Biomedical subjects

Gary T Schwartz

Publications and source records attributed to Gary T Schwartz.

6 recordsLinked to original sources

PubMed2006

Foot bones from Omo: implications for hominid evolution.

We reanalyze a hominid talus and calcaneus from Omo dating to 2.2 mya and 2.36 mya, respectively. Although both specimens occur at different localities and times, both tarsals articulate well together, suggesting a single taxon on the basis of size and function. We attribute these foot bones to early Homo on the basis of their morphology. The more modern-like tarsal morphology of these Omo foot bones makes them very similar to a talus from Koobi Fora (KNM-ER 813), a specimen attributed to Homo rudolfensis or Homo erectus. Although the Omo tarsals are a million years younger than the oldest known foot bones from Hadar, both localities demonstrate anatomical differences representing two distinct morphological patterns. Although all known hominid tarsals demonstrate clear bipedal features, the tarsal features noted below suggest that biomechanical changes did occur over time, and that certain features are associated with different hominid lineages (especially the robust australopithecines).

Anatomy, Comparative↗

PubMed2005

Dental development in Megaladapis edwardsi (Primates, Lemuriformes): implications for understanding life history variation in subfossil lemurs.

Teeth grow incrementally and preserve within them a record of that incremental growth in the form of microscopic growth lines. Studying dental development in extinct and extant primates, and its relationship to adult brain and body size as well as other life history and ecological parameters (e.g., diet, somatic growth rates, gestation length, age at weaning), holds the potential to yield unparalleled insights into the life history profiles of fossil primates. Here, we address the absolute pace of dental development in Megaladapis edwardsi, a giant extinct lemur of Madagascar. By examining the microstructure of the first and developing second molars in a juvenile individual, we establish a chronology of molar crown development for this specimen (M1 CFT = 1.04 years; M2 CFT = 1.42 years) and determine its age at death (1.39 years). Microstructural data on prenatal M1 crown formation time allow us to calculate a minimum gestation length of 0.54 years for this species. Postnatal crown and root formation data allow us to estimate its age at M1 emergence (approximately 0.9 years) and to establish a minimum age for M2 emergence (>1.39 years). Finally, using reconstructions or estimates (drawn elsewhere) of adult body mass, brain size, and diet in Megaladapis, as well as the eruption sequence of its permanent teeth, we explore the efficacy of these variables in predicting the absolute pace of dental development in this fossil species. We test competing explanations of variation in crown formation timing across the order Primates. Brain size is the best single predictor of crown formation time in primates, but other variables help to explain the variation.

Age Determination by Teeth↗

PubMed2005

Sexual dimorphism in modern human permanent teeth.

On average, males possess larger tooth crowns than females in contemporary human populations, although the degree of dimorphism varies within different populations. In previous studies, different amounts of either enamel or dentine were implicated as the cause of this dimorphism. In this study, we attempt to determine the nature of sexual dimorphism in the crowns of permanent modern human teeth and to determine if two contrasting tooth types (permanent third molars and canines) show identical patterns of dimorphism in enamel and dentine distribution. We estimated the relative contributions of both enamel and dentine to total crown size, from buccolingual sections of teeth. Our sample consisted of a total of 144 mandibular permanent third molars and 25 permanent mandibular canines of known sex. We show that sexual dimorphism is likely due, in part, to the presence of relatively more dentine in the crowns of male teeth. However, whatever the underlying cause, dimorphism in both tooth root and tooth crown size should produce measurable dimorphism in tooth weight, though this has not been previously explored. Therefore, we provide some preliminary data that indicate the usefulness of wet tooth weight as a measure of sexual dimorphism. Both male permanent third molars and canines are significantly heavier than those of females. The weight dimorphism reported here for both classes of teeth may prove a useful finding for future forensic studies. In particular, weights of canines may be more useful as a means of sexing modern human skeletal material than linear or area measurements of teeth.

Anthropology, Physical↗

PubMed2004

Developmental processes and canine dimorphism in primate evolution.

Understanding the evolutionary history of canine sexual dimorphism is important for interpreting the developmental biology, socioecology and phylogenetic position of primates. All current evidence for extant primates indicates that canine dimorphism is achieved through bimaturism rather than via differences in rates of crown formation time. Using incremental growth lines, we charted the ontogeny of canine formation within species of Eocene Cantius, the earliest known canine-dimorphic primate, to test whether canine dimorphism via bimaturism was developmentally canalized early in primate evolution. Our results show that canine dimorphism in Cantius is achieved primarily through different rates of crown formation in males and females, not bimaturism. This is the first demonstration of rate differences resulting in canine dimorphism in any primate and therefore suggests that canine dimorphism is not developmentally homologous across Primates. The most likely interpretation is that canine dimorphism has been selected for at least twice during the course of primate evolution. The power of this approach is its ability to identify underlying developmental processes behind patterns of morphological similarity, even in long-extinct primate species.

Animals↗

PubMed2003

Preliminary investigation of dental microstructure in the Yuanmou hominoid (Lufengpithecus hudienensis), Yunnan Province, China.

Fieldwork in the Yuanmou Basin of southern China has uncovered a large assemblage of late Miocene hominoid fossils assigned to Lufengpithecus hudienensis. Two mandibular first molars from this species were made available for histological analysis as part of a larger ongoing study on the ontogeny of dental development in Miocene to Recent hominoids. Results are compared with published and unpublished data on tooth growth in a wide range of extant and extinct hominoids. The Yuanmou molars are smaller than those of Lufengpithecus lufengensis and have markedly shorter crown formation times, overlapping slightly with Pan, but most similar to Proconsul and Dryopithecus. In other aspects of molar development (including enamel extension rates and enamel thickness), L. hudienensis shows similarities with all extant hominoids, in particular, Pongo. Ultimately, charting the ontogeny of molar crown formation may help shed light on the relationship of Lufengpithecus hudienensis to orangutans, and other Miocene to Recent hominoids.

Animals↗

PubMed2002

Dental microstructure and life history in subfossil Malagasy lemurs.

When compared with their recently extinct relatives, living lemurs represent a mere fraction of a broad radiation that occupied unique niches in the recent past. Among living lemurs, indrids exhibit the fastest rates of dental development. This dental precocity is tightly correlated with rapid pace of postnatal dental eruption, early replacement of the deciduous teeth, high dental endowment at weaning, and relatively slow somatic growth. This pattern is in stark contrast to that seen in extant lemurids, where somatic development is highly accelerated and dental development is relatively slow. We report on the pace of dental development in one species of palaeopropithecid, the sister group to extant indrids. Like much smaller modern indrids, the chimpanzee-sized Palaeopropithecus ingens was dentally precocious at birth as evidenced by the advanced state of molar crown formation. This finding implies a pattern characteristic of Propithecus and other indrids-rapid dental development despite relatively prolonged gestation. Gestation length in this one species of subfossil lemur was likely greater than 9 months. Our results demonstrate that large body size in primates does not preclude exceedingly rapid dental development.

Animals↗