clypeus

UKAFH fossil hunt to Cross hands quarry, Warwickshire on 21st October 2018

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We were blessed with a warm sunny day on the 21st October to Cross Hands Quarry which is located on private land owned by Mr Newman. Mr Newman had kindly created a couple of fresh spoil heaps especially for our trip, so our party had fresh pickings as will be seen below lots of fossils were found.  As the trip is now centred around these spoil heaps, this location is perfect for families to visit. Therefore we had quite a few families on our trip. 

The quarry was once used to supply building stone for the local town of Chipping Norton, which is located in the Cotswolds famous for its rich honey coloured stone buildings.

Cross Hands Quarry is a Site of Special Scientific Interest (SSSI) for its geological features. The rocks exposed in the quarry faces are mostly limestones, formed from the remains of shelly creatures living in the warm tropical seas which covered large areas of England in the Middle Jurassic Period, about 175 million years ago.

GEOLOGY

Cross Hands Quarry exposes rocks of Middle Jurassic age that were deposited in a shallow marine environment, not too dissimilar to that of the modern-day Bahamas. These rocks belong to the Inferior Oolite and comprise the Clypeus Grit, overlain by the Chipping Norton Limestone and the Hook Norton Limestone.

Towards the end of Upper Lias times sea levels fell somewhat, bringing a change of conditions which initiated the Middle Jurassic. Low sea levels persisted for 15 million years and in clear, warm, shallow waters the most important sediment was calcium carbonate. The accumulations of carbonate mud and carbonate sand have been transformed into a variety of limestones which are grouped into two series called the Inferior Oolite and the Great Oolite.

The word Oolite refers to a rock containing a proportion of polite. These are little spheres of calcium carbonate, typically half to one millimetre in diameter. The name comes from the Greek word on – meaning egg – because a densely oolitic limestone has the appearance of fish eggs.

The Inferior Oolite group of formations is so called not because of any inferior quality but because it’s rocks are older than, and therefore stratigraphically below, those of the Great Oolite. This limestone makes excellent building material as has been used in the Cotswolds to give the buildings there distinctive golden yellow colour.

SSI Noticeboard showing Geology and typical fossils that can be found at location.

During the Inferior Oolite and Great Oolite times this area was low-lying between shallow sea to the south-west and a swampy, coastal region to the north-east. In these shallow, variable environments the deposition of sediment varied greatly in amount and type from place to place and time to time. As a result the strata exhibit rapid lateral changes in thickness and character and some beds may be restricted to small areas.

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Geology Map of Whiltshire

In the early 1960’s remains of a partial right femur from a Cruxicheiros(meaning “cross hand”) is a genus of tetanuran theropod dinosaurwhich lived in the Middle Jurassic of England. The type species is C. newmanorum,described by Roger Benson and Jonathan Radley in 2010.  The 2010 paper recognized differences between the Cross Hands Quarry discovery and those attributed to Megalosaurus. These differences include lower and broader spines along the animal’s back, and differences in leg and hip bones. The authors renamed the Cross Hands Quarry specimens Cruxicheiros newmanorum; the generic name Cruxicheiros comes from a mixture of Latin and Greek, Latin crux meaning “cross” and Greek cheir meaning “hand,” in reference to the Cross Hands Quarry locality where the fossils were discovered. The specific name newmanorum honors the Newman family, who own the quarry. Cruxicheiroswas a large theropod, but the known material is very limited. The holotype, catalogued as WARMS G15770, is a partial right femur. Additional material from the site probably comes from the same individual as the holotype, based on examination of the matrix of sandy limestone and calcite which make up all the fossils. The additional material consists of “an anterior dorsal or posterior cervical vertebra; a dorsal neural arch; a partial dorsal vertebra; the anterior half of a middle-distal caudal vertebra; a partial right scapulocoracoid; a partial left ilium; the proximal end of a left pubis; [and] numerous rib and bone fragments”. The specimens are now stored at Warwickshire Museum Service (Source Wikipedia).

FINDS

Typical fossils found at this location are bivalves, brachiopods, gastropods, echinoids (such as Clypeus ploti).

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Nice big articulated bivalve – Pholadomya fidiculá
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Coral
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Lovely piece of Coral showing polyps
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Echinoid probably a Clypeus Ploti
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Echinoid – possibly a Nucleolites sp
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Gastropod- Bactroptyxis bacillus
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Gastropod – Purpuroidea morrisea

Many thanks to Mr Newman for allowing our party to visit his quarry.

UKAFH fossil hunt at Withington, Gloucestershire on Sunday 30th September 2018

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We were blessed with a warm, dry day for our first hunt at Withington in ploughed farm fields. The proximity of the topsoil to the Inferior Oolite below in this locality means that ploughing brings rock to the surface which contains a large variety of fossils. A field hunt really is an excellent way to find fossils with little effort other than to look patiently and “get your eye in”. The weather conditions were dry and overcast, making it an ideal day to fossil hunt as the rock was relatively clean and easy to spot so plenty of finds were made.

 

Once assembled at the farm, which we obtained special permission to visit, our guest leader and local expert for the day, Mark Baggott, gave us an introduction to the local geology and fossils. After enjoying Mark’s display and introduction to the site the group spread out to hunt across the two newly cleared fields and finds were quickly being made.  The lower field produced a good variety of abundant brachiopods, bivalves and regular echinoids and the upper field yielded complete and partial Clypeus ploti.  A number of members also found ammonites, which was unexpected as ammonite finds at this location have historically been quite uncommon.  Some beautiful gastropods were found and one lucky hunter even found a flint scraper!

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Mark Baggott’s fantastic display of Withington finds and geology and ID sheets

The Cotswold Escarpment rocks are almost exclusively marine and were deposited mainly in warm tropical seas. Plate tectonics has transported this part of the Earth’s crust northward over the last 150-200 million years to its current location. The Middle Jurassic rocks here are the characteristic ‘Cotswold Limestone’; soft, yellow, sandy limestone at the base of the Inferior Oolite (literally egg stone), a sedimentary rock formed from ooids, spherical grains composed of concentric layers. Towards the top of the Inferior Oolite the limestone becomes more fossiliferous and is referred to as ‘grits’ due to its coarser texture. Such an Inferior Oolite exposure is exposed at the farm and the fossils that this limestone contains date from between 167 to 175 million years ago at a time when this farm was at the bottom of a warm tropical sea. The rocks exposed near the farm comprise the Salperton and Aston Limestone and, from a fossil perspective, the most interesting layers are the Grits (Clypeus, Upper Trigonia Grit and Lower Trigonia Grit).

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Clypeus ploti found by Sam Caethoven

The commonest fossil found at this location is the sea urchin (echinoid) Clypeus ploti. These are more commonly known as Chedworth Buns (after the nearby village where they were often found) or Pound Stones, because their weight was usually a good approximation to 1lb. Clypeus lived in burrows on the seafloor, and burrowed their way through the sediment to get nutrients. They had fine hair-like spines and are an example of what is known as an “irregular” echinoid because they are shaped, not rounded.  Because these irregular echinoids lived in the sediment, they didn’t need the spiky and sometimes poisonous spines that the spiny sea urchins (known as regular echinoids) that we can see on the seafloor today have for protection. As well as the Clypeus Ploti we find other echinoid species which are “regular” and would have had sharp spines.  Unfortunately the spines rarely fossilize still attached, but they can frequently be found individually in the same sediment.

Trigonia bivalves gave their name to the second grit since they are very common at this horizon.  Trigonia are a family of saltwater clams, noticeable because the exterior of the shell is highly ornamented. Other fossils to be found comprise of brachiopods, bivalves and gastropods. Brachiopods are a marine animal that had hard valves (shells) on the upper and lower surfaces.  They are distinguished from bivalves which also have two valves/shells but in a left/right arrangement rather than upper and lower.

Brachiopods are bottom dwelling marine animals and, although rare today, in Jurassic times they dominated the sea floor and were frequently found in large colonies. One characteristic unique to brachiopods is the pedicle, which is a long, thin fleshy appendage which is used to burrow into the sea floor as an anchor while the brachiopod could feed clear of the silt.  Although the fleshy pedicle itself does not preserve in the fossils, the opening at the top of the animal from whence the pedicle connected (known as the foramen) is clearly visible. Brachiopods are filter feeders, gathering microscopic organisms and bits of organic matter from the water that flows by them using a specialized organ called a lophophore. This is a tube like structure with cilia (hair like projections). The cilia move food particles down the lophophore to the mouth.

Brachiopods are often known as lamp shells as the curved shell of the some classes look rather like Roman pottery lamps. There are two main groups of articulate brachiopods from the Jurassic, terebratulids and rhynchonellids. A common example of a telebratulid brachiopod found here is Stiphrothyris tumida; their main characteristics are their ovoid/circular shape, presence of a clear hinge line and a circular pedicle opening located in the beak. This brachiopod is a type example for lower Trigonia grit. The second of the main orders of articulate brachiopods is the rhynchonellids. The main characteristics are their strongly ribbed wedge shape, the absence of a clear hinge line, the line between the valves/shells is often zigzagged and a circular pedicle opening located in the beak. An example of such a brachiopod found here is Burmirhynchia sp.  The rhynchonellids were able to extrude their lophophore out of the shell in water, whereas the terebratulids maintained their lophophore within the shell.  The ability to extrude the lophophore led to more efficient food-gathering and is probably why rhynchonellids survived the mass extinction events better than the terebratulids.

Bivalves include such animals as clams, oysters, cockles, mussels, and scallops are also found at Withington. The majority are filter feeders and often they bury themselves in sediment where they can be safe from predators. Others lie on the sea floor or attach themselves to rocks or other hard surfaces, a few such as scallops are able to propel themselves through the water. The shell consists of two usually similar valves, and is joined at the hinge line by a flexible ligament with interlocking teeth on each valve. This arrangement allows the shell to be opened and closed for feeding without the two halves becoming disarticulated. Bivalves found here include Thracia (a member of the clam family) and Pleuromya (a member of the mussel family).

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Large ammonite found by Sarah Scott

If we compare brachiopods and bivalves, although they resemble bivalves, brachiopods are not even molluscs.  They are so unique that they have been placed in their own phylum, Brachiopoda. Brachiopods are shelled marine organisms that superficially resembled bivalves in that they are of similar size and have a hinged shell in two parts. However, brachiopods evolved from a very different ancestral line, and the resemblance to bivalves only arose because of a similar lifestyle. The differences between the two groups are due to their separate ancestral origins. Different initial structures have been adapted to solve the same problems, a case of convergent evolution. In modern times, brachiopods are not as common as bivalves. Brachiopod shells are often made of calcium phosphate as well as calcium carbonate, whereas bivalve shells are composed entirely of calcium carbonate.

Also to be found at Withington are gastropods. They are called univalves because they build a single coiled shell to protect their soft bodies. Ancient fossilized gastropods are related to living gastropods of today and are snails. Gastropods can be carnivorous or herbivorous. Their tongue is covered with thousands of tiny teeth to tear apart food.

UKAFH would like to sincerely thank the landowner for allowing us access to the fields.  Special thanks also to Mark Baggott for organising the visit and providing a fantastic display of local fossils and information for us all to refer to throughout the day.  Thank you also to Mark and to Alan Banyard for bringing along some very nice examples of undamaged and prepared Clypeus ploti and ammonites from nearby locations for members to take home.