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).

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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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The Ashdown Maniraptoran

This May's edition of the journal Cretaceous Research saw a paper by Darren Naish and Steve Sweetman of the School of Earth and Environmental Sciences, University of Portsmouth in which they describe a neck vertebrae from a maniraptoran dinosaur. This was discovered by fossil collector Dave Brockhurst, at the Ashdown Brickworks Quarry at Bexhill in Sussex, and has been dated as belonging to the Valanginian Age (Early Cretaceous), or as being about 140 million years old(ish). If the identification is correct then this will be both the first maniraptoran dinosaur discovered in the UK, and one of the smallest non-avian dinosaurs (i.e. dinosaurs that are not birds) ever discovered.

The single bone from which the Ashdown Maniraptoran was described, seen from a variety of angles.

Dinosaur finds have a reputation for being the only kind of scientific discovery that sells newspapers, and the story duly appeared in a number of papers (e.g. the Mirror, The Daily Mail, Huffington Post), though most of these relied heavily on the University's original press release. This is fair enough, vertebrate palaeontology is a fairly technical subject, and few papers have science corespondents equipped for in depth analysis. The fossil has also been discussed on specialist palaeontology blogs, such as those of senior author Darren Naish or dinosaur artist Matt Martyniuk, though these tend to be quite technical and aimed at other vertebrate palaeontologists. Thus there is a lack of a non-technical discussion of this (fairly remarkable) paper; one which having a background in palaeontology (though not dinosaur palaeontology) I shall attempt to fill.

What is a Maniraptoran?
The maniraptorans are a group of predominantly small theropod dinosaurs, including birds and those groups closely related to them; as such the maniraptorans are not extinct, though all non-avian groups died out during the Late Mesozoic (at least 65 million years ago). When palaeontologists talk about this fossil being the first maniraptoran fossil from the UK they are referring to non-avian maniraptorans.

The maniraptorans are distinguished by having a calcified breast bone (as opposed to a cartilage sternal plate), an elongated, three fingered hand, and a half-moon shaped bone in the wrist. The non-avian maniraptorans were the first group of dinosaurs in which feathers were well described, from remarkably well preserved fossils from Lioaning Province in China, although these have now been found in a wider range of dinosaur groups. Several groups have backwards-facing, bird-like hips.

The maniraptorans comprise six main groups, and a few extra specimens which don't fall precisely into any group. Their classification is likely to change in the future; many maniraptorans are small and known only from fragmentary remains, it is quite possible that new specimens will emerge that cause dinosaur specialists to review their current classification.

The Therizinsaurs are a poorly understood group with bird-like hips and adaptations for herbivory, they had three large, elongate claws on each hand, and specimens with feathers have been found. The smallest therizinsaurs were slightly over two meters long, the largest about twelve meters, and weighed over six tonnes (big for a theropod dinosaur. They were known only from fragmentary remains till the mid 1990s, and subsequently were variously classified as theropods, prosauropods of ornithischians.

Reconstruction of the Therizinsaur Beipiaosaurus, by artist Matias Bofarull Oddo.

The Alvaresaurs are a group of small dinosaurs with elongated rear limbs and snouts (with very small teeth), and reduced fore-limbs, though their structure suggests these fore-limbs were very powerful, with large arm and breast muscles. Feathered specimens have been found. It has been suggested that the Alvaresaurs were adapted to digging into termite mounds or similar insect structures. Many Alvaresaurs are only known from partial specimens, making detailed reconstructions difficult, but those that can be reconstructed ranged in size from about 40 cm to about 2.5 meters.

Reconstruction of the Alvaresaur Achillesaurus by artist Hodari Nundu.


The Oviraptosaurs are a group of maniraptorans with short tails, few or no teeth, and beak-like jaws. They ranged in size from about 15 cm to about 8 meters, though as with other groups many species are known only from fragmentary remains, so this range might be extended. Feathers have been discovered on specimens of several species. Some dinosaur specialists consider the Oviraptosuars should be classified as being true birds.

An artists impression of the Oviraptosaur Hagryphus giganteus. Illustration by dinosaur artist Michael W. Skrepnick.

The Avialans are the birds (Aves) plus a couple of specimens considered so close to the birds they are placed with them rather than in any other group. The two non-bird members of the group are Scansoriopteryx and Epidexpteryx. It is possible that these are the same species as Scansoriopteryx is only known from two juvenile specimens and Epidexpteryx from a single adult specimen, although Scansoriopteryx has a very long tail and Epidexpteryx a very short one, which would be an unusual growth pattern.

A reconstruction of Epidexipteryx by Qui Ji and Xing Lida.

The Troodontids have larger brains for their size than any other known dinosaur group, comparable to modern birds and larger than Mesozoic forms. They also had large forward facing eyes, and enlarged middle ears, implying acute senses. The ears were slightly asymmetric, a trait also known from owls, which suggests that they may have had a similar ability to pinpoint sounds. They had retractable or semi-retractable claws on their second toes. Troodontids ranged in size from about 30 cm to about 2.4 meters in length. Feathers are well documented in the Troodontids.

Illustration of Troodon by Dr Anne Musser of the Australian Museum.

The Dromaeosaurs were bipedal dinosaurs with well developed retractable claws on their second toes, fore-limbs that could be folded close against their bodies and elongate, rigid tails. Most if not all species appear to have been feathered and some species may have been able to fly. They ranged in size from about 70 cm to over 8 meters in length. The Dromeosaurs are often encountered in popular culture as 'raptors'.

A reconstruction of Velociraptor by dinosaur artist Michael W. Skrepnick.


Where did it live?
The fossil was found in the Ashdown Formation, part of the Wealden Group, a Lower Cretaceous sequence of rocks from southern and eastern England and running out under the North Sea. The Wealden Group represent subtropical coastal plains and shallow marine environments. The Ashdown Formation are sand and mudstones from braided river formation, possibly an environment similar to the Lower Mississippi, Senegal, Ganges or Murray-Darling river basins. On the whole smaller vertebrates have lower preservational potential than large ones, since their skeletons are more easily broken up. In an environment such as a river basis whole skeletons from small animals are unlikely to survive, but individual bones stand a good chance of being fossilized.

How confident is the diagnosis?
Claiming a dinosaur group was present in an area where it was previously unknown from a single bone is a bold step, and mistakes have been made in the past, but this does not mean the diagnosis is wrong. In the nineteenth century it was common to describe each new bone discovered and formally name it. Since precedence is important in naming fossils, this meant that a palaeontologist who had painstakingly reconstructed a complete animal could well lose the right to name this to an amateur fossil hunter who had found a single bone. Unsurprisingly this way of doing things fell into disrepute, and for much of the twentieth century palaeontologists ignored isolated bones. Naish and Sweetman are clearly aware of this, and have declined to name a species on the basis of this bone. They have, however, been able to make some deductions from the bone.

Dinosaur vertebrae are quite distinctive to a specialist, and bones from the different parts of the spine can also be identified with confidence, so the diagnosis of the bone as a neck bone from a small dinosaur is probably reasonable. The bone appears to have stopped growing, supporting their analysis of it as an adult bone, suggesting that this does indeed come from a small species, though exactly how small is hard to judge; a reconstruction based upon a typical maniraptoran gives a length of 30-50 cm, though if it belongs to a different group then this would be different. The bone has a distinctive 'x' shaped profile, which has been observed in Oviraptorsaurs, on the basis of which Naish and Sweetman conclude that the bone is a dinosaur bone, most likely to be a Oviraptosaur, and if not then likely to be a closely related Maniraptoran. It is possible that the bone does come from a different form of dinosaur, but this would be a more remarkable find, since no other dinosaur group is known to produce specimens this small, or with this bone structure.

See also An Australian Spinosaurid,
Head-Butting in a Pachycephalosaur
and Dinosaurs on Sciency Thoughts YouTube.