Microcomputed tomography of the holotype of the early tetrapod Ichthyerpeton bradleyae (Huxley in Wright and Huxley, 1866) from the Pennsylvanian of Ireland

Ichthyerpeton bradleyae (Huxley in Wright and Huxley, 1866) is one of the seven tetrapods originally described by Huxley from the Jarrow Assemblage (Pennsylvanian, Langsettian Regional Substage equated with the Bashkirian International Stage) in south-eastern Ireland. The holotype, one of only two specimens considered to represent the taxon, consists of the postcranial skeleton, which has been highly compressed and has undergone extensive replacement of bone by carbonaceous material. The holotype is studied using microcomputed tomography, which reveals that the vertebral column has at least 25 diplospondylous vertebrae with cylindrical centra. Neural arches and a haemal arch are described for the first time. Neural arches in the caudal region are paired and neural spines only contact one another dorsally. The hemal arch is fused and wraps around the ventral margin of the centrum. A stout femur and tibia are described. The morphology of the femur is unique for early tetrapods, with fibular and tibial condyles of similar length and lacking an adductor crest. The morphology of the femur, and its length relative to the tibia, suggests that the holotype of I. bradleyae preserves an immature individual. The tibia is a flat bone characteristic of stem tetrapods. Phalanges from the right and left pes are present. Because the phalanges are disarticulated, a phalangeal count cannot be determined. Despite the new anatomical information, the systematic position of I. bradleyae is still difficult to establish; however, it does not belong within the colosteids, temnospondyls, or embolomeres, to which it has previously been assigned.

Microanatomy and growth of the mesosaurs <i>Stereosternum tumidum</i> and <i>Brazilosaurus sanpauloensis</i> (Reptilia, Parareptilia)

Histology and microanatomy of vertebrae, ribs, haemal arch, and humeri and femora of 10 individuals of Stereosternum and two dorsal ribs of 1 individual of Brazilosaurus were studied. All individuals had achieved a body length of 50 cm (equal to 65 % of the maximum known body length) or larger. All sampled bones are highly osteosclerotic due to the reduction of medullary cavities and the filling of medullary regions by endosteal bone. Calcified cartilage occurs – if at all – only locally in small clusters in the medullary regions of midshaft and in higher amounts only in non-midshaft sections of long bones and towards the medio-distal rib shaft, respectively. The primary bone tissue consists of highly organized parallel-fibred tissue and/or lamellar tissue, which is in most samples relatively lightly vascularized or even avascular. If present, vascular canals are mainly longitudinally oriented; some show a radial orientation. Simple vascular canals as well as primary osteons occur. Some of the latter are secondarily altered, i.e. widened. Remodelling of the periosteal cortex is only documented by few scattered erosion cavities and secondary osteons. The tissue is regularly stratified by lines of arrested growth (LAGs), which usually appear as double or multiple rest lines, indicating strong dependence on exogenous and endogenous factors. Because of the inhibition of periosteal remodelling the growth record is complete and no inner cycles are lost. Individuals of Stereosternum show a poor correlation of body size and number of growth marks, which might be the result of developmental plasticity. Brazilosaurus shows a highly organized, avascular lamellar tissue and a high number of regularly deposited rest lines throughout the cortex of the ribs. The medullary region in the ribs of Brazilosaurus is distinctly larger when compared to ribs of Stereosternum. However, strong osteosclerosis is obvious in both taxa, pointing to a high degree of aquatic adaption. Ribs of Stereosternum, Brazilosaurus, and Mesosaurus are clearly distinguishable from each other by the distribution of the periosteal and endosteal territory. Furthermore, Brazilosaurus differs in its growth pattern (i.e. spacing of rest lines) when compared to Stereosternum and Mesosaurus.

The postcranial skeleton ofDiscosauriscusKuhn, a seymouriamorph tetrapod from the Lower Permian of the Boskovice Furrow (Czech Republic)

The description of the postcranial skeleton of larval, metamorphic and early juvenile specimens of the genusDiscosauriscusis based on three-dimensional material and includes a description of the ontogeny of the swollen neural arches and the central elements of the vertebrae.Discosauriscushas 24 (or 23) presacral vertebrae. The morphology of the atlas–axis complex is similar to that inSeymouria sanjuanensis. The neural arches start to swell slightly in specimens of late larval stage; they are completely swollen immediately after metamorphosis. There are about 40 caudal vertebrae and one sacral vertebra. The atlantal pleurocentrum is paired in metamorphic individuals. In postmetamorphic individuals, the pleurocentra 2–5 are not completely closed dorsally; the pleurocentra 6–30 form complete discs. The first haemal arch is situated on the sixth caudal vertebra. The atlantal rib is present. There are six caudal ribs.Discosauriscushas an anocleithrum which is the first record of this dermal pectoral element within seymouriamorphs and Lower Permian tetrapods. The scapula and the coracoid are separate elements. The phalangeal formula of the manus is 2, 3, 4, 5, 3. The iliac blade has a massive, almost horizontally oriented posterior process; the anterior process is absent. The phalangeal formula of the pes is 2, 3, 4, 5, 3. Rounded ventral scales are present. The comparison and evaluation of the available postcranial elements ofDiscosauriscus, Utegenia, AriekanerpetonandSeymouriasupport the view thatDiscosauriscusandAriekanerpeton, forming the family Discosauriscidae, are immediately related genera.

Seymouria sanjuanensis (Amphibia, Batrachosauria) from the Lower Permian Cutler Formation of north-central New Mexico and the occurrence of sexual dimorphism in that genus questioned

Six specimens of the amphibian Seymouria, preserved in a single block of matrix from the Lower Permian Cutler Formation of north-central New Mexico, are described and referred to Seymouria sanjuanensis Vaughn. They are the only Seymouria specimens known from New Mexico and provide a more extensive definition of the species. It is interpreted that the specimens from New Mexico were collected from an early to middle Wolfcampian horizon and therefore represent the earliest known members of the genus. Evidence is presented that challenges previous explanations for the variability of several features of the skull and axial skeleton in specimens of Seymouria baylorensis and S. sanjuanensis as an indication of sexual dimorphism. Differences in the number of maxillary teeth, depth of the maxilla, and development of the maxillary dentition, particularly in the "canine" region, are interpreted as closely related morphological trends in Seymouria. Although no satisfactory explanation is offered for differences in the serial position of the first haemal arch and in the interorbital breadth, sexual dimorphism is considered very unlikely.

Memoirs: The Structure and Development of the Tail in the Plaice (Pleuronectes Platessa) and the Cod (Gadus Morrhua)

1. The structure and development of the tails of Pleuronectes platessa L. and of Gadus morrhua L. are described, with some reference to Gadus merlangus L. 2. In Pleuronectes: (a) seven hypural cartilages are identified, the most posterior becoming a dorsal element in the definitive tail while the next three fuse to form the upper of the two hypural bones borne by the terminal vertebra; (b) the penultimate vertebra comes to bear two dorsal and two ventral arches, probably basalia, as a result of their fusion during development; (c) the last neural and haemal arch bear traces of transient interbasalia at their first appearance. 3. In Gadus: (a) the tail resembles that of Pleuronectes in certain respects, including the general arrangement of the dorsal and ventral skeletal elements, the appearance of transient interbasalia in connexion with the last neural and haemal arch, and the fusion of two dorsal and ventral arches, although these come to be borne by the ante-penultimate vertebra instead of by the penultimate vertebra; (b) the embryonic axial lobe of the fin is median (instead of dorsal as it is in Pleuro-nectes) as a result of the upper half of the definitive fin being formed by the backgrowth of a dorsal fin-fold. 4. The two types of fin are compared, and it is concluded that the Gadoid fin could have been derived by the fusion of a homocercal fin, of a type more primitive than that of Pleuro-nectes, with a dorsal and ventral fin-fold, accompanied by the loss of the most terminal of the hypurals characteristic of the normal homocercal fin.