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20-524: Certain rocks of marine origin of this age in Europe are called " Lias " and that name was used for the period, as well, in 19th-century geology. In southern Germany rocks of this age are called Black Jurassic . There are two possible origins for the name Lias: the first reason is it was taken by a geologist from an English quarryman 's dialect pronunciation of the word "layers"; secondly, sloops from north Cornish ports such as Bude would sail across

40-475: A huge diversity of new forms with complex suture patterns (the ammonites proper). Ammonites evolved so rapidly, and their shells are so often preserved, that they serve as important zone fossils . There were several distinct waves of ammonite evolution in Europe alone. The Early Jurassic was an important time in the evolution of the marine reptiles. The Hettangian saw the already existing Rhaetian ichthyosaurs and plesiosaurs continuing to flourish, while at

60-566: Is Gaelic for "flat stone". There has been some debate over the actual base of the Hettangian Stage, and so of the Jurassic System itself. Biostratigraphically , the first appearance of psiloceratid ammonites has been used; but this depends on relatively complete ammonite faunas being present, a problem that makes correlation between sections in different parts of the world difficult. If this biostratigraphical indicator

80-604: Is a lithostratigraphic unit (a sequence of rock strata ) found in a large area of western Europe, including the British Isles , the North Sea , the Low Countries and the north of Germany . It consists of marine limestones , shales , marls and clays . Lias is a Middle English term for hard limestone, used in this specific sense by geologists since 1833. In the past, geologists used Lias not only for

100-794: Is divided into the following formations (from top to base): In South Wales only the Blue Lias is present. The Lias is underlain by the Late Triassic Penarth Group , and overlain by the Inferior Oolite in most of England and the Dogger Formation or Ravenscar Group in the Cleveland Basin. In some areas there is a stratigraphic hiatus , and the rocks are overlain by Cretaceous marine sediments. There are restricted outcrops of Lias rocks on

120-871: Is used, then technically the Lias Group —a lithostratigraphical division—spans the Jurassic / Triassic boundary. There are extensive Liassic outcrops around the coast of the United Kingdom , in particular in Glamorgan , North Yorkshire and Dorset . The 'Jurassic Coast' of Dorset is often associated with the pioneering work of Mary Anning of Lyme Regis . The facies of the Lower Jurassic in this area are predominantly of clays , thin limestones and siltstones , deposited under fully marine conditions. Lias Group strata form imposing cliffs on

140-657: The Bristol Channel to the Vale of Glamorgan to load up with rock from coastal limestone quarries (lias and Carboniferous limestone from South Wales was used throughout North Devon/North Cornwall as it contains calcium carbonate to 'sweeten' (i.e.neutralise) the acidic Devonian and Carboniferous soils of the West Country ); the Cornish would pronounce the layers of limestone as ' laiyers ' or ' lias '; leac

160-677: The Teleosauroidea and Metriorhynchoidea . Teleosauroids are not greatly specialised for oceanic life, with back osteoderms similar to other crocodyliformes. Within Metriorhynchoidea, the Metriorhynchidae displayed extreme adaptions for life in the open ocean, including the transformation of limbs into flippers, the development of a tail fluke , and smooth, scaleless skin, and probably gave live birth , seemingly uniquely among archosaurs. The term Thalattosuchia

180-446: The Vale of Glamorgan coast, in southern Wales . Stretching for around 14 miles (23 km) between Cardiff and Porthcawl , the remarkable layers of these cliffs, situated on the Bristol Channel are a rhythmic decimetre scale repetition of limestone and mudstone formed as a late Triassic desert was inundated by the sea. During this period, ammonoids , which had almost died out at the end-of-Triassic extinction, radiated out into

200-551: The coelophysoids , prosauropods and the sauropods that had continued over from the Triassic. Accompanying them as small carnivores were the sphenosuchian and protosuchid crocodilians. In the air, new types of pterosaurs replaced those that had died out at the end of the Triassic. But in the undergrowth were various types of early mammals, as well as tritylodont synapsids , lizard-like sphenodonts , and early lissamphibians . Lias Group The Lias Group or Lias

220-584: The Early Jurassic ( Sinemurian ) of Chile and France. However, they cannot be assigned to the group with confidence as they lack diagnostic characters. In 2023 a basal teleosauroid was reported from the earliest Jurassic (Hettangian-Sinemurian) of Morocco, representing one of the oldest known thalattosuchians. Turnersuchus from the Pliensbachian of England appears to be basal to both Teleosauroidea and Metriorhynchoidea. While abundant during

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240-815: The Netherlands and the southern North Sea. In northern Germany, the Lias Group consists of nine formations (from top to base): Thalattosuchia Thalattosuchia is a clade of mostly marine crocodylomorphs from the Early Jurassic to the Early Cretaceous that had a cosmopolitan distribution . They are colloquially referred to as marine crocodiles or sea crocodiles , though they are not members of Crocodilia and records from Thailand and China suggest that some members lived in freshwater. The clade contains two major subgroupings,

260-501: The end of the Early Jurassic, the thalattosuchians (marine " crocodiles ") appeared, as did new genera of ichthyosaurs ( Stenopterygius , Eurhinosaurus , and the persistently primitive Suevoleviathan ) and plesiosaurs (the elasmosaurs (long-necked) Microcleidus and Occitanosaurus , and the pliosaur Hauffiosaurus ). On land, a number of new types of dinosaurs—the heterodontosaurids , scelidosaurs , stegosaurs , and tetanurans —appeared, and joined those groups like

280-650: The name "Lias" for a unit of time is therefore slowly disappearing. In southern England , the Lias Group is often divided into Lower, Middle and Upper subgroups. In Southern England the Lias is divided into the following formations (from top to base): In the East Midlands Shelf the Lias is divided into the following formations (from top to base): In the Cleveland Basin in Yorkshire the Lias

300-465: The same time a number of new types of these marine reptiles appeared, such as Ichthyosaurus and Temnodontosaurus among the ichthyosaurs, and Eurycleidus , Macroplata , and Rhomaleosaurus among the plesiosaurs (all Rhomaleosauridae , although as currently defined this group is probably paraphyletic ). All these plesiosaurs had medium-sized necks and large heads. In the Toarcian , at

320-471: The sequence of rock layers, but also for the timespan during which they were formed. It was thus an alternative name for the Early Jurassic epoch of the geologic timescale . It is now more specifically known that the Lias is Rhaetian to Toarcian in age (over a period of c. 20 million years between 200 to 180 million years ago ) and thus also includes a part of the Triassic . The use of

340-810: The similarities to neosuchians being as a result of convergent evolution . Mark T. Young and colleagues in 2024 defined Thalattosuchia in the PhyloCode as "the largest clade within Crocodylomorpha containing Macrospondylus bollensis and Thalattosuchus superciliosus , but not Protosuchus richardsoni , Notosuchus terrestris , Peirosaurus tormini , Anteophthalmosuchus hooleyi , Deltasuchus motherali , Pholidosaurus schaumburgensis , Dyrosaurus phosphaticus , and Crocodylus niloticus (the Nile crocodile )". 'The group contains basal taxa such as Turnersuchus and Plagiophthalmosuchus and

360-538: The two main groups Metriorhynchoidea and Teleosauroidea . Those two groups are united in the clade Neothalattosuchia, named and defined in the PhyloCode by Mark T. Young and colleagues in 2024 as "the smallest clade within Thalattosuchia containing Macrospondylus bollensis , Platysuchus multiscrobiculatus , Pelagosaurus typus , and Thalattosuchus superciliosus . Indeterminate remains possibly belonging to thalattosuchians have been reported from

380-648: The west coast of Scotland where, in the Sea of the Hebrides depositional basin on Skye , Raasay and Mull , the Broadford Beds Formation , Pabay Shale Formation and overlying Scalpay Sandstone Formation are assigned to the Lias Group. In Dutch lithostratigraphy, the name Lias has no official status, however, it is often used for the lower part of the Altena Group in the subsurface of

400-542: Was coined by Fraas in 1901. Various authors considered Thalattosuchia an infraorder or a suborder within " Mesosuchia ". However, the term "Mesosuchia" is a paraphyletic group, and as such is no longer used. For consistency, the Thalattosuchia are here placed at suborder rank, although the order that contains it is unnamed. The exact phylogenetic position of Thalattosuchia is uncertain, with them either being interpreted as members of Neosuchia alongside other aquatic crocodylomorphs, or more basal members of Crocodylomorpha, with

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