Impact of Sauropod Dinosaurs on Lagoonal Substrates in the Broome Sandstone (Lower Cretaceous), Western Australia

PLOS ONE, Dec 2019

Existing knowledge of the tracks left by sauropod dinosaurs (loosely ‘brontosaurs’) is essentially two-dimensional, derived mainly from footprints exposed on bedding planes, but examples in the Broome Sandstone (Early Cretaceous) of Western Australia provide a complementary three-dimensional picture showing the extent to which walking sauropods could deform the ground beneath their feet. The patterns of deformation created by sauropods traversing thinly-stratified lagoonal deposits of the Broome Sandstone are unprecedented in their extent and structural complexity. The stacks of transmitted reliefs (underprints or ghost prints) beneath individual footfalls are nested into a hierarchy of deeper and more inclusive basins and troughs which eventually attain the size of minor tectonic features. Ultimately the sauropod track-makers deformed the substrate to such an extent that they remodelled the topography of the landscape they inhabited. Such patterns of substrate deformation are revealed by investigating fragmentary and eroded footprints, not by the conventional search for pristine footprints on intact bedding planes. For that reason it is not known whether similar patterns of substrate deformation might occur at sauropod track-sites elsewhere in the world.

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Impact of Sauropod Dinosaurs on Lagoonal Substrates in the Broome Sandstone (Lower Cretaceous), Western Australia

Western Australia. PLoS ONE 7(5): e36208. doi:10.1371/journal.pone.0036208 Impact of Sauropod Dinosaurs on Lagoonal Substrates in the Broome Sandstone (Lower Cretaceous), Western Australia Tony Thulborn 0 Andrew A. Farke, Raymond M. Alf Museum of Paleontology, United States of America 0 Kenmore , Queensland , Australia Existing knowledge of the tracks left by sauropod dinosaurs (loosely 'brontosaurs') is essentially two-dimensional, derived mainly from footprints exposed on bedding planes, but examples in the Broome Sandstone (Early Cretaceous) of Western Australia provide a complementary three-dimensional picture showing the extent to which walking sauropods could deform the ground beneath their feet. The patterns of deformation created by sauropods traversing thinly-stratified lagoonal deposits of the Broome Sandstone are unprecedented in their extent and structural complexity. The stacks of transmitted reliefs (underprints or ghost prints) beneath individual footfalls are nested into a hierarchy of deeper and more inclusive basins and troughs which eventually attain the size of minor tectonic features. Ultimately the sauropod trackmakers deformed the substrate to such an extent that they remodelled the topography of the landscape they inhabited. Such patterns of substrate deformation are revealed by investigating fragmentary and eroded footprints, not by the conventional search for pristine footprints on intact bedding planes. For that reason it is not known whether similar patterns of substrate deformation might occur at sauropod track-sites elsewhere in the world. - Before the 1990s there was very little evidence of dinosaurs in the western half of Australia. That vast geographic region, roughly equivalent in area to the western half of the continental USA, had produced only a few reports of some three-toed dinosaur tracks in sandstone beds at Gantheaume Point (Minyirr), near the town of Broome [1,2], in the remote Kimberley region of Western Australia. Subsequently the sandstones at Gantheaume Point were designated the type section of the Broome Sandstone unit and were estimated to be of Early Cretaceous age (probably Valanginian, c. 130135 My). The near-horizontal beds of the Broome Sandstone underlie the whole of the Dampier Peninsula, to the north of Broome (Figure 1), but there are few inland exposures and the unit is accessible mainly in a string of headlands and rocky foreshores along the peninsulas western coast [27]. By the 1990s it was apparent that those coastal exposures of the Broome Sandstone contain a rich dinosaurian ichnofauna, including the tracks of sauropods, theropods, ornithopods and quadrupedal ornithischians provisionally identified as thyreophorans (armoured dinosaurs, perhaps stegosaurs) [811]. Ongoing research has revealed at least 16 distinct morphological types of dinosaur tracks in the Broome Sandstone, some referable to existing ichnotaxa and others certainly indicative of new ones. By world standards this is an outstandingly rich and diverse dinosaurian ichnofauna, and as sites elsewhere in Western Australia have yielded only a few fragments of dinosaur bone [12,13], the Broome Sandstone remains the principal source of information about dinosaurs in this region of the globe. The most abundant and conspicuous of the Broome Sandstone dinosaur tracks are those of sauropods (Figure 2), members of the clade Sauropoda, which included the biggest terrestrial animals of all time and are familiar to most people in the form of huge quadrupedal plant-eaters such as Apatosaurus (popularly known as Brontosaurus) and Diplodocus [14,15]. The sauropod tracks in the Broome Sandstone are the first and only examples recorded in the Australasian region. Persistent reports of a sauropod footprint in the Walloon Coal Measures (Middle Jurassic, Bajocian) of Queensland (e.g.[1619]) are erroneous. Some of them derive from a misreading of a catalogue of fossil reptiles in Queensland [20] whereas others refer to a putative example which bears no resemblance to any known sauropod track and was originally attributed to a stegosaur [21]. That putative example clearly originated from an ornithischian dinosaur of some sort and is definitely not the work of a sauropod [22]. There are no such uncertainties about the sauropod tracks of the Broome Sandstone, which in some cases would qualify as textbook examples. The sauropod tracks have been reviewed in preliminary fashion elsewhere [9], and this present report is concerned not so much with the tracks themselves as with some of the remarkable sedimentary structures associated with them. Fullgrown sauropods were big animals, sometimes estimated to have weighed 3060 tons, or even more [14,15,19], and it is not surprising that they should have left all manner of disturbances in their wake, not just footprints. However, the extent to which a walking sauropod might deform the ground beneath its feet seems never to have been investigated very thoroughly. (...truncated)


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Tony Thulborn. Impact of Sauropod Dinosaurs on Lagoonal Substrates in the Broome Sandstone (Lower Cretaceous), Western Australia, PLOS ONE, 2012, Volume 7, Issue 5, DOI: 10.1371/journal.pone.0036208