Uragasaurus kalasinensis: A new species of Mamenchisaurid Sauropod from the Late Jurassic of northeastern Thailand.

The Mamenchisaurids were a group of non-Neosauropod Sauropods (i.e. outside the largest grouping of Sauropods) which formed a significant part of the fauna of East Asia during the Middle and Late Jurassic. They can be distinguished by highly pneumatised and elongated cervical vertebrae, as well as procoelous vertebrae on the front part of the caudal spine (procoelous vertebrae have a convex forward disk and a concave rear disk), which were distinctive during the Jurassic, but evolved convergently in several Neosauropod groups during the Cretaceous. 

Most known Mamenchisaurids come from China, and for a long time they were thought to be restricted to that country. However, in 2005 fragmentary remains attributed to the group were recovered from the Middle to Late Jurassic Khlong Min Formation of Krabi Province in southern Thailand. In 2013, further fragmentary remains were found in the Late Jurassic to Early Cretaceous Phu Kradung Formation of northeastern Thailand, and in 2019 a new species of Mamenchisaurid, Wamweracaudia keranjei, was described on the basis of a partial skeleton from the Late Jurassic Tendaguru Formation of southeastern Tanzania, establishing the presence of the group in Africa.

In a paper published in the journal Scientific Reports on 8 July 2026, Apirut Nilpanapan of the Department of Biology at Mahasarakham University, Sita Manitkoon of the Palaeontological Research and Education Centre and Vertebrate Palaeontology and Evolution Research Unit at Mahasarakham University, Varavudh Suteethorn of the Khon Kaen Geopark Association, and Komsorn Lauprasert also of the Department of Biology and Evolution Research Unit at Mahasarakham University, describe a new species of Mamenchisaurid Sauropod from the Phu Kradung Formation of northeastern Thailand.

The new species is named on the basis of material collected from the Phu Noi Locality, which is in the village of Ban Din Chi in the Kham Muang District of Kalasin Province, in the northeast of Thailand. Here an outcrop of the Phu Kradung Formation has produced one of the most diverse non-marine Vertebrate fossil assemblages in Southeast Asia. The Phu Kradung Formation comprises a series of sandstones, siltstones, and mudstones laid down in a fluvial environment, within a continental basin. 

Locality map and section diagram of Phu Noi Locality. (a) Map of Thailand; (b) location of Phu Noi Locality and the distribution of Phu Kradung Formation, the northeastern region with the outline of Kalasin Province; (c) diagrammatic section of Phu Noi. Nilpanapan et al. (2026).

The precise age of the Phu Kradung Formation is unknown, as it lacks any radiometrically datable horizons. However, regional stratigraphic correlations, Vertebrate assemblages, and detrital zircon data suggest that it is of Late Cretaceous origin, possibly with the uppermost part of the formation extending into the earliest Cretaceous.

The Phu Noi outcrop comprises numerous channel horizons formed within a braided river, rather than horizontal layers extending across the whole site. It has three fossil-bearing horizons. The lowest of these is a grey conglomeratic sandstone laid down in a channel bottom. About 10 m above this, the middle horizon is a brownish-purple and greenish-grey sandy siltstone and mudstone. At the same level as this, but about 400 m to the southwest, the upper horizon is a greyish siltstones within proximal floodplain deposit.

The specimen from which Nilpanpan et al. describe the new species comes from the middle horizon, which is particularly rich in Vertebrate remains, having previously yielded Hybodont Sharks,  Ginglymodian Fish, Lungfish, Eucryptodiran Turtles, Teleosaurid Crocodyliformes, and Neornithischian Dinosaurs, and with Brachyopid Temnospondyl, Rhamphorynchoidea Pterosaur, Tyrannosauroid and Metriacanthosaurid Theropod specimens currently being studied. This faunal assemblage shows a strong affinity to the Late Jurassic and earliest Cretaceous faunas of the Junggar, Turpan, and Sichuan basins of China.

The new species is named Uragasaurus kalasinensis, where 'Uragasaurus' derives from 'Uraga' (उरग) the Sanskrit word for Snake, in reference to the long, serpentine, neck of Sauropod Dinosaurs, plus '-saurus' (σαύρος), the Greek for Lizard, a common suffix in Dinosaur names, and 'kalasinensis' means 'from Kalasin' in reference to the province where the specimen was discovered. 

Uragasaurus kalasinensis is described from a single isolated anterior dorsal vertebra (PRC 460), from the Phu Noi locality, which is housed in the collection of the Palaeontological Research and Education Centre at Mahasarakham University. A number of other Sauropod elements were found close to this specimen in the same level, which Nilpanpan et al. refer to Uragasaurus kalasinensis. However, because these elements cannot be assigned to the same original Animal with 100% confidence, and they do not have overlapping diagnostic features, they are not included in the formal description of the species. This material includes two anterior dorsal neural arches (KS 34-581 & KS 34-586), a left coracoid (KS 34-587), a left fibula (KS 34-588), a middle cervical vertebra (KS 34-602a), a right cervical rib (KS 34-602b), a middle-to-posterior dorsal vertebra (KS 34-692), and a posterior dorsal vertebra (PN 13-23).

Holotype of Uragasaurus kalasinensis (PRC 460) and associated materials in the quarry map. PRC 460 Anterior dorsal vertebra in anterior view (a), KS 34-581 anterior dorsal neural arch in anterior view (b), KS 34-602a middle cervical vertebra in ventral view (c), KS 34-586 anterior dorsal neural arch in anterior view, attached by KS 34-588 fibula (d), KS 34-587 coracoid in lateral view (e), KS 34-602b right cervical rib in lateral view (f). Quarry map showing the spatial distribution of the holotype and associated materials from the Phu Noi Locality (g). PRC 460, representing the new taxon Uragasaurus kalasinensis, is indicated in red. Associated Sauropod elements include KS 34-586, KS 34-587, KS 34-588, and KS 34-602a–b, highlighted in yellow, green, blue, purple, and pink, respectively. The inset shows a close-up of the excavation grid highlighting the relative positions of the holotype and nearby associated materials. Each grid square represents 0.75 × 0.75 m. Nilpanpan et al. (2026).

The anterior dorsal vertebra assigned to Uragasaurus kalasinensis has a prominent, elongated teardrop-shaped pneumatic fossae on the distal portion of the transverse processes, intraprezygapophyseal laminae meeting ventromedially to form a Y-shaped configuration in anterior view, incorporating a single vertical intraprezygapophyseal lamina, and a shallow, subtriangular pleurocoel lacking an internal septum. 

The holotype anterior dorsal vertebra of Uragasaurus kalasinensis (PRC 460) in anterior (a) and posterior (b) views. Digital rendering of the specimen in anterior (c), posterior (d), right lateral (e), left lateral (f), dorsal (g), and ventral (h) views. Asterisk refers to an autapomorphic character. The blue highlight indicates the pneumatic fossa and pleurocoel. Abbreviations: Cpol, centropostzygapophyseal lamina; cprl, centroprezygapophyseal lamina; di, diapophysis; ns, neural  spine; pa, parapophysis; pcdl, posterior centrodiapophyseal lamina; pl, pleurocoel; pnfo, pneumatic  fossa; po, postzygapophysis; podl, postzygodiapophyseal lamina; ppdl, paradiapophyseal lamina; prdl, prezygodiapophyseal lamina; posl, postspinal lamina; prsl, prespinal lamina; spof, spinopostzygapophyseal fossa; spol, spinopostzygapophyseal lamina; sprf, spinoprezygapophyseal fossa; sprl, spinoprezygapophyseal  lamina; stprl, single interprezygapophyseal lamina; tp, transverse process; tpol, intrapostzygapophyseal lamina;  tprl, intraprezygapophyseal lamina. Nilpanpan et al. (2026).

A computed tomography of specimen PRC 460 showed that its centrum has a camellate internal pneumatic structure composed of numerous small, irregular chambers separated by thin bony septa, although it was not possible to accurately measure the dimensions of these cavities due to mineral infilling. This is a structure unique to advanced Mamenchisaurids, which differs from the procamerate internal structure seen in certain Neosauropods and the camerate condition found in Macronarians and Diplodocoids. 

Computed tomography scan of the anterior dorsal vertebra of Uragasaurus kalasinensis (holotype PRC 460). Three￾dimensional reconstructions of the vertebra in anterior view (a) and right lateral view (c). Corresponding computed tomography sections in anterior view (b) and right lateral view (d). The section plane corresponds to the dashed line separating the grey (anterior) and blue (posterior) regions in the 3D reconstructions. White boxes highlight camellate pneumatic cavities within the centrum. Red arrows indicate polygonal camellae within the camellate internal structure of the centrum. Black-and-white arrows indicate anatomical orientation in each panel. Nilpanpan et al. (2026).

Phylogenetic analysis of the taxonomic affinities of Uragasaurus kalasinensis consistently recovered the species as a basal Mamenchisaurid, although its precise placement within this group was hard to determine, which is unsurprising given the limited nature of the material. However, the same analysis consistently found Rhomaleopakhus turpanensis, a species formerly classified as being a member of the Mamenchisauridae, as being outside the group, which Nilpanpan et al. suggest may indicate the need to re-evaluate the taxonomic status of a number of Late Jurassic Asian Sauropods. 

A previous Mamenchisaurid specimen, KS26-4, was described from the Phu Dan Ma locality in 2013. This specimen comprises a nearly complete posterior cervical vertebra and two fragmentary ribs. Since this material did not contain any elements considered reliably diagnostic within the Mamenchisauridae, it was not formally described as a new species. This lack of diagnostic features, combined with a lack of shared elements with the material assigned to Uragasaurus kalasinensis leads Nilpanpan et al. to refrain from assessing whether it belongs to the same species.

Posterior cervical vertebra of Mamenchisaurus sp. from Phu Dan Ma, Kalasin Province,Thailand,Phu Kradung Formation, Late Jurassic–Early Cretaceous. Vertebra (SM KS26−4), right rib (SMKS26−2), and left rib (SMKS26−3) in anterior (A; A₂, close−up view of neural spine showing attachment scar for interspinal elastic ligament), left lateral (B), posterior (C), right lateral (D; D₂, close−up view of articular condyle showing a cancellous internal structure), and dorsal (E) views. Suteethorn et al. (2013).

See also...

Parafaveoloolithus xixiaensis: A new oospecies of Dinosaur eggs from the Upper Cretaceous of HenanProvince, China.

The Xixia Basin lies in Henan Province, China, to the east of the Qinling Mountains of southern Shaanxi Province, and extends roughly 100 km east-to-west, covering an area of about 518 km². Within this basin a series of Upper Cretaceous fluvial deposits overlie a Devonian basement. These deposits have been divided into three formations, the middle one of which, known as the Zhaoying (or sometimes Majiacun) Formation, comprises a 2120 m thick sequence beginning with motley argillaceous siltstones, sandstones and mudstones at the base, and grading into an upper layer comprising reddish mudstones and sandstones. This sequence, considered to have been laid down in a fluvio-lacustrine basin in an area with a generally arid climate, is noted for its production of preserved Dinosaur eggs, with at least seven described oospecies (because eggs are a record of a part of the life-cycle of an animal, can seldom be related to a species defined from body fossils, they are described under a parataxonomic system as oospecies, which are then organised into oogenera and oofamilies) from the Zhaoying and underlying Zoumagang (or Gaogou) formation, as well as ichnofossils (trace fossils), Dinosaur bones, Turtle eggs, and fossil Bivalves, Gastropods, Ostracods, Spinicaudatans, and Plants.

In a paper published in the journal Acta Palaeontologica Polonica on 17 December 2025, Qing He and Shutong Li of the School of Resources and Environmental Engineering at Anhui University, Shukang Zhang of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, Yifan Huang of the Prevention and Control Center for Geological Disasters at the Henan Geological Bureau, Xiqiang Cao of the Henan Scientific Academy of Land and Resources, and Hongqing Li and Mengyuan Zhu, also of the School of Resources and Environmental Engineering at Anhui University, describe a new oospecies of Dinosaur eggs from the Zhaoying Formation of Xixia County.

The new species is placed in the oogenus Parafaveoloolithus, and given the specific name 'xixiaensis' meaning 'from Xixia'. The species is described from a clutch of 13 subspherical eggs arranged in a radial pattern. The individual eggs are 123.3–142.6 mm by 97.2–127.2 mm, with shells 123.3–142.6 mm and 97.2–127.2 mm thick. The shells have a single structural layer with no visible growth lines and a honeycomb structure with straight pore canals. 

A clutch of Dinosaur egg oospecies Parafaveoloolithus xixiaensis. YJYM-01–13 (each egg has unique repository number), from the Upper Cretaceous of the Xixia Basin, Henan Province, China. He et al. (2025).

The oogenus Parafaveoloolithus belongs to the oofamily Faveoloolithidae, which includes six genera from the Late Cretaceous of China, Mongolia, and South Korea. No fossil eggs from outside East Asia have been assigned to the oofamily (some 'Titanosaur eggs' from the Late Cretaceous of Argentina have been suggested as possible members of the family, but this is doubtful), suggesting that the egg-layers had a limited geographical distribution, although they are found in a variety of different palaeoenvironments.

Thin sections (SREE X13-01) of Dinosaur eggshell Parafaveoloolithus xixiaensis, (YJYM-13) from the Upper Cretaceous of the Xixia Basin, Henan Province, China. (A₁) A single structural layer composed of loosely arranged eggshell units and the straight pore canals between eggshell units; arrows indicate the secondary eggshell units. (A₂) A line drawing showing the eggshell units in radial section. (A₃) Enlargement of the gathered egg￾shell units; arrow points to the single eggshell unit. (A₄) Growth centres of the gathered eggshell units; arrows point to the six growth centres. He et al. (2025).

Very few eggs belonging to the Faveoloolithidae have been found in clutches to date, and Parafaveoloolithus xixiaensis is probably the best known example to date. The radial pattern in which the eggs are arranged suggests that this is a true representation of how they were deposited, rather than a result of transportation and redeposition. He et al. suggest that the pattern and porosity of the eggs implies the female Dinosaur would have deposited the eggs in a roughly circular arrangement, before covering them over with sand - something which would also have aided there preservation. 

See also...

Athenar bermani: A new species of Diplodocoid Sauropod from the Late Jurassic Morrison Formation of Dinosaur National Monument, Utah.

In 1913 prolific American fossil hunter Earl Douglass excavated a Sauropod braincase and partial skull roof from the Carnegie Quarry site at Dinosaur National Monument in Utah. This made its way to the collection of the Carnegie Museum in Pittsburgh, where it was given the specimen number CM 26552, and was largely forgotten for half a century (to give some context, Douglass excavated over 300 tonnes of material, including many complete skeletons of Jurassic Dinosaurs such as Diplodocus, Dryosaurus, Stegosaurus, Barosaurus, Camarasaurus, and Brontosaurus, which may have left museum staff a bit busy).

In 1978, palaeontologists David Berman and John McIntosh published a study in which they revised the genera Diplodocus and Camarasaurus, in which they assigned the specimen CM 26552 to Diplodocus. However, since that time our understanding of Sauropods as a group has developed significantly, notably in that proportional differences, which could change significantly as Sauropods grew, are no longer given the same emphasis, with modern palaeontologists instead placing more emphasis on character states (discrete features which can be present or absent).

In a paper published in the journal Palaeontologica Electronica in October 2025, John Whitlock of the Department of Science and Mathematics at Mount Aloysius College and the Section of Vertebrate Paleontology at the Carnegie Museum of Natural History, Juan Pablo Garderes and Pablo Gallina of the Consejo Nacional de Investigaciones Científicas y Técnicas, the Argentina and Fundación de Historia Natural Félix de Azara, and the Centro de Ciencias Naturales, Ambientales y Antropológicas at Universidad Maimónides, and Matthew Lamanna, also of the Section of Vertebrate Paleontology at the Carnegie Museum of Natural History, formally redescribe specimen CM 26552, assigning it to a new species and genus.

Based upon their inspection of CM 26552, Whitlock et al. conclude that it should be placed in the Dicraeosauria, a sub-group of the Diplodocidae, the skulls of which can be determined by (1) the presence of postparietal and frontoparietal fenestrae, (2) the exclusion of the basioccipital from the dorsal margin of the occipital condyle by the exoccipitals, (3) the presence of a distinct prong on the squamosal, (4) the contribution of the frontal to the margin of the supratemporal fenestra, (5) an expanded crista prootica, (6) a free dorsal margin of the antotic process, (7) the presence of a “shelf” overhanging the foramen for the trigeminal (V) nerve, and (8) the flat distal margin of the paroccipital process.

Braincase CM 26552 in anterior (A), (C) and posterior (B), (D) views. Abbreviations: BO, basioccipital; BS, basisphenoid; BT, basal tubera; CPR, crista prootica; EO-OP, exoccipitalopisthotic; F, frontal; LS, laterosphenoid; OS, orbitosphenoid; P, parietal; PO, postorbital; POP, paroccipital process; PR, prootic; S, shelf overhanging the opening for cranial nerve V; SOC, supraoccipital; SQ, squamosal; I, opening for cranial nerve I; II, opening for cranial nerve II; IV, opening for cranial nerve IV; V, opening for cranial nerve V. Whitlock et al. (2025).

Within that group, however, specimen CM 26552 shows a unique combination of features, not seen in any other genera, plus one unique character state,  the presence of a ‘tooth’ in the parietal/opisthotic suture, which has not previously been seen. For this reason, Whitlock et al. assign CM 26552 to a new species and genus, under the name Athenar bermani, where 'Athenar' honours the musician Athenar, 'for whom no better palaeontological comparison exists than a broken skull', and 'bermani' honours palaeontologist David Berman, who did so much of the fundamental modern work on Diplodocoid skulls at Carnegie Museum of Natural History and was responsible for the initial description of the specimen. 

Given the limited material available, no size estimate is made for Athenar bermani, although Whitlock et al. note that it appears to have been larger than the mature Diplodocus CM 11161, but shows incomplete fusion of many of the sutures of the braincase, suggesting that the specimen was a subadult at its time of death. 

See also...

Using drones to hunt for Lichens - as a way to find Dinosaur bones.

The progress of palaeontology as a science is reliant upon the discovery of new fossils, but this process is in itself often close to random. Palaeontologists can identify places they where believe fossils are likely to be found, but this is no guarantee of results, with success often more reliant on luck than science. Anecdotal evidence has suggested for some time that types of red Lichen grow preferentially on the bones of Vertebrates in the Dinosaur beds of western North America, but this has not previously been put to any actual scientific test.

In a paper published in the journal Current Biology on 3 November 2025, Brian Pickles of the School of Biological Sciences at the University of Reading, Caleb Brown of the Royal Tyrrell Museum of Palaeontology, the Department of Earth Sciences at the University of Manitoba, and the Department of Biological Sciences at the University of Alberta, Sean Herridge-Berry of the Department of Geography and Environment and Institute for Geospatial Inquiry, Instruction and Innovation at the University of Lethbridge, Cameron Martin, Melissa Dergousoff, and Teri Gilmar, also of the School of Biological Sciences at the University of Reading, Phil Bell of the Palaeoscience Research Centre at the University of New England, and Derek Peddle, also of the Department of Geography and Environment and Institute for Geospatial Inquiry, Instruction and Innovation at the University of Lethbridge, present the results of a study in which they used drones to search for Dinosaur fossils by searching for pigments produced by Lichens.

Pickles et al. targeted three multi-taxic microfossil bonebeds within the Dinosaur Park Formation at exposures in Dinosaur Provincial Park, in Alberta, Canada. Within these beds, fossil bone made up between 2% and 6% of the total volume, which ironstone made up between 92% and 98%. The presence of two Lichens, Rusavskia elegans and Xanthomendoza trachyphylla, was found to be strongly correlated with the presence of bone, with no discernible correlation between Lichen presence and ironstone density. Lichen density appeared to correlate well with the amount of time bones had been exposed on the surface.

Lichen were found to be more reflective within the near-infrared to shortwave infrared regions of the electromagnetic spectrum (800–1400 nm), but less reflective in the blue part of the spectrum (400-500 nm) than substrates such as fossil bones, fossil teeth, ironstone, or sandstone, whereas the spectra of by fossil bone was not notably different from that of the surrounding sediment. 

Preferential colonisation of Dinosaur bones by Lichens with distinctive spectral profiles and their detection in drone images. (A) Photograph of mixed faunal bonebed (BB 209) in Dinosaur Provincial Park showing extensive Lichen colonisation of two exposed Hadrosaur limb bones (white arrows), but absent on surrounding sediment (2005). (B) Photograph of the Centrosaurus Bonebed (BB 43) in Dinosaur Provincial Park prior to excavation showing extensive Lichen colonisation of abundant surface-exposed Dinosaur bones (1979). (C) Spectral reflectance plot of major components of the Alberta badlands environment including rock substrates, Rusavskia elegans Lichen, and Vertebrate fossils. (D)–(F) Drone image of mixed faunal bonebed (BB23) in Dinosaur Provincial Park acquired at 30 m above ground; (E) and (F) are zoomed in with each pixel measuring ~2.5 x 2.5 cm and blue pixels in (F) indicating positive detection of Lichen-colonised fossil Dinosaur bones from the spectral classification. Pickles et al. (2025).

Pickles et al. were able to detect Lichen growing on Vertebrate fossils, primarily the bones of Ornithischian Dinosaurs, in images taken using a Remotely Piloted Aircraft System hovering 30 m above the ground, at which distance each pixel represented an area of 2.5 cm x 2.5 cm. The contrast between the Lichen and the background was sufficient to allow detection by an unsupervised algorithm.

Lichen was found to be most heavily concentrated upon the bones of large Ornithischian Dinosaurs, such as Hadrosaurs and Ceratopsids, and in particular on the large limb bones of these, although it was unclear whether this was a true ecological preference, due to the high porosity and large surface area of these bones, or a sampling bias brought about by the abundance of these bones compared to those of other Dinosaurs. Likewise, Lichen were found to grown more abundantly on the lower part of the exposure, but this was also where Dinosaur bones were most abundant. 

Pickles et al. suspect that the preference shown for Dinosaur bones by the Lichen reflects a preference for alkaline, calcareous substrates. They further believe that the porous nature of Dinosaur bones makes more of this substrate readily available, and note that the Lichen were not colonising teeth (other than the cementum of the tooth root), which have a similar chemical makeup to bone, but are denser and less porous. The also note that the more porous bone might be retaining more water than other exposed substrates in the Alberta badlands, which might be another factor promoting Lichen growth.

See also...

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...