{{Short description|Geological formation in England}} {{Infobox rockunit|name=Helsby Sandstone Formation|image=Ladram Bay , Ladram Rock ^ Point - geograph.org.uk - 1831352.jpg|caption=Exposures of Helsby Sandstone Formation at Ladram Bay, Devon, England|type=[[Formation (stratigraphy)|Formation]]|age=[[Middle Triassic]] {{fossilrange|Anisian}}|period=Anisian|prilithology=[[sandstone]]|otherlithology=[[siltstone]], [[mudstone]], [[Conglomerate (geology)|conglomerate]]|namedfor=|namedby=|region=[[England]]|country=[[United Kingdom]]|coordinates=|unitof=[[Sherwood Sandstone Group]] (SSG)|subunits=|overlies=[[Wilmslow Sandstone Formation]] (disconformity) or [[Sherwood Sandstone Group|Chester Formation]] (unconformity)|thickness=Up to 500 m|extent=|area=|map=|map_caption=}}
The '''Helsby Sandstone Formation''', formerly known as the '''Otter Sandstone''' (in [[Devon]] and [[Somerset]]), is a geological formation in England. It was deposited during the [[Anisian]] stage of the [[Middle Triassic]] epoch. It primarily consists of [[Grain size|medium to fine grained]] [[sandstone]], both [[Cross-bedded|cross bedded]] and flat bedded, with minor [[Lenticular bedding|lenticular beds]] of [[siltstone]] and [[mudstone]] and thin beds of [[Conglomerate (geology)|conglomerate]] in some areas. It is found across England, from [[Devon]] on the southern coast to [[Cumbria]] in the north, with some exposures in [[Worcestershire]] and in [[Staffordshire]] in the Midlands.<ref>{{Cite web |title=Helsby Sandstone Formation- BGS Lexicon of Named Rock Units |url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=HEY |access-date=2025-09-10 |website=webapps.bgs.ac.uk}}</ref> It was deposited in a hot, [[Desert climate|arid climate]], when Britain formed part of the [[Pangaea|Pangean]] interior<ref>{{Cite journal |last1=Bloomfield |first1=J. P. |last2=Moreau |first2=M. F. |last3=Newell |first3=A. J. |date=January 2006 |title=Characterization of permeability distributions in six lithofacies from the Helsby and Wilmslow sandstone formations of the Cheshire Basin, UK |url=https://www.lyellcollection.org/doi/10.1144/GSL.SP.2006.263.01.04 |journal=Geological Society, London, Special Publications |language=en |volume=263 |issue=1 |pages=83–101 |doi=10.1144/GSL.SP.2006.263.01.04 |bibcode=2006GSLSP.263...83B |issn=0305-8719}}</ref><ref name=":0">{{Cite journal |last1=Coram |first1=Robert A. |last2=Radley |first2=Jonathan D. |last3=Benton |first3=Michael J. |date=June 2019 |title=The Middle Triassic (Anisian) Otter Sandstone biota (Devon, UK): review, recent discoveries and ways ahead |url=https://linkinghub.elsevier.com/retrieve/pii/S0016787817300925 |journal=Proceedings of the Geologists' Association |language=en |volume=130 |issue=3–4 |pages=294–306 |doi=10.1016/j.pgeola.2017.06.007 |bibcode=2019PrGA..130..294C |url-access=subscription |hdl=1983/e9769a73-9c34-43b5-813c-bdd2b9478d9f |hdl-access=free }}</ref> with the cross-bedded sandstones of the formation representing remnants of [[Aeolian processes|wind-blown]] [[sand dunes]],<ref>{{Cite journal |last1=Mountney |first1=Nigel P. |last2=Thompson |first2=David B. |date=August 2002 |title=Stratigraphic evolution and preservation of aeolian dune and damp/wet interdune strata: an example from the Triassic Helsby Sandstone Formation, Cheshire Basin, UK |url=https://onlinelibrary.wiley.com/doi/10.1046/j.1365-3091.2002.00472.x |journal=Sedimentology |language=en |volume=49 |issue=4 |pages=805–833 |doi=10.1046/j.1365-3091.2002.00472.x |bibcode=2002Sedim..49..805M |issn=0037-0746|url-access=subscription }}</ref> while the majority of the formation was produced by the action of [[braided river]]s, specifically the ‘Budleighensis’ river system, which flowed northwards from what is now northern France through southern and central England, before draining into what is now the [[Irish Sea]]. The environment of the formation likely also included [[pond]]s and small [[lake]]s, with surrounding vegetation including [[conifer]]s (whose roots are preserved as [[concretion]]s) and [[Equisetaceae|horsetails]]. Towards the top of the formation, the rocks indicate that the conditions became more arid, with the overlying formations showing the disappearance of rivers and the existence of [[Endorheic basin|playa]]-like conditions.<ref name=":0" />
The formation in its southern coastal exposure in Devon in the vicinity of [[Sidmouth]] has provided important fossils of contemporary vertebrates, including [[Temnospondyli|temnospondyl]] amphibians, [[Procolophonidae|procolophonid]], [[rhynchosauria]]n, [[sphenodontian]] and [[archosaur]]ian reptiles, as well as fish.<ref name=":0" />
== Paleobiota == Taken from<ref name=":0" /> unless otherwise noted: === Fish === {| class="wikitable" ! colspan="6" |Fish of the Helsby Sandstone |- !Genus !Species !Locality !Materials !Notes !Image |- |''[[Dipteronotus]]'' |''D. cyphus'' | | |A small {{Convert|10-12|cm}} long [[perleidiform]] fish, known from complete specimens, most common fish in the formation.<ref name=":1">{{Cite journal |last1=Coram |first1=Robert A. |last2=Radley |first2=Jonathan D. |last3=Benton |first3=Michael J. |date=September 2021 |title=Triassic tragedy—a bone bed in the Otter Sandstone of East Devon, south-west England |url=https://onlinelibrary.wiley.com/doi/10.1111/gto.12367 |journal=Geology Today |language=en |volume=37 |issue=5 |pages=176–183 |doi=10.1111/gto.12367 |bibcode=2021GeolT..37..176C |issn=0266-6979|url-access=subscription }}</ref> |[[File:Dipteronotus aculeatus.jpg|thumb|Fossil of the closely related species ''D. aculeatus'']] |- |[[Actinopterygii]] |Indeterminate | | |Includes a fish known from a single mostly complete {{Convert|10-12|cm}} long undescribed specimen distinct from ''Dipteronotus'', as well as the jaw of a pike-like fish, as well as other indeterminate microvertebrate remains.<ref name=":1" /> | |- |[[Elasmobranchii]] |Indeterminate | | |Freshwater shark known from a fin spine | |- |} === Amphibians === {| class="wikitable" ! colspan="6" |[[Amphibians]] of the Helsby Sandstone |- !Genus !Species !Locality !Materials !Notes !Image |- |''[[Eocyclotosaurus]]'' | | | |A [[capitosauria]]n [[Temnospondyli|temnospondyl]] |[[File:Eocyclotosaurus wellesi1DB.jpg|center|frameless]] |- |''[[Mastodonsaurus]]?'' | | | |A [[capitosauria]]n [[Temnospondyli|temnospondyl]], some authors have consider the remains indeterminate and not belonging to the genus<ref name=":0" /> |[[File:Mastodontosaurus spp DB24.jpg|center|frameless]] |- |[[Temnospondyli]] |Indeterminate | | |Includes a third taxon apparently distinct from ''Eocyclotosaurus'' and ''Mastodonsaurus'', as well as indeterminate fragmentary material | |- |}
=== Reptiles === {| class="wikitable" ! colspan="6" |[[Reptile]]s of the Helsby Sandstone |- !Genus !Species !Locality ! Materials !Notes !Image |- |''[[Agriodontosaurus]]'' <ref>{{Cite journal |last1=Marke |first1=Daniel |last2=Whiteside |first2=David I. |last3=Sethapanichsakul |first3=Thitiwoot |last4=Coram |first4=Robert A. |last5=Fernandez |first5=Vincent |last6=Liptak |first6=Alexander |last7=Newham |first7=Elis |last8=Benton |first8=Michael J. |date=2025-09-10 |title=The oldest known lepidosaur and origins of lepidosaur feeding adaptations |url=https://www.nature.com/articles/s41586-025-09496-9 |journal=Nature |pages=1–10 |language=en |doi=10.1038/s41586-025-09496-9 |issn=0028-0836|doi-access=free |pmc=12629995 }}</ref> |''A. helsbypetrae'' | | |A [[sphenodontian]] reptile known from a partial skeleton |[[File:Agriodontosaurus (holotype, BRSUG 29950-14).png|center|frameless]] |- |''[[Bentonyx]]'' |''B. sidensis'' | | |A [[rhynchosauria]]n reptile, known from a skull |[[File:Bentonyx sidensis.jpg|center|frameless]] |- |''[[Coartaredens]]'' |''C. isaaci'' | | |A reptile of uncertain classification, possibly a lepidosauromorph, but this is uncertain as it is only known from fragmentary remains | |- style="background:#FEF6E4; |''[[Chirotherium]]'' | | | |[[Ichnogenus]] of fossil trackways thought to be produced by "[[rauisuchia]]ns" | |- |''[[Feralisaurus]]''<ref>{{Cite journal |last1=Cavicchini |first1=Iacopo |last2=Zaher |first2=Marta |last3=Benton |first3=Michael J. |date=2020-05-03 |title=An Enigmatic Neodiapsid Reptile from the Middle Triassic of England |url=https://www.tandfonline.com/doi/full/10.1080/02724634.2020.1781143 |journal=Journal of Vertebrate Paleontology |language=en |volume=40 |issue=3 |article-number=e1781143 |doi=10.1080/02724634.2020.1781143 |bibcode=2020JVPal..40E1143C |issn=0272-4634|doi-access=free }}</ref> |''F. corami'' | | |A small [[Lepidosauromorpha|lepidosauromorph]] known from specimen BRSUG 29950-12, an articulated skeleton | |- |''[[Fodonyx]]'' |''F. spenceri'' | | |A [[rhynchosauria]]n reptile, known from postcranial and skull material |[[File:Fodonyx NT.jpg|center|frameless]] |- |''[[Kapes (reptile)|Kapes]]'' |''K. bentoni'' | | |A small lizard-like [[Procolophonidae|procolophonid]] reptile, known from a partial skeleton |[[File:Kapes full skeleton.jpg|center|frameless]] |- |[[Archosauria]] | rowspan="3" |Indeterminate | | |Remains include vertebrae, teeth and a jaw fragment, which may represent [[Poposauroidea|poposauroids]], with the jaw fragment suggested to possibly represent ''[[Bromsgroveia]].'' | |- |[[Rhynchosauria]] | | |Most common vertebrates in the formation, remains include a headless skeleton | |- |[[Procolophonidae]] | | |Several indeterminate procolophonids distinct from ''Kapes'', one of which has similarities to ''[[Sclerosaurus]]'' | |}
== References == {{Reflist}}
[[Category:Geologic formations of England]] [[Category:Geology of Devon]] [[Category:Geology of Worcestershire]] [[Category:Geology of Staffordshire]] [[Category:Anisian Stage]] [[Category:Sandstone formations]] [[Category:Triassic England]] [[Category:Triassic System of Europe]]