Trace Fossil - Evolution

Evolution

The earliest complex trace fossils, not including microbial traces such as stromatolites, date to 2,000 to 1,800 million years ago. This is far too early for them to have an animal origin, and they are thought to have been formed by amoedae. Putative "burrows" dating as far back as 1,100 million years may have been made by animals which fed on the undersides of microbial mats, which would have shielded them from a chemically unpleasant ocean; however their uneven width and tapering ends make a biological origin so difficult to defend that even the original author no longer believes they are authentic.

The first evidence of burrowing which is widely accepted dates to the Ediacaran (Vendian) period, around 560 million years ago. During this period the traces and burrows basically are horizontal on or just below the seafloor surface. Such traces must have been made by motile organisms with heads, which would probably have been bilateran animals. The trace observed imply simple behaviour, and point to organisms feeding above the surface and burrowing for protection from predators. Contrary to widely circulated opinion that Ediacaran burrows are only horizontal the vertical burrows Skolithos are also known. The producers of burrows Skolithos declinatus from the Vendian (Ediacaran) beds in Russia with date 555.3 million years ago have not been found, they might have been filter feeders subsisting on the nutrients from the suspension. The density of these burrows is up to 245 burrows/dm2. Some Ediacaran trace fossils have been found directly associated with an body fossils. Yorgia and Dickinsonia are often found at the end of long pathways of trace fossils matching their shape. The feeding was performed in a mechanical way, supposedly the ventral side of body these organisms was covered with cilia. The potential mollusc related Kimberella is associated with scratch marks, perhaps formed by a radula, further traces from 555 million years ago appear to imply active crawling or burrowing activity.

As the Cambrian got underway, new forms of trace fossil appeared, including vertical burrows (e.g. Diplocraterion) and traces normally attributed to arthropods. These represent a “widening of the behavioural repertoire”, both in terms of abundance and complexity.

Trace fossils are a particularly significant source of data from this period because they represent a data source that is not directly connected to the presence of easily-fossilized hard parts, which are rare during the Cambrian. Whilst exact assignment of trace fossils to their makers is difficult, the trace fossil record seems to indicate that at the very least, large, bottom-dwelling, bilaterally symmetrical organisms were rapidly diversifying during the early Cambrian.

Further, less rapid diversification occurred since, and many traces have been converged upon independently by unrelated groups of organisms.

Trace fossils also provide our earliest evidence of animal life on land. The earliest arthropod trackways date to the Cambro-Ordovician, and trackways from the Ordovician Tumblagooda sandstone allow the behaviour of these organisms to be determined. The enigmatic trace fossil Climactichnites may represent an earlier still terrestrial trace, perhaps made by a slug-like organism.