ABSTRACT

A magnetostratigraphic study was undertaken to improve the age control of the middle Eocene Coldwater Sandstone in southern California. The results show that, although magnetic overprinting is common in Coldwater rocks, most samples yield a stable characteristic remanence that passes reversal and fold tests and shows a clockwise rotation of about 100° ± 17° (consistent with other pre-Miocene units in the western Transverse Ranges). Magnetic stratigraphy and refined chronostratigraphy of the late Uintan and Duchesnean mammals found within redbeds of the Coldwater show that the upper Cozy Dell Shale-Coldwater Sandstone-Sespe Formation succession in central Ventura County spans Chrons C19r-C18n (approximately 39.5-42.5 Ma). These data do not corroborate the sequence stratigraphic interpretations of Campion et al. (1994) or Clark (1994). We suggest that local tectonic forces may have been more important than eustatic sea level fluctuations in causing sequence boundaries in the Coldwater and Sespe formations.

ABSTRACT

Our understanding of the early evolution of the Camelidae has long been confused by poor specimens, bad taxonomy, and dubious methodology, but the excellent specimens in the Frick Collection have clarified much of the confusion. The earliest known camelid is Poebrodon, from the early and late Uintan of Utah, Wyoming, and California. Hidrosotherium is not a camel at all, but a leptomerycid. Four valid species of Poebrotherium are recognized: P. chadronense, n. sp., and P. franki from the Chadronian of Texas; P. eximium from the Chadronian and early Orellan of the High Plains; and P. wilsoni, from the Chadronian to Whitneyan of the High Plains. “Poebrotherium” labiatum, from the early Orellan, is transferred to Paratylopus, which previously included only the type species, the Whitneyan camel Paratylopus primaevus. The long misunderstood camels known as “Protomeryx cedrensis,” “Protomeryx campester,” and “Paralabis matthewi” are now combined as Paralabis cedrensis.

The highly specialized, extremely hypsodont, gazellelike stenomyline camels of the late Arikareean-Barstovian have long been phylogenetically isolated, but Frick specimens show that Pseudolabis dakotensis (from the Whitneyan-Arikareean) and Miotylopus (from the Arikareean) are sister-taxa to the Stenomylini. Miotylopus includes three species: the small, Stenomylus-like M. leonardi; the medium-sized M. gibbi (including Dyseotylopus); and a large new species, M. taylori. Gentilicamelus sternbergi is the only valid species of this wastebasket genus, and is the sister-taxon of Nothokemas. The bizarre, long-snouted floridatragulines are closely related to higher camels.

ABSTRACT

Magnetostratigraphic sampling was conducted on key fossiliferous exposures of the upper Eocene-lower Oligocene (Chadronian-Whitneyan) White River Group in North Dakota, the Pine Ridge area of Nebraska, the North Platte River-Wildcat Ridge area of Nebraska and Wyoming, and the Cedar Creek area of northeastern Colorado. Together with results from other White River outcrops reported elsewhere, a general magnetostratigraphic pattern can be seen. Most sections are of predominantly reversed polarity, but there are two short zones of normal polarity in the middle and late Chadronian, another in the early Orellan, and another in the late Whitneyan. Based on 40Ar/39Ar dates and correlation to the Berggren et al. (1995) time scale, the middle Chadronian normal magnetozone correlates with Chron C16n (35.4-35.6 Ma), the late Chadronian normal magnetozone with Chron C15n (35.7-35.9 Ma), the early Orellan normal magnetozone with Chron C13n (33.0-33.5 Ma), and the late Whitneyan with Chron C12n (30.5-30.9 Ma). The Chadronian spans the interval from about 37 Ma to about 33.8 Ma; the Orellan from 33.8-32 Ma; the Whitneyan from 32 to about 30 Ma.

ABSTRACT

The Uinta Formation in northeastern Utah was the original basis of the Uintan land mammal “age.” Magnetostratigraphic studies were conducted in four sections in the northeastern, north-central, and northwestern Uinta Basin. The uppermost Evacuation Creek Member of the Green River Shale, and all of unfossiliferous Uinta Formation unit “A,” was of normal polarity. This normal interval probably correlates with Chron C21n (46.3-47.8 Ma), as originally suggested by Prothero and Swisher (1992). Most of Uinta “B” was reversed (= Chron C20r, 43.8-46.3 Ma). A short normal zone spanning upper Uinta “B” and lower Uinta “C” probably correlates with Chron C20n (42.5-43.8 Ma). The upper part of the Uinta “C” and the lowermost portion of the Duchesne River Formation were also reversed (= C19r, 41.4-42.5 Ma), with normal (= C19n, 41.1-41.4 Ma) and reversed (= C18r, 40.0-41.1 Ma) magnetozones in the higher part of the Brennan Basin Member.

Although the original biostratigraphic data for most Uinta Basin collections are very poor, distinctions between the faunas of Uinta “B1,” “B2,” and “C” are possible. Uinta “B1” (the “Metarhinus zone” of Osborn, 1929) spans the interval 45–46 Ma, and is characterized by overlapping ranges of the brontotheres Sthenodectes and Metarhinus, the rhinocerotoids Hyrachyus eximius, Forstercooperia grandis and Triplopus obliquidens, and the agriochoerid oreodont Protoreodon parvus. Uinta “B2” (the “Eobasileus-Dolichorhinus zone” of Osborn, 1929), including White River Pocket, spans the interval 43-45 Ma, and is characterized by the overlapping ranges of the brontotheres Sphenocoelus, Metarhinus, Eotitanotherium, the chalicothere Eomoropus, the horse Epihippus gracilis, the creodont Oxyaenodon, and the artiodactyls Diplobunops, Oromeryx, and Leptotragulus.

ABSTRACT

Detailed magnetostratigraphic and biostratigraphic studies of Eocene-Oligocene deposits in southwestern Montana have greatly refined the dating of the fossil mammal assemblages known from the region. In the Jefferson Basin, the middle Chadronian Pipestone Springs l.f. correlates with Chrons C15r to C16r1 (35.0-35.5 Ma). The assemblages known from the middle to late(?) Chadronian Little Pipestone Creek localities correlate at least in part with Chrons C15r to C16n1 (35.0-35.4 Ma). The late Chadronian West Easter Lily l.f. and the succeeding early Orellan Easter Lily l.f. span much of Chron C13r (33.5-34.3 Ma). In the Beaverhead Basin, the early Chadronian McCarty's Mountain fauna correlates with Chrons C16r1 to C17n1 (35.5-36.7 Ma). The late Duchesnean Diamond O Ranch l.f. may correlate with part of the long episode of normal polarity during Chron C17n. In the Sage Creek Basin, the late Uintan Dell beds have produced the stratigraphically successive Douglass Draw and Hough Draw local faunas, both of which appear to fall in Chron C18r (40.2-41.0 Ma). The Matador Ranch and Cook Ranch local faunas from the stratigraphically higher Cook Ranch Formation are correlated with the Orellan part of Chron C12r (32.0-33.0 Ma).

The mammalian local faunas of southwestern Montana strongly reflect the significant faunal provincialism that characterized western North America during the Eocene-Oligocene transition. A varying percentage (generally 20-40%) of the mammalian species known from the late Uintan through late Orellan local faunas of the region appear to have been endemic to southwestern Montana.