USING PALEOSOL CARBONATE TO LOCATE THE EARLY EOCENE H2 HYPERTHERMAL IN A MCCULLOUGH PEAKS FOSSIL LOCALITY, BIGHORN BASIN, WYOMING

The mammal fauna of the Willwood Formation, central Bighorn Basin, Wyoming, is ideal for paleoecological analysis because it is extensive, well studied, and continuously distributed over sediments representing the first 3 Myr of the early Eocene. The geology of the Bighorn Basin is also well known, providing a precise temporal framework and climatic context for the Willwood mammals. Previous analysis identified three “biohorizons,” based on simple counts of the first and last appearances of species. This study uses species diversity and appearance rates calculated from more extensive collections to approximate the ecological dynamic of the ancient fauna and assess whether the biohorizons were significant turnover events related to recently described climatic variation. Diversity and appearance data collected for this project are extensively corrected for uneven sampling, which varies by two orders of magnitude. Observed, standardized appearance and diversity estimates are subsequently compared with predicted background frequencies to identify significant variation. Important coincident shifts in the biotic parameters demonstrate that ecological change was concentrated in two discrete intervals ≤300 Kyr each that correspond with two of the original biohorizons. The intervals coincide with the onset and reversal of an episode of climate cooling identified directly from Bighorn Basin floras and sediments. Ecological changes inferred from the diversity and turnover patterns at and following the two biohorizons suggest short- and long-term faunal response to shifts in mean annual temperature on the order of 5–8°C.

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Palynofloral dynamics of the late Palaeocene to early Eocene from the Bighorn Basin, northwest Wyoming, USA

GFF ◽

10.1080/11035890001221067 ◽

2000 ◽

Vol 122 (1) ◽

pp. 67-68 ◽

Cited By ~ 1

Author(s):

Guy J. Harrington

Keyword(s):

Bighorn Basin ◽

Early Eocene

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Cladogenesis and replacement in the fossil record of Microsyopidae (?Primates) from the southern Bighorn Basin, Wyoming

Biology Letters ◽

10.1098/rsbl.2020.0824 ◽

2021 ◽

Vol 17 (2) ◽

Author(s):

Mary T. Silcox ◽

Keegan R. Selig ◽

Thomas M. Bown ◽

Amy E. Chew ◽

Kenneth D. Rose

Keyword(s):

Fossil Record ◽

Evolutionary Change ◽

Local Extinction ◽

Bighorn Basin ◽

Early Eocene ◽

Evolutionary Transitions ◽

Continuous Record ◽

Extinction Event ◽

Nearly Continuous

The early Eocene of the southern Bighorn Basin, Wyoming, is notable for its nearly continuous record of mammalian fossils. Microsyopinae (?Primates) is one of several lineages that shows evidence of evolutionary change associated with an interval referred to as Biohorizon A. Arctodontomys wilsoni is replaced by a larger species, Arctodontomys nuptus , during the biohorizon interval in what is likely an immigration/emigration or immigration/local extinction event. The latter is then superseded by Microsyops angustidens after the end of the Biohorizon A interval. Although this pattern has been understood for some time, denser sampling has led to the identification of a specimen intermediate in morphology between A. nuptus and M. angustidens , located stratigraphically as the latter is appearing. Because specimens of A. nuptus have been recovered approximately 60 m above the appearance of M. angustidens , it is clear that A. nuptus did not suffer pseudoextinction. Instead, evidence suggests that M. angustidens branched off from a population of A. nuptus , but the latter species persisted. This represents possible evidence of cladogenesis, which has rarely been directly documented in the fossil record. The improved understanding of both evolutionary transitions with better sampling highlights the problem of interpreting gaps in the fossil record as punctuations.

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Sandstone body character, river styles, and geomorphology of the lower Eocene Willwood Formation, Bighorn Basin, Wyoming, USA

10.31223/x5qp63 ◽

2021 ◽

Author(s):

Youwei WANG ◽

Timothy Baars ◽

Hiranya Sahoo ◽

Joep Storms ◽

Allard Martinius ◽

...

Keyword(s):

Water Discharge ◽

Bighorn Basin ◽

Early Eocene ◽

Average Width ◽

Lower Eocene ◽

Braided Rivers ◽

Braided Channel ◽

Facies Associations ◽

Sinuous Channel ◽

Willwood Formation

The lower Eocene Willwood Formation of the intermontane Bighorn Basin, Wyoming, USA, is an alluvial red bed succession with a sand content of ca. 20%-25%. The formation has been studied intensively for paleontology, paleoclimate, and sedimentary reconstruction. However, alluvial sandstone bodies and their corresponding river styles remain little characterized and documented. Here, efforts are made to study the characteristics and river styles of sandstone bodies through ca. 300 m of alluvial stratigraphy in the McCullough Peaks outcrop area based on the field data and a georeferenced 3-D photogrammetric model. Four channel facies associations are recognized, and they are ascribed to four river planform styles: distributary channel, massive trunk-shaped channel, braided channel, and sinuous channel, with the latter two styles being the more abundant. The channel sandstone bodies that show the character of sinuous rivers and those of braided rivers differ significantly in average thickness (6.1 m versus 9.0 m) and insignificantly in average width (on average 231 m) and paleoflow directions (on average N003). Braided-character dominated and sinuous-character dominated river styles are seen to alternate in the outcrop, while they show no spatial dependency in the 10 km2 study area. Bighorn Basin margins varied in the early Eocene, with differing tectonic, geological, and topographic characteristics. The observed mixture of river styles may be attributed to differential influences of axial and transverse river systems and/or climate change that controls water discharge and sediment load. An early Eocene geomorphologic reconstruction is constructed summarizing these new and earlier results.

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