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'Worm' burrow
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Most creatures that lived in the past didn't survive in the form of fossil remains after they died. They rotted, and whatever remnants they left were eroded and scattered. There's nothing now to find. But a few have left traces in other ways - by the impressions and traces they made when they were living. These are known as trace fossils. They're almost always interesting because of the puzzle they pose: what left these marks? Footprints are perhaps the most exciting finds, but for animals without feet the next best thing is the tracks they made as they burrowed or wriggled around. This specimen is a beautiful example, exposed on a cleavage plane in a block of sandstone from the Permian, in the Roding River valley, near Nelson. Though it's not immediately evident from the image, the feature is three-dimensional, and tube-like, not just a shallow impression in the rock. That, and the characteristic lenses that run across the tube suggest that it is a burrow, made by an animal moving through the sediment, rather than a track left on its surface.
The form of the burrow is distinctive and perhaps diagnostic: a meandering, unbranched track, about 6 mm in diameter, lacking a ditinct wall, and filled with cresecent-shaped lenses of darker material (ca 3 mm in maximum diameter), different from the surrounding rock, alternating with thin (0.5-1.0 mm) strands of grey sand. There is faint evidence of lobate disturbance of the sediment on either side of the track, perhaps as a consequence of compression or micro-slumping of the sand as the burrow was created. This is especially marked on the outer side of the curves, where the trace makes an almost right-angle turn (right centre, and left top of the image). The fossil lies on a bedding plane in the sandstone, suggesting that the burrow was horizontal.
The form of the burrow is distinctive and perhaps diagnostic: a meandering, unbranched track, about 6 mm in diameter, lacking a ditinct wall, and filled with cresecent-shaped lenses of darker material (ca 3 mm in maximum diameter), different from the surrounding rock, alternating with thin (0.5-1.0 mm) strands of grey sand. There is faint evidence of lobate disturbance of the sediment on either side of the track, perhaps as a consequence of compression or micro-slumping of the sand as the burrow was created. This is especially marked on the outer side of the curves, where the trace makes an almost right-angle turn (right centre, and left top of the image). The fossil lies on a bedding plane in the sandstone, suggesting that the burrow was horizontal.
Exactly what made the track is uncertain. Not only worms, but many other organisms, including bivalves, gastropods and echinoids, burrowed in the soft sediments on the sea-floor during the Permian. Based on the form of the trace, however, it might be tentatively suggested that it is a Taenidium. First defined by Heer, in 1887, these are described as 'unlined or thinly-lined, cylindrical to subcylindrical straight to sinuous tubes with meniscate backfill'. They tend to be formed parallel or oblique to bedding and tend not to cross-sect other tubes, unless made by different organisms. They are believed to be the feeding burrows of deposit-feeders, and have been interpreted as the creation of annelid worms. Examples have been reported from marine environments, ranging from shallow to deep water, as well as from fluvial enviornments, and they range in age from Cambrian to Pleistocene. An examples illustrated on the informative, though sadly incomplete, KU Ichnology website is shown to the right, for comparison.
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Taenidium - a trace fossil of an annelid worm,
from KU Ichnology |
If this intrerpretation is correct, the dark lenses are probably clumps of faecal material excreted by the creature as it burrowed. As indicated by the shape of the lenses in the infill, the direction of movement in this specimen was probably from right to left on the slab as shown here.
Further Reading:
KU Ichnology: http://ichnology.ku.edu/tracefossils.html
Bann, K.L. et al. 2004 Differentiation of estuarine and offshore marine deposits using integrated ichnology and sedimentology: Permian Pebbley Beach Formation, Sydney Basin, Australia. In: McIlroy, D. (editor) The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, pp 179-211.
KU Ichnology: http://ichnology.ku.edu/tracefossils.html
Bann, K.L. et al. 2004 Differentiation of estuarine and offshore marine deposits using integrated ichnology and sedimentology: Permian Pebbley Beach Formation, Sydney Basin, Australia. In: McIlroy, D. (editor) The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, pp 179-211.