User:NGPezz/British fissure deposits

British fissure deposits are a series of Late Triassic to Early Jurassic sedimentary deposits in southwest England and the Glamorgan region of Wales. These deposits are fissure fills, terrestrial sediments trapped within fractures in the underlying bedrock and preserved through geological processes. The British fissure fills represent sediments from a Triassic-Jurassic archipelago which were preserved within Carboniferous limestone bedrock. Fossils can be common, with individual fissure fills often preserving unique faunas. Among the animal groups represented in the deposits are rhynchocephalians, kuehneosaurids, early mammaliaforms, and early sauropodomorph dinosaurs.

Durdham Down
The quarry at Durdham Down (ST 573747) was the first of the British fissure deposits to be exploited by paleontologists. It was utilized through the 1830s, after which it was filled in to make room for the expanding Bristol urban area. Among the most famous fossils from this quarry were the earliest specimens of Thecodontosaurus antiquus, a basal sauropodomorph. Thecodontosaurus was the first Triassic dinosaur to be named and only the sixth known worldwide, although its dinosaurian status would not be reported until 1870. Other fossils have been found in rocks collected from the quarry, including but not limited to various sphenodonts, theropod dinosaur remains, and marine fauna. As with the nearby Tytherington quarry, the most common animal is the sphenodont Diphydontosaurus. The Durdham Down fissure fills are mostly dolomitic conglomerates and breccia which were geologically very similar to Tytherington deposits, though they formed within the younger Clifton Down Limestone rather than the Black Rock Limestone. Some authors have historically considered the blocky Triassic deposits of Durdham Down to have formed in a low basin or depression rather than a water-induced fissure, though the most common interpretation held by modern paleontologists is that the deposit represents a collapsed cave. Palynomorphs are unknown from Durdham Down itself, but a diverse assemblage akin to that of Tytherington has been found in a small fissure deposit at the Clifton suspension bridge, which is located only ~1.5 km away. Planocephalosaurus fossils are known at the Barnhill quarry (ST 725826) north of Bristol in Chipping Sodbury.

Emborough and Batscombe
Several fissure fills are known from quarries in the Mendip Hills. Some of these are mammaliamorph-dominated, while others are sauropsid-dominated. Emborough quarry (ST 623505) has the only sauropsid-dominated fissure deposits known to produce mammaliamorph remains as well. By far the most abundant fossils at Emborough belong to Kuehneosaurus, a gliding kuehneosaurid reptile. Other notable reptile remains include the only known specimens of Variodens, a "tricuspisaur" (trilophosaurid or procolophonid) closely related to Tricuspisaurus. The mammaliamorph remains consist of a pair of Kuehneotherium teeth. The Emborough fissure represents a collapsed cave which formed within the Hotwells Limestone (also known as the Oxwich Head Limestone Formation). The lower part of the fissure contains red marls, while the upper portion is predominantly limestone-derived conglomerate incorporating remarkably well-preserved feldspar chips and fossils. Rhaetian marine sediments overlying the fissure have been used to argue that this fissure formed prior to the Rhaetian, but this reasoning has been criticized.

The formerly abandoned Batscombe quarry (ST 460550) near Cheddar, now known as the Batts Combe quarry, preserved some of the best stalactites and cave pearls among the regional fissure fills. Although the fissure fill in this quarry may now be fully quarried away, at one time fossils were abundant enough to form bonebeds. The Batscombe fissure was low in diversity; the only named taxon known from the quarry is the kuehneosaurid Kuehneosuchus. This gliding reptile was originally equated with the "Emborough lepidosaur" (Kuehneosaurus) before receiving its own genus, although this separation is not universally accepted. The nearby Highcroft (Gurney Slade) quarry has been known to produce Clevosaurus fossils.

Holwell and Windsor Hill
The quarry complex at Holwell has produced among the most historically significant British fissure fills. The original Holwell fissure was discovered by Charles Moore in 1860. It was the first of the deposits to be recognized as a fissure fill, even though the Durdham Down fissure was the first to be used as a paleontological resource. Moore's original site produced mammaliaform teeth which were studied in-depth by paleontologists such Hermann Von Meyer and Sir Richard Owen.

The Windsor Hill Quarry (ST 615452) also has a mammaliamorph-dominated fissure deposit ("Mendip 14"), though its fossils are far less diverse. More than 2000 isolated bones have been reported, but all of them are attributed to Oligokyphus, a tritylodont cynodont. The fissure of Windsor Hill is a thin and vertically-oriented neptunian dike which formed with a west-end-trend likely due to tectonics rather than karst system, though it has also been interpreted as a doline (sinkhole). Its geology and fauna are similar to Holwell, but Windsor Hill differs in that it formed in the Black Rock Limestone rather than the Clifton Down Limestone. The Oligokyphus fossils are stained black due to marine chemical influences, an unusual feature also observed in fossils at the Woodleaze quarry.

Pant-y-ffynon, Ruthin, and Twyn-yr-Odyn
The quarry at Ruthin (SS 975796) is similar to Pant-y-ffynon in its fossil content. Insular dwarfism, procolophonids, etc. (Skinner et al 2020) Various reptile fossils are known from its fissure, though these fossils are heavily weathered and poorly described. Among these include sphenodont material and teeth of Tricuspisaurus, a strong-jawed reptile which was originally described as a trilophosaurid, though is more likely a procolophonid. Ruthin's geology is also ambiguous, with the fissure forming at an angle within heavily-faulted strata of the underlying Hunts Bay Oolite bedrock. Another Welsh quarry at Twyn-yr-Odyn (ST 116738) near Cardiff possesses freshwater fish fossils. Many of these are well-preserved, including the only specimens of Legnonotus recovered from the British fissure deposits. Unfortunately, the Twyn-yr-Odyn collections seem to be lost.

St. Brides
Six Welsh quarries in the vicinity of St Brides Major are considered to preserve material from a single early Jurassic island, the St. Brides paleo-island. Five of these (Ewenny, Pant, Pontalun, Duchy, and Cnap Twt quarries) have produced diagnostic fossils, while only scraps and fragments are known from the sixth (Longlands Quarry). The St. Brides quarries are considered Early Jurassic in age, similar to the Holwell and Windsor Hill quarries. The paleo-island may have been larger or had higher terrain than the other islands in the area, as evidenced by modern limestone topography. However, at some points in history it may have been low enough for seawater to seep into the limestone and form large caverns. The fossils of St. Brides are preserved in colorful clays or marls alongside abundant hematite ooids. They represent a distinct tropical ecosystem, which has been termed the Hirmeriella association. The most common fossils are carbonized remnants of the conifer tree Hirmeriella muensteri. The most common animal is the rhynchocephalian reptile Gephyrosaurus, followed by the mammaliaforms Morganucodon and Kuehneotherium. Pant 4, a St. Brides fissure deposit discovered in 1968, has a more diverse association than previously known for the area. Clevosaurus convallis is much more common at Pant 4 than Gephyrosaurus, and the deposit seems to have a larger variety of mammaliaforms as well.