Neurocranial development of the coelacanth and the evolution of the sarcopterygian head.

The neurocranium of sarcopterygian fishes was originally divided into an anterior (ethmosphenoid) and posterior (otoccipital) portion by an intracranial joint, and underwent major changes in its overall geometry before fusing into a single unit in lungfishes and early tetrapods1. Although the pattern of these changes is well-documented, the developmental mechanisms that underpin variation in the form of the neurocranium and its associated soft tissues during the evolution of sarcopterygian fishes remain poorly understood. The coelacanth Latimeria is the only known living vertebrate that retains an intracranial joint2,3. Despite its importance for understanding neurocranial evolution, the development of the neurocranium of this ovoviviparous fish remains unknown. Here we investigate the ontogeny of the neurocranium and brain in Latimeria chalumnae using conventional and synchrotron X-ray micro-computed tomography as well as magnetic resonance imaging, performed on an extensive growth series for this species. We describe the neurocranium at the earliest developmental stage known for Latimeria, as well as the major changes that the neurocranium undergoes during ontogeny. Changes in the neurocranium are associated with an extreme reduction in the relative size of the brain along with an enlargement of the notochord. The development of the notochord appears to have a major effect on the surrounding cranial components, and might underpin the formation of the intracranial joint. Our results shed light on the interplay between the neurocranium and its adjacent soft tissues during development in Latimeria, and provide insights into the developmental mechanisms that are likely to have underpinned the evolution of neurocranial diversity in sarcopterygian fishes.

A microanatomical and histological study of the scales of the Devonian sarcopterygian Miguashaia bureaui and the evolution of the squamation in coelacanths.

Coelacanths have traditionally been described as morphologically conservative throughout their long evolutionary history, which spans more than 400 million years. After an initial burst during the Devonian, a morphological stasis was long thought to have prevailed since the Carboniferous, as shown by the extant Latimeria. New fossil discoveries have challenged this view, with punctual and sometimes unusual departures from the general coelacanth Bauplan. The dermal skeleton is considered to represent one, if not the main, example of morphological stasis in coelacanth evolution and as a consequence, has remained poorly surveyed. The lack of palaeohistological data on the dermoskeleton has resulted in a poor understanding of the early establishment and evolution of the coelacanth squamation. Here we describe the scales of Miguashaia bureaui from the Upper Devonian of Miguasha, Québec (Canada), revealing histological data for a Palaeozoic coelacanth in great detail and adding to our knowledge on the dermal skeleton of sarcopterygians. Miguashaia displays rounded scales ornamented by tubercules and narrow ridges made of dentine and capped with enamel. At least two generations of superimposed odontodes occur, which is reminiscent of the primitive condition of stem osteichthyans like Andreolepis or Lophosteus, and onychodonts like Selenodus. The middle vascular layer is well developed and shows traces of osteonal remodelling. The basal plate consists of a fully mineralised lamellar bone with a repetitive rotation pattern every five layers indicating a twisted plywood-like arrangement of the collagen plies. Comparisons with the extant Latimeria and other extinct taxa show that these features are consistently conserved across coelacanth evolution with only minute changes in certain taxa. The morphological and histological features displayed in the scales of Miguashaia enable us to draw a comprehensive picture of the onset of the coelacanth squamation and to propose and discuss evolutionary scenarios for the coelacanth dermoskeleton.

Development and growth of the pectoral girdle and fin skeleton in the extant coelacanth Latimeria chalumnae.

The monobasal pectoral fins of living coelacanths and lungfishes are homologous to the forelimbs of tetrapods and are thus critical to investigate the origin thereof. However, it remains unclear whether the similarity in the asymmetrical endoskeletal arrangement of the pectoral fins of coelacanths reflects the evolution of the pectoral appendages in sarcopterygians. Here, we describe for the first time the development of the pectoral fin and shoulder girdle in the extant coelacanth Latimeria chalumnae, based on the tomographic acquisition of a growth series. The pectoral girdle and pectoral fin endoskeleton are formed early in development with a radially outward growth of the endoskeletal elements. The visualization of the pectoral girdle during development shows a reorientation of the girdle between the fetus and pup 1 stages, creating a contact between the scapulocoracoids and the clavicles in the ventro-medial region. Moreover, we observed a splitting of the pre- and post-axial cartilaginous plates in respectively pre-axial radials and accessory elements on one hand, and in post-axial accessory elements on the other hand. However, the mechanisms involved in the splitting of the cartilaginous plates appear different from those involved in the formation of radials in actinopterygians. Our results show a proportional reduction of the proximal pre-axial radial of the fin, rendering the external morphology of the fin more lobe-shaped, and a spatial reorganization of elements resulting from the fragmentation of the two cartilaginous plates. Latimeria development hence supports previous interpretations of the asymmetrical pectoral fin skeleton as being plesiomorphic for coelacanths and sarcopterygians.

A complete insect from the Late Devonian period.

After terrestrialization, the diversification of arthropods and vertebrates is thought to have occurred in two distinct phases, the first between the Silurian and the Frasnian stages (Late Devonian period) (425-385 million years (Myr) ago), and the second characterized by the emergence of numerous new major taxa, during the Late Carboniferous period (after 345 Myr ago). These two diversification periods bracket the depauperate vertebrate Romer's gap (360-345 Myr ago) and arthropod gap (385-325 Myr ago), which could be due to preservational artefact. Although a recent molecular dating has given an age of 390 Myr for the Holometabola, the record of hexapods during the Early-Middle Devonian (411.5-391 Myr ago, Pragian to Givetian stages) is exceptionally sparse and based on fragmentary remains, which hinders the timing of this diversification. Indeed, although Devonian Archaeognatha are problematic, the Pragian of Scotland has given some Collembola and the incomplete insect Rhyniognatha, with its diagnostic dicondylic, metapterygotan mandibles. The oldest, definitively winged insects are from the Serpukhovian stage (latest Early Carboniferous period). Here we report the first complete Late Devonian insect, which was probably a terrestrial species. Its 'orthopteroid' mandibles are of an omnivorous type, clearly not modified for a solely carnivorous diet. This discovery narrows the 45-Myr gap in the fossil record of Hexapoda, and demonstrates further a first Devonian phase of diversification for the Hexapoda, as in vertebrates, and suggests that the Pterygota diversified before and during Romer's gap.

Environmental constraints drive the partitioning of the soundscape in fishes.

The underwater environment is more and more being depicted as particularly noisy, and the inventory of calling fishes is continuously increasing. However, it currently remains unknown how species share the soundscape and are able to communicate without misinterpreting the messages. Different mechanisms of interference avoidance have been documented in birds, mammals, and frogs, but little is known about interference avoidance in fishes. How fish thus partition the soundscape underwater remains unknown, as acoustic communication and its organization have never been studied at the level of fish communities. In this study, passive acoustic recordings were used to inventory sounds produced in a fish community (120 m depth) in an attempt to understand how different species partition the acoustic environment. We uncovered an important diversity of fish sounds, and 16 of the 37 different sounds recorded were sufficiently abundant to use in a quantitative analysis. We show that sonic activity allows a clear distinction between a diurnal and a nocturnal group of fishes. Moreover, frequencies of signals made during the day overlap, whereas there is a clear distinction between the different representatives of the nocturnal callers because of a lack of overlap in sound frequency. This first demonstration, to our knowledge, of interference avoidance in a fish community can be understood by the way sounds are used. In diurnal species, sounds are mostly used to support visual display, whereas nocturnal species are generally deprived of visual cues, resulting in acoustic constraints being more important.

An exceptionally preserved Late Devonian actinopterygian provides a new model for primitive cranial anatomy in ray-finned fishes.

Actinopterygians (ray-finned fishes) are the most diverse living osteichthyan (bony vertebrate) group, with a rich fossil record. However, details of their earliest history during the middle Palaeozoic (Devonian) 'Age of Fishes' remains sketchy. This stems from an uneven understanding of anatomy in early actinopterygians, with a few well-known species dominating perceptions of primitive conditions. Here we present an exceptionally preserved ray-finned fish from the Late Devonian (Middle Frasnian, ca 373 Ma) of Pas-de-Calais, northern France. This new genus is represented by a single, three-dimensionally preserved skull. CT scanning reveals the presence of an almost complete braincase along with near-fully articulated mandibular, hyoid and gill arches. The neurocranium differs from the coeval Mimipiscis in displaying a short aortic canal with a distinct posterior notch, long grooves for the lateral dorsal aortae, large vestibular fontanelles and a broad postorbital process. Identification of similar but previously unrecognized features in other Devonian actinopterygians suggests that aspects of braincase anatomy in Mimipiscis are apomorphic, questioning its ubiquity as stand-in for generalized actinopterygian conditions. However, the gill skeleton of the new form broadly corresponds to that of Mimipiscis, and adds to an emerging picture of primitive branchial architecture in crown gnathostomes. The new genus is recovered in a polytomy with Mimiidae and a subset of Devonian and stratigraphically younger actinopterygians, with no support found for a monophyletic grouping of Moythomasia with Mimiidae.

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.

Pulmonary arteries in coelacanths shed light on the vasculature evolution of air-breathing organs in vertebrates.

To date, the presence of pulmonary organs in the fossil record is extremely rare. Among extant vertebrates, lungs are described in actinopterygian polypterids and in all sarcopterygians, including coelacanths and lungfish. However, vasculature of pulmonary arteries has never been accurately identified neither in fossil nor extant coelacanths due to the paucity of fossil preservation of pulmonary organs and limitations of invasive studies in extant specimens. Here we present the first description of the pulmonary vasculature in both fossil and extant actinistian, a non-tetrapod sarcopterygian clade, contributing to a more in-depth discussion on the morphology of these structures and on the possible homology between vertebrate air-filled organs (lungs of sarcopterygians, lungs of actinopterygians, and gas bladders of actinopterygians).

A reevaluation of the anatomy of the jaw-closing system in the extant coelacanth Latimeria chalumnae.

The coelacanth Latimeria is the only extant representative of the Actinistia, a group of sarcopterygian fishes that originated in the Devonian. Moreover, it is the only extant vertebrate in which the neurocranium is divided into an anterior and a posterior portion that articulate by means of an intracranial joint. This joint is thought to be highly mobile, allowing an elevation of the anterior portion of the skull during prey capture. Here we provide a new description of the skull and jaw-closing system in Latimeria chalumnae in order to better understand its skull mechanics during prey capture. Based on a dissection and the CT scanning of an adult coelacanth, we provide a detailed description of the musculature and ligaments of the jaw system. We show that the m. adductor mandibulae is more complex than previously reported. We demonstrate that the basicranial muscle inserts more anteriorly than has been described previously, which has implications for its function. Moreover, the anterior insertion of the basicranial muscle does not correspond to the posterior tip of the tooth plate covering the parasphenoid, questioning previous inferences made on fossil coelacanths and other sarcopterygian fishes. Strong ligaments connect the anterior and the posterior portions of the skull at the level of the intracranial joint, as well as the notochord and the catazygals. These observations suggest that the intracranial joint is likely to be less mobile than previously thought and that its role during feeding merits to be reexamined.

Life history and ossification patterns in Miguashaia bureaui reveal the early evolution of osteogenesis in coelacanths.

The study of development is critical for revealing the evolution of major vertebrate lineages. Coelacanths have one of the longest evolutionary histories among osteichthyans, but despite access to extant representatives, the onset of their weakly ossified endoskeleton is still poorly understood. Here we present the first palaeohistological and skeletochronological study of Miguashaia bureaui from the Upper Devonian of Canada, pivotal for exploring the palaeobiology and early evolution of osteogenesis in coelacanths. Cross sections of the caudal fin bones show that the cortex is made of layers of primary bone separated by lines of arrested growth, indicative of a cyclical growth. The medullary cavity displays remnants of calcified cartilage associated with bony trabeculae, characteristic of endochondral ossification. A skeletochronological analysis indicates that rapid growth during a short juvenile period was followed by slower growth in adulthood. Our new analysis highlights the life history and palaeoecology of Miguashaia bureaui and reveals that, despite differences in size and habitat, the poor endoskeletal ossification known in the extant Latimeria chalumnae can be traced back at least 375 million years ago.

Development and growth of the pelvic fin in the extant coelacanth Latimeria chalumnae.

The ontogeny of the paired appendages has been extensively studied in lungfishes and tetrapods, but remains poorly known in coelacanths. Recent work has shed light on the anatomy and development of the pectoral fin in Latimeria chalumnae. Yet, information on the development of the pelvic fin and girdle is still lacking. Here, we described the development of the pelvic fin and girdle in Latimeria chalumnae based on 3D reconstructions generated from conventional and X-ray synchrotron microtomography, as well as MRI acquisitions. As in other jawed vertebrates, the development of the pelvic fin occurs later than that of the pectoral fin in Latimeria. Many elements of the endoskeleton are not yet formed at the earliest stage sampled. The four mesomeres are already formed in the fetus, but only the most proximal radial elements (preaxial radial 0-1) are formed and individualized at this stage. We suggest that all the preaxial radial elements in the pelvic and pectoral fin of Latimeria are formed through the fragmentation of the mesomeres. We document the progressive ossification of the pelvic girdle, and the presence of a trabecular system in the adult. This trabecular system likely reinforces the cartilaginous girdle to resist the muscle forces exerted during locomotion. Finally, the presence of a preaxial element in contact with the pelvic girdle from the earliest stage of development onward questions the mono-basal condition of the pelvic fin in Latimeria. However, the particular shape of the mesomeres may explain the presence of this element in contact with the girdle.

Palaeogeography: Devonian tetrapod from western Europe.

Several discoveries of Late Devonian tetrapods (limbed vertebrates) have been made during the past two decades, but each has been confined to one locality. Here we describe a tetrapod jaw of about 365 million years (Myr) old from the Famennian of Belgium, which is the first from western continental Europe. The jaw closely resembles that of Ichthyostega, a Famennian tetrapod hitherto known only from Greenland. The environment of this fossil provides information about the conditions that prevailed just before the virtual disappearance of tetrapods from the fossil record for 20 Myr.

Lung anatomy and histology of the extant coelacanth shed light on the loss of air-breathing during deep-water adaptation in actinistians.

Lungs are specialized organs originated from the posterior pharyngeal cavity and considered as plesiomorphic for osteichthyans, as they are found in extant basal actinopterygians (i.e. Polypterus) and in all major groups of extant sarcopterygians. The presence of a vestigial lung in adult stages of the extant coelacanth Latimeria chalumnae is the result of allometric growth during ontogeny, in relation with long-time adaptation to deep water. Here, we present the first detailed histological and anatomical description of the lung of Latimeria chalumnae, providing new insights into its arrested differentiation in an air-breathing complex, mainly represented by the absence of pneumocytes and of compartmentalization in the latest ontogenetic stages.

The homology and function of the lung plates in extant and fossil coelacanths.

The presence of a pulmonary organ that is entirely covered by true bone tissue and fills most of the abdominal cavity is hitherto unique to fossil actinistians. Although small hard plates have been recently reported in the lung of the extant coelacanth Latimeria chalumnae, the homology between these hard structures in fossil and extant forms remained to be demonstrated. Here, we resolve this question by reporting the presence of a similar histological pattern-true cellular bone with star-shaped osteocytes, and a globular mineralisation with radiating arrangement-in the lung plates of two fossil coelacanths (Swenzia latimerae and Axelrodichthys araripensis) and the plates that surround the lung of the most extensively studied extant coelacanth species, L. chalumnae. The point-for-point structural similarity of the plates in extant and fossil coelacanths supports their probable homology and, consequently, that of the organ they surround. Thus, this evidence questions the previous interpretations of the fatty organ as a component of the pulmonary complex of Latimeria.

Placoderm Assemblage from the Tetrapod-Bearing Locality of Strud (Belgium, Upper Famennian) Provides Evidence for a Fish Nursery.

The placoderm fauna of the upper Famennian tetrapod-bearing locality of Strud, Belgium, includes the antiarch Grossilepis rikiki, the arthrodire groenlandaspidid Turrisaspis strudensis and the phyllolepidid Phyllolepis undulata. Based on morphological and morphometric evidence, the placoderm specimens from Strud are predominantly recognised as immature specimens and this locality as representing a placoderm nursery. The Strud depositional environment corresponds to a channel in an alluvial plain, and the presence of a nursery in such environment could have provided nutrients and protection to the placoderm offspring. This represents one of the earliest pieces of evidence for this sort of habitat partitioning in vertebrate history, with adults living more distantly from the nursery and using the nursery only to spawn or give live birth.

A 365-Million-Year-Old Freshwater Community Reveals Morphological and Ecological Stasis in Branchiopod Crustaceans.

Branchiopod crustaceans are represented by fairy, tadpole, and clam shrimps (Anostraca, Notostraca, Laevicaudata, Spinicaudata), which typically inhabit temporary freshwater bodies, and water fleas (Cladoceromorpha), which live in all kinds of freshwater and occasionally marine environments [1, 2]. The earliest branchiopods occur in the Cambrian, where they are represented by complete body fossils from Sweden such as Rehbachiella kinnekullensis [3] and isolated mandibles preserved as small carbonaceous fossils [4-6] from Canada. The earliest known continental branchiopods are associated with hot spring environments [7] represented by the Early Devonian Rhynie Chert of Scotland (410 million years ago) and include possible stem-group or crown-group Anostraca, Notostraca, and clam shrimps or Cladoceromorpha [8-10], which differ morphologically from their modern counterparts [1, 2, 11]. Here we report the discovery of an ephemeral pool branchiopod community from the 365-million-year-old Strud locality of Belgium. It is characterized by new anostracans and spinicaudatans, closely resembling extant species, and the earliest notostracan, Strudops goldenbergi [12]. These branchiopods released resting eggs into the sediment in a manner similar to their modern representatives [1, 2]. We infer that this reproductive strategy was critical to overcoming environmental constraints such as seasonal desiccation imposed by living on land. The pioneer colonization of ephemeral freshwater pools by branchiopods in the Devonian was followed by remarkable ecological and morphological stasis that persists to the present day.

Redescription of the hyoid apparatus and associated musculature in the extant coelacanth Latimeria chalumnae: functional implications for feeding.

The coelacanth Latimeria is the only extant vertebrate in which the neurocranium is divided into an anterior and a posterior portion which articulate by means of an intracranial joint. This articulation is thought to allow an elevation of the snout up to 20-degree angle, which is supposed to enhance mouth gape and velocity, in turn allowing for a powerful suction. Several functional models have been proposed to explain the skull movement in Latimeria, but they disagree on the mechanisms responsible for mandibular depression and intracranial elevation, and more precisely on the role and mobility of the hyoid apparatus during these processes. We here show that the m. coracomandibularis spans ventrally to the palate-mandible joint, and is likely involved in mandibular depression. The hyoid apparatus is sheathed by several layers of ligaments, rendering extensive movements of the hyoid bones in the anteroposterior direction unlikely. Together with the manipulation of the 3D virtual model of the skull, these observations suggest that the hyoid arch is less mobile than previously proposed, and that the movements proposed in previous models are unlikely. In the light of our new observations, we suggest that the mechanisms proposed for explaining the intracranial elevation are incomplete. Moreover, we suggest that the extensive movements of the hyoid arch elements, which were thought to accompany intracranial elevation, are unlikely. In the absence of intracranial elevation, we propose that the movements of the hyoid mainly take place in the transverse plane, allowing the lateral expansion of the orobranchial chamber.

Bite force in the extant coelacanth Latimeria: the role of the intracranial joint and the basicranial muscle.

The terrestrialization process involved dramatic changes in the cranial anatomy of vertebrates. The braincase, which was initially divided into two portions by the intracranial joint in sarcopterygian fishes, became consolidated into a single unit in tetrapods and lungfishes [1-3]. The coelacanth Latimeria is the only extant vertebrate that retains an intracranial joint, which is associated with a unique paired muscle: the basicranial muscle. The intracranial joint has long been thought to be involved in suction feeding by allowing an extensive elevation of the anterior portion of the skull, followed by its rapid depression driven by the basicranial muscle [4-7]. However, we recently challenged this hypothesis [8, 9], and the role of the basicranial muscle with respect to the intracranial joint thus remains unclear. Using 3D biomechanical modeling, we show here that the basicranial muscle and the intracranial joint are involved in biting force generation. By flexing the anterior portion of the skull at the level of the intracranial joint, the basicranial muscle increases the overall bite force. This likely allows Latimeria to feed on a broad range of preys [10, 11] and coelacanths to colonize a wide range of environments during their evolution [4]. The variation in the morphology of the intracranial joint observed in Devonian lobe-finned fishes would have impacted to various degrees their biting performance and might have permitted feeding specializations despite the stability in their lower jaw morphology [12]. VIDEO ABSTRACT.

Allometric growth in the extant coelacanth lung during ontogenetic development.

Coelacanths are lobe-finned fishes known from the Devonian to Recent that were long considered extinct, until the discovery of two living species in deep marine waters of the Mozambique Channel and Sulawesi. Despite extensive studies, the pulmonary system of extant coelacanths has not been fully investigated. Here we confirm the presence of a lung and discuss its allometric growth in Latimeria chalumnae, based on a unique ontogenetic series. Our results demonstrate the presence of a potentially functional, well-developed lung in the earliest known coelacanth embryo, and its arrested growth at later ontogenetic stages, when the lung is clearly vestigial. The parallel development of a fatty organ for buoyancy control suggests a unique adaptation to deep-water environments. Furthermore, we provide the first evidence for the presence of small, hard, flexible plates around the lung in L. chalumnae, and consider them homologous to the plates of the 'calcified lung' of fossil coelacanths.