Korth, William W.The Tertiary record of rodents in North America / William W. Korth. p. cm. --(Topics in geobiology ; v. 12) Includes bibliographical references and index.1. Rodents, Fossil--North America. 2. Paleontology--Tertiary.

The John Day Formation of Oregon is one of the richest and best studied assemblages in North America. Including seven members and spanning about 20 million years, there are over 150 vertebrate species known from the John Day Formation. The rodent faunas of John Day have not been as well studied as larger mammals, with many families having received little attention in the last hundred years. Here, twenty one species of rodents are described, based on new discoveries and previously unpublished specimens from the John Day Formation. The new material includes four new genera and species; the eutypomyid Allotypomys pictus, the anchitheriomyine castorid Microtheriomys brevirhinus, the eomyid Proapeomys condoni, the heteromyid Bursagnathus aterosseus; and six new species: the sciurine sciurid Miosciurus covensis, the eomyids Apeomys whistleri and Neoadjidaumo arctozophus, the heteromyids Proheteromys latidens and Trogomys oregonensis, and the sicistine dipodid Plesiosminthus fremdi. The problematical eomyid species, "Florentiamys" lulli Wood, 1936a, is referred questionably as a new combination, ?Proapeomys lulli (Wood, 1936a). Emended diagnoses of Miosciurus ballovianus (Cope, 1881) and Proheteromys thorpei Wood, 1935b, are offered based upon substantially more complete material than was previously known for either. Biostratigraphic ranges of the newly described rodents allow more direct comparison with other contemporaneous faunas from North America. These new rodents also allow better reconstruction of past ecosystems and are vital for understanding how faunas have changed through time in the region.

All species of North American marsupials from the Chadronian through Hemingfordian are reviewed. Most North American species of didelphids previously referred to Peratherium are allocated to Herpetotherium. Five species of Herpetotherium are recognized.The Chadronian Peratherium donahoei Hough is synonymized with Herpetotherium valens (Lambe). The Orellan species Herpetotherium stevensoni Cope is resurrected and referred, along with the Chadronian Peratherium titanelix Matthew, to a new genus, Copedelphys. Both Copedelphys and Herpetotherium can be derived from known Duchesnean species of Peratherium from North America.Two genera of peradectids are recognized, Nanodelphys McGrew and Didelphidectes Hough. Nanodelphys minutus is synonymized with “Herpetotherium” hunti Cope and a second species of Nanodelphys is recognized from the Arikareean, though not named.

Two principal modes of origin have been suggested for microvertebrate deposits -1) the scatological hypothesis, and 2) the fluvial hypothesis. These two hypotheses were tested by 1) examining remains found in modern carnivore feces and owl pellets, 2) experimentally determining the susceptibility of various skeletal elements of small mammals to stream sorting, and 3) analyzing microvertebrate fossil assemblages.The abraded rather than sharply broken condition of the fossil bone and the frequency of preservation of individual elements most closely resembled that of the bone that had been experimentally abraded rather than bone from predator scats.Although the hydraulic properties of skeletal elements are dependent to some extent on shape, empirically derived settling curves show that the settling velocity of mammal bones is strongly size dependent. As predicted from the curves, it was found that the sediment particle size of the fossil bearing deposits had equivalent hydraulic properties to the bone contained in the sediment. It was concluded that the accumulations investigated were of alluvial origin.The paleoecologic applications of alluvially sorted microvertebrate fossil accumulations are limited because the effects of stream flow can markedly alter the composition and relative representation of members of the fauna. Korth-Microvertebrate Taphonomy 243Korth, William W. 1979. "Taphonomy of microvertebrate fossil assemblages."

The dermopteran basicranium combines a primitively constructed and oriented auditory bulla formed by ectotympanic, rostral entotympanic, and tubal cartilage with derived features of the middle ear transformer and internal carotid circulation. Living dermopterans possess a primitive eutherian auditory region that has been structurally modified to perceive a lower frequency sound spectrum than probably was utilized by ancestral Mesozoic therians. Perception of the low to midfrequency range is enhanced in Dermoptera by reducing stiffness in the mechanical transformer while maintaining low mass of the component parts. Stiffness has been reduced by (1) development of an epitympanic sinus about four times the volume of the middle ear cavity proper, (2) detachment of the anterior process of the malleus from the ectotympanic, and (3) by delicate suspension of the ear ossicles within the middle ear. We apply to dermopterans a measure of hearing efficiency derived from recent functional studies of the mammalian middle ear that regards the middle ear mechanism as an impedance matching transformer. Calculation of the impedance transformer ratio for Dermoptera suggests that these mammals are relatively efficient in comparison to other eutherians in their ability to match the impedance of cochlear fluids to that of air at the eardrum. Dermopterans theoretically are capable of using over 90% of incident sound energy striking the eardrum at the resonant or natural frequency. Mechanical impedance of the middle ear transformer exerts a minimal influence on hearing efficiency due to low mass, little stiffness, and little frictional resistance. Analysis of measurements of the middle ear transformer published by Gerald Fleischer and integration of these data with current theory on the peripheral hearing mechanism in mammals allow us to propose a model that describes the structural and functional evolution of the mammalian middle ear transformer. Structural changes appear to be correlated with alteration in function from primitive small mammals with stiff middle ear transformers and high frequency dominated hearing to mammals with a wider range in body size with more mobile middle ear transformers and a greater range of frequency perception, often including improved sensitivity to lower frequencies. Mammals employ different anatomical strategies in attainment of increased hearing efficiency and sensitivity. Efficiency is improved by adjustment of lever and areal ratios of the middle ear transformer to achieve an optimum impedance match of external air and cochlear fluids. Sensitivity over a broad frequency spectrum is attained by minimizing mass, stiffness, and frictional resistance of the transformer. The morphology of the auditory region of both living and fossil mammals seems explicable in terms of selection pressure directed toward these ends.