Biology subjects

Dearden, R. P.

Publications and source records attributed to Dearden, R. P..

9 recordsLinked to original sources

bioRxiv2026

Skeletal remains of a freshwater jalodontid chondrichthyan from the Upper Triassic Arden Sandstone Formation of Warwickshire, UK.

The Triassic was a significant period in the evolution of modern chondrichthyans (sharks, rays and chimaeras), bridging the gap between Paleozoic chondrichthyans and more recognizably modern Mesozoic forms. Although only a handful of Triassic skeletal fossils of chondrichthyans are known worldwide their calcified cartilages provide rare insight into their anatomy, evolution, and lifestyle. Here we use computed tomographic methods to describe six chondrichthyan skeletal fossils from the Carnian (Upper Triassic) Arden Sandstone Formation of Shrewley, Warwickshire, UK, consisting of a partial braincase, jaws, pectoral fin radials, caudal fin hypurals, and dorsal fin basal cartilages, and characterize their paleoenvironment. We assign the braincase and jaws, and tentatively assign the post-cranial remains, to the jalodontid taxon Keuperodus brodiei, previously described on the basis of teeth including those from the same locality. This represents the first known occurrence of articulated chondrichthyan skeletal remains from the Upper Triassic and the first interpretable cranial cartilages of the Order Jalodontiformes. The partial braincase of K. brodiei preserves an unusual basicranial circulation, almost completely embedded in the basicranium, and the jaws have an elongate extended shape; both features are akin to Paleozoic phoebodontid chondrichthyans. Phylogenetic analysis recovers K. brodiei as a stem-group elasmobranch with a close relationship to phoebodonts, one of several Paleozoic shark lineages to have independently survived the end-Permian mass extinction. However, unlike Phoebodontiformes and all other Jalodontiformes, K. brodiei occupied freshwater lake shorelines and associated fluvial feeder channels, revealing a possible freshwater refugium for Jalodontiformes towards at the end of their stratigraphic range.

paleontology↗

bioRxiv2026

Early Ontogenetic Development of Tessellated Calcified Cartilage in Chondrichthyans

Tessellated calcified cartilage (TCC) is a hallmark of the chondrichthyan skeleton, yet its development early in ontogeny across the four major groups (batoids, galeomorphs, squalomorphs, and holocephalans) remains poorly understood. Specialised traits of TCC, such as multi-layered TCC and internal mineralised trabeculae, typically develop in response to feeding mechanics. In this study, we evaluated TCC morphology in the jaws of 12 representative taxa to observe its structure at an early ontogenetic stage to determine whether these specialised features had yet developed. Batoids consistently exhibited well-developed, homogeneous, polygonal tesserae early in ontogeny regardless of jaw morphology or feeding habit. In contrast, galeomorphs displayed high morphological heterogeneity. Notably, we document the first report of an extensive internal trabecular network in a non-batoid elasmobranch, observed in Ginglymostoma cirratum, which may serve to resist the mechanical pressures of specialised suction feeding. Furthermore, we identified voussoir tesserae in galeomorphs for the first time, extending their documented presence across all elasmobranch groups, where they display an inverted aspect ratio (wider than tall) compared to mature forms. The durophagous Mustelus mustelus exhibited surprisingly poor TCC development despite being a durophagous feeder, pointing to a pronounced ontogenetic lag. In Squatina oculata, TCC was characterised by large and thick tesserae and extensive fused tesseral regions which may relate to its explosive ambush predation mode, whereas the holocephalan Chimaera exhibited a poorly mineralized, mesh-like structure without resolvable discrete tesserae or trabeculae-matching findings from previous studies. Across all specimens, multi-layered TCC was absent, confirming that multi-layering develops later in ontogeny. These results demonstrate that generalised models of TCC development based on one group or a few taxa fail to capture the broader diversity of TCC morphology. It also opens up many exciting avenues for further study, and forms the basis for comparisons with fossil chondrichthyans, to investigate the evolution of TCC.

zoology↗

bioRxiv2025

Three-dimensional fossils of a Cretaceous collared carpet shark (Parascylliidae, Orectolobiformes) shed light on skeletal evolution in galeomorphs

A rich fossil record of teeth shows that many living shark families origins lie deep in the Mesozoic. Skeletal fossils of the sharks to whom these teeth belonged are far rarer and when they are preserved are often flattened, hindering understanding of the evolutionary radiation of living shark groups. Here we use computed tomography to describe two articulated Upper Cretaceous shark skeletons from the Chalk of the UK preserving three- dimensional neurocrania, visceral cartilages, pectoral skeletons and vertebrae. These fossils display skeletal anatomies characteristic of the Parascylliidae, a family of Orectolobiformes now endemic to Australia and the Indo Pacific. However, they differ in having a more heavily mineralised braincase and a tri-basal pectoral fin endoskeleton, while their teeth can be attributed to a new species of the problematic taxon Pararhincodon. Phylogenetic analysis of these new fossils confirms that Pararhincodon is a stem-group parascylliid, providing insight into the evolution of parascylliids distinctive anatomy during the late Mesozoic-Cenozoic shift in orectolobiform biodiversity from the Northern Atlantic to the Indo Pacific. Meanwhile both Pararhincodon and extant parascylliids have a distinctive vertebral morphology previously described only in Carcharhiniformes, contributing a skeletal perspective to the picture emerging from macroevolutionary analyses of coastal, small-bodied origins for galeomorphs.

paleontology↗

bioRxiv2024

The Paleozoic assembly of the holocephalian body plan far preceded post-Cretaceous radiations into the ocean depths

Among cartilaginous fishes, Holocephali represents the species-depauperate, morphologically conservative sister to sharks, rays, and skates and the last survivor of a once far greater Paleozoic and Mesozoic diversity. Currently, holocephalian diversity is concentrated in deep-sea species, suggesting this lineage might contain relictual diversity that persisted in the ocean depths. Yet, the relationships of living holocephalians to their extinct relatives and the timescale of diversification of living species remains unclear. Here, we reconstruct the evolutionary history of holocephalians using comprehensive morphological and DNA sequence datasets. Our results suggest that living holocephalians entered and diversified in deep (>1000 m) ocean waters after the Cretaceous-Paleogene mass extinction, contrasting with the hypothesis that this ecosystem has acted as a refugium of ancient cartilaginous fishes. These invasions were decoupled from the evolution of key features of the holocephalian body plan, including crushing dentition, a single frontal clasper, and holostylic jaw suspension, in the Paleozoic Era, and considerably postdated the appearance of the living familes by 150 million years ago during a major period of biotic turnover in oceans termed the Mesozoic Marine Revolution. These results clarify the origins of living holocephalians as the recent diversification of a single surviving clade among numerous Paleozoic lineages.

evolutionary biology↗

bioRxiv2024

Evolution of the Batoidea Pectoral Fin Skeleton: Convergence, Modularity, and Integration Driving Disparity Trends

Batoids (skates and rays) are the most speciose group of cartilaginous fishes. Their body plan represents diverse ecologies and swimming modes. Early skeletal fossil remains, and recent phylogenetic analyses suggest that convergence has occurred within the batoids several times independently. The drivers for such disparity patterns and possible association with modularity and phenotypic integration among batoids are not fully understood. Here we used geometric morphometrics and phylogenetic comparative methods to characterize the evolutionary trends of the basal fin skeleton of batoids and sharks. Our analyses show that the morphological variation has a strong phylogenetic signal. Interestingly, the most speciose orders of batoids display low morphological disparity. Reef and freshwater species, show increased evolutionary rates. Meanwhile, the swimming mode shows different rates depending on the fin structure analyzed. A higher modularity and integration signal suggest that the pectoral fin of batoids has experienced mosaic evolution. The low morphological disparity might be associated with high integration. We find support for convergence between Jurassic, Cretaceous, and Extant guitarfishes, however, not completely between sharks and batoids. Our findings suggest that habitats and swimming mode have shaped the pectoral fin evolution among batoids, and at the same time batoids have constrained their basal fin skeleton.

evolutionary biology↗

bioRxiv2023

The pharynx of the iconic stem-group chondrichthyan Acanthodes (Agassiz, 1833) revisited with micro computed tomography.

Acanthodes has long been the primary source of information on the pharyngeal skeleton of acanthodians, a stem-group chondrichthyan grade. Because of this its anatomy has played an outsized role in attempts to understand the evolution of the jawed vertebrate pharynx and the clade as a whole. However, the anatomy of the pharynx of Acanthodes remains poorly understood and subject to several competing interpretations. We use computed tomography (CT) to image the articulated pharyngeal skeletons of three specimens of Acanthodes confusus from Lebach, Germany. Acanthodes had a melange of osteichthyan-like and chondrichthyan-like morphologies in its pharyngeal skeleton. Like other stem-chondrichthyans, Acanthodes had a basihyal with no hypohyals, and four pairs of posteriorly oriented pharyngobranchials. Like osteichthyans, Acanthodes possessed an interhyal, but lacked the separate infra- and supra-pharyngobranchial elements present in osteichthyans and the crown-chondrichthyan Ozarcus. Using this new data we build and animated a digital 3D model of the pharyngeal endoskeleton in Acanthodes, showing that the jaws would have swung outwards during the opening cycle, increasing the anteriorly facing area of the gape for suspension feeding. These new data provide a more definitive picture of the anatomy of a taxon that has long been of great significance in early vertebrate palaeontology.

paleontology↗

bioRxiv2023

The three-dimensionally articulated oral apparatus of a Devonian heterostracan sheds light on feeding in Palaeozoic jawless fishes

Attempts to explain the origin and diversification of vertebrates have commonly invoked the evolution of feeding ecology, contrasting the passive suspension feeding of invertebrate chordates and larval lampreys with active predation in living jawed vertebrates. Of the extinct jawless vertebrates that phylogenetically intercalate these living groups, the feeding apparatus is preserved only in the early diverging stem-gnathostome heterostracans and its anatomy remains poorly understood. Here we use X-ray microtomography to characterise the feeding apparatus of the pteraspid heterostracan Rhinopteraspis dunensis (Roemer, 1855). We show that the apparatus is composed of thirteen plates arranged approximately bilaterally, the majority of which articulate from the postoral plate. Our reconstruction of the apparatus shows that the oral plates would have been capable of movement within the dorso-ventral plane, but their degree of movement was limited. The functional morphology of the apparatus in Rhinopteraspis precludes all proposed interpretations of feeding except for suspension/deposit feeding and we interpret the apparatus as having served primarily to moderate the oral gape. This is consistent with evidence that at least some early jawless gnathostomes were suspension feeders and runs contrary to macroecological scenarios that envisage early vertebrate evolution as characterised by a directional trend towards increasingly active food acquisition.

paleontology↗

bioRxiv2023

Hagfish genome illuminates vertebrate whole genome duplications and their evolutionary consequences

Whole genome duplications (WGDs) are major events that drastically reshape genome architecture and are causally associated with organismal innovations and radiations1. The 2R Hypothesis suggests that two WGD events (1R and 2R) occurred during early vertebrate evolution2, 3. However, the veracity and timing of the 2R event relative to the divergence of gnathostomes (jawed vertebrates) and cyclostomes (jawless hagfishes and lampreys) is unresolved4-6 and whether these WGD events underlie vertebrate phenotypic diversification remains elusive7. Here we present the genome of the inshore hagfish, Eptatretus burgeri. Through comparative analysis with lamprey and gnathostome genomes, we reconstruct the early events in cyclostome genome evolution, leveraging insights into the ancestral vertebrate genome. Genome-wide synteny and phylogenetic analyses support a scenario in which 1R occurred in the vertebrate stem-lineage during the early Cambrian, and the 2R event occurred in the gnathostome stem-lineage in the late Cambrian after its divergence from cyclostomes. We find that the genome of stem-cyclostomes experienced two additional, independent genome duplications (herein CR1 and CR2). Functional genomic and morphospace analyses demonstrate that WGD events generally contribute to developmental evolution with similar changes in the regulatory genome of both vertebrate groups. However, appreciable morphological diversification occurred only after the 2R event, questioning the general expectation that WGDs lead to leaps of morphological complexity7.

genomics↗

bioRxiv2020

The morphology and evolution of chondrichthyan cranial muscles: a digital dissection of the elephantfish Callorhinchus milii and the catshark Scyliorhinus canicula.

The anatomy of sharks, rays, and chimaeras (chondrichthyans) is crucial to understanding the evolution of the cranial system in vertebrates, due to their position as the sister group to bony fishes (osteichthyans). Strikingly different arrangements of the head in the two constituent chondrichthyan groups - holocephalans and elasmobranchs - have played a pivotal role in the formation of evolutionary hypotheses targeting major cranial structures such as the jaws and pharynx. However, despite the advent of digital dissections as a means of easily visualizing and sharing the results of anatomical studies in three dimensions, information on the musculoskeletal systems of the chondrichthyan head remains largely limited to traditional accounts, many of which are at least a century old. Here we use synchrotron tomography acquire 3D data which we used to carry out a digital dissection of a holocephalan and an elasmobranch widely used as model species: the elephantfish, Callorhinchus milii, and the small-spotted catshark, Scyliorhinus canicula. We describe and figure the skeletal anatomy of the head, labial, mandibular, hyoid, and branchial cartilages in both taxa as well as the muscles of the head and pharynx. We make new observations, particularly regarding the branchial musculature of Callorhinchus, revealing several previously unreported or previously ambiguous structures. Finally, we review what is known about the evolution of chondrichthyan cranial muscles from their fossil record and discuss the implications for muscle homology and evolution, broadly concluding that the holocephalan pharynx is likely derived from a more elasmobranch-like form. This dataset has great potential as a resource, particularly for researchers using these model species for zoological research, functional morphologists requiring models of musculature and skeletons, as well as for palaeontologists seeking comparative models for extinct taxa.

evolutionary biology↗