Biomedical subjects

Tanya M Smith

Publications and source records attributed to Tanya M Smith.

3 recordsLinked to original sources

PubMed2004

An examination of dental development in Graecopithecus freybergi (=Ouranopithecus macedoniensis).

This study examined enamel thickness and dental development in Graecopithecus freybergi (=Ouranopithecus macedoniensis), a late Miocene hominoid from Greece. Comparative emphasis was placed on Proconsul, Afropithecus, Dryopithecus, Lufengpithecus, and Gigantopithecus, fossil apes that vary in enamel thickness and patterns of development. In addition, comparisons were made with Paranthropus to investigate reported similarities in enamel thickness. Several sections of a right lower third molar were generated, from which enamel thickness and aspects of the enamel and dentine microstructure were determined. Data from parallel sections shed light on the effects of section obliquity, which may influence determination of both enamel thickness and crown formation time. Graecopithecus has relatively thick enamel, greater than any fossil ape but less than Paranthropus, with which it does show similarity in prism path and Hunter-Schreger band morphology. Aspects of enamel microstructure, including the periodicity and daily secretion rate, are similar to most extant and fossil apes, especially Afropithecus. Total crown formation time was estimated to be 3.5 years, which is greater than published values for modern Homo, similar to Pan, and less than Gigantopithecus. Data on dentine secretion and extension rates suggest that coronal dentine formation was relatively slow, but comparative data are very limited. Graecopithecus shares a crown formation pattern with several thick-enamelled hominoids, in which cuspal enamel makes up a very large portion of crown area, is formed by a large cell cohort, and is formed in less than half of the total time of formation. In Paranthropus, this pattern appears to be even more extreme, which may result in thicker enamel formed in an even shorter time. Developmental similarities between Paranthropus and Graecopithecus are interpreted to be parallelisms due to similarities in the mechanical demands of their diets.

Animals↗

PubMed2003

Age at first molar emergence in early Miocene Afropithecus turkanensis and life-history evolution in the Hominoidea.

Among primates, age at first molar emergence is correlated with a variety of life history traits. Age at first molar emergence can therefore be used to broadly infer the life histories of fossil primate species. One method of determining age at first molar emergence is to determine the age at death of fossil individuals that were in the process of erupting their first molars. This was done for an infant partial mandible of Afropithecus turkanensis (KNM-MO 26) from the approximately 17.5 Ma site of Moruorot in Kenya. A range of estimates of age at death was calculated for this individual using the permanent lateral incisor germ preserved in its crypt, by combining the number and periodicity of lateral enamel perikymata with estimates of the duration of cuspal enamel formation and the duration of the postnatal delay in the inception of crown mineralization. Perikymata periodicity was determined using daily cross striations between adjacent Retzius lines in thin sections of two A. turkanensis molars from the nearby site of Kalodirr. Based on the position of the KNM-MO 26 M(1)in relation to the mandibular alveolar margin, it had not yet undergone gingival emergence. The projected time to gingival emergence was estimated based on radiographic studies of M(1)eruption in extant baboons and chimpanzees. The estimates of age at M(1)emergence in KNM-MO 26 range from 28.2 to 43.5 months, using minimum and average values from extant great apes and humans for the estimated growth parameters. Even the absolute minimum value is well outside the ranges of extant large Old World monkeys for which there are data (12.5 to <25 months), but is within the range of chimpanzees (25.7 to 48.0 months). It is inferred, therefore, that A. turkanensis had a life history profile broadly like that of Pan. This is additional evidence to that provided by Sivapithecus parvada (Function, Phylogeny, and Fossils: Miocene Hominoid Evolution and Adaptations, 1997, 173) that the prolonged life histories characteristic of extant apes were achieved early in the evolutionary history of the group. However, it is unclear at present whether life-history prolongation in apes represents the primitive catarrhine pace of life history extended through phyletic increase in body mass, or whether it is derived with respect to a primitive, size-adjusted life history that was broadly intermediate between those of extant hominoids and cercopithecoids. Life history evolution in primates as a whole may have occurred largely through a series of grade-shifts, with the establishment of fundamental life-history profiles early in the histories of major higher taxa. These may have included shifts that were largely body mass dependent, as well as those that occurred in the absence of significant changes in body mass.

Adaptation, Physiological↗

PubMed2003

Enamel thickness, microstructure and development in Afropithecus turkanensis.

Afropithecus turkanensis, a 17-17.5 million year old large-bodied hominoid from Kenya, has previously been reported to be the oldest known thick-enamelled Miocene ape. Most investigations of enamel thickness in Miocene apes have been limited to opportunistic or destructive studies of small samples. Recently, more comprehensive studies of enamel thickness and microstructure in Proconsul, Lufengpithecus, and Dryopithecus, as well as extant apes and fossil humans, have provided information on rates and patterns of dental development, including crown formation time, and have begun to provide a comparative context for interpretation of the evolution of these characters throughout the past 20 million years of hominoid evolution. In this study, enamel thickness and aspects of the enamel microstructure in two A. turkanensis second molars were quantified and provide insight into rates of enamel apposition, numbers of cells actively secreting enamel, and the time required to form regions of the crown. The average value for relative enamel thickness in the two molars is 21.4, which is a lower value than a previous analysis of this species, but which is still relatively thick compared to extant apes. This value is similar to those of several Miocene hominoids, a fossil hominid, and modern humans. Certain aspects of the enamel microstructure are similar to Proconsul nyanzae, Dryopithecus laietanus, Lufengpithecus lufengensis, Graecopithecus freybergi and Pongo pygmaeus, while other features differ from extant and fossil hominoids. Crown formation times for the two teeth are 2.4-2.6 years and 2.9-3.1 years respectively. These times are similar to a number of extant and fossil hominoids, some of which appear to show additional developmental similarities, including thick enamel. Although thick enamel may be formed through several developmental pathways, most Miocene hominoids and fossil hominids with relatively thick enamel are characterized by a relatively long period of cuspal enamel formation and a rapid rate of enamel secretion throughout the whole cusp, but a shorter total crown formation time than thinner-enamelled extant apes.

Animals↗