The rocks of the Winton Formation in Queensland, Australia, date to around 97-95 million years ago (MYA), which was during the very earliest part of the Late Cretaceous. At this time, a large but shallow inland sea was in the process of slowly retreating from the land, as sea levels dropped and the Australian continent was slowly uplifted. Few good dinosaur skeletons have been discovered in the Winton Formation sediments. The best fossils are of sauropods, the largest of dinosaurs. These include several specimens that appear to be a species of Austrosaurus, and "Elliot", which may also be related to these animals. The reason that only large dinosaurs are known from these deposits is because of the slow rate of erosion in much of outback Australia.
Erosion is a double-edged sword for palaeontologists. On the one hand, it uncovers new fossils so they can be discovered. On the other hand, once the fossils are uncovered those same erosional forces begin to wear away at the fossils themselves. If erosion is too fast, then the window of opportunity between the fossils being uncovered and their destruction may be small. In a continent as large and unpopulated as Australia, being in the right place at the right time can be next to impossible. Ironically, slow erosional forces can be more disastrous for palaeontologists than faster rates of erosion. Australia's rate of erosion in inland areas is extremely slow, which means that fossils are not uncovered quickly. Instead they tend to spend a lot of time in the soil zone between the surface and the underlying rock. While they are there, chemical and physical forces may break the fossils up into small unrecognisable pieces before they are uncovered at the surface. By the time fossils are visible, they have often been destroyed beyond repair.
Large bones, like those of sauropods, tend to fare better. Even if a sauropod bone is broken up into many pieces, those individual pieces may still be fist-sized and recognisable as fossil bone. It was once thought that good dinosaur material would never be recovered from the Winton Formation because of the slow rate of erosion. However, with the success of the "Elliot" dig, other known sites with tiny bone fragments scattered about the surface may be revisited to see whether anything more substantial is still preserved deeper below ground.
The Age of Dinosaurs dig around the Elliot site has since yielded the scattered remains of small theropods, the first such skeletal remains from these deposits. Small dinosaurs are not well represented from Queensland sites, since erosional forces usually destroy such tiny fragile bones. At least two well-preserved (and tiny) claws were recovered over just two days of the dig.
There are other clues as to the type of dinosaurs that lived along side (and usually beneath) the enormous sauropods. Lark Quarry is a site 113 km south west of Winton, and boasts what may be the world's only footprint site resulting from a dinosaur stampede. The water level of the stream seems to have dropped, exposing a large expanse of mudflats besides a stream leading into a lake. The tracks of hundreds of small bipedal (two-legged) dinosaurs have been preserved, all of them running quickly across this ancient mudflat. The generally accepted theory is that small ornithopods and theropods were spooked by the presence of a single large theropod about 10 metres (33 feet) long.
I have another possible scenario; a large theropod may have taken a creche of juveniles down to the water to drink, where they discovered a group of small plant-eaters and did what any young carnivore would - chased them about. Some bird and crocodilian species form creches, which are groups of young offspring from several different parents. One or two adult animals are all that is required to take care of the creche, while other adults go out and look for food. Alternatively, a single adult theropod may have looked after its own numerous offspring by itself, although it is difficult to imagine a single female laying enough eggs to account for the number of small theropod tracks known at Lark Quarry.
Beneath the tracks of the main stampede are some older traces. These were made by medium-sized dinosaurs swimming in the lake and kicking the bottom at irregular intervals. There is also a larger trackway of an ostrich-sized ornithopod. This trackway has been partially eroded, probably by running water as the stream bed drained dry. The site reveals that there were more than just sauropod dinosaurs living in Queensland at the time. A whole dinosaur ecosystem, from the smallest plant eaters to large predators, called the shores of the inland sea home.
The "Elliot" site itself reveals other clues to the world of the Winton Formation. Many of Elliot's bones have fossilised plant remains stuck to them, indicating that the animal probably died at a bend in a river where plant material was deposited on a sand bar. So far the remains of conifer cones and scales, cycads, ferns, angiosperms (flowering plants), ginkgoes (maiden hair trees), and horsetails (like modern scouring rushes) have been found. Some of the conifer remains are almost identical to the Wollemi pine (left), that still exists in isolated pockets in Australia today. It is thought that the area was an open floodplain dominated by scattered trees, a transitional forest between seed-fern dominated plants and flowering plants.
Freshwater bivalves have also been found at the site, known scientifically as Prohyria macmichaeli. They resemble modern freshwater mussels, that still live in rivers, streams and lakes in Australia today. They were preserved in the same position as they would have been in life, indicating they were covered up quickly by sediments that smothered them to death. The same flooding event that covered up and killed the mussels probably also covered "Elliot's" body, preserving it so that it could become fossilised.
Only a few kilometres from the "Elliot" site is another scattering of surface fossils known as the Lungfish Site. The remains of lungfish, small sharks, more mussels, and scattered dinosaur material have been found here. The large number of aquatic animals in the area attests to the large annual rainfall, which has been estimated at a metre or more a year. The landscape was probably dotted with lakes and bisected by rivers as the water headed for the nearby sea shore.
Gurnis, M., R.D.Müller & L.Moresi 1998 Cretaceous Vertical Motion of Australia and the Australian-Antarctic Discordance. Science 279:1499-1504
Thulborn, R.A. & M.Wade 1984 Dinosaur trackways in the Winton Formation (mid-Cretaceous) of Queensland. Memoirs of the Queensland Museum 21(2): 413-517
Wade, M. & R.E.Molnar 2000 Winton Dinosaur Trackways. Queensland Museum Leaflet
Catalyst television program transcript. Digging Elliot (Part 1) Thursday, 22 August 2002
Catalyst television program transcript. Elliot's World (Part 2) Thursday, 29 August 2002