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Shark tooth or
The three types of
ostracod valve observed.
(see text for explanation)
The Langport Member of the Lilstock Formation is the topmost part of the Triassic Penarth Group and is composed of mainly limestones and marls which change abrubtly to the dark laminated shales of the Pre-planorbis beds at the base of the Blue Lias formation. At the top of the Langport Member a transgression is believed to have occured that ended the pattern of deposition dominated by near-shore facies with intermittent periods of sub-aerial exposure, replacing them with offshore facies of the Lias.
These beds are are exposed in a number of locations in South Wales, South Gloucestershire, Somerset and Devon, each locality recording a slightly different pattern of deposition linked to its position within the basin. I've looked at the Aust outcrop in South Gloucesterhire, and some of those on the Somerset coast. The Aust outcrop is high in the cliff, and it's only possible to examine fallen material on the beach. The literature says that the Langport member is missing here, the topmost beds of the Penarth Group being the Cotham Member. At the top of this, marking the boundary with the Pre-planorbis beds, is purportedly a narrow band of limestone containing ripped-up flakes of the underlying beds that represents the start of a transgression. Indeed, blocks of limestone of that description can be found quite readily on the beach.
In Somerset, there is not such an obvious unconformity, or erosion surface. At the outcrop we're concerned with here (several miles to the east of that sampled in Swift (2003)) the top of the Langport member is marked by an approx 150mm thickness of marl containing abundant shell debris. Near the base of this is a thin limestone band, and at the top, the base of the laminated shales of the Pre-planorbis beds. The marl sits on a limestone bed the interface with which is uneven and seems to represent an erosion surface that could correlate with the Aust one. I've found reading about this especially interesting, so thought I'd take a closer look! The log at the base of this piece is from measurements I took at the outcrop. I also managed to get a sample of the marl at the top of the Langport Member from a fallen block, and was able to process it to extract microfossils that yield some information about the environment at the time of deposition.
This piece illustrates the microfossils I found, though the photos aren't especially brilliant due to the optical limitations of my microscope and photographic equipment. I've also briefly outlined the stratigraphical context both in the text, and with a diagram and some photos. However, there's a lot more I'd like to say about that, but need to do some more reading, and looking at the outcrops, first.
The sample came from the area marked "marl with thin limestone band near base" on the log below. This is sandwiched between the dark laminated shales of the Pre-planorbis beds, and the irregular top of a limestone bed. The thin limestone band about 20mm thick is around 40mm above this irregular surface, and the sample included material from either side. I've described it as marl on the basis that it fizzes vigorously in 10% HCl, though that may be due to the amount of shell debris present. Perhaps biocastic mudstone would be another suitable description. The sample was covered in hot water with a tablespoon of Calgon added, then left to soak for a few days. It was then put through a 1mm, then a 0.25mm sieve, the material retained on the latter being dried in the oven on a low heat. The process was then repeated three more times. It was then possible to pick specimens using a low-power stereo microscope.
Specimens, in the form of ostracods, were relatively prolific. When the material was thinly scattered over a 3cm x 3cm area there was usually at least one ostracod valve present. Also, abundant shell debris, and echinoderm spines. A single shark tooth, or possibly dermal denticle, was found, as well. Some elongate black fibres were present that I assume must be plant debris. Altogether around 150 ostracod specimens (valves or whole carapaces) were picked,and these constituted the only group of specimens identifiable to genus or species level. Besides the previously mentioned finds, Other unidentifiable fragments were present in the sample. In particular, some small spheres, that I assume are pyrite (I'll have to read more about that). However, the material in the fallen block was weathered, and could have been contaminated by contemporary material.
The photos on the right illustrate some of the fossils found. The tooth (or dermal denticle), and echinoid spines are self-explanatory. The ostracods, however, need further explanation. They are crustaceans whose carapace is formed of two valves. Either individual valves, or the complete carapace are preserved as fossils. Identification of the specimen is made based on the morphology of those components; that can include things like size, shape or ornamentation, and includes detail on both the internal and external part of each valve. The difficulties for an amateur such as myself are getting the specimens clean so that particles don't obscure fine detail. Also, as I've a fairly budget-priced microscope, the optics aren't exactly brilliant so that can hinder resolving fine detail. This is exacerbated by problems with depth of field when photographing specimens. The final hurdle, is in getting literature to make an identification. Despite that, I have been able to take a stab.
The ostracod valves observed can be split into three main categories that are illustrated in the sketch, on the right, towards the bottom of the page. The first type is roughly oval, and has ridges on the outside of the valve, including along a portion of the margin. The second has no surface ornamentation, and is an elongate oval shape. The last one, again, has no ornamentation and is a more round oval shape. To confuse things, there are various degrees of each of these basic descriptions, and identification is not necessarily clear cut.
Swift (2003), and Swift et al. (1999) were used to identify the specimens. The one with the ridges appears to be Cytherelloidea praepulchella. A photo is shown to the right at the top -- you can just make out a peripheral rib, though the others aren't so clear. The sketch at the bottom illustrates how it appears under the microscope by eye. Several of these were found. The most common specimen is shown in the second photo. Ogmoconchella bristolensis, I think. It corresponds to the second type in the previous paragraph. Several of the third type were found, too, which I think is Cytherella plattensis.
Those identifications are rather tentative. It's also pointing out that Swift (2003) revises the naming of certain of the species, but I don't want to take up any more space with that here. Another interesting point, is that in that same paper two species of Eucytherura are described from a little further along the coast. I didn't see any of those in the material I sampled, or at least none were identifiable. They are smaller, so that may have been a contributary factor; indeed, they may have passed through the 0.25mm sieve.
So, what to make of the evidence? Some of the ostracods were preserved as whole carapaces, suggesting they were actually living in the sediment -- transportation would have disarticulated them. In fact, even when disarticulated, most of the valves were undamaged. The shell debris indicates a high energy environment. The presence of echinoderm spines suggests normal salinity. So, the ostracods were likely living in the sediment, where they were protected from currents or wave action, in a near-shore setting. This suggest a transitional environment between the tidal flats of the Cotham Member and the offshore deposition of the Blue Lias Formation.
Log from measurements at outcrop showing Langport Member and base of the Pre-planorbis Beds.
The outcrop. The ruler (170mm long) sits on the eroded limestone bed, its top is in the base of the laminated shales.
The top of the Langport member showing the shell debris.
Benton, M.J et al. 2002. Permian and Triassic Red Beds and the Penarth Group of Great Britain, Geological Conservation Review Series, No.24, Joint Nature Conservation Committee, Peterborough.
Swift, A. 2003. An ostracod fauna from the Upper Langport Member (Penarth Group: Rhaetian, Upper Triassic) near Watchet, west Somerset (UK), including two new species of Eucytherura and Cythelloidea praepulchella n. nom. Journal of Micropalaeontology, 22: 127-136.
Swift, A. et al. 1999. Fossils of the Rhaetian Penarth Group. The Palaeontological Association.