Edward B. Daeschler

A Devonian tetrapod-like fish and the evolution of the tetrapod body plan

The relationship of limbed vertebrates (tetrapods) to lobe-finned fish (sarcopterygians) is well established, but the origin of major tetrapod features has remained obscure for lack of fossils that document the sequence of evolutionary changes. Here we report the discovery of a well-preserved species of fossil sarcopterygian fish from the Late Devonian of Arctic Canada that represents an intermediate between fish with fins and tetrapods with limbs, and provides unique insights into how and in what order important tetrapod characters arose. Although the body scales, fin rays, lower jaw and palate are comparable to those in more primitive sarcopterygians, the new species also has a shortened skull roof, a modified ear region, a mobile neck, a functional wrist joint, and other features that presage tetrapod conditions. The morphological features and geological setting of this new animal are suggestive of life in shallow-water, marginal and subaerial habitats.

The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb.

Wrists, ankles and digits distinguish tetrapod limbs from fins, but direct evidence on the origin of these features has been unavailable. Here we describe the pectoral appendage of a member of the sister group of tetrapods, Tiktaalik roseae, which is morphologically and functionally transitional between a fin and a limb. The expanded array of distal endochondral bones and synovial joints in the fin of Tiktaalik is similar to the distal limb pattern of basal tetrapods. The fin of Tiktaalik was capable of a range of postures, including a limb-like substrate-supported stance in which the shoulder and elbow were flexed and the distal skeleton extended. The origin of limbs probably involved the elaboration and proliferation of features already present in the fins of fish such as Tiktaalik.

A Devonian Tetrapod from North America

An early tetrapod fossil from the Upper Devonian of Pennsylvania (Catskill Formation) extends the temporal range of tetrapods in North America and suggests that they attained a virtually global equatorial distribution by the end of the Devonian. Derived features of the shoulder girdle indicate that appendicular mechanisms of support and propulsion were well developed even in the earliest phases of tetrapod history. The specialized morphology of the pectoral skeleton implies that the diversity of early tetrapods was great and is suggestive of innovative locomotor patterns in the first tetrapods.

EARLY TETRAPOD JAWS FROM THE LATE DEVONIAN OF PENNSYLVANIA, USA

Abstract Recent paleontological fieldwork in the Upper Devonian Catskill Formation at Red Hill in Clinton County, Pennsylvania, USA, has produced a diverse assemblage of vertebrate fossils including early tetrapods. The tetrapod Hynerpeton bassetti was described from the site in 1994 and a recently recognized partial lower jaw of that taxon is described here. Additionally, this paper describes a new Late Devonian tetrapod, Densignathus rowei new genus and species, based on a well-preserved lower jaw. This new taxon is characterized by dramatic widening of the jaw anterior of the adductor fossa, a pronounced twist in the orientation of ventral margin of the jaw, an uninterrupted exposure of Meckelian bone on the mesial surface, and weakly-developed radiating ornament on the lateral surface of the infradentaries. Although phylogenetic resolution within stem tetrapods is lacking, Densignathus rowei, n. gen. and sp., informs several topics including the sequence of character acquisition in the lower jaw, morphological diversity, and paleoecology of the earliest tetrapods.

A NEW SPECIMEN OF SAURIPTERUS TAYLORI (SARCOPTERYGII, OSTEICHTHYES) FROM THE FAMENNIAN CATSKILL FORMATION OF NORTH AMERICA

Abstract A new specimen of the rhizodontid sarcopterygian, Sauripterus taylori (Sarcopterygii, Osteichthyes), from the Late Devonian (Famennian) Catskill Formation of Pennsylvania consists of a well-preserved right pectoral fin, girdle, and associated scales. Rhizodontid affinity is supported by the unique pattern of overlap between the clavicle and cleithrum, the robustness of the pectoral girdle, the presence of elongate and unjointed lepidotrichia in the pectoral fin, and the pattern of connectivity between the endochondral bones of the pectoral fin. The new specimen of Sauripterus provides a three-dimensional and articulated view of basal rhizodontid pectoral girdle and fin anatomy and preserves the first known scapulocoracoid for a Devonian rhizodontid. The glenoid fossa faces posteroventrally, in contrast to the more posterolaterally facing glenoids of other tetrapodomorph fish. The humerus is short proximodistally and possesses an expansive ventral flange for the attachment of the adductor, flexor, and rotatory musculature of the fin. Together, these data suggest that the pectoral fins of Sauripterus were optimized for adduction and flexion against the water column or a solid substrate. To maximize the propulsive force of these motions the distal fin was stiffened by the presence of eight radials, similar in position to the autopodials of tetrapods. However, unlike the digits of tetrapods, the unit that interacts with the environment in rhizodontids is a combined structure consisting of both dermal and endochondral fin skeletons. As recent phylogenetic hypotheses do not support a sister-group relationship between rhizodontids and tetrapods, the similarities between the endoskeletal radials of Sauripterus and the autopodium of tetrapods are independently derived.

Fish with fingers

Fingers and toes were long thought to be novelties associated with the invasion of land by tetrapods. The recent identification of a variety of aquatic specializations in some early tetrapods has provoked a debate on whether digits arose in primarily terrestrial or aquatic animals,. We recently discovered a pectoral fin of a lobe-finned fish (Fig. 1a, b) that is remarkably similar to tetrapod limbs. This discovery reveals that major tetrapod novelties are also seen in the paddles of some closely related fish and therefore need not have arisen to meet the demands of a terrestrial existence.

The late Devonian lungfish Soederberghia (Sarcopterygii, Dipnoi) from Australia and North America, and its biogeographical implications

Abstract A new species of rhynchodipterid lungfish, Soederberghia simpsoni, sp. nov., is described on the basis of a complete skull roof, and an incomplete but articulated head plus body, from the Upper Devonian Mandagery Sandstone near Canowindra, New South Wales (NSW, Fig. 1A,), Australia. It is compared with Soederberghia material from the upper part of the Catskill Formation, Pennsylvania (Fig. 1B), and the Cloghnan Shale at Jemalong near Forbes, NSW, Australia. The Catskill and Cloghnan Shale material, consisting of two incomplete skull roofs, appears to be conspecific with the type species, Soederberghia groenlandica from the Famennian Remigolepis Series of Greenland. This is congruent with a recent suggestion that the Mandagery Sandstone is late Frasnian in age, whereas the upper Catskill Formation and Cloghnan Shale are Famennian. At the Famennian localities, Soederberghia is associated with tetrapods. Articulated postcranial material of Soederberghia, including the pectoral girdle, cranial and pleural ribs, unpaired fins and fin supports, is described for the first time. The presence of cranial ribs and a parasphenoid with a posterior stalk suggest that Soederberghia gulped air and probably inhabited a non-marine or shallow near-shore marine environment. The occurrence of Soederberghia groenlandica in the Famennian Old Red Sandstone of North America, Greenland and Australia thus furnishes evidence for contact or close proximity between Gondwana and Laurussia at this time, in conflict with some recent paleomagnetic data. The sister group of Soederberghia is Rhynchodipterus elginensis from the Famennian of Rosebrae near Elgin, Scotland; together they form the Family Rhynchodipteridae. Griphognathus, previously included in the Rhynchodipteridae, lacks a stalked parasphenoid and cranial ribs, and has a quite different dermal bone pattern from Soederberghia and Rhynchodipterus. We remove it from the group on these grounds, and interpret it as less crownward than the Rhynchodipteridae.

The cranial endoskeleton of Tiktaalik roseae

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.

CRITICAL ISSUES OF SCALE IN PALEOECOLOGY

In mid-September 2007, 32 paleontologists gathered at the Smithsonian Institution to spend four days discussing research frontiers in paleoecology, particularly at the interface with neoecology. They represented expertise throughout the Phanerozoic and in all major groups of fossilizable organisms. This meeting was timely, given the increasing evidence of the impact of climate change on ecosystems in our modern world. The vast repository of paleoecological data on past environmental change and concomitant ecological responses, observed at many different spatial, temporal, and taxonomic scales, is of potentially great value for understanding and predicting how modern ecosystems will respond to climate change. Of particular interest to the participants of this meeting were questions of how ecological data collected at different scales could be reconciled so that our knowledge of ecological change in the past can better inform our understanding of the present and our predictions of how ecosystems will change in the future. Certainly, this is one of the most exciting research frontiers in paleoecology. Data collected for different ecological studies (both paleoecological and neoecological) encompass a wide range of spatial, temporal, and taxonomic scales. Understanding the scales inherent in an ecological research question is critical to designing a sampling protocol that will yield data capable of resolving that question, yet these scales are often not adequately evaluated or presented in published paleoecological reports. Furthermore, for any body of paleoecological research to be rescued from isolation and integrated with other studies, the various scales encompassed by the research questions and data must be understood and reported. The greatest barrier to communicating and collaborating with neoecologists is not that data collected from extant ecosystems are necessarily different or more complete than paleoecological data but, rather, that these two data sets commonly represent or are collected at different scales. If such differences of scale can …

NEW FLAT-BACKED ARCHIPOLYPODAN MILLIPEDES FROM THE UPPER DEVONIAN OF NORTH AMERICA

Abstract Two new flat-backed archipolypodan millipede taxa, Orsadesmus rubecollus n. gen. and sp. and Zanclodesmus willetti n. gen. and sp., are described from the Upper Devonian of Clinton County, Pennsylvania, and Miguasha, Québec, respectively. These new taxa are placed in the new family Zanclodesmidae within the order Archidesmida. Archidesmida have previously only been described from the United Kingdom, and their presence in North America confirms a geographic continuum in the millipede fauna across the Old Red Sandstone Continent during the Devonian.