Per Ahlberg

All living tetrapods have a one-to-two branching pattern in the embryonic proximal limb skeleton, with a single element at the base of the limb (the humerus or femur) that articulates distally with two parallel radials (the ulna and radius or the tibia and fibula). This pattern is also seen in the fossilized remains of stem-tetrapods, including the fishlike members of the group, in which despite the absence of digits, the proximal parts of the fin skeleton clearly resemble those of later tetrapods. However, little is known about the developmental mechanisms that establish and canalize this highly conserved pattern. We describe the well-preserved pelvic fin skeleton of Rhizodus hibberti, a Carboniferous sarcopterygian (lobe-finned) fish, and member of the tetrapod stem group. In this specimen, three parallel radials, each robust with a distinct morphology , articulate with the femur. We review this unexpected morphology in a phylogenetic and developmental context. It implies that the developmental patterning mechanisms seen in living tetrapods, now highly constrained, evolved from mechanisms flexible enough to accommodate variation in the zeugopod (even between pectoral and pelvic fins), while also allowing each element to have a unique morphology. zeugopod | pelvis | limb patterning | tetrapodomorph | rhizodontid

The numerous cushion-shaped tooth-bearing plates attributed to the stem group osteichthyan Lophosteus superbus, which are argued here to represent an early form of the osteichthyan inner dental arcade, display a previously unknown and presumably primitive mode of tooth shedding by basal hard tissue resorption. They carry regularly spaced, recumbent, gently recurved teeth arranged in transverse tooth files that diverge towards the lingual margin of the cushion. Three-dimensional reconstruction from propagation phase-contrast synchrotron microtomography (PPC-SRµCT) reveals remnants of the first-generation teeth embedded in the basal plate, a feature never previously observed in any taxon. These teeth were shed by semi-basal resorption with the periphery of their bases retained as dentine rings. The rings are highly overlapped, which evidences tooth shedding prior to adding the next first-generation tooth at the growing edge of the plate. The first generation of teeth is thus diachronous. Successor teeth at the same sites underwent cyclical replacing and shedding through basal resorption, producing stacks of buried resorption surfaces separated by bone of attachment. The number and spatial arrangement of resorption surfaces elucidates that basal resorption of replacement teeth had taken place at the older tooth sites before the addition of the youngest first-generation teeth at the lingual margin. Thus, the replacement tooth buds cannot have been generated by a single permanent dental lamina at the lingual edge of the tooth cushion, but must have arisen either from successional dental laminae associated with the individual predecessor teeth, or directly from the dental epithelium of these teeth. The virtual histological dissection of these Late Silurian microfossils broadens our understanding of the development of the gnathostome dental systems and the acquisition of the osteichthyan-type of tooth replacement.

Fin spines are commonly known from fossil gnathostomes (jawed vertebrates) and are usually associated with paired and unpaired fins. They are less common among extant gnathostomes, being restricted to the median fins of certain chondrichthyans (cartilaginous fish), including chimaerids (elephant sharks) and neoselachians (sharks, skates, and rays). Fin spine growth is of great interest and relevance but few studies have considered their evolution and development. We investigated the development of the fin spine of the chimaerid Callorhinchus milii using stained histological sections from a series of larval, hatchling, and adult individuals. The lamellar trunk dentine of the Callorhinchus spine first condenses within the mesenchyme, rather than at the contact surface between mesenchyme and epithelium, in a manner more comparable to dermal bone formation than to normal odontode development. Trabecular dentine forms a small component of the spine under the keel; it is covered externally with a thin layer of lamellar trunk dentine, which is difficult to distinguish in sectioned adult spines. We suggest that the distinctive characteristics of the trunk dentine may reflect an origin through co-option of developmental processes involved in dermal bone formation. Comparison with extant Squalus and a range of fossil chondrichthyans shows that Callorhinchus is more representative than Squalus of generalized chondrichthyan fin-spine architecture, highlighting its value as a developmental model organism.

The recent developments of phase-contrast synchrotron imaging techniques have been of great interest for paleontologists, providing three-dimensional (3D) tomographic images of anatomical structures, thereby leading to new paleobiological insights and the discovery of new species. However, until now, it has not been used on features smaller than 5–7 μm voxel size in fossil bones. Because much information is contained within the 3D histological architecture of bone, including an ontogenetic record, crucial for understanding the paleobiology of fossil species, the application of phase-contrast synchrotron tomography to bone at higher resolutions is potentially of great interest. Here we use this technique to provide new 3D insights into the submicron-scale histology of fossil and recent bones, based on the development of new pink-beam configurations, data acquisition strategies, and improved processing tools. Not only do the scans reveal by nondestructive means all of the major features of the histology at a resolution comparable to that of optical microscopy, they provide 3D information that cannot be obtained by any other method.

The origin of tetrapods is one of the key events in vertebrate history. The oldest tetrapod body fossils are Late Devonian (Frasnian–Famennian) in age, most of them consisting of rare isolated bone elements. Here we describe tetrapod remains from two Famennian localities from Belgium: Strud, in the Province of Namur, and Becco, in the Province of Liège. The newly collected material consists of an isolated complete postorbital, fragments of two maxillae, and one putative partial cleithrum, all from Strud, and an almost complete maxilla from Becco. The two incomplete maxillae and cleithrum from Strud, together with the lower jaw previously recorded from this site, closely resemble the genus Ichthyostega, initially described from East Greenland. The postorbital from Strud and the maxilla from Becco do not resemble the genus Ichthyostega. They show several derived anatomical characters allowing their tentative assignment to a whatcheeriid-grade group. The new tetrapod records show that there are at least two tetrapod taxa in Belgium and almost certainly two different tetrapod taxa at Strud. This locality joins the group of Devonian tetrapod-bearing localities yielding more than one tetrapod taxon, confirming that environments favourable to early tetrapod life were often colonized by several tetrapod taxa.

Dupret, V., S. Sanchez, D. Goujet, P. Tafforeau, and P. Ahlberg. 2011: Intracranial anatomy of Romundina stellina Ørvig 1975 (Vertebrata, Placodermi, Acanthothoraci) revealed by phase contrast synchrotron imaging The 2nd Wiman Meeting on Scandinavian-Baltic Palaeontology, Uppsala, Sweden, 2011.

Dupret, V., S. Sanchez, D. Goujet, P. Tafforeau, and P. Ahlberg. 2012: Structures intra-crâniennes de Romundina stellina Ørvig 1975 (Vertebrata, Placodermi, Acanthothoraci) révélé par tomographie synchrotron en contraste de phase. Vème Rencontres de l'Ichtyologie en France, Paris, France, 2012.


"Les placodermes acanthothoracides sont parmi les vertébrés gnathostomes les plus basaux phylogénetiquement et morphologiquement. Néanmoins, une bonne connaissance anatomie crânienne fait défaut, et à ce jour un seul genre (Brindabellaspis) a été décrit en détails. Nous présentons le modèle en 3 dimensions d’un crâne presque complet de Romundina stellina, un petit acanthothoracide du Dévonien inférieur de l’Archipel Arctique Canadien, décrit originellement par Ørvig (1975). Le spécimen a été microtomographié sur la ligne de faisceau ID 19 de l’ESRF de Grenoble (European Synchrotron Radiation Facility), en protocole de contraste de phase, avec un voxel isotrope de 7,45 micromètres.
Malgré une cassure oblique, la plupart des structures peuvent être reconstruites par symétrie. Chaque nerf crânien peut être suivi entre la cavité encéphalique et les murs du neurocrâne composés d’os périchondral. Il en est de même pour les vaisseaux sanguins. La détermination des homologies en est donc facilitée, tout en assurant la non destruction du spécimen. Les hypothèses d’homologies formulées par Ørvig peuvent être traitées en toute confidence.
La couche d’os périchondrale entourant la cavité encéphalique n’est pas homogène mais présente un aspect en dentelle entre les nerfs trijumeaux (V) et vague (X) ; il en est de même pour les oreilles internes, dont les canaux semi-circulaires ne sont pas ossifiés du tout latéralement et dorsalement. Cet aspect en dentelle n’est ni un artefact de fossilisation, de préservation ou de modélisation, et n’a jamais été retrouvé sur aucun autre vertébré (mais l’échantillonnage à cette résolution fait encore cruellement défaut).
Les canalicules nerveux reliés aux neuromastes de la ligne latérale permettent de retracer leur origine à une branche du nerf facial (VII). Les deux oreilles internes ont été reconstruites avec précision et montrent une morphologie primitive.
Le réseau vasculaire de l’os dermique a été reconstruit en détails, et permet de mettre en évidence les limites de plaques du toit crânien, invisibles autrement. Ce réseau vasculaire est relié à des veines drainant la bordure de la boîte crânienne ou à une branche de la veine jugulaire. La courbure de ces vaisseaux autour de l’oreille interne pourrait démarquer la limite entre la capsule otique et l’arc hyoïdien qui s’y attachait.
D’un point de vue général, la morphologie de la boîte crânienne et de ses structures associées paraît moins primitive (et moins extrême) que celle de Brindabellaspis, mais rappelle au contraire plus les structures observées chez le placoderme arthrodire Kujdanowiaspis, plus dérivé.
Ces différences mettent en lumière les premiers stades de l’évolution du crâne des placodermes, donc des gnathostomes. "

"Dupret, V., S. Sanchez, D. Goujet, P. Tafforeau, and P. Ahlberg. 2012: Dans la peau de Romundina stellina Ørvig, 1975 (Vertebrata, Placodermi, Acanthothoraci): Anatomie crânienne d'un des premiers gnathostomes révélée par tomographie synchrotron en contraste de phase - Being Romundina stellina Ørvig, 1975 (Vertebrate, Placodermi, Acanthothoraci): Intracranial anatomy of one of the deepest gnathostomes revealed by synchrotron tomograpy in phase contrast protocole. Quatrième Symposium "Georges Cuvier": Fossiles, Evolution, Mouvement, Montbéliard, France, 2012.
"

Dupret, V., S. Sanchez, D. Goujet, P. Tafforeau, and P. Ahlberg. 2011: Internal structures of the skull of Romundina stellina Ørvig 1975 (Vertebrata, Placodermi, Acanthothoraci) revealed by phase contrast synchrotron scanning. 13th Conference on Australasian Vertebrate Evolution Palaeontology and Systematics, Perth, Australia, 2011.


"The acanthothoracid placoderms (armored fishes) are considered as among the most phylogenetically basal and morphologically primitive gnathostomes (jawed vertebrates; Janvier 1996). However, their endocranial morphology is poorly understood. So far only the anatomy of the genus Brindabellaspis has been described in detail (Young 1980).
Here we present the 3D reconstruction of a nearly complete skull of Romundina stellina, first described by Ørvig (1975), from the Lochkovian of Prince of Wales Island, Canadian Arctic Archipelago. The specimen was scanned at the European Synchrotron Radiation Facility in Grenoble, France, at a 7.45 µm voxel resolution, using Phase Contrast protocol (Tafforeau et al. 2006). An oblique crack obscures part of the model, but most features are remarkably preserved and most of the missing structures can be virtually rebuilt by symmetry.
Each postethmoid cranial and craniospinal nerve can be followed between the well-preserved endocranial cavity and the walls of the perichondrally ossified neurocranium. The vasculature of the dermal bones is rendered in detail, and allows hypotheses regarding skeletogenesis (Dupret et al. 2010). Special attention is focused on the orbital area of the neurocranium. One of the advantages of this non-destructive technology is its ability to connect with certainty all the foramina of the orbit to the vascularization and the cranial nerves, and hence identify accurate homologies. Factually, we can now correct Ørvig’s original assumptions. This is crucial for understanding the first steps of the gnathostome evolutionary history, since the most basal gnathostomes have very peculiar orbital structures (supernumerary foramina, myodomes, etc).
The high resolution of the model reveals some unusual features in the perichondral bone cover of the endocranial cavity: it appears that between the trigeminal and vagus nerve (and the inner ears), the perichondral bone shows a “lace” pattern, unknown so far in vertebrates (presumably because of the lack of data). The significance of this character is unclear, but it is definitely not an artifact of taphonomy or scanning.

References
DUPRET V., SANCHEZ S., GOUJET D., TAFFOREAU P., & AHLBERG P. 2010. - Bone vascularization and growth in placoderms (Vertebrata): the example of the premedian plate of Romundina stellina Ørvig, 1975 Comptes Rendus Palevol 9: 369–375.
JANVIER P. 1996. - Early Vertebrates. Clarendon Press Edition, in CHARNOCK H., DEWEY J. F., PRICE R. A., CONWAY MORRIS S., NAVROTSKY A., SKINNER B. J. & OXBURGH E. R. (eds.), Oxford Monographs On Geology And Geophysics, Volume 1. Oxford Science Publications, Oxford, 393 p.
TAFFOREAU P., BOISTEL R., BOLLER E., BRAVIN A., BRUNET M., CHAIMANEE Y., CLOETENS P., FEIST M., HOSZOWSKA J., JAEGER J.-J., KAY R. F., LAZZARI V., MARIVAUX L., NEIL A., NEMOZ C., THIBAULT X., VIGNAUD P., & ZABLER S. 2006. - Applications of X-ray synchrotron microtomography for non-destructive 3D studies of paleontological specimens. Applied Physics A - Materials Science & Processing 83: 195–202.
YOUNG G. C. 1980. - A new Early Devonian placoderm from New South Wales, Australia, with a discussion of placoderm phylogeny. Palaeontographica (A): 10–76.
ØRVIG T. 1975. - Description, with special reference to the dermal skeleton, of a new Radotinid arthrodire from the Gedinnian of Arctic Canada. Extrait des Colloques internationaux du Centre National de la Recherche Scientifique - Problèmes actuels de Paléontologie - Evolution des Vertébrés 218: 41–71.""

Dupret, V., S. Sanchez, D. Goujet, P. Tafforeau, and P. Ahlberg. 2010: The cranial anatomy of Romundina stellina Ørvig, 1975 (Vertebrata, Placodermi, Acanthothoraci) revealed by phase contrast synchrotron scanning 3rd International Palaeontological Congress, London, U.K., 2010.


The acanthothoracid placoderms are among the most phylogenetically basal and morphologically primitive gnathostomes. However, their endocranial anatomy is not well understood; only one genus, Brindabellaspis, has been described in detail. Here we present a near-complete three-dimensional skull of Romundina stellina, a small Early Devonian acanthothoracid from the Canadian Arctic Archipelago, scanned at the European Synchrotron Radiation Facility, Grenoble, France, at a 7.45 µm resolution. Despite some loss of material along an oblique crack, most of the internal structures are remarkably preserved. Each postethmoid cranial and craniospinal nerve can be followed between the well-preserved endocranial cavity and the walls of the perichondrally ossified neurocranium. The minute nerve canals that supplied the neuromast organs of the sensory line system are preserved and can in the postorbital area be traced directly to a branch of the facial nerve. Both inner ears are present. The vascular mesh of the dermal bones has been reconstructed in detail, rendering visible the dermal plate boundaries of the skull roof, and is shown to connect to larger internal veins that drain to the edge of the braincase or into the jugular vein canal. The curvature of the latter vessels parallels the outer surface of the inner ear and may demarcate the boundary between otic capsule proper and applied hyoid arch material. Overall, the braincase morphology appears less extreme (and less primitive?) than that of Brindabellaspis, in some respects more reminiscent of a primitive arthrodire such as Kujdanowiaspis. These differences may illuminate the earliest stages of placoderm cranial evolution.