August 26, 2010
When I think of dinosaur bones, the rocky and shrub-flecked expanses of western North America immediately come to mind, but it should not be forgotten that some of the first dinosaurs recognized by science were discovered across the Atlantic in England. Paleontologists have been searching for dinosaurs there longer than anywhere else, and among the localities scientists keep returning to are the Early Cretaceous strata on the Isle of Wight. In fact, paleontologists Steven Sweetman and Allan Insole have recently published a new study in Palaeogeography, Palaeoclimatology, Palaeoecology about what the island was like during the time of the dinosaurs, and it also identifies the unusual mechanism by which the remains of the dinosaurs were trapped in stone.
Part of a larger geologic unit called the Wealden Supergroup, the Wessex Formation contains records of an ancient river valley from a time when the Isle of Wight was a bit further south and dinosaurs indicative of the Jurassic were giving way to groups that became prominent during the Cretaceous. At this time—approximately 130 million years ago—the area experienced a warmer, seasonal climate with both wet and dry seasons, and much of this is known from geologic layers created along the edges of an ancient, winding river. About 4 percent of these deposits consist of plant debris (with vertebrate bones in the same beds) which were thought to have been laid down by regular floods, but in their new study Sweetman and Insole propose a different scenario.
Contrary to previous hypotheses that the area was subject to frequent, seasonal flooding, Sweetman and Insole took the content and pattern of the plant debris deposits to indicate that the area was subject to relatively rare depositional events which moved plant material and animal remains a short distance before settling. In fact, the details of each plant bed appear to indicate that the plant debris was not sloshed about in a large quantity of water, but instead was transported in what is known as a debris flow—a damp slurry of moving plant debris and other organic matter. In the scenario proposed by the authors, forest fires may have removed trees and low-lying plants that would have trapped water and stabilized the soil on hillsides. When heavy rain fell on these areas, logs, plants, and soil material would have spilled down the hill in a wet sort of landslide, picking up dinosaur carcasses and other objects before settling near the riverbank.
While the fossilized remains within the debris beds are a bit messy, by sampling through them paleontologists have been able to pick out a pretty good representation of what kinds of animals lived during the time. As indicated by scraps of plants, the vegetation of the time was a mix of the ancient and modern, with cycads and gingkoes existing alongside conifers and flowering plants. Altogether, the plant community appears to represent a savanna-like environment in which only the occasional stand of conifer trees stood up among large open areas of lower vegetation.
The riverside-savanna habitat the authors reconstruct for the area was apparently very productive, for, in addition to invertebrate remains, the bones of lizards, turtles, crocodiles, small mammals, pterosaurs and numerous kinds of dinosaurs have been found within the debris beds. Ankylosaurs, sauropods, theropods and representatives of other dinosaur groups have been found here, leading the authors to conclude that—at least during times when plants were abundant—the floodplain area was able to support a diverse array of dinosaurs. (It is worth noting, however, that dinosaurs have been found in this area outside of debris beds, meaning that different skeletons became preserved by different means.) When the debris flows came down the hill, they carried the bones from whatever dinosaur carcasses were already present on the hillside, thus creating a representative mix of the animals which modern paleontologists can use to measure the biodiversity of a long-lost habitat.
Sweetman, S., & Insole, A. (2010). The plant debris beds of the Early Cretaceous (Barremian) Wessex Formation of the Isle of Wight, southern England: their genesis and palaeontological significance Palaeogeography, Palaeoclimatology, Palaeoecology, 292 (3-4), 409-424 DOI: 10.1016/j.palaeo.2010.03.055
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