Anthony P. Russell

Integrative functional morphology of the gekkotan adhesive system (reptilia: gekkota).

Abstract Climbing assisted by adhesive subdigital pads in gekkotan lizards has been the subject of intrigue and study for centuries. Many hypotheses have been advanced to explain the mechanism of adhesion, and recently this phenomenon has been investigated at the level of individual setae. The ability to isolate, manipulate and record adhesive forces from individual setae has provided new insights, not only into the mechanism of attachment, but also into the physical orientation of these structures necessary to establish attachment, maximize adhesive force, and effect subsequent release. This, in turn, has enabled a reassessment of the overall morphology and mode of operation of the adhesive system. Digital hyperextension has often been noted as a behavioral characteristic associated with the deployment of the gekkotan adhesive system—this is now understandable in the context of setal attachment and release kinematics, and in the context of the evolution of this pattern of digital movement from the primitive pattern of saurian digital kinematics. The perpendicular and parallel preloads associated with setal attachment are now reconcilable with other morphological aspects of the gekkotan adhesive system—the lateral digital tendon complex and the vascular sinus network, respectively. Future investigations of the integrated adhesive system will help to further elucidate the interdependence of its structural and functional components.

Repeated Origin and Loss of Adhesive Toepads in Geckos

Geckos are well known for their extraordinary clinging abilities and many species easily scale vertical or even inverted surfaces. This ability is enabled by a complex digital adhesive mechanism (adhesive toepads) that employs van der Waals based adhesion, augmented by frictional forces. Numerous morphological traits and behaviors have evolved to facilitate deployment of the adhesive mechanism, maximize adhesive force and enable release from the substrate. The complex digital morphologies that result allow geckos to interact with their environment in a novel fashion quite differently from most other lizards. Details of toepad morphology suggest multiple gains and losses of the adhesive mechanism, but lack of a comprehensive phylogeny has hindered efforts to determine how frequently adhesive toepads have been gained and lost. Here we present a multigene phylogeny of geckos, including 107 of 118 recognized genera, and determine that adhesive toepads have been gained and lost multiple times, and remarkably, with approximately equal frequency. The most likely hypothesis suggests that adhesive toepads evolved 11 times and were lost nine times. The overall external morphology of the toepad is strikingly similar in many lineages in which it is independently derived, but lineage-specific differences are evident, particularly regarding internal anatomy, with unique morphological patterns defining each independent derivation.

The Taphonomy of a Centrosaurus (Ornithischia: Certopsidae) Bone Bed from the Dinosaur Park Formation (Upper Campanian), Alberta, Canada, with Comments on Cranial Ontogeny

Abstract Bone bed 43 is one of at least eight paucispecific Centrosaurus bone beds located in the Dinosaur Park Formation (Upper Campanian) in Dinosaur Provincial Park, Alberta, Canada. It long has been used as a case example for evidence of herding and social behavior in dinosaurs, but a detailed analysis of the site has not been presented until this study. The bone bed is dominated by the disarticulated, mostly fragmentary and slightly abraded remains of Centrosaurus apertus, with minor occurrences of other taxa, notably teeth from the large tyrannosaurid Albertosaurus libratus. Fossils occur in a stacked to amalgamated succession of lag deposits, deposited and reworked at the erosional base of a paleochannel. The most parsomonious scenerio suggests that Centrosaurus material represents part of a large aggregation of animals (possibly numbering in the thousands) that died by drowning on the alluvial plain. Disarticulation occurred at a point upriver from the bone-bed site. Scavenging by theropods, primarily Albertosaurus, at or near the original site of death is suggested by the high number of shed theropod teeth. A subsequent event prior to fossilisation moved the material to its present location removing many juvenile-sized and hydrodynamically light elements from the original death assemblage. Evidence for distinct size classes amongst the preserved elements is not supported by the data, but the size range of elements preserved are representative of living individuals that would have ranged from small juveniles to mature adults. The large data base of specimens from bone bed 43 allows for the illustration of the ontogenetic changes that occurred in the diagnostic cranial elements of Centrosaurus.

Parallelism and Integrated Design in the Foot Structure of Gekkonine and Diplodactyline Geckos

The Gekkonidae is divided into four subfamilies (Eublepharinae, Diplodactylinae, Gekkoninae and Sphaerodactylinae) and various interrelationships of these taxa have been proposed. On a cladistic basis the Diplodactylinae may be regarded either as the sister group of the Gekkoninae or the sister group of the Gekkoninae and Sphaerodactylinae. With respect to diversity within the family the diplodactylines and gekkonines have occupied the greatest number of habitat types and their locomotor systems have developed independently but in similar ways. These two subfamilies have radiated largely in isolation from each other and in many instances parallelisms have occurred. With respect to locomotor mechanisms these parallelisms may be linked with the possession of a key innovation-a spinose Oberhautchen. Much of the apparent similarity seen is due to the way of life of the organisms concerned and within the family near duplicates have been produced in isolation. This is particularly evident in foot structure and reflects the method of contact of the organism with the substrate and exemplifies the interdependence of substrate type, locomotor pattern, morphology and behavior. The present paper attempts to delineate some functional correlates associated with different foot types. Subdigital, pilose pads have probably evolved on several occasions in the Gekkoninae but appear to have arisen only once in the Diplodactylinae. In both subfamilies secondary pad loss appears to be correlated with secondary terrestrialism but there has also been an adaptive radiation of primitively padless forms in both groups. Comparison of the structure of subdigital pads in both subfamilies shows how the various parts of the system are interrelated and interdependent and illustrates which parts of the system are fundamental to the successful operation of this type of adhesive mechanism and which parts are reflective of independent solutions of the same basic problems. The mechanical units of the foot are closely functionally integrated and with respect to the process of adhesion exhibit a continuity of pattern, through morphotypic series, reflecting continued functional efficiency in the transition between locomotor modes. The various attempts to solve similar mechanical problems have resulted in slightly different configurations of mechanical units. Similar selective pressures in different regions have presumably resulted in similar adaptive radiations in which many apomorphic character states occur in only slightly different configurations.

Terminonaris (Archosauria: Crocodyliformes): new material from Saskatchewan, Canada, and comments on its phylogenetic relationships

Abstract A newly-discovered skeleton of Terminonaris (=Teleorhinus) robusta (=robustus) from the early Turonian marine deposits of east-central Saskatchewan affords the opportunity for a detailed description of the anatomy of this species and its placement in the context of other known species of this genus. The specific diagnosis of T. robusta and the generic diagnosis of Terminonaris are established. Terminonaris robusta differs from the type species T. browni mainly in size-related features. Teleorhinus mesabiensis Erickson, 1969 is placed into the synonymy of Terminonaris robusta. The genus can be diagnosed by features of the anterior end of the snout, sutural relationships of the bones surrounding the orbit, and the structure of the occiput. A phylogenetic analysis based upon data drawn from the new material suggests that Terminonaris is not a pholidosaurid but is, instead, closely related to Dyrosaurus. This analysis continues to support the hypothesis that long-snouted crocodyliforms of the traditional “mesosuchian grade” form a clade.

Paleobiogeography of the Cretaceous Western Interior Seaway of North America: the vertebrate evidence

Abstract Distribution patterns of marine vertebrates of the Cretaceous Western Interior Seaway of North America are analysed in a biogeographical context. The analysis is based on fossil vertebrate assemblages from the following Lower Campanian localities: (1) Anderson River fauna, N.W.T. (2) Pembina fauna, southern Manitoba (3) Sharon Springs fauna, South Dakota-Wyoming (4) Niobrara fauna, Kansas (5) Mooreville fauna, Alabama. Due to the cosmopolitan nature of many marine vertebrates, relative abundance is used in preference to presence-absence data in identifying faunal distribution patterns. The analysis demonstrates the presence of two faunal subprovinces in the seaway. The Northern Interior Subprovince is characterized by a low diversity in all groups and is dominated by plesiosaurs, hesperornithiforms and the mosasaur Platecarpus. The Southern Interior Subprovince is characterized by a high diversity in all groups and is dominated by sharks, turtles and the mosasaur Clidastes. These biogeographical subdivisions support the hypothesis of warm temperature and cool temperate zones in the seaway that have been proposed on the basis of invertebrate faunas.

Biomechanics and kinematics of limb-based locomotion in lizards: review, synthesis and prospectus☆

The sprawling pattern of locomotion in lizards is kinematically intriguing and is underpinned by a distinctive pattern of appendicular morphology. The statics of the sprawling posture dictate fundamental design principles, and these place constraints on the three-dimensional kinematics of the limbs and body axis as locomotion is effected. The fore and hind limbs accommodate these constraints and dictates in fundamentally similar, but positionally different ways, resulting in different kinematic profiles for these two appendages. Recent kinematic investigations have helped to clarify earlier generalizations about lizard locomotion and have revealed that kinematic patterns are more variable than was previously supposed. Such analyses, and attendant detailed studies of the anatomy of the locomotor system, promise a new synthesis and enhanced understanding of evolutionary patterns of locomotion of lizards and adjustment to various locomotor substrata and modes of progression.

A new angle on clinging in geckos: incline, not substrate, triggers the deployment of the adhesive system

Lizards commonly climb in complex three-dimensional habitats, and gekkotans are particularly adept at doing this by using an intricate adhesive system involving setae on the ventral surface of their digits. However, it is not clear whether geckos always deploy their adhesive system, given that doing so may result in decreased (i.e. reduction in speed) locomotor performance. Here, we investigate circumstances under which the adhesive apparatus of clinging geckos becomes operative, and examine the potential trade-offs between speed and clinging. We quantify locomotor kinematics of a gecko with adhesive capabilities (Tarentola mauritanica) and one without (Eublepharis macularius). Whereas, somewhat unusually, E. macularius did not suffer a decrease in locomotor performance with an increase in incline, T. mauritanica exhibited a significant decrease in speed between the level and a 10° incline. We demonstrate that this results from the combined influence of slope and the deployment of the adhesive system. All individuals kept their digits hyperextended on the level, but three of the six individuals deployed their adhesive system on the 10° incline, and they exhibited the greatest decrease in velocity. The deployment of the adhesive system was dependent on incline, not surface texture (600 grit sandpaper and Plexiglas), despite slippage occurring on the level Plexiglas substrate. Our results highlight the type of sensory feedback (gravity) necessary for deployment of the adhesive system, and the trade-offs associated with adhesion.

Migration in amphibians and reptiles: An overview of patterns and orientation mechanisms in relation to life history strategies

Differences in integumentary permeability dictate alternative life history strategies in amphibians and reptiles. Limiting resources for amphibians are chiefly associated with availability of water and, as a consequence, amphibian migrations are chiefly associated with movements to and from aquatic breeding habitats. These cyclic migrations from breeding to overwintering sites may be direct, or may be interrupted by periods of residence at foraging sites. In general, migrations take place over relatively short distances and are constrained by the problem of water balance associated with exposure during longer overland journeys. Many amphibians exhibit complex mechanisms of orientation involving multiple sensory modalities and are capable of precise homing abilities.

An embryonic staging table for in ovo development of Eublepharis macularius, the leopard gecko.

Squamates constitute a major vertebrate radiation, representing almost one‐third of all known amniotes. Although speciose and morphologically diverse, they remain poorly represented in developmental studies. Here, we present an embryonic staging table of in ovo development for the basal gekkotan Eublepharis macularius (the leopard gecko) and advocate this species as a laboratory‐appropriate developmental model. E. macularius, is a hardy and tractable species of relatively large body size (with concomitantly relatively large eggs and embryos), that is widely available and easy to maintain and propagate. Additionally, E. macularius displays a body plan appropriate to the study of the plesiomorphic quadrupedal condition of early pentadactylous terrestrial amniotes. Although not unexpected, it is worth noting that the morphological events characterizing limb development in E. macularius are comparable with those described for the avian Gallus gallus. Therefore, E. macularius holds great promise as a model for developmental studies focusing on pentadactyly and the formation of digits. Furthermore, it is also attractive as a developmental model because it demonstrates temperature‐dependent sex determination. The staging table presented herein is based on an all‐female series and represents the entire 52 day in ovo period. Overall, embryogenesis of E. macularius is similar to that of other squamates in terms of developmental stage attained at the time of oviposition, patterns of limb and pharyngeal arch development, and features of the appearance of scalation and pigmentation, indicative of a conserved developmental program. Anat Rec, 292:1198–1212, 2009.