Thermal physiology and the origin of terrestriality in vertebrates

The adaptive reasons for the evolutionary transition between obligatorily aquatic lobe-finned fish and facultatively terrestrial early tetrapods have long been debated. The oldest adequately known amphibians, Acanthostega and Ichthyostega , from the final stage in the Upper Devonian (Famennian), can be clearly distinguished from the most advanced choanate sarcopterygian fish from the next older stage (Frasnian) by the presence of large pectoral and pelvic girdles, limbs generally resembling those of later Palaeozoic land vertebrates, and the absence of bones linking the back of the skull with the shoulder girdle. Upper Devonian and most Lower Carboniferous amphibians, like their aquatic predecessors, differed significantly from modern amphibians in their much larger size, up to a metre or more in length. Animals of this size, resembling modern crocodiles and the marine iguana, could have raised their body temperatures by basking in the sun and sustained them upon re-entry into the water. It is hypothesized that the physiological advantages of thermoregulation were a major selective force that resulted in the increasing capacity for the ancestors of tetrapods to move into shallow water, and later to support their bodies against the force of gravity and increase the size and locomotor capacities of the limbs.  © 2005 The Linnean Society of London, Zoological Journal of the Linnean Society , 2005, 143 , 345–358.     

The braincase of Typhlops and Leptotyphlops (Reptilia: Serpentes)

The structure of the braincases and associated nerves and blood-vessels is described. Typhlops combines primitive lacertilian features, notably the retention of a palatine artery, with specialisations such as the structure of the recessus scalae tympani. Leptotyphlops is more completely snake-like, but the two genera show in common a specialised intracranial course of the hyomandibular branch of the facial nerve, due to lateral closure of the juxtastapedial recess by the overgrowing crista circumfenestralis. The lateral closure of the juxtastapedial recess is considered as an adaptation to burrowing habits. The Vidian canal in scolecophidians is poorly defined, due to the lack of lateral ascending wings of the parasphenoid. This might constitute an archaic character contradicting the descent of snakes from any Recent lacertilian group.     

The braincase of the Middle Triassic shark Acronemus tuberculatus (Bassani, 1886)

Abstract: The chondrocranium of the enigmatic Middle Triassic shark Acronemus tuberculatus is investigated using computerized tomography scanning and 3‐D digital reconstitution techniques. The braincase reveals some autapomorphies, plus other features that suggest a phylogenetic relationship to both hybodontiform and neoselachian elasmobranchs, including evidence of features implicated in low‐frequency semi‐directional phonoreception. Acronemus can no longer be classified as a ‘ctenacanthiform’, although its relationships remain elusive and it presents supposedly hybodontiform and neoselachian features that have not previously been found in combination.     

Comparable disparity in the appendicular skeleton across the fish–tetrapod transition,and the morphological gap between fish and tetrapod postcrania

Appendicular skeletal traits are used to quantify changes in morphological disparity and morphospace occupation across the fish–tetrapod transition and to explore the informativeness of different data partitions in phylogeny reconstruction. Anterior appendicular data yield trees that differ little from those built from the full character set, whilst posterior appendicular data result in considerable loss of phylogenetic resolution and tree branch rearrangements. Overall, there is a significant incongruence in the signals associated with pectoral and pelvic data. The appendicular skeletons of fish and tetrapods attain similar levels of morphological disparity (at least when data are rarefied at the maximum sample size for fish in our study) and occupy similarly sized regions of morphospace. However, fish appear more dispersed in morphospace than tetrapods do. All taxa show a heterogeneous distribution in morphospace, and there is a clear separation between fish and tetrapods despite the presence of several evolutionarily intermediate taxa.     

Devonian tetrapod trackways and trackmakers; a review of the fossils and footprints

The earliest tetrapods are known from the Upper Devonian. Their remains are becoming better known from increasing numbers of specimens, localities, environments and ichnofossils. Each of the eight (or possibly nine) genera now represented by skeletal fossils is reviewed in its sedimentological, faunal and stratigraphic context, with an assessment of what might be inferred about the habitus and locomotory capabilities of each. Fossil trackways and their interpretations are then re-examined in the context of the known body forms, and consideration given to the degree of fit between the skeletal fossils, the trackways and their interpretations. The currently known Devonian tetrapods are unlikely to have made any of the known tracks, unless they were produced under water. Neither the skeletal fossils nor the trackways show good evidence of terrestrial locomotion among Devonian tetrapods. When the fossil material and recent phylogenetic analyses are taken in combination, it appears that neither tetrapods nor limbs with digits are likely to have arisen before the Frasnian. This should be borne in mind in palaeoecological studies of these animals.     

Morphology of the palate and braincase of Dimetrodon milleri

The palate and partial braincase of the holotype of Dimetrodon milleri (MCZ 1365) are preserved in three dimensions, but have yet to be described in detail. Here, we describe these structures for the first time for this species, and compare them with the better-known specimens of D. limbatus. Interesting characteristics of the morphology include the patterns of articulation of the palatal elements, including the palatine and vomer, and anatomy of the pterygoid in the posterior region of the palatal vacuities. Dimetrodon milleri is found to differ from D. limbatus in the lack of teeth on the ectopterygoid, the shape of the basal process of the epipterygoid, and the anterior extent of the palatine and pterygoid. The two species are similar in the relative position of the basicranial articulation, but differ significantly from that in other sphenacodontids, including Secodontosaurus and Sphenacodon. The evolution of these cranial features will be the subject of future phylogenetic analyses of sphenacodontids.     

An onychodont fish (Osteichthyes,Sarcopterygii) from the Early Devonian of China,and the evolution of the Onychodontiformes

Although the superbly preserved specimens of Onychodus jandemarrai have greatly advanced our understanding of the Onychodontiformes, a primitive sarcopterygian group with large parasymphysial tooth whorls, the scarcity of the otoccipital material in the group hampers further morphological comparisons between onychodonts and other sarcopterygian groups. Here we report a new onychodont Qingmenodus yui gen. et sp. nov. from the Early Devonian (Pragian) of South China that comprises well-ossified otoccipital and upper and lower jaw material. As one of the oldest known onychodonts, Qingmenodus shows for the first time the nearly complete structure of the otoccipital in onychodonts and provides an additional basis to address the phylogenetic position of the group. Its elongated otic shelf exhibits the posterior shift of the attachment for the basicranial muscle as in coelacanths and sheds light on the feeding mechanism of onychodonts. Qingmenodus displays a mosaic of primitive and derived onychodont features. The phylogenetic analysis places Qingmenodus immediately basal to the clade comprising Onychodus and Grossius.     

Observations on two rhipidistian fishes from the Upper Devonian of Lode, Latvia

New finds of extremely well preserved skeletons of two rhipidistian crossopterygians, Panderichthys rhombolepis (Gross) and Laccognathus panderi Gross, in the Gauja beds (Upper Devonian of Latvia) make possible nearly complete reconstructions of these fishes, and amplify our knowledge of the morphology of the two main orders of Rhipidistia, the Osteolepidida and Holoprvchiida. Panderichthys rhambolepis is distinguished from other osteolepids by consolidation of the bones of the skull roof and cheek; a single attachment surface between the scapulocoracoid and cleithrum; unusual vertebrae; partial fusion of the distal elements in the endoskeleton of the pectoral fin; and the absence of scutes at the bases of the fins. P. rhombolepis may represent a new order of Rhipidistia. L. panderi (litters from known holoptychiids only in details. One unusual feature is the presence of a single large external narial opening, limited posteriorly by a previously unknown bone, the prelachrvinal.     

The Upper Devonian tetrapodomorph Gogonasus andrewsae from Western Australia: Reconstruction of the shoulder girdle and opercular series using X-ray Micro-Computed Tomography

The tetrapodomorph fish, Gogonasus andrewsae is a three dimensionally well-preserved sarcopterygian from the Gogo Formation (Frasnian, early Upper Devonian, ∼380 million years ago) in Western Australia. High-resolution X-ray Micro-Computed Tomography and 3D printouts were used to obtain a digital reconstruction of its shoulder girdle and opercular series. Our new findings show the opercular series in a close fit against the upper bones of the shoulder girdle only if the anocleithrum, supracleithrum and post-temporal are aligned more horizontally than in previous reconstructions. The lowermost subopercular bone also differs, in partly covering the clavicle of the shoulder girdle. The ascending process of the clavicle, and the ventral process of the anocleithrum, do not fit closely inside the cleithrum, and perhaps functioned for ligamentous attachment. A rugose area on the anocleithral process is in a similar relative position to the attachment of a muscle ligament on the shoulder girdle of various living actinopterygians. Our manipulation of 3D printouts permits testing of the morphological fit of extremely fragile acid-etched bones, and indicates a new way to investigate the constructional morphology of one or more mechanical units of the vertebrate skeleton. It is suggested that Micro-CT imaging, reconstruction, visualisation and 3D printing techniques will provide a rigorous new test leading to modification of previous reconstructions of extinct vertebrates that were based on graphical methods and 2D imaging.     

Deciphering morphological variation in the braincase of caecilian amphibians (gymnophiona)

High levels of morphological homoplasy have hindered progress in understanding morphological evolution within gymnophione lissamphibians. Stemming from the hypothesis that the braincase has the potential to yield phylogenetic information, the braincases of 27 species (23 genera) of gymnophione amphibians were examined using high‐resolution micro‐computed tomography and histologically prepared specimens. Morphology of the brain and its relationship to features of the braincase is described, and it is shown that eight different patterns exist in the distribution of foramina in the antotic region. The distribution of variants is congruent with molecule‐based phylogeny. Additionally, all variants are shown to correspond directly to stages along developmental continua, suggesting that the evolutionary truncation of development in the antotic region at various stages has driven the evolution of morphology in this region. Attempts to correlate the observed morphology with proxies of putative heterochronic events (including those attributable to burrowing, life history, and size) fail to explain the distribution of morphology if each proxy is considered separately. Thus, it is concluded that either currently unrecognized causes of heterochrony or combinations thereof have influenced morphology in different lineages independently. These data identify clades whose morphology can now be reconsidered in light of previously unrecognized heterochronic events, thereby providing a foundation for future analyses of the evolution of morphology within Gymnophiona as a whole. Most significantly, these data confirm, for the first time in a lissamphibian group, that the braincase can preserve important phylogenetic information that is otherwise obscured in regions of the skull that experience strong influences from functional constraints. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Morphological and histological changes of dermal scales during the fish‐to‐tetrapod transition

Witzmann F. (2011). Morphological and histological changes of dermal scales during the fish‐to‐tetrapod transition. —Acta Zoologica (Stockholm) 92 : 281–302. The gastral scales of limbed tetrapodomorphs evolved from the ‘elpistostegid’‐type of scale by an enlargement and differentiation of the articulation facets and a shortening and broadening of the keel. These changes caused a tighter connection between gastral scales within a scale row and a greater overlap between the rows. Dorsal round scales of limbed tetrapodomorphs developed from a gastral scale‐type by an alteration of the ontogenetic pathway. The posterolateral direction of scale rows in ‘elpistostegids’ was retained in the gastral scalation of most limbed tetrapodomorphs, whereas the arrangement of round dorsal scales is modified to a transverse orientation. Both gastral and dorsal scales of limbed tetrapodomorphs consist solely of parallel‐fibred bone with circumferential growth marks. The proportionally larger overlap surfaces of gastral scales and their mode of articulation in the ventral midline indicate that the body of limbed tetrapodomorphs might have been more flexible than that of their finned relatives. The alteration of dermal scales was one of the most rapid morphological changes during the fish‐to‐tetrapod transition. Once established, gastral and dorsal scales were retained as a conservative character in different lineages of basal tetrapods, in both the amphibian and the amniote lineages.     

The stapes of Edops craigi and ear evolution in the lissamphibian stem group

The Lower Permian temnospondyl Edops craigi exemplifies an early and plesiomorphic condition of the single ear ossicle or stapes among the temnospondyls, the probable stem group of lissamphibians. In Edops, the 11-cm-long bone is more massive than in other temnospondyls, has a distinct neck, a dorsal crest and incompletely subdivided footplate and ventral process. Despite a range of invariances, temnospondyl stapes were much more diverse than previously conceived. A survey of described stapes gives insight into character evolution of the ear ossicle in the lissamphibian stem group. These include alternative patterns of paedomorphosis, proportional size change, morphology of tympanic region and reorientation of the auditory apparatus.     

Bystrow’s Paradox – gills,fossils, and the fish‐to‐tetrapod transition

Schoch, R.R. and Witzmann, F. 2011. Bystrow’s Paradox – gills, fossils, and the fish‐to‐tetrapod transition. —Acta Zoologica (Stockholm) 92 : 251–265. The issue of which breathing mechanism was used by the earliest tetrapods is still unsolved. Recent discoveries of stem tetrapods suggest the presence of internal gills and fish‐like underwater breathing. The same osteological features were used by Bystrow to infer a salamander‐like breathing through external gills in temnospondyl amphibians. This apparent contradiction – here called Bystrow’s Paradox – is resolved by reviewing the primary fossil evidence and the anatomy of the two gill types in extant taxa. Rather unexpectedly, we find that internal gills were present in a range of early crown tetrapods (temnospondyls), based on the anatomy of gill lamellae and location of branchial arteries on the ventral side of gill arch elements (ceratobranchials). Although it remains to be clarified which components are homologous in external and internal gills, both gill types are likely to have been present in Palaeozoic tetrapods – internal gills in aquatic adults of some taxa, and external gills in the larvae of these taxa and in larvae of numerous forms with terrestrial adults, which resorbed the external gills after the larval phase. Future developmental studies will hopefully clarify which mechanistic pathways are involved in gill formation and how these might have evolved.     

The evolution of the basicranium in the Henophidia (Reptilia: Serpentes)

In lizards, a short Vidian canal pierces the base of the basipterygoid process. In snakes, the anterior opening of the primitively short Vidian canal lies on the dorsal surface of the basisphenoid, trapped in an intracranial position by downgrowths of the frontal and parietal which meet the lateral edges of the basisphenoid-parasphenoid complex. This condition is observed in anilioid snakes which retain other primitive features in the braincase: the paroccipital process and the spheno-occipital tubercle (Aniliidae only) and participation of the basioccipital in the apertura lateralis of the recessus scalae tympani.
Subsequent evolution within booid snakes shows a shift of the anterior opening of the Vidian canal around the anterior edge of the basisphenoid, thus acquiring a secondary extracranial position. This occurs in parallel within boine and pythonine snakes. Dinilysia shows a parallel development of die condition observed in advanced booid snakes. Pseudoboa , with its short Vidian canal opening in-tracranially, demonstrates that caenophidians originated from a basal henophidian or pre-henophidian stock. The Acrochordidae show a basicranium that can be interpreted as either primitive henophidian or primitive caenophidian.