Impact of Sauropod Dinosaurs on Lagoonal Substrates in the Broome Sandstone (Lower Cretaceous), Western Australia
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Impact of Sauropod Dinosaurs on Lagoonal Substrates in the Broome Sandstone (Lower Cretaceous), Western Australia Tony Thulborn* Kenmore, Queensland, Australia Abstract 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. Citation: Thulborn T (2012) Impact of Sauropod Dinosaurs on Lagoonal Substrates in the Broome Sandstone (Lower Cretaceous), Western Australia. PLoS ONE 7(5): e36208. doi:10.1371/journal.pone.0036208 Editor: Andrew A. Farke, Raymond M. Alf Museum of Paleontology, United States of America Received January 12, 2012; Accepted March 28, 2012; Published May 25, 2012 Copyright: ß 2012 Tony Thulborn. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The author has no funding or support to report. Competing Interests: The author has declared that no competing interests exist. * E-mail: [email protected] Introduction [12,13], the Broome Sandstone remains the principal source of information about dinosaurs in this region of the globe. Before the 1990s there was very little evidence of dinosaurs in The most abundant and conspicuous of the Broome Sandstone the western half of Australia. That vast geographic region, roughly dinosaur tracks are those of sauropods (Figure 2), members of the equivalent in area to the western half of the continental USA, had clade Sauropoda, which included the biggest terrestrial animals of produced only a few reports of some three-toed dinosaur tracks in all time and are familiar to most people in the form of huge sandstone beds at Gantheaume Point (Minyirr), near the town of quadrupedal plant-eaters such as Apatosaurus (popularly known as Broome [1,2], in the remote Kimberley region of Western Brontosaurus) and Diplodocus [14,15]. The sauropod tracks in the Australia. Subsequently the sandstones at Gantheaume Point Broome Sandstone are the first and only examples recorded in the were designated the type section of the Broome Sandstone unit Australasian region. Persistent reports of a sauropod footprint in and were estimated to be of Early Cretaceous age (probably the Walloon Coal Measures (Middle Jurassic, Bajocian) of Valanginian, c. 130–135 My). The near-horizontal beds of the Queensland (e.g.[16–19]) are erroneous. Some of them derive Broome Sandstone underlie the whole of the Dampier Peninsula, from a misreading of a catalogue of fossil reptiles in Queensland to the north of Broome (Figure 1), but there are few inland [20] whereas others refer to a ‘putative’ example which bears no exposures and the unit is accessible mainly in a string of headlands resemblance to any known sauropod track and was originally and rocky foreshores along the peninsula’s western coast [2–7]. By attributed to a stegosaur [21]. That putative example clearly the 1990s it was apparent that those coastal exposures of the originated from an ornithischian dinosaur of some sort and is Broome Sandstone contain a rich dinosaurian ichnofauna, definitely not the work of a sauropod [22]. including the tracks of sauropods, theropods, ornithopods and There are no such uncertainties about the sauropod tracks of quadrupedal ornithischians provisionally identified as thyreophor- the Broome Sandstone, which in some cases would qualify as ans (armoured dinosaurs, perhaps stegosaurs) [8–11]. Ongoing textbook examples. The sauropod tracks have been reviewed in research has revealed at least 16 distinct morphological types of preliminary fashion elsewhere [9], and this present report is dinosaur tracks in the Broome Sandstone, some referable to concerned not so much with the tracks themselves as with some of existing ichnotaxa and others certainly indicative of new ones. By the remarkable sedimentary structures associated with them. Full- world standards this is an outstandingly rich and diverse grown sauropods were big animals, sometimes estimated to have dinosaurian ichnofauna, and as sites elsewhere in Western weighed 30–60 tons, or even more [14,15,19], and it is not Australia have yielded only a few fragments of dinosaur bone surprising that they should have left all manner of disturbances in their wake, not just footprints. However, the extent to which a PLoS ONE | www.plosone.org 1 May 2012 | Volume 7 | Issue 5 | e36208 Substrates Deformed by Cretaceous Dinosaurs Figure 2. Cretaceous sauropod tracks and their potential rack- makers. A, silhouette of Diamantinasaurus, a titanosaur or related Figure 1. Map showing location of sites mentioned in text. The sauropod from the Winton Formation (Albian-Cenomanian) of Queens- entire peninsula is composed of flat-lying beds of the Broome land (after Hocknull et al. [40]); scale bar indicates 1 metre. B, silhouette Sandstone (Early Cretaceous). of Brachiosaurus (after Farlow [19]); undescribed skeletal fragments of a doi:10.1371/journal.pone.0036208.g001 similar sauropod are also known to occur in the Rolling Downs Group of Queensland; scale bar indicates 1 metre. C, right manus-pes couple (at right) and D, part of a trackway (at left), of Brontopodus birdi,a walking sauropod might deform the ground beneath its feet seems distinctive form of sauropod track from the Trinity Group (Early never to have been investigated very thoroughly. In fact, there is Cretaceous, Comanchean) of Texas and Arkansas; after Farlow et al. barely a mention of this subject in even the most comprehensive [38]); long suspected to be the track of the contemporary brachiosaur studies of the sauropod fossil record [19,23]. By good fortune Pleurocoelus, but more recently attributed [58] to Paluxysaurus,a relative of Brachiosaurus; length of the pes print ranges from 40–50 cm many sauropod tracks in the Broome Sandstone are preserved and to more than 100 cm. E, a sample of sauropod tracks from the Broome exposed in a such a way that they show very clearly the patterns of Sandstone, Western Australia, to illustrate their diversity in size and disturbance and deformation that might be created by a walking shape; three isolated pes prints (at left) and three manus-pes couple (at sauropod. Some of those patterns appear to be unprecedented in right) are shown at uniform scale; scale bar (extreme left) is 1 metre. their size and structural complexity, and they reveal that doi:10.1371/journal.pone.0036208.g002 sauropods, like living elephants, were instrumental in remodelling the topography of their habitats. microplankton, miospores and bivalves [6,25,26]. Plant remains are diverse and abundant, including araucarian conifers, cycads, Palaeoenvironmental setting ferns, bennettitalean seed-ferns and lycophytes, though there is no The sauropod tracks and sedimentary structures described below evidence of angiosperms or gingkos [27–29]. All the palaeonto- were observed in exposures of the Broome Sandstone at sites along logical evidence indicates that the sediments of the Broome the western coast of the Dampier Peninsula, which extends Sandstone accumulated during the Early Cretaceous, with more northwards from Broome for a distance of about 200 km precise estimates of age having ranged from Berriasian through (Figure 1). Only one of those scattered exposures, Gantheaume Valanginian to Hauterivian (i.e. the ‘Neocomian’ of older Point, near Broome, reveals a stratigraphic section thicker than 12– literature). On balance the Broome Sandstone ichnofauna seems 13 m, but borehole samples indicate that the Broome Sandstone most likely to be of Valanginian age (c. 130–135 My) and, thus, attains a maximum thickness of at least 300 m [4,6,24,25]. The base roughly contemporaneous with the Wealden dinosaur faunas of of the unit is seen only in borehole cores and its upper boundary is southern England and Germany, but distinctly more ancient than always an erosion surface. The Broome Sandstone is composed the better-known dinosaur faunas of Early Cretaceous age (Aptian- entirely of clastic sedimentary rocks, mainly fine-grained to coarse- Albian) in Queensland and Victoria [7,30]. Dinosaur faunas of this grained sandstones, with subordinate siltstones and occasional particular age are poorly represented in the southern hemisphere conglomerates. There are also rare seams of pure white porcellanite, and otherwise unknown in the Australasian