Thursday, 20 October 2011

The Other Oxford Clay Fauna

The Oxford Clay Formation in England has yielded a diverse community of marine reptiles and fish for well over 150 years now and has featured in this blog many times, as regular readers will know. What does not get mentioned so often is the fact that there is also a significant invertebrate fauna, trace fossils and, as I have mentioned previously, plant fossils.

The bed that we study most intensely is known as Bed 10 which is Middle Callovian in the Lower Oxford Clay – more familiarly known as the Jason Zone. Bed 10 is at the very base of the clay quarries that have been dug out over recent years and is as far as excavations take place. This is primarily due to the fact that the bed is full of concretions that would easily damage very valuable machinery and interrupt the brick making process.

The other issue would be the fact that further penetration would break into the underlying Kellaways Formation that has no commercial value although this formation has produced some excellent vertebrate fossils where it has been exposed on occasion. The only other excavations into Bed 10 are to dig drainage ditches which help control the water level in the quarries although these are not as common as they once were. This is unfortunate since many excellent finds have been made in and around these ditches over the years.


The most abundant fossils in the clay, that are readily apparent to the naked eye, are the ammonites and these are dominated by the Kosmoceratidae – specifically Kosmoceras jason, from which the zone epithet is derived, but there are others represented such as K. medea and other species such as Sigaloceras. Preservation of ammonites, in these quarries, is often spectacular to look at in the clays and shales but are impossible to collect since they are nearly always crushed flat and are wafer thin. They can occasionally be found in three dimensions in nodules but, again, are almost impossible to extract.

Belemnites are also abundant and, on occasion, are spectacularly preserved. These, in contrast to the ammonites, are easily collected since the rostra fossilise well. Belemnite rostra, like the nodules, also cause problems for the brick makers and will often explode in the curing process so they are always removed where possible.

The most common species are Cylindroteuthis puzosiana and if you are fortunate to visit a quarry after fresh clay extraction, then you can find many undamaged rostra in superb condition. Some are giants, approaching 25cm long, but are more likely to be 15 – 20cm long although you can find some that are as small as 25mm as well. Fragments suggest even bigger specimens exist but these are very elusive and it is apparent that their size and length make them vulnerable to extreme damage.


Cylindroteuthis (left) & Belemnopsis (right). 

Phragmacones are fairly common and, every now and then, you can observe the badly preserved outline of the soft parts at the head of the animal, including the tentacles, in the clay. Hooklets are occasionally found but I personally have yet to see one. The other locally common species, Belemnopsis bessina, is much smaller with the rostra probably reaching a maximum of 75 to 80mm and these are sometimes mistaken for fish bone.

Crustaceans are present in the shales and are meant to be locally abundant, especially at the top of the Jason zone, but I have never seen one until this year when a specimen of Mecocheirus pearcei was recovered. However, just because I have not seen them does not mean they are not there since I am looking for vertebrate remains more often than not.

In my eyes, some of the most remarkable and delicate fossils are those of Genicularia vertebralis, an annelid worm of the class Polychaetia. The “fossils” are actually the casts of the burrows these worms created and are represented by calcified shells. Some are wonderfully preserved, albeit somewhat fragile and the preservation of the ornamentation never fails to delight.

Genicularia

Bivalves are well represented in the quarries and are dominated by Gryphaea dilobotes and these, although generally abundant throughout, can often be found in large dense encrustations. Some can get really big, with some recovered pieces suggesting shells over 100mm long but they are exceptional and most are around half that size with every other size and morphology in between. They are quite fragile as well and need gentle handling when a good specimen is found.

Gryphaea

Other bivalves include Pinna mitis which are often only found as imprints in newly uncovered clay. These are often mistaken for the imprints of fish fins or tails by those who are unfamiliar with them and it is easy to see why. Pinna are elongate fan-like bivalves with multiple, densely packed ribbing and, when crushed, really do look like fish fins. Another bivalve I’ve seen is Trigonia Sp. But this is likely to have been derived from the Middle Oxford Clay.

Plant fossils are not so well known from the clay and yet there is evidence for them throughout the quarries. They are all land plants that have drifted out to sea, most likely carried by rivers in flood that have eventually sunk to the sea floor. Large chunks of wood that have become carbonised can be found all over the place although once exposed to air they quickly crumble and often turn to dust. Some of us have attempted to save a few of the better preserved pieces but it takes vast amounts of consolidant to stabilise them and they need constant review otherwise they are prone to deterioration at a rapid rate.

Whole logs are sometimes uncovered but they are seldom removed, as you may remember from an earlier post. But every now and then, an odd branch or frond may fossilise well and these are extremely rare. My very first visit to Quarry 4, some years ago, revealed a freshly exposed piece of tree bark - the only example I have ever seen. It has taken a lot of maintenance to keep it intact but it is a wonderful piece and displays superb ripple-like marks in the bark.

Tree Bark

There is one other fossil worth a mention and that is that there is an abundance of fish coprolites in the clay. These are often ignored by some who do not recognise them as such and there are those who ignore them anyway but they are missing out in my opinion. The vast majority are small, only 5 to 10mm on average, pretty nondescript and are a dirty white in colour. They tend to be like this because they are largely composed of phosphate derived from digested fish bone.

However, some have quite distinct forms and the bigger ones are much more interesting since closer inspection reveals the remnants of what had been digested and passed through the gut such as small fish bones and remains of crustaceans. Rarest of all are coprolites of the marine reptiles but, every now and then, one is found and the inclusions in these are much larger and, although almost impossible to identify, are suggestive of bone and ammonite pieces, and are quite intriguing.

Coprolites displaying inclusions


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