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...

Hominin remains from the Late Pliocene of Ledi-Geraru, Ethiopia.

The genera Homo and Paranthropus are common in the fossil record from about 2.0 million years ago. Both are thought to have derived from an earlier Australopithecus ancestor, with the most likely ancestor for the genus Homo generally thought to be Australopithecus afarensis. However, Australopithecus afarensis is not known after 2.95 million years ago, with Hominid fossils being rare over the intervening interval, spanning the latest Pliocene and earliest Pleistocene. Examples of Paranthropus have recently been described from 2.7 million-year-old deposits in the Omo-Turkana Basin of Ethiopia and Nyayanga in Kenya, and the 2.66 million-year-old deposits at Laetoli in Tanzania, while a jawbone attributed to the genus Homo has been found at Ledi-Geraru in Ethiopia which has been dated to 2.78 million years ago, pushing the presence of both these 'Pleistocene' genera back into the latest Pliocene, while a new species of Australopithecus, Australopithecus garhi, has recently been described from 2.5 million-year-old (earliest Pleistocene) deposits in the Afar region of Ethiopia.

In a paper published in the journal Nature on 13 August 2025, a team of scientists led by Brian Villmoare of the Department of Anthropology at the University of Nevada Las Vegas describe a series of recent Hominin finds made by the Ledi-Geraru Research Project in the Afar Basin of Ethiopia.

The Ledi-Geraru sites are located at the northern end of the palaeoanthropological sites of the Afar Basin, and has produced the only known evidence of the genus Australopithecus surviving after 2.95 million years ago, as well as the earliest evidence for the appearance of the genus Homo. Paranthropus has not been found in this area, but it is unclear whether this represents a genuine absence. The two sites of Ledi-Geraru, Lee Adoyta and Asboli are to the west of the Awash River, in an area cut through by the Mille and Geraru rivers and their tributaries. The deposits here are between 2.5 and 3.0 million years old, and have been dated by Argon-Argon radioisotope stratigraphy of volcanic layers, as well as magnitostratigraphy.

Map of the Ledi-Geraru Research Project area. (a) The  Ledi-Geraru Research Project area (yellow star) is located towards the northern  extent of palaeontological sites (red circles) in the Afar depression, Afar Region,  Ethiopia. (b) Within the Ledi-Geraru project area, the Lee Adoyta and Asboli fossil  sites are located approximately 12.5 km apart. Villmoare et al. (2025).

Argon-Argon dating relies on determining the ratio of radioactive Argon⁴⁰ to non-radioactive Argon³⁹ within minerals from igneous or metamorphic rock (in this case volcanic ash) to determine how long ago the mineral cooled sufficiently to crystallise. The ratio of Argon⁴⁰ to Argon³⁹ is constant in the atmosphere, and this ratio will be preserved in a mineral at the time of crystallisation. No further Argon³⁹ will enter the mineral from this point, but Argon⁴⁰ is produced by the decay of radioactive Potassium⁴⁰, and increases in the mineral at a steady rate, providing a clock which can be used to date the mineral.

Magnitostratigraphy uses traces of ancient magnetic fields preserved in iron minerals in rocks to trace ancient pole reversals; the poles only have two possible orientations (north pole in the north/south pole in the south or south pole in the north/north pole in the south) and these occasionally flip, with the poles exchanging positions. Pole reversals happen more-or-less at random, with periods between reversals occurring at intervals ranching from tens of thousands to millions of years, and reflected across the globe. This creates a pattern of magnetic reversals in sedimentary rocks that can be matched in different rocks across the globe.

The first specimen described by Villmoare et al. comes from the Gurumaha Sedimentary Package, which outcrops in narrow fault-bounded exposures in the central Lee Adoyta basin and in drag-faulted blocks adjacent to basalt ridges  bounding the basin to the east. This sedimentary package is cut through by the Gurumaha Tuff, which has been dated to 2.782 million years before the present. This is the unit which previously produced specimen LD 350, a 2.78 million-year-old mandible which is the oldest fossil assigned to the genus Homo. 

The specimen derived from this unit, LD 302-23, is a third right lower premolar found 22 m to the southwest and 7 m bellow specimen LD 350, but still above the Gurumaha Tuff layer. This tooth measures 11.5 mm in length and 10.5 mm in width, and has a fragment of enamel missing from its lingual corner, being otherwise well-preserved. The shape of the premolar is consistent with that seen in some examples from Australopithecus afarensis, but the pattern of cusps is quite different to anything seen in any member of the genus, making it unlikely that this tooth came from an Australopithecus. It also falls within the size range of both species of Paranthropus, but is quite different in shape. Third premolars from early members of the genus Homo are quite variable, but clearly differ from both those of Australopithecus and Paranthropus. Since this tooth falls within the size and shape variation found in these early Homo specimens, Villmoare et al. assign it to the genus Homo. 

New Hominin dentition from the Ledi-Geraru Research project. Right, from top: LD 302-23 P₃,  LD 750 P₄, AS 100 M¹ and AS 100 M².  Left, images show the LD 760 assemblage  (top, from left: maxillary molar, I², I¹, maxillary canine; bottom, mandibular molars). Villmoare et al. (2025).

The second specimen described, LD 750-115670, is an isolated lower fourth premolar, found at the base of an 8 m exposure of fossiliferous mudstones and sandstones at site LD 750. This site is located stratigraphically between the 2.63 million-year-old Lee Adoyta Tuffs and the 2.59 million-year-old Giddi Sands Tuff. 

The tooth crown is unworn, with all cusps preserved, although the root is broken off, giving a maximum root height of about 2 mm. The lack of wear may imply that the tooth was unerupted at the time of death. The tooth is 12.4 mm long and 11.4 mm wide, placing it at the upper end of the size range for Australopithecus afarensis or Australopithecus africanus, and too large for Australopithecus anamensis. No lower jaw or teeth are known for Australopithecus garhi, but the specimen is within a plausible size range for the species. It also falls within the size range of both Paranthropus species, but again is quite different in shape. It does resemble several fourth premolars attributed to early Homo, though Villmoare et al. note that these attributions are provisional, and that it is difficult to distinguish between early Homo and Australopithecus fourth premolars. Since this tooth lacks any distinctively Homo features, Villmoare et al. provisionally assign it to aff. Australopithecus sp..

A set of five associated lower molars were discovered at a site identified as LD 760, a flat sandy area approximately below the 2.63 million-year-old Lee Adoyta Tuffs. These are worn, with dentine exposed on their outer cusps, and wide for their lengths, giving them a squarish profile. Notably, the third molar is larger than the second molar, and the second molar is larger than the first, the first and second molars a relatively square, and the first and second molars lack a seventh cusp, all traits compatible with Australopithecus afarensis, but not early Homo. However, these teeth also differ from those of Australopithecus afarensis in several ways; they do not taper towards the rear, and lack a distinctive bilobate buccal contour. 

LD 760 molars compared to Australopithecus afarensis. Left molars from Ledi-Geraru specimen LD 760 (left) and Hadar specimen A.L. 400-1 (right). Measurements in mm of the LD 760 molars (length × width): LM1: 13.3 × 13.4, LM2: 14.5 × 14.6, LM3: 14.0 (estimated) × 15.7, RM1: 13.2 × 13.1, RM2: 14.8 × 15.2. Specimens are oriented with their buccal surfaces to the left and mesial surfaces up. Villmoare et al. (2025).

A partial upper molar was also recovered from this locality. This preserves the lingual grove of the tooth, which appears to be quite distinct. In the upper molars of Australopithecus garhi this groove is indistinct. Furthermore, the upper molars of Australopithecus garhi has a greatly reduced hypercone cusp, which leads the protocone cusp to take on a triangular shape. The hypercone is absent in the LD 760 specimen, but the protocone is present, and shows no sign of modification due to a reduced hypercone. However, the sample size for Australopithecus gahri is small, so this cannot be ruled out as a natural variation within the species.

Also found at the LD760 site were a right maxillary (upper) canine, a complete left maxillary lateral incisor and a left fragmentary maxillary central incisor. Thecanine (LD 760-115979) is well  preserved, lacking only the tip of the root. This has mesial and distal interproximal contact facets (wear marks caused by contact between teeth), with the interproximal contact facet having a matching distal interproximal contact facet on the second incisor. This means that the second incisor and canine were in contact, something typical of the the genus Homo, and unlike the situation in many Australopithecus specimens (including the Australopithecus gahri maxilla BOU-VP-12/130) where these two teeth are separated by a diastema (this trait is variable in Australopithecus afarensis, which may-or-may not have a diastema, so this could conceivably also be the case in Australopithecus gahri). The canine is also notably large, towards the upper end of the size range seen in Australopithecus (and much larger than anything seen in Paranthropus).

Comparative maxillary canine morphology. (a) Lingual (left) and  labial (right) views of the Ledi-Geraru LD 760-115979 canine (left) with Hadar Australopithecus afarensis specimens A.L. 763-1 (middle) and A.L. 333x-3 (right). Note that the  LD 760 canine is a right canine, whereas the Australopithecus afarensis canines are from the  left and are mirrored in these images. (b)–(d) LD 760-115979 ((b) shown in lingual view) contrasted with Hadar Australopithecus afarensis specimen A.L. 199-1 ((c) right canine  shown; distal to the upper right) and Bouri Australopithecus garhi specimen BOU-VP-12/130  (d) left canine, mirrored; distal is to the right). Note the simple mesial–distal  chisel-like wear pattern on the LD 760 canine (b) in contrast to the complex  multi-faceted wear pattern of Australopithecus afarensis (c) and the broad curved basin on the  distal side of the Australopithecus garhi upper canine ((d) this is seen on both left and right  canines). Images are oriented differently to emphasise the distinctive relevant morphology. Images in (b)–(d) are not to scale. Vallmoare et al. (2025).

The canine of Australopithecus gahri has a unique structure, with a shallow distal basin reminiscent of a talon which is contiguous with a wide wear furrow which runs along the entire post-canine dental row, something not seen in any other Hominin, and absent in the LD 760 canine. Morphologically, this tooth resembles those of Australopithecus afarensis, however, it has different wear patterns. In Australopithecus afarensis wear is mostly seen on the distal crest, whereas in the the LD 760 canine it is predominantly on the apex, suggesting a difference in diet and/or lifestyle.

The LD 760 individual clearly does not belong to Paranthropus, and is not morphologically consistent with any described species of Australopithecus. However, since it resembles Australopithecus afarensis more closely than anything else, Vallmoare et al. refer it to Australopithecus sp. indet.

The final specimens discussed come from the Giddi Sands unit in the Asboli region. These were found immediately below the 2.593 million-year-old Giddi Sands Tuff, and comprise a partial upper left first molar, and two fragments of an upper left second molar, which can be assembled to form a whole crown. They show little wear, and are close in shape to those of Australopithecus afarensis, although they lack the pronounced lingual occlusocervical sloping and general 'puffy' appearance of the molars of that species. They closely resemble the molars of early Homo specimens such as the 2.3 million-year-old specimen from the Busidima Formation at Hadar, or the 2.5-2.4 million-year-old specimen from Mille-Logya. It is quite different in form from the molars of Australopithecus gahri, and is small compared to the molars of either Australopithecus gahri or Paranthropus. 

Vallmoare et al. believe that although the Ledi-Geraru material is very limited, it provides clear evidence that both Australopithecus and Homo were present during the 3.0-2.5 million years ago interval, suggesting that multiple non-robust Hominin lineages were present in East Africa before 2.5 million years ago. 

The molar fragments from Asboli sufficiently resemble the Homo specimens from Hadar and Mille-Logya that Vallmoare et al. are confident that they represent the same species. However, they predate the older of these specimens (Mille-Logya) by at least 150 000 years. This adds to the evidence for an early appearance of Homo as Ledi-Geraru previously established by the LD 350-1 mandible, as does the LD 302-23 premolar which Vallmoare et al. describe from the Gurumaha sedimentary package.

The LD 750 and LD 760 material both come from the Lee Adoyta sedimentary package, although they are separated by 24 m of strata and the 2.63 million-year-old Lee Adoyta Tuffs. Nevertheless, both appear to represent a single species of Australopithecus (an assumption based in part upon the unlikelyhood of two similar species of Austrlopithecus coexisting in the same area). 

Vallmoare et al. consider four potential explanations for this material. Firstly, they might represent a late surviving population of Australopithecus afarensis (approximately 350 000 younger than the current youngest member of the species, from the Kada Hadar 2 Submember at Hadar). Secondly, they may represent an unknown species of Australopithecus ancestral to Paranthropus; the presence of Homo in this region implies that the Homo and Paranthropus lineages had diversified by this time, but Paranthropus itself appears to be absent. However, the oldest currently accepted member of the genus coming from about 2.7 million years ago from Nyayanga in Kenya and 2.66 million years ago from the Upper Ndolanya Beds at Laetoli in Tanzania, which makes this scenario less likely. Thirdly, they may represent earlier examples of Australopithecus gahri, which have not yet developed the distinctive features of the 2.5 million-year-old specimen BOU-VP-12/130, although the lack of similarities makes this unlikely. Finally, these specimens represent a new, as yet undescribed, species of Australopithecus.

Villmoare et al. conclude that were at least three species of Hominins present in the Afar Region between 3.0 and 2.5 million years ago, Australopithecus gahri, an unknown species of Homo, and an unknown species of Australopithecus. At the same time, Australopithecus africanus was still present in South Africa, and Paranthropus had already appeared in Kenya, Tanzania, and southern Ethiopia. The Ledi-Geraru environment was drier and more open than was typical for Australopithecus, very much the sort of environment associated with appearance and proliferation of the genus Homo, suggesting that, at least locally, Australopithecus may have been able to adjust to these more open environments, at least for a while.

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Tavachelydra stevensoni: A new species of Snapping Turtle from the Palaeocene of Colorado.

Snapping Turtles, Chelydroidea, are found today from northern South America to Southern Canada, forming an important component of many North American freshwater ecosystems. Despite being widespread and numerous, there are only five species alive today. Stem group-Chelydroids (i.e. species which are more closely related to living Chelydroids than to any other group), known as pan-Chelydroids, first appeared in the Late Cretaceous, although many fossils are fragmentary, as the shells of Snapping Turtles are less heavily fused than other Turtle groups and tend to disarticulate soon after death, limiting our understanding of this group. Due to this no Cretaceous pan-Chelydroids have been described to date, although post-Cretaceous species have been described from across Laurasia.

In a paper published in the Swiss Journal of Palaeontology on 5 August 2025, Tyler Lyson, Holger Petermann,Salvador Bastien,Natalie Toth,Evan Tamez‑Galvan, and Sadie Sherman of the Department of Earth Sciences at the Denver Museum of Nature & Science, and Walter Joyce of the Department of Geosciences at the University of Fribourg, describe a new species of pan-Chelydroid Turtle from the Early Palaeocene Corral Bluffs of the Denver Basin in Colorado.

The Coral Bluffs are a series of outcrops of Latest Cretaceous to Eocene outcrops in El Paso County in the southern Denver Basin, to the east of Colorado Springs. The sequence is well-dated, with three documented magnetic reversals 30n/29r, 29r/29n, and 29n/28r), a pollen-defined Cretaceous/Palaeocene boundary, and a volcanic ash layer which has been dated using lead and uranium isotopes. These bluffs have produced abundant Vertebrate remains from the Puercan North American Land Mammal Age, including Denverus middletoni, one of two known Early Palaeocene pan-Chelydroid Turtles, which together form the earliest described members of the group.

Geography, chronostratigraphy, and biostratigraphy of the Corral Bluffs Study Area within the Denver Basin from which specimens of Tavachelydra stevensoni were collected. (A) Map of Late Cretaceous through Eocene sediments within the Denver Basin showing the location  of the Corral Bluffs Study Area within Colorado Springs (highlighted by box and enlarged in part (B)) in the southwestern portion of the basin. (B)  High-resolution photogrammetric model of the eastern portion of the Corral Bluffs Study Area overlain on Google Earth with geographic locations  of Tavachelydra stevensoni denoted by red stars: (1) DMNH EPV.141854/DMNH Loc.19258; (2) DMNH EPV.143100/DMNH Loc. 20,053; (3) DMNH. EPV.134087/DMNH Loc. 7082; (4) DMNH. EPV.136265/DMNH Loc. 18,852; 5, DMNH EPV.143200/DMNH Loc. 6284. (C) Age, magnetostratigraphic, lithostratigraphic,  and biostratigraphic logs showing the stratigraphic placement of Tavachelydra stevensoni localities (red stars; see numbers from (B)). Stratigraphy is tied to the Geomagnetic Polarity Time Scale using remnant magnetisation of the rocks at the Corral Bluffs Study Area, two chemical abrasion–isotope dilution–thermal ionisation mass spectrometry uranium/lead-dated volcanic ash beds (yellow star; two dated ash samples represent the same volcanic ash locality and thus only one yellow star), and the palynologically defined K/Pg boundary (italicised dates). The lithostratigraphic log is a composite and shows that the sequence is dominated by intercalated mudstone and sandstone, reflecting a loosely anastomosing fluvial  environment. Pollen biozones are defined by diversification of Momipites spp. (fossil Juglandaceous pollen). Abbreviations: Ma, million years  ago; K/Pg, Cretaceous-Paleogene boundary. Lyson et al. (2025).

The new species is named Tavachelydra stevensoni, where 'Tavachelydra' is a combination of 'Tava' from the Ute/Nuuchiu name for Pike's Peak Tavá-Kaavi, which can be directly translated as 'Sun Mountain'), and -chelydra, a common suffix for Turtles, which derives from the Greek 'khéludros', meaning 'water serpent', while 'stevensoni' honours the late Brandon Stevenson, a dear friend of Tyler Lyson andlong-time  supporter of the Corral Bluffs project.

The new species is described from five specimens, DMNH EPV.141854, the holotype, which consists of a disarticulated,  but associated, skeleton, comprising a nearly complete carapace and plastron and a complete pelvis, and three paratypes, DMNH EPV.143100, an articulated complete carapace  and partial right hypo- and hypoplastron, DMNH EPV.143200, a hypo- and xiphiplastra, and DMNH. EPV.134087, a poorly preserved, but complete cranium. 

Tavachelydra stevensoni, DMNH EPV.141854 (DMNH Loc.19258), holotype, external view of shell. (A) Photograph and (B) interpretive line drawing of the carapace. (C) Photograph and (D) interpretive line drawing of the plastron. Abbreviations: Ab abdominal scale, An anal scale, Ce cervical scale, co costal, ent entoplastron, epi epiplastron, Fe femoral scale, Gu gular scale, Hu humeral scale, hyo hyoplastron, hypo hypoplastron,  Ig intergular scale, Im inframarginal scale, Ma marginal scale, nu nuchal, per peripheral, Pl pleural scale, pn postneural, spy suprapygal, py pygal, Ve vertebral scale, xi xiphiplastron. Arabic numerals denote neurals. Lyson et al. (2025).

Specimens of Tavachelydra  stevensoni are large, with carapace reaching almost 50 cm in length. This is four times the size of Denverus middletoni, the other pan-Chelydroid Turtle from the Denver Basin. Within the Coral Bluffs fauna only one Turtle, Axestemys infernalis, is larger, while a second, Neurankylus sp., is about the same size. It also appears to be one of the rarer species in a Turtle-rich fauna, and since other thin-shelled species, such as Hoplochelys clark, are relatively abundant, this appears to be a reflection of actual rarity rather than a reflection of poor preservational potential. All the known specimens of Tavachelydra  stevensoni have been found in deposits associated with ponds, rather than river channels (the most abundant environment in the Coral Bluffs deposits), suggesting that they preferred such an environment in life. The skull of Tavachelydra  stevensoni is large and broad, with flat biting surfaces, which suggests a durophagous diet (eating hard food, such as shellfish). This is noteworthy, as there is evidence that durophagous species and groups may have preferentially survived the End Cretaceous Extinction.

Reconstruction of Tavachelydra stevensoni basking on a log in a ponded water environment. Andrey Atuchin in Lyson et al. (2025).

A phylogenetic tree constructed by Lyson et al. placed Tavachelydra  stevensoni as the sister species to the extant Snapping Turtles, with later European and Asian species less closely related. This is unsurprising, as while these species are more recent, they are less likely to be ancestral to modern species restricted to North America. Denverus middletoni was also recovered as only distantly related to extant Chelydrids, indicating it was a member of a lineage which did not survive till today.

Cladogram of Chelydroid Turtles mapped  against the stratigraphic ranges for each taxon (black, type strata,  grey, age of referred material). Strict consensus tree from six most  parsimonious trees. Lyson et al. (2025).

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Malformations in Trilobites from the Silurian and Devonian of Europe.

Malformations in fossils, such as pathologies caused by infections, scars left by recovered injuries, or teratologies caused by developmental problems, can tell us a lot about how extinct organisms grew and interacted with their environments, although when analysing these is clearly preferable to have access to non-malformed examples of the same species, or at least a close relative.

Trilobites were a diverse and abundant group of Arthropods which appeared early in the Cambrian, and survived until the End Permian Extinction. Their dorsal exoskeleton was heavily biomineralized, and was shed periodically to allow the animals to grow. This has lead to an extensive fossil record with Trilobites being extremely common in many Palaeozoic marine deposits. This abundant fossil record makes Trilobites an excellent candidate group for the study of malformations.

In a paper published in the journal Acta Palaeontologica Polonica on 22 April 2025, Russell Bicknell of the Division of Paleontology (Invertebrates) at the American Museum of Natural History, and the Palaeoscience Research Centre at the University of New England, Patrick Smith of the Palaeontology Department at the Australian Museum Research Institute, and the Department of Biological Sciences at Macquarie University, Lisa Amati of Paleontology at the New York State Museum, and Melanie Hopkins, also of the Division of Paleontology (Invertebrates) at the American Museum of Natural History, describe malformations in European Silurian and Devonian Trilobite specimens from the collections of the Natural History Museum in London and the New York State Museum.

The first specimen examined by Bicknell et al., NYSM 19739, is an isolated cephalon (head part) from a Harpetid Trilobite, Lioharpes venulosus, from the Early Devonian Koněprusy Limestone of the Czech Republic, in the collection of the New York State Museum. The cephalon is 26.2 mm long and 22.3 mm wide with a u-shaped indentation on its right marginal rim. This indentation is 5.6 mm long and extends 1.9 mm towards the midline. The marginal rim is covered in small circular pits, which around the indentation are irregular, ovate, and occasionally fused into larger pits.

Malformed Harpetid Trilobite Lioharpes venulosus, NYSM 19739 from the Koněprusy Limestone, Pragian, Lower Devonian, Koněprusy, Czech Republic. (A₁) complete cephalon; (A₂) close up showing U-shaped indentation (arrow). Specimen coated in ammonium chloride sublimate. Bicknell et al. (2025).

Bicknell et al. note that malformations to the cephalic fringes of Harpetid Trilobites have been recorded before, and that these are usealy attributed to injuries, an analysis with which they concur. However, they also observe that injuries can happen in a variety of ways, with fringe injuries having previously attributed to problems during moulting, failed predation attempts, or unknown causes. They suggest that a moulting injury is the most likely explanation for the injury to the Koněprusy specimen, with the delicate fringe likely torn during moulting, and the enlarged and fused pits being a result of fusion of the torn margin during healing. Various purposes have been suggested for the cephalic fringes of Harpetid Trilobites, including filtering for food, sensory roles, sediment ploughing, hydrostatic support, cephalic reinforcement, burrowing, and enhancing hydrodynamic efficiency. Whatever the purpose of this organ, an injury to it is likely to have been detrimental to the living Trilobite, and presumably repairing this injury would have been a priority during subsequent moults.

The second specimen, NHMUK PI In 65061, looked at is a Phacopid Trilobite, Calymene blumenbachii, from the Early Silurian Much Wenlock Limestone Formation of Shropshire, England, in the collection of the Natural History Museum. This specimen comprises a partial cephalon, thorax, and pygidium, with a total length of 92.9 mm and a width of 48.6 mm. The second thoracic axial ring (middle part of the second segment of the thorax) of this specimen is covered by a structure with closely spaced openings, which has an elevated round crater at its right extremity, with an opening 1.7 mm across.

Bicknell et al. interpret this as an encrusting Trepostome Bryozoan covering the 3rd thoracic tergite, with the larger opening being an ovate zoarium (specially modified zooid which produced eggs). The restriction of the encrustation to one tergite strongly suggests that this happened while the Trilobite was alive, and that the Bryozoan colony was therefore unable to overgrow the articulations between tergites.

Abnormal Calymenid Trilobites Calymene blumenbachii from the Much Wenlock Limestone Formation, Homerian, Wenlock, Silurian, England, UK. (A) NHMUK PI In 65061, (A₁) complete specimen; (A₂) close up showing the large bryozoan growth. (B) NHMUK PI In 19857 showing pygidial ribs that terminate early (white arrows) and are fused proximal to the medial lobe (black arrow).  Bicknell et al. (2025).

The next specimen examined, NHMUK PI In 19857, is another example of Calymene blumenbachii from the Much Wenlock Limestone. This specimen is a partial pygidium (tail part) 13.2 mm long and 18.2 mm wide. On the right side of this specimen the pygidial ribs are disrupted and irregular, with two ribs terminating 1.6 mm short of the pygidial margin, while another two fuse 1.2 mm from the pygidial axis.

Bicknel et al. observe that similar deformations to the pygidia have been observed in a wide range of other Trilobites, including Dalmanities pleuroptyx, Dechenella macrocephalus, Niobina sp., and Prionopeltis archiaci. They attribute these deformations to genetic or developmental issues, but suggest that the limited disruption to the pygidium they caused did not represent a major handicap.

Specimen NYSM 19740 is an Acastid Trilobite, Treveropyge sp., from the early Devonian Saint Céneré Formation of Mayenne in northwest France. this specimen is another isolated pygidium, s 11.6 mm long and 17.9 mm wide. It has a deformation to the axial lobe, which is asymmetric, with two of the axial rings malformed and curving to the right.

Malformed Acastid Trilobite Treveropyge sp., NYSM 19740 from the Saint Céneré Formation, Lochkovian, Lower Devonian, Mayenne, France. (A₁) Complete pygidium; (A₂) close up showing asymmetrical axial lobe and incomplete axial ring (arrows). Specimen coated in ammonium chloride sublimate. Bicknell et al. (2025).

Again, Bicknell et al. note that similar deformations have been seen in other Trilobites, such as Calliops marginatus, Dolicholeptus licticallis, and Sceptaspis lincolnensis. They suggest that these malformations are caused by genetic deformations, leading to incomplete development or non-functional somites.

The final specimen examined, NHMUK PI I 1108, is an external impression of a partial pygidium belonging to the Styginid Trilobite Scutellum (Scutellum) pardalios, from the Middle Devonin Barton Limestone Member of Devon, England. This impression is t is 59.5 mm long and 44.0 mm wide. On the right side of this impression (i.e. on the left side of the Trilobite) two ribs fuse into a single rib 29.1 mm from the pygidial axis. This single rib then terminates 4.8 mm from the pygidial margin.

Malformed Styginid Trilobite Scutellum (Scutellum) pardalios, NHMUK PI I 1108 from the Barton Limestone Member, Torquay Limestone Formation, Givetian, Middle Devonian, England, UK. (A₁) Pygidium preserved as external impression; (A₂) close up showing fused pygidial pleurae (arrows). Bicknell et al. (2025).

Bicknell et al. note that the surface of the pygidium was covered with ornamentation, with no visible break in this, which appears to  rule out the malformation having been formed by an unsuccessful predation attempt, or similar injury. Recovery from injury is the most commonly sited reason for malformations seen in Styginid Trilobites, followed by parasitic infections during early development, which seems equally unlikely. Bicknell et al. suggest instead that this deformity might be the result of a difficult moult, or possibly a genetic aberration. They do not believe this minor deformity would have significantly affected the living Trilobite.

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