Crocodilians are the last surviving reptilian representatives of the subclass Archosauria. The analysis of proteins, lipids, and nucleic acids and the most recent morphological studies agree in aligning crocodilians with birds and dinosaurs. Therefore, crocodilians and birds represent the only surviving archosaurian clades among the modern vertebrates. Crocodylia are traditionally placed into three subcategories: Protosuchia, Mesosuchia, and Eusuchia. Three morphological characteristics are used to distinguish these suborders: the development of the secondary palate, the morphology of the vertebral centra, and the exclusion of the pubis from the acetabulum. A major problem in resolving the systematics and evolution of the eusuchian crocodilians is their tendency toward general morphological conservatism and convergence/parallelism in cranial morphology. The morphological conservatism is explicit in the postcranial region, where few reliable characters can be used for phylogenetic studies. Therefore, most traditional assessments of crocodilian phylogeny are based on analysis of the numerous differences in head morphology and skull structure among different species.

CHAPTER 10 – Feeding in Crocodilians

The Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton & Charig, 1962: cranial anatomy, functional morphology, taxonomy, and relationships

Studies of the zebrafish (Danio rerio) promise to contribute much to an understanding of the developmental genetic mechanisms underlying diversification of the vertebrate dentition. Tooth development, structure, and replacement in the zebrafish largely reflect the primitive condition of jawed vertebrates, providing a basis for comparison with features of the more extensively studied mammalian dentition. A distinctive derived feature of the zebrafish dentition is restriction of teeth to a single pair of pharyngeal bones. Such reduction of the dentition, characteristic of the order Cypriniformes, has never been reversed, despite subsequent and extensive diversification of the group in numbers of species and variety of feeding modes. Studies of the developmental genetic mechanism of dentition reduction in the zebrafish suggest a potential explanation for irreversibility in that tooth loss seems to be associated with loss of developmental activators rather than gain of repressors. The zebrafish and other members of the family Cyprinidae exhibit species-specific numbers and arrangements of pharyngeal teeth, and extensive variation in tooth shape also occurs within the family. Mutant screens and experimental alteration of gene expression in the zebrafish are likely to yield variant tooth number and shape phenotypes that can be compared with those occurring naturally within the Cyprinidae. Such studies may reveal the relative contribution to trends in dental evolution of biases in the generation of variation and sorting of this variation by selection or drift.

Zebrafish dentition in comparative context

Lower Cretaceous Mesoeucrocodylians from Scandinavia (Denmark and Sweden)

Ichthyosaurs, an extinct group of Mesozoic marine diapsids, show a relatively small range of tooth crown morphologies. With few exceptions, members of the group bear a large number of conical teeth and show only minor heterodonty within a jaw. This uniformity in gross morphology masks a high degree of variation in both the quantity and arrangement of the mineralized tooth tissues. Here, we describe tooth tissue structure and distribution in derived ichthyosaurs. We synthesize these new observations with the historical literature, to map changes in the quantity and arrangement of tooth tissues. These changes affected tooth attachment, tooth replacement, plicidentine morphology, and the amount and distribution of cellular cementum. The amount of variation detected in features relating to ichthyosaurian dentition is not surprising given the geological longevity and morphological disparity of the group, but does emphasize the importance of extensive taxon sampling in studies of tooth histology and evolution. This study is important in that it incorporates morphological and histological information in a phylogenetic and developmental context, something that is rarely done for marine reptile dentitions.

Tooth histology, attachment, and replacement in the Ichthyopterygia reviewed in an evolutionary context

Patterns of tooth replacement in the Nile crocodile Crocodylus niloticus were evaluated in the lateral radiographs of six dried skulls of 1-year-old individuals. Additionally, the ontogeny of the succession of teeth in this species was investigated in four embryos and a hatchling, which were serially sectioned. Three-dimensional, computer-generated reconstructions of these individuals suggested that there was a regular pattern of odontogenic progression. Three morphogenetic zones could be identified in each of the age classes: an incisor, a canine, and a molar region. It was thus concluded that the Nile crocodile is hetero- rather than homodont. Zahnreihen, or tooth rows, with a periodicity of one were easily identified. © 1993 Wiley-Liss, Inc.

Heterodonty and patterns of tooth replacement in Crocodylus niloticus

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Heterodonty and patterns of tooth replacement in Crocodylus niloticus