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 evolution of tetrapods from sarcopterygian fish is one of the major transformations in the history of life and involved numerous structural and functional innovations, including new modes of locomotion, respiration and hearing. Fish and tetrapod fossils across this transition can reveal how these innovations were assembled. During the origin of tetrapods in the Late Devonian (385-359 million years ago), the proportions of the skull were remodelled, the series of bones connecting the head and shoulder was lost, and the region that was to become the middle ear was modified. At the same time, robust limbs with digits evolved, the shoulder girdle and pelvis were altered, the ribs expanded, and bony connections between vertebrae developed. Few of these features, however, are seen in the closest relatives of tetrapods-the elpistostegalian fishes-which are incompletely known. Elpistostege, for example, is represented only by two partial dermal skull roofs and a segment of the axial skeleton from the early Frasnian Escuminac Formation in Quebec 1-3 . The best-known elpistostegalian, Panderichthys, consists of complete specimens of Middle to Late Devonian age (late Givetian and early Frasnian stages) mostly from the Lode quarry in Latvia 4-10 . Panderichthys possesses relatively few tetrapod synapomorphies, and provides only partial insight into the origin of major features of the skull, limbs and axial skeleton of early tetrapods. In view of the morphological gap between elpistostegalian fish and tetrapods, the phylogenetic framework for the immediate sister group of tetrapods has been incomplete and our understanding of major anatomical transformations at the fish-tetrapod transition has remained limited.The discovery of a new elpistostegalian sarcopterygian from the Fram Formation in Nunavut Territory, Canada (Fig. 1) significantly enhances our knowledge of the fish-tetrapod transition. Many articulated specimens from a single site are used to describe a taxon that is a remarkable intermediate between Panderichthys and early tetrapods. The material provides oppor...

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.

The excursions of the scapulocoracoid and forelimb and the activity of 18 shoulder muscles were studied by simultaneous cineradiography and electromyography in Savannah Monitor lizards (Varanus exanthematicus) walking on a treadmill at speeds of 0.7-1.1 km/hour. During the propulsive phase, the humerus moves anteroposteriorly 40-55° and rotates a total of 30-40°. Simultaneously, the coracoid translates posteriorly along the tongue-and-groove coracosternal joint by a distance equivalent to about 40% the length of the coracoid. Biceps brachii, coraco-brachialis brevis and longus, the middle and posterior parts of the latissimus dorsi and pectoralis, serratus anterior, serratus anterior superficialis, subscapularis, supracoracoideus, and triceps usually become active during the late swing phase and continue activity throughout most or all of propulsion. The anterior part of the latissimus dorsi is active during the transition from propulsive to swing phases. Brachialis, deltoideus scapularis, levator scapulae, the anterior part of pectoralis, scapulo-humeralis posterior, and subcoracoideus are active primarily during the swing phase; they are occasionally active during propulsion. Deltoideus clavicularis, scapulo-humeralis posterior, sternocoracoideus, and the posterior part of the trapezius are biphasic, with activity in both the propulsive and swing phases. A number of shoulder muscles in Varanus exanthematicus and Didelphis virginiana (the Virginia opossum) are similar in attachments, in activity patterns with respect to phases of the step cycle, and in apparent actions. These similarities are interpreted as a pattern inherited from the ancestors of higher tetrapods. The sliding coracosternal joint permits an increase in step length without demanding greater excursion at the shoulder and elbow joints.

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Three-dimensional skeletal movement is often impossible to accurately quantify from external markers. X-ray imaging more directly visualizes moving bones, but extracting 3-D kinematic data is notoriously difficult from a single perspective. Stereophotogrammetry is extremely powerful if bi-planar fluoroscopy is available, yet implantation of three radio-opaque markers in each segment of interest may be impractical. Herein we introduce scientific rotoscoping (SR), a new method of motion analysis that uses articulated bone models to simultaneously animate and quantify moving skeletons without markers. The three-step process is described using examples from our work on pigeon flight and alligator walking. First, the experimental scene is reconstructed in 3-D using commercial animation software so that frames of undistorted fluoroscopic and standard video can be viewed in their correct spatial context through calibrated virtual cameras. Second, polygonal models of relevant bones are created from CT or laser scans and rearticulated into a hierarchical marionette controlled by virtual joints. Third, the marionette is registered to video images by adjusting each of its degrees of freedom over a sequence of frames. SR outputs high-resolution 3-D kinematic data for multiple, unmarked bones and anatomically accurate animations that can be rendered from any perspective. Rather than generating moving stick figures abstracted from the coordinates of independent surface points, SR is a morphology-based method of motion analysis deeply rooted in osteological and arthrological data.

The discovery of a turtle in the Early Jurassic(185 million years before present) Kayenta Formation of northeastern Arizona provides significant evidence about the origin of modern turtles. This new taxon possesses many of the primitive features expected in the hypothetical common ancestor of pleurodires and cryptodires, the two groups of modern turtles. It is identified as the oldest known cryptodire because of the presence of a distinctive cryptodiran jaw mechanism consisting of a trochlea over the otic chamber that redirects the line of action of the adductor muscle. Aquatic habits appear to have developed very early in turtle evolution. Kayentachelys extends the known record of cryptodires back at least 45 million years and documents a very early stage in the evolution of modern turtles.

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Major stages in the structural and functional evolution of the mammalian humero-ulnar joint are described on the basis of paleontological and cineradiographic evidence. In pelycosaurs (the earliest known fossil reptiles with mammalian affinities), the humerus projected laterad and more or less horizontally; locomotor movements were principally rotation about its proximodistal axis. Because the forearm moved in a plane perpendicular to this axis, the flexed elbow was subjected to substantial torque. The humero-ulnar joint consisted of two pairs of facets that engaged upon humeral rotation and was principally a stabilizing rather than a flexion-extension mechanism.Cynodonts (advanced mammal-like reptiles ancestral to mammals) possessed an ulnar condyle rather than a trochlea. A condylar humero-ulnar articulation, usually with a spiral configuration, was retained by early mammals and persists in slightly modified form among modern prototherians. The spiral joint allows the ulna to extend in a sagittal plane as the humerus rotates, adducts, and elevates.The primitive therian trochlea evolved by enlargement of the intercondylar groove separating the ulnar and radial condyles and by retention of part of the ulnar condyle mechanism. Cineradiography demonstrates the relationship of diverse types of mammalian humero-ulnar joints to limb posture and excursion characteristics.