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.

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 Produçao Mineral. Brazil.

Tuesday 8 May 2012

Laridae vs Columbidae - guess who won.

We took a trip out yesterday to Chester and the North Wales coast. Having never visited Chester before I was looking forward to seeing the famous medieval Rows and visit the cathedral. The Rows were fascinating, the cathedral we didn't get to see as it cost 6 quid to get in if you weren't a worshipper; as we're not religious we departed.

What we did get to see was slightly less aesthetically pleasing to the eye but equally fascinating. In a nearby area hard to the city walls called the Kaleyards we spotted a Lesser Black-backed Gull attacking and eating a feral pigeon; avian dinosaur predation in an urban centre.

Feral pigeons seek respite from the weather in the lee of the city walls.
The large stones in the bottom of the image are Roman.

This area (named as the spot was where monks grew vegetables in days of yore) has an impressive coop and the area is home a large number of feral pigeons (and the odd duck, not in shot) all of whom use the city walls as shelter from the elements. When we arrived the gull had already disabled the pigeon, and whether it had attacked a healthy bird or picked on an injured or weak one we did't see.

Gull attacking still-alive pigeon.

The pigeon was still very much alive however, and the gull was pecking towards the rear of the bird. After several minutes the unfortunate pigeon gave up the ghost and the gull continued to feed, before a parking motorist scared it off. It waited a while on a nearby lamppost for a while, then flew out of sight and then returned after a few minutes and continued to feed. Lesser Black-Backed gulls have been noted as predators, and intraspecific predation in colonies of the species have been recorded by Davis et al.

The prey had been dragged a few metres away by now, and had expired.

It was a gully day. We saw Herring gulls nesting on the roof of the railway bridge, Herring gulls nesting in a window seat in a tower at Conwy castle and lotsa gulls in Llandudno, as you would expect. Here's some pics for all admirers of the Laridae out there.

Room with a view: a brooding Herring Gull at the magnificent Conwy Castle.

Adult Herring Gull on the pier at Llandudno. Ever the opportunists, these gulls are
quite bold when scavenging from the food discarded by humans.
Immature Herring Gull, also at Llandudno and keeping a close eye on the activities
of the anglers fishing of the end of the pier.

J.W.F. Davis, E.K. Dunn (1976). Intraspecific predation in Lesser Black-Backed Gulls Larus fuscus. International Journal of Avian Science 118, 65-77.