Protostegid sea turtles - credit Ms Maree Maxwell.
Etymology
Notochelone costata - the genus name Notochelone means ‘back turtle’ in Greek, and the species name costata means ‘ribbed’ in Latin. The combined meaning is ‘ribbed back turtle’.
Cratochelone berneyi - the genus name Cratochelone means ‘strong turtle’ in Greek. The species name berneyi is in honour of Mr Frederic Berney who collected the holotype specimen.
Bouliachelys suteri – the genus name Buliachelys means ‘Boulia turtle’ after Boulia where many specimens were initially recovered. The species name suteri is named after Mr Richard and Mr John Suter who collected many of these specimens.
Relationships
The sea turtles of the Eromanga Sea belong to the Chelonioidea, a group which also contains modern sea turtles. Members of the Chelonioidea have heads that cannot retract into their shells, flattened fusiform (spindle-shaped) bodies and paddle-like flippers. Within the Chelonioidea, the sea turtles of the Eromanga Sea belong to an extinct family knowns as the Protostegidae. Protostegids were among the oldest sea turtles, with the earliest representatives from South America including the 120 million year old Desmatochelys padillai and the 110 million year old Santanachelys gaffneyi. The last protostegids died at the end of the Cretaceous during a mass extinction event, with the leatherback sea turtle being their closest surviving relative.
Discoveries
The first Australian protostegid to be described was Notochelone by the eminent British palaeontologist Sir Richard Owen in 1882. Owen initially used the name Notochelys in describing the holotype specimen, a partial carapace, plastron and limb bones collected from an unknown locality along the Flinders River. However, the name Notochelys was already in use for the extant Malayan flat-shelled turtle. Mr Richard Lydekker subsequently erected the genus Notochelone for this material in 1889. Following this initial research, over 50 specimens of Notochelone have been collected by various Australian museums. Despite Notochelone material representing the most commonly collected tetrapod fossils from the Toolebuc Formation, much of its anatomy has not been described.
The only specimen of Cratochelone was found on Sylvania Station, near Hughenden. These fragmentary remains, including parts of a humerus, radius, ulna, scapula, coracoid, entoplastron and hyoplastron were described by Mr Heber Longman in 1915 and redescribed by Dr Benjamin Kear in 2006. Further fieldwork on Sylvania Station has failed to yield more remains of Cratochelone.
The presence of this third protostegid from the Eromanga Sea was initially suggested by Dr Ralph Molnar in 1991. This was confirmed with the description of Bouliachelys suteri by Dr Benjamin Kear and Dr Michael Lee in 2006 based on cranial specimens collected from Hughenden and Boulia. Despite the purported presence of unique characters from the skull of Bouliachelys, Dr Troy Myers suggested the possible synonymy of Notochelone and Bouliachelys based on morphometric evidence. Further study of Notochelone and Bouliachelys material from Richmond, Hughenden, Julia Creek and Boulia is required to resolve this dichotomy.
Geology
The protostegids of the Eromanga Sea are best known from the Late Albian Toolebuc Formation and Allaru Mudstone (approximately 100 million years old). Material from Kronosaurus Korner has also been collected from the younger Mackunda Formation. These units belong to the Rolling Downs Group of Northwest Queensland.
Description
Protostegids have relatively large heads tipped with strong beaks. In Bouliachelys, the beak is furthermore described as being hooked and serrated.
Similar to modern leatherback sea turtles, the shells of protostegids are reduced compared to those of other sea turtles. Regarding the carapace (upper shell) of Notochelone, a series of small rectangular bones called neurals form a low keel down the midline. Long pointed bones called pleurals originate next to the neurals. The ends of the pleurals insert inside small notched bones called peripherals which form the rim of the shell. As with other turtles, the outer layer of the carapace and plastron (bottom shell) was covered in modified scales called scutes. These scutes were made of keratin and did not fossilise.
The limbs of protostegids were not adapted for deep diving. In primitive protostegids, the digits within the limbs were not completely fused together as in more advanced sea turtles.
The shell of Notochelone measures less than one metre in length, similar in size to a modern green sea turtle. The shell of Bouliachelys is reported to be slightly larger than that of Notochelone. Based on fragmentary material, the shell of Cratochelone is estimated to be four metres in length, similar in size to the gigantic North American protostegid Archelon – the largest recorded sea turtle.
Protostegid remains from Richmond - left to right: skull, hyoplastron, peripheral and humerus.
Palaeobiology
Cratochelone had highly vascular bone tissue indicative of increased metabolic rates. It may have travelled long distances in the open ocean and only occasionally strayed within the confines of the Eromanga Sea. This notion may be supported by the rarity of Cratochelone fossils from the Rolling Downs Group.
The remains of baby protostegids have been found in Richmond’s Free Fossil Hunting Sites. Like their modern relatives, female protostegids probably ventured on land to lay eggs. After hatching and entering the water, baby protostegids were vulnerable to attacks from marine predators. The stomach contents of some Platypterygius specimens contain the remains of baby protostegids.
Adult protostegid remains have been found in association with the bones of Kronosaurus queenslandicus, suggesting a predator-prey relationship. Protostegid fossils from overseas also contain bite marks from sharks.
Some protostegid specimens from Richmond contain Inoceramus clam shells inside their stomach contents. It is thought that these protostegids consumed thin-shelled bivalves through biting with their strong jaws. Possible protostegid coprolites also contain the remains of bivalve shells.
Turtle tucker? Compacted Inoceramus clam shells in cross section.
References
Cadena, E. A. and Parham, J. F. 2015. Oldest known marine turtle? A new protostegid from the Lower Cretaceous of Colombia. PaleoBios 32. ucmp_paleobios_28615.
Gaffney, E. S. 1981. A review of the fossil turtles of Australia. American
Museum Novitates 2720: 1-38.
Kear, B. P. 2006. Reassessment of Cratochelone Berneyi Longman, 1915, a giant
sea turtle from the Early Cretaceous of Australia. Journal of Vertebrate Paleontology 26: 779-783.
Kear. B. P. and Lee, M. S. Y. 2006. A primitive protostegid from Australia and early sea turtle evolution. Biology Letters 2:116-119.
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.