BRAINCASE OF THE UPPER DEVONIAN SHARK CLADODOIDES WILDUNGENSIS (CHONDRICHTHYES, ELASMOBRANCHII), WITH OBSERVATIONS ON THE BRAINCASE IN EARLY CHONDRICHTHYANS

Reconstructing the subcephalic musculature in Pucapampella and Ichthyostega

We present new reconstructions of subcephalic musculature for the stem chondrichthyan Pucapampella, the tetrapodomorph fish Eusthenopteron, and the Devonian tetrapod Ichthyostega. These reconstructions are based on macroscopic dissections of the head muscles of an archaic shark Heptranchias and an archaic actinopterygian Polypterus, that are combined with functional considerations and a reappraisal of not widely known theoretical concepts from the past. The subcephalic, as well as the supracephalic, musculature is formed by four anterior myomeres. They are continuous with subsequent myomeres of the trunk, but are innervated by ventral nerve roots of the medulla oblongata and thus belong to the head. The fourth subcephalic myomere ends with its posterior myoseptum on the occiput in osteichthyans, but on the first vertebra in chondrichthyans. The original function of subcephalic and supracephalic muscles in basal gnathostomes supposedly was to hold together anterior and posterior parts of the neurocranium during interaction with prey, such as the backward-ripping prey dissection, hypothesized for Pucapampella. In sarcopterygian osteichthyans, subcephalic musculature is involved in active depression of the anterior part of the neurocranium; specialization of this mechanism resulted in a complete separation of m. subcephalicus from trunk myomeres in Latimeria. Fusion of anterior and posterior parts of the neurocranium has resulted in reduction of the subcephalic musculature in the majority of cartilaginous and bony fishes. However, hexanchid sharks retain three posterior subcephalic myomeres for backward-ripping prey dissection. Polypterus and Chauliodus have retained the subcephalic musculature, but its function has shifted to a depression of the whole neurocranium.

Broad snouted cladoselachian with sensory specialization at the base of modern chondrichthyans

Throughout the Silurian and Devonian, cartilaginous fish successively evolved their specialized skeletal and dental characteristics, and increasingly refined their sensory systems. The Late Devonian shark taxon Maghriboselache mohamezanei gen. et sp. n. from the eastern Anti-Atlas of Morocco is known from multiple specimens preserving most of its skeletal features, which in some instances are preserved in three dimensions. Key details of the dentition, jaws, and pectoral skeleton are shared with the iconic genus Cladoselache. Phylogenetic analyses place the family Cladoselachidae as the sister group of symmoriiforms and these groups as sister group of the holocephalans. Further phylogenetic results corroborate that the initial evolutionary radiation of crown chondrichthyans occurred within or before the Late Devonian. Remarkably, this new stem holocephalan is equipped with a wide snout and large laterally separated nasal capsules: the earliest known example of this condition in the chondrichthyan and (perhaps) gnathostome record. This suggests sensory specialization approaching that of extant broad-rostrum elasmobranchs and represents a significant addition to increasingly apparent ecomorphological diversity among early chondrichthyans.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1186/s13358-023-00266-6.

Endocasts of ornithopod dinosaurs: Comparative anatomy

Ornithopod dinosaurs were a successful group before they became extinct at the end of the Cretaceous. They were present on every continent, though they were rare in the Southern Hemisphere. We present the results of our work on the brain of these dinosaurs as an attempt to determine which evolutionary trends affected it. Old and new technologies allow us to peer into the skull of long extinct animals and retrieve information about their brain. First we provide a short description of the brain of ornithopod dinosaurs from Europe and Asia, then we sum up the characteristics that can be gathered from it. The presence of valleculae helps us to assess the actual size of the brain with more confidence. The olfactory peduncles are large and these animals had a good sense of smell. There is a trend toward an increase in the size of the cerebral hemispheres, and a more straight-lined brain. The latter can be the result of the ontogeny and the size achieved by the adult animal on the development of the brain. Other characteristics, like the development of the cerebral hemispheres and the encephalization quotient, allude to Hadrosauridae having had cognitive abilities more developed than previously assumed. This is in adequacy with other data from the physical characteristics (e.g., crests) and the social life (e.g., living in herds, communal nests) of these dinosaurs, which denote high and complex behaviors like care for their young, sexual courtship, and gregariousness.

Architectural and ultrastructural features of tessellated calcified cartilage in modern and extinct chondrichthyan fishes

Tessellated calcified cartilage (TCC) is a distinctive kind of biomineralized perichondral tissue found in many modern and extinct chondrichthyans (sharks, rays, chimaeroids and their extinct allies). Customarily, this feature has been treated somewhat superficially in phylogenetic analyses, often as a single "defining" character of a chondrichthyan clade. TCC is actually a complex hard tissue with numerous distinctive attributes, but its use as a character complex for phylogenetic analysis has not yet been optimized. This study attempts to improve this situation by presenting new terminology for certain aspects of tesseral architecture, including single-monolayered, multiple-monolayered, polylayered and voussoir tesserae; new histological data, including thin sections of TCC in several Palaeozoic taxa, and new proposals for ways in which various characters and states (many of which are defined here for the first time) could be applied in future phylogenetic analyses of chondrichthyan fishes. It can be concluded that many, but not all, of the unique attributes of modern TCC evolved by the Early Devonian (ca. 400 before present (bp)). The globular calcified cartilage reported in Silurian sinacanthids and the so-called subtessellated perichondral biomineralization (with irregular and ill-defined geometries of a layer or layers of calcified cartilage blocks) of certain extinct "acanthodians" (e.g., Climatius, Ischnacanthus, Cheiracanthus) could represent evolutionary precursors of TCC, which seems to characterize only part of the chondrichthyan total group. It is hypothesized that heavily biomineralized "layer-cake" TCC in certain Palaeozoic chondrichthyans perhaps served a dual physiological role, as a phosphate sink and in providing increased skeletal density in very large (>7 m) Devonian-Permian marine sharks such as ctenacanths and as an adaptation to calcium-deficient environments among Permo-Carboniferous non-marine sharks such as xenacanths. By contrast, the equivalent tissue in modern elasmobranchs probably serves only to reinforce regions of cartilage (mostly in the jaws) subjected to high loading. It is also noted that much of the variation observed in tesseral architecture (including localized remodelling), ultrastructure and histology in modern and extinct chondrichthyans is confined to the perichondrally facing cap zone (where Type-1 collagen matrix predominates in modern TCC), whereas the main body of the tessera (where Type-2 collagen matrix predominates) exhibits comparatively little evidence of remodelling and histological or structural variation.

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 proportions^1,18 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.

Tracing the genetic footprints of vertebrate landing in non-teleost ray-finned fishes

Rich fossil evidence suggests that many traits and functions related to terrestrial evolution were present long before the ancestor of lobe- and ray-finned fishes. Here, we present genome sequences of the bichir, paddlefish, bowfin, and alligator gar, covering all major early divergent lineages of ray-finned fishes. Our analyses show that these species exhibit many mosaic genomic features of lobe- and ray-finned fishes. In particular, many regulatory elements for limb development are present in these fishes, supporting the hypothesis that the relevant ancestral regulation networks emerged before the origin of tetrapods. Transcriptome analyses confirm the homology between the lung and swim bladder and reveal the presence of functional lung-related genes in early ray-finned fishes. Furthermore, we functionally validate the essential role of a jawed vertebrate highly conserved element for cardiovascular development. Our results imply the ancestors of jawed vertebrates already had the potential gene networks for cardio-respiratory systems supporting air breathing.

A Bayesian approach to dynamic homology of morphological characters and the ancestral phenotype of jawed vertebrates

Phylogenetic analysis of morphological data proceeds from a fixed set of primary homology statements, the character-by-taxon matrix. However, there are cases where multiple conflicting homology statements can be justified from comparative anatomy. The upper jaw bones of placoderms have traditionally been considered homologous to the palatal vomer-dermopalatine series of osteichthyans. The discovery of 'maxillate' placoderms led to the alternative hypothesis that 'core' placoderm jaw bones are premaxillae and maxillae lacking external (facial) laminae. We introduce a BEAST2 package for simultaneous inference of homology and phylogeny, and find strong evidence for the latter hypothesis. Phenetic analysis of reconstructed ancestors suggests that maxillate placoderms are the most plesiomorphic known gnathostomes, and the shared cranial architecture of arthrodire placoderms, maxillate placoderms and osteichthyans is inherited. We suggest that the gnathostome ancestor possessed maxillae and premaxillae with facial and palatal laminae, and that these bones underwent divergent evolutionary trajectories in placoderms and osteichthyans.

The early elasmobranch Phoebodus: phylogenetic relationships, ecomorphology and a new time-scale for shark evolution

Anatomical knowledge of early chondrichthyans and estimates of their phylogeny are improving, but many taxa are still known only from microremains. The nearly cosmopolitan and regionally abundant Devonian genus Phoebodus has long been known solely from isolated teeth and fin spines. Here, we report the first skeletal remains of Phoebodus from the Famennian (Late Devonian) of the Maïder region of Morocco, revealing an anguilliform body, specialized braincase, hyoid arch, elongate jaws and rostrum, complementing its characteristic dentition and ctenacanth fin spines preceding both dorsal fins. Several of these features corroborate a likely close relationship with the Carboniferous species Thrinacodus gracia, and phylogenetic analysis places both taxa securely as members of the elasmobranch stem lineage. Identified as such, phoebodont teeth provide a plausible marker for range extension of the elasmobranchs into the Middle Devonian, thus providing a new minimum date for the origin of the chondrichthyan crown-group. Among pre-Carboniferous jawed vertebrates, the anguilliform body shape of Phoebodus is unprecedented, and its specialized anatomy is, in several respects, most easily compared with the modern frilled shark Chlamydoselachus. These results add greatly to the morphological, and by implication ecological, disparity of the earliest elasmobranchs.

An early chondrichthyan and the evolutionary assembly of a shark body plan

Although relationships among the major groups of living gnathostomes are well established, the relatedness of early jawed vertebrates to modern clades is intensely debated. Here, we provide a new description of Gladbachus, a Middle Devonian (Givetian approx. 385-million-year-old) stem chondrichthyan from Germany, and one of the very few early chondrichthyans in which substantial portions of the endoskeleton are preserved. Tomographic and histological techniques reveal new details of the gill skeleton, hyoid arch and jaws, neurocranium, cartilage, scales and teeth. Despite many features resembling placoderm or osteichthyan conditions, phylogenetic analysis confirms Gladbachus as a stem chondrichthyan and corroborates hypotheses that all acanthodians are stem chondrichthyans. The unfamiliar character combination displayed by Gladbachus, alongside conditions observed in acanthodians, implies that pre-Devonian stem chondrichthyans are severely under-sampled and strongly supports indications from isolated scales that the gnathostome crown group originated at the latest by the early Silurian (approx. 440 Ma). Moreover, phylogenetic results highlight the likely convergent evolution of conventional chondrichthyan conditions among earliest members of this primary gnathostome division, while skeletal morphology points towards the likely suspension feeding habits of Gladbachus, suggesting a functional origin of the gill slit condition characteristic of the vast majority of living and fossil chondrichthyans.

Neurocranial anatomy of an enigmatic Early Devonian fish sheds light on early osteichthyan evolution

The skull of 'Ligulalepis' from the Early Devonian of Australia (AM-F101607) has significantly expanded our knowledge of early osteichthyan anatomy, but its phylogenetic position has remained uncertain. We herein describe a second skull of 'Ligulalepis' and present micro-CT data on both specimens to reveal novel anatomical features, including cranial endocasts. Several features previously considered to link 'Ligulalepis' with actinopterygians are now considered generalized osteichthyan characters or of uncertain polarity. The presence of a lateral cranial canal is shown to be variable in its development between specimens. Other notable new features include the presence of a pineal foramen, the some detail of skull roof sutures, the shape of the nasal capsules, a placoderm-like hypophysial vein, and a chondrichthyan-like labyrinth system. New phylogenetic analyses place 'Ligulalepis' as a stem osteichthyan, specifically as the sister taxon to 'psarolepids' plus crown osteichthyans. The precise position of 'psarolepids' differs between parsimony and Bayesian analyses.

Neurocranial anatomy of the petalichthyid placoderm Shearsbyaspis oepiki Young revealed by X-ray computed microtomography

Stem-group gnathostomes reveal the sequence of character acquisition in the origin of modern jawed vertebrates. The petalichthyids are placoderm-grade stem-group gnathostomes known from both isolated skeletal material and rarer articulated specimens of one genus. They are of particular interest because of anatomical resemblances with osteostracans, the jawless sister group of jawed vertebrates. Because of this, they have become central to debates on the relationships of placoderms and the primitive cranial architecture of gnathostomes. However, among petalichthyids, only the braincase of Macropetalichthys has been studied in detail, and the diversity of neurocranial morphology in this group remains poorly documented. Using X-ray computed microtomography, we investigated the endocranial morphology of Shearsbyaspis oepiki Young, a three-dimensionally preserved petalichthyid from the Early Devonian of Taemas-Wee Jasper, Australia. We generated virtual reconstructions of the external endocranial surfaces, orbital walls and cranial endocavity, including canals for major nerves and blood vessels. The neurocranium of Shearsbyaspis resembles that of Macropetalichthys, particularly in the morphology of the brain cavity, nerves and blood vessels. Many characters, including the morphology of the pituitary vein canal and the course of the trigeminal nerve, recall the morphology of osteostracans. Additionally, the presence of a parasphenoid in Shearsbyaspis (previously not known with confidence outside of arthrodires and osteichthyans) raises some questions about current proposals of placoderm paraphyly. Our detailed description of this specimen adds to the known morphological diversity of petalichthyids, and invites critical reappraisal of the phylogenetic relationships of placoderms.

Bayesian Morphological Clock Methods Resurrect Placoderm Monophyly and Reveal Rapid Early Evolution in Jawed Vertebrates

The phylogeny of early gnathostomes provides an important framework for understanding one of the most significant evolutionary events, the origin and diversification of jawed vertebrates. A series of recent cladistic analyses have suggested that the placoderms, an extinct group of armoured fish, form a paraphyletic group basal to all other jawed vertebrates. We revised and expanded this morphological data set, most notably by sampling autapomorphies in a similar way to parsimony-informative traits, thus ensuring this data (unlike most existing morphological data sets) satisfied an important assumption of Bayesian tip-dated morphological clock approaches. We also found problems with characters supporting placoderm paraphyly, including character correlation and incorrect codings. Analysis of this data set reveals that paraphyly and monophyly of core placoderms (excluding maxillate forms) are essentially equally parsimonious. The two alternative topologies have different root positions for the jawed vertebrates but are otherwise similar. However, analysis using tip-dated clock methods reveals strong support for placoderm monophyly, due to this analysis favoring trees with more balanced rates of evolution. Furthermore, enforcing placoderm paraphyly results in higher levels and unusual patterns of rate heterogeneity among branches, similar to that generated from simulated trees reconstructed with incorrect root positions. These simulations also show that Bayesian tip-dated clock methods outperform parsimony when the outgroup is largely uninformative (e.g., due to inapplicable characters), as might be the case here. The analysis also reveals that gnathostomes underwent a rapid burst of evolution during the Silurian period which declined during the Early Devonian. This rapid evolution during a period with few articulated fossils might partly explain the difficulty in ascertaining the root position of jawed vertebrates.

The internal cranial anatomy of Romundina stellina Ørvig, 1975 (Vertebrata, Placodermi, Acanthothoraci) and the origin of jawed vertebrates-Anatomical atlas of a primitive gnathostome

Placoderms are considered as the first jawed vertebrates and constitute a paraphyletic group in the stem-gnathostome grade. The acanthothoracid placoderms are among the phylogenetically most basal and morphologically primitive gnathostomes, but their neurocranial anatomy is poorly understood. Here we present a near-complete three-dimensional skull of Romundina stellina, a small Early Devonian acanthothoracid from the Canadian Arctic Archipelago, scanned with propagation phase contrast microtomography at a 7.46 μm isotropic voxel size at the European Synchrotron Radiation Facility, Grenoble, France. This is the first model of an early gnathostome skull produced using this technique, and as such represents a major advance in objectivity compared to past descriptions of placoderm neurocrania on the basis of grinding series. Despite some loss of material along an oblique crack, most of the internal structures are remarkably preserved, and most of the missing structures can be reconstructed by symmetry. This virtual approach offers the possibility to connect with certainty all the external foramina to the blood and nerve canals and the central structures, and thus identify accurate homologies without destroying the specimen. The high level of detail enables description of the main arterial, venous and nerve canals of the skull, and other perichondrally ossified endocranial structures such as the palatoquadrate articulations, the endocranial cavity and the inner ear cavities. The braincase morphology appears less extreme than that of Brindabellaspis, and is in some respects more reminiscent of a basal arthrodire such as Kujdanowiaspis.

A new method for reconstructing brain morphology: applying the brain-neurocranial spatial relationship in an extant lungfish to a fossil endocast

Lungfish first appeared in the geological record over 410 million years ago and are the closest living group of fish to the tetrapods. Palaeoneurological investigations into the group show that unlike numerous other fishes-but more similar to those in tetrapods-lungfish appear to have had a close fit between the brain and the cranial cavity that housed it. As such, researchers can use the endocast of fossil taxa (an internal cast of the cranial cavity) both as a source of morphological data but also to aid in developing functional and phylogenetic implications about the group. Using fossil endocast data from a three-dimensional-preserved Late Devonian lungfish from the Gogo Formation, Rhinodipterus, and the brain-neurocranial relationship in the extant Australian lungfish, Neoceratodus, we herein present the first virtually reconstructed brain of a fossil lungfish. Computed tomographic data and a newly developed 'brain-warping' method are used in conjunction with our own distance map software tool to both analyse and present the data. The brain reconstruction is adequate, but we envisage that its accuracy and wider application in other taxonomic groups will grow with increasing availability of tomographic datasets.

The hyoid arch and braincase anatomy of Acanthodes support chondrichthyan affinity of 'acanthodians'

Solving the evolutionary relationships of the acanthodians is one of the key problems in reconstructing ancestral anatomical conditions for the jawed vertebrates (gnathostomes). Current debate concerns whether acanthodians are an assemblage of stem chondrichthyans, or a more generalized grade encompassing some early stem osteichthyans. The skull anatomy of Acanthodes bronni has been pivotal in these debates, owing to tension between chondrichthyan- and osteichthyan-like models of reconstruction. We use computed tomography scanning and traditional palaeontological techniques to resolve the long-standing debate about the anatomy of the jaw suspension. We establish the correct length of the hyomandibula and show that it attaches to a process on the ventrolateral angle of the braincase below the jugular vein groove. This condition corresponds precisely to that in chondrichthyans. This character represents an unambiguously optimized synapomorphy with chondrichthyans given current gnathostome phylogenies, corroborating the growing consensus of the chondrichthyan affinity of acanthodians.

Ontogenetic Shape Change in the Chicken Brain: Implications for Paleontology

Paleontologists have investigated brain morphology of extinct birds with little information on post-hatching changes in avian brain morphology. Without the knowledge of ontogenesis, assessing brain morphology in fossil taxa could lead to misinterpretation of the phylogeny or neurosensory development of extinct species. Hence, it is imperative to determine how avian brain morphology changes during post-hatching growth. In this study, chicken brain shape was compared at various developmental stages using three-dimensional (3D) geometric morphometric analysis and the growth rate of brain regions was evaluated to explore post-hatching morphological changes. Microscopic MRI (μMRI) was used to acquire in vivo data from living and post-mortem chicken brains. The telencephalon rotates caudoventrally during growth. This change in shape leads to a relative caudodorsal rotation of the cerebellum and myelencephalon. In addition, all brain regions elongate rostrocaudally and this leads to a more slender brain shape. The growth rates of each brain region were constant and the slopes from the growth formula were parallel. The dominant pattern of ontogenetic shape change corresponded with interspecific shape changes due to increasing brain size. That is, the interspecific and ontogenetic changes in brain shape due to increased size have similar patterns. Although the shape of the brain and each brain region changed considerably, the volume ratio of each brain region did not change. This suggests that the brain can change its shape after completing functional differentiation of the brain regions. Moreover, these results show that consideration of ontogenetic changes in brain shape is necessary for an accurate assessment of brain morphology in paleontological studies.

First shark from the Late Devonian (Frasnian) Gogo Formation, Western Australia sheds new light on the development of tessellated calcified cartilage

BACKGROUND

Living gnathostomes (jawed vertebrates) comprise two divisions, Chondrichthyes (cartilaginous fishes, including euchondrichthyans with prismatic calcified cartilage, and extinct stem chondrichthyans) and Osteichthyes (bony fishes including tetrapods). Most of the early chondrichthyan ('shark') record is based upon isolated teeth, spines, and scales, with the oldest articulated sharks that exhibit major diagnostic characters of the group--prismatic calcified cartilage and pelvic claspers in males--being from the latest Devonian, c. 360 Mya. This paucity of information about early chondrichthyan anatomy is mainly due to their lack of endoskeletal bone and consequent low preservation potential.

METHODOLOGY/PRINCIPAL FINDINGS

Here we present new data from the first well-preserved chondrichthyan fossil from the early Late Devonian (ca. 380-384 Mya) Gogo Formation Lägerstatte of Western Australia. The specimen is the first Devonian shark body fossil to be acid-prepared, revealing the endoskeletal elements as three-dimensional undistorted units: Meckel's cartilages, nasal, ceratohyal, basibranchial and possible epibranchial cartilages, plus left and right scapulocoracoids, as well as teeth and scales. This unique specimen is assigned to Gogoselachus lynnbeazleyae n. gen. n. sp.

CONCLUSIONS/SIGNIFICANCE

The Meckel's cartilages show a jaw articulation surface dominated by an expansive cotylus, and a small mandibular knob, an unusual condition for chondrichthyans. The scapulocoracoid of the new specimen shows evidence of two pectoral fin basal articulation facets, differing from the standard condition for early gnathostomes which have either one or three articulations. The tooth structure is intermediate between the 'primitive' ctenacanthiform and symmoriiform condition, and more derived forms with a euselachian-type base. Of special interest is the highly distinctive type of calcified cartilage forming the endoskeleton, comprising multiple layers of nonprismatic subpolygonal tesserae separated by a cellular matrix, interpreted as a transitional step toward the tessellated prismatic calcified cartilage that is recognized as the main diagnostic character of the chondrichthyans.

Osteichthyan-like cranial conditions in an Early Devonian stem gnathostome

The phylogeny of Silurian and Devonian (443-358 million years (Myr) ago) fishes remains the foremost problem in the study of the origin of modern gnathostomes (jawed vertebrates). A central question concerns the morphology of the last common ancestor of living jawed vertebrates, with competing hypotheses advancing either a chondrichthyan- or osteichthyan-like model. Here we present Janusiscus schultzei gen. et sp. nov., an Early Devonian (approximately 415 Myr ago) gnathostome from Siberia previously interpreted as a ray-finned fish, which provides important new information about cranial anatomy near the last common ancestor of chondrichthyans and osteichthyans. The skull roof of Janusiscus resembles that of early osteichthyans, with large plates bearing vermiform ridges and partially enclosed sensory canals. High-resolution computed tomography (CT) reveals a braincase bearing characters typically associated with either chondrichthyans (large hypophyseal opening accommodating the internal carotid arteries) or osteichthyans (facial nerve exiting through jugular canal, endolymphatic ducts exiting posterior to the skull roof) but lacking a ventral cranial fissure, the presence of which is considered a derived feature of crown gnathostomes. A conjunction of well-developed cranial processes in Janusiscus helps unify the comparative anatomy of early jawed vertebrate neurocrania, clarifying primary homologies in 'placoderms', osteichthyans and chondrichthyans. Phylogenetic analysis further supports the chondrichthyan affinities of 'acanthodians', and places Janusiscus and the enigmatic Ramirosuarezia in a polytomy with crown gnathostomes. The close correspondence between the skull roof of Janusiscus and that of osteichthyans suggests that an extensive dermal skeleton was present in the last common ancestor of jawed vertebrates, but ambiguities arise from uncertainties in the anatomy of Ramirosuarezia. The unexpected contrast between endoskeletal structure in Janusiscus and its superficially osteichthyan-like dermal skeleton highlights the potential importance of other incompletely known Siluro-Devonian 'bony fishes' for reconstructing patterns of trait evolution near the origin of modern gnathostomes.

The characters of Palaeozoic jawed vertebrates

Newly discovered fossils from the Silurian and Devonian periods are beginning to challenge embedded perceptions about the origin and early diversification of jawed vertebrates (gnathostomes). Nevertheless, an explicit cladistic framework for the relationships of these fossils relative to the principal crown lineages of the jawed vertebrates (osteichthyans: bony fishes and tetrapods; chondrichthyans: sharks, batoids, and chimaeras) remains elusive. We critically review the systematics and character distributions of early gnathostomes and provide a clearly stated hierarchy of synapomorphies covering the jaw-bearing stem gnathostomes and osteichthyan and chondrichthyan stem groups. We show that character lists, designed to support the monophyly of putative groups, tend to overstate their strength and lack cladistic corroboration. By contrast, synapomorphic hierarchies are more open to refutation and must explicitly confront conflicting evidence. Our proposed synapomorphy scheme is used to evaluate the status of the problematic fossil groups Acanthodii and Placodermi, and suggest profitable avenues for future research. We interpret placoderms as a paraphyletic array of stem-group gnathostomes, and suggest what we regard as two equally plausible placements of acanthodians: exclusively on the chondrichthyan stem, or distributed on both the chondrichthyan and osteichthyan stems.

Holocephalan embryo provides new information on the evolution of the glossopharyngeal nerve, metotic fissure and parachordal plate in gnathostomes

The phylogenetic relationships between the different groups of Paleozoic gnathostomes are still debated, mainly because of incomplete datasets on Paleozoic jawed vertebrate fossils and ontogeny of some modern taxa. This issue is illustrated by the condition of the glossopharyngeal nerve relative to the parachordal plate, the otic capsules and the metotic fissure in gnathostomes. Two main conditions are observed in elasmobranchs (shark and rays) and osteichthyans (bony fishes and tetrapods). The condition in the other chondrichthyan taxon, the holocephalans, is still poorly known, and without any information on this taxon, it remains difficult to polarize the condition in gnathostomes. Based on the anatomical study of an embryo of the holocephalan Callorhinchus milii by means of propagation X-Ray Synchrotron phase contrast microtomography using both holotomography and single distance phase retrieval process, we show that, contrary to what was previously inferred for holocephalans (i.e. an osteichthyan-like condition), the arrangement of the glossopharyngeal nerve relative to the surrounding structure in holocephalans is more similar to that of elasmobranchs. Furthermore, the holocephalan condition represents a combination of plesiomorphic characters for gnathostomes (e.g., the glossopharyngeal nerve leaves the braincase via the metotic fissure) and homoplastic characters. By contrast, the crown osteichthyans are probably derived in having the glossopharyngeal nerve that enters the saccular chamber and in having the glossopharyngeal foramen separated from the metotic fissure.

The buccohypophyseal canal is an ancestral vertebrate trait maintained by modulation in sonic hedgehog signaling

Background

The pituitary gland is formed by the juxtaposition of two tissues: neuroectoderm arising from the basal diencephalon, and oral epithelium, which invaginates towards the central nervous system from the roof of the mouth. The oral invagination that reaches the brain from the mouth is referred to as Rathke’s pouch, with the tip forming the adenohypophysis and the stalk disappearing after the earliest stages of development. In tetrapods, formation of the cranial base establishes a definitive barrier between the pituitary and oral cavity; however, numerous extinct and extant vertebrate species retain an open buccohypophyseal canal in adulthood, a vestige of the stalk of Rathke’s pouch. Little is currently known about the formation and function of this structure. Here we have investigated molecular mechanisms driving the formation of the buccohypophyseal canal and their evolutionary significance.

Results

We show that Rathke’s pouch is located at a boundary region delineated by endoderm, neural crest-derived oral mesenchyme and the anterior limit of the notochord, using CD1, R26R-Sox17-Cre and R26R-Wnt1-Cre mouse lines. As revealed by synchrotron X-ray microtomography after iodine staining in mouse embryos, the pouch has a lobulated three-dimensional structure that embraces the descending diencephalon during pituitary formation. Polaris^fl/fl; Wnt1-Cre, Ofd1^-/- and Kif3a^-/- primary cilia mouse mutants have abnormal sonic hedgehog (Shh) signaling and all present with malformations of the anterior pituitary gland and midline structures of the anterior cranial base. Changes in the expressions of Shh downstream genes are confirmed in Gas1^-/- mice. From an evolutionary perspective, persistence of the buccohypophyseal canal is a basal character for all vertebrates and its maintenance in several groups is related to a specific morphology of the midline that can be related to modulation in Shh signaling.

Conclusion

These results provide insight into a poorly understood ancestral vertebrate structure. It appears that the opening of the buccohypophyseal canal depends upon Shh signaling and that modulation in this pathway most probably accounts for its persistence in phylogeny.

What is an 'elasmobranch'? The impact of palaeontology in understanding elasmobranch phylogeny and evolution

The Subclass Elasmobranchii is widely considered nowadays to be the sister group of the Subclass Holocephali, although chimaeroid fishes were originally classified as elasmobranchs along with modern sharks and rays. While this modern systematic treatment provides an accurate reflection of the phylogenetic relationships among extant taxa, the classification of many extinct non-holocephalan shark-like chondrichthyans as elasmobranchs is challenged. A revised, apomorphy-based definition of elasmobranchs is presented in which they are considered the equivalent of neoselachians, i.e. a monophyletic group of modern sharks and rays which not only excludes all stem and crown holocephalans, but also many Palaeozoic shark-like chondrichthyans and even close extinct relatives of neoselachians such as hybodonts. The fossil record of elasmobranchs (i.e. neoselachians) is reviewed, focusing not only on their earliest records but also on their subsequent distribution patterns through time. The value and limitations of the fossil record in answering questions about elasmobranch phylogeny are discussed. Extinction is seen as a major factor in shaping early elasmobranch history, especially during the Triassic. Extinctions may also have helped shape modern lamniform diversity, despite uncertainties surrounding the phylogenetic affinities of supposedly extinct clades such as cretoxyrhinids, anacoracids and odontids. Apart from these examples, and the supposed Cretaceous extinction of 'sclerorhynchids', elasmobranch evolution since the Jurassic has mostly involved increased diversification (especially during the Cretaceous). The biogeographical distribution of early elasmobranchs may be obscured by sampling bias, but the earliest records of numerous groups are located within the Tethyan realm. The break-up of Gondwana, and particularly the opening of the South Atlantic Ocean (together with the development of epicontinental seaways across Brazil and Africa during the Cretaceous), provided repeated opportunities for dispersal from both eastern (European) and western (Caribbean) Tethys into newly formed ocean basins.

Digital cranial endocast of Hyopsodus (Mammalia, "Condylarthra"): a case of paleogene terrestrial echolocation?

We here describe the endocranial cast of the Eocene archaic ungulate Hyopsodus lepidus AMNH 143783 (Bridgerian, North America) reconstructed from X-ray computed microtomography data. This represents the first complete cranial endocast known for Hyopsodontinae. The Hyopsodus endocast is compared to other known "condylarthran" endocasts, i. e. those of Pleuraspidotherium (Pleuraspidotheriidae), Arctocyon (Arctocyonidae), Meniscotherium (Meniscotheriidae), Phenacodus (Phenacodontidae), as well as to basal perissodactyls (Hyracotherium) and artiodactyls (Cebochoerus, Homacodon). Hyopsodus presents one of the highest encephalization quotients of archaic ungulates and shows an "advanced version" of the basal ungulate brain pattern, with a mosaic of archaic characters such as large olfactory bulbs, weak ventral expansion of the neopallium, and absence of neopallium fissuration, as well as more specialized ones such as the relative reduction of the cerebellum compared to cerebrum or the enlargement of the inferior colliculus. As in other archaic ungulates, Hyopsodus midbrain exposure is important, but it exhibits a dorsally protruding largely developed inferior colliculus, a feature unique among "Condylarthra". A potential correlation between the development of the inferior colliculus in Hyopsodus and the use of terrestrial echolocation as observed in extant tenrecs and shrews is discussed. The detailed analysis of the overall morphology of the postcranial skeleton of Hyopsodus indicates a nimble, fast moving animal that likely lived in burrows. This would be compatible with terrestrial echolocation used by the animal to investigate subterranean habitat and/or to minimize predation during nocturnal exploration of the environment.

A new paleozoic Symmoriiformes (Chondrichthyes) from the late Carboniferous of Kansas (USA) and cladistic analysis of early chondrichthyans

Background

The relationships of cartilaginous fishes are discussed in the light of well preserved three-dimensional Paleozoic specimens. There is no consensus to date on the interrelationship of Paleozoic chondrichthyans, although three main phylogenetic hypotheses exist in the current literature: 1. the Paleozoic shark-like chondrichthyans, such as the Symmoriiformes, are grouped along with the modern sharks (neoselachians) into a clade which is sister group of holocephalans; 2. the Symmoriiformes are related to holocephalans, whereas the other Paleozoic shark-like chondrichthyans are related to neoselachians; 3. many Paleozoic shark-like chondrichthyans, such as the Symmoriiformes, are stem chondrichthyans, whereas stem and crown holocephalans are sister group to the stem and crown neoselachians in a crown-chondrichthyan clade. This third hypothesis was proposed recently, based mainly on dental characters.

Methodology/Principal Findings

On the basis of two well preserved chondrichthyan neurocrania from the Late Carboniferous of Kansas, USA, we describe here a new species of Symmoriiformes, Kawichthys moodiei gen. et sp. nov., which was investigated by means of computerized X-ray synchrotron microtomography. We present a new phylogenetic analysis based on neurocranial characters, which supports the third hypothesis and corroborates the hypothesis that crown-group chondrichthyans (Holocephali+Neoselachii) form a tightly-knit group within the chondrichthyan total group, by providing additional, non dental characters.

Conclusions/Significance

Our results highlight the importance of new well preserved Paleozoic fossils and new techniques of observation, and suggest that a new look at the synapomorphies of the crown-group chondrichthyans would be worthwhile in terms of understanding the adaptive significance of phylogenetically important characters.

The phyllolepid placoderm Cowralepis mclachlani: insights into the evolution of feeding mechanisms in jawed vertebrates

Remarkably preserved specimens of Cowralepis mclachlani Ritchie, 2005 (Proc Linn Soc NSW 126:215-259) (Phyllolepida, Placodermi) represent a unique ontogenetic sequence adding to our understanding of anatomy, function, and phylogeny among basal jawed vertebrates (gnathostomes). A systematic review demonstrates that the Phyllolepida are a subgroup of the Arthrodira. Consideration of visceral and neurocranial characters supports the hypothesis that placoderms are the sister group to remaining gnathostomes. Placoderms possess, as adult plesiomorphic features, a number of characters that are only seen in the development of extant gnathostomes-a peramorphic shift relative to placoderms. Developmental evidence in vertebrates leads to a revised polarity of character transitions. These include 1) hyomandibula-neurocranium and ventral parachordal-palatoquadrate articulations (vertebrate synapomorphies); 2) jointed pharynx, paired basibranchials, anterior ethmoidal-palatoquadrate articulation, short trabeculae cranii, and anterior and posterior neurocranial fissures (gnathostome synapomorphies); and 3) fused basibranchials, dorsal palatoquadrate-neurocranium articulation, loss of the anterior neurocranial fissure, elongated trabeculae cranii, and transfer of the ventral parachordal-palatoquadrate articulation to the trabeculae (crown group gnathostomes). The level of preservation in C. mclachlani provides the basis for a reinterpretation of phyllolepid anatomy and function. Cowralepis mclachlani possesses paired basibranchials allowing the reinterpretation of the visceral skeleton in other placoderms. Mandible depression in C. mclachlani follows an osteichthyan pattern and the ventral visceral skeleton acts as a functional unit. Evidence for hypobranchial musculature demonstrates the neural crest origin of the basibranchials and that Cowralepis was a suction feeder. Finally, the position of the visceral skeleton relative to the neurocranium in placoderms parallels the condition in selachians and osteichthyans, but differs in the elongation of the occiput. The cucullaris fossa of placoderms (interpreted as a site of muscle attachment) is shown to represent, in part, the parabranchial chamber.

Skull and brain of a 300-million-year-old chimaeroid fish revealed by synchrotron holotomography

Living cartilaginous fishes, or chondrichthyans, include numerous elasmobranch (sharks and rays) species but only few chimaeroid (ratfish) species. The early history of chimaeroids, or holocephalans, and the modalities of their divergence from elasmobranchs are much debated. During Carboniferous times, 358-300 million years (Myr) ago, they underwent a remarkable evolutionary radiation, with some odd and poorly understood forms, including the enigmatic iniopterygians that were known until now from poorly informative flattened impressions. Here, we report iniopterygian skulls found preserved in 3 dimensions in approximately 300-Myr-old concretions from Oklahoma and Kansas. The study was performed by using conventional X-ray microtomography (muCT), as well as absorption-based synchrotron microtomography (SR-muCT) [Tafforeau P, et al. (2006) Applications of X-ray synchrotron microtomography for non-destructive 3D studies of paleontological specimens. Appl Phys A 83:95-202] and a new holotomographic approach [Guigay P, Langer M, Boistel R, Cloetens P (2007) Mixed transfer function and transport of intensity approach for phase retrieval in the Fresnel region. Opt Lett 32:1617-1619], which revealed their peculiar anatomy. Iniopterygians also share unique characters with living chimaeroids, suggesting that the key chimaeroid skull features were already established 300 Myr ago. Moreover, SR-muCT of an articulated skull revealed a strikingly brain-shaped structure inside the endocranial cavity, which seems to be an exceptional case of soft-tissue mineralization of the brain, presumably as a result of microbially induced postmortem phosphatization. This was imaged with exceptional accuracy by using holotomography, which demonstrates its great potential to image preserved soft parts in dense fossils.

The Dogfish Scyliorhinus canicula: A Reference in Jawed Vertebrates

INTRODUCTIONDue to their large size and long generation times, chondrichthyans have been largely ignored by geneticists. However, their key phylogenetic position makes them ideal subjects to study the molecular bases of the important morphological and physiological innovations that characterize jawed vertebrates. Such analyses are crucial to understanding the origin of the complex genetic mechanisms unraveled in osteichthyans. The small spotted dogfish Scyliorhinus canicula, a representative of the largest order of extant sharks, presents a number of advantages in this context. Due to its relatively small size among sharks, its abundance, and easy maintenance, the dogfish has been an important model in comparative anatomy and physiology for more than a century. Recently, revived interest has occurred with the development of large-scale transcriptomic and genomic resources, together with the establishment of facilities allowing massive egg and embryo production. These new tools open the way to molecular analyses of the elaborate physiological and sensory systems used by sharks. They also make it possible to take advantage of unique characteristics of these species, such as organ zonation, in analyses of cell proliferation and differentiation. Finally, they offer important perspectives to evolutionary developmental biology that will provide a better understanding of the origin and diversifications of jawed vertebrates. The dogfish whole-genome sequence, which may shortly become accessible, should establish this species as an essential shark reference, complementary to other chondrichthyan models. These analyses are likely to reveal an organism of an underestimated complexity, far from the primitive prototypical gnathostome anticipated in gradistic views.

Skeletal elements in the vertebrate eye and adnexa: morphological and developmental perspectives

Although poorly appreciated, the vertebrate eye and adnexa are relatively common sites for skeletogenesis. In many taxa, the skeleton contributes to internal reinforcement in addition to the external housing of the eye (e.g., the circumorbital bones and eyelids). Eyeball elements such as scleral cartilage and scleral ossicles are present within a broad diversity of vertebrates, albeit not therian mammals, and have been used as important models for the study of condensations and epithelial-mesenchymal interactions. In contrast, other elements invested within the eye or its close surroundings remain largely unexplored. The onset and mode of development of these skeletal elements are often variable (early versus late; involving chondrogenesis, osteogenesis, or both), and most (if not all) of these elements appear to share a common neural crest origin. This review discusses the development and distribution of the skeletal elements within and associated with the developing eye and comments on homology of the elements where these are questionable.