Systematics and evolution of Pseudhipparion (Mammalia, Equidae) from the late Neogene of the Gulf Coastal Plain and the Great Plains
S. David Webb, Richard C. Hulbert, JR., 1986. "Systematics and evolution of Pseudhipparion (Mammalia, Equidae) from the late Neogene of the Gulf Coastal Plain and the Great Plains", Vertebrates, Phylogeny, and Philosophy, Kathryn M. Flanagan, Jason A. Lillegraven
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The seven species of Pseudhipparion are, from oldest to youngest, as follows: unnamed species; P. retrusum; P. curtivallum, new combination; P. hessei, new species; P. gratum; P. skinneri, new species; and P. simpsoni, new species. They range from late Barstovian through latest Hemphillian in the Great Plains and the Gulf Coastal Plain. Both the phylogenetic relationships among these species and the phylogenetic position of this genus among other hipparionine horses are discussed.
A notable feature of Pseudhipparion evolution is its prevailing tendency toward dwarfing. All crown dimensions except unworn height decrease through time, although analysis is complicated by the fact that sample means are larger in the Great Plains than in contemporaneous Gulf Coast samples, in keeping with Bergmann’s Rule. Rates of change in several dental measurements between various Pseudhipparion species pairs, calculated over intervals of about one million years, have a mean value of 0.11 darwins, which is equal to or greater than in other hipparionine species pairs. In P. simpsoni, the final late Hemphillian species, root formation was delayed ontogenetically, producing extremely high-crowned (incipiently hypsodont) cheek teeth and incisors. Potential crown heights are 85 mm in upper premolars and 110 mm in upper molars; and the enamel patterns are greatly simplified. Such extreme hypsodonty was attained during no more than 1.5 million years within the late Hemphillian at a rate of at least 0.58 darwins, roughly six times the normal rate of crown height increase in hipparionine horses.
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Vertebrates, Phylogeny, and Philosophy
Dr. George Gaylord Simpson, one of the most important evolutionary biologists of the Twentieth Century, was born on June 16, 1902 and died on October 6, 1984. His contributions to science include not only a modern synthesis of evolutionary thought, but original research on anthropology, mammalogy, paleontology, general biology, and statistics. His prolific writings were intended for scientific and non—scientific communities alike. He helped and encouraged many who now work in the fields of paleontology and evolutionary biology. Contributors to this book dedicate their efforts as tribute to his memory.
Included authors are colleagues, former students, and friends of Dr. Simpson’s. They represent but a few of the people he would have included in these categories. The book is intended to suggest only a sampling of the diversity of George Gaylord Simpson's impact on present vertebrate paleontology, from its most senior to its very junior participants.
Ms. Flanagan’s letter of invitation entreated the following from potential authors: "In the spirit of Dr. Simpson’s own writings, we encourage imaginative contributions that would be just a little different from items expected in a regular scientific journal." The title of the volume (Vertebrates, Phylogeny, and Philosophy) reflects that request. Though individual articles deal almost exclusively with fossil mammals, emphases cross the spectrum of evolutionary biology, including systematic paleontology, considerations of adaptation, ontogeny, analyses of evolutionary tem— po and mode, biogeographic procedure, and paleogeography. Philip Gingerich’s contribution stresses the crucial importance of solid empirical research to the foundations upon which theoretical/philosophical writings should be based. Mesozoic and Cenozoic taxa are considered, and two articles discuss the modern union of molecular biology, genetics, and paleontology. Most articles benefited directly from the pioneering writings of George Simpson, yet the breadth of concerns of this volume covers only a small fraction of the interests exhibited in his lifetime of evolutionary research.
Kathryn Flanagan served as principal correspondent with authors and reviewers. Jason Lillegraven had principal responsibility for manuscript editing and considerations of production.
We take this opportunity to thank the thirty-two authors for their contributions. Similarly, more than fifty individuals served as unpaid reviewers, and we give our most sincere thanks for their generosity of time and effort. Also, we thank Linda E. Lillegraven for creating the cover design.