Species longevity, stasis, and stairsteps in rhizomyid rodents
Published:January 01, 1986
New data from the middle and late Miocene Siwalik deposits of Pakistan provide accurate estimates of real temporal durations of extinct species of rhizomyid rodents. Most early rhizomyid species survive on the order of millions of years, with at least two spanning about five million years, and display apparent stasis in most characters. Average species duration for all Rhizomyidae of the Potwar Plateau is about 1.2 million years, a figure in line with other estimates for all Mammalia. Three closely related species show sharp differences in hypsodonty, while other traits remain static in each species or change slowly within the clade, on a scale above the species level. Evolution of this lineage shows at least one step in a staircase pattern, with descendants replacing ancestors, and entails an abrupt morphological change that provides a nonarbitrary definition for species boundaries. One ancestral morphotype appears to survive for a short time with its daughter species. Whereas early nonburrowing Rhizomyidae display longterm stasis, later species, some with burrowing adaptations, are shorter lived and at least one rhizomyine shows rapid, perhaps continuous phyletic change.
“The time at which an evolving population became different enough from its ancestry to be called a different species cannot, even in theory, be a precise, naturally defined date unless the new, descendant species arose in a single, abrupt step.”
G. G. Simpson, 1953, p. 35
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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.