# Wessex Formation

The Wessex Formation is a fossil-rich English geological formation that dates from the Berriasian to Barremian stages of the Early Cretaceous. It forms part of the Wealden Group and underlies the younger Vectis Formation and overlies the Durlston Formation. 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

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

## Stratigraphy and lithology
### Introduction

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.

#### Cornubian Massif

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.

### 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, Lulworth Cove and Worbarrow Bay. 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. The transition to the Vectis Formation at Swanage is obscured by a landslip.

### The Wessex Formation of the Isle of Wight

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 Hauterivian to Barremian. A radiometric date has been obtained from the lower part of the succession, estimated to be 127.3 ± 2.7 million years ago. 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. The climate at the time of deposition is considered to be semi-arid, based on the presence of pedogenic calcrete nodules within the mudstones. The "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.

#### Plant debris beds

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 Pseudofrenelopsis 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. 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. 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. Plant debris beds also exist within the Swanage section, and one of these horizons has yielded microvertebrate remains.

#### "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. 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.

## Palaeoenvironment

The palaeoenvironment of the Wessex Formation is considered to have been semi-arid, and has variously been compared to chaparral or macchia Mediterranean shrubland. The dominant trees were conifers of the extinct family Cheirolepidiaceae belonging to the genera Pseudofrenelopsis and Watsoniocladus, both of which have reduced xerophytic leaves adapted to arid conditions. Tree cover is thought to have been thin, and concentrated near waterways. The ground cover is thought to have consisted of xerophytic ferns. Wildfires were common occurrences, as evidenced by preserved charred vegetation.

## Fauna

The Wessex Formation likely spans several million years of deposition, and not all taxa are likely to have been contemporaneous.

### Invertebrates

Invertebrates are commonly preserved in the Wessex Formation. Freshwater bivalves can be found including unionids such as Margaritifera, Nippononaia, and Unio. These bivalves help reconstruct 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 Libanodiamesa simpsoni, both chironomid midges, as well as Embolemopsis maryannae, a embolemid parasitic wasp. However a table of undescribed taxa has been given, and several images of some of the undescribed taxa have been released from various sources, including multiple chironomids, and a therevid dipteran.
| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |

#### Arthropods

Arthropods of the Wessex Formation :
| Genus | Species | Location | Stratigraphic Position | Abundance | Note | Images |
| Cypridea | C. wicheri, C. brevirostrata | Dungy Head | Base of the formation | | An ostracod belonging to Cypridoidea | |
| Cretamygale | C. chasei | Chilton Chine | L6 plant debris bed | Single specimen | Mygalomorph spider, has been described from a specimen found in amber. | See article |
| Dungeyella | D. gavini | Chilton Chine | L6 plant debris bed | Multiple specimens | A tiny buchonomyiine chironomid midge | See article |
| Embolemopsis | E. maryannae | 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. | |
| Protopangu | P. valdensis | Chilton Chine | L6 plant debris bed | Single specimen | A panguid parasitic stinging wasp | |
| Libanodiamesa | L. simpsoni | 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. | |

### Vertebrates
#### Dinosaurs
##### Ornithischians
###### Ankylosaurs

Ankylosaurs reported from the Wessex Formation :
| Genus | Species | Synonyms | Stratigraphic Position | Material | Notes | Images |
| 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. | An ankylosaur | |
| Vectipelta | V. barretti | | L5 | Partial skeleton | An ankylosaur. Referred to as the "Spearpoint ankylosaur" prior to its original description. Only distantly related to Polacanthus. | |

###### Neornithischians

Neornithischians reported from the Wessex Formation :
| Genus | Species | Synonyms | Stratigraphic Position | Material | Notes |
| Brighstoneus | B. simmondsi | | L9 | Partial skeleton | A hadrosauriform iguanodontian |
| Comptonatus | C. chasei | | Compton Bay | Partial skeleton | An iguanodontian dinosaur |
| Hypsilophodon | H. foxii | | Hypsilophodon bed | Many partial skeletons | A hypsolophodontid |
| Heterodontosauridae? | Indeterminate | | Yaverland bed 38 | Teeth | Resemble the cheek teeth of the heterodontosaurid Tianyulong. |
| Iguanodon | I. bernissartensis | | | | A hadrosauroid iguanodontian. Specimens classified as Iguanodon seelyi are referable to this species |
| Mantellisaurus | M. atherfieldensis | Dollodon bampingi, Proplanicoxa? | | | A hadrosauroid iguanodontian |
| Valdosaurus | V. canaliculatus | | Recovered from various parts of the formation | Multiple partial skeletons | A dryosaurid |
| Vectidromeus | V. insularis | | Lower part of the formation | Associated elements of the dorsal vertebrae, pelvis, hindlimbs, and tail, from a juvenile specimen | A hypsilophodontid |

##### Saurischians
###### Sauropods

Sauropods reported from the Wessex Formation :
| Genus | Species | Stratigraphic Position | Material | Notes |
| Chondrosteosaurus | C. gigas | | "Several cervical vertebrae." | |
| Eucamerotus | E. foxi | | "Dorsal vertebra." | Perhaps a somphospondylan? |
| Iuticosaurus | I. valdensis | | "Several caudal vertebrae." | |
| Oplosaurus | O. armatus | | "Tooth." | |
| Ornithopsis | O. eucamerotus | | "Ischia and pubis.", regarded as an "undiagnostic titanosauriform of uncertain affinities." | |
| Ornithopsis | O. hulkei | | One dorsal vertebra. | |
| Undescribed Sauropod (Barnes High sauropod) | Indeterminate | | "Partial postcranial skeleton, including presacral vertebrae, anterior caudal vertebrae, girdle and limb elements" | Currently in private collection and unavailable to researchers. Known informally as "the Barnes High sauropod". Possibly the same as Eucamerotus |
| "Angloposeidon" (informal) | Indeterminate | L1 | One cervical vertebra and possible associated centrum | Represents a large animal 20 metres or greater in length. Known informally as "Angloposeidon". Perhaps a somphospondylan? |
| Rebbachisauridae | Indeterminate | Scapula found approximately around L11 | Scapula, radius and ulna, several cervical sacral and caudal vertebrae and isolated teeth from multiple individuals | Taxon has close affinities with Demandasaurus and Nigersaurus |
| 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 | Material | Notes |
| Avialae | Indeterminate | | Isolated teeth | Possibly an Enantiornithe and a Hesperornithid present |
| Aristosuchus | A. pusillus | | "Sacrum and pubis." | A compsognathid |
| Calamosaurus | C. foxi | | "Vertebrae." | A compsognathid |
| Calamospondylus | C. oweni | | "Vertebrae." | A possible oviraptorosaur |
| Ceratosuchops | C. inferodios | | Partial skull material | A spinosaurid, formerly considered remains of Baryonyx. |
| Eotyrannus | E. lengi | Chilton Chine | "Partial skull and skeleton." | A tyrannosauroid |
| Neovenator | N. salerii | Chilton | 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 | A spinosaurid, formerly considered remains of Baryonyx. |
| Tetanurae | Indeterminate | Chilton Chine | Partial pubis and femur | Distinct from Neovenator and Baryonyx. Currently in private collection. |
| Thecocoelurus | T. daviesi | | "Cervical vertebrae." | A theropod of uncertain classification, possibly an ornithomimosaur (this assignment has been questioned by both Mortimer and Naish). |
| Undescribed coelurosaur | Indeterminate | | Partial associated skeleton | Apparently small. In private collection and undescribed. Referred to as "That Which Cannot Be Named" by Darren Naish Has been suggested to be a tyrannosauroid. |
| Coelurosauria (Calamospondylus?) | Indeterminate (or possibly C. oweni) | | Tibia | Previously referred to Hypsilophodon, Aristosuchus, Calamosaurus, Ornithomimosauria indet. and Coelurosauria indet. |
| Velociraptorinae | Indeterminate | | Isolated teeth | May belong to a proceratosaurid, or alternatively they may belong to Vectiraptor. |
| 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. |

#### Fish
##### Cartilaginous fishes

Cartilaginous fishes of the Wessex Formation :
| Genus | Species | Stratigraphic Position | Abundance | Note |
| Egertonodus | H. basanus | | | Hybodontid hybodont shark |
| Hybodus | Indeterminate 1 | Yaverland bed 38, L9 | | Hybodontid hybodont shark |
| Hybodus | Indeterminate 2 | Yaverland bed 38, L9 | | Hybodontid hybodont shark |
| Hybodus | Indeterminate 3 | Yaverland bed 38, L9 | | Hybodontid hybodont shark |
| Lonchidion | L. breve | Yaverland bed 38 | | Lonchidiid hybodont shark |
| Lonchidion | L. striatum | Yaverland bed 38, L9 | 51.43% of total chondrichthyan taxa in L9 | Lonchidiid hybodont shark |
| Lonchidion | Indeterminate 1 | Yaverland bed 38, L9 | | Lonchidiid hybodont shark |
| Lonchidion | Indeterminate 2 | Yaverland bed 38, L9 | 35.86% of total chondrichthyan taxa in Yaverland bed 38 | Lonchidiid hybodont shark |
| Lonchidion | Indeterminate 3 | Yaverland bed 38, L9 | | Lonchidiid hybodont shark |
| Hylaeobatis | H. problematica | | | Lonchidiid hybodont shark |
| Palaeoscyllium | Indeterminate | | | Catshark |
| Parvodus | P. heterodon | | | Lonchidiid hybodont shark |
| Protospinax | Indeterminate | | | Selachimorph shark |
| Vectiselachos | V. ornatus | | | Lonchidiid hybodont shark |

##### Ray-finned fish

Ray-finned fishes of the Wessex Formation :
| Genus | Species | Abundance | Notes | Images |
| Belonostomus | Indeterminate | Aspidorhynchiformes | Known from isolated teeth | Full skeleton of the related Belonostomus tenuirostris |
| Caturus | Indeterminate | Amiiform fish | | |
| Coelodus | Indeterminate | Pycnodontid fish | | |
| Coccolepis | Indeterminate | Coccolepidid fish | | |
| Lepidotes | Indeterminate | | | |
| Ocloedus | Indeterminate | | | |
| Pachythrissops | Indeterminate | | | |
| Pycnodontiformes | Indeterminate | | | |
| Scheenstia | Indeterminate | Lepidotid fish | | |

#### Mammals

Mammals of the Wessex Formation :
| Genus | Species | Location | Stratigraphic Position | Material | Notes |
| Eobaatar | E. clemensi | Multiple debris beds | L9, Yaverland bed 38 and CL3 | Teeth | Eobaatarid multituberculate |
| ?Gobiconodon | Indeterminate | Yaverland | Yaverland bed 38 | A lower right premolariform | Gobiconodontid |
| Loxaulax | ?L. valdensis | | | Teeth | Eobaatarid multituberculate |
| Yaverlestes | Y. gassoni | Multiple debris beds | Mandible from Yaverland bed 38, isolated teeth from L2, L9 and L14 | Mandible Fragment and isolated teeth | Spalacotheriid |
| Dryolestidae | 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) | |
| Eutriconodonta | Indeterminate | Yaverland | Yaverland bed 38 | Lower left molariform | |
| Zatheria | Indeterminate | | | Premolars | Eutheria? |

#### Lissamphibians

Amphibians of the Wessex Formation :
| Genus | Species | Location | Stratigraphic Position | Abundance | Notes |
| Wesserpeton | W.evansae | | L2 and Yaverland 38 | | Albanerpetontid amphibian |
| Anura | Indeterminate | Multiple | 1: Yaverland 38 2: L2 and Yaverland 38 3: L2 and Yaverland 38 4: Yaverland 38 5: Yaverland 38 | At least 5 distinct taxa distinguished by characters in their ilium | |
| Urodela | Indeterminate | Multiple | 1: L2, L14 and Yaverland 33 and 38 2: L9 and Yaverland 38 3: unnamed bed in Compton bay and Yaverland 38 | At least 3 distinct taxa distinguished by their atlas vertebrae | |

#### Reptiles
##### Squamates

Squamates of the Wessex Formation :
| Genus | Species | Location | Stratigraphic Position | Abundance | Notes |
| Meyasaurus | Indeterminate | Yaverland | Yaverland bed 38 | Teeth and a partial lower jaw | Genus also known from Spain |
| Anguimorpha | indeterminate | Multiple plant debris beds | 1st taxon L14 and L2, 2nd L2 and Yaverland bed 38, 3rd Yaverland bed 38 | At least 3 distinct taxa represented by isolated teeth, maxilla and lower jaw fragments, alongside a possible fourth taxon. | |
| Scincomorpha | indeterminate | Multiple plant debris beds | 1: L14 and Yaverland 38 2: Yaverland 38, 3: L2 and Yaverland 38 4,5: Yaverland bed 38 6:L2, L14 and Yaverland 38 7,8,9: Yaverland 38 10: L2, L14 and Yaverland 38 | At least 10 distinct taxa represented by isolated teeth, maxilla and lower jaw fragments, some of which are paramacellodids based on the common occurrence of osteoderms typical of this clade. | |

##### Neosuchians

Neosuchians of the Wessex Formation :
| Genus | Species | Location | Stratigraphic Position | Abundance |
| Anteophthalmosuchus | A. hooleyi | | Disputed, either uppermost Wessex or lowermost Vectis | |
| Anteophthalmosuchus | A. epikrator | Hanover Point | | Mostly complete skull and partial dentaries and associated postcranial material |
| Bernissartia | Indeterminate | | | 40 Isolated teeth |
| Hulkepholis | H. willetti | | | |
| Koumpiodontosuchus | K. aprosdokiti | | | |
| Theriosuchus | Indeterminate | | | |
| Vectisuchus | V. leptognathus | Barnes High | Just below base of Vectis formation | "Partial semi-articulated skeleton" |

##### Plesiosaurs

Plesiosaurs of the Wessex Formation :
| Genus | Species | Location | Stratigraphic Position | Abundance | Notes | Images |
| Elasmosauridae | Indeterminate | Compton Bay | | Vertebra | | |
| Plesiosauroidea | Indeterminate | Tie pits, atherfield | | Propodial | Leptocleididae? | |

##### Turtles

Turtles of the Wessex Formation :
| Genus | Species | Material | Notes |
| Helochelydra | H. nopcsai | Shell and skull material | Helochelydrid |
| Brodiechelys | B. brodiei | Shell fragments | Xinjiangchelyid, previously known by the synonyms Plesiochelys brodiei Lydekker, 1889; Plesiochelys valdensis Lydekker, 1889 and Plesiochelys vectensis Hooley, 1900 |
| Eodortoka | E. cf. morellana | Partial shell with associated vertebrae and limb bones | A dortokid panpleurodiran turtle. |

#### Pterosaurs

Pterosaurs of the Wessex Formation :
| Genus | Species | Stratigraphic Position | Material | Notes |
| 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" | Estimated wingspan of 5.6 metres |
| Uktenadactylus | U. rodriguesae | | Rostrum fragment | An ornithocheirid, distinct from Coloborhynchus |
| Wightia | W. declivirotris | | Rostrum fragment | A tapejarid |

## Flora
### Spermatophytes

Spermatophytes reported from the Wessex Formation :
| Family | Genus | Species | Material | Notes |
| Araucariaceae | Agathoxylon? | Indeterminate | Wood | Reported as "Dadoxylon" Considered a dubious referral |
| Araucariaceae | Brachyphyllum | B. obesum | Leaves | |
| Cheirolepidiaceae | Pseudofrenelopsis | P. parceramosa | Leaves and abundant, occasional segments of trunk | |
| Cheirolepidiaceae | Watsoniocladus | W. valdensis | Leaves | |
| Cupressaceae | Sphenolepis | | Leaves | "taxodiaceous" |
| Pinaceae | Pityites | P. solmsii | | |

### Pteridophytes

Pteridophytes reported from the Wessex Formation :
| Genus | Species | Material | Notes |
| Polypodiopsida | Indeterminate | | |
| Tempskya | Indeterminate | | May actually originate from the underlying Purbeck Group. |
| Weichselia | W. reticulata | Multiple specimens | |