Uniquely preserved gut contents illuminate trilobite palaeophysiology.

Trilobites are among the most iconic of fossils and formed a prominent component of marine ecosystems during most of their 270-million-year-long history from the early Cambrian period to the end Permian period1. More than 20,000 species have been described to date, with presumed lifestyles ranging from infaunal burrowing to a planktonic life in the water column2. Inferred trophic roles range from detritivores to predators, but all are based on indirect evidence such as body and gut morphology, modes of preservation and attributed feeding traces; no trilobite specimen with internal gut contents has been described3,4. Here we present the complete and fully itemized gut contents of an Ordovician trilobite, Bohemolichas incola, preserved three-dimensionally in a siliceous nodule and visualized by synchrotron microtomography. The tightly packed, almost continuous gut fill comprises partly fragmented calcareous shells indicating high feeding intensity. The lack of dissolution of the shells implies a neutral or alkaline environment along the entire length of the intestine supporting digestive enzymes comparable to those in modern crustaceans or chelicerates. Scavengers burrowing into the trilobite carcase targeted soft tissues below the glabella but avoided the gut, suggesting noxious conditions and possibly ongoing enzymatic activity.

Ancient vertebrate dermal armor evolved from trunk neural crest.

Bone is an evolutionary novelty of vertebrates, likely to have first emerged as part of ancestral dermal armor that consisted of osteogenic and odontogenic components. Whether these early vertebrate structures arose from mesoderm or neural crest cells has been a matter of considerable debate. To examine the developmental origin of the bony part of the dermal armor, we have performed in vivo lineage tracing in the sterlet sturgeon, a representative of nonteleost ray-finned fish that has retained an extensive postcranial dermal skeleton. The results definitively show that sterlet trunk neural crest cells give rise to osteoblasts of the scutes. Transcriptional profiling further reveals neural crest gene signature in sterlet scutes as well as bichir scales. Finally, histological and microCT analyses of ray-finned fish dermal armor show that their scales and scutes are formed by bone, dentin, and hypermineralized covering tissues, in various combinations, that resemble those of the first armored vertebrates. Taken together, our results support a primitive skeletogenic role for the neural crest along the entire body axis, that was later progressively restricted to the cranial region during vertebrate evolution. Thus, the neural crest was a crucial evolutionary innovation driving the origin and diversification of dermal armor along the entire body axis.

Response to comment on "Exceptional preservation of organs in Devonian placoderms from the Gogo largerstätte".

Jensen et al. (1) question evidence presented of a chambered heart within placoderms, citing its small size and apparently ventral atrium. However, they fail to note the belly-up orientation of the placoderm within one nodule, and the variability of heart morphology within extant taxa. Thus, we remain confident in our interpretation of the mineralized organ as the heart.

The Bothriolepis (Placodermi, Antiarcha) material from the Valentia Slate Formation of the Iveragh Peninsula (middle Givetian, Ireland): Morphology, evolutionary and systematic considerations, phylogenetic and palaeogeographic implications.

Material of the antiarch placoderm Bothriolepis from the middle Givetian of the Valentia Slate Formation in Iveragh Peninsula, Ireland, is described and attributed to a new species, B. dairbhrensis sp. nov. A revision of the genus Bothriolepis is proposed, and its taxonomic content and previous phylogenetic analyses are reviewed, as well as the validity of morphologic characteristics considered important for the establishment of the genus, such as the shape of the preorbital recess of the neurocranium. A series of computerised phylogenetic analyses was performed, which reveals that our new species is the sister taxon to the Frasnian Scottish form B. gigantea. New phylogenetic and biogeographic analyses of the genus Bothriolepis together with comparisons between faunal assemblages reveal a first northward dispersal wave from Gondwana to Euramerica at the latest in the mid Givetian. Other Euramerican species of Bothriolepis seem to belong to later dispersal waves from Gondwana, non-excluding southward waves from Euramerica. Questions remain open such as the taxonomic validity and stratigraphic constraints for the most ancient forms of Bothriolepis in China, and around the highly speciose nature of the genus.

A high latitude Gondwanan species of the Late Devonian tristichopterid Hyneria (Osteichthyes: Sarcopterygii).

We describe the largest bony fish in the Late Devonian (late Famennian) fossil assemblage from Waterloo Farm near Makhanda/Grahamstown, South Africa. It is a giant member of the extinct clade Tristichopteridae (Sarcopterygii: Tetrapodomorpha) and most closely resembles Hyneria lindae from the late Famennian Catskill Formation of Pennsylvania, USA. Notwithstanding the overall similarity, it can be distinguished from H. lindae on a number of morphological points and is accordingly described as a new species, H. udlezinye sp. nov. The preserved material comprises most of the dermal skull, lower jaw, gill cover and shoulder girdle. The cranial endoskeleton appears to have been unossified and is not preserved, apart from a fragment of the hyoid arch adhering to a subopercular, but the postcranial endoskeleton is represented by an ulnare, some semi-articulated neural spines, and the basal plate of a median fin. The discovery of H. udlezinye shows that Hyneria is a cosmopolitan genus extending into the high latitudes of Gondwana, not a Euramerican endemic. It supports the contention that the derived clade of giant tristichopterids, which alongside Hyneria includes such genera as Eusthenodon, Edenopteron and Mandageria, originated in Gondwana.

Exceptional preservation of organs in Devonian placoderms from the Gogo lagerstätte.

The origin and early diversification of jawed vertebrates involved major changes to skeletal and soft anatomy. Skeletal transformations can be examined directly by studying fossil stem gnathostomes; however, preservation of soft anatomy is rare. We describe the only known example of a three-dimensionally mineralized heart, thick-walled stomach, and bilobed liver from arthrodire placoderms, stem gnathostomes from the Late Devonian Gogo Formation in Western Australia. The application of synchrotron and neutron microtomography to this material shows evidence of a flat S-shaped heart, which is well separated from the liver and other abdominal organs, and the absence of lungs. Arthrodires thus show the earliest phylogenetic evidence for repositioning of the gnathostome heart associated with the evolution of the complex neck region in jawed vertebrates.

The oldest complete jawed vertebrates from the early Silurian of China.

Molecular studies suggest that the origin of jawed vertebrates was no later than the Late Ordovician period (around 450 million years ago (Ma))1,2. Together with disarticulated micro-remains of putative chondrichthyans from the Ordovician and early Silurian period3-8, these analyses suggest an evolutionary proliferation of jawed vertebrates before, and immediately after, the end-Ordovician mass extinction. However, until now, the earliest complete fossils of jawed fishes for which a detailed reconstruction of their morphology was possible came from late Silurian assemblages (about 425 Ma)9-13. The dearth of articulated, whole-body fossils from before the late Silurian has long rendered the earliest history of jawed vertebrates obscure. Here we report a newly discovered Konservat-Lagerstätte, which is marked by the presence of diverse, well-preserved jawed fishes with complete bodies, from the early Silurian (Telychian age, around 436 Ma) of Chongqing, South China. The dominant species, a 'placoderm' or jawed stem gnathostome, which we name Xiushanosteus mirabilis gen. et sp. nov., combines characters from major placoderm subgroups14-17 and foreshadows the transformation of the skull roof pattern from the placoderm to the osteichthyan condition10. The chondrichthyan Shenacanthus vermiformis gen. et sp. nov. exhibits extensive thoracic armour plates that were previously unknown in this lineage, and include a large median dorsal plate as in placoderms14-16, combined with a conventional chondrichthyan bauplan18,19. Together, these species reveal a previously unseen diversification of jawed vertebrates in the early Silurian, and provide detailed insights into the whole-body morphology of the jawed vertebrates of this period.

Morphometric analysis of lungfish endocasts elucidates early dipnoan palaeoneurological evolution.

The lobe-finned fish, lungfish (Dipnoi, Sarcoptergii), have persisted for ~400 million years from the Devonian Period to present day. The evolution of their dermal skull and dentition is relatively well understood, but this is not the case for the central nervous system. While the brain has poor preservation potential and is not currently known in any fossil lungfish, substantial indirect information about it and associated structures (e.g. labyrinths) can be obtained from the cranial endocast. However, before the recent development of X-ray tomography as a palaeontological tool, these endocasts could not be studied non-destructively, and few detailed studies were undertaken. Here, we describe and illustrate the endocasts of six Palaeozoic lungfish from tomographic scans. We combine these with six previously described digital lungfish endocasts (4 fossil and 2 recent taxa) into a 12-taxon dataset for multivariate morphometric analysis using 17 variables. We find that the olfactory region is more highly plastic than the hindbrain, and undergoes significant elongation in several taxa. Further, while the semicircular canals covary as an integrated module, the utriculus and sacculus vary independently of each other. Functional interpretation suggests that olfaction has remained a dominant sense throughout lungfish evolution, and changes in the labyrinth may potentially reflect a change from nektonic to near-shore environmental niches. Phylogenetic implications show that endocranial form fails to support monophyly of the 'chirodipterids'. Those with elongated crania similarly fail to form a distinct clade, suggesting these two paraphyletic groups have converged towards either head elongation or truncation driven by non-phylogenetic constraints.

Feeding ecology has shaped the evolution of modern sharks.

Sharks are iconic predators in today's oceans, yet their modern diversity has ancient origins. In particular, present hypotheses suggest that a combination of mass extinction, global climate change, and competition has regulated the community structure of dominant mackerel (Lamniformes) and ground (Carcharhiniformes) sharks over the last 66 million years. However, while these scenarios advocate an interplay of major abiotic and biotic events, the precise drivers remain obscure. Here, we focus on the role of feeding ecology using a geometric morphometric analysis of 3,837 fossil and extant shark teeth. Our results reveal that morphological segregation rather than competition has characterized lamniform and carcharhiniform evolution. Moreover, although lamniforms suffered a long-term disparity decline potentially linked to dietary "specialization," their recent disparity rivals that of "generalist" carcharhiniforms. We further confirm that low eustatic sea levels impacted lamniform disparity across the end-Cretaceous mass extinction. Adaptations to changing prey availability and the proliferation of coral reef habitats during the Paleogene also likely facilitated carcharhiniform dispersals and cladogenesis, underpinning their current taxonomic dominance. Ultimately, we posit that trophic partitioning and resource utilization shaped past shark ecology and represent critical determinants for their future species survivorship.

Tooth morphology elucidates shark evolution across the end-Cretaceous mass extinction.

Sharks (Selachimorpha) are iconic marine predators that have survived multiple mass extinctions over geologic time. Their prolific fossil record is represented mainly by isolated shed teeth, which provide the basis for reconstructing deep time diversity changes affecting different selachimorph clades. By contrast, corresponding shifts in shark ecology, as measured through morphological disparity, have received comparatively limited analytical attention. Here, we use a geometric morphometric approach to comprehensively examine tooth morphologies in multiple shark lineages traversing the catastrophic end-Cretaceous mass extinction-this event terminated the Mesozoic Era 66 million years ago. Our results show that selachimorphs maintained virtually static levels of dental disparity in most of their constituent clades across the Cretaceous-Paleogene interval. Nevertheless, selective extinctions did impact apex predator species characterized by triangular blade-like teeth. This is particularly evident among lamniforms, which included the dominant Cretaceous anacoracids. Conversely, other groups, such as carcharhiniforms and orectolobiforms, experienced disparity modifications, while heterodontiforms, hexanchiforms, squaliforms, squatiniforms, and †synechodontiforms were not overtly affected. Finally, while some lamniform lineages disappeared, others underwent postextinction disparity increases, especially odontaspidids, which are typified by narrow-cusped teeth adapted for feeding on fishes. Notably, this increase coincides with the early Paleogene radiation of teleosts as a possible prey source, and the geographic relocation of disparity sampling "hotspots," perhaps indicating a regionally disjunct extinction recovery. Ultimately, our study reveals a complex morphological response to the end-Cretaceous mass extinction and highlights an event that influenced the evolution of modern sharks.

Exceptionally preserved beetles in a Triassic coprolite of putative dinosauriform origin.

The Triassic was a crucial period for the early evolution and diversification of insects, including Coleoptera1-3-the most diverse order of organisms on Earth. The study of Triassic beetles, however, relies almost exclusively on flattened fossils with limited character preservation. Using synchrotron microtomography, we investigated a fragmentary Upper Triassic coprolite, which contains a rich record of 3D-preserved minute beetle remains of Triamyxa coprolithica gen. et sp. nov. Some specimens are nearly complete, preserving delicate structures of the legs and antennae. Most of them are congruent morphologically, implying that they are conspecific. Phylogenetic analyses suggest that T. coprolithica is a member of Myxophaga, a small suborder of beetles with a sparse fossil record, and that it represents the only member of the extinct family Triamyxidae fam. nov. Our findings highlight that coprolites can contain insect remains, which are almost as well preserved as in amber. They are thus an important source of information for exploring insect evolution before the Cretaceous-Neogene "amber time window." Treated as food residues, insect remains preserved in coprolites also have important implications for the paleoecology of insectivores, in this case, likely the dinosauriform Silesaurus opolensis.

Fossilized cell structures identify an ancient origin for the teleost whole-genome duplication.

Teleost fishes comprise one-half of all vertebrate species and possess a duplicated genome. This whole-genome duplication (WGD) occurred on the teleost stem lineage in an ancient common ancestor of all living teleosts and is hypothesized as a trigger of their exceptional evolutionary radiation. Genomic and phylogenetic data indicate that WGD occurred in the Mesozoic after the divergence of teleosts from their closest living relatives but before the origin of the extant teleost groups. However, these approaches cannot pinpoint WGD among the many extinct groups that populate this 50- to 100-million-y lineage, preventing tests of the evolutionary effects of WGD. We infer patterns of genome size evolution in fossil stem-group teleosts using high-resolution synchrotron X-ray tomography to measure the bone cell volumes, which correlate with genome size in living species. Our findings indicate that WGD occurred very early on the teleost stem lineage and that all extinct stem-group teleosts known so far possessed duplicated genomes. WGD therefore predates both the origin of proposed key innovations of the teleost skeleton and the onset of substantial morphological diversification in the clade. Moreover, the early occurrence of WGD allowed considerable time for postduplication reorganization prior to the origin of the teleost crown group. This suggests at most an indirect link between WGD and evolutionary success, with broad implications for the relationship between genomic architecture and large-scale evolutionary patterns in the vertebrate Tree of Life.

Endocast and Bony Labyrinth of a Devonian "Placoderm" Challenges Stem Gnathostome Phylogeny.

Our understanding of the earliest evolution of jawed vertebrates depends on a credible phylogenetic framework for the jawed stem gnathostomes collectively known as "placoderms".1 However, their relationships, and whether placoderms represent a single radiation or a paraphyletic array, remain contentious.2-13 This uncertainty is compounded by an uneven understanding of anatomy across the group, particularly of the phylogenetically informative braincase and brain cavity-endocast. Based on new tomographic data, we here describe the endocast and bony labyrinth of Brindabellaspis stensioi from the Early Devonian of New South Wales.14 The taxon was commonly recovered as branching near the base of placoderms.5-9,11,12,15-17 Previous studies of Brindabellaspis emphasized its resemblances with fossil jawless fishes in the braincase anatomy14 and endocast proportions1,18 and its distinctive features were interpreted as autapomorphies, such as the elongated premedian region.19 Although our three-dimensional data confirmed the resemblance of its endocast to those of jawless vertebrates, we discovered that the inner ear and endolymphatic complex display a repertoire of previously unrecognized characters close to modern or crown-group jawed vertebrates, including a pronounced sinus superior and a vertical duct that connects the endolymphatic sac and the labyrinth cavity. Both parsimony and Bayesian analyses suggest that prevailing hypotheses of placoderm relationships are unstable, with newly revealed anatomy pointing to a radical revision of early gnathostome evolution. Our results call into question the appropriateness of arthrodire-like placoderms as models of primitive gnathostome anatomy and raise questions of homology relating to key cranial features.

The developmental relationship between teeth and dermal odontodes in the most primitive bony fish Lophosteus.

The ontogenetic trajectory of a marginal jawbone of Lophosteus superbus (Late Silurian, 422 Million years old), the phylogenetically most basal stem osteichthyan, visualized by synchrotron microtomography, reveals a developmental relationship between teeth and dermal odontodes that is not evident from the adult morphology. The earliest odontodes are two longitudinal founder ridges formed at the ossification center. Subsequent odontodes that are added lingually to the ridges turn into conical teeth and undergo cyclic replacement, while those added labially achieve a stellate appearance. Stellate odontodes deposited directly on the bony plate are aligned with the alternate files of teeth, whereas new tooth positions are inserted into the files of sequential addition when a gap appears. Successive teeth and overgrowing odontodes show hybrid morphologies around the oral-dermal boundary, suggesting signal cross-communication. We propose that teeth and dermal odontodes are modifications of a single system, regulated and differentiated by the oral and dermal epithelia.

Human teeth are an example of odontodes: hard structures made of a material called dentine that are sometimes coated in enamel. Teeth are the only odontodes humans have, but other vertebrates (animals with backbones) have tooth-like scales on their skin. These structures are called dermal odontodes, and sharks and rays, for example, are covered with them. How these structures evolved, and whether teeth or dermal odontodes developed first, continues to spark great discussion among palaeontologists. Some researchers think that teeth evolved from dermal odontodes, a theory known as the 'scales-to-teeth' hypothesis. Others think dermal odontodes are distinct from teeth because they lack the same spatial organization. To investigate this further, palaeontologists are looking at the earliest examples of odontodes they can find: fossils of early vertebrates that carry both teeth and dermal odontodes. Here, Chen et al. have studied Lophosteus, one of the earliest bony fishes that lived more than 400 million years ago, to explore early tooth evolution and growth patterns. Chen et al. digitally dissected a fossilized Lophosteus jawbone using submicron X-ray imaging, a technique with resolution to less than one millionth of a metre. Imaging thin sections of the specimen, found in Estonia, Chen et al. reconstructed the entire sequence of odontode development in the bony fish in 3D. The analysis showed that teeth and dermal odontodes initially take shape together but differentiate as they grow, presumably instructed to do so by various developmental signals. However, at a later stage, the two types of odontodes become similar in appearance again, suggesting that they respond to each other's signals. For example, as the jawbone grows, dermal odontodes overgrow the earliest formed teeth. These younger odontodes resemble teeth, while the new teeth developing near the dermal odontodes take after dermal odontodes. These findings suggest that teeth and dermal odontodes are not wholly separate systems but, instead, are closely related on a molecular level. The results also show that contrary to the 'scale-to-teeth' hypothesis, teeth do not evolve from fully formed dermal odontodes, rather the two types of odontodes form out of one founder. This research builds on our knowledge from modern sharks and points to a previously unrecognised evolutionary relationship between teeth and dermal odontodes. It also furthers our understanding of how molecular regulation controls development.

Specialized Craniofacial Anatomy of a Titanosaurian Embryo from Argentina.

The first dinosaur embryos found inside megaloolithid eggs from Auca Mahuevo, Patagonia, were assigned to sauropod dinosaurs that lived approximately 80 million years ago. Discovered some 25 years ago, these considerably flattened specimens still remain the only unquestionable embryonic remains of a sauropod dinosaur providing an initial glimpse into titanosaurian in ovo ontogeny. Here we describe an almost intact embryonic skull, which indicates the early development of stereoscopic vision, and an unusual monocerotic face for a sauropod. The new fossil also reveals a neurovascular sensory system in the premaxilla and a partly calcified braincase, which potentially refines estimates of its prenatal stage. The embryo was found in an egg with thicker eggshell and a partly different geochemical signature than those from the egg-bearing layers described in Auca Mahuevo. The cranial bones are comparably ossified as in previously described specimens but differ in facial anatomy and size. The new specimen reveals significant heterochrony in cranial ossifications when compared with non-sauropod sauropodomorph embryos, and demonstrates that the specialized craniofacial morphology preceded the postnatal transformation of the skull anatomy in adults of related titanosaurians.

The smallest known Devonian tetrapod shows unexpectedly derived features.

A new genus and species of Devonian tetrapod, Brittagnathus minutus gen. et sp. nov., is described from a single complete right lower jaw ramus recovered from the Acanthostega mass-death deposit in the upper part of the Britta Dal Formation (upper Famennian) of Stensiö Bjerg, Gauss Peninsula, East Greenland. Visualization by propagation phase contrast synchrotron microtomography allows a complete digital dissection of the specimen. With a total jaw ramus length of 44.8 mm, Brittagnathus is by far the smallest Devonian tetrapod described to date. It differs from all previously known Devonian tetrapods in having only a fang pair without a tooth row on the anterior coronoid and a large posterior process on the posterior coronoid. The presence of an incipient surangular crest and a concave prearticular margin to the adductor fossa together cause the fossa to face somewhat mesially, reminiscent of the condition in Carboniferous tetrapods. A phylogenetic analysis places Brittagnathus crownward to other Devonian tetrapods, adjacent to the Tournaisian genus Pederpes. Together with other recent discoveries, it suggests that diversification of 'Carboniferous-grade' tetrapods had already begun before the end of the Devonian and that the group was not greatly affected by the end-Devonian mass extinction.

Filter feeding in Late Jurassic pterosaurs supported by coprolite contents.

Diets of pterosaurs have mainly been inferred from indirect evidence such as comparative anatomy, associations of co-occurring fossils, and functional morphology. Gut contents are rare, and until now there is only a single coprolite (fossil dropping), with unidentified inclusions, known. Here we describe three coprolites collected from a palaeosurface with numerous pterosaur tracks found in early Kimmeridgian (Hypselocyclum Zone) intertidal deposits of the Wierzbica Quarry, Poland. The specimens' morphology and association to the tracks suggest a pterosaur producer. Synchrotron scans reveal numerous small inclusions, with foraminifera making up the majority of the identifiable ones. Other small remains include shells/carapaces (of bivalves, ostracods, and other crustaceans/arthropods) and bristles (some possibly of polychaete worms). The high density of the small shelly inclusions suggest that they were not accidently ingested, but constituted an important food source for the pterosaur(s), perhaps together with unpreserved soft-bodied animals. The combined evidence from the tracks and coprolites suggest a filter-feeding ctenochasmatid as the most likely tracemaker. If true, this significantly expands the bromalite record for this pterosaur group, which was previously only known from gastroliths. Moreover, this study also provides the first direct evidence of filter feeding in Jurassic pterosaurs and shows that they had a similar diet to the recent Chilean flamingo (Phoenicopterus chilensis).

Beetle-bearing coprolites possibly reveal the diet of a Late Triassic dinosauriform.

Diets of extinct animals can be difficult to analyse if no direct evidence, such as gut contents, is preserved in association with body fossils. Inclusions from coprolites (fossil faeces), however, may also reflect the diet of the host animal and become especially informative if the coprolite producer link can be established. Here we describe, based on propagation phase-contrast synchrotron microtomography (PPC-SRµCT), the contents of five morphologically similar coprolites collected from two fossil-bearing intervals from the highly fossiliferous Upper Triassic locality at Krasiejów in Silesia, Poland. Beetle remains, mostly elytra, and unidentified exoskeleton fragments of arthropods are the most conspicuous inclusions found in the coprolites. The abundance of these inclusions suggests that the coprolite producer deliberately targeted beetles and similar small terrestrial invertebrates as prey, but the relatively large size of the coprolites shows that it was not itself a small animal. The best candidate from the body fossil record of the locality is the dinosauriform Silesaurus opolensis Dzik, 2003, which had an anatomy in several ways similar to those of bird-like neotheropod dinosaurs and modern birds. We hypothesize that the beak-like jaws of S. opolensis were used to efficiently peck small insects off the ground, a feeding behaviour analogous to some extant birds.

Tyrannosaurid-like osteophagy by a Triassic archosaur.

Here we present evidence for osteophagy in the Late Triassic archosaur Smok wawelski Niedzwiedzki, Sulej and Dzik, 2012, a large theropod-like predator from Poland. Ten medium to large-sized coprolites are matched, by their dimensions and by association with body fossils and footprints, to S. wawelski. The coprolites contain fragments of large serrated teeth as well as up to 50 percent by volume of bone fragments, with distinct fragmentation and angularity, from several prey taxa. This suggests pronounced osteophagy. Further evidence for bone-crushing behaviour is provided by isolated worn teeth, bone-rich regurgitalites (fossil regurgitates) and numerous examples of crushed or bite-marked dicynodont bones, all collected from the same bone-bearing beds in the Lipie Slaskie clay-pit. Several of the anatomical characters related to osteophagy, such as a massive head and robust body, seem to be shared by S. wawelski and the tyrannosaurids, despite their wide phylogenetic separation. These large predators thus provide evidence of convergence driven by similar feeding ecology at the beginning and end of the age of dinosaurs.

Soft-tissue evidence for homeothermy and crypsis in a Jurassic ichthyosaur.

Ichthyosaurs are extinct marine reptiles that display a notable external similarity to modern toothed whales. Here we show that this resemblance is more than skin deep. We apply a multidisciplinary experimental approach to characterize the cellular and molecular composition of integumental tissues in an exceptionally preserved specimen of the Early Jurassic ichthyosaur Stenopterygius. Our analyses recovered still-flexible remnants of the original scaleless skin, which comprises morphologically distinct epidermal and dermal layers. These are underlain by insulating blubber that would have augmented streamlining, buoyancy and homeothermy. Additionally, we identify endogenous proteinaceous and lipid constituents, together with keratinocytes and branched melanophores that contain eumelanin pigment. Distributional variation of melanophores across the body suggests countershading, possibly enhanced by physiological adjustments of colour to enable photoprotection, concealment and/or thermoregulation. Convergence of ichthyosaurs with extant marine amniotes thus extends to the ultrastructural and molecular levels, reflecting the omnipresent constraints of their shared adaptation to pelagic life.