Late Eocene-Oligocene nonmarine mollusks of the northern Kishenehn Basin, Montana and British Columbia

Abstract

The Kishenehn Basin is a narrow, asymmetric graben, developed during an episode of crustal extension occurring from middle Eocene to early Miocene time . The Flathead listric normal fault system defines the northeast margin of the basin. The southwest margin is either bounded by antithetic normal faults or is onlapped by Kishenehn Basin strata. High rates of basin subsidence and synextensional sedimentation resulted in an immense thickness of middle Eocene-early Miocene strata (stratigraphic thickness, -5000 m) and a relatively uninterupted depositional record. Provenance and paleocurrent studies indicate that the ancestral Livingston, Lewis, and Clark ranges to the east contributed not only vast amounts of detritus to the Basin, but also ample runoff that included seasonal snow-melt. Sediment ages of outcrops within the basin are linearly banded, with the course of the Flathead River exposing late Eocene-early Oligocene sediments (Chadronian-Orellan NALMA's), and with the oldest sediments on the southwest and youngest on the east margins of the basin, respectively. An unnamed tephra at the north end of the basin provides a date of 33.0±1.0 Ma (early Oligocene, Orellan NALMA).The molluscan fauna is extremely diverse, 55 taxa (35 terrestrial, 20 aquatic), of which 32 taxa are new, and seven are revised taxonomically. This fauna is clearly divisible into three Groups. Group I is a relict tropical wet fauna, existing along waterways in the sheltered environment of the basin. It may typify the pre-middle Eocene (early Duchesnean NALMA) tropical climate of the Western Interior of North America. Modern analogs of these taxa are found in the Caribbean, Central and South America. Group II is a semitropical, semiarid fauna that developed after the climatic changes of the middle Eocene. Modern analogs are found in a band from the north coast of the Gulf of Mexico across the southern United States and northern Mexico to Southern and Baja California. Essentially 100% of the Group I and II analog taxa are now extra-limital, displaced to the south. Group III taxa developed in uplands along the east margin of the basin. Modern analogs of these taxa are common in the east-central United States. The coexistence of these climatically disparate groups of mollusks is a reflection of the large paleotopographic relief that existed between the basin floor and the eastern mountain peaks, which rose at least 2,000 meters above the basin floor, and permitted microclimates that varied from arid subtropical in the basin to humid temperate in the uplands. At the older localities. Group III taxa were, probably, transported into the basin by run-off from the mountains, but during the climatic cooling of the latest Eocene-early Oligocene, Groups I and II taxa were replaced by Group III taxa in the low-lands. Group III analog taxa are 87% endemic, although their modern centers of distribution are displaced eastward into an area of greater precipitation.The following taxa are described herein as new:

De novo genome assembly and genome skims reveal LTRs dominate the genome of a limestone endemic Mountainsnail (Oreohelix idahoensis)

“…3 A). Our results show that O. idahoensis split from C. unifasciata between 82.3 and 93.7 MYA (95% highest posterior density estimate), which is consistent with the earliest fossil evidence of Oreohelicidae in Cretaceous-Tertiary boundary deposits [ 41 ].

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Diverse fruits and seeds of the mid-Eocene Kishenehn Formation, northwestern Montana, USA, and their implications for biogeography

“…Ages estimates for the middle sequence of the Coal Creek Member are 46.2 ± 0.4 Ma based on 40 Ar/ 39 Ar analysis and 43.5 ± 4.9 Ma based on fission-track analysis, corresponding to the Lutetian stage of the Eocene (Constenius 1996). Pollen, mammal teeth and mollusks support these ages (Constenius et al 1989, Pierce and Constenius 2001, 2014, Harbach and Greenwalt 2012, Dawson and Constenius 2018. These dates coincide with the Challis Magmatic Episode (Dawson and Constenius 2018).…”

The fossil record of freshwater Gastropoda – a global review

“…Important centres of diversity in Europe include the Paris Basin in France (Pacaud & Le Renard, 1995; Pacaud, 2008) and the London and Hampshire basins in England (Wenz, 1923–1930; Symonds, 2002; Munt, 2014). In North America, a diverse Eocene fauna was reported from the Kishenehn Basin in Montana and British Columbia (Pierce & Constenius, 2001, 2014). Numerous rich faunas have furthermore been documented from China, specifically from Anhui (Yü et al ., 1982), Hebei (Yü & Pan, 1982), and Shandong (Youluo, 1978).…”