The Acipenseridae comprises the largest group of non-teleostean actinopterygians and has served a central role in evolutionary studies of the Actinopterygii, and yet no comprehensive study of morphology and systematics of the family has ever been made. The family includes twenty-four species within four traditionally recognized genera, but the traditional generic designations lack rigorous definition within a cladistic scheme and are confounded by plesiomorphic characters. As background, revised diagnoses, synonymy lists, and overviews of life history are provided for every acipenserid species and genus. The systematics of the Acipenseridae are addressed with emphasis on generic-level relationships based on characters from examinations of skeletal morphology. A complete description of the skeleton of the shovelnose sturgeon Scaphirhynchus platorynchus provides a template of the skeleton of acipenserids. Skeletal comparisons with representatives of the remaining three acipenserid genera (Huso, Acipenser, and Pseudoscaphirhynchus), as well as Polyodontidae, $\dagger$Chondrosteidae, $\dagger$Peipiaosteidae, and other actinopterygians were used to identify, define, and test putative synapomorphies. Ninety-six synapomorphies are identified defining all major nodes within Acipenseriformes. A novel phylogeny of Acipenseridae is proposed, with Huso recognized as the sister group to a newly redefined subfamily Acipenserinae comprising all other acipenserids. The shovelnose sturgeons (genera Scaphirhynchus and Pseudoscaphirhynchus) compose a monophyletic Scaphirhynchini. No synapomorphies are recognized defining Acipenser, leaving the genus as a potentially untenable assemblage within Acipenseridae. The proposed phylogeny suggests that evolution within Acipenseridae followed a markedly different course than typically assumed, with progressive increase in morphological specializations for benthic life. Huso and polyodontids (as immediate outgroups) define an outgroup morphology and life history founded on mid-water habitats and piscivory. In contrast, most acipenserines, and more markedly in scaphirhynchines, are more benthic as exemplified by their ventral jaws and substrate-oriented diets. This phylogeny further suggests that peramorphosis, not paedomorphosis, played a central role in acipenserid evolution, exemplified by progressive acquisition of novel bones, increased scalation, and stronger dermal ornament. While putative paedomorphic characters are coincident with these peramorphic characters, addition of characters through peramorphosis belies the historical idea that paedomorphic or degenerate features dominate acipenserid evolution.

Armadillos are fossorial mammals found primarily in South America. Extant genera vary in size, behavior, and habitat. Armadillos use their forelimbs to break the soil when digging; hindlimb use varies. Some use hindlimbs purely for bracing; others actively excavate with their hindlimbs. Use of the hindlimbs in excavation is related to burrow creation. Three armadillo species were examined in the present study: Dasypus novemcinctus, Tolypeutes matacus, and Chaetophractus vellerosus. Dramatic differences were present in both the osteology and myology of the hindlimb. Identification of muscles was a non-trivial task. Among the difficulties was identification of divisions of the gluteus superficialis. This muscle traditionally is divided into the gluteus maximus and femorococcygeus. In armadillos the natural division appears to be tripartite. The hindlimb of Tolypeutes is dramatically different from that of Dasypus. Chaetophractus has a more intermediate morphology, but generally resembles Dasypus. Many anatomical differences appear related to burrowing. Tolypeutes is the least fossorial, but most cursorial of extant armadillos. Both Dasypus and Chaetophractus are accomplished burrowers, and share several features which appear to be specializations for using the hindlimb to excavate soil. The tibia and fibula are more robust in Dasypus than in Tolypeutes. A well developed flange, site of origin for the peronei, is present on the proximal fibula in Dasypus. The tibia-fibula of Chaetophractus is intermediate. The muscles of Dasypus, particularly the gluteals and hamstrings, are better developed than those of Chaetophractus, which in turn, has better developed muscles than Tolypeutes. Two muscles, the caudofemoralis and crurococcygeus, are present in Dasypus, but absent in the other two armadillos. The sartorius and tenuissimus are present in Chaetophractus and Dasypus, but absent in Tolypeutes. Intrinsic pedal musculature is better differentiated and developed in Dasypus and Chaetophractus. Fossoriality has yet to be directly measured in armadillos. Behavioral descriptions have suggested Chaetophractus is more fossorial than Dasypus. Hindlimb anatomy suggests Dasypus is the better burrower. Extending the hindlimb analysis to a controversial fossil armadillo, †Peltephilus, suggests it was not a burrower.

Despite a rich fossil record and much attention from paleontologists, the phylogeny of mammalian order Perissodactyla (including extant horses, rhinoceroses, and tapirs) is poorly understood. There are at present no well-supported phylogenetic hypotheses of the interrelationships of perissodactyls, nor has the position of this order among other eutherian orders been established. The main hindrance to this understanding is a lack of thorough and rigorous analysis of characters and taxa. This study attempts to overcome this problem by analyzing characters from all aspects of the skeleton in a broad diversity of taxa for one major perissodactyl lineage, the Tapiromorpha. Studies of interordinal and intraordinal relationships of the Perissodactyla have previously been effectively incompatible, because these two types of studies utilized different types of characters. Interordinal studies focused on cranial and postcranial characters, whereas intraordinal studies relied heavily on tooth morphology. This study focuses on cranial and postcranial osteology. The cranial and postcranial skeleton of a broad range of tapiromorphs is described. These descriptions have two purposes: (1) to identify potentially useful characters for use in a phylogenetic analysis of tapiromorphs; and (2) to provide a reference on tapiromorph osteology that will facilitate the inference of primitive tapiromorph character states for higher-level analyses. Putative tapiromorphs include "tapiroids," rhinocerotoids, and chalicotherioids. Numerous representatives of these groups are compared with other perissodactyls (equoids and brontotheres) and eutherians thought to be closely related to perissodactyls. Cranial and postcranial characters drawn from the afore-mentioned descriptions are combined with selected dental characters in a phylogenetic analysis of tapiromorph genera. For the sake of clarity, higher-level taxonomic groups are defined a priori. The results of the phylogenetic analysis demonstrate that there is no unequivocal evidence for including chalicotherioids or the traditional primitive "tapiroid" Homogala in the Tapiromorpha (as defined here). The analysis supports the close relationship between tapirs and rhinoceroses relative to horses, as well as clades that fulfill the definitions of Tapiroidea and Rhinocerotoidea. The results of the phylogenetic analysis provide the basis for a revised classification of the Tapiromorpha.

Canids have long been considered to be conservative in their postcranial anatomy, so there are few studies examining individual canid taxa for locomotor adaptations. Canids are generally considered to be the most cursorial of the carnivorans. The limbs of large canids are generally adapted for rapid terrestrial locomotion, as these animals frequently rely on speed for prey capture. The prey animal is captured and killed using the jaws and teeth. Smaller canids, such as the red fox Vulpes and gray fox Urocyon, do not use their limbs primarily for fast running. The red fox appears to have many adaptations for running, including long slender legs, but these foxes do not run in their daily activities except when chased. The red fox uses its forelimbs to help in prey capture and its hind legs for leaping. The gray fox is an unusual canid since it regularly climbs trees. The limbs of the gray fox, especially the forelimb, are utilized in climbing. This dissertation contains a detailed description of the postcranial osteology and myology Vulpes and Urocyon cinercoargenteus and includes an analysis of these anatomical features in a functional framework. An examination of both the osteology and myology of the fore and hind limbs of these two foxes reveals that their behavior is reflected in a number of anatomical characters. Adaptations for leaping in the red fox include the presence of unusually long hind legs relative to the front legs, and an increase in the length of the distal bony limb elements relative to more proximal ones. In addition, the limb bones are very slender. Muscle bellies of tarsal and digital flexors and extensors are restricted to a proximal position on the limb, and muscles in general are emphasized that act along the long axis of the limbs. Adaptations of the gray fox for climbing include the presence of relatively short legs, a greater ability to rotate the radius on the ulna relative to other canids, and a relatively greater ability to abduct the hind limb. In addition, both red and gray foxes are able to retract their claws, an ability that is not generally associated with canids.