New hominin dental remains from the ∼2.04-1.95 Ma Drimolen Main Quarry, South Africa.

BACKGROUND: The Drimolen Palaeocave site is situated within the UNESCO Fossil Hominid Sites of South Africa World Heritage Area and has yielded numerous hominin fossils since its discovery in 1992. Most of these fossils are represented by isolated dental elements, which have been attributed to either of two distinct hominin genera, Paranthropus and Homo. AIM: This paper provides morphological descriptions for a further 19 specimens that have been recovered from the ∼2.04-1.95 Ma Drimolen Main Quarry (DMQ) deposits since 2008. This paper also discusses the two primary hypotheses used to explain Paranthropus robustus variation: sexual dimorphism, and micro-evolution within a lineage. SUBJECTS AND METHODS: These 19 fossils are represented by 47 dental elements and expand the sample of DMQ early Homo from 13 to 15, and the sample of Paranthropus robustus from 69 to 84. RESULTS: The evidence presented in this paper was found to be inconsistent with the sexual dimorphism hypothesis. CONCLUSION: Some support was found for the micro-evolution hypothesis.

New discoveries of skeletal elements of Hadropithecus stenognathus from Andrahomana Cave, southeastern Madagascar.

Remains of what appears to be a single, subadult Hadropithecus stenognathus were recovered from a previously unexcavated site at Andrahomana Cave (southeastern Madagascar). Specimens found comprise isolated teeth and cranial fragments (including the frontal processes of the orbits), as well as a partial postcranial skeleton. They include the first associated fore- and hind-limb bones, confirming the hind-limb attributions made by Godfrey and co-workers in 1997, and refuting earlier attributions by Lamberton in 1937/1938. Of particular interest here are the previously unknown elements, including a sacrum, other vertebrae and ribs, some hand bones, and the distal epiphysis of a femur. We briefly discuss the functional implications of previously unknown elements. Hadropithecus displayed a combination of characters reminiscent of lemurids, others more like those of the larger-bodied Old World monkeys, and still others more like those of African apes. Yet other characteristics appear unique. Lemurid-like postcranial characteristics may be primitive for the Archaeolemuridae. Hadropithecus diverges from the Lemuridae in the direction of Archaeolemur, but more extremely so. Thus, for example, it exhibits a stronger reduction in the size of the hamulus of the hamate, greater anteroposterior compression of the femoral shaft, and greater asymmetry of the femoral condyles. Nothing in its postcranial anatomy signals a close relationship to either the Indriidae or the Palaeopropithecidae.

Growth processes in teeth distinguish modern humans from Homo erectus and earlier hominins.

A modern human-like sequence of dental development, as a proxy for the pace of life history, is regarded as one of the diagnostic hallmarks of our own genus Homo. Brain size, age at first reproduction, lifespan and other life-history traits correlate tightly with dental development. Here we report differences in enamel growth that show the earliest fossils attributed to Homo do not resemble modern humans in their development. We used daily incremental markings in enamel to calculate rates of enamel formation in 13 fossil hominins and identified differences in this key determinant of tooth formation time. Neither australopiths nor fossils currently attributed to early Homo shared the slow trajectory of enamel growth typical of modern humans; rather, both resembled modern and fossil African apes. We then reconstructed tooth formation times in australopiths, in the approximately 1.5-Myr-old Homo erectus skeleton from Nariokotome, Kenya, and in another Homo erectus specimen, Sangiran S7-37 from Java. These times were shorter than those in modern humans. It therefore seems likely that truly modern dental development emerged relatively late in human evolution.

Ontogeny of canine dimorphism in extant hominoids.

Many behavioral and ecological factors influence the degree of expression of canine dimorphism for different reasons. Regardless of its socioecological importance, we know virtually nothing about the processes responsible for the development of canine dimorphism. Our aim here is to describe the developmental process(es) regulating canine dimorphism in extant hominoids, using histological markers of tooth growth. Teeth preserve a permanent record of their ontogeny in the form of short- and long-period incremental markings in both enamel and dentine. We selected 52 histological sections of sexed hominoid canine teeth from a total sample of 115, from which we calculated the time and rate of cuspal enamel formation and the rate at which ameloblasts differentiate along the future enamel-dentine junction (EDJ) to the end of crown formation. Thus, we were able to reconstruct longitudinal growth curves for height attainment in male and female hominoid canines. Male hominoids consistently take longer to form canine crowns than do females (although not significantly so for our sample of Homo). Male orangutans and gorillas occasionally take up to twice as long as females to complete enamel formation. The mean ranges of female canine crown formation times are similar in Pan, Gorilla, and Pongo. Interspecific differences between female Pan canine crown heights and those of Gorilla and Pongo, which are taller, result from differences in rates of growth. Differences in canine crown heights between male Pan and the taller, more dimorphic male Gorilla and Pongo canines result both from differences in total time taken to form enamel and from faster rates of growth in Gorilla and Pongo. Although modern human canines do not emerge as significantly dimorphic in this study, it is well-known that sexual dimorphism in canine crown height exists. Larger samples of sexed modern human canines are therefore needed to identify clearly what underlies this.

Enamel thickness and the helicoidal wear plane in modern human mandibular molars.

Helicoid occlusion has long been recognized as a feature characterizing the human dentition and has been viewed as an important morphological marker in the transition from Australopithecus to Homo. The hallmark of helicoidal wear is a buccal wear slope in anterior mandibular molars (and a corresponding lingual slope of wear in anterior maxillary molars) reversing to a flat or lingual-oriented one in posterior mandibular molars. If localized increases in enamel thickness are taken as evidence of an adaptation to increased wear resistance, then data on enamel thickness in unworn molars can be used to assess whether the region of greatest wear changes from anterior to posterior in such a way as to provide evidence for the helicoidal wear plane being a structural feature of the orofacial skeleton. Such a hypothesis was supported in a previous study on enamel thickness in modern human maxillary molars. As maxillary and mandibular precisely interdigitate, it is reasonable to expect that a similar pattern of enamel thickness distribution should be present in mandibular molars. To test this, data on the distribution of enamel thickness across functionally relevant regions of the crown were collected on a sample of twenty-nine completely unworn mandibular molars. Results suggest that enamel thickness increases slightly posteriorly but no evidence exists for morphological changes along the mandibular molar series of modern humans to follow a trend towards providing additional tooth material in areas under greater wear in accordance with a helicoidal wear model. This suggests that the patterning of enamel thickness must be viewed in conjunction with other features, such as the biomechanical behaviour of molars during occlusion and axial molar angulation, to ascertain the precise anatomical determinants of this unique feature of the human dentition.

Taxonomic and functional aspects of the patterning of enamel thickness distribution in extant large-bodied hominoids.

One of the few uncontested viewpoints in studies of enamel thickness is that the molars of the African apes, Pan and Gorilla, possess "thin" enamel, while Pongo and modern humans possess varying degrees of "thick" enamel, even when interspecific differences in overall body or tooth size are taken into account. Such studies focus primarily on estimates of the total volume of enamel relative to tooth size (i.e., "relative" enamel thickness), as this is thought to bear directly on questions concerning dietary proclivities and phylogenetic relationships. Only recently have studies shifted focus to examining differences in the distribution of enamel across the tooth crown, i.e., the patterning of enamel thickness, as this may contribute to more refined models of tooth function and dietary adaptations in extant hominoids. Additionally, this feature has been suggested to be a reliable indicator of taxonomic affinity in early hominins, though no study has specifically addressed whether species-specific patterns exist among known phena. The aims of this paper were to test more explicitly whether enamel thickness patterning provides valuable taxonomic, functional, and/or phylogenetic information for maxillary molars of large-bodied extant hominoids. A series of seven linear enamel thickness measurements was recorded in the plane of the mesial cusps in cross sections of a total of 62 maxillary molars of P. troglodytes, G. gorilla, P. pygmaeus, and H. sapiens to estimate the patterning of enamel thickness distribution. Results from a discriminant function analysis reveal that, overall, this trait reclassifies extant hominoid maxillary molars with 90% accuracy: 100% of extant Homo, 75. 0% of Pongo, 83.3% of Pan, and 66.7% of Gorilla are reclassified correctly, indicating that this feature possesses a strong taxonomic signal. Furthermore, differences in the structure of the enamel cap are evident among hominoids: modern humans differ from Pongo in possessing proportionally thicker enamel in areas of the crown associated with shearing activity; Pan molars are better designed than those of Gorilla for generating a greater component of crushing/grinding loads. Thus, African ape molars are structurally dissimilar, even though they are both considered to belong to a morphologically homogeneous "thin-enameled" group. Simple developmental mechanisms can be invoked to explain the sometimes subtle differences in the achievement of adult morphology. For instance, human and orangutan molar cusps possess a similar degree of enamel thickness, but the possibility exists that despite similarities in morphology, each species follows a different sequence of secretory activity of enamel to achieve the final, albeit similar, degree of enamel thickness. Such a finding would suggest that the shared possession of "thick" or "thin" enamel among species may be phylogenetically uninformative, as it would not represent a developmental synapomorphy.

Enamel thickness and the topography of the enamel-dentine junction in South African Plio-Pleistocene hominids with special reference to the Carabelli trait.

This study explores the internal morphology of early hominid teeth using high-resolution computed tomography. Data on Carabelli feature size, enamel thickness, and the topography of the enamel-dentine junction are considered together in order to examine the relationship among these variables in the maxillary molars of gracile and robust australopithecines from South Africa. In particular, one aim is to investigate the degree to which Carabelli feature size influences enamel thickness in the plane of the mesial cusps. The results demonstrate that maxillary molars attributed to Australopithecus africanus from Sterkfontein, Taung and Makapansgat possess larger Carabelli features and thinner enamel along the lingual wall of the protocone than do specimens attributed to Paranthropus robustus from Swartkrans and Kromdraai. Distinct differences in the position of the Carabelli feature at the level of both the enamel-dentine junction and tooth crown surface between early hominid species may help explain the observed disparity in enamel thickness at that region of the tooth crown as well as offer clues to the functional role of Carabelli's cusp. As the size and position of the Carabelli feature affects the linear thickness of enamel at this one particular region of the tooth crown, future comparative studies focusing on taxa that possess moderate to strong development of the Carabelli complex should use the linear thickness of enamel taken close to the protoconal dentine horn or at the maximum projection of the Carabelli's cusp.

A histological reconstruction of dental development in the common chimpanzee, Pan troglodytes.

Much is known about the dental development of Pan compared with that for other extant great apes. The majority of information available has concentrated either on the emergence times of teeth or on the sequence of mineralization stages of the teeth as revealed from radiographs. However, the problems of defining stages of tooth formation sufficiently accurately on radiographs are only now becoming recognized. All of the data available to date suggest the presence of a more variable picture for the timing of mineralization stages in chimpanzees than for the timing of tooth emergence. In particular, arguments persist in the literature over the time of initial mineralization and the time it takes to form each anterior tooth crown in chimpanzees. Therefore we attempt to provide a more precise chronological time scale for dental development in our closest living relative. Furthermore, we examine the sequence of molar cusp formation relative to enamel formation times related specifically to those cusps and to try to tie these data in with information from functional studies of molar crowns. Histological sections of 14 maxillary and 28 mandibular teeth from four chimpanzee (Pan troglodytes) individuals and three molar teeth from three chimpanzees of unknown origin were prepared in accordance with a well-established protocol. By combining data on short-period and long-period incremental lines (including daily secretion rates, periodicity, prism lengths and enamel thickness) in both enamel and dentine, we reconstruct times for the onset and duration of crown formation as well as construct a schedule for the pattern and timing of dental development in this one hominoid species. Interestingly, our histologically-derived data confirms that the data from radiographic studies underestimate crown formation times by the following amounts for each tooth type: I1 2.5 years, I2 3.1 years, C 1.6 years, P3 1.9 years, P4 0.1 years, M1 0.8 years, M2 1.1 years and M3 0.3 years. When combined with data on gingival emergence, it seems that chimpanzee teeth have a greatly reduced time for root growth before emergence occurs and that the major differences between Homo sapiens and Pan lie in the first part of the root formation rather than in the total period of crown formation. Maxillary and mandibular molar functional cusps take longer to complete enamel formation to the cervix than any other cusp in that same tooth, which makes sense as these cusps are thick enamelled. These results suggest that new links can be made between developmental aspects, occlusal morphology and tooth function.

Cross-sectional geometric properties of the Otavipithecus mandible.

Cross-sectional geometric properties of the postcanine mandibular corpus are determined for the only known specimen of Otavipithecus namibiensis, a middle Miocene hominoid from southern Africa. It is shown that Otavipithecus is unique in that several important mechanical properties of its mandible, including maximum and minimum moments of inertia and distribution of cortical bone, differ from patterns seen in both extant hominoids and the early hominids Australopithecus africanus and Australopithecus (Paranthropus) robustus. This is particularly apparent in the mechanical design of the posterior portion of the mandibular corpus for resisting increased torsional and transverse bending moments. Cortical index values at the level of M2 also reveal that both Otavipithecus and A. africanus are similarly designed to resist increased masticatory loads with relatively less cortical bone area, a highly efficient mechanical design.