Critter of the......evening. Liebea.

This is Liebea, a Permian bivalve mollusc similar to modern mussels. They were epibyssate suspension feeders, meaning that they anchored themselves to rock or seaweed and filtered food particles out of the water. Liebea  commonly formed clumps containing only that species.

The fossil above is Liebea squamosa  and is one of the bivalves I found in the Ashfield Brick-clay Pit in Conisbrough. Like the brachiopods found there, the Liebea  fossils appear to be quite small (about half the size they should be) but not to the same extent as the brachiopods. This could be interpreted as having a high infant mortality rate (due to overcrowding or a soft substrate) or small adult size due to overcrowding.

Today's Critter: Dielasma

Today my tutor lent me the book Zechstein Reef Fossils and Their Palaeoecology  by Hollingworth and Pettigrew, so expect more post about the Zechstein in future. Sadly it only covers an area way up near Middlesbrough, but it does give a lot of insight into my own locality (Ashfield brick-clay pit). I'm going to see if I can order it online as it is a useful and potentially cheap resource. It contains some decent information on the different fossils, so I thought I would start with the easiest I identified from Conisbrough and add the info from the book. Here is Dielasma elongatum, a Permian brachiopod which was abundant in the Zechstein:

The brachiopods pictured are bigger than most of the ones I found, which can be seen below:

Many which I found appear to be juveniles or smaller individuals, perhaps suggesting a high infant mortality rate. It may also suggest that the substrate was not firm or that there was crowding and therefore increased competition. The Dielasma specimen in the book is five times bigger than my smallest and around twice the size of my largest. Although I haven't compared yet, this seems to be true of the bivalves I have as well. This would seem to support the idea that there was crowding, though perhaps this did also affect infant mortality.

I won't list the taxonomic details from the book, as those change often and the book was published in the late 80s. For any interested, the description given is:

Shell is oval in outline, pedicle and brachial valves convex, pedicle valve with broad shallow sulcus and brachial valve with corresponding broad fold. Ornament of fine growth lamellae.

Dielasma  was an epifaunal suspension feeder, meaning that it lived on the sea floor (as opposed to within it) and sifted out its food from the water (using an appendage known as a lophophore). They are the ultimate couch potatoes of the sea. They had a thick, fully functional pedicle, which is the bit which attaches to the substrate so that the brachiopod stays put (also indicating that it is epifaunal; without it would suggest it was part of the infauna). The pedicle may have divided into rootlets to give an even stronger hold. Dielasma are often found as nests and with a wide range in size. 

It would be interesting to find Dielasma attached to other organisms, such as a bryozoan reef, as this sort of thing often happened. Many fossils have little brachiopods attached to them, particularly sponges, even way back in the Cambrian. 

More From the Zechstein Basin

It isn't every day that a specific area of interest comes up in a lecture multiple times. I was already paying quite a lot of attention during this particular sedimentology lecture, but managed to pay even more when the Zechstein sea was brought up. I thought I would share the information I got from it.



The first mention simply came with the mention of commercial uses of evaporite deposits, as salt from the Zechstein sea is mined for industrial purposes and human consumption. Boulby mine in East Yorkshire is apparently the deepest in the UK and extends a mile out to sea.

The Zechstein basin is a saline giant, extensive marine evaporite deposits (deposited when sea water evaporates and leaves minerals) thousands of metres thick in what are known as intracratonic basins (a craton is a part of the Earth's crust that has not been split or merged). There are no modern equivalents of saline giants. Below is a picture of major Phanerozoic evaporite deposits.

Saline giants present a conundrum for geologists as evaporating sea water in the laboratory does not produce natural evaporite minerals and produces them in the wrong volumes (not enough anhydrite, too much sylvite/carnallite). Evaporating a 300m column of sea water only produces less than 5m of salt, yet in Jurassic marine deposits in America the desposits are 3500m thick, which would require the evaporation of a 240km deep marine basin!

On the left is the data predicted compared with the actual amounts found in the Zechstein basin. Below is the same data plotted in a graph. As can be seen, the predictions do not match what is produced.

The explanation for this seems to be that these basins were repeatedly replenished with sea water, possibly before the sea had fully dried out. Doncaster during the Permian was in a sea which was constantly evaporating and being refilled. This required special conditions. There needs to have been a barrier to the open ocean permitting replenishment of sea water (which could be continual or episodic); there needs to have been prolonged subsidence; and there needs to have been prolonged aridity and thermo-haline stratification (this relates to the temperature and salt content of the water).

There are different types of saline giant basins. The Zechstein was a deep barred basin, as shown on the left. These have the same mineralogy across the basin at any given time, overlying anoxic muds, with a straitified water column in 10s to 100s of metres cycles.


Hopefully future lectures will reveal more about the Zechstein basin, though I suspect that I will have to apply information from other examples in order to work out more about it.

Exploring the Zechstein Sea

I've lived in Conisbrough, a large village on the outskirts of Doncaster, for most of my life. Even so, I knew very little about the geology of the area until recently. As a child I knew that there were no dinosaurs to be found and that impressive fossils like ammonites were to be found at the coast. I also knew that I was in the heart of the South Yorkshire coal fields (Cadeby Colliery used to be in walking distance and Maltby still has a pit) so I could have guessed that Carboniferous rocks were nearby. Apart from that I also knew that there was a working dolomite quarry nearby, but not what age it was.

This Easter I decided to rectify that. Before returning home I had a look at some of the books in the university library, not really expecting to find much but still hopeful. I looked at the few books they had on the geology of South Yorkshire, mostly getting little information. Eventually I stumbled on a battered looking book from the '60s titled Geological Excursions in the Sheffield Region which contained a chapter titled "Conisborough". The spelling mistake irked me a little, but I read on, only to immediately happen upon them stating that fossils were rare in Conisbrough. Unperturbed I read on. Fortunately the book mentioned just one locality in Conisbrough where fossils could be found and I was determined to find it. The map given was poor and the descriptions of the route were heavily outdated, but still I checked it out of the library in hope.

Below is a better map which I found after finding the place for myself:

I set out looking for it with my friend Johnathan, we had plans to walk somewhere else afterwards but first we would look for fossils as it was on the way. The poorer map had us looking around 150 metres away at some bare rocks which matched none of the descriptions and clearly held no fossils. We had almost given in. As I used to go walking around here I knew of a place at the top of the hill which might just have been it, so we decided it was worth looking. When we got there we were not sure and we looked at a lot of the rocks up close but to no avail.

Up close there was a lot of interesting features on the rocks, but not the fossils we were looking for. We may have found shell casts, but with no actual shells we were not convinced. On the left is some of the interesting layering we found, including a lovely brick red colour at the top. It was around this point that Johnathan seemed to have given in. Continuing to look seemed futile and the place was not easy to get around. We were later to realise that this was the old Ashfield Brick-clay Pit, which is now only used by fly-tippers. This was useful for me in my youth as I travelled there in search of things to build dens out of; now it is a bit of a nuisance. Whilst Johnathan wandered off getting bored, I kept looking, having to do a little climbing in the slippery mud. I did find a couple of interesting features:

Above is a picture taken in what could be described as a small cave if I was being generous. What was interesting about it are the mineral precipitations from the ceiling. I can't help but wonder if any of the kids who burn their fires and drink there have ever stopped to admire this. On the right is an example of a quartz vein in the rocks there. They are surprisingly common when you start looking at the rocks, yet I reckon I am one of a handful of people who have stopped to appreciate them.

At this point I was enjoying looking at the rocks and the surprises they held. When I reached the place in the picture on the left I decided to have a look at some of the higher rocks and found myself climbing a very wobbly tree. I found some interesting clays and more quartz veins whilst I sifted through little rocks I found, hoping to find a fossil. Somehow I did it. I picked up a small rock and realised that I was probably holding an ancient shell. I shouted to Johnathan that I thought I had found something, but I really wasn't sure as I had a niggling feeling that I was seeing what I wanted to see.

It was then that I found another, which I sadly do not have a picture of as I gave it to Johnathan as a souvenir of our little expedition. I felt almost triumphant, I had succeeded. In my hands I held two little pieces of ancient history, long dead bivalve molluscs that have been fortunate to fossilise and survive hundreds of millions of years. I've had this feeling before as these were not my first fossil finds, but this meant more to me. Conisbrough will always feel like home to me and now I have a piece of its ancient history. Our fossil hunting was over for the day, so we continued on our walk with some sense of achievement.

My automatic reaction to this is to do some more research. I classified both of these specimens as belonging to genus Liebea but I may be wrong. It certainly does occur around here, but so does Bakevellia and to be honest I can't tell which is which in some examples. I am certain some of my later finds are Liebea, however, it was those that made me think the first specimen might not be. During my research I found this paper which I wish I had in the first place. It has a more detailed description of the rock exposure, is more up to date, and has a map which would have led me to instantly recognise the place.

A few days later I was playing guitar in church on Easter Sunday when I spotted Andrew, a friend I hadn't seen for a long time. After church we went over to the church hall for a cup of tea and a bit of a catch up. I can't remember how we got onto the subject of fossils, but when I said I had found some in Conisbrough his eyes lit up and his mouth dropped. Fossil hunting had become a hobby for him recently and he was even doing a project on them (he is an artist). I think he felt as I did, the thought of finding fossils in Conisbrough, the place of our youth, was exhilarating. It didn't take long before we were both thinking of heading there straight away, I feel like we read each others' minds. So off we set, myself with a guitar on my back, off to the Ashfield pit in search of ancient life.

When we got to the place I had found fossils Andi was using his keen observational skills. He suggested we looked in the area where the rocks had fallen for fossils which had come away from the rock face. I can't believe I had not done this myself, but then I am used to being told to look right up at a rock in order to identify sedimentary structures. As soon as he had suggested surveying the floor I spotted one, a lovely Liebea specimen. I showed Andi so that he knew what to look for and he went on to find four fossils of his own. I went up to my spot up the tree and came across my first brachiopod find, identified as Dielasma, along with a bivalve mollusc which has me baffled at the moment, as it does not fit with anything listed in the paper. Below are my finds up to that point. On the left is my first specimen. The other three are what I found with Andi. They are Liebea (the one which makes me want to reassess the first), Dielasma, and the unidentified bivalve.

After this I did more research. I already knew that I was looking at Permian rocks, so this put the fossils at around 250,000,000 years old. During the late Permian the Earth looked like this, as seen on the picture to the right. Conisbrough sits in an area which used to be the Zechstein sea, an area which stretches to Poland. The Zechstein sea was shallow and land-locked, containing many reef systems, particularly bryozoan reefs in this area it seems.

Predictably I decided I would go back at least one more time before heading to Portsmouth again. This time I went more prepared as I had a bag for specimens, a notepad and pen, and a camera to take pictures of the location. Partly simply to have documented it, partly in case others would like to explore the ancient history of Conisbrough.

This time I barely had to put in any effort. Spotting the fossils became so easy. I brought home around 20 this time and left a few there, keeping only my first finds and the most interesting ones. 

I'd love to spend ages putting up pictures of all of these, but I won't. On the left is a bit of substrate which I have kept in its original condition as it contains the mould of a Liebea specimen, along with two other shells, one of which is definitely a bivalve, but the other I cannot see enough of. After this are the bivalve specimens, one of which has what appears to be the original colouration. Over on the right are the brachiopods I collected, including the smallest which is a tiny 4mm in length. 

After I had found all of these I went to look at more of the rocks. I found no fossils but the rocks were interesting to look at. I also found a spot where an owl or another bird of prey might rest to eat occasionally, as I found numerous small bones. Judging by the jaw bones it was a rodent. Sadly the batteries in the camera died as I tried to take pictures of it, so I packed up and came home.

When I got home I documented them and eventually put them in a safe place (a biscuit tin). Over summer I will be returning to the place as there are species listed in the papers I have looked at which I have not found. For now I will have to wait though as I return to university in just two days.