aptian of Australia, QM F18041 represents one of the earliest polycotylid specimens in the world. Based on this evidence, it has been suggested that the Polycotylidae may have evolved in Gondwana.
'Penny the Plesiosaur' - polycotylid specimen QM F18041.
Discovery
While fragmentary polycotylid material has been discovered from other localities in Queensland, New South Wales and South Australia, QM F18041 represents the most complete plesiosaur, if not large vertebrate fossil discovered from Australia.
The remains of QM F18041 were discovered by Mr Ian Ievers in 1989 on Marathon Station near Richmond. While feeding cattle near the homestead, Mr Ievers noticed a piece of the rostrum (snout) protruding from a dry creek bank. Soil was removed from around the rest of the skull by Mr Ievers and his brothers Robert, Peter and Ross. Several vertebrae were also discovered in articulation behind the skull.
Upon realising the potential significance of the specimen, the Ievers family invited staff from the Queensland Museum in Brisbane to help excavate the remaining skeleton. The Queensland Museum team was led by the late palaeontologist Dr Mary Wade.
Over the course of 12 days, approximately one metre of overlying soil was carefully removed from the postcranial skeleton. The specimen was coated in a protective plaster jacket and lifted out of the ground to be transported to the Queensland Museum for preparation.
The specimen was donated to the Queensland Museum by the Ievers family. It has since been loaned to Kronosaurus Korner as the centrepiece of the Rob Ievers Gallery.
QM F18041 - left: Dr Mary Wade and Ms Cathy Mobbs during the excavation on Marathon Station; right: the specimen on display today at Kronosaurus Korner.
Geology
The remains of QM F18041 are known from the approximately 100 million year old Late Albian Toolebuc Formation of the Rolling Downs Group.
Description
The morphology of QM F18041 has not been described, but is currently under study by Dr Benjamin Kear. At 4.25 metres in length, QM F18041 represents a relatively large polycotylid compared to related forms from overseas. The skull is elongate, measuring approximately one metre in length. Although slightly worn, the teeth are conical (cone-like) in shape, slightly recurved and covered around the circumference in longitudinal ridges. A possible bit mark is present on the tip of the rostrum.
The neck is relatively short compared to other plesiosauroids. Both the neck and pectoral region show signs of abnormal bone growth, possible sustained after an injury. The streamline body has four large paddle-like limbs and a short tail.
The skull of QM F18041 - note the arrow pointing to a potential bite mark.
Palaeobiology
Polycotylids were probably incapable of hearing, as their skulls lack anatomy associated with eardrums. In contrast, their relatively large eyes probably provided excellent vision.
It is likely that polycotylids were fast-moving, manoeuvrable swimmers, with the ability to hunt small agile prey. This may have brought polycotylids into completion with ichthyosaurs over food resources, which may have contributed to the extinction of the latter.
The stomach contents of some polycotylid specimens from the Northern Hemisphere contain the beaks of small ammonites (shelled relatives of octopuses). These polycotylids may have swallowed small ammonites whole, as their teeth were poorly adapted for crushing shells.
Evidence of live birth in plesiosaurs is based on pregnant polycotylid specimens. One notable specimen contains a single large foetus that measures at least a third the size of its mother. This suggests that polycotylids had relatively long pregnancies and gave birth to one offspring at a time. This may have been coupled with substantial maternal care, similar in behaviour to dolphins and whales.
References
Benson, R. B. J. and Druckenmiller, P. S. 2014. Faunal turnover of marine tetrapods during the Jurassic-Cretaceous transition. Biological Reviews 89: 1-23.
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 B 270: S206–S208.
O’Keefe, F. R. and Chiappe, L. M. 2011. Viviparity and K-Selected Life History in a Mesozoic Marine Plesiosaur (Reptilia, Sauropterygia). Science 333: 870-873.
Sato, T. and Tanabe, K. 1998. Cretaceous plesiosaurs ate ammonites. Nature 394: 629-630.
Satoa, T., Wu, X. C., Tirabasso, A. and Bloskie, P. 2011. Braincase of a Polycotylid Plesiosaur (Reptilia: Sauropterygia) from the Upper Cretaceous of Manitoba, Canada. Journal of Vertebrate Paleontology 31: 313-329.