Figure 1.
Map showing location of sites mentioned in text.
The entire peninsula is composed of flat-lying beds of the Broome Sandstone (Early Cretaceous).
Figure 2.
Cretaceous sauropod tracks and their potential rack-makers.
A, silhouette of Diamantinasaurus, a titanosaur or related sauropod from the Winton Formation (Albian-Cenomanian) of Queensland (after Hocknull et al. [40]); scale bar indicates 1 metre. B, silhouette of Brachiosaurus (after Farlow [19]); undescribed skeletal fragments of a 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 distinctive form of sauropod track from the Trinity Group (Early Cretaceous, Comanchean) of Texas and Arkansas; after Farlow et al. [38]); long suspected to be the track of the contemporary brachiosaur Pleurocoelus, but more recently attributed [58] to Paluxysaurus, a relative of Brachiosaurus; length of the pes print ranges from 40–50 cm to more than 100 cm. E, a sample of sauropod tracks from the Broome Sandstone, Western Australia, to illustrate their diversity in size and shape; three isolated pes prints (at left) and three manus-pes couple (at right) are shown at uniform scale; scale bar (extreme left) is 1 metre.
Figure 3.
Variation in colour of Broome Sandstone and its sauropod dinosaur tracks.
A, freshly-exposed and conspicuous example of a pes (hindfoot) print; the thinly layered sediments are characteristic of lagoonal substrates in the Broome Sandstone, though the vivid coloration is often subdued by weathering; scale is 1 ft (c. 31 cm) wooden ruler. B, pes print impressed in, and filled by, blue-grey siltstone; examples such as this are difficult to detect when sea-water has evaporated from the erosion pits along the interface between cast and mould; scale indicated by camera lens cap (diameter 6.7 cm) at lower left.
Figure 4.
Part of a sauropod trackway in thinly-bedded lagoonal deposits of the Broome Sandstone.
The deep footprints have controlled the development of fractures traversing the shore platform. Note the deep near-vertical walls of the prints.
Figure 5.
Variation in topographic expression of sauropod tracks in the Broome Sandstone.
A, a shallow dish-like recess in exposed bedding plane (concave epirelief); the footprint’s filling is slightly more susceptible to erosion than the surrounding rock. B, with sediment filling being eroded at about same rate as the surrounding rock surface (see also Figure 3). C, footprints filled and capped by erosion-resistant filling persist as pedestals while less durable surrounding rock has been removed by erosion. All footprints shown are between 30 and 40 cm in length.
Figure 6.
Interior of a deeply impressed sauropod pes print.
Note the flat floor, representing the footprint sensu stricto, and the steep boundary wall (largely in shadow) which curves over at the top to merge into the undisturbed bedding plane. The smoothly curved transitions between the floor, the wall and the bedding plane make it extremely difficult to identify an objective limit for the footprint’s extent.
Figure 7.
A scattering of sauropod tracks endows this bedding plane with an undulating appearance.
The exact size of the individual prints is difficult to determine on account of their smoothly rounded margins. Note how the distribution of footprints has controlled the development of fractures through this paper-thin sheet of rock.
Figure 8.
Lateral and superficial disturbances of substrate caused by impact of sauropod feet.
A, shallow dish-like print surrounded by extensive ripple-like disturbances. The actual footprint (impressed directly by underside of the track-maker’s foot) has probably been lost to erosion, but nevertheless these sub-surface features convey a good idea of the extent to which impact of a sauropod’s foot could disturb the surrounding substrate. Note the very faint ripple-like disturbance at extreme left. B, undersurface of rock slab that formerly overlay and filled a sauropod pes print. Large oval feature at centre is that footprint’s rock filling (natural cast). The gutter surrounding it indicates that the original footprint was encircled by a raised rim of displaced sediment. Evidence of a second but much smaller print (the manus?) is at upper right, partly concealed by adherent rock. As the surface of this overturned slab is convex, it must have overlain a substrate that was concave - as explained diagrammatically in Figure 9. The regular dimpled texture results from two intersecting sets of ripple-marks. 10 cm scale to right of pes print.
Figure 9.
Series of diagrams explaining origin of the specimen shown in Figure 8B.
A, sauropod footprint impressed into substrate; the footprint, a natural mould (concave epirelief) is bordered by a raised rim of displaced sediment. B, the footprint mould lies at the centre of a larger depression, apparently a zone of subsidence or down-warping created by the impact of the track-maker’s foot. C, the area is buried by an influx of sediment which fills the footprint mould to form the natural cast. D, much later, after lithification, the two layers of rock are separated by natural breakage and erosion. E, the upper layer is overturned by waves to expose its convex lower surface with the footprint cast surrounded by a gutter. Smaller features in Figure 8B (manus print and ripple-marks) are omitted for the sake of clarity.
Figure 10.
Manus-pes couple impressed into ripple-marked surface and surrounded by a raised rim of displaced sediment.
Slightly obscured by modern wind-blown beach sand. The small shelf to right is a remnant of the shallow manus print, which was partly overtrodden and obliterated by the much bigger and deeper pes print. Scale indicated by geological hammer at lower right.
Figure 11.
Thinly-layered lagoonal sediments pushed up into a rounded fold by a deeply-impressed sauropod footprint.
Figure 12.
Sauropod pes prints each with raised rim of displaced sediment (arrowed) at its leading edge.
Example in foreground is viewed from behind; example in mid-background shows the raised rim at front of pes (larger puddle) tilted forwards to overhang rear part of manus print (smaller puddle).
Figure 13.
Crater-like area of subsidence produced by impact of sauropod feet.
This example is situated high up on the beach, where extensive subaerial erosion has destroyed traces of sauropod track(s) in the interior - except possibly for some remnants (rounded notches) at extreme left. The flat interior and the steep boundary wall are characteristic features.
Figure 14.
Basin of deformed substrate formed by sauropod trampling.
A, the flat interior of the basin with shallow sauropod tracks, each outlined by a rim of displaced sediment. B, another view of the same example, showing curvature of underlying beds. C, closer view of one end, showing the steep boundary wall, overlapping pes prints and one clearly-defined manus print (centre foreground).
Figure 15.
Sauropod footprints in thinly-layered sediments responsible for ‘onion-ring’ effect in broken or weathered specimens.
A, example broken obliquely, showing extent of laminations enclosing the entire footprint. B, example broken horizontally, revealing typical ‘onion-ring’ pattern of laminations surrounding the footprint.
Figure 16.
Transmitted reliefs and their effects on the preservation and appearance of sauropod footprints.
A, vertical section of thinly-layered substrate showing transmitted reliefs stacked beneath a sauropod footprint (natural mould). B, same example buried by influx of sediment which fills the footprint to form its natural cast. C, following lithification (transformation of soft wet sediments to hard dry rock), erosion to level of dotted line will produce the ‘onion-ring’ pattern which is characteristic of many sauropod footprints in the Broome Sandstone - a remnant of the natural cast encircled by the eroded edges of transmitted reliefs (Figures 3,15). D, the natural cast proves more durable than the surrounding rock, which is removed by erosion to leave a rock pillar - a stack of transmitted reliefs capped and protected by a remnant of the natural cast (Figures 5B,C and 21). Further erosion undercuts the upper part of the stack (red arrows). E, the upper part of the rock pillar breaks free and is rolled over by wave action, finally coming to rest on the beach as a ‘turtle-back’ boulder (Figures 18C,D).
Figure 17.
Sauropod tracks showing extent of transmitted reliefs and effects of erosion.
A, near-vertical section through a sauropod pes print with stack of transmitted reliefs beneath it. As this section is through the comparatively shallow lateral part of footprint, it does not reveal the maximum depth of the transmitted reliefs (which lie beneath the deeper medial part of the print). The erosion-resistant capping of ironstone is 61 cm long. B, part of trackway containing the same footprint (at centre left), viewed obliquely from above. Note how durable ironstone capping has protected the footprints while the intervening areas of rock have been eroded more rapidly.
Figure 18.
Transmitted reliefs: some examples of unusual appearance and preservation.
A, thin sheets of sandstone moulded into the form of transmitted reliefs and exposed by natural erosion. The long sheet of sandstone in the background is a fragment of transmitted relief from a large basin-like feature (Figures 20–22); it has been rolled over by wave action to expose its convex sole and one very conspicuous transmitted relief of a single sauropod pes print. B, a characteristic turtle-back boulder - the core of a durable footprint cast, encased in transmitted reliefs, which has been freed by erosion and come to rest on the beach. Scale is 1 foot (c. 31 cm). C, a double turtle-back boulder comprising the cores of two footprint casts. D, a basin of transmitted relief with figure-8 outline; derived from the stacks of transmitted reliefs below two near-coincident footprints.
Figure 19.
Hierarchy of transmitted reliefs: the basic elements.
Two sauropod footprints, each underlain by its own stack of transmitted reliefs, are enclosed in a single larger basin of transmitted reliefs. Scale is 1 foot (c. 31 cm), but tilted and foreshortened. This specimen encapsulates the basis of hierarchical pattern - two stacks of transmitted reliefs nested into a single larger basin of transmitted reliefs.
Figure 20.
Hierarchy of transmitted reliefs: a saddle-shaped basin.
A,B, two views of single saddle-shaped basin of deformation containing residual stacks of transmitted reliefs from two sauropod footprints. The two photographs were taken on different occasions and in the interim a storm removed some of the obscuring beach sand.
Figure 21.
Hierarchy of transmitted reliefs: an entire sauropod trackway.
A trough of deformed substrate extending from upper right to lower left betrays the route taken by a sauropod dinosaur. Arrows indicate the steeply dipping flanks of the trough; vertical pointers identify much-eroded stacks of transmitted reliefs representing individual pes prints. Scale indicated by 1 ft (c. 31 cm) ruler at lower right. This complex pattern of substrate deformation cannot be detected by conventional search for pristine (‘museum-grade’) footprints on an intact bedding plane; it is revealed only in broken and eroded specimens which are often deemed to be of inferior quality.
Figure 22.
Basins and channels produced by the impact of sauropod feet.
A, short stretch of coast with evidence of much sauropod traffic; along the seaward margin two large basins, resembling shallow synclines, are separated by an eroded area resembling a minor anticline. B, closer view of the larger basin shown above; note remnants of distinct sauropod footprints at extreme left.
Figure 23.
Part of shore platform viewed from cliff-top at low tide.
A, crest of a ‘monoclinal’ fold flanking a trough trodden down by sauropod dinosaurs (near side, with remnants of numerous footprints among the puddles of sea-water); beds on far side of the crest are folded down to a lower level than those on the near side. B, crater-like area of subsidence shown in Figure 13. C–D, area trodden by sauropods (but largely concealed by rubble); there is no definite border corresponding to A–A. E, a smaller ‘monoclinal’ fold, with correspondingly small trough to near side. F, wide but shallow basin containing sauropod tracks (resembling that in Figure 22, but broken into two parts). G, another basin with sauropod tracks, about to be inundated by the rising tide.
Figure 24.
Composite panorama of James Price Point, about 60 km north of Broome.
Looking northwards from the southern side, at evening low tide, 18∶46, July 24 2009. A, elevated area of flat-lying beds, trodden only by small bipedal dinosaurs (see Figure 25), not by sauropods. B, corresponding elevated area. C, concave sloping flank of the low-lying area trodden down by sauropod dinosaurs; the slope has been slightly exaggerated by erosional undercutting, collapse and slipping, but is nonetheless the curved flank of a basin or trough (see Figure 27). D, arcuate end of a basin (strike of the bedding indicated by dotted line). E, end of one long water-filled channel (extending from mid-left) representing a thoroughfare or heavily trampled route used by sauropods; the landward part of shore is obscured by rubble and sand. F, flat-lying beds exposed at the core of a low-amplitude ‘anticlinal’ fold which intervenes between the water-filled channels G and H; this represents a less-heavily trodden area between two major dinosaurian thoroughfares. Although the terrain has been somewhat reduced by modern erosion, it still conveys a reasonably faithful impression of the Early Cretaceous topography.
Figure 25.
Thoroughfares and troughs produced by sauropod dinosaurs.
A, residual hummock or ‘anticlinal’ fold of lagoonal sediments lying between two dinosaurian thoroughfares (with axes indicated by dashed lines). The thoroughfares are so deeply trodden that they have exposed the underlying beds - red palaeosols (weathered grey) with vestiges of sauropod tracks; south of James Price Point. B, a similar but smaller feature at James Price Point, at the very margin of the lower-lying areas shown in Figure 24. The two water-filled areas at left and right have been trodden down by sauropods to leave an ‘anticlinal’ fold between them.
Figure 26.
The middle of a dinosaurian thoroughfare, thoroughly trampled by sauropods.
Examples such as these, to the south of James Price Point, tend to be ephemeral, as the thinly-bedded rock is rapidly stripped away and broken up during the annual cyclone season. A few moderately large (30–35 cm) three-toed tracks of predaceous theropod dinosaurs (ichnogenus Megalosauropus) have been found in these severely trampled areas, but the somewhat smaller three-toed tracks of plant-eating ornithopod dinosaurs (e.g. ichnogenus Wintonopus, in Figure 28) appear to be completely absent.
Figure 27.
The curved flank of a dinosaurian thoroughfare.
The area shown here is at the margin of the elevated region A in Figure 24. Transmitted reliefs of sauropod tracks are visible in foreground.
Figure 28.
Left pes print of small ornithopod dinosaur, cf. ichnogenus Wintonopus.
Tracks of this type are found on the elevated areas of the shore at James Price Point (e.g. A,B in Figure 24), but not in the lower-lying areas that were trodden by sauropods. It is tempting to suppose that these smaller dinosaurs preferred higher ground, thereby avoiding the heavy traffic of sauropods.
Figure 29.
Crumpled bedding - the result of trampling by sauropods.
Previous reports of contorted bedding in the Broome Sandstone may well be based on similar occurrences. Individual sauropod footprints are still discernible, despite the severe trampling.
Figure 30.
Sauropod pes print, cf. ichnogenus Brontopodus, in silicified carpet of plant debris overlying red palaeosol.
This non-layered substrate does not register any transmitted reliefs. Note conspicuous traces of claws along the lateral edge of the print.