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In 1952, Erik Jarvik diagnosed and offered a brief description of the large-bodied, highly nested tristichopterid taxon Eusthenodon, and its type species E. wangsjoi, from fossils recovered from the Britta Dal Formation (Famennian) of Gauss Halvø ( = Peninsula) and Ymer Ø ( = Island) in East Greenland. The original diagnosis for Eusthenodon only needed to distinguish the tristichopterid taxon from the two others known at the time, Eusthenopteron and Tristichopterus, both of them small-bodied forms with anatomy now recognised to be primitive within the clade. Following that publication, no new large-bodied tristichopterids with Eusthenodon-like characteristics would be introduced until the description of Mandageria fairfaxi in 1997. In the 45 interim years, the limited descriptive details and insufficient diagnosis of Eusthenodon turned the name into a broadly applicable taxonomic label for large-bodied tristichopterid discoveries. Recent efforts to rediagnose the taxon and reconsider its global distribution of referred materials have improved the taxonomic utility of the name. However, no complete description of type species E. wangsjoi has yet been written. This is despite a type series of specimens that includes complete and articulated skulls that remain available for study in the collections of the Natural History Museum of Denmark (NHMD). The work presented here fulfils the need for a complete comparative description of E. wangsjoi in the context of the many highly nested tristichopterid species that have been described in the last three decades. New figures of the E. wangsjoi type series of specimens are the first to offer views of the fossils unobscured by the superimposition of interpretive line drawings. The new description is accompanied by a descriptive inventory of all the potential E. wangsjoi fossils at the NHMD and a new phylogenetic analysis of clade Tristichopteridae that includes revised character data for E. wangsjoi and adds one recently described species.

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.

A new species of Langlieria is described from the Upper Devonian (Frasnian) Irish Valley Member of the Catskill Formation. The type material was collected from a road cut exposure on the north side of the westbound lanes of Pennsylvania Route 322 west of Port Matilda, Centre County, Pennsylvania, U.S.A. The new species of Langlieria, the second from the Catskill Formation, is represented by high quality cranial material including parietal and postparietal shields, cheek, principal gular, and lower jaw; fin material; and body scales. The new species is the fourth tristichopterid to be described from the Catskill Formation (after Hyneria lindae, Langlieria radiatus, and Eusthenodon bourdoni) and the first from the Irish Valley Member. It is also the first species of Langlieria with a known record in the Frasnian. Sedimentological and stratigraphic data suggest that the discovery site represents a distal, tide-dominated location within the Catskill Delta system with a greater marine influence than has been reconstructed for the previously described Catskill Formation tristichopterids. A new phylogenetic analysis of Tristichopteridae is presented that is the first to include the new species of Langlieria described here and E. bourdoni from the Cogan House Exit Ramp locality (Famennian, Duncannon Member) along U.S. Route 15/Interstate 99. The 50% majority-rule consensus tree from the analysis supports a highly-nested clade of tristichopterids that includes all of the considered species from the Famennian, with the exception of Heddleichthys dalgleisiensis, and additionally includes the Frasnian Langlieria species that is described here.

New material from the Upper Devonian (Famennian) Catskill Formation of Pennsylvania, USA, reveals anatomical details previously lacking from the enigmatic fossil species that Newberry (1889, U.S. Geological Survey Monograph 16:1–228) named Holoptychius ? radiatus. The original material (scales and lepidotrichia) was attributed to Holoptychius (Sarcopterygii, Porolepiformes) according to the ridged ornament on the exposed portion of the scales. Scale and cranial anatomy revealed by the new material supports a reassignment of the species to Langlieria (Sarcopterygii, Tristichopteridae). The emended diagnosis of the new combination includes the grooved scale ornament that was noted, though described differently by Newberry, and a posterior oblique pitline groove of the parietal that extends to the caudal margin of the element. Along with Hyneria lindae and Eusthenodon sp. indet., there are now at least three known species of Tristichopteridae from the Catskill Formation. Temporal/spatial separation of these species, along with supporting anatomical characteristics, suggests that tristichopterids commonly filled the ecological niche of top predator in the Famennian-age nonmarine ecosystems of the Catskill Delta Complex. The presence of a Langlieria species in the Catskill Formation lends further support to a biogeographic connection between Pennsylvania, USA, and Belgium during the Famennian Stage of the Upper Devonian.

We analyze a new sample of very small Bothriolepis from the Catskill Formation (Upper Devonian, Famennian Stage) in Tioga County, Pennsylvania, USA. The material was collected in a series of rock slabs, each of which carries as many as thirty-one individuals in densely-packed groups with little to no overlap and with a preferred orientation. Although the taxonomy of Catskill Bothriolepis does not allow for a species diagnosis, morphometric comparison between the new sample and a growth series of Bothriolepis canadensis supports the juvenile status of the individuals in the new Catskill sample. By comparing the new material with other worldwide occurrences of small antiarchs, we recognize anatomical features that indicate juvenile status in antiarchs, including a large median ventral fenestra lacking a median ventral skeletal element, especially large medial serrations on the pectoral appendages, and a strongly convex rostral margin of the postpineal element. The geological and taphonomic context of the new material suggests death events of Bothriolepis hatchlings due to anoxia and desiccation in marginal aquatic environments. We highlight the implications for reproductive strategy in antiarchs.

Thomson (1968, Postilla 124:1–13) described and diagnosed the tristichopterid Hyneria lindae from parts of a disarticulated skull and isolated scales recovered from the Upper Devonian (Famennian) Catskill Formation of Pennsylvania, U.S.A. Since the publication of that description, knowledge of the tristichopterid clade has grown significantly, with numerous new descriptions of Famennian taxa from around the world. Additionally, a concerted effort to collect vertebrates from the Catskill Formation in Pennsylvania has produced significant new material of Hyneria lindae from the type locality at Red Hill, Clinton County, Pennsylvania. The new material serves as the basis for a rediagnosis and redescription of Hyneria lindae. The species is uniquely diagnosed by features including scales with a fringed free margin, a wide and blunt snout, and short intertemporals. The redescription of H. lindae gives occasion to review all of the Catskill Formation tristichopterid material that requires further diagnosis. This includes a near-complete cranial specimen collected near Red Hill that was originally diagnosed as Eusthenodon wängsjöi by Thomson (1976; Journal of Paleontology 50:1203–1208) and is here revised as Hyneria cf. lindae and figured for the first time. The emerging sample of tristichopterids from the Catskill Formation informs the diversity and paleobiogeography of these large predators in the fluvial ecosystems of the Red Hill site and within the Catskill Delta complex.

New material from the Upper Devonian (Famennian) Catskill Formation of Pennsylvania, U.S.A., indicates the presence of a new species of Megalichthys. A large collection of complete and well-preserved fossil material from the Red Hill field locality enables a thorough anatomical description that is not commonly available in Megalichthyidae. The cranial material includes skull roof, partial cheek, lower jaw, opercular-gular elements, palatoquadrate, and dermal palatal elements. The postcranial material includes pectoral fin elements including cleithrum, mesomeres, and lepidotrichia; pelvic fin including lepidotrichia; and body scales in articulation. The new species is distinguished from other species of Megalichthys by the presence of a pineal series of bones, a triangular-shaped principal gular, and a lack of sensory canal pores on the postparietal. We review the taxonomic history and status of Megalichthyidae and Megalichthys and provide revised diagnoses for each that include only discrete features demonstrated to be sufficient for taxonomic assignment. We discuss additional cosmine-covered osteolepiform fossils from other Catskill Formation exposures. These include the type and only specimen of Sterropterygion brandei and seven new specimens (including a partial snout, a partial parietal shield, a partial postparietal shield, a partial cheek, and complete lower jaws) that potentially represent megalichthyid species in addition to the one at the focus of this study. With this description of new megalichthyid material, the Catskill Formation continues to produce, in some cases, the only known Devonian occurrence of otherwise younger taxa.

A new species of Eusthenodon from the Upper Devonian (Famennian) Catskill Formation was collected from talus blocks at the base of a road cut along the Cogan House Exit Ramp on U.S. 15/Interstate 99 in Lycoming County, Pennsylvania, U.S.A. The new species is represented by a nearly complete, articulated skull (ANSP 23748) and additional cranial bones, body scales, and pectoral girdle elements. The breadth of anatomy represented by the new fossils, and the quality of their preservation and preparation, enable a detailed anatomical description of a new species. The original diagnosis of Eusthenodon distinguished the taxon from Tristichopterus and Eusthenopteron, the only other described tristichopterids at the time. The descriptive qualities of the Eusthenodon wangsjoi type material do not diagnose Eusthenodon from among the highly nested tristichopterid taxa that have been discovered since the original publication. In an effort to strengthen the taxonomic status and phylogenetic utility of Eusthenodon, we propose a revised diagnosis that uses only discrete, commonly preserved features that are unlikely to be altered by deformation. The new species is the third described Catskill Formation tristichopterid along with Hyneria lindae and Langlieria radiatus. All three are large-bodied and in a highly nested phylogenetic position within the group. The presence of a single tristichopterid species at each site highlights temporal and spatial regionalization of the Catskill fauna and may, along with a suite of anatomical features, support the hypothesis that tristichopterids occupied the role of top predator in the freshwater ecosystems of the Catskill Delta complex.

New material from the Upper Devonian (Frasnian) Fram Formation of Ellesmere Island, Nunavut, Canada, represents the second holoptychiid (Porolepiformes) species to be described from the formation. The quality of preservation and preparation of the material enables description of underrepresented anatomy for Holoptychius including the braincase (ethmosphenoid and otico-occipital). The new species of Holoptychius is diagnosed by, a quadrate articulation positioned on the lingual surface of the lower jaw, the presence of a marginal tooth field on all three coronoids, and the presence of a third mandibular sensory canal located dorsal to the infradentary foramina. Traditional use of the name Holoptychius has resulted in an overabundance of species. In recent years, efforts have been made to re-diagnose Holoptychius according to discrete cranial features rather than more widely distributed aspects of scale morphology. The new species is diagnosed as Holoptychius without any scales attributed to it With this new description, the recent Nunavut Paleontological Expeditions continue to expand the Fram Formation's diverse vertebrate fauna, one that additionally includes the elpistostegalian Tiktaalik roseae.

New material from the Upper Devonian (Famennian) Catskill Formation of Pennsylvania, U.S.A., indicates the presence of a new species of Megalichthys. A large collection of complete and well-preserved fossil material from the Red Hill field locality enables a thorough anatomical description that is not commonly available in Megalichthyidae. The cranial material includes skull roof, partial cheek, lower jaw, opercular-gular elements, palatoquadrate, and dermal palatal elements. The postcranial material includes pectoral fin elements including cleithrum, mesomeres, and lepidotrichia; pelvic fin including lepidotrichia; and body scales in articulation. The new species is distinguished from other species of Megalichthys by the presence of a pineal series of bones, a triangular-shaped principal gular, and a lack of sensory canal pores on the postparietal. We review the taxonomic history and status of Megalichthyidae and Megalichthys and provide revised diagnoses for each that include only discrete features demonstrated to be sufficient for taxonomic assignment. We discuss additional cosmine-covered osteolepiform fossils from other Catskill Formation exposures. These include the type and only specimen of Sterropterygion brandei and seven new specimens (including a partial snout, a partial parietal shield, a partial postparietal shield, a partial cheek, and complete lower jaws) that potentially represent megalichthyid species in addition to the one at the focus of this study. With this description of new megalichthyid material, the Catskill Formation continues to produce, in some cases, the only known Devonian occurrence of otherwise younger taxa.