Mary C. Maas

Mammalian generic diversity and turnover in the late Paleocene and early Eocene of the Bighorn and Crazy Mountains Basins, Wyoming and Montana (USA)

Abstract Patterns of mammalian generic turnover, richness, and faunal composition were investigated for faunas from 17 biostratigraphic zones in middle Paleocene through early Eocene deposits of the Bighorn and Clarks Fork basins of northern Wyoming and the Crazy Mountains Basin of south-central Montana. Significance of turnover was evaluated (1) by comparison of observed turnover to expected turnover (calculated from the multiple regression of turnover on zone duration and generic richness), and (2) by comparison of observed turnover to a bootstrapped turnover distribution. Patterns of turnover and richness also were assessed in light of relative sampling quality of each faunal zone. The analysis identified four intervals of significant faunal change: the Torrejonian Tiffanian transition, the late Tiffanian, the earliest Wasatchian, and the middle-to-late Wasatchian. The first interval was characterized by a high number of last occurrences in the latest Torrejonian, resulting in a decrease in standing generic richness in the earliest Tiffanian, but no major changes in ordinal composition. During the next interval of significant turnover, the late Tiffanian, higher-than-expected first occurrences resulted in an increase in standing richness and a change in faunal composition, most probably reflecting the immigration of taxa from outside North America. The third, and most dramatic, interval of significant generic turnover took place in the earliest Wasatchian and was distinguished by a high number of first occurrences, but relatively few last occurrences. This led to a marked increase in generic richness, a pattern similar to that for the early Wasatchian of North America as a whole. The major change in faunal composition, as in the late Tiffanian, was largely composed of immigrants from other continents. The pattern of faunal change during the early Wasatchian of the Bighorn Basin, along with evidence for global warming at the Paleocene-Eocene boundary, supports previous interpretations associating this episode in mammalian evolution with the opening of high latitude intercontinental dispersal routes. During the fourth interval of interest, the middle to late Wasatchian, the Bighorn Basin fossil record shows a drop in generic richness. This differs from the overall North American pattern, and may be, in part, an artifact of still inadequate sampling for the latest part of the stratigraphic sequence in the Bighorn Basin.

Dental microwear and microstructure in early oligocene primates from the Fayum, Egypt: Implications for diet

Textbook descriptions usually portray the Fayum anthropoideans as frugivores, with Parapithecus grangeri including a folivorous component in its diet and Apidium a component of hard-object feeding. Recent work with modern mammals has shown that analyses of both dental microwear and dental microstructure may yield insights into diet and tooth use. The purpose of this study was to combine these two techniques to gain a better perspective on the paleobiology of the Fayum higher primates. Dental microwear analyses involved the use of high resolution epoxy casts of Aegyptopithecus, Parapithecus, and Apidium housed in the Duke University Primate Center. Scanning electron micrographs were taken at x500, and all microwear features in each micrograph were digitized. For microstructure analyses, molar teeth were sectioned in a variety of planes, lightly etched, and photographed in the SEM. Results of the dental microwear analyses indicate that the three Fayum anthropoideans all clustered with modern primate frugivores but that there were also significant differences between Aegyptopithecus and the other two Fayum genera. By contrast, dental microstructure analyses showed important differences between Apidium and the other two genera. The reason for these differences probably lies in a combination of body size and dietary differences, with Aegyptopithecus occasionally feeding on hard objects and Apidium maximizing wear resistance through a unique emphasis of radial (rather than decussating) enamel.

A scanning electron-microscopic study of in vitro abrasion of mammalian tooth enamel under compressive loads

Microscopic tooth-wear (microwear) patterns can be an important tool for assessing modes and rates of abrasive tooth wear, but their analysis and interpretation is complicated by the fact that microwear is influenced by many factors. Three of these factors were here tested under conditions of compressive loading: (1) species differences in enamel structure, (2) abrasive particle size and (3) magnitude of force. Teeth of four species (Homo sapiens, Lemur fulvus, Ovis aries and Crocodylus rhombifer) were abraded in vitro using three sizes of abrasive silicon-carbide grit (average diameters 73, 23 and 14 microns), at two loads (50 and 100 kg). Microwear features were assessed by scanning electron microscopy of lightly etched enamel surfaces and epoxy replicas. Microwear pits (length:width < 4:1) were the predominant feature type. Factorial analysis of variance of rank-transformed, feature-area measurements demonstrated that, under conditions of compressive loading, the size of abrasive particles was the primary determinant of microwear size. These results contrast with previous experimental tests of abrasion by predominantly shearing loads, where feature size was influenced by interaction among experimental factors, including the microscopic orientation of enamel crystallites. Although magnitude of compressive force was not a factor in microwear size variation, it may be a critical factor in explaining the presence or absence of microwear on tooth surfaces. The relatively small compressive bite force generated during typical chewing may not consistently produce abrasive pitting. These experiments demonstrate that, as the same abrasive regime can produce both large and small pits, the mechanism by which wear features are formed (i.e. compression or adhesion) cannot be determined from the size of features alone. Nevertheless, the dependence of pit size on abrasive particle size demonstrates that metrical variation in wear features can elucidate important attributes of an animals diet.

Comparative paleoecology of Paleogene and Neogene mammalian faunas: Trophic structure and composition

Abstract Trophic structure and composition are examined in two important biotic records, one the Paleogene of Wyoming and Montana, and the other from the Neogene of Pakistan. The Paleogene sequence spans approximately 10 million years and encompasses four North American Land Mammal Ages (Torrejonian, Tiffanian, Clarkforkian, and Wasatchian). The Neogene sequence spans approximately 17 m.y. and includes most of the Miocene and Pliocene with the best documented interval spanning from 16 to 7 Ma. Five basic trophic categories (primary consumers: herbivores, frugivores, omnivores: secondary consumers: insectivores, carnivores) are recognized for Paleogene and Neogene mammals based on tooth morphology, body size, and analogy with modern mammalian groups. The Paleogene mammalian biota is characterized as one in which both trophic structure and taxonomic composition change through the history of the record. The Neogene mammalian biota maintains a relatively consistent trophic structure through most of the record, although taxonomic composition changes substantially through time. Based on comparisons of trophic structure from the Paleogene and Neogene records with that of selected modern mammalian faunas, Paleogene habitats fluctuated between closed, humid forests and more open, drier woodlands. Neogene trophic structure indicates that savanna woodlands were the typical habitat present through most of the sequence. Only after 7 Ma did these woodlands give way to more open grasslands in Pakistan.

Comparative paleoecology of Paleogene and Neogene mammalian faunas: body-size structure

Abstract Species size is correlated with many aspects of life history, ecology, and behavior, which means that size changes within species, lineages, and faunas represent an important component of evolutionary paleoecology. Comparison of Paleogene mammalian faunas from the Bighorn, Clarks Fork, and Crazy Mountains basins of Wyoming and Montana with Neogene mammalian faunas from the Siwalik Group of northern Pakistan reveals similarities and differences in patterns of size change through intervals of 10 m.y. Two approaches to size change are presented. The first is to evaluate changes in the size distribution of faunas over three time intervals in each sequence. Rank-ordered size distributions, or cenograms, are used to depict faunal size structure for non-carnivorous species. The slopes and gaps in different regions of the size spectrum reflect conditions of vegetation and climate, by analogy with modern mammalian faunas (Legendre, 1986, 1989). For the Paleogene and Neogene faunas, subtle changes over time in size structure reflect changes in local vegetation and climate. The Paleogene cenograms suggest a habitat shift from mesic to humid forest, and the Neogene cenograms suggest a shift from open woodland to savannah scrub. These interpretations are supported by concurrent changes in trophic structure, faunal turnover, and in floral and geologic indicators. The second approach focuses on size change within species and lineages in several families of predominantly herbivorous species. For 60 Paleogene species and 39 Neogene species, change in average species size over successive biostratigraphic intervals is assessed by a criterion of doubling or halving of body mass relative to the preceding interval. New occurrences are compared to established species of the same genus and of the same family. In both records, size increases occur slightly more often than size decreases. The size distribution of groups changes more often through appearances of species of more than double or less than half the size of established species of the same group or by disappearances, rather than through rapid change of size within species. The pattern of change in median size and size range of contemporaneous species varies among families in both records. Three causes of evolutionary size change—climatic change, competition, and predation—are evaluated. In both records, climatic change and interspecific competition are considered the principle mechanisms for the observed changes.

Paleogeography, Paleobiogeography, and Anthropoid Origins

The study of anthropoid origins has long been tied to hypotheses postulating Eocene dispersal of early anthropoids or their precursors into Africa from Europe (e.g., Gingerich, 1975; Rasmussen and Simons, 1988), from Asia (e.g., Gingerich, 1980; Ciochon and Ghiarelli, 1980; Giochon et al., 1985; Rosenberger, 1986), or from South America (e.g., Szalay, 1976). Similarly, investigations of platyrrhine origins have focused on either a North American (e.g., Gingerich, 1980; Hoffstetter, 1972; Wood, 1980; Rosenberger, 1986) or an African source (e.g., Lavocat, 1974, 1980; Giochon and Chiarelli, 1980; Fleagle, 1986). These different scenarios have been based in large part on putative ancestor-descendant relationships and the identification of early anthropoids or protoanthropoids in the presumed source areas but also have relied on current understanding of temporal relationships between faunas and reconstructions of Eocene paleogeography and paleobiogeography to determine the probable timing, mode, and route of dispersal.

Enamel microstructure of Pakicetus (Mammalia: Archaeoceti)

The tooth enamel of the earliest cetacean, Pakicetus , is described and compared to enamel of a primitive artiodactyl and a variety of primitive ungulate families. Pakicetus enamel organization, which is considered primitive for Cetacea, consists of a combination of radial and decussating enamel types. Prism patterns include prisms with open (horseshoe-shaped) and closed (circular) boundaries. Pakicetus enamel is similar to that of many primitive ungulates, including Diacodexis , the earliest artiodactyl, and Mesonychidae, an archaic ungulate family that often is considered close to the ancestry of Cetacea. This finding is consistent with the hypothesis, originally proposed on the basis of other aspects of morphology, that artiodactyls, cetaceans, and Mesonychidae are closely related.

Mammalian turnover and community structure in the Paleocene of North America

The effects of faunal turnover on mammalian community structure are evaluated for 17 faunal zones of the North American Paleocene through early Eocene land mammal ages (Puercan through early Wasatchian). Generic disappearances were significantly high at the end of the Puercan, Torrejonian, and Tiffanian land mammal ages, but appearances were significantly high only during the early Puercan. Generic richness rose rapidly in the early Puercan, remained stable throughout most of the Paleocene, and increased from the late Paleocene into the early Eocene. The null hypothesis that generic turnover clustered preferentially according to dentally defined trophic or body size categories could be rejected or attributed to sampling problems for all but the early (Pu0) and late Paleocene (Ti5‐Cf2). Early Paleocene change in community structure most probably represented endemic radiation of mammals into previously unoccupied niches. Community restructuring in the late Paleocene reflected a complex of causes, including ...