Imbricated shell sculpture in benthic bivalves.

Molluscan shells display a high diversity of external sculpture. Sculptural elements may be symmetrical, where both edges of an element are morphologically similar, or asymmetrical, where one edge is steeper than the other. Asymmetrical sculpture can be ratcheted, with the leading edges (those in the direction of locomotion or growth) less steep than the trailing edges, or imbricated (leading edges steeper than trailing edges). While the ratcheted sculpture is better known, the diversity of imbricated sculpture has remained largely unexplored. In a survey of extant benthic shell-bearing molluscs, we document imbricated sculpture primarily in epifaunal bivalves or on the exposed sectors of shells of semi-infaunal bivalves. Imbricated sculpture is particularly widespread in pteriomorphian bivalves, but it is absent in the subclade Mytiloidea as well as in highly mobile Pectinidae. It also occurs in many carditid bivalves (Archiheterodonta) and in phylogenetically scattered euheterodonts. In several infaunal bivalves including species of Cardites (Carditidae), Hecuba (Donacidae), and Chione (Veneridae), comarginal elements on the posterior sector are imbricated whereas anterior comarginal ridges are ratcheted. Imbricated sculpture in bivalves tends to be concentrated on the upper (left) valves of pectinids or on the posterior sector of both valves in archiheterodonts and euheterodonts. Imbricated sculpture is uncommon in gastropods, even in epifaunal species, but does occur in the collabral ridges in some Vasidae and a few other groups. Expression of imbricated sculpture does not depend on shell mineral composition or microstructure. The ecological distribution and within-shell pattern of expression of imbricated sculpture point to the likelihood that this type of asymmetrical sculpture is both widespread and potentially functional. Additionally, we present a potential methodology whereby shell sculpture categories (symmetrical, ratcheted, and imbricated) may be quantified by comparing the lengths of corresponding leading and trailing edges across the shell surface.

Morphological relationships between vertebrate claw unguals and sheaths and the functional morphology of these structures.

The link between claw morphology and function has been historically difficult to quantify, analyze, and interpret. A confounding factor is the ambiguous morphological relationship between the ungual and the sheath and whether one structure or the other is more useful for inferring function from morphology. In this study, the functional morphology of vertebrate claws is analyzed using sheath and ungual measurements taken from modern claw specimens spanning birds and mammals. Claw measurements were chosen for their potential biomechanical significance and a revised, expanded categorization of claw function is used. When corresponding claw measurements from the ungual and sheath are compared independently, some features are highly correlated whereas others are not. A principal component analysis of the claw measurements reveals that some of the morphological disparity is related to functional differences; however, different functional categories are not clearly separated based solely on morphology. A linear discriminant analysis incorporating a supervised dimensionality reduction method (J-function) successfully classifies 94.52% of the claw specimens to their documented functional categories. When the posterior probabilities of each classification are examined, and the next highest probabilities are considered, the analysis can successfully classify 98.63% of the claw specimens. Sheath measurements perform better than ungual measurements but combining measurements from both structures perform better than considering either structure individually. Both structures contribute valuable morphological information when it comes to inferring claw function from morphology.

Functional morphology of vertebrate claws investigated using functionally based categories and multiple morphological metrics.

The link between claw morphology and function has been historically difficult to quantify, analyze, and interpret. In this study, the functional morphology of vertebrate claws is analyzed using measurements taken from 80 modern claw specimens spanning birds, mammals, and one reptile. Claw measurements were chosen for their potential biomechanical significance and a revised, expanded categorization of claw function is defined and used. This categorization scheme is the result of an extensive literature review and is based on the observed mechanics of claw function rather than the animal's overall ecology, an important departure from the norm followed in previous studies. A principal component analysis of the claw measurements reveals that some of the morphological disparity is related to functional differences; however, different functional categories are not clearly separated based solely on morphology. A linear discriminant analysis successfully classifies 81.25% of the claw specimens to their documented functional categories. When the posterior probabilities of each classification are examined, and the next highest probabilities are considered, the analysis is able to successfully classify 96.25% of the claw specimens. Expressing angle measurements in terms of lengths prior to analysis and incorporating cross-sectional shape data both serve to reduce the misclassification rate. The use of biomechanically meaningful claw measurements and categories based on function (rather than ecology) improves confidence in the ability to infer claw function based on morphology using discriminant analysis methods. While overall claw morphology is most certainly the result of multiple factors (e.g., growth, size, etc.), this study establishes that it reflects mechanical function more than previously demonstrated.

Three-Legged Locomotion and the Constraints on Limb Number: Why Tripeds Don't Have a Leg to Stand On.

Three-legged animals do not exist today and such an animal is not found in the fossil record. Which constraints operate to result in the lack of a triped phenotype? Consideration of animal locomotion and robotic studies suggests that physical constraints would not prevent a triped from being functional or advantageous. As is reviewed here, the strongest constraint on the evolution of a triped is phylogenetic: namely, the early genetic adoption of a bilaterally symmetrical body plan occurring before the advent of limbs. Presumably, this would greatly constrain any three-legged animal from ever evolving. Tripedalism is employed only by a few animals, but many use a tripod stance while engaged in a variety of activities. Because terms are often used interchangeably in the literature, a standardization of locomotion terminology is proposed. Understanding the constraints behind "forbidden" phenotypes forces us to confront gaps in our evolutionary understanding of which we may be unaware.