Development of the Osteocranium in Natrix natrix (Serpentes,Colubridae) Embryogenesis II: Development of the jaws,palatal complex and associated bones

Snakes differ from the other vertebrates with their hyperkinetic skull. To establish the developmental features of the skull bones, involved in prey capture and ingestion, the Grass snake Natrix natrix (Serpentes, Colubridae) embryos are studied at all the successive stages of embryogenesis. Thirty-five N. natrix embryos are examined. Twenty embryos are studied with histological methods; fifteen embryos are cleared and double-stained with alizarin red and alcian blue. The sequence of appearance and formation of the upper and lower jaw bones, palatal complex and associated bones is described in accordance with the table of developmental stages. New features in the ossification mode of some bones are revealed: each bone, namely, the vomer, septomaxilla and maxilla, is formed from three separate ossification centres. Three ossification centres in the maxilla, two ossification centres in the bodies of the septomaxilla and vomer, as well as the unknown additional ossification centre in the vomer had not been previously described in snake embryos. The new data can be used in further comparative research on the reptile skull development and vertebrate phylogeny.     

A heterochronic interpretation of the origin of digging adaptations in the northern water vole, Arvicola terrestris (Rodentia: Arvicolidae)

The Palaearctic genus Arvicola includes two species: the south‐western water vole A. sapidus, and the northern water vole A. terrestris. The latter has semiaquatic and/or subterranean populations, while populations of A. sapidus are always semiaquatic. According to the current phylogenetic and palaeontological data, adaptation to semiaquatic life is plesiomorphic for the genus Arvicola. We studied the ontogenetic allometry of skull and long bones of the semiaquatic A. sapidus, a semiaquatic population of A. terrestris (A. t. italicus), and two fossorial populations of A. terrestris (A. t. scherman and A. t. monticola). Animals from fossorial populations were smaller than were those from semiaquatic populations. We found that most of the ontogenetic allometric exponents of characters linked to digging in the skull and in the long bones were significantly higher in A. t. monticola, a fossorial clade, than they were in the semiaquatic populations. On the other hand, there may have been an evolutionary lag between invasion of the hypogeic habitat and the acquisition of fossorial adaptations in A. t. scherman. We showed statistically that the morphological differences linked to the invasion of a hypogeic habitat are already present in juvenile animals and, according to these results, suggest that these morphological differences are the direct expression of genetic changes rather than the outcome of epigenetic factors of mechanical origin. Moreover, we tried to ascertain whether the apomorphic shape of the skull and long bones in the fossorial populations of A. terrestris (compared with the primitive condition that would have been retained by the semiaquatic A. sapidus) are the outcome of a heterochronic process. Optimization by squared change parsimony supported the hypothesis of an apomorphic reduction of body size linked to the invasion of the subterranean habitat. The comparison of the ontogenetic trajectories of both skull shape and long bone shape suggested that a heterochronic process was involved in this morphological transformation. By using the ‘clock model’ method, this mechanism was identified as ‘accelerated dwarfism’ affecting both the skull and long bones. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 87 , 381–391.     

Human fossil remains from the Mousterian levels of Artenac (Charente)

The cave of Artenac is located about 20 km northeast of Angouleme, in the commune of Saint-Mary (Charente). In 1995 and 1996, two fragmentary hominine skull bones, a maxilla and a frontal, were uncovered during excavations in Mousterian levels of the cave. Biostratigraphic analyses of the abundant mammalian fauna place the Mousterian occupation of the cave at the beginning of Oxygen Isotope Stage 5. Although both bones come from adults, aspects of their morphology indicate they could derive from two individuals of different ages at death. The two bones are relatively fragmentary, but sufficient detail has been preserved, including an inflated maxillary region lacking an infra-orbital depression (or fossa canina) and the shape of the forehead, to establish their identity as Neandertals.     

The identification of the dermal bones of the head

The discovery of the ichthyostegid Amphibia in Upper Devonian rocks by Säve-Söderbergh (1932) introduced further difficulties into the already complex problems of the dermal bones of the skull roof. For some years previously ideas about the origin of the tetrapods had been dominated by Watson's (1926) Croonian Lecture in which he had demonstrated beyond reasonable doubt that the crossopterygian fishes and not the Dipnoi were their ancestors, and had attempted to show that many of the features of the Carboniferous labyrinthodonts were a direct inheritance from these fishes. It was to be expected, therefore, that any Amphibia from the Upper Devonian would be intermediate in their structures between the Middle Devonian osteolepids and the Carboniferous labyrinthodonts, but when discovered the ichthyostegids did not conform at all well to this expectation. While their skulls showed some very primitive features which might have been expected, the pattern of the dermal bones did not conform to plan, for these new animals had lost, it seemed, the intertemporals, bones found in both the osteolepids and nearly all early labyrinthodonts, and had a single postparietal bone in place of the paired bones of all other early Amphibia. The osteolepid skull had many more bones than these earliest Amphibia.     

Craniometry of cranial vault bones in newborn infants

10 craniometric measurements of the skull bones' fornix have been performed on 89 dried infant skulls. The authors have shown that the craniometric variability of the skull bone's fornix is connected with the skull length rather than with its height. The skull length is determined mostly by the squama of the temporal bone and occipital squama. The shape of the skull vault is determined mainly by parietal bone and squama of the frontal bone. Parietal bones shut unpaired fontanelle, but the remaining bones, under investigation, contribute to the closure of anterolateral and postero-lateral fontanelle.     

New information on the anatomy of the Late Permian gliding reptile<Emphasis Type="Italic">Coelurosauravus</Emphasis>

A new almost complete specimen ofCoelurosauravus is reported from the Kupferschiefer (Upper Permian) at Mansfeld, Sachsen/Anhalt (Germany). The well preserved posterior half of the skull differs from the previous finds by the large parietals and squamosals. The latter are interpreted as Single elements, rather than as combination of squamosal and supratemporal, and it is supposed that they may have been partially mobile, features of a male individual, and could be deployed in order to produce a threat display. Thin-sections of the patagial spars show the typical anatomical structures of lamellar bones with rather sparsely distributed osteocyte lacunae. Compared to earlier reconstructions a new restoration of the complete skeleton ofCoelurosauravus has a shorter trunk and “wings” with a delta-shaped outline and a feather-like “foresail”. What may be an impression of the patagium is preserved on one specimen in association with the second spar. The presumed mechanism of unfolding and folding the patagium is tried to be reconstructed.     

The density of long limb bones and the percentage ash weight of the skeleton of Macaca mulatta

The gravimetric density of humeri, radii, femora and tibiae from a series of 274 male and female skeletons of rhesus monkeys, Macaca mulatta, was determined for fetal, young and adult periods. The ages of 171 of the animals were known: they ranged from 57 days of gestation to 13.6 years; the ages of an additional 103 skeletons were estimated. The mean density of the fetal bones was found to increase linearly with age and was higher for males than females, and higher for the superior than for the inferior limb bones. During the young period the pattern of increase in density can be represented by a power-type curve, and the density is significantly higher in females than in males and in superior than in inferior limb bones. The densities of the long limb bones of the adult skeletons show a slight, but not significant, negative trend with increasing age. In this age group the mean densities are higher for males than females and higher for the superior than for the inferior limb bones. The percentage ash weight was determined for the total skeleton and for 21 subdivisions of 23 postnatal skeletons with estimated ages. The skull and long limb bones were found to have higher mean percentage ash weights than the vertebral segments and the sternum. Both the density and the percentage ash weight of the Macaca mulatta skeletons examined exceed those found in our earlier studies of the human skeleton.     

Observations on the skulls of sturgeons (Acipenseridae): shared similarities of <Emphasis Type="Italic">Pseudoscaphirhynchus kaufmanni</Emphasis> and juvenile specimens of <Emphasis Type="Italic">Acipenser stellatus</Emphasis>

A novel hypothesis uniting Acipenser stellatus and Pseudoscaphirhynchus as sister groups has recently been proposed based on analysis of DNA sequences. In this paper I compare specimens of A. stellatus and P. kaufmanni, and show that they share several putatively derived similarities in the structure of their skulls, including: the presence of large spines on the dermal bones of skull; lateral extrascapular bones that enclose the confluence of the posttemporal, supratemporal, and otic sensory canals; elongate dorsal rostral bones; border rostral bones distinct in shape from dorsal rostral bones; greatly enlarged jugal that lacks a median flange; rostral canal bones that loop posteriorly at the anterior commissure of the rostral sensory canals; and the presence of an elongate, flat, and broad posterior ventral rostral bone. These similarities support a close relationship between A. stellatus and Pseudoscaphirhynchus, but still remain to be critically tested.     

The erectile cheek-spine apparatus in the bristlenose catfish Ancistrus (Loricariidae, Siluriformes), and its relation to the formation of a secondary skull roof

In the South American catfish family Loricariidae, the opercle has been decoupled from the lower jaw, and has also lost its function in expiration. While many loricariid species have a small and slightly mobile opercle with reduced opercular musculature, within the hypostomine subfamily a novel opercular mechanism has developed that erects a tuft of enlarged odontodes anterior to the opercle. This defensive mechanism is examined in Ancistrus cf. triradiatus. The opercle has a prominent anterior process and the orientation of the reinforced articulation hinge to the hyomandibular bone has shifted. The opercular musculature is well developed, with a hypertrophied dilatator operculi that extends deep inside the skull roof bones and toward the midline, over the brain, but below the superficial skull roof. Hence the frontal, sphenotic, parieto-supraoccipital and compound pterotic bones consist of a dorsal, superficial part and a deeper part separating the brain from the muscle: two functional skull roofs are thus formed. The impact on the path of the cranial sensory canals is substantial, moving canals away from the skull surface. Hypertrophy of cranial muscles is known from many teleosts, but the invasion of such large muscles into the skull, which is drastically modified and literally hollowed out, has never been described before. These cranial modifications are greater in males than in females, related to the territorial behavior of the former, in which the erectile spines are usually used.     

The subdivisions and fusions of the exoskeletal skull bones ofDiscosauriscus austriacus (Makowsky 1876) and their possible homologues in rhipidistians

Two localities in the Boskovice Furrow region of Moravia (Czecho-Slovakia) have produced new, well preserved material of the Lower Permian tetrapodDiscosauriscus austriacus (Makowsky 1876). A relatively large number of specimens have been found with some dermal skull roof bones partly or fully subdivided, and/or fused. These are the first records of such subdivisions and fusions in this tetrapod and the following bones are discussed: frontal, postfrontal, parietal, intertemporal, supratemporal, tabular and postparietal. The subdivided bones within the skull roof ofDiscosauriscus (and some dissorophoids) are situated at the same places as those found in Devonian rhipidistians, and may relate to the homology of the bones of the cranial exoskeletal roof inDiscosauriscus and in osteolepiforms. It shows that the “orthodox” terminology of the skull roof bones used in osteolepiforms is correct.     

Mhox and vertebrate skeletogenesis: The long and the short of it

The development of the vertebrate skeleton is under complex genetic control, and good progress is being made towards identifying the genes responsible. A recent paper⁽¹⁾ contributes to this progress by describing transgenic mice in which the homeobox-containing MHox gene has been disrupted. MHox(?/?) mice have a range of skeletal defects, involving loss or shortening of structures in the skull, face and limb. Puzzling features of the MHox(?/?) mutation, which has similar effects on bones with very different embryological origins and yet spares other bones completely, may hold clues to the mechanisms that shape the skeleton. MHox(?/?) mice, used in conjunction with other skeletal mutants, will be important tools for exploring these mechanisms further.