No abstract

The mammalian dentition is uniquely characterized by a combination of precise occlusion, permanent adult teeth and a unique tooth attachment system. Unlike the ankylosed teeth in most reptiles, mammal teeth are supported by a ligamentous tissue that suspends each tooth in its socket, providing flexible and compliant tooth attachment that prolongs the life of each tooth and maintains occlusal relationships. Here we investigate dental ontogeny through histological examination of a wide range of extinct synapsid lineages to assess whether the ligamentous tooth attachment system is unique to mammals and to determine how it evolved. This study shows for the first time that the ligamentous tooth attachment system is not unique to crown mammals within Synapsida, having arisen in several non-mammalian therapsid clades as a result of neoteny and progenesis in dental ontogeny. Mammalian tooth attachment is here re-interpreted as a paedomorphic condition relative to the ancestral synapsid form of tooth attachment.

The paper deals with the distribution of the axons that arise in the fascia dentata and go to the hippocampus proper. Lesions were placed in the fascia dentata and degenerating mossy fibers were mapped by silver impregnation, mostly according to the method of Fink and Heimer ('67). The fascia dentata and the layer of mossy fibers in the rat form a curved and twisted structure extending from the level of the posterior aspect of the septum to the temporal tip. The horizontal serial sections used for this study were, therefore, in part interpreted by means of three-dimensional reconstructions.Mossy fibers do not enter the fimbria at any point. Degenerating fibers can be traced to the limit between regio superior (CA 1 ) and regio inferior. This dentato-hippocampal connection displays a pattern of precise localization, each level of fascia dentata projecting onto a restricted level of regio inferior. The bands of degenerated mossy fibers produced by lesions in the fascia dentata were largely oriented in the transverse direction (slightly in a temporal direction) and were somewhat wider distally than at the origin from the hilus. Very narrow bands were seen in a few animals with particularly small lesions.

Dental ankylosis was part of the normal development of the stem mammal periodontium for millions of years prior to the evolution of a permanent gomphosis in mammals. Mammals may have evolved a permanent gomphosis by delaying the processes that produced dental ankylosis in stem mammals. Pathological ankylosis may represent a reversion to the ancestral condition, which now only forms via advanced ageing and pathology.

Tooth morphology and development can provide valuable insights into the feeding behaviour and evolution of extinct organisms. The teeth of Theropoda, the only clade of predominantly predatory dinosaurs, are characterized by ziphodonty, the presence of serrations (denticles) on their cutting edges. Known today only in varanid lizards, ziphodonty is much more pervasive in the fossil record. Here we present the first model for the development of ziphodont teeth in theropods through histological, SEM, and SR-FTIR analyses, revealing that structures previously hypothesized to prevent tooth breakage instead first evolved to shape and maintain the characteristic denticles through the life of the tooth. We show that this novel complex of dental morphology and tissues characterizes Theropoda, with the exception of species with modified feeding behaviours, suggesting that these characters are important for facilitating the hypercarnivorous diet of most theropods. This adaptation may have played an important role in the initial radiation and subsequent success of theropods as terrestrial apex predators.

No abstract

A well-preserved skull from a previously unknown growth stage for the lambeosaurine hadrosaurid Hypacrosaurus stebingeri from the Campanian of western North America is described. This skull is equivalent in size to the smallest known growth stages for Corythosaurus, Hypacrosaurus altispinus and Lambeosaurus, and allows for a direct comparison of the juvenile growth stage of all four taxa for the first time. H. stebingeri is not diagnosed by any recognised autapomorphies, and the morphology of juvenile skulls is similar for all four taxa. Juveniles and sub-adults of H. stebingeri are characterised by an incipient cranial crest formed predominantly by the nasal, elongate narial openings with a reduced premaxilla-nasal fontanelle and a conspicuous bifurcation at the rostral end of the nasal. The bifurcation of the nasal is virtually identical to juveniles and sub-adults of Corythosaurus. These cranial morphologic similarities support the phylogenetic hypothesis that Hypacrosaurus is more closely related to Corythosaurus than to Lambeosaurus.

Paleozoic sphenacodontid synapsids are the oldest known fully terrestrial apex predators. Dimetrodon and other sphenacodontids are the first terrestrial vertebrates to have strong heterodonty, massive skulls and well-developed labio-lingually compressed and recurved teeth with mesial and distal cutting edges (carinae). Here we reveal that the dentition of Dimetrodon and other sphenacodontids is diverse. Tooth morphology includes simple carinae with smooth cutting edges and elaborate enamel features, including the first occurrence of cusps and true denticles (ziphodonty) in the fossil record. A time-calibrated phylogenetic analysis indicates that changes in dental morphology occur in the absence of any significant changes in skull morphology, suggesting that the morphological change is associated with changes in feeding style and trophic interactions in these ecosystems. In addition, the available evidence indicates that ziphodonty evolved for the first time in the largest known species of the genus Dimetrodon and independently from the ziphodont teeth observed in some therapsids.