Lawrence B. Martin

Species, Species Concepts, and Primate Evolution

What, If Anything, Is a Species N. Eldridge. Species Concepts F.S. Szalay. Primates and Paradigms J. Masters. Species, Subspecies, and Baboon Systematics C.J. Jolly. Speciation in Living Hominoid Primates C.P. Groves. Geographic Variation in Primates G.H. Albrecht, J.M.A. Miller. Speciation and Morphological Differentiation in the Genus Lemur I. Tattersall. Squirrel Monkey (Genus Saimiri) Taxonomy R.K. Costello, et al. Measures of Dental Variation as Indicators of Multiple Taxa in Samples of Sypatric Cercopithecus Species D.A. Cope. Catarrhine Dental Variability and Species Recognition in the Fossil Record J.M. Plavcan. Multivariate Craniometric Variation in Chimpanzees B.T. Shea, et al. Species Concepts and Species Recognition in Eocene Primates K.D. Rose, T.M. Brown. Anagenetic Angst L. Krishtalka. Cladistic Concepts and the Species Problem in Hominoid Evolution T. Harrison. 7 additional articles. Index.

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.

Species Recognition in Middle Miocene Hominoids

This chapter addresses the issue of species recognition in fossil samples. The particular example that is examined is the extensive collection of dental and gnathic remains from the middle Miocene locality at Pasalar, Turkey. This site is unusual in that available sedimentological and taphonomic evidence indicate that it was accumulated extremely rapidly and from a very localized area (Al-pagut et al., 1990a; Andrews and Alpagut, 1990; Andrews and Ersoy, 1990; Bestland, 1990), so that the mammalian fauna is comparable to a museum collection of modern animals made from one locality. In fact, the fauna may be sampled from within 3–5 km of the site, which would include only a few hundred meters of altitude at most. The sediments may have been deposited in hours or days, so that the temporal range of the fauna is determined by the period of predepositional skeletal preservation. Therefore, it might be anticipated that a species at Pasalar might be less variable than modern comparative samples collected from a variety of geographical localities. The Pasalar sample thus provides an unusual opportunity to examine analytical methods that may be used to determine species numbers in such an assemblage.

New perspectives on chimpanzee and human molar crown development

Previous histological studies of small samples of chimpanzee and human molars suggested similarities in crown formation time, which is surprising given substantial life history differences. As part of an on-going study of hominoid molar development, we report on the largest-known sample of chimpanzee and human molars, including re-evaluation of previously examined histological sections. Variation of incremental features within and between genera is examined, including Retzius line periodicity, daily secretion rate, and Retzius line number. Differences due to population-level variation and sexual dimorphism are also considered. Significant increasing trends in daily secretion rates were found from inner to outer cuspal enamel, ranging from approximately 3–5 microns/day in chimpanzees. Humans demonstrate slightly lower and higher mean values at the beginning and end of cuspal formation, respectively, but both genera show an overall average of approximately 4 microns/day. Retzius line periodicity ranges from 6–7 days within chimpanzees and 6–12 days within humans. Within upper molars, mesiopalatal cusps (protocones) show thicker cuspal enamel and longer crown formation time than mesiobuccal cusps (paracones). Within lower molars, mesiobuccal cusps (protoconids) show greater Retzius line numbers, longer imbricational formation time, and thicker cuspal enamel than mesiolingual cusps (metaconids), resulting in longer formation times. A negative correlation was found between Retzius line number and periodicity in the human sample, resulting in similar crown formation times within cusp types, despite the range of individual periodicities. Few sex differences were found, but a number of developmental differences were apparent among human populations. Cusp-specific formation time in chimpanzees ranges from 2–3 years on average. Within specific cusp types, humans show greater average formation times than chimpanzees, due to higher mean periodicity values and/or thicker cuspal enamel. However, formation time within specific cusp types varies considerably, and the two genera show overlapping ranges, which has implications for the interpretation of small samples.

Quantification of dentine shape in anthropoid primates

The external shape and thickness of the enamel component of primate molars have been employed extensively in phylogenetic studies of primate relationships. The dentine component of the molar crown also has been suggested to be indicative of phylogenetic relationships, but few studies have quantified dentine morphology in order to evaluate this possibility. To explore the utility of dentine shape as an indicator of phylogenetic affinity, a two-dimensional geometric morphometric analysis (EDMA-II) was performed using nine homologous landmarks on a sample of sectioned maxillary molars of extant ceboid, cercopithecoid, and hominoid primates. Results indicate that dentine shape (the configuration of the enamel-dentine junction, or EDJ) can distinguish taxa at every taxonomic level examined, including superfamilies, subfamilies, and closely related genera and species. This supports the idea that dentine morphology may be useful for phylogenetic studies. It is further suggested that the morphology of the EDJ may be more conservative than enamel morphology, and perhaps better-suited to phylogenetic studies. Among the samples studied, cercopithecoid primates have a unique dentine shape, and it is suggested that the development of bilophodont molars may be related to the distinctive EDJ configuration in cercopithecoids.

Hominoid primates from a new Miocene locality named Meswa Bridge in Kenya

The remains of a species of Proconsul from early Miocene deposits at Meswa Bridge are described. Associated dental, mandibular, maxillary and cranial specimens represent several immature individuals. They are equivalent in size to Proconsul nyanzae but differ morphologically, particularly in the greater breadth and degree of flare of the deciduous and permanent molars, but lack of adult individuals from Meswa Bridge makes it difficult to compare them adequately with existing species. They are therefore left unassigned as an indeterminate species of Proconsul .

Micro-computed tomography of primate molars: Methodological aspects of three-dimensional data collection

Phylogenetic, paleodietary, and developmental studies of hominoid primates frequently make use of the post-canine dentition, in particular molar teeth. To study the thickness and shape of molar enamel and dentine, internal dental structures must be revealed (e.g., the location of dentine horn apices), typically necessitating the production of physical sections through teeth. The partially destructive nature of such studies limits sample sizes and access to valuable fossil specimens, which has led scholars to apply several methods of radiographic visualization to the study of teeth. Radiographic methods aimed at visualizing internal dental structures include lateral flat-plane X-rays, ultrasound, terra-hertz imaging, and computed tomography. Each of these techniques has resolution limitations rendering them inadequate for accurately reconstructing both the enamel-dentine junction and the outer enamel surface; the majority of studies are thus performed using physical sections of teeth. A comparatively new imaging technique, micro-computed tomography (mCT), accurately portrays the enamel-dentine junction of primate molars, and provides accurate measurements of enamel cap thickness and morphology. The research presented here describes methodological parameters pertinent to mCT studies of molars (slice thickness and pixel resolution), and the observable impact on measurement accuracy when these parameters are altered. Measurements taken on a small, taxonomically diverse sample of primate molars indicate that slice thickness should be conservatively set at approximately 3.45 % of specimen length, and image resolution should be maximized (ideally, greater than or equal to 2048 × 2048 pixels per image) in order to ensure measurement accuracy. After discussing this base-line protocol for future mCT studies of the primate dentition, illustrative applications of this imaging technology are presented.

Species and Speciation

When confronted with the bewildering array of life forms, whether living or extinct, the evolutionary scientist must attempt to create order out of apparent chaos before addressing any other of the myriad questions that stem from a dedication to understanding the organic world. As “the lowest level of genuine discontinuity above the level of the individual [organism]” (Mayr, 1982, p. 251), the species is the basic unit of evolutionary diversity. One would think, therefore, that there is a large degree of unanimity regarding the nature of the species and the role it plays in evolutionary theory. But this is not the case. As amply illustrated by the contributions to this book, there remains a great deal of disagreement on the so-called “species problem.” Questions such as (1) what are species in the context of evolutionary theory, (2) what processes are responsible for species’ origins (and extinctions), and (3) what role do species play in evolution over geological time have occupied the center of debate among evolutionary biologists for decades, and the discussion shows no signs of abatement as we close in on the millennium.