Eromangasaurus - credit Ms Janet Fickling
Etymology
The genus name Eromangasaurus, meaning ‘Eromanga lizard’, was named after the Eromanga Sea. The species name australis is Latin for ‘southern’.
Relationships
Eromangasaurus australis is a member of the Plesiosauria, an extinct group of marine reptiles commonly known as plesiosaurs. The ancestors of the Plesiosauria can be traced back through the Early-Middle Triassic to small lizard-like creatures that were adapted for semi-aquatic lifestyles. The first true plesiosaurs that evolved during the Late Triassic were larger, had long necks, broad bodies, four flippers and short tails. Plesiosaurs continued to thrive throughout the Jurassic and Cretaceous, surviving for over 140 million years.
Eromangasaurus belongs to a diverse group of plesiosaurs known as the Plesiosauroidea. Within the Plesiosauroidea, Eromangasaurus is a primitive member of the long-necked Elasmosauridae. The Elasmosauridae were among the last plesiosaurs to be become extinct 66 million years ago at the end of the Cretaceous.
The genus Eromangasaurus contains only one species E. australis, which is known from a single specimen – the holotype QM F11050.
Discoveries
The holotype of Eromangasaurus QM F11050 consists of a crushed skull broken into two pieces, an atlas-axis complex and three additional cervical (neck) vertebrae. These elements were collected by different individuals on separate occasions in the 1970s. The posterior part of the skull and associated vertebrae were discovered by Mr Ted Noonan near Maxwelton and donated to the Geological Survey of Queensland. Subsequent fieldwork at the site by Dr Ralph Molnar, Dr Tony Thulborn and Dr Mary Wade uncovered the anterior part of the skull. The anterior part was registered with the Australian Museum, where Dr Alex Ritchie determined that it belonged to the same specimen found by Mr Noonan. The holotype is currently registered within the Queensland Museum, with a high quality cast on display at Kronosaurus Korner.
QM F11050 was initialled placed with the genus ‘Woolungasaurus’, which had been described from postcranial remains from other Queensland localities. Research by Dr Benjamin Kear subsequently found postcranial remains referred to ‘Woolungasaurus’ to be indeterminate elasmosaurid material, thus rendering the name a nomen dubium (doubtful name). Kear also found the reassignment of postcranial remains from ‘Woolungasaurus’ to Styxosaurus to be unfounded.
While QM F11050 was briefly assigned to the New Zealand form Tuarangisaurus, Kear noted sufficient unique features in QM F11050 to designate it as the holotype of a new form Eromangasaurus australis.
The first elasmosaurid material from Australia was collected circa 1865 by Mr James Sutherland and David Carson from Marathon Station near Richmond. While many other partial postcranial specimens have been collected from Richmond, Hughenden, Prairie, Chillagoe, Julia Creek, Barcaldine and Boulia in Queensland, none are associated with sufficient cranial material, making it difficult to compare them with the Eromangasaurus holotype. It is possible that the heads of elasmosaurids would easily detached from decaying carcasses and drift away – resulting in the lack of well-preserved skulls. The head region may have also been an easy target for predators.
Left: the holotype of Eromangasaurus australis QM F11050; right: reconstruction of the skull of Eromangasaurus australis (modified from Sachs 2005).
Geology
The Eromangasaurus holotype is known from 100 million year old Late Albian deposits of the Toolebuc Formation from Outback Queensland. Other elasmosaurid specimens are known from the Toolebuc Formation, Allaru Mudstone, Mackunda Formation and the Doncaster Member of the Wallumbilla Formation of the Rolling Downs Group.
Description
The holotype specimen of Eromangasaurus is badly crushed and broken into two pieces. Most of the teeth are badly eroded but would have been elongate, slender and conical (cone-like) in shape. Several large indentations are present on both sides of the skull, which has been interpreted by Dr Tony Thulborn and Dr Susan Turner as possible bite marks from Kronosaurus queenslandicus.
Information on the body of Eromangasaurus can be inferred from other elasmosaurids. It is likely that Eromangasaurus grew between 9-10 metres in length, perhaps half of which would consist of neck. Some elasmosaurids have over 70 cervical vertebrae, more than that of any other vertebrate. In comparison, humans only have 7 neck vertebrae. Like other elasmosaurids, the vertebrae of Eromangasaurus have small foramina (holes) on the ventral surface.
The body of Eromangasaurus was probably broad, with the paddle-like fore and hind limbs being equal in size. The bones in the middle of the limbs are generally flat and disk-like, while those towards the end are hourglass-like in shape.
Possible elasmosaurid tooth and flipper bones collected from Richmond's Free Fossil Hunting Sites.
Palaeobiology
It is possible that Eromangasaurus fed on a range of small creatures. This may have included bottom-dwelling invertebrates such as bivalves, gastropods, crinoids and crustaceans. The remains of these creatures have been found inside the fossilised stomach contents of Queensland elasmosaurids, along with small amounts of fish material and belemnites. Having a varied diet may have helped Eromangasaurus survive when specific food sources became depleted within the Eromanga Sea.
As with other plesiosaurs, Eromangasaurus probably swallowed gastroliths or stomach stones. Gastroliths may have been used to grind up food in the stomach, as the teeth of Eromangasaurus were not adapted for crushing hard objects. Eromangasaurus may have also swallowed gastroliths for buoyancy control. Some Queensland elasmosaurids contain gastroliths originating over 300 kilometres away from their source, suggesting that these slow-swimming creatures travelled long distances during their lives.
The neck of Eromangasaurus was probably held straight in front of its body while swimming forward, but was capable of moderate downwards and sideways movement while turning and feeding. Eromangasaurus may have used its neck to capture prey by stealth, reach inside tight spaces or attract potential mates.
References
McHenry, C. R., Cook, A. G. and Wroe, S. 2005. Bottom-Feeding Plesiosaurs. Science 310: 75. Thulborn, T. and Turner, S. 1993. An elasmosaur bitten by a pliosaur. Modern Geology 18: 489-501.
Kear, B. P. 2007. Taxonomic clarification of the Australian elasmosaurid genus Eromangasaurus, with reference to other austral elasmosaur taxa. Journal of Vertebrate Paleontology 27: 241-246.
Zammit, M., Daniels, C. B. and Keat, B. P. 2008. Elasmosaur (Reptilia: Sauropterygia) neck flexibility: Implications for feeding strategies. Comparative Biochemistry and Physiology Part A 150: 124-130.