Monday, March 8, 2010

New Ediacaran Trace Fossils

ResearchBlogging.orgA paper in Geology has just reported a new type of trace fossils from the Ediacaran of Mistaken Point, Newfoundland.

This is interesting because until now trace fossil diversity in the Ediacaran has been very limited, with only three recognised traces fossil types:

1) Helminthoidichnites
The most common from, composed of simple groove traces with levees. Probably formed in the topmost 10mm of the sediment. They are commonly preserved as negative epireliefs or negative hyporeliefs, indented into the bottom of the overlying beds. Sediment is commonly displaced to form marginal raised ridges. There were probable tubes indicating organisms that may have been round. Some directional meandering is evident.

A form with fine ridges, arranged in fans and associated with Kimberella in a few instances. Broadly analogous to mollusc radula-like grazing. Kimberella occurs at the apex of the fans and appears to have scraped bio-material from the sediment with a proboscis.

3) Resting traces, particularly Dickinsonia
Evidence of serial mat "feeding" dissolution by creeping mat-like animals. These are outlines of organisms that have been impressed into algal mats covering the sediment, often in association with similar-sized body fossils. These are often found as positive epirelief - sticking out from the under side of the bed, as apposed to normal Dickinsonia body fossils which are found as negative epirelief - an indentation into the bottom of the bed.

The latest finds from Newfoundland have been found on the top of green mudstone overlain by a volcanic tuff which has protected the traces. If should be noted that the preservation at Mistaken point differs from most Ediacaran sites in that the fossils are preserved on the top of beds, under a volcanic tuff which blanketed the forms when alive and protected them. Most Ediacaran fossils occur on the base of coarse sandstone beds which smothered the organisms (see An Introduction to the Ediacaran Fauna).

Over 70 straight traces, ranging from 1.5 to 17.2 cm in length and up to 13 mm in width have been found. The surfaces of the traces are marked by regular crescentic internal divisions, formed by thin ridges of siltstone with a spacing of approx. 1 mm.
Locomotion trace from Mistaken Point Formation, Newfoundland.
A) Largest observed trail on bedding plane. B–D are
close-up images of crescentic internal divisions in A. B) Distal end of
trail. Note pyrite crystals embedded in ash surrounding trail. C) Central
section of trail. D: Proximal section of trail with terminal circular
impression. Scale bars = 1 cm.

Each trace typically bears marginal ridges which the authors claim provides key evidence for movement of an object along the surface of the sediment, and can be used to distinguish trace fossils from abiogenic structures. At the far end of several specimens, a negative circular impression can also be seen, which the authors interpret as the mould of the trace maker itself.

The largest trace on the trace-bearing bedding plane, Mistaken Point Formation, Mistaken Point Ecological Reserve, Newfoundland, Canada. Note the clear terminal disc (at right), prominent marginal levees indicating displacement of sediment, and the positive internal crescentic ridges.

The authors interpret the trace as being made by a cnidarian-like organisms similar to Urticina, a modern sea anemone.

Modern actinian (Urticina) trails in mud, produced in our
marine aquaria. Note concave-forward hemispherical structures
(at left) and positive marginal ridges (right).
Scale bar = 3 cm.

This is a big claim - that a cnidarian-grade organism was crawling around the Ediacaran. Of course a number of people have been claiming that this level of organisation was around then, but this would be an important step in supporting evidence.

There are a few issues however.

These traces are extremely rare, and are not found in other locations. An explanation for that is the differing preservatonal styles, but even so, some similar finds would be expected. Also a number of finds of tube-like remains have been found along with the 'stitch and groove' pattern shown by the Mistaken Point forms, and have been interporeted as body fossils. So there is still some uncertainty here. Of course, finding a sea-anemone-like form at the end of one of these trails would be nice, but there is still a lot we need to understand about stitch and groove forms before we can say with any degree of certainty that these forms were made by cnidarian-grade organisms

Dickinsonia trace:

Liu, A., Mcllroy, D., & Brasier, M. (2010). First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point Formation, Newfoundland Geology, 38 (2), 123-126 DOI: 10.1130/G30368.1