Kronosaurus queenslandicus

Introduction

Kronosaurus queenslandicus, from the Early Cretaceous of Queensland, was the largest known pliosaurid. These huge marine carnivores plied the oceans and inland seas during the Mesozoic, using both pairs of limbs to 'fly' underwater. Kronosauruswould have hunted large fish, invertebrates such as giant squid and ammonites, and perhaps even other large reptiles like elasmosaurids and ichthyosaurs.

Identification

Plesiosaurs (pliosaurids and elasmosaurids) were secondarily marine tetrapods that evolved from a group of terrestrial sauropterygian reptiles during the latest Triassic. The limbs of plesiosaurs were developed into paddles, and the limb girdles formed large ventral plates akin to the plastron of turtles. Pliosaurids were short-necked with proportionately large heads, while elasmosaurids generally had small heads and long necks (the 'pliosaurid' body plan apparently evolved independently in at least three plesiosaur lineages). The skulls of pliosaurids were massive, leading to overestimates of their total lengths.

Kronosaurus was the largest of the pliosaurids. Its skull was at least 2.4m long (proportionately large for the estimated body length of no more than ten meters). References to a length for Kronosaurus of nearly 13 meters are exaggerated. The teeth of Kronosaurus are conical, crushing teeth like those of pliosaurids that fed on the plentiful, hard-shelled ammonites of Cretaceous seas.

Kronosaurus queenslandicus was most closely related to the only other known species of Kronosaurus, K. boyacensis from the Early Cretaceous of the Boyaca region, northern Colombia.

Habitat

Kronosaurus lived in the cool, high-latitude Eromanga Sea - an inland sea that covered vast areas of inland Australia from 120-90 million years ago (Aptian-Albian-Cenomanian). Glendonites and boulders that may have been rafted by ice are evidence of cold to near-freezing conditions during the austral winter.

Distribution

Kronosaurus queenslandicus was found at Army Downs near Hughenden in north-central Queensland. Isolated teeth referable to Kronosaurus are reported from the Walumbilla Formation, White Cliffs, New South Wales and from the Bulldog Shale, northern South Australia.

Life history cycle

Plesiosaurs propelled themselves through the water using lift-based appendicular locomotion, as in birds, sea lions and sea turtles. However, only plesiosaurs used both forelimbs and hindlimbs in locomotion. Pliosaurids (unlike elasmosaurids) were able to cruise at high speeds for long periods of time.

Fossils description

Kronosaurus queenslandicus was named in 1901 from a jaw fragment with six teeth found near Hughenden, north-central Queensland. This material was originally described as ichthyosaur by Longman (1921) who later revised this as pliosaur (1924). The most well known fossils of Kronosaurus are those of an individual (a skull and most of the skeleton) found at 'Army Downs' near Hughenden in central Queensland by an expedition from Harvard University (1930 -- 1931). This skeleton was taken back to Harvard, where it is still held by the Museum of Comparative Zoology (MCZ). A reconstruction of Kronosaurus incorporating the fossil material is displayed at the MCZ. There has been no complete description or comparisons with the type material in Australia (the type specimen is held by the Queensland Museum in Brisbane). It is therefore possible that the material held by the MCZ will need to be revised.

The skeleton on display at the MCZ (nicknamed 'Plasterosaurus') is one-third plaster, and liberties have been taken in reconstructing its size. The reconstructed skeleton, although impressive, is 12.8 metres long, about three metres too long. The modeled skeleton took almost thirty years to produce, the original bones having been discovered in 1931.

Evolutionary relationships

Plesiosauria (pliosaurs and plesiosaurs) were secondarily marine tetrapods that evolved from a group of terrestrial sauropterygian reptiles during the latest Triassic. Both pliosaurs and plesiosaurs survived until the Late Cretaceous, becoming extinct at the end of the era in the end-Cretaceous extinction event.

Relationships of Kronosaurus are debated, as are the relationships of pliosaurs in general.

References

• Hampe, O. 1992. Ein grosswuechsiger pliosauride (Reptilia, Plesiosauria) aus der Unterkreide (oberes Aptium) von Kolumbien. Courier Forschungsinstitut Senckenberg 145, 1-32.
• Longman, H. A.1924. A new gigantic marine reptile from the Queensland Cretaceous, Kronosaurus queenslandicus new genus and species. Memoirs of the Queensland Museum 8, 26-28.
• Longman, H. A. 1930. Kronosaurus queenslandicus. A gigantic Cretaceous pliosaur. Memoirs of the Queensland Museum 10, 1-7.
• Longman, H. A. 1932. Restoration of Kronosaurus queenslandicus. Memoirs of the Queensland Museum 10, 98.
• Kear, B. P. 2003. Cretaceous marine reptiles of Australia: a review of taxonomy and distribution. Cretaceous Research 24, 277-303.
• Kear, B. P., Schroeder, N. I., Vickers-Rich, P, and Rich, T. H. 2006. Early Cretaceous high latitude marine reptile assemblages from southern Australia. Paludicola 5, 200-205.
• Molnar, R. E., 1991. Chapter 18: Fossil reptiles in Australia. Pp. 605-702 in Vickers-Rich, P., Monaghan, J. M., Baird, R. F. and Rich, T. H. (eds) Vertebrate Palaeontology of Australasia. Pioneer Design Studio, Melbourne.
• Romer, A. S. and Lewis, A. D. 1959. A mounted skeleton of the giant plesiosaur Kronosaurus. Breviora 112, 1-15.
• Sachs, S. 2005. Tuarangisaurus australis sp. nov. (Plesiosauria: Elasmosauridae) from the Lower Cretaceous of northeastern Queensland, with additional notes on the phylogeny of the Elasmosauridae. Memoirs of the Queensland Museum 50, 425-440.
• Thulborn, T. and Turner, S. 1993. An elasmosaur bitten by a pliosaur. Modern Geology 18, 489-501.
• White, T. E. 1935. On the skull of Kronosaurus queenslandicus Longman. Occasional Papers of the Boston Society of Natural History 8, 219-228.

Further reading

• Dinosaurs of Australia and New Zealand and Other Animals of the Mesozoic Era, by J. A. Long (1998). New South Wales University Press, Sydney.
• Wallace, D. R. 2007. Neptune's Ark: from Ichthyosaurs to Orcas. University of California Press, Berkeley.