Cliff and foreshore exposures between Seven Rock Point and Golden Cap, Dorset
The Charmouth Mudstone Formation is a geological formation in England, dating to the Early Jurassic (Sinemurian–Pliensbachian).[1] It forms part of the lower Lias Group. It is most prominently exposed at its type locality in cliff section between Lyme Regis and Charmouth (alongside the underlying Blue Lias) but onshore it extends northwards to Market Weighton, Yorkshire, and in the subsurface of the East Midlands Shelf and Wessex Basin. The formation is notable for its fossils, including those of ammonites and marine reptiles and rare dinosaur remains. The formation played a prominent role in the history of early paleontology, with its Lyme Regis-Charmouth exposure being frequented by fossil collectors including Mary Anning.
Stratigraphy
Shales With Beef Member
The Shales With Beef Member is around 28–30 metres thick in the Lyme Regis-Charmouth region and predominantly consists of thinly bedded medium to dark grey mudstone, blocky calcareous pale-weathering mudstone and brown-grey organic-rich mudstones with frequent bedding parallel veins of fibrous calcite ("beef"), that are usually less than 10 centimetres thick. Several beds of nodular and tabular limestone are also present. It is the lowest unit of the formation and directly overlies the Blue Lias Formation, with the boundary being marked by a prominent bioturbated horizon. Notable persistent marker beds within the member include the laminated calcareous siltstone "Fish Bed", "Table Ledge", which consists of lens beds of limestones with mud content with nests of rhynchonellidbrachiopods, the Devonshire Head and the Spittles limestones and the Birchi Nodules (which are septarian concretions) The upper boundary with the Black Ven Marl Member is marked by the prominent laterally persistent limestone Birchi Tabular Bed.[2]
Black Ven Marl Member
The Black Ven Marl Member is around 43 metres thick consists of thinly bedded dark mudstones,[3] with several laterally persistent cementstone horizons, notable horizons include the Lower and Upper Cement beds and the Stellare nodules.
Belemnite Marl Member
The Belemnite Marl Member is around 20 to 27 metres thick, and consists of interbedded pale and dark grey calcareous mudstone, with numerous belemnites, hence the name. The top of the member is marked by the Belemnite Stone Bed[4]
Green Ammonite Member
The Green Ammonite Member is up to 31 metres thick predominantly consists of medium grey mudstones, with 3 limestone horizons, Lower Limestone; Red Band, and Upper Limestone, it is conformably overlain by the Dyrham Formation in some areas,[5] but in the Charmouth area there is an erosive unconformable boundary with the much younger Early Cretaceous (Albian) aged Gault clays.
Paleobiota
Vertebrate fauna of the Charmouth Mudstone Formation
A marine sauropterygian, type member of the family Plesiosauroidea inside Plesiosauria. A juvenile specimen that resembles those assigned to the genus Plesiosaurus. Its assignation is dubious.
A marine sauropterygian, type member of the family Plesiosauroidea inside Plesiosauria. The typical plesiosaur, and one of the most common found on the formation.
LRM 2021/45, a partial skeleton including cranial material, cervical, dorsal, and caudal vertebrae, ribs, right pectoral girdle, partial limb bones, and an osteoderm
A basal dimorphodontid pterosaur. The relatively deep rostrum and extreme size dimorphism in the dentition show clear similarities to Dimorphodon, also from the Lias of Dorset. The distinctive dentition, in which the first four pairs of rostral teeth and two pairs of mandibular teeth are relatively large and fang-like, while the remaining teeth are remarkably small and short.[21]
NHMUK OR43062. Shaft of a large humerus, crushed proximal femur, fibula, fragmentary radius?, other large bone fragments
NHMUK OR46011. Probable rib fragment – indet.
BRSMG Ce12785. Partial skull and some disarticulated postcranial elements
BRSMG Ce12787. Partly eroded femur and vertebra
BRSMG Ce12788. cervical osteoderms.
BRSMG Ce12789. Vertebra and bone fragments
BRSMG Cf2781. nine middle-distal caudal vertebrae and some haemal arches with some organic (kerogenized) remains of the skin preserved.
BRSMG LEGL 0004. Articulated skeleton
BRSMG LEGL 0005. Articulated partial skeleton
DORCM G.7542. Osteoderms
DORCM G.10817. Osteoderms
CAMSM X39256. Cranial: disarticulated skull and jaw elements
A basal thyreophoran, and the type member of the family Scelidosauridae. One of the best known armored dinosaurs of the lower Jurassic, also the most complete found to date, with specimens preserving even the skin and dermal armour.[25]
A theropod, possible basal member of Tetanurae. Initially considered part of Scelidosaurus holotype, considered Tetanurae indet by Carrano 2012. It was named "Merosaurus" by Welles, Powell and Pickering in 1995.[32] But due to lack description was considered invalid.[33]
A theropod, possibly a member of Coelophysidae inside Neotheropoda. Invalid and non diagnostic. It was identified as Zanclodon(?) sp. b., Megalosaurus lydekkeri and reassigned as Magnosaurus(?) lydekkeri.[36] Mortimer found that was changed to "Megalosaurus" woodwardi later.[36] It was found on mostly of recent works to be non diagnostic, probably a coelophysoid.[35]
A theropod, possibly a basal member of Neotheropoda. Previously misidentified as a referred specimen of the pterosaur Dimorphodon macronyx.[18] NHMUK PV R36855 differs from those of all other Early Jurassic-Late Triassic, theropods in its combination of features, but bears no discernible autapomorphies.[37] It is similar to Tawa, Dracoraptor and Liliensternus, being considered an early branching neotheropod, with a size range similar to Coelophysis bauri, representing an individual of approximately 10 kg, approximately the same size as Coelophysis bauri.[37]
A sauropodomorph, possibly a member of Massopoda. While it was not associated with BMNH 39496 or GSM 109560 and was not assigned to "Merosaurus" it was classified as coming from an indeterminate theropod. Mortimer found closer resemblance with pedal unguals of basal massopods like Blikanasaurus and Jingshanosaurus.[33]
Fish
Numerous fish species are known from the Charmouth Mudstone and underlying Blue Lias, from such horizons as the "Fish Bed" of the Shales With Beef Member.
^R. Owen. 1865. Part I. Sauropterygia. Monographs on the fossil Reptilia of the Liassic formations 17(75):1-40
^W. J. Sollas. 1881. On a new species of Plesiosaurus (P. Conybeari) from the Lower Lias of Charmouth; with observations on P megacephalus, Stutchbury, and P. brachycephalus, Owen. Quarterly Journal of the Geological Society of London 37:440-480
^G. W. Storrs. 1995. A juvenile specimen of ? Plesiosaurus sp. from the Lias (Lower Jurassic, Pliensbachian) near Charmouth, Dorset, England. Proceedings of the Dorsal Archaeological and Natural History Society 116:71-76
^W. D. Conybeare. 1824. On the discovery of an almost perfect skeleton of the Plesiosaurus. Transactions of the Geological Society of London, series 2 1:381-389
^Bennett, S.P.; Barrett, P.M.; Collinson, M.E.; Moore-Fay, S.; Davis, P.G.; Palmer, C.P. (January 2012). "A new specimen of Ichthyosaurus communis from Dorset, UK, and its bearing on the stratigraphical range of the species". Proceedings of the Geologists' Association. 123 (1): 146–154. doi:10.1016/j.pgeola.2011.07.001.
^Martill, D. M. (2014). Dimorphodon and the Reverend George Howman's noctivagous flying dragon: the earliest restoration of a pterosaur in its natural habitat. Proceedings of the Geologists' Association, 125(1), 120-130.
^ abSteel, L. (2012). The pterosaur collection at the Natural History Museum, London, UK: an overview and list of specimens, with description of recent curatorial developments. Acta Geologica Sinica‐English Edition, 86(6), 1340-1355.
^ abUnwin, D. M. (2003). On the phylogeny and evolutionary history of pterosaurs. Geological Society, London, Special Publications, 217(1), 139-190.
^Barrett, P. M., Butler, R. J., Edwards, N. P., & Milner, A. R. (2008). Pterosaur distribution in time and space: an atlas. Zitteliana, 61-107.
^ abUnwin, D. M. (2011): A new dimorphodontid pterosaur from the Lower Jurassic of Dorset, southern England. 59th Annual Symposium of Vertebrate Palaeontology and Comparative AnatomyLyme Regis, Dorset, UK
^ abB.H. Newman The Jurassic dinosaur Scelidosaurus harrisoni, Owen Palaeontology, 11 (1968), pp. 40-43
^David B Norman, FLS, Scelidosaurus harrisonii from the Early Jurassic of Dorset, England: cranial anatomy, Zoological Journal of the Linnean Society, Volume 188, Issue 1, January 2020, Pages 1–81, [1]
^David B Norman, FLS, Scelidosaurus harrisonii from the Early Jurassic of Dorset, England: the dermal skeleton, Zoological Journal of the Linnean Society, Volume 190, Issue 1, September 2020, Pages 1–53, [2]
^P. C. Ensom. 1989. New scelidosaur remains from the Lower Lias of Dorset. Proceedings of the Dorset Natural History and Archaeological Society 110:166-167
^Anonymous. 1955. British Museum (Natural History). Nature 176(4487):815-816
^Maidment, S.C.R., Porro, L.B., 2010, "Homology of the palpebral and origin of the supraorbital ossifications in ornithischian dinosaurs", Lethaia, 43: 95-111
^Rixon AE. 1968. The development of the remains of a small
Scelidosaurus from a Lias nodule. Museums Journal 67:
315–321
^R.B.J. Benson, P.M. Barrett Dinosaurs of Dorset: Part 1, the carnivorous dinosaurs (Saurischia, Theropoda) Proceedings of the Dorset Natural History and Archaeological Society, 130 (2009), pp. 133-147
^M.T. Carrano, R.B.J. Benson, S.D. Sampson The phylogeny of Tetanurae (Dinosauria: Theropoda) Journal of Systematic Palaeontology, 10 (2012), pp. 211-300
^Pickering, 1995. Jurassic Park: Unauthorized Jewish Fractals in Philopatry. A Fractal Scaling in Dinosaurology Project, 2nd revised printing. Capitola, California. 478 pp.
^ abMortimer, M.(2010): Pickering's taxa 4: Merosaurus newmani. The Theropod Database Blog
^R. Lydekker Catalogue of the Fossil Reptilia and Amphibia in the British Museum (Natural History). Part I. British Museum (Natural History), London 309 pp
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^ abM.T. Carrano, S.D. Sampson A review of coelophysoids (Dinosauria: Theropoda) from the Early Jurassic of Europe, with comments on the late history of the Coelophysoidea Neues Jahrbuch für Geologie und Paläontologie, Monatshefte, 2004 (2004), pp. 537-558
^ abMortimer, M.(2012):"New" name for Megalosaurus? lydekkeri. The Theropod Database Blog
^ abcChoinierea J. N., Wills, S., Bennett, S.C., Barrett P.M.(2020):A small theropod dinosaur from the Lower Jurassic Lias Group of Charmouth, Dorset. Proceedings of the Geologists' Association. 14 September 2020
^Benson, 2010. The osteology of Magnosaurus nethercombensis (Dinosauria, Theropoda) from the Bajocian (Middle Jurassic) of the United Kingdom and a re-examination of the oldest records of tetanurans. Journal of Systematic Palaeontology. 8(1), 131-146.
United Kingdom
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