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.
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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)