Structure and histogenesis of tooth plates in Sagenodus inaequalis Owen considered in relation to the phylogeny of post-Devonian dipnoans

Growth and patterning in the conodont skeleton

Recent advances in our understanding of conodont palaeobiology and functional morphology have rendered established hypotheses of element growth untenable. In order to address this problem, hard tissue histology is reviewed paying particular attention to the relationships during growth of the component hard tissues comprising conodont elements, and ignoring a priori assumptions of the homologies of these tissues. Conodont element growth is considered further in terms of the pattern of formation, of which four distinct types are described, all possibly derived from a primitive condition after heterochronic changes in the timing of various developmental stages. It is hoped that this may provide further means of unravelling conodont phylogeny. The manner in which the tissues grew is considered homologous with other vertebrate hard tissues, and the elements appear to have grown in a way similar to the growing scales and growing dentition of other vertebrates.

On the amphibian Crassigyrinus scoticus watson from the carboniferous

The holotype of Crassigyrinus scoticus Watson from the Viséan (Lower Carboniferous) of Edinburgh shows the side of the skull of a very primitive amphibian with fish-like proportions, an osteolepiform fish configuration of bones round the nostril and a preopercular bone on the cheek. ‘ Macromerium scoticum ’ Lydekker from the same locality and horizon proves to be a Crassigyrinus mandibular ramus. This is corroborated by discovery of a skull and anterior skeleton of Crassigyrinus from the Namurian (basal Upper Carboniferous) of Cowdenbeath, Fife. The skull of Crassigyrinus is also shown to have a loosely articulated basioccipital which did not form a finished occipital condyle and a mandible with coronoid teeth. However, it shares a number of derived (synapomorph) characters with the anthracosauroid amphibia of the Carboniferous and early Permian, notably the characteristic tabular horn, the probable absence of posttemporal fossae, the nature of the dermal ornament, the histology of the teeth and a true basipterygoid articulation. The last character may also indicate relations to loxommatid and seymouriamorph amphibia and amniotes. The pattern of bones of the Crassigyrinus skull table, however, is the primitive tetrapod (‘temnospondyl’) one. The postcranial skeleton is both primitive and degenerate. The vertebrae each have a single crescent-shaped centrum (‘intercentrum ’) and neural arches as poorly ossified, unfused bilateral halves. Prezygapophyses are unbuttressed facets and postzygapophyses totally lacking. There is room for a virtually unconstricted notochord. The diameter of the centra increases posteriorly from the small (partly reconstructed) atlas-axis complex. Ribs are long, well-ossified and cylindrical, but lack well-ossified rib-heads. The fore-limb is minute, with a typical primitive tetrapod humerus, which, however, retains some foramina otherwise seen only in Ichthyostega and fishes. The elongate ventral scales are unlike those of any ‘labyrinthodont’ amphibia. It is suggested that the apparent ‘otic notches’ of Crassigyrinus may mark the position of persistent spiracles, while the stapes, not preserved in any specimen, may have been like that known in the Coal Measure anthracosaurs and in the primitive temnospondyl Greererpeton . Combined with an air-filled spiracular cleft the stapes could have been tuned to underwater rather than aerial hearing. Crassigyrinus appears to have been a large Amphiuma -like underwater predator. A case is made for the ‘sister-group’ relation of Crassigyrinus to the anthracosauroids and a cladogram presented of the subgroups involved. It is, however, difficult to make a case for the close relationship of Crassigyrinus and the Seymouriamorpha and the closeness of relationship of the latter to anthracosauroids is questioned. Crassigyrinus shares several primitive characters with Ichthyostega , but they are only distantly related, so that the loss of those characters in all other tetrapods must have been polyphyletic. There are other characters in which one or the other is clearly the more primitive, but the polarity of a number of alternative character states in the two genera is equivocal. The cladistic use of out-group comparison is impotent to solve the problem because rival sister-groups for the Tetrapoda have been proposed using, inter alia , the disputed characters.

Comparative morphology, histology and growth of the dental plates of the Devonian dipnoan Chirodipterus

The dental plates of the Devonian lungfish Chirodipterus australis Miles (Osteichthyes; Dipnoi) are shown to have achieved their characteristic morphology by a growth process different from that assumed for the plates of genera such as Dipterus. Each plate was thickened by the addition of layers of bone that also extended the plate labially, thus providing the base on which and into which dentine grew. Distinctive features of the dentition are: (a) labial increase of the dentine mass by the addition of blister-like denticles of simple enamel-covered dentine, which is initially ingrown by pleromic dentine and subsequently resorbed and replaced by petrodentine; (b) increase in the midline by a similar process that results in the addition of one (or possibly two) new ridges; (c) resorption of the posterior edge of the pterygoid plates and the posterior and posteromedial edges of the prearticular plates, with subsequent development over the resorbed surfaces of several generations of simple regenerative dentine; (d) resorption and redeposition of pleromic dentine and bone in a triangular region posteromedially on the pterygoid plates; (e) the formation of tuberosities that simulate teeth at a short distance in from the labial edge, by four processes: formation of an undulating plate margin, differential growth of petrodentine (hard compact dentine) within the pulp cavity, differential wear of the petrodentine and the adjacent bone plus pleromic dentine, and slightly greater growth of the petrodentine towards the occlusal surface relative to the adjacent bone and dentine; (f) expansion of the large flat surfaces of the plates by gradual replacement of the bone and dentine at the proximal ends of the furrows and also by the development of linkages of petrodentine across the furrows; (g) development of isolated tuberosities on the flat posterolateral parts of the plates. The petrodentine of the ridges, tuberosities and plateaus of the plates is indistinguishable structurally and in its mode of growth from the petrodentine in extant species of dipnoans. Plates similar to those of C. australis have been observed in Stomiahykus, Archaeonectes, Conchodus, Palaedaphus and Sunwapta, as well as several species usually referred to as Dipterus. Sunwapta may be congeneric with C. australis.(ABSTRACT TRUNCATED AT 400 WORDS)

The relationships of Uronemus: a carboniferous dipnoan with highly modified tooth plates

Previous accounts of the dentition of the Carboniferous dipnoan Uronemus have stressed the significance of the scattered small denticles. These, together with the marginal teeth and ridges, have been interpreted as primitive characters of the dipnoan dentition shared with three other genera: the Devonian Uranlophus and Griphognathus and the Carboniferous to Permian Conchopoma. Genera with tooth plates have been considered to be a monophyletic group in which tooth plates are a derived character; Uronemus has been excluded from this group in all previous investigations dealing with the significance of the dentition for determining relationships among dipnoans. The macromorphology of the dentition of Uronemus has been re-examined and correlated with the histology of all the dental tissues. Optical study of thin sections and scanning electron microscope study of the adjacent cut surfaces has shown that the hard, wear-resistant dentine of the teeth and ridges is petrodentine. The arrangement, growth, wear and histology of the dental tissues have been compared with those of denticulated and tooth-plated genera. The arrangement of new teeth relative to the tooth ridge, the pattern of wear along the ridge, and the type of dentine and its growth indicate that the dentition of Uronemus is best interpreted as a tooth plate with one long lingual tooth ridge and reduced lateral tooth rows. Therefore the marginal tooth ridges are not considered to be homologous with those of denticulate dipnoans such as Uranolophus. The presence of petrodentine, a tissue type only found in forms with tooth plates, is consistent with the view that the dentition is derived by modification of a radiate tooth plate. The denticles covering restricted regions of the palate and lower jaw are considered to have been a secondary acquisition. The suggestion that Conchopoma is a close relative of Uronemus is not accepted, and possible new relationships have been proposed. New data on Scaumenacia and Phaneropleuron, two other genera previously compared with Uronemus, are presented. Rhinodipterus, a form with elongate lingual ridges, is also discussed. Phaneropleuron is shown to have radiate tooth plates and not a marginal row of conical teeth as previously described. It is proposed that the tooth plate of Uronemus is derived from a dipterid type of plate. A discussion of some of the other factors involved in determining the relationships of the genus is given.(ABSTRACT TRUNCATED AT 400 WORDS)