Wessex Formation

Wessex Formation Stratigraphic range: Late Berriasian–Barremian, 140–125 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Exposure of the Wessex Formation west of Chilton Chine
Type	Geological formation
Unit of	Wealden Group
Sub-units	Coarse Quartz Grit (in Dorset)
Underlies	Vectis Formation
Overlies	Durlston Formation
Thickness	up to 1000 m near Swanage
Lithology
Primary	mudstone
Other	sandstone, ironstone & conglomerate
Location
Region	Southern England
Country	UK
Extent	Dorset, Isle of Wight, offshore Wessex Basin
Type section
Named for	Wessex
Named by	Daley and Stewart
Location	Bacon Hole, Mupe Bay
Year defined	1979
Exposure of the Wessex and Vectis Formations on the South Coast of the Isle of Wight, shown in turquoise, exposures in Dorset not shown.

The Wessex Formation is a fossil-rich English geological formation that dates from the Berriasian to Barremian stages (about 145–125 million years ago) of the Early Cretaceous. It forms part of the Wealden Group and underlies the younger Vectis Formation and overlies the Durlston Formation.^[1] The dominant lithology of this unit is mudstone with some interbedded sandstones. It is part of the strata of the Wessex Basin, exposed in both the Isle of Purbeck and the Isle of Wight. While the Purbeck sections are largely barren of vertebrate remains, the Isle of Wight sections are well known for producing the richest and most diverse fauna in Early Cretaceous Europe.

Nomenclatural History[]

The Wessex Formation has historically alternately been called the "Variegated Marls And Sandstones", a name used by W. J. Arkell in his 1947 map of the Isle of Purbeck^[2] as well as the "Wealden Marls" ^[3] It was given its current formal name by Daley and Stewart in 1979^[4]

Stratigraphy and Lithology[]

Introduction[]

Position of the Wessex Formation within the Wessex Basin succession

The Wessex Formation forms part of the Wealden Group within the Wessex Basin, an area of subsidence since Permo-Triassic times. The basin is located along southern half of the Isle of Wight and Purbeck, extending offshore into the English Channel. The Wealden Group is also exposed significantly in the Weald Basin, which has a separate stratigraphic succession. The Wealden Group is not widely present elsewhere in Britain, as these areas were tectonic highs where little to no sediment deposition was taking place. The formation has limited exposure as it has been deeply buried beneath the subsequent Lower Greensand. Selbourne and Chalk Groups, as well as being very vulnerable to erosion. It has been exposed at the surface due to the creation of anticlinal structures as a distant effect of the formation of the Pyrenees as part of the Alpine Orogeny during the Paleogene.^[5] The major source rocks for the sediments were from the Cornubian Massif to the west, an upland region roughly equivalent to the extent of Cornwall and Devon, with occasional large dropstones transported in tree roots being found in Wealden sediments over 100 kilometres from where they originated.^[6]

The Wessex Formation in the Isle of Purbeck[]

The exposure in of the Wessex Formation in the Isle of Purbeck is largely confined to a thin belt on the south side of the Purbeck Ridge and is best exposed at Swanage,^[7] Lulworth Cove^[8] and Worbarrow Bay.^[9] One notable persistent horizon within the Purbeck sections of the formation is the "Coarse Quartz Grit", an up to 6 metre thick sequence of conglomeratic ironstone, with many beds including numerous centimetre sized subangular to rounded pebbles predominantly of vein derived quartz, hence the name. This horizon is present throughout the Purbeck outcrops of the Wessex Formation.^[9]

The Wessex Formation of the Isle of Wight[]

Wessex Formation mudstones

The Isle of Wight succession has two primary exposures, The major one being the several kilometre long section along the South West coastline around Brighstone Bay, and another smaller exposure on the South East coast near Yaverland. While the formation taken as a whole dates from the Berriasian to the Barremian, only the uppermost part of the formation is exposed on the Isle of Wight. With less than 200 metres of exposed composite stratigraphic thickness, and which dates from latest Hauterivian to Barremian. The boundary between the two ages lies near the Pine Raft horizon.^[10] This makes the formation coeval with upper portion of the Weald Clay in the Weald Basin. The primary lithology of the exposed portion of the formation on the Isle of Wight consists of featureless purple-red overbank mudstone, interbedded with sandstones. The environment of deposition was a floodplain within a narrow. east-west oriented valley.^[11] The climate at the time of deposition is considered to be semi-arid, based on the presence of pedogenic calcrete nodules within the mudstones.^[12] The aforementioned "Pine Raft" horizon found near the base of the exposed portion of the formation includes calcitized conifer trunks up to metre in diameter and 2–3 metres long.^[13]

Cast of a dinosaur footprint, Wessex Formation

Plant Debris Beds[]

Close up of L6 plant debris bed, showing charcoalised plant matter suspended in grey mudstone

A notable feature of the formation are the so-called "Plant debris beds". These consist of a basal matrix supported conglomerate, grading upwards into grey mudstone with lignitic plant debris, including large trunk fragments of the extinct conifer present in the upper portion. These were formed by sheet flood deposits induced by storms that filled pre-existing topographic lows like oxbow lakes and abandoned channels in the floodplain depositional environment.^[14] The debris beds do not form a continuous horizon throughout the formation, but are laterally extensive over tens of metres. Many of the wood fragments in the debris beds are cemented together with large nodules of pyrite, suggesting depositional conditions were anoxic.^[15] Most fossils within the formation are associated with the debris beds. Vertebrate fossils are mostly disarticulated individual bones and teeth, suggesting a long subaerial exposure prior to burial, though the bones lack abrasion, suggesting that they had not been significantly transported. Partial skeletons also sometimes occur, but are uncommon. Autochthonous siderite nodules are also present, which encase some of the fossils.^[14] Plant debris beds also exist within the Swanage section, but are noticeably sparse in fossils.^[16]

"Hypsilophodon bed"[]

While most fossils are associated with the plant debris beds, a notable exception is the "Hypsilophodon bed" present near the top of the formation, an up to 1 metre thick bed of silty red-green mudstone, with two separate horizons that have produced almost exclusively over a hundred complete and articulated skeletons of the dinosaur Hypsilophodon, sometimes even with preserved tail tendons. The bed is laterally extensive, being persistent for over a kilometre. It has been recently suggested that the accumulation of skeletons were a mass mortality event caused by a crevasse splay.^[17] Just above the "Hypsilophodon bed" the red mudstones of the Wessex Formation change to the transitional light coloured sandstone "White rock" and overlying laminated grey mudstones of the Vectis Formation, caused by the changing of environmental conditions from that of a floodplain to coastal lagoon conditions, initiated by the early Aptian marine transgression. The transition to the Vectis Formation is also present in Swanage but thins out and disappears westward, having an erosive unconformity with the Lower Greensand Group or the Gault in these areas, often with a basal pebble bed.

Paleoenvironment[]

The paleoenvironment of the Wessex Formation is considered to have been semi-arid, and has variously been compared to chaparral^[11] or macchia^[16] Mediterranean shrubland. The dominant trees were conifers of the extinct family Cheirolepidiaceae belonging to the genera and , both of which have reduced xerophytic leaves adapted to arid conditions.^[16] Tree cover is thought to have been thin, and concentrated near waterways.^[18] The ground cover is thought to have consisted of xerophytic ferns.^[16]

Fauna[]

Invertebrates[]

Invertebrates are commonly preserved in the Wessex Formation. Freshwater bivalves can be found including unionids such as Margaritifera, , and Unio. These bivalves are helpful in reconstructing what the freshwater paleoenvironment may have been like during the formation's deposition. Specimens of Viviparus, a genus of freshwater snail, have also been found. While compression fossils of insects are found in the overlying Vectis Formation, all insect fossils in the Wessex formation are found as inclusions in amber. Amber can be found present as a rare component in plant debris beds in the Wessex formation both on the Isle of Wight and the Isle of Purbeck, however the only significant concentration and where all of the inclusions have been found is a lag channel in the L6 plant debris horizon just south-east of Chilton Chine. Only four species from the amber have been formally described, Cretamygale chasei a mygalomorph spider, Dungeyella gavini^[19] , both chironomid midges, as well as , a embolemid parasitic wasp. However a table of undescribed taxa has been given,^[19] and several images of some of the undescribed taxa have been released from various sources, including multiple chironomids, and a therevid dipteran.^[20]

Arthropods[]

Arthropods of the Wessex Formation
Genus	Species	Location	Stratigraphic Position	Abundance	Note	Images
Cretamygale	C. chasei[21]	Chilton Chine	L6 Plant debris bed	Single specimen	Mygalomorph spider, has been described from a specimen found in amber.	See article
Dungeyella	D. gavini[22]	Chilton Chine	L6 Plant debris bed	Multiple specimens	A tiny buchonomyiine chironomid	See article
	E. maryannae[23]	Chilton Chine	L6 Plant debris bed	Single specimen	Embolemid parasitic wasp, genus also known from the Zaza Formation. Specimen previously described as a dryinid in a 2015 conference abstract.[20]
	L. simpsoni[24]	Chilton Chine	L6 Plant debris bed	Single specimen	Prodiamesinae chironomid, genus also known from the Lebanese amber
Chironomidae	Indeterminate	Chilton Chine	L6 Plant debris bed	44% of all arthropod inclusions	Uncertain how many taxa represented, likely at least two. At least one is a tanypodian with a wingspan twice that of Dungeyella.
Therevidae	Indeterminate	Chilton Chine	L6 Plant debris bed	At least one specimen	At least one specimen figured
Diptera	Indeterminate	Chilton Chine	L6 Plant debris bed	Multiple specimens	Uncertain how many taxa represented
Hymenoptera	Indeterminate	Chilton Chine	L6 Plant debris bed	Multiple specimens	Uncertain how many taxa represented
Blattodea	Indeterminate	Chilton Chine	L6 Plant debris bed	Single specimen
Coleoptera	Indeterminate	Chilton Chine	L6 Plant debris bed	Single specimen
Curculionoidea?	Indeterminate				Represented by a boring in a gymnospermous seed.[25]

Cartilaginous fishes[]

Ray-finned fish[]

Ray-finned fishes of the Wessex Formation
Genus	Species	Location	Stratigraphic Position	Abundance	Notes	Images
Belonostomus	Indeterminate			Aspidorhynchiformes
Caturus	Indeterminate			Amiiform fish
Coelodus	Indeterminate			Pycnodontid fish
Coccolepis	Indeterminate
Lepidotes	Indeterminate
	Indeterminate
Pachythrissops	Indeterminate
Pycnodontiformes	Indeterminate
Scheenstia	Indeterminate			Lepidotid fish

Lissamphibians[]

Squamates[]

Turtles[]

Neosuchians[]

Plesiosaurs[]

Mammals[]

Mammals of the Wessex Formation
Genus	Species	Location	Stratigraphic Position	Material	Notes	Images
Eobaatar	E. clemensi[34]	Multiple debris beds	L9, Yaverland bed 38 and CL3	Teeth	Eobaatarid multituberculate
?Gobiconodon	Indeterminate	Yaverland	Yaverland bed 38	A lower right premolariform[35]	Gobiconodontid
Loxaulax	?L. valdensis[34]			Teeth	Eobaatarid multituberculate
Yaverlestes[36]	Y. gassoni	Multiple debris beds	Mandible from Yaverland bed 38, isolated teeth from L2, L9 and L14	Mandible Fragment and isolated teeth	Spalacotheriid
Dryolestidae[27]	Indeterminate	Multiple debris beds	Yaverland bed 38 and L9	A lower right molar (NHMUK.M45558) C, an upper left molar (NHMUK.M45564)., a tentatively referred lower left or upper right premolar (NHMUK.M45484)[35]
Eutriconodonta	Indeterminate	Yaverland	Yaverland bed 38	Lower left molariform
Zatheria[27]	Indeterminate			Premolars	Eutheria?

Pterosaurs[]

Pterosaurs of the Wessex Formation
Genus	Species	Location	Stratigraphic Position	Material	Notes	Images
Caulkicephalus	C. trimicrodon			Skull and rostrum fragments	An ornithocheirid
Ctenochasmatidae	Indeterminate			Tooth, potentially a gnathosaurine
Istiodactylus	I. latidens			Partial skeleton and skull, and referred mandible fragment	An istiodactylid
Istiodactylidae	Indeterminate		Found throughout the sub basin	Teeth	Two other species, distinct from I.latidens
Neoazhdarchia	Indeterminate			Humerus
Ornithocheiridae	Indeterminate			"partial distal left metacarpal IV"[37]	Estimated wingspan of 5.6 metres
Uktenadactylus	U. rodriguesae			Rostrum fragment	An ornithocheirid, distinct from Coloborhynchus
Wightia[38]	W. declivirotris			Rostrum fragment	A tapejarid

Dinosaurs[]

Ornithischians[]

Ornithischians reported from the Wessex Formation
Genus	Species	Synonyms	Stratigraphic Position	Material	Notes	Images
Hypsilophodon	H. foxii		Hypsilophodon bed	Many partial skeletons
Heterodontosauridae?	Indeterminate			Teeth	Resemble the cheek teeth of the heterodontosaurid Tianyulong.[28]
Brighstoneus	B. simmondsi			Partial skeleton[39]
Iguanodon	I. bernissartensis				Specimens classified as Iguanodon seelyi are referable to this species
Mantellisaurus	M. atherfieldensis	Dollodon bampingi, Proplanicoxa?[40]
Valdosaurus	V. canaliculatus			Multiple partial skeletons	A dryosaurid
Ankylosauria	Undescribed			Partial skeleton	Referred to as the "Spearpoint ankylosaur". Part of the skeleton is in Dinosaur Isle (IWCMS 1996.153), while other parts in a private collection. Likely represents a distinct taxon from Polacanthus.[41]
Polacanthinae	Indeterminate				Distinct from P. foxii.[42]
Polacanthus	P. foxii	Polacanthoides ponderosus?			Dorsal vertebrae, dorsosacral vertebrae, sacral vertebrae, caudosacral vertebra, caudal vertebrae, dorsal ribs, ilia, pubes, ischia, femora, tibia, distal end of fibula, astragalus, metatarsals, ungual phalanx, sacral shield and osteoderms.

Sauropods[]

Sauropods reported from the Wessex Formation
Genus	Species	Location	Stratigraphic Position	Material	Notes	Images
Chondrosteosaurus	C. gigas			"Several cervical vertebrae."[43]
Eucamerotus	E. foxi			"Dorsal vertebra."[43]	Perhaps a somphospondylan?[44]
Iuticosaurus	I. valdensis			"Several caudal vertebrae."[43]
Oplosaurus	O. armatus			"Tooth."[45]
Ornithopsis	O. eucamerotus			"Ischia and pubis.",[43] regarded as an "undiagnostic titanosauriform of uncertain affinities."
	O. hulkei			One dorsal vertebra.[46]
Undescribed Sauropod (Barnes High sauropod)	Indeterminate			"Partial postcranial skeleton, including presacral vertebrae, anterior caudal vertebrae, girdle and limb elements"[47]	Currently in private collection and unavailable to researchers. Known informally as "the Barnes High sauropod". Possibly the same as Eucamerotus[47]
"Angloposeidon" (informal)	Indeterminate			One cervical vertebra and possible associated centrum[48]	Represents a large animal 20 metres or greater in length. Known informally as "Angloposeidon". Perhaps a somphospondylan?
Rebbachisauridae	Indeterminate			Scapula, radius and ulna, several cervical sacral and caudal vertebrae and isolated teeth from multiple individuals	Taxon has close affinities with Demandasaurus and Nigersaurus[49]
Titanosauria	Indeterminate			Two isolated large middle caudal vertebra, one isolated large cervical vertebra, BMNH R5333, two articulated caudal vertebrae with an articulated fragment of a third

Theropods[]

Theropods reported from the Wessex Formation
Genus	Species	Location	Stratigraphic Position	Material	Notes	Images
Avialae	Indeterminate			Isolated teeth	Possibly an Enantiornithe and a Hesperornithid present
Aristosuchus	A. pusillus			"Sacrum and pubis."[50]	A compsognathid
Calamosaurus	C. foxi			"Vertebrae."[51]	A compsognathid
Calamospondylus	C. oweni			"Vertebrae."[51]	A possible oviraptorosaur
Ceratosuchops	C. inferodios			Partial skull material[52]	A spinosaurid, formerly considered remains of Baryonyx.
Eotyrannus	E. lengi			"Partial skull and skeleton."[53]	A tyrannosauroid
Neovenator	N. salerii			Cranial and postcranial remains of at least four individuals.	A carcharodontosaurian
Ornithodesmus	O. cluniculus				A species of dromaeosaurid, once misidentified as a pterosaur
Richardoestesia?	Indeterminate			Teeth	Dubious, uncertain referral on the genus level
Riparovenator	R. milnerae			Partial skull and referred caudal material[52]	A spinosaurid, formerly considered remains of Baryonyx.
Tetanurae	Indeterminate	Chilton Chine		Partial pubis and femur[54]	Distinct from Neovenator and Baryonyx. Currently in private collection.
Thecocoelurus	T. daviesi			"Cervical vertebrae."[55]	A theropod of uncertain classification, possibly an ornithomimosaur (this assignment has been questioned by both Mortimer and Naish).[56][57]
Undescribed coelurosaur	Indeterminate			Partial associated skeleton	Apparently small. In private collection and undescribed. Referred to as "That Which Cannot Be Named" by Darren Naish[56] Has been suggested to be a tyrannosauroid.[58][28]
Velociraptorinae	Indeterminate			Isolated teeth	May belong to a proceratosaurid.[59]
Vectiraptor	V. greeni			Dorsal vertebrae and sacral vertebrae	A large bodied Eudromaeosaur
Yaverlandia	Y. bitholus	Yaverland		Known from a "partial skull roof comprising both frontals and parts of the right postorbital and left orbitosphenoid". A second specimen is known but has not been formally published.	A maniraptoran of uncertain classification, originally identified as an ornithischian.

Flora[]

Spermatophytes[]

Pteridophytes[]

Pteridophytes reported from the Wessex Formation
Genus	Species	Location	Stratigraphic Position	Material	Notes	Images
Polypodiopsida	Indeterminate
Tempskya	Indeterminate				May actually originate from the underlying Purbeck Group.[16]
Weichselia	W. reticulata			Multiple specimens

See also[]

• List of fossil sites
• List of stratigraphic units with dinosaur body fossils
• Dinosaurs of the Isle of Wight

References[]

• Batten, D. J. (ed.) 2011. English Wealden Fossils. The Palaeontological Association, London.

External links[]

• Discovery of shark teeth