Istiorachis macarthurae: A new species of Iguanodontian Dinosaur from the Early Cretaceous Wessex Formation of the Isle of Wight.

The Iguanodontians were a highly successful group of Ornithischian Dinosaurs, which included groups such as the Dryosaurids, Iguanodonts, and Hadrosaurs. The oldest known Iguanodontian is Callovosaurus leedsi, a Dryosaurid from the Middle Jurassic Oxford Clay Formation of England, while by the End of the Cretaceous they were the most numerous group of Herbivorous Dinosaurs in Laurasia. The Iguanodontians were a relatively minor part of Late Jurassic faunas, but underwent a significant evolutionary radiation in the Early Cretaceous, becoming a numerous and morphologically diverse group, which they remained throughout the Cretaceous Period.

In a paper published in the journal Papers in Palaeontology on 21 August 2025, Jeremy Lockwood of the Fossil Reptiles, Amphibians and Birds Section at the Natural History Museum, and the School of the Environment and Life Sciences at the University of Portsmouth, David Martill, also of the School of the Environment and Life Sciences at the University of Portsmouth, and Sussanah Maidment, also of the Fossil Reptiles, Amphibians and Birds Section at the Natural History Museum, describe a new species of Iguanodontian Dinosaur from the Early Cretaceous Wessex Formation of the Isle of Wight.

The new species is named Istiorachis macarthurae, where 'Istiorachis' means 'sail-spine' in reference to the long neural spines on the vertebrae of the species, which may have given it a 'sail-backed' appearance, and 'macarthurae' honours  Dame Ellen MacArthur, an English sailor who in 2005 set a world record forthe fastest solo non-stop voyage around the world on her first attempt and who also founded the Ellen MacArthur Cancer Trust for young people on the Isle of Wight. It is described from a single, partial specimen recovered from the 1.5 m thick 'Black Band' which outcrops about 100 m to the east of Grange Chine (a chine on the Isle of White is a steep-sided gorge cut into a cliff made from a soft sediment by a stream immediately before reaching the sea). This 'Black Band' overlies the Grange Chine Sandstone within the Wessex Formation.

Locality and stratigraphy of Istiorachis macarthurae. (A) Generalised stratigraphic log and  schematic lithological logs of Wealden Group exposure between Sudmoor and Atherfield on the Isle of Wight,  showing excavation sites of the holotypes of the new Dinosaur (MIWG 6643), Brighstoneus simmondsi (MIWG 6344) and Mantellisaurus atherfieldensis (NHMUK PV R 5764). (B) Simplified geological map of the Isle of Wight. (C) Enlarged area showing the site of the excavation of MIWG 6643 in the Black Band (arrowed) (50.63354 N, 1.40654 W). Abbreviation: SS, sandstone. Note that the dashed line in (A) dividing the Wessex Formation into exposed and unexposed, applies only to the Isle of Wight exposures. Lockwood et al. (2025).

During the excavation of the specimen from which Istiorachis macarthurae is described,  MIWG 6643 was discovered and excavated by the late Nick Chase, a prolific Isle of Wight Dinosaur-hunter, who died of cancer in 2019. Unfortunately, during this excavation, the site was poached, and an unknown amount of material removed. The remaining material comprises one cervical vertebra, eight dorsal vertebrae, three dorsal rib heads, a partial sacrum, seven caudal vertebrae, both pubes and both ischia. Despite these loses, the remaining material is in good condition, with little distortion and good surface preservation.

Istiorachis macarthurae, holotype (MIWG 6643). Skeletal reconstruction. Scale bar represents 500 mm. Lockwood et al. (2025).

The most notable feature of Istiorachis macarthurae is the extremely long neural spines on its dorsal vertebrae. This is a trait which arose independently multiple times in Cretaceous Iguanodontians, although its purpose is unclear. 

Istiorachis macarthurae, holotype (MIWG 6643). 12th dorsal vertebra from early posterior series. (A)–(D), 12th dorsal vertebra in: (A) anterior; (B) left lateral; (C) posterior; (D) right lateral view. (E) reconstruction to show two consecutive vertebrae in lateral view. Abbreviations: cle, cleft; para, parapophysis; ri, ridge. Scale bar represents 50 mm. Lockwood et al. (2025).

A variety of modern Lizards have distinctive sails on their backs formed by elongation of the neural spines, but this is a feature associated with sexual selection and found only in the males. Such a role cannot be ruled out in Iguanodontians, but no evidence of sexual dimorphism has been found within the group (i.e., as far as we are aware, the males and females looked essentially the same). 

A Green Basilisk Lizard, Basiliscus plumifrons, in Alajuela Province, Costa Rica. This species has a distinctive crest, formed by elongation of the neural spines, but this trait is only seen in the males. Connor Long/Wikimedia Commons.

Large sails formed by elongated neural spines are also known from several groups of Carboniferous and Permian Tetrapods, including Sphenacodontids such as Dimetrodon limbatus and Echinerpeton intermedium, and Edaphosaurids, such as Edaphosaurus pogonias. In these Late Palaeozoic Tetrapods large sails appear to have been linked to temperature regulation. This cannot be ruled out in Iguanodontians, but where this the case crests would be expected to be a fixed trait (i.e. arising once then found in all subsequent members of the group), instead they seem to have appeared several times within different Iguanodontian lineages, with some groups having apparently gained and then lost crests.

Reconstructed skeleton of Dimetrodon incisivum from the Permian of Texas, in the collection of the Staatliches Museum für Naturkunde Karlsruhe in Germany. Such crests are thought to have been used for thermoregulation, and are a fixed trait within the Family Sphenacodontidae. Wikimedia Commons.

American Bison, Bison bison, have elongated neural spines on the forward part of their dorsal spine, which help to support the musculature needed to carry and move their large heads. Again, it is possible that the crests of Iguanodontians served a similar purpose, supporting extra musculature, but the distribution of elongated neural spines appears unrelated to size in the group, and it is unclear why this would have been present in some species, and absent in other, similar sized, species.

The skeleton of an American Bison, Bison bison, showing a crest made from elongated neural spines on the forward part of the dorsal spine. Museu de Anatomia Veterinária de Universidade de São Paulo/Wikimedia Commons.

Iguanodontians were secondarily quadrupedal grazing herbivores (that is to say, they descended from bipedal ancestors, but adopted a quadruped posture to facilitate grazing close to the ground). This required their spine be held in a horizontal position, and as they evolved to large sizes, to be able to bear large weights. To facilitate this, Iguanodontians evolved a system of ossified tendons, providing additional support for the spine, but also making it ridged and inflexible. 

Lockwood et al. speculate that elongated neural spines in Iguanodontians would have increased the efficiency of these ossified tendons, allowing for a reduction in muscle mass, which in turn would have enabled functions such as bending or running to be more efficient and less energetic. However, they are unable to explain why this would have been advantageous to some Iguanodontians but not others, and therefore the patchy distribution of the trait within the group.

Artist's impression of Istiorachis macarthurae in life. James Brown in Lockwood et al. (2025).

See also...

Vectipelta barretti: A new species of Ankylosaurian Dinosaur from the Early Cretaceous of the Isle of Wight.

The Ankylosaurs are Ornithischian Dinosaurs noted for their broad body, short limbs and extensive covering of dermal armour on the head, body and tail. They are a diverse group, found on all continents, but reaching their greatest diversity on the continents of the former Laurasian landmass, i.e. North America, Europe, and Asia. They split from their sister group, the Stegosaurs, in the Early or early Middle Jurassic, but appear to have remained a minor component of most ecosystems until the Early Cretaceous, when they underwent a significant radiation, remaining a significant member of the herbevore community until their demise in the End Cretaceous extinction. 

A number of Ankylosaurs have been described from the UK, from the Middle Jurassic onwards, although the group reach their maximum abundance in the UK in the Early Cretaceous Wealden Group of southern England. For a long time, all Ankylosaurs from the Wealden Group were referred to two species, Polacanthus foxii and Hylaeosaurus armatus, but this is now thought likely to hide considerable diversity, as the Wealden Group is thought to cover at least 8 million years of time, including all of the Barremian Stage and part of the Aptian.

Traditionally, all Ankylosaur material from the Isle of Wight was referred to Polacanthus foxii, but a recent study of Ankylosaur diversity within the Wealden Group, led by Thomas Raven of the Fossil Reptiles, Amphibians and Birds Section at the Natural History Museum, has suggested that this name should be restricted to the holotype specimen, NHMUKPV R175, and other specimens should be considered taxonomically uncertain until re-examined. One notable specimen examined in Raven et al.'s study was IWCMS 1996.153, known as the Spearpoint Ankylosaur, which was shown to be particularly different to the Polacanthus foxii holotype, and not likely to be closely related.

In a paper published in the Journal of Systematic Palaeontology on 15 June 2023, Stuart Pond, also of the Fossil Reptiles, Amphibians and Birds Section at the Natural History Museum, Sarah-Jane Strachan of the Department of Earth Sciences at University College London, Thomas Raven (again), Martin Simpson of Lansdowne on the Isle of Wight, Kirsty Morgan of Fayetteville in Arkansas, and Susannah Maidment, again of the Fossil Reptiles, Amphibians and Birds Section at the Natural History Museum, formally describe the Spearpoint Ankylosaur as the holotype of a new species.

The Wessex Group consists of a series of terrestrial, lacustrine, flu-vial and lagoonal deposits which outcrop in the Wessex Sub-basin of the Isle of Wight and the Weald Sub-basin of south-east England. On the Isle of Wight the group comprises two formations, the Wessex Formation, considered Barremian in age, and the Vectis Formation, thought to be late Barremian to early Aptian. The Wessex Formation is a sequence about 180 m thick, consisting of variegated mudstones and interbedded sandstones deposited as river channel, floodplain and point bar deposits by a high-sinuosity river system flowing west to east. The overlying Vectis Formation is thought to have been laid down in a shallow lagoon. The largest exposures of both formations are on the southwestern coast of the Isle of Wight, with smaller exposures at Yaverland on the south-east coast.

The Wessex Formation is thought to have been laid down in a Mediterranean climate, with distinctive dry and wet seasons, the latter associated with periodic flood events in which debris flows including large trees and Dinosaur remains were swept down from the (now lost) highlands from which the river descended, and were deposited on the flood plain. This formation is rich in fossils, producing numerous Plant (particularly Gymnosperms), Invertebrates, trace fossils, and a diverse Vertebrate assemblage, including Bony Fish, Turtles, Crocodilians, Pterosaurs and Dinosaurs.

The Spearpoint Ankylosaur was discovered within the Compton Chine to Steephill Cove Site of Special  Scientific Interest, about 50 m to the west of Chilton Chine. Chines on the Isle of Wight, are gullies which cut into the overlying chalk grassland by seasonal streams, and emerging from the cliffs as waterfalls. These features never erode down to beach level, as the cliffs are eroded backwards by the action of the sea faster than the chines are eroded downwards by the streams, and form a unique environment with their own distinctive flora and fauna, including many rare Plants and Insects.

In November 1993 fossil hunter Gavin Leng found several Ankylosaur vertebrae on beach, which he later donated to the Dinosaur Isle Museum. The following spring another fossil hunter Lin Spearpoint, discovered the remainder of the skeleton in the cliffs above, on land belonging to dairy farmer Richard Fisk. Fisk gave permission for Lin Spearpoint, Dick Spearpoint, and Martin Simpson, to excavate the skeleton, which was then prepared by Lin Spearpoint, housed at the University of Southampton, where it was the subject of several MSci, MRes and PhD projects, some of which were carried out by Stuart Pond, Sarah-Jane Strachan, and Kirsty Morgan, temporarily displayed at the SeaCity  Museum,  Southampton, and eventually acquired by the Dinosaur Isle Museum. 

Pond et al. formally describe the Spearpoint Ankylosaur as the holotype of a new species, Vectipelta barretti, where 'Vectipelta' means 'Vectis-shield' (Vectis was the name used by the Romans for the Isle of Wight), and 'barretti' honours Paul Barrett of the Natural History Museum, in recognition of his major and ongoing contributions to Dinosaurian Vertebrate palaeontology, and his importance to Stuart Pond, Thomas Raven and Susannah Maidment as a mentor, supervisor, colleague and friend.

Geological setting and stratigraphy. (A) Map of Isle of Wight with the extent of Wessex Sub-basin marked in orange, and the site of the Vectipelta barretti (IWCMS 1996.153 and IWCMS 2021.75) excavation indicated by the black arrow. (B) Schematic stratigraphical log of the outcrop between Sudmore Point and Ship Ledge showing the approximate stratigraphical location of Vectipelta barretti (IWCMS 1996.153 and IWCMS 2021.75). Pond et al. (2023).

The new species is described from two specimens, both believed to have come from the same individual and therefore treated as one. WCMS1996.153 comprises portions of four cervical vertebrae and six  free dorsal vertebrae discovered on the foreshore by Gavin Leng in 1993, while IWCMS 2021.75 comprises five fused dorsal vertebrae, forming the sacral rod, fragments of several dorsal ribs, a fused sacrum made up of four and a half fused vertebrae, thirteen caudal vertebrae, and fragments of four more, the proximal part of the left scapula, the proximal and distal ends of the left humerus, a possible ulna fragment, multiple pieces of the iliosacral block, including parts both the left and right ilia, both pubes, fused to the anteromedial margins of the acetabula, a more-or-less complete left ischium, some possible fibula fragments, and second metatarsal, the sacral shield, a large number of osteoderms, and three recurved spines. Specimen IWCMS 2021.75 was found lying on its back, with its (missing) head pointing towards the cliff face, leading to the assumption that the cervical and upper dorsal vertebrae of WCMS1996.153 came from the same individual. Pond et al, note that several years prior to the discovery of WCMS1996.153, a beach-rolled Ankylosaur braincase was found at the same location by fossil hunter David Cooper, and acquired by the University of Cambridge's Sedgwick Museum, and a partial femur and posterior part of an ilium were found nearby by David Richards in the 1990s and donated to the Natural History Museum. These specimens may also relate to the Spearpoint Ankylosaur, but all are heavily eroded, making it impossible to assess them with any confidence, so they were excluded from the study.

Holotype material of Vectipelta barretti, IWCMS 1996.153 and IWCMS 2021.75 in (A) dorsal view; (B) left lateral view; (C) dorsal view of the osteoderms only; (D) complete skeletal reconstruction, in left lateral view. Scale baris 1m. Pond et al. (2023).

The recurved spines of the Spearpoint Ankylosaur are notable, as NHMUKPV R175, the holotype of Polacanthus foxii, lacks any such structure, supporting the idea that the two are quite different Animals. Some spines assigned to Polacanthus sp. have been described from Early Cretaceous of Soria, Spain, but these are also unlikely to derive from that genus. Recurved spines are known in a wide range of Ankylosaurs, including the other Wealden Group species, Hylaeosaurus armatus, but the form of the Spearpoint Ankylosaur's spines is different to any of these, supporting the idea that it should be described as a new species.

Vectipelta barretti, IWCMS 2021.75. Examples of recurved spines. Orientation presumed: (A)–(C) proximal; (D)–(F) distal; (G)–(I) left lateral; (J)–(L) right lateral; (M)–(O) anterior; (P)–(R) posterior. Scale bar is 10 cm. Pond et al. (2023).

Understanding the diversity of Early Cretaceous Ankylosaurs is not just important because we are interested in Dinosaurs; it has implications for our understanding of Mesozoic ecosystems and geology as a whole. The Jurassic is believed to have ended with a mass extinction event, although the nature of this is poorly understood. Terrestrial deposits with good fossil preservation are rare from the Early Cretaceous, leading to the apparent emergence of a quite new ecosystem in the Middle Cretaceous. However, the clustering of multiple specimens from diverse groups under a few poorly constrained species names in Early Cretaceous deposits, as has happened with Wealden Ankylosaurs (and, it has also been suggested, Iguanodontids), could be hiding a more gradual faunal turnover during this interval.

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A new species of Albanerpetontid Amphibian from the Early Cretaceous of the Isle of Wight.

The Albanerpetontid Amphibians were a distinct order of Lisamphibians (the group that includes all modern Amphibians and anything else descended from their most recent common ancestor) known in the fossil record from the Middle Jurassic to the late Pliocene. They were morphologically conservative, resembling Salamanders in shape (though they were distinctive). A single known specimen with preserved skin was scaly and had eyelids, implying a dryer habitat preference than Salamanders, though it is not known if this applied to the whole group.

In a paper published in the April 2013 edition of the journal Acta Palaeontologica Polonica, Steven Sweetman of the School of Earth and Environmental Sciences at the University of Portsmouth and James Gardner of the Royal Tyrrell Museum of Palaeontology describe a new species of Albanerpetontid Amphibian from the Early Cretaceous Wessex Formation of the Isle of Wight.

The new species is named Wesserpeton evansae, where 'Wesserpeton' means 'creeping animal from Wessex' and 'evansae' honours Susan Evans of University College London, a distinguished microvertebrate palaeontologist who has make important contributions to our understanding of the Albanerpetontids.

Wesserpeton evansae is described from a series of disarticulated, mostly partial, skeletons comprising frontal (forehead) bones, premaxillae (front part of the upper jaw), maxillae (remainder of the upper jaw), fragments of dentary, a number of fragmentary articular and angular bones (bones connecting the lower jaw to the skull, fused in Albanerpetontid Amphibians and moved to the inner ear in Mammals), a single prearticular, a variety of vertebrae, one complete and several partial humeri and several partial ilia.

Scanning electron micrographs of two nearly complete dentaries of the Albanerpetontid Amphibian Wesserpeton evansae from the Barremian Wessex Formation at Yaverland on the Isle of Wight. (A) substantially complete left dentary missing most of area for attachment of post−dentary bones in lingual (A₁), labial (A₂), and dorsal (A₃) views. (B) Right dentary preserving similar amount of bone, but with pathological anterior portion in lingual (B₁) and dorsal (B₂) views; enlargement of the anterior end in ventral view (B₃), showing bony callous and distortion in region of possible healed fracture and re−modelled symphyseal surface. Sweetman & Gardner (2013).

See also A new species of Crested Newt from the Balkan and Anatolian Peninsulas, Stomach contents in Jurassic Salamanders, A new species of Toad from the coastal restingas of southern Brazil, Two new species of Aromobatid Frog from Panama and New species of Frog from Japan,  

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