Monday, 28 May 2012

Neoichnology: bioturbation around ephemeral ponds

Randomly oriented dinosaur track assemblages occur frequently in the fossil record (Thulborn, 1990) and form under a variety of conditions. Thulborn states that these random assemblages can appear to represent a patch of ground with high dinosaur traffic that are passing through, but some sites have been interpreted as being the margins around water holes (Martill and Naish, 2001).

I went to check out a local ephemeral pond situated on farmland to see if there was evidence of bioturbation, and was able to do some photogrammetry on what I found. All images were taken between 1pm - 1.30pm on a bright, sunny day (at last!) in late May.

Ephemeral pond in grass crop grown for silage.

The pond is located in a field used for growing grass for silage, after each cutting (which occurs three times a year) cattle are allowed to graze briefly before being excluded again as the crop resumes growth. The small pond is present during the winter months and occasionally re-appears after extended wet spells over summer, however the pond gradually dries up as spring progresses. The pond appears to be fed by water draining off adjacent fields; no streams fed into the pond and it is situated in a shallow natural hollow.

I have often seen mallards at the site of this pond, and fully expected to find evidence of their presence in the form of footprints and tracks, as this site is close to where I recorded duck tracks in mud alongside a small stream; this photograph is taken from the foot of the levee which separates the stream from the field (this stream feeds into a reservoir around half a mile away and never floods into the adjacent field).

Zones of substrate consistency surrounding the pond.
As can be seen from the photo above there are three possible zones of substrate consistency. The outer margin is dry and hard, and coated with slurry applied by the farmer when wetter conditions persisted. A dried out microbial mat might also be present here. Red Deer footprints are present in this zone and these too were made when the substrate was wetter.  There was one canine trackway which was too big for a Red Fox and probably represents a pet dog. Goose and other bird droppings were present. The middle zone consists of slightly dryer area containing some deer footprints and is covered by a microbial mat which has formed over the decomposing slurry. The layer constituting the pond margin is waterlogged, and like the rest of the site consists of fine-grained sediment. There is no evidence of slurry or microbial mat in this zone. So we have a homogenous substrate with varying consistencies with a surface covered with fibrous, decomposing vegetable material and a microbial mat present in at least one zone.

3D model generated by photogrammetry.
Bioturbation in the pond margin, consisting of monotypic, randomly orientated bird footprints.

The pond margin zone was heavily bioturbated. The tridactyl tracks are randomly orientated, monotypic and all of uniform morphology. It's likely this represents a single species of bird visiting the pond to drink or feed, although no signs of probing appears present in the substrate. The term for a fossil association of tracks preserved in a single horizon is ichnocoenosis (Leonardi, 1987), I so I guess this assemblage would be termed a neoichnocoenosis.

Close-up of untextured mesh which shows the chaotic nature of the bioturbation.
Footprints and partial footprints are overlain
Textured version of the mesh. Note quality of mesh is so good the individual nodes
of each digit are clearly visible in some footprints, especially the central print.
The two images above are close-ups of the mesh, untextured and textured. The print in the middle of the image clearly shows the nodes of each digit, as well as displacement rims and is impressed deeply into the wet substrate.

The tracks are different form the duck tracks recorded in the local area. They are smaller, show no sign of interdigital webbing (as can be seen, the footprint impressions are deep) and there is a large hallux present. However, the prints are large enough to rule out many of the smaller birds to be found in the local area, and it might be that the track maker was a corvid, gull or pheasant, all of which are abundant locally. The pheasant can probably be ruled out as it has a very short hallux (see image below), and no interdigital webbing would rule out gulls as track makers. The most likely candidates are corvids and the pond lies below the route which rooks use twice daily between the local roost and the fields where they feed, and jackdaws, magpies and common crows are all present.

Of course, the prints we're looking at here are concave epireliefs, or moulds. It would be impossible to see the cast (or convex hyporelief) of these prints in the real world, but photogrammetry allows us to look underneath and view the bioturbated surface from below. The ability to look at tracks as moulds with adjustable light angles is a useful tool in footprint morphology interpretation.

The underside of the mesh, showing the tracks as casts instead of moulds.
References:

Thulborn, R.A. 1990. Dinosaur Tracks, 310pp. Chapman and Hall, London.

Martill, D. and Naish, D (Eds). 2001. Dinosaurs of the Isle of Wight, 319pp. The Palaeontological Association, London.

Leonardi, G. 1987. Glossary and Manual of Tetrapod Footprint Palaeoichnology, 43pp. Departamento Nacional de Produ├žao Mineral. Brazil.

2 comments:

  1. The tracks with the long hallux impressions are very likely those of corvids - crows or magpies. BTW, very nice visualizations with the photogrammetry

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  2. Very neat!

    and I just couldn't resist the stupid joke on G+ - sorry

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