Polar dinosaur tracks of the Wonthaggi Formation (Lower Cretaceous), Victoria, Australia and their palaeontological significance

Abstract

The Wonthaggi Formation (Valanginian-Aptian) in Victoria, Australia has one of the best assemblages of polar dinosaur body fossils in the Southern Hemisphere, but most remains are preserved as disarticulated bones and teeth, representing post-mortem transport in fluvial systems. In contrast, the recent discovery of at least 24 tridactyl dinosaur tracks in the Wonthaggi Formation in the same places as their body fossils shows dinosaurs were living in these environments. Nearly all dinosaur tracks are preserved as isolated structures in marine-platform exposures of overbank and floodplain facies, but with one partial theropod trackway recording two consecutive steps. Based on forms and sizes, most tracks were made by medium to large theropods (0.8–1.9 m hip heights), with only a few small ornithopods (40–48 cm hip heights) represented. Relative abundances of theropod and ornithopod tracks are the inverse of Wonthaggi dinosaur body fossils, which are dominated by ornithopods, yet the ornithopod tracks are the first reported from the Wonthaggi Formation. Two tracks (44 and 47 cm long) are the largest known Early Cretaceous theropod footprints in southern Australia, representing theropod sizes comparable to the Australian megaraptoran Australovenator. These and other >30 cm-long tracks further affirm the presence of large polar theropods previously inferred from scanty skeletal material. Because tracks are preserved in facies adjacent to channel sandstones, they were likely made by theropods and ornithopods traveling through those environments after spring-thaw flooding, such as during polar summers. Anthony J. Martin* [geoam@emory.edu], Department of Environmental Sciences, Emory University, Atlanta, Georgia, 30322, USA; Melissa Lowery [melissalouiselowery@gmail.com], School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, 3800, Australia Michael Hall [mike.hall@monash.edu], School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, 3800, Australia Thomas H. Rich [trich@museum.vic.gov.au], Sciences Department, Museums Victoria, Box 666, Melbourne, Victoria, 3001 Australia Doris E. Seegets-Villiers [dseegetsvilliers@swin.edu.au], PrimeSci! Science Education, Swinburne University of Technology, Wantirna, Victoria, 3152, Australia Peter Swinkels [peterswinkels@optusnet.com.au], School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, 3800, Australia John Broomfield [jbroom@museum.vic.gov.au], Department of Photography and Imaging, Museums Victoria, Box 666, Melbourne, Victoria, 3001 Australia Patricia Vickers-Rich [pat.rich@monash.edu], School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, 3800, Australia.