Kronosaurus queenslandicus - credit Ms Lisa Wallace.
Etymology
The genus name Kronosaurus, meaning ‘Kronos lizard’, was named after the Greek titan Kronos. Kronos committed many savage acts, including eating his own children and castrating his father Uranus. The species name queenslandicus is named after Queensland where the type specimen was discovered.
Relationships
Kronosaurus queenslandicus belongs to an extinct group of marine reptiles known as the Plesiosauria, commonly referred to as plesiosaurs. With over 100 recognised genera from around the world, most plesiosaurs generally had short or long necks, broad bodies, four flippers and short tails. The first plesiosaurs evolved over 205 million years ago in the Late Triassic, with different forms diversifying in the Jurassic and Cretaceous. The last known plesiosaurs perished during a mass extinct event at the end of the Late Cretaceous 65 million years ago.
Within the Plesiosauria, Kronosaurus belongs to a group known as the Pliosauridae. Pliosaurs like Kronosaurus which had long heads, large teeth, short necks and large bodies are generally placed within the Thalassophonea (Greek for ‘sea slayer’). Members of the Thalassophonea were the dominant predators of marine environments throughout the Middle Jurassic to the start of the Late Cretaceous.
Kronosaurus is a member of the Brachaucheninae, the dominant group of thalassophoneans to survive into the Cretaceous. From the Early Cretaceous, the Brachaucheninae includes the primitive form Makhaira from Russia, ‘Brachauchenius’ sp., ‘Kronosaurus’ boyacensis from Colombia and Kronosaurus queenslandicus from Australia. From the Late Cretaceous, the Brachaucheninae includes Polyptychodon interruptus from multiple European countries, P. hudsoni from the USA, Megacephalosaurus eulerti from the USA and Brachauchenius lucasi from the USA and Morocco. The disappearance of the Brachaucheninae from the fossil record coincides with the emergence of large predatory mosasaurs.
While the closest relative of Kronosaurus queenslandicus appears to be ‘Kronosaurus’ boyacensis, there are a number of significant differences between the two forms (e.g. number of premaxillary teeth, humerus/femur length ratio, vertebral count, etc.). The revision of such features is required to determine whether ‘Kronosaurus’ boyacensis belongs in a separate genus.
Left: reconstruction of the skull of Kronosaurus queenslandicus (modified from McHenry 2009). Right: reconstruction of the skull of ‘Kronosaurus’ boyacensis (modified from Hampe 1992).
Discoveries
Compared to the remains of other large vertebrates from the Eromanga Sea, fossils of Kronosaurus queenslandicus are relatively rare. Over a dozen substantial skeletal specimens are known from Richmond, Hughenden, Julia Creek and Boulia. Isolated or fragmentary material has been found elsewhere in Queensland, New South Wales and South Australia.
The first remains of Kronosaurus queenslandicus to be discovered was the holotype specimen - a fragmentary mandibular symphysis collected by Mr Andrew Crombie in 1899 from an unknown locality near Hughenden. This new form was described by Mr Heber Longman in 1924, who correctly noted affinities between Kronosaurus and pliosaurs from the northern hemisphere.
Perhaps the most famous specimen of Kronosaurus queenslandicus was discovered by Mr Ralph Thomas in 1926 from Army Downs Station near Richmond. This specimen, consisting of parts of the skull, vertebral column, girdle and limb bones was collected by the American Mr William E. Schevill from the Museum of Comparative Zoology of Harvard University in 1931-32. The limestone encasing the specimen was exploded into manageably-sized pieces with dynamite and shipped to Harvard University. Taking over 25 years to prepare, heavily weathered bones were coated with plaster to enhance the specimen for display. A large sagittal crest was added to the top of the skull, which is now known to be inaccurate. These alterations earned the specimen the nickname ‘Plasterosaurus’. Several artificial backbones were added to the vertebral column, increasing the length of the specimen to 12.8 metres from approximately 10.5 metres. This more realistic size still places ‘Plasterosaurus’ as one of the largest known specimen of Kronosaurus found to date.
Another large Kronosaurus specimen was found by Mr Marlin Entriken in 1978 from Toronto Park Station near Richmond. The specimen, collected by Dr Alan Bartholomai for the Queensland Museum, currently resides at Kronosaurus Korner. The articulated skeleton represents the most complete specimen of Kronosaurus queenslandicus on display in Australia.
The best preserved mandibles of Kronosaurus queenslandicus were discovered by Mr Robert Hacon in 2014 on Euraba Station near Nelia. Mr. Hacon noticed parts of the mandibles lying on the ground while he was poisoning prickly acacia. He uncovered the rest of the specimen by scraping away layers of soil with a loader. The specimen is on display at Kronosaurus Korner along with several other Kronosaurus specimens.
Left to right: Mr Robert Hacon discovering parts of the Euraba Kronosaurus; uncovering more of the specimen with a loader; and sitting with the specimen at Kronosaurus Korner.
Geology
Most specimens of Kronosaurus queenslandicus have been discovered from units within the Rolling Downs Group of Outback Queensland. This includes material from Late Aptian Doncaster Member of the Wallumbilla Formation and the Late Albian Toolebuc Formation and Allaru Mudstone. Dr Ralph Molnar suggested that Kronosaurus material from the Doncaster Member (circa 115 million years ago) could represent a different species to specimens from the Toolebuc Formation (circa 100 million years ago) . However, Dr Colin McHenry suggested that taphonomy could be responsible for differences between these specimens
Description
Based on research by Dr Colin McHenry, Kronosaurus queenslandicus was a relatively large pliosaur, measuring approximately 10-11 metres in length and weighing 11,000 kilograms. In comparison to other thalassophonean pliosaurs, it would have been surpassed in size by some species of Pliosaurus. Typical of other thalassophoneans, Kronosaurus has a large head with elongate jaws. The largest recorded Kronosaurus skull measures 2.2 metres in length with a mandible stretching 2.7 metres long. Estimated bite forces for the skull of Kronosaurus arestimated to be rces for the skull of Kronosaurus some form of small fin. up to 30,000 newtons — approximately twice as powerful as a large saltwater crocodile. Simulations suggest that Kronosaurus was capable of simple ‘open and shut’ biting, but was poorly suited for twisting prey.
The teeth of Kronosaurus are anisodont – being uneven in shape and size throughout the jaws. Enormous fang-like teeth within the maxilla can measure up to 30 cm from the crown tips to the bottom of the roots. Teeth are mostly conical (cone-like) in shape, although some specimens are slightly recurved towards the tip. The surfaces lack carinae (sharp edges) and are covered around the circumference by prominent longitudinal ridges. Some Kronosaurus teeth exhibit wear from feeding on hard objects or rubbing against other teeth.
The neck of Kronosaurus is short but was relatively flexible, perhaps an adaptation for hunting small evasive prey. The compact body is fusiform, being tapered towards both ends and streamlined in shape. Gastralia (abdominal ribs) supporting the stomach would have kept the body firm. It is unknown whether the relatively small tail had some form of small fin as postulated for some plesiosaurs.
Four strong, paddle-like limbs would have propelled Kronosaurus through the water. The front limbs were outsized by hind limbs, which could span approximately two metres in diameter.
Left to right: Kronosaurus tooth found by Mr Mike D'Arcy; broken Kronosaurus tooth with a worn tip found by Dr Tim Holland; and a section of Kronosaurus mandible found by Mr Robert Hacon.
Palaeobiology
Kronosaurus queenslandicus was a predator with a varied diet. The fossilised stomach contents of some specimens contain the bones of other marine reptiles such as small sea turtles and elasmosaurids. Shark vertebrae have also been preserved with a Kronosaurus specimen, possibly indicating a predator-prey relationship. It is likely that Kronosaurus also ate smaller prey items such as teleost fish and cephalopods.
Bite marks from Kronosaurus are also present on several fossils from the Rolling Downs Group. Possible tooth marks have been reported on the gladii of the squid-like Boreopeltis soniae, while the holotype specimen of Eromangasaurus also displays crushing and punctures from a probable Kronosaurus bite. The bones of some sub-adult specimens of Kronosaurus queenslandicus display possible bite marks from their own species – potential evidence of intraspecific aggression or cannibalism.
Some Kronosaurus specimens contain gastroliths (rounded stones) in the stomach area. These stones may have been swallowed to control buoyancy or process food in the digestive system (Kronosaurus lacked teeth capable of chewing its food into small pieces). It is also possible that gastroliths were accidentally swallowed from the sea floor while Kronosaurus attempted to feed on other animals.
Based on evidence from other plesiosaurs, Kronosaurus would have given birth to live young underwater. Baby Kronosaurus would have been birthed headfirst to prevent drowning before swimming to the surface to breath. Similar to modern whales, Kronosaurus was adapted for life at sea and would be immobilised underneath its own weight if stranded.
References
Fischer, V., Arkhangelsky, M. S., Stenshin, I. M., Uspensky, G. N., Zverkov, N. G. and Benson, R. B. J. 2015 Peculiar microphagous adaptations in a new Cretaceous pliosaurid. Royal Society Open Science 2: 150552.
McHenry, C. R. 2009. ‘Devourer of the Gods’ The palaeoecology of the Cretaceous pliosaur Kronosaurus queenslandicus. PhD thesis/dissertation, University of Newcastle.