LITERATURE CITED 72 PENNSYLVANIAN DIAPSID REPTILE ly immature. KUVP 995S, left clavicle, slightly immature. KUVP 9959, seven separate blocks containing badly preserved subadult skull, interclavicle, immature, lower jaws, scattered \ertebrae and ribs, forelimbs, hindlimbs, both ilia, very immature. KUVP 9961, left pelvis, mature. KUVP 9962, ilium with first sacral rib attached, immature. KU\T 10320, badly preser\ed specimen containing skull fragments, vertebrae, hindlimbs, immature. KUVP 33602, isolated left maxilla, slightly immature. KUVP 33603, fragments of skull material, distal end of humerus, immature. KUVP 33604, scattered skull material, atlasaxis complex, cleithrum, distal end of humerus, radius, ulna, mature. KUVP 33605, seven posterior dorsal vertebrae and first sacral vertebra, first sacral rib, all articulated, slightly immature. KUVP 33606, partial skull, posterior dorsal, sacral and anterior caudal vertebrae, some associated ribs, clavicle, partial scapulacoracoid, humerus, partial pelvis, right hindlimb with scattered pes, mature. KUVP 33607, slightly dissociated skull, first three cervical vertebrae, cleithrum, right scapulacoracoid, olecranon, partial carpus, mature. KUVP 33608, poorly preserved skull and cervical vertebrae. KUVP 33609, right scapulacoracoid, mature. OSTEOLOGY Skull By using the available material, it has been possible to obtain a composite, fairly accurate reconstruction of the skull, in which most of the external details of the dermal roof, occiput, palate, the lateral and medial aspects of the mandible can be discerned. The quality of preservation prevents any reconstructions of the braincase beyond those seen in Figs. 3b and 4.

The origin of extant amphibians (Lissamphibia: frogs, salamanders and caecilians) is one of the most controversial questions in vertebrate evolution, owing to large morphological and temporal gaps in the fossil record. Current discussions focus on three competing hypotheses: a monophyletic origin within either Temnospondyli or Lepospondyli, or a polyphyletic origin with frogs and salamanders arising among temnospondyls and caecilians among the lepospondyls. Recent molecular analyses are also controversial, with estimations for the batrachian (frog-salamander) divergence significantly older than the palaeontological evidence supports. Here we report the discovery of an amphibamid temnospondyl from the Early Permian of Texas that bridges the gap between other Palaeozoic amphibians and the earliest known salientians and caudatans from the Mesozoic. The presence of a mosaic of salientian and caudatan characters in this small fossil makes it a key taxon close to the batrachian (frog and salamander) divergence. Phylogenetic analysis suggests that the batrachian divergence occurred in the Middle Permian, rather than the late Carboniferous as recently estimated using molecular clocks, but the divergence with caecilians corresponds to the deep split between temnospondyls and lepospondyls, which is congruent with the molecular estimates.

▪ Abstract  A critical reexamination of turtle relationships continues to support a sister-group relationship of turtles with a clade of marine reptiles, Sauropterygia, within crown-group Diapsida (Sauria). The high Homoplasy Index raises concerns about the phylogenetic information content of various morphological characters in broad-scale phylogenetic analyses. Such analyses may also suffer from inadequate statements of primary homology. Several such statements that have played an important role in the analysis of turtle relationships (dermal armor, acromion, astragalo-calcaneal complex, hooked fifth metatarsal) are reviewed in detail. An evolutionary scenario for the origin of the turtle bauplan suggests an aquatic origin of turtles, which is supported not only by their sauropterygian relationships, but also by paleobiogeographic and stratigraphic considerations. However, turtle relationships remain labile, and further investigations of their relationships are required, involving molecular and physiological data.

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Tooth implantation provides important phylogenetic and functional information about the dentitions of amniotes. Traditionally, only mammals and crocodilians have been considered truly thecodont, because their tooth roots are coated in layers of cementum for anchorage of the periodontal ligament, which is in turn attached to the bone lining the alveolus, the alveolar bone. The histological properties and developmental origins of these three periodontal tissues have been studied extensively in mammals and crocodilians, but the identities of the periodontal tissues in other amniotes remain poorly studied. Early work on dental histology of basal amniotes concluded that most possess a simplified tooth attachment in which the tooth root is ankylosed to a pedestal composed of “bone of attachment”, which is in turn fused to the jaw. More recent studies have concluded that stereotypically thecodont tissues are also present in non-mammalian, non-crocodilian amniotes, but these studies were limited to crown groups or secondarily aquatic reptiles. As the sister group to Amniota, and the first tetrapods to exhibit dental occlusion, diadectids are the ideal candidates for studies of dental evolution among terrestrial vertebrates because they can be used to test hypotheses of development and homology in deep time. Our study of Permo-Carboniferous diadectid tetrapod teeth and dental tissues reveal the presence of two types of cementum, periodontal ligament, and alveolar bone, and therefore the earliest record of true thecodonty in a tetrapod. These discoveries in a stem amniote allow us to hypothesize that the ability to produce the tissues that characterize thecodonty in mammals and crocodilians is very ancient and plesiomorphic for Amniota. Consequently, all other forms of tooth implantation in crown amniotes are derived arrangements of one or more of these periodontal tissues and not simply ankylosis of teeth to the jaw by plesiomorphically retaining “bone of attachment”, as previously suggested.