Claw retraction and protraction in the carnivora: Skeletal microvariation in the phalanges of the Felidae

All carnivorans retract and protract their claws. In felids and some viverrids the claws of digits II through V of both the manus and pes have a larger arc of rotation than those of other carnivorans; the claws retract to the lateral side of the middle phalanx rather than onto its dorsal surface as in most other carnivorans. This condition should be termed hyper-retraction. Morphological features of the middle and distal (ungual) phalanges that have been purported to be necessary for hyper-retraction in felids vary considerably among digits within the manus and pes. These features include the lateral projection of the distal head and the asymmetry of the shaft of the middle phalanx, and the oblique orientation of the articular surface on the distal phalanx. None of these features is necessary in every instance for hyperretraction, and some of the variation in these features is associated instead with protraction. Differences among digits in the orientation of the articular surface on the distal phalanx are associated with differences in the degree to which the claws must move laterally to rotate from the protracted to the retracted position. Differences in the orientation of the distal head on the middle phalanx are associated with the spreading of the claws during protraction. The manual claws are hook-shaped, whereas the pedal claws are more blade-like; this morphological difference is associated with differences in function between the manus and pes. In the manus the medial claws have a larger radius of curvature and a smaller angle of arc as compared to the more lateral claws; in the pes, the claws on digits III and IV have larger radii of curvature and smaller angles of arc. Digit I of the manus lacks the hyper-retraction mechanism; nonetheless, this digit shares many of the attributes that are associated with this mechanism. © 1996 Wiley-Liss, Inc.     

Range of Movement in Ray I of Manus and Pes and the Prehensility of the Autopodia in the Early Permian to Late Cretaceous Non-Anomodont Synapsida

The mobility of ray I was analysed in seventy-eight Early Permian to Late Cretaceous specimens of non-mammalian Synapsida and one extant mammal. In all non-mammaliamorph Synapsida investigated, ray I formed a digital arcade. The first phalanx was maximally extendable to the zero position in the metapodiophalangeal joint I. Metapodiale I was the functional equivalent to a basal phalanx of digits II–V. In contrast, there was no digital arcade in ray I in Mesozoic Mammaliamorpha. Phalanx 1 I was dorsally extendable and metapodiale I was functionally part of the metapodium. During the propulsion phase, autopodial rotation occurred in the majority of Synapsida with abducted limb posture. Regarding ray I, the reduction of autopodial rotation can be estimated, e.g., from the decrease of lateral rotation and medial abduction of the first phalanx in the metapodiophalangeal joint I. Autopodial rotation was high in Titanophoneus and reduced in derived Cynodontia. In Mammaliamorpha the mobility of the first ray suggests autopodial rolling in an approximately anterior direction. Most non-mammaliamorph Therapsida and probably some Mesozoic Mammaliamorpha had prehensile autopodia with an opposable ray I. In forms with a pronounced relief of the respective joints, ray I could be opposed to 90° against ray III. A strong transverse arch in the row of distalia supported the opposition movement of ray I and resulted in a convergence of the claws of digits II–V just by flexing those digits. A tight articular coherence in the digital joints of digits II–V during strong flexion supported a firm grip capacity. Usually the grip capacity was more pronounced in the manus than in the pes. Prehensile autopodia of carnivorous Therapsida may have been utilized to hold prey while biting, thus helping to avoid fractures of the laterally compressed fangs.     

The Mississippian Tetrapod Footprint Ichnogenus Palaeosauropus: Extramorphological Variation and Ichnotaxonomy

Palaeosauropus primaevus is a tetrapod footprint ichnotaxon first described from the Upper Mississippian (Visean) Mauch Chunk Formation near Pottsville, Pennsylvania, United States. Our relocation of the type locality and stratigraphic horizon of P. primaevus, a long-available but unstudied collection of tetrapod footprints from these strata, and our new collections allow a much fuller characterization of this ichnotaxon and the range of extramorphological variation encompassed by it. P. primaevus is characterized as the footprints of a quadruped with a pentadactyl pes and a tetradactyl manus, in which the pes frequently oversteps the manus and with which tail drags are common. In the manus, all digits are relatively broad and have rounded tips, digit III is longest, and digit IV is more widely separated from digit III than the other digits are from each other. The pes has five digits that are also wide and blunt-tipped, digit IV is longest, and digit V projects nearly laterally. P. primaevus is the track of a relatively large temnospondyl (～400 mm gleno-acetabular length) and documents the Mississippian presence of such large amphibians long before their body fossil record. Palaeosauropus also occurs in Mississippian strata in Indiana and is distinguished from the geologically younger but similar temnospondyl footprint ichnogenus Limnopus by its relatively narrower manus and pes that lack broad and rounded sole impressions.     

Patterns of variation in the phalangeal formulae of land tortoises (Testudinidae): developmental constraint, size, and phylogenetic history

Documentation of variation in phalangeal formulae in land tortoises combined with ontogenetic information from turtles in general were used, in a phylogenetic context, to infer the potential effect of size and developmental constraints upon patterns of morphological variation. A sample of 201 specimens and published illustrations of 37 tortoise species were examined, representing all but one living genera and most species of the Testudinidae. Specimens were either articulated dry skeletons or preserved animals that were x-rayed. The patterns of digital and phalangeal loss in tortoises were predicted from developmental studies of the manus and pes in other turtles. If a digit is lost, it is the first digit, which is the last one to develop. If a digit has a single phalanx, it is usually the fifth digit. The primitive phalangeal formula for land tortoises is probably 2-2-2-2-1, the most common pattern found in living testudinid species. The presence of a second phalanx in the fifth digit evolved independently many times and usually in large tortoises. Such additions were interpreted as instances of peramorphosis. Many small tortoises have a full complement of digits (five) and phalanges (two in each digit); nevertheless, phalangeal and digital loss is associated with small size. Small and medium size tortoises exhibit greater variation in phalangeal number than do large tortoises. We hypothesize that epigenetic processes, and not simply adaptation, played a major role in the evolution of the variation in phalangeal formulae in tortoises.     

Rate heterogeneity, ancestral character state reconstruction, and the evolution of limb morphology in Lerista (Scincidae, Squamata)

Rates of phenotypic evolution derive from numerous interrelated processes acting at varying spatial and temporal scales and frequently differ substantially among lineages. Although current models employed in reconstructing ancestral character states permit independent rates for distinct types of transition (forward and reverse transitions and transitions between different states), these rates are typically assumed to be identical for all branches in a phylogeny. In this paper, I present a general model of character evolution enabling rate heterogeneity among branches. This model is employed in assessing the extent to which the assumption of uniform transition rates affects reconstructions of ancestral limb morphology in the scincid lizard clade Lerista and, accordingly, the potential for rate variability to mislead inferences of evolutionary patterns. Permitting rate variation among branches significantly improves model fit for both the manus and the pes. A constrained model in which the rate of digit acquisition is assumed to be effectively zero is strongly supported in each case; when compared with a model assuming unconstrained transition rates, this model provides a substantially better fit for the manus and a nearly identical fit for the pes. Ancestral states reconstructed assuming the constrained model imply patterns of limb evolution differing significantly from those implied by reconstructions for uniform-rate models, particularly for the pes; whereas ancestral states for the uniform-rate models consistently entail the reacquisition of pedal digits, those for the model incorporating among-lineage rate heterogeneity imply repeated, unreversed digit loss. These results indicate that the assumption of identical transition rates for all branches in a phylogeny may be inappropriate in modeling the evolution of phenotypic traits and emphasize the need for careful evaluation of phylogenetic tests of Dollo's law.     

Trackways of the American Crocodile (Crocodylus acutus) in Northwestern Costa Rica: Implications for Crocodylian Ichnology

We documented trackways of free-living Crocodylus acutus on beaches at the mouths of Tamarindo and Ventanas estuaries, Costa Rica. Our crocodiles had estimated total lengths of 1–3 meters or more. Manus prints have five digits, with digits I–III bearing claw marks. Pes prints have four digits, with claw marks on digits I–III. The pes is plantigrade. Claws generally dig into the substrate. Apart from claw marks, digit I and the heel of the pes are usually the most deeply impressed parts of footprints. Trackways are wide-gauge. Pes prints are usually positioned just behind ipsilateral manus prints of the same set and may overlap them. Manus and pes prints angle slightly outward with respect to the crocodile's direction of movement. Claw-bearing digits of both the manus and pes may create curved, concave-toward-the-midline drag marks as the autopodium is protracted. The tail mark varies in depth and clarity, and in shape from nearly linear to markedly sinuous. Sometimes the tail mark hugs the trackway midline, but sometimes it is closer to, or even cuts across, prints of one side. American crocodile footprints and trackways are similar to those observed in other extant crocodylian species, indicating substantial trackway conservatism across the group.     

The Developmental Origins of Mosaic Evolution in the Primate Limb Skeleton

The central hypothesis of this paper is that basic properties of vertebrate limb development bias the generation of phenotypic variation in certain directions, and that these biases establish focal units, or regions, of evolutionary change within the primate hand and foot. These focal units include (1) a preaxial domain (digit I, hallux or pollex, metapodial and proximal phalanx), (2) a postaxial domain (metapodials and phalanges of digits II?CV), and (3) a digit tip domain (terminal phalanges and nails/claws of rays I?CV). The existence of these focal units therefore provides a mechanistic basis for mosaic evolution within the hand and foot, and can be applied to make specific predictions about which features of the limb skeleton are most likely to be altered in primate adaptive radiations over time. Examination of the early primate fossil record provides support for this model, and suggests that the existence of variational tendencies in limb development has played a major role in guiding the origin and evolution of primate skeletal form.     

Autopodial skeleton evolution in side-necked turtles (Pleurodira)

Carpal and tarsal anatomy was documented based on the observation of dry skeletons of adult specimens representing 25 species in 15 genera and on data taken from the literature. In addition, histological sections and cleared and double‐stained autopodia of recently hatched and juvenile specimens representing seven chelid and pelomedusoid species were studied. There is much more morphological diversity in the manus than in the pes. Variation in autopodial skeletons includes: the astragalus and calcaneum are either separated or fused; fusion of distal carpals 3–4−5 or just 4–5; number of centralia in the carpus; and presence/absence of a pisiform and of an accessory radial element. The widespread and probably basal phalangeal formula for Pleurodira is 2.3.3.3.3. Deviations are Pelomedusa subrufa, exhibiting a reduction to 2.2.2.2.2, Pelusios spp. with one phalanx less in digit I and for one species in digit V as well, and Acanthochelys pallidipectoris with an additional phalanx in the fourth finger. Six discrete characters itemizing some of the morphological variation observed were plotted on a composite pleurodire phylogeny, revealing not only homoplastic patterns but also the utility of some characters in supporting the monophyly of several clades. The pisiform is the last carpal element to ossify in Chelus fimbriatus. We hypothesize that the so‐called fifth hooked metatarsal represents the fusion of distal tarsal 5 with metatarsal V. The accessory radial element that was occasionally present in the turtles examined may represent an atavism of the otherwise lost radiale of turtles.     

Limb and digit orientation during vertical clinging in Bibron's gecko,Chondrodactylus bibronii (A. Smith, 1846) and its bearing on the adhesive capabilities of geckos

Geckos with subdigital adhesive pads can scale smooth vertical surfaces in defiance of gravity. The deployment of the adhesive system is activated by the musculoskeletal system during active traverses of such surfaces, but adhesion on such substrata can also be achieved by passive means, with the body weight of the gecko applying tensile loading to the adhesive setae, maintaining prolonged, static contact with the surface. To investigate whether passively induced adhesion is employed by geckos holding station on smooth vertical surfaces, we investigated the magnitude of shear force generation for the manus and pes, and the positioning of the limb segments and digits in Chondrodactylus bibronii in freely selected resting postures (head‐up, head‐down and facing laterally to the left and right). Our results indicate that different subsets of digits occupy positions consistent with them being passively loaded in different body orientations. Limb segment and digit orientation are consistent within, and differ between, the resting postures, and relatively few of the 20 digits are positioned to take advantage of gravitationally induced loading in any posture. The pedal digits have greater adhesive potential than the manual ones and, more frequently, capitalize on passive loading than do manual digits. This is especially evident in the commonly adopted head‐down resting posture.     

Pterosaur tracks and the terrestrial ability of pterosaurs

Unusual tracks of a quadrupedal animal with a three-digit (occasionally four-digit) manus print and four-digit pes print were first interpreted as those of pterosaurs in the 1950s. In the 1980s these tracks were reinterpreted as crocodilian, but new material shows that the original identification was correct. Two features: evidence for elongate penultimate phalanges in digits two to four of the pes, and manus trackways up to three times the width of pes trackways, can only be attributed to pterosaurs. Recent improvements in understanding of pterosaur anatomy and functional morphology explain remaining difficulties regarding the interpretation of ich-nites such as the orientation of the manus digits and the absence of some expected ichnological features. Pteraichnus and Pteraichnus-like tracks show that, when grounded, some, perhaps all, pterosaurs were plantigrade, quadrupedal, and had a semi-erect stance and gait. This is consistent with some functional interpretations of pterosaur anatomy and resolves a long-running debate regarding the terrestrial ability of this group.     

Plausibility of inferred ancestral phenotypes and the evaluation of alternative models of limb evolution in scincid lizards

Phylogenetic approaches to inferring ancestral character states are becoming increasingly sophisticated; however, the potential remains for available methods to yield strongly supported but inaccurate ancestral state estimates. The consistency of ancestral states inferred for two or more characters affords a useful criterion for evaluating ancestral trait reconstructions. Ancestral state estimates for multiple characters that entail plausible phenotypes when considered together may reasonably be assumed to be reliable. However, the accuracy of inferred ancestral states for one or more characters may be questionable where combined reconstructions imply implausible phenotypes for a proportion of internal nodes. This criterion for assessing reconstructed ancestral states is applied here in evaluating inferences of ancestral limb morphology in the scincid lizard clade Lerista. Ancestral numbers of digits for the manus and pes inferred assuming the models that best fit the data entail ancestral digit configurations for many nodes that differ fundamentally from configurations observed among known species. However, when an alternative model is assumed for the pes, inferred ancestral digit configurations are invariably represented among observed phenotypes. This indicates that a suboptimal model for the pes (and not the model providing the best fit to the data) yields accurate ancestral state estimates.     

Quantitative genetic variation in static allometry in the threespine stickleback

The common pattern of replicated evolution of a consistent shape-environment relationship might reflect selection acting in similar ways within each environment, but divergently among environments. However, phenotypic evolution depends on the availability of additive genetic variation as well as on the direction of selection, implicating a bias in the distribution of genetic variance as a potential contributor to replicated evolution. Allometry, the relationship between shape and size, is a potential source of genetic bias that is poorly understood. The threespine stickleback, Gasterosteus aculeatus, provides an ideal system for exploring the contribution of genetic variance in body shape allometry to evolutionary patterns. The stickleback system comprises marine populations that exhibit limited phenotypic variation, and young freshwater populations which, following independent colonization events, have often evolved similar phenotypes in similar environments. In particular, stickleback diversification has involved changes in both total body size and relative size of body regions (i.e., shape). In a laboratory-reared cohort derived from an oceanic Alaskan population that is phenotypically and genetically representative of the ancestor of the diverse freshwater populations in this region, we determined the phenotypic static allometry, and estimated the additive genetic variation about these population-level allometric functions. We detected significant allometry, with larger fish having relatively smaller heads, a longer base to their second dorsal fin, and longer, shallower caudal peduncles. There was additive genetic variance in body size and in size-independent body shape (i.e., allometric elevation), but typically not in allometric slopes. These results suggest that the parallel evolution of body shape in threespine stickleback is not likely to have been a correlated response to selection on body size, or vice versa. Although allometry is common in fishes, this study highlights the need for additional data on genetic variation in allometric functions to determine how allometry evolves and how it influences phenotypic evolution.     

Body size and scaling of the hands and feet of prosimian primates

The hands and feet of primates fulfill a variety of biological roles linked with food acquisition and positional behavior. Current explanations of shape differences in cheiridial morphology among prosimians are closely tied to body size differences. Although numerous studies have examined the relationships between body mass and limb morphology in prosimians, no scaling analysis has specifically considered hand and foot dimensions and intrinsic proportions. In this study, we present such an analysis for a sample of 270 skeletal specimens distributed over eight prosimian families. The degree of association between size and shape was assessed using nonparametric correlational techniques, while the relationship between each ray element length and body mass (from published data and a body mass surrogate) was tested for allometric scaling. Since tarsiers and strepsirrhines encompass many taxa of varying degrees of phylogenetic relatedness, effective degrees of freedom were calculated, and comparisons between families were performed to partially address the problem of statistical nonindependence and "phylogenetic inertia." Correlational analyses indicate negative allometry between relative phalangeal length (as reflected by phalangeal indices) and body mass, except for the pollex and hallux. Thus, as size increases, there is a significant decrease in the relative length of the digits when considering all prosimian taxa sampled. Regression analyses show that while the digital portion of the rays scales isometrically with body mass, the palmar/plantar portion of the rays often scales with positive allometry. Some but not all of these broadly interspecific allometric patterns remain statistically significant when effective degrees of freedom are taken into account. As is often the case in interspecific scaling, comparisons within families show different scaling trends in the cheiridia than those seen across families (i.e., lorisids, indriids, and lemurids exhibit rather different allometries). The interspecific pattern of positive allometry that appears to best characterize the metapodials of prosimians, especially those of the foot, parallels differences found in the morphology of the volar skin. Indeed, relatively longer metapodials appear to covary with flatter and more coalesced volar pads, which in turn slightly improve frictional force for animals that are at a comparative disadvantage while climbing because of their larger mass. Despite the essentially isometric relationship found between digit length and body mass across prosimians, examination of the residual variation reveals that tarsiers and Daubentonia possess, relative to their body sizes, remarkably long fingers. Such marked departures between body size and finger length observed in these particular primates are closely linked with specialized modes of prey acquisition and manipulation involving the hands.     

Morphological correlates of the secondarily symmetrical pes of gekkotan lizards

Gekkotan lizards, belonging to the family Gekkonidae, especially those that bear subdigital adhesive pads, tend to exhibit a pes that is symmetrical in appearance, rather than the asymmetrical configuration of lizards in general. Such a symmetrical morphology is reminiscent of that of primitive tetrapods, but is secondarily derived within lizards, as shown by way of qualitative assessments and quantitative comparisons of length and angular measurements of the pedal skeleton. The results reveal that trends towards secondary symmetry of the pes, including spreading of the metatarsal rays, relative shortening of the metatarsals, especially the fourth, and of the digits are already present in primitive gekkotans. It thus appears that such trends are precursors to the bearing of subdigital adhesive pads and not modifications to this end. The configuration of subdigital adhesive pads in gekkonoids appears to have resulted from exapting a syndrome of morphological modifications already present in the basal gekkotan stock.     

On the manual morphology of Compsognathus longipes and its bearing on the diagnosis of Compsognathidae

Compsognathus longipes sits at an important point in theropod evolution at the base of Coelurosauria . Despite its relative completeness and oft-cited morphology, however, the manual morphology has been unclear. This work provides the first detailed study of the morphology of the manus of Compsognathus longipes . It shows that Compsognathus longipes had two fully formed functional digits as well as a reduced, perhaps even non-functional, third digit. That conclusion runs counter to the usual interpretation that Compsognathus longipes had only two phalanges, rather than the expected complement of three, in digit II. This work also identifies a unique suite of metacarpal I morphologies that are used to diagnose a subclade among species often referred to as 'Compsognathidae'. These features are used to construct an apomorphy-based definition of a new clade name: Compsognathidae.  © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 149 , 569–581.     

A resampling approach and implications for estimating the phalangeal index from unassociated hand bones in fossil primates

Primate fossil assemblages often have metacarpals and phalanges from which functional/behavioral interpretations may be inferred. For example, intrinsic hand proportions can indicate hand function and substrate use. But, estimates of intrinsic hand proportions from unassociated hand elements can be imperfect due to digit misattribution. Although isolated metacarpals can be identified to a specific digit, phalanges are difficult to assign to a specific ray. We used a resampling approach to evaluate how estimates of intrinsic hand proportions are affected by such uncertainty. First, the phalangeal index—intermediate phalanx length plus proximal phalanx length divided by metacarpal length—for the third digit was calculated for associated specimens of terrestrial, semiterrestrial, and arboreal taxa. We then used resampling procedures to generate distributions of “composite digits” based on resampled ratios in which phalanges from the second, fourth, and fifth rays, and from different individuals, were chosen randomly. Results confirm that the phalangeal index for associated third digits significantly discriminates groups. We also found that resampled ratios had significantly lower means, indicating that using composite digits is prone to systematic underestimation. Resampled ratios also generated distributions with greater variance around the means that obscured distinctions between groups, although significant differences between the most arboreal and terrestrial taxa are maintained. We conclude that using unassociated phalanges to calculate a phalangeal index is prone to sampling bias. Nevertheless, a resampling approach has the potential to inform estimates of hand proportions for fossil taxa, provided that the comparative sample is constrained to mimic the fossil composition. Am J Phys Anthropol 151:280–289, 2013. © 2013 Wiley Periodicals, Inc.     

Forearm Range of Motion in Australovenator wintonensis (Theropoda,Megaraptoridae)

The hypertrophied manual claws and modified manus of megaraptoran theropods represent an unusual morphological adaptation among carnivorous dinosaurs. The skeleton of Australovenator wintonensis from the Cenomanian of Australia is among the most complete of any megaraptorid. It presents the opportunity to examine the range of motion of its forearm and the function of its highly modified manus. This provides the basis for behavioural inferences, and comparison with other Gondwanan theropod groups. Digital models created from computed tomography scans of the holotype reveal a humerus range of motion that is much greater than Allosaurus, Acrocanthosaurus, Tyrannosaurus but similar to that of the dromaeosaurid Bambiraptor. During flexion, the radius was forced distally by the radial condyle of the humerus. This movement is here suggested as a mechanism that forced a medial movement of the wrist. The antebrachium possessed a range of motion that was close to dromaeosaurids; however, the unguals were capable of hyper-extension, in particular manual phalanx I-2, which is a primitive range of motion characteristic seen in allosaurids and Dilophosaurus. During flexion, digits I and II slightly converge and diverge when extended which is accentuated by hyperextension of the digits in particular the unguals. We envision that prey was dispatched by its hands and feet with manual phalanx I-2 playing a dominant role. The range of motion analysis neither confirms nor refutes current phylogenetic hypotheses with regards to the placement of Megaraptoridae; however, we note Australovenator possessed, not only a similar forearm range of motion to some maniraptorans and basal coelurosaurs, but also similarities with Tetanurans (Allosauroids and Dilophosaurus).     

Limb ossification patterns of the ichthyosaur Stenopterygius, and a discussion of the proximal tarsal row of ichthyosaurs and other neodiapsid reptiles

Limb ossification patterns for the Lower Jurassic (Toarcian) ichthyosaur, Stenopterygius , are described. It is found that limb ossification follows a continuous proximal to distal sequence from the propodial elements through to the terminal elements of 1st to 4th digit in the manus and the 1st to 3rd digit in the pes. The 5th manal and 4th pedal digit begin ossification later than more preaxial digits and also show evidence of proximal addition of elements near the distal mesopodial row in a manner consistent with delayed ossification of the 5th distal mesopodial in other diapsids. Ossification of manal elements in the Supernumerary 3–4 (S3-4) digit and the 5th digit appear interdependent; if one or the other is highly ossified, ossification of the other is retarded. The 1st pedal digit is considered to be lost in Stenopterygius and the 4th pedal digit is identified as the 5th digit. Delayed ossification of the mesopodium is not observed. The most preaxial proximal tarsal is identified as the centralc; the remaining proximal tarsals are the astragalus and calcaneum, and it is inferred that the astragalus and calcaneum ossified from within a single proximal cartilage.