Sophie Sanchez

A primitive placoderm sheds light on the origin of the jawed vertebrate face

Extant vertebrates form two clades, the jawless Cyclostomata (lampreys and hagfishes) and the jawed Gnathostomata (all other vertebrates), with contrasting facial architectures. These arise during development from just a few key differences in the growth patterns of the cranial primordia: notably, the nasal sacs and hypophysis originate from a single placode in cyclostomes but from separate placodes in gnathostomes, and infraoptic ectomesenchyme migrates forward either side of the single placode in cyclostomes but between the placodes in gnathostomes. Fossil stem gnathostomes preserve cranial anatomies rich in landmarks that provide proxies for developmental processes and allow the transition from jawless to jawed vertebrates to be broken down into evolutionary steps. Here we use propagation phase contrast synchrotron microtomography to image the cranial anatomy of the primitive placoderm (jawed stem gnathostome) Romundina, and show that it combines jawed vertebrate architecture with cranial and cerebral proportions resembling those of cyclostomes and the galeaspid (jawless stem gnathostome) Shuyu. This combination seems to be primitive for jawed vertebrates, and suggests a decoupling between ectomesenchymal growth trajectory, ectomesenchymal proliferation, and cerebral shape change during the origin of gnathostomes.

Fossil musculature of the most primitive jawed vertebrates

From Jawless to Jawed The earliest vertebrates were jawless. Past reconstructions have assumed that the primitive jawed condition was much like that found in sharks. Trinajstic et al. (p. 160, published online 13 June; see the Perspective by Kuratani) describe fossil musculature from the early jawed placoderms (an extinct class of armored prehistoric fish) that show that the basal structure was distinct from that found in sharks, having a notable dermal joint between the skull and shoulder girdle. Fossilized muscles in an ancient armored fish reveal a distinctive neck musculature that differs from that of sharks. [Also see Perspective by Kuratani] The transition from jawless to jawed vertebrates (gnathostomes) resulted in the reconfiguration of the muscles and skeleton of the head, including the creation of a separate shoulder girdle with distinct neck muscles. We describe here the only known examples of preserved musculature from placoderms (extinct armored fishes), the phylogenetically most basal jawed vertebrates. Placoderms possess a regionalized muscular anatomy that differs radically from the musculature of extant sharks, which is often viewed as primitive for gnathostomes. The placoderm data suggest that neck musculature evolved together with a dermal joint between skull and shoulder girdle, not as part of a broadly flexible neck as in sharks, and that transverse abdominal muscles are an innovation of gnathostomes rather than of tetrapods.

3D Microstructural Architecture of Muscle Attachments in Extant and Fossil Vertebrates Revealed by Synchrotron Microtomography

Background Firm attachments binding muscles to skeleton are crucial mechanical components of the vertebrate body. These attachments (entheses) are complex three-dimensional structures, containing distinctive arrangements of cells and fibre systems embedded in the bone, which can be modified during ontogeny. Until recently it has only been possible to obtain 2D surface and thin section images of entheses, leaving their 3D histology largely unstudied except by extrapolation from 2D data. Entheses are frequently preserved in fossil bones, but sectioning is inappropriate for rare or unique fossil material. Methodology/Principal Findings Here we present the first non-destructive 3D investigation, by propagation phase contrast synchrotron microtomography (PPC-SRµCT), of enthesis histology in extant and fossil vertebrates. We are able to identify entheses in the humerus of the salamander Desmognathus from the organization of bone-cell lacunae and extrinsic fibres. Statistical analysis of the lacunae differentiates types of attachments, and the orientation of the fibres, reflect the approximate alignment of the muscle. Similar histological structures, including ontogenetically related pattern changes, are perfectly preserved in two 380 million year old fossil vertebrates, the placoderm Compagopiscis croucheri and the sarcopterygian fish Eusthenopteron foordi. Conclusions/Significance We are able to determine the position of entheses in fossil vertebrates, the approximate orientation of the attached muscles, and aspects of their ontogenetic histories, from PPC-SRµCT data. Sub-micron microtomography thus provides a powerful tool for studying the structure, development, evolution and palaeobiology of muscle attachments.

Developmental plasticity of limb bone microstructural organization in Apateon : histological evidence of paedomorphic conditions in branchiosaurs.

SUMMARY Apateon, a key genus among Branchiosauridae from the Carboniferous—Permian of Europe, is often considered closely related to salamanders on the basis of developmental similarities, anatomical features, and life history. The current work deals with recognition of heterochronies among three “time‐averaged populations” of Apateon based on inference from histological features already studied in extant urodeles. Our study is performed on the long bones of 22 specimens of Apateon pedestris and Apateon caducus. Histological observations show that diaphyseal and epiphyseal ossification patterns of Apateon are similar to those of urodeles. From skeletochronological analysis, the identification of the age of sexual maturity allows us to discriminate juveniles from adults and to confirm the previous hypothesis of a paedomorphic condition based on anatomical data among these species. The current study also suggests a paedomorphic condition of each “population” at the histological level. This heterochrony may have been linked to peculiar ecological conditions such as hypoxic and fresh water environment. Functional reasons may also be invoked to explain differences of ossification between fore‐ and hindlimbs of the “populations” from Odernheim and Niederkirchen because paleoecological conditions are very different from one locality to another. This study illustrates the role that the acquisition of heterochronic features plays at the microevolutionary scale.

Vertebral architecture in the earliest stem tetrapods

The construction of the vertebral column has been used as a key anatomical character in defining and diagnosing early tetrapod groups. Rhachitomous vertebrae—in which there is a dorsally placed neural arch and spine, an anteroventrally placed intercentrum and paired, posterodorsally placed pleurocentra—have long been considered the ancestral morphology for tetrapods. Nonetheless, very little is known about vertebral anatomy in the earliest stem tetrapods, because most specimens remain trapped in surrounding matrix, obscuring important anatomical features. Here we describe the three-dimensional vertebral architecture of the Late Devonian stem tetrapod Ichthyostega using propagation phase-contrast X-ray synchrotron microtomography. Our scans reveal a diverse array of new morphological, and associated developmental and functional, characteristics, including a possible posterior-to-anterior vertebral ossification sequence and the first evolutionary appearance of ossified sternal elements. One of the most intriguing features relates to the positional relationships between the vertebral elements, with the pleurocentra being unexpectedly sutured or fused to the intercentra that directly succeed them, indicating a ‘reverse’ rhachitomous design. Comparison of Ichthyostega with two other stem tetrapods, Acanthostega and Pederpes, shows that reverse rhachitomous vertebrae may be the ancestral condition for limbed vertebrates. This study fundamentally revises our current understanding of vertebral column evolution in the earliest tetrapods and raises questions about the presumed vertebral architecture of tetrapodomorph fish and later, more crownward, tetrapods.

Limb-bone histology of temnospondyls: implications for understanding the diversification of palaeoecologies and patterns of locomotion of Permo-Triassic tetrapods

The locomotion of early tetrapods has long been a subject of great interest in the evolutionary history of vertebrates. However, we still do not have a precise understanding of the evolutionary radiation of their locomotory strategies. We present here the first palaeohistological study based on theoretical biomechanical considerations among a highly diversified group of early tetrapods, the temnospondyls. Based on the quantification of microanatomical and histological parameters in the humerus and femur of nine genera, this multivariate analysis provides new insights concerning the adaptations of temnospondyls to their palaeoenvironments during the Early Permian, and clearly after the Permo‐Triassic crisis. This study therefore presents a methodology that, if based on a bigger sample, could contribute towards a characterization of the behaviour of species during great evolutionary events.

The humerus of Eusthenopteron :a puzzling organization presaging the establishment of tetrapod limb bone marrow

Because of its close relationship to tetrapods, Eusthenopteron is an important taxon for understanding the establishment of the tetrapod body plan. Notably, it is one of the earliest sarcopterygians in which the humerus of the pectoral fin skeleton is preserved. The microanatomical and histological organization of this humerus provides important data for understanding the evolutionary steps that built up the distinctive architecture of tetrapod limb bones. Previous histological studies showed that Eusthenopterons long-bone organization was established through typical tetrapod ossification modalities. Based on a three-dimensional reconstruction of the inner microstructure of Eusthenopterons humerus, obtained from propagation phase-contrast X-ray synchrotron microtomography, we are now able to show that, despite ossification mechanisms and growth patterns similar to those of tetrapods, it also retains plesiomorphic characters such as a large medullary cavity, partly resulting from the perichondral ossification around a large cartilaginous bud as in actinopterygians. It also exhibits a distinctive tubular organization of bone-marrow processes. The connection between these processes and epiphyseal structures highlights their close functional relationship, suggesting that either bone marrow played a crucial role in the long-bone elongation processes or that trabecular bone resulting from the erosion of hypertrophied cartilage created a microenvironment for haematopoietic stem cell niches.

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.

Homogenization of sample absorption for the imaging of large and dense fossils with synchrotron microtomography

Propagation phase-contrast synchrotron radiation microtomography (PPC-SRμCT) has proved to be very successful for examining fossils. Because fossils range widely in taphonomic preservation, size, shape and density, X-ray computed tomography protocols are constantly being developed and refined. Here we present a 1-h procedure that combines a filtered high-energy polychromatic beam with long-distance PPC-SRμCT (sample to detector: 4–16 m) and an attenuation protocol normalizing the absorption profile (tested on 13-cm-thick and 5.242 g cm−3 locally dense samples but applicable to 20-cm-thick samples). This approach provides high-quality imaging results, which show marked improvement relative to results from images obtained without the attenuation protocol in apparent transmission, contrast and signal-to-noise ratio. The attenuation protocol involves immersing samples in a tube filled with aluminum or glass balls in association with a U-shaped aluminum profiler. This technique therefore provides access to a larger dynamic range of the detector used for tomographic reconstruction. This protocol homogenizes beam-hardening artifacts, thereby rendering it effective for use with conventional μCT scanners.

Life history of the stem tetrapod Acanthostega revealed by synchrotron microtomography

The transition from fish to tetrapod was arguably the most radical series of adaptive shifts in vertebrate evolutionary history. Data are accumulating rapidly for most aspects of these events, but the life histories of the earliest tetrapods remain completely unknown, leaving a major gap in our understanding of these organisms as living animals. Symptomatic of this problem is the unspoken assumption that the largest known Devonian tetrapod fossils represent adult individuals. Here we present the first, to our knowledge, life history data for a Devonian tetrapod, from the Acanthostega mass-death deposit of Stensiö Bjerg, East Greenland. Using propagation phase-contrast synchrotron microtomography (PPC-SRμCT) to visualize the histology of humeri (upper arm bones) and infer their growth histories, we show that even the largest individuals from this deposit are juveniles. A long early juvenile stage with unossified limb bones, during which individuals grew to almost final size, was followed by a slow-growing late juvenile stage with ossified limbs that lasted for at least six years in some individuals. The late onset of limb ossification suggests that the juveniles were exclusively aquatic, and the predominance of juveniles in the sample suggests segregated distributions of juveniles and adults at least at certain times. The absolute size at which limb ossification began differs greatly between individuals, suggesting the possibility of sexual dimorphism, adaptive strategies or competition-related size variation.