NORTHERN SUBURBS MAY MEETING REPORT
The first scientific report on Broome’s dinosaur footprints was published in our club’s journal in 1952, (The Western Australian Naturalist, Vol. 3, p82-83). Mr Ludwig Glauert, Director of the WA Museum, reported the discovery by Walter Jones of Broome in 1945 of large impressions “shaped like giant emu tracks”, approximately 13 inches (33 cm) long, in the Broome sandstone at extreme low spring tide near the Point Gantheaume Lighthouse.
So it was very fitting to have as our May speaker Mr Rob Seggie, to present the results of his research which has been ongoing since 2005. While the footprints discovered by Walter Jones were those of a theropod dinosaur, now named Megalosauropus broomensis, Rob has concentrated his research on the more circular footprints and other features of the sauropods.
Rob explained that the Broome Sandstone is an Early Cretaceous rock formation, deposited about 140 million years ago. It outcrops sporadically over 300 km, both north and south of Broome. One of its characteristics is its amazing lack of ‘normal’ sedimentary structures. Instead it exhibits widespread disturbance which is interpreted as single or superimposed footprints and other depressions left by large sauropods as they trekked across the soft sediments of ancient deltas and coastal plains. This form of bioturbation has been labelled ‘dinoturbation’.
We were shown photos of sauropod footprints from a number of locations, with the largest footprint being 1.5 m across. Other researchers have found them up to 1.7 m across. As a comparison, adult elephant footprints are only 0.3 m in diameter.
In some places, sufficient tracks are exposed to allow the stride length of individuals to be measured—1.4 m footprints were made by a sauropod moving with a 4 m stride, while a half-metre footprint still had a 3 m stride, which could indicate the sauropod was galloping.
The depth of penetration depended on the weight of the sauropod and the hardness of the surface it was crossing.
Some foot withdrawal features are up to 2 m deep (above, R Seggie)
Rob puzzled for a long time on why the footprints of the back legs were always circular but those of the
front feet were often semi-circular (above, R Seggie). After examining profiles of footprints in exposed cliffs, it occurred to him that as sauropods probably had a diagonal gait, the front footprint—while initially circular—was deformed by the mud that was squeezed forward when the hind leg, on the same side, sank into the mud on its next step (below, R Seggie). A larger example of the squeezing of mud could be the shallow anticlines and synclines (folds) observed along the coast at numerous locations. One theory for their formation is that, over time, the weight of successive herds of sauropods following the same pathways squeezed the liquefied mud into gentle folds.
Profile of back foot (left), squeezed rollover feature (centre) and disturbed front foot (right). (R Seggie)
Rob concluded his talk by showing how, using the knowledge he gained from examining the footprints on exposed surfaces, he was able to interpret dinoturbation in shallow engineering cores at a known footprint site and extrapolate this to deep cores cut over 3 km below the sea in wells drilled for oil and gas.
A summary of other research on the Broome Sandstone dinosaurs can be found in Appendix 2 of the WA Department of State Development’s report:
Palaeontology Survey of the Broome Sandstone-Browse LNG Precinct Report