Something told me I'd picked the wrong day to check out the view from the top of the Eiffel Tower . . .
Wednesday, February 2, 2011
The type section for the Cambrian-Precambrian boundary is at Fortune Head, Burin Peninsula, Newfoundland, as is marked by the first appearance of the trace fossil Treptichnus pedum (the trace fossil formally known as Phycoides pedum).
Treptichnus pedum is a distinctive burrow pattern comprised of a series of lobes set along a central, sometimes curved burrow, thought to represent successive probes into the sediment searching for food. The traces are thought to represent an organism more complex that those of the Ediacaran, but that is not necessarily true, as no actual animal has been found is association with the burrows, so we don't know what made them (although modern priapulid worms make similar burrows).
However, we should not confuse the somewhat complicated pattern of Treptichnus pedum with the first occurrence of well-developed, fairly complex metazoan animals.
While the start of the Cambrian is marked by the first appearance of Treptichnus pedum, this is not the first appearance of trace fossils probably from metazoans. A number of trace fossil types exist in the preceding Ediacaran rocks. In fact they show a neat line in increasing complexity from simple traces early in the Ediacaran to traces as complex as T. pedum towards the end of the Ediacaran.
The earliest traces we have, and the most simple, is Planolites
This is a lowermost Cambrian specimen (picture from NCSE), but it shows you what they are like. The fossil is of simple, cylindrical, unbranched and unlined burrows. They are usually sinuous and undulatory, and often appear as small knobs or discontinuous segments on bedding planes. The small image below is what they typically look like in Ediacaran rocks
Further up in the Ediacaran, more traces appear. This is Helminthopsis
Helminthopsis are much more meandering burrows that appear to represent shallow feeding burrows. the distinctive whorls and loops are considered to represent a different feeding strategy to that of Planolites.
Towards the top of the Ediacaran, things get a bit more complex. As well as Planolites and Helminthopsis, we get the feeding traces of Kimberella.
Here, the feeding traces are marked "R" for radula, and the animal itself is marked "K". A Dickinsonia is also present. "K'" and "D'" mark the resting trace of Kimberella and Dickinsonia, which comprise, of course, another form of trace fossil.
Towards the top of the Ediacaran, things start to get crowded. Not only do we have all the previous traces, but they are joined by Mattaia miettensis.
This specimen comes from the Kessyusa Formation, from Khorbusuonka in northern Siberia. The trace is filled with two bands of sediment, raised on the sides and depressed in the middle, where the bands are separated by a vertical fissure filled with clay. A possible culprit is a pripulid worm such as the Burgess Shale form Louisella pedunculata.
So by the time you get to the Cambrian-Precambrian boundary there were quite a few traces around. Sure in the Cambrian the traces get much more diverse, but they don't start at the boundary, there is a steady increase in trace fossils through the Ediacaran.
Aleksey Nagovitsyn has kindly informed me that the age range for Mattaia miettensis in Palaoeporn 22 is incorrect, and Mattaia miettensis is actually to be found in the Cambrian Tommotian Stage of the Cambrian.
So we need to remove Mattaia miettensis from the list of trace fossils found at the top of the Ediacaran.