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TXV24 posted a topic in Micro-paleontologyHi, I've recently fully processed some matrix from the Lower Hamstead Mbr. that I collected back in November, and I thought I'd share some of my finds in a similar way to my Bembridge Marls Mbr. material. The matrix originates from a 'shelly' horizon in the Lower Hamstead Mbr. and was collected from fallen blocks at the base of a low cliff exposure at Bouldnor Cliff. The Lower Hamstead Mbr. overlays the late Eocene Bembridge Marls and dates from the very earliest Oligocene epoch, approximately 33.75 - 33.5 million years ago. To put the finds into an environmental context the Lower Hamstead Mbr. was deposited during a period of rapid global cooling and drop in sea levels associated with the onset of antarctic glaciation (Oi-1). The cooling and eustatic change had begun in the late Eocene, with the palaeo-environments of the Bembridge Marls becoming increasingly terrestrial towards the Eocene/Oligocene boundary. By the Lower Hamstead Member the southern Hampshire Basin was a low lying coastal plain with extensive wetlands, lakes, ponds and sluggish rivers flowing south east towards the early channel (at this time the channel was more a large embayment with only occasional connection to the North Sea). The dense sub-tropical forests of the late Eocene had disappeared and the landscape was dominated by open woodlands of pine, sequoia, and oak. The environment was much cooler and annual rainfall had significantly dropped since the Eocene, although temperatures would begin to rise again further into the rupelian and Hamstead Mbrs. The basin was surrounded by areas of chalk upland (still existing today) with forests of sequoia and broadleaf species. This dramatic climate change is likely what triggered the Grande Coupure, in which endemic Eocene mammals like the palaeotheres disappeared and were replaced with Asian groups such as carnivorans, rhinocerotids, anthracotheres, and a variety of other artiodactyls. The mammals of the dense tropical Eocene forests simply couldn't adapt fast enough to the new open environments of the Oligocene and ultimately failed to compete against the better adapted migrants. By the Upper Hamstead Member the mammals on the Hampshire Basin coastal plain are almost entirely of Asian origin. Therefore the micro-vertebrates lived in an environment of large scale climatic and ecological change, which I think adds another level of interest to collecting from this member of the Bouldnor Fm. The material I've collected so far is a lot more varied than the Bembridge Marls, but overall is less abundant. So far it's produced at least 3 fish taxa, 2 mammals, and an indeterminate piece of jaw which may be reptilian or mammal. 1. A skull element from a Bowfin (Amia sp.), these fish are very common in most horizons of the Bouldnor Fm. 2. A vertebra from a Bowfin (Amia sp.) 3. A damaged lateral scute from a Sturgeon (Acipenser sp.) showing the transition to a freshwater environment 4. An indeterminate piece of a tiny jaw, may be crocodilian although I'm not sure. 5. The nicest find of the lot, a lower incisor from the theridomyid rodent Isoptychus (ID'd by Jerry hooker from the NHM). These rodents looked similar to modern kangaroo rats, hopping along the ground on large rear legs. Bite marks on Isoptychus bones collected from Thorness Bay suggest that they were common prey for the bear-dog Cynodictis. 6. Finally 2 images of an unidentified mammal tooth. I'm unsure as to whether this is part of the tooth or the entire crown, but it doesn't appear to be from a rodent. Hope you all enjoyed the finds, Theo
Hi, I thought I'd show some of my first micro-vertebrate fossils from the Bembridge Marls Mbr. of the Bouldnor Fm. I collected around 2kg of matrix from one of the 'shelly' estuarine horizons in the lower part of the member at Hamstead Ledge, and am really pleased the results so far! The Bembridge Marls form the basal member of the Bouldnor Fm. and were deposited between 34.0 and 33.75 million years representing the final 250,000 years of the Eocene epoch. The depositional environment varies throughout the member and many beds are laterally discontinuous (like the Insect Bed, which produces finely preserved insects, feathers, leaves, and lizard skin impressions). Generally however, the Bembridge Marls were laid down in a sluggish lagoonal/estuarine environment with areas of wetland and adjacent sub-tropical/tropical forests, in the southern regions of the Hampshire Basin. To the south were forested chalk uplands that are now the downs of the Isle Of Wight. There was also some fluvial influence from rivers flowing from the west, draining the uplands around Dartmoor in Devon. Fauna-wise vertebrates like fish and freshwater turtles are common, and mammal remains are rarely found (in comparison to the overlying Hamstead members which are rich in post and pre-grande coupure mammals), these include palaeotheres, creodonts, rodents, anoplotheres, choeropotamids, xiphodonts, and primates. So far I've only searched through a small amount of the matrix but it has produced indeterminate teleost vertebra, Bowfin teeth, fin spines, indeterminate fish premaxillae, and a very nice crocodilian tooth. (The quality of the images isn't always fantastic but I'm trying to find a way to work around it in the microscope's program) Isolated fish vertebra from teleosts are by far the most common micro-fossil, and I've collected more than 10 so far. Here's a nice example: Bowfin teeth are also quite common and vary in size from 2-7.5mm in length. Bowfins would have been ambush predators feeding on smaller fish and other vertebrates in the lagoons and estuaries. Based on vertebra I've found ex-situ on the beach it seems some of these fish were very large. (Close up of one the teeth) These pre-maxillae also seem to turn up from time to time and appear to be from some form of teleost. The closest match I can find is with some kind of Gadiform? And finally the best find so far, a crocodilian tooth crown. I spotted this on the surface of one of the matrix blocks. It's most likely from the alligatoroid Diplocynodon which was very common in the wetlands and rivers of Europe from the Palaeocene to the Miocene. Diplocynodon has also been found in the early Eocene marine deposits of the London Clay suggesting that they frequented both freshwater and brackish/coastal habitats. The matrix is nowhere near fully sieved and sorted through yet so hopefully there's a lot more micro-vertebrates in there! Hope this was of interest, Theo