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John Strong Newberry described three species of the lobe-fin fish Onychodus (Osteichthyes, Sarcopterygii, Onychodontida) based on parasymphysial teeth, or tusks. Two species, Onychodus sigmoides Newberry, 1857 (type species of the genus) and Onychodus hopkinsi Newberry, 1857, were described from the “fish beds” in the Delaware Limestone (Middle Devonian, Eifelian) of Delaware, Ohio, USA; and one species, Onychodus ortoni Newberry, 1889, was described from the Ohio Shale, Huron Member (Upper Devonian, Famennian) of Perry Township, Franklin County, Ohio. In 1873, Newberry replaced the original species-group definition of O. hopkinsi with a definition based on teeth of different morphology from the West Falls Group (Upper Devonian, Frasnian) of Franklin, New York. Specimens of Newberry’s original Onychodus material, including the primary types, which were long assumed to be lost, have been rediscovered in a 19th-century collection. They show O. hopkinsi to be a junior synonym of O. sigmoides and clarify the species definition of O. sigmoides. Onychodus sigmoides, which is recognized from Middle Devonian strata of the Appalachian Basin in the United States and Canada, shows two end-member shapes of teeth on the parasymphysial whorl: procurved (arcuate) or nearly so proximally and recurved distally (anteriorly). Small teeth are commonly more slender than large teeth, which are robust. Parasymphysial teeth from the Upper Devonian of Ohio and New York are referred to O. ortoni.

Panderichthys toes the line

Panderichthys toes the line Early hypotheses suggested that the digits of tetrapods (land vertebrates) were homologues of the fin radials of fish, but this idea fell out of favour based on developmental studies and also on the fin of Panderichthys , an extinct fish closely related to land vertebrates, which appeared to lack distal digit-like fin radials. Evidence that fish had homologues of digits early in their evolutionary history has mounted, but Panderichthys stood in the way of full acceptance. Now a new computerized tomography study of a classic Panderichthys specimen shows that the old interpretation was in error. It did indeed have digit-like radials: the way is open to the era of fish fingers. Early hypotheses suggested that the digits of tetrapods (land vertebrates) were homologues of fin radials, but this idea fell out of favour on the basis of developmental studies and also on the fin of Panderichthys , a fish closely related to land vertebrates, which appeared to lack distal digit-like fin radials. A new CT study of a classic specimen of Panderichthys shows that the old interpretation was in error. Panderichthys did indeed have digit-like radials: nothing stands in the way of the era of fish fingers. One of the identifying characteristics of tetrapods (limbed vertebrates) is the presence of fingers and toes. Whereas the proximal part of the tetrapod limb skeleton can easily be homologized with the paired fin skeletons of sarcopterygian (lobe-finned) fish, there has been much debate about the origin of digits. Early hypotheses 1 interpreted digits as derivatives of fin radials, but during the 1990s the idea gained acceptance that digits are evolutionary novelties without direct equivalents in fish fin skeletons. This was partly based on developmental genetic data 2 , but also substantially on the pectoral fin skeleton of the elpistostegid (transitional fish/tetrapod) Panderichthys , which appeared to lack distal digit-like radials 3 . Here we present a CT scan study of an undisturbed pectoral fin of Panderichthys demonstrating that the plate-like ‘ulnare’ of previous reconstructions is an artefact and that distal radials are in fact present. This distal portion is more tetrapod-like than that found in Tiktaalik 4 and, in combination with new data about fin development in basal actinopterygians 5 , sharks 6 and lungfish 7 , makes a strong case for fingers not being a novelty of tetrapods but derived from pre-existing distal radials present in all sarcopterygian fish.

Among the morphological changes that occurred during the ‘fish-to-tetrapod’ transition was a marked reorganization of the cranial endoskeleton. Details of this transition, including the sequence of character acquisition, have not been evident from the fossil record. Here we describe the braincase, palatoquadrate and branchial skeleton of Tiktaalik roseae , the Late Devonian sarcopterygian fish most closely related to tetrapods. Although retaining a primitive configuration in many respects, the cranial endoskeleton of T. roseae shares derived features with tetrapods such as a large basal articulation and a flat, horizontally oriented entopterygoid. Other features in T. roseae , like the short, straight hyomandibula, show morphology intermediate between the condition observed in more primitive fish and that observed in tetrapods. The combination of characters in T. roseae helps to resolve the relative timing of modifications in the cranial endoskeleton. The sequence of modifications suggests changes in head mobility and intracranial kinesis that have ramifications for the origin of vertebrate terrestriality. Fins to limbs: joining the dots Details of the morphological changes involved in the vertebrate transition to land — including the order in which they occurred — are sparse. The fossil record is too sketchy to give a feature-by-feature timeline, but by examining multiple specimens of the braincase of the Devonian fossil Tiktaalik roseae , a transitional form between fishes and tetrapods, Downs et al. have filled in some of the gaps. Although primitive in many respects, some of T. roseae' s features nod to the tetrapod state. And by reference to this new view of Tiktaalik morphology, the transition from finned to limbed fossil forms can be more readily understood. The Devonian fossil Tiktaalik roseae represents a transitional form between fishes and tetrapods. This paper presents a detailed examination of the braincase of this creature. Although primitive in many respects, some features nod to the tetrapod state.

Background Pharyngeal segmentation is a defining feature of vertebrate embryos and is apparent as a series of bulges found on the lateral surface of the embryonic head, the pharyngeal arches. The ancestral condition for gnathostomes is to have seven pharyngeal segments: jaw, hyoid, and five posterior branchial arches. However, within the sarcopterygians, the pharyngeal region has undergone extensive remodelling that resulted in a reduction in the number of pharyngeal segments, such that amniotes have only five pharyngeal arches. The aim of this study is to probe the developmental basis of this loss of pharyngeal segments. Results We have therefore compared the development of the pharyngeal arches in an amniote, the chick, which has five segments, with those of a chondrichthyan, the catshark, which has seven segments. We have analysed the early phase of pharyngeal segmentation and we find that in both the most anterior segments form first with the posterior segments being added sequentially. We also documented the patterns of innervation of the pharynx in several vertebrates and note that the three most anterior segments receive distinct innervation: the first arch being innervated by the Vth nerve, the second by the VIIth and the third by the IXth. Finally, we have analysed Hox gene expression, and show that the anterior limit of Hoxa2 aligns with the second pouch and arch in both chick and catshark, while Hoxa3 is transiently associated with the third arch and pouch. Surprisingly, we have found that Hoxb1 expression is spatially and temporally dynamic and that it is always associated with the last most recently formed pouch and that this domains moves caudally as additional pouches are generated. Conclusion We propose that the first three pharyngeal segments are homologous, as is the posterior limit of the pharynx, and that the loss of segments occurred between these two points. We suggest that this loss results from a curtailment of the posterior expansion of the pharyngeal endoderm in amniotes at relatively earlier time point, and thus the generation of fewer segments.

Hox genes are known to play a key role in shaping the body plan of metazoans. Evolutionary dynamics of these genes is therefore essential in explaining patterns of evolutionary diversity. Among extant sarcopterygians comprising both lobe-finned fishes and tetrapods, our knowledge of the Hox genes and clusters has largely been restricted in several model organisms such as frogs, birds and mammals. Some evolutionary gaps still exist, especially for those groups with derived body morphology or occupying key positions on the tree of life, hindering our understanding of how Hox gene inventory varied along the sarcopterygian lineage. We determined the Hox gene inventory for six sarcopterygian groups: lungfishes, caecilians, salamanders, snakes, turtles and crocodiles by comprehensive PCR survey and genome walking. Variable Hox genes in each of the six sarcopterygian group representatives, compared to the human Hox gene inventory, were further validated for their presence/absence by PCR survey in a number of related species representing a broad evolutionary coverage of the group. Turtles, crocodiles, birds and placental mammals possess the same 39 Hox genes. HoxD12 is absent in snakes, amphibians and probably lungfishes. HoxB13 is lost in frogs and caecilians. Lobe-finned fishes, amphibians and squamate reptiles possess HoxC3. HoxC1 is only present in caecilians and lobe-finned fishes. Similar to coelacanths, lungfishes also possess HoxA14, which is only found in lobe-finned fishes to date. Our Hox gene variation data favor the lungfish-tetrapod, turtle-archosaur and frog-salamander relationships and imply that the loss of HoxD12 is not directly related to digit reduction. Our newly determined Hox inventory data provide a more complete scenario for evolutionary dynamics of Hox genes along the sarcopterygian lineage. Limbless, worm-like caecilians and snakes possess similar Hox gene inventories to animals with less derived body morphology, suggesting changes to their body morphology are likely due to other modifications rather than changes to Hox gene numbers. Furthermore, our results provide basis for future sequencing of the entire Hox clusters of these animals.

The evolutionary trajectories of growth hormone-releasing hormone (GHRH) receptor remain enigmatic since the discovery of physiologically functional GHRH-GHRH receptor (GHRHR) in non-mammalian vertebrates in 2007. Interestingly, subsequent studies have described the identification of a GHRHR(2) in chicken in addition to the GHRHR and the closely related paralogous receptor, PACAP-related peptide (PRP) receptor (PRPR). In this article, we provide information, for the first time, on the GHRHR in sarcopterygian fish and amphibians by the cloning and characterization of GHRHRs from lungfish (P. dolloi) and X. laevis. Sequence alignment and phylogenetic analyses demonstrated structural resemblance of lungfish GHRHR to their mammalian orthologs, while the X. laevis GHRHR showed the highest homology to GHRHR(2) in zebrafish and chicken. Functionally, lungfish GHRHR displayed high affinity towards GHRH in triggering intracellular cAMP and calcium accumulation, while X. laevis GHRHR(2) was able to react with both endogenous GHRH and PRP. Tissue distribution analyses showed that both lungfish GHRHR and X. laevis GHRHR(2) had the highest expression in brain, and interestingly, X. laevis(GHRHR2) also had high abundance in the reproductive organs. These findings, together with previous reports, suggest that early in the Sarcopterygii lineage, GHRHR and PRPR have already established diverged and specific affinities towards their cognate ligands. GHRHR(2), which has only been found in xenopus, zebrafish and chicken hitherto, accommodates both GHRH and PRP.