Photo Caption: A type of branching burrow system that first appear at the base of the Cambrian (circa 545 million years before the present). The trace consists of a series of curved open tunnels that extended into the muddy sea floor. The tunnels were later filled with sand and the mud weathered away resulting in a cast of burrow system. The producer of this trace fossil is not known but these trace fossil nevertheless are important in that they mark the beginning of the sea floor being churned by sediment processing animals. Compared to the much simpler trace fossils in older rocks they also bear witness to the appearance of more complex animal behavior. The figured specimen is from the Lower Cambrian of Sweden. The length of each curved element is about 7 mm.
Photo Caption: A trace fossil made on the top of the sediment surface. The trace is about 1-2 mm in width. This is a common form found in Late Proterozoic sedimentary rocks. The figured specimen is from Flinders Range, South Australia.
Study suggests macroscopic bilaterian animals did not appear until 555 million years ago
The traces left behind by ancient animals may hold the key to determining when macroscopic bilaterians -- animals that are symmetric about a central axis, with a body divided into equivalent right and left halves, and with an anterior-posterior polarity (e.g., this includes worms, ants, and ranging up to humans) -- first appeared. A team led by Dr. Mary Droser, professor of geology at the University of California, Riverside, studied "trace" fossils, e.g., burrows, trails and tracks left behind by the earliest bilaterian animals. Results from their study suggest that bilaterian animals did not appear until approximately 555 million years ago.
The authors publish their findings in a paper entitled "Trace fossils and substrates of the terminal Proterozoic-Cambrian transition: Implications for the record of early bilaterians and sediment mixing" in the Proceedings of the National Academy of Sciences (PNAS). They report that these trace fossils, found in many different locations around the world, were preserved very well in sediment beds from the Early Cambrian (544 to 510 million years ago), both in terms of quality of detail and in preserving traces made close to this sediment-water interface. Trace fossils can shed light on an organism’s behavioral activity.
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