Dinosaurs - Leaellynasaura amicagraphica

Introduction

Leaellynasaura amicagraphica was a tiny ornithopod from the Early Cretaceous of Victoria (perhaps a juvenile because of its small size). During the Early Cretaceous southeast Victoria lay south of the palaeo-Antarctic Circle, and Leaellynasaura may have had special adaptations to thrive at such high latitudes. Once thought to be a hypsilophodontid, the position of Leaellynasaura within Ornithopoda is currently unresolved.

Identification

Leaellynasaura amicagraphica was described from an upper jaw with teeth, a skull roof with a partial brain endocast, as well as postcranial material. Several isolated upper leg bones (femora) were later referred to Leaellynasaura based on the original postcranial material.

As in hypsilophodontids, and unlike the basal ornithischian Lesothosaurus, the upper teeth of Leaellynasaura are recessed under a lateral process of the maxilla. The maxillary teeth of Leaellynasaura have five equally developed ridges present on both sides of unworn teeth, less than in either Qantassaurus or Atlascopcosaurus, two other small Victorian ornithopods (both of which have at least eight ridges on upper teeth). The teeth of Leaellynasaura also lack the prominent primary ridge seen in Qantassaurus and Atlascopcosaurus. The skull roof referred to Leaellynasaura has enlarged eye sockets and the endocast has enlarged optic lobes, possible adaptations for enhanced vision in low-light conditions. The femur attributed to Leaellynasaura as part of the type series exhibits the primitive feature of the anteroposterior compression of the greater trochanter like that of Lesothosaurus, and additionally is anteroposteriorly compressed distally. Additional femora assigned to Leaellynasauraall have a femur that is mediolaterally compressed distally.

Histological study of long bones referred to Leaellynasaura and at least two other Victorian ornithopod species show that growth was continuous throughout the year. These bones have no LAGs (Lines of Arrested Growth) to indicate either winter hibernation or slowed growth caused by a decreased food supply. Leaellynasaura and other small ornithopods may have therefore been active through most (if not all) of the winter months and, arguably, may have even been warm-blooded.

Habitat

Southern Victoria during the Early Cretaceous was a rift valley formed between Australia and Antarctica as the two continents slowly split apart. Braided streams and rivers formed extensive floodplains. Large fossilized logs found along the Victorian coast suggest nearby forests of evergreen araucarian conifers and podocarps. Deciduous ginkgoes were also important forest trees. The understorey would have included pentoxylaeans, ferns, bryophytes and mosses, and horsetails and reeds would have lined the streambanks.

Victoria lay south of the palaeo-Antarctic Circle in the Cretaceous and, although the global climate was much warmer than it is today, the region would have experienced the extremes of daylight typical of polar regions. Evidence for rafting ice in the Eromanga Sea and studies of cryoturbation structures from the area (deformations in sediments caused by the seasonal freezing and thawing of soils) suggest that the climate may at times have been quite cold, with winter ice and seasonally frozen ground. However, the presence of such species as lungfish and crocodilians in Victoria, now known only from warmer regions, suggests a generally milder climate.

Distribution

Leaellynasaura is known only from Dinosaur Cove on the Victorian coast (Eumeralla Formation, Otway Group).

Feeding and diet

Leaellynasaura was a small herbivore that would have fed on edible low-growing Early Cretaceous plants such as ferns and horsetails.

Other behaviours and adaptations

Small ornithopods were remarkably common in Cretaceous Australia, particularly in southern Victoria, and may have traveled in small herds or flocks. Dinosaur burrows, possibly those of small ornithopods, were recently discovered in Victoria. This suggests that at least some small ornithopods may have spent the coldest months of the year underground.

Fossils description

Leaellynasaura amicagraphica was described from a toothed maxilla (the holotype, NMV P185991); a skull roof with large eye sockets and associated endocast of the brain (NMV P185990); and postcranial material including femora (Rich and Rich 1989). Several isolated femora from Dinosaur Cove were later referred to Leaellynasaura(Rich and Vickers-Rich 1999). As with most of the Victorian Cretaceous material, these fossils were not found in association, and assignment of all of this material to Leaellynasaura amicagraphica is problematic. All material is held by Museum Victoria in Melbourne.

Evolutionary relationships

Leaellynasaura was first assigned to ‘Hypsilophodontidae’, a group of small ornithopod dinosaurs from the Early Cretaceous of Europe whose relationships are uncertain. The position of Leaellynasaura within ornithopods has since been subject to differing interpretations, and is currently under review (as are the relationships of other small Australian ornithopods).

Based on its dentition, Leaellynasaura appears to differ from other small ornithopods from southern Victoria, Qantassaurus and Atlascopcosaurus, both of which are more similar to each other than to Leaellynasaura.

References

• Butler, R. J., Upchurch, P. and Norman, D. B., 2008. The phylogeny of the ornithischian dinosaurs. Journal of Systematic Palaeontology 6(1), 1-40.
• Chinsamy, A., Rich, T. H. and Vickers-Rich, P., 1998. Polar dinosaur bone histology. Journal of Vertebrate Paleontology 18, 385-390.
• Huene, F. von, 1932. Die fossile Reptil-Ordnung Saurischia, ihre Entwicklung und Geschichte. Monograph in Geologie und Paläontologie (Series 1), 1-361.
• Molnar, R. E., 1980b. Australian late Mesozoic terrestrial tetrapods, some implications. Memoirs of the Soc. Geol. Fr. 139, 131-143.
• Molnar, R. E., 1991. Chapter 18: Fossil Reptiles in Australia. Pp. 605-701 in Vickers-Rich, P., Monaghan, J. M., Baird, R. F. and Rich, T. H. (1991) Vertebrate Palaeontology of Australasia. Pioneer Design Studio, Lilydale, Victoria.
• Molnar, R. E. and Galton, P. M., 1986. Hypsilophodontid dinosaurs from Lightning Ridge, New South Wales, Australia. Geobios 19, 231-239.
• Rich, T. H., 1996. Significance of polar dinosaurs in Australia. Memoirs of the Queensland Museum 39, 711-717.
• Rich, T. H. V. and Rich, P. V., 1989. Polar dinosaurs and biotas of the Early Cretaceous of southeastern Australia. National Geographic Research 5, 15-53.
• Rich, T. H. and Vickers-Rich, P., 1999. The Hypsilophodontidae from southeastern Australia. Pp. 167-180 in Tomida, Y., Rich, T. H. and Vickers-Rich (eds) Proceedings of the Second Symposium on Gondwanan Dinosaurs. National Science Museum of Japan, Tokyo.
• Rich, T. H., Vickers-Rich, P. and Gangloff, R. A., 2002. Polar dinosaurs. Science295, no. 5557, 979-980.

Further reading

• Flannery, T. F. and Rich, T. H., 1981. Dinosaur digging in Victoria. Australian Natural History 20, 195-198.
• Long, J. A., 2002. Dinosaurs of Australia and New Zealand and Other Animals of the Mesozoic Era. New South Wales University Press, Sydney.
• Rich, T. H. and Vickers-Rich, P., 2002. Dinosaurs of Darkness. Indiana University Press, Bloomington.
• K. Carpenter (ed), 2006. Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs. Indiana University Press, Bloomington.
• Vickers-Rich, P. and Rich, T. H., 1993. Wildlife of Gondwana. Reed Books, Sydney.