752
Journal of Paleontology 92(5):751–767 A trilobite family that is particularly dominant in the fauna
of the biomere gives the biomere its name, thus the biomere coinciding with the Sunwaptan Stage is the Ptychaspid biomere (Longacre, 1970) and the Skullrockian Stage corresponds to the Symphysurinid biomere (Stitt, 1983). These extinctions and the environmental changes associated with them have been traced globally (Öpik, 1966, 1967; Miller, 1984; Saltzman et al., 2000; Westrop and Adrain, 2013; Landing and Westrop, 2015). The timing of these extinction events suggests that they
played some role in ending the Cambrian Explosion, and with their cessation, allowed the Great Ordovician Biodiversification Event to proceed (Adrain, 2010; Saltzman et al., 2015), but despite much work on the nature of the sedimentary and trilobite fossil record across these intervals (e.g., Ludvigsen and Westrop, 1983; Westrop, 1988, 1989, 1990; Loch et al., 1993; Osleger and Read, 1993; Saltzman, 1999; Taylor et al., 1999, 2004; Westrop and Cuggy, 1999; Adrain et al., 2009; Saltzman et al., 2004; Landing et al., 2011) and the geochemical sig- natures of the events (e.g., Saltzman et al., 1995, 1998, 2000, 2004; Perfetta et al., 1999; Saltzman, 1999; Montañez et al., 2000; Taylor et al., 2004; Gill et al., 2011; Saltzman et al., 2015; Gerhardt and Gill, 2016) their cause remains unclear. Incomplete documentation of the effect the events
had on other members of the Cambrian benthic community hampers our understanding of these extinctions. Linguliform brachiopods are members of the Cambrian Fauna along with trilobites (Sepkoski, 1979, 1981) and are abundant in Cambrian strata, yet few studies have considered the impact of the extinction on brachiopods (Freeman and Stitt, 1996; Westrop, 1996; Landing and Westrop, 2015). Rowell and Brady (1976) suggested that linguliform brachiopods were not significantly affected by the events causing repeated extinction among trilobites.
Field area and methods
Our brachiopod collections are from limestones deposited in a shallow subtidal environment on the Laurentian platform (Lochman-Balk, 1970; Barnes and Bell, 1977; Osleger and Read, 1993). The samples were collected from the Cambrian– Ordovician Wilberns Formation (uppermost Point Peak Mem- ber and lowermost San Saba Member) and Ordovician Tanyard Formation exposed around the Llano Uplift in central Texas (Fig. 1). The Wilberns Formation includes abundant grainstones, indicating high-energy, shallow conditions. Brachiopods from samples of the Wilberns Formation are generally fragmental. The overlying type section of the Threadgill Member of the Tanyard Formation is primarily lime mudstone and yielded specimens that were somewhat more complete, although fre- quently crushed. The interval includes parts of two North American upper Cambrian–lowermostOrdovician stages (Fig. 2), the Sunwaptan and Skullrockian stages, approximately correlating to the global Cambrian Furongian stages 9–10 and the Lower Ordovician Tremadocian Stage. Samples from these strata were collected at two measured
sections that form a composite section. The sections are on two private ranches near Doss, Texas (Fig. 1), and permission from the owners is necessary to visit them. The section mea- surements in meters correspond to previously published section
QK
COws Ott K
783
Quaternary and Cretaceous deposits, undifferentiated
Cretaceous strata, undifferentiated
Threadgill Member of Tanyard Formation
San Saba Member of Wilberns Formation
Paved Ranch Roads
648 Lange
LR Pathway of Lange
Ranch Section
Unpaved Pasture Road
Crenwelge Managers/owners of Lange Ranch
property
Pathway of Threadgill Creek
Section TC
Cr Cr 3373000mN QK TC
Crenwelge Lange
LR 783 QK Ott LR COws COws Ott 10 Ott QK QK QK
0 0
3370000mN Feet
Meters SCALE
4000 1000 3369000mN QK QK DOSS 648 to Hilltop UTM ZONE 14
Figure 1. Location of two measured sections near the Llano Uplift. LR is Lange Ranch Section; TC is Threadgill Creek Section.
measurements (reviewed in Miller et al., 2012). The accessible part of the Threadgill Creek section (TC) under consideration begins at 30.485309N, -99.121655W (Datum WGS84), although only the upper part of the section was collected for this project. The Lange Ranch section (LR) begins a little farther upstream on Threadgill Creek at 30.474924N, -99.123782W and continues up Mormon Creek. These sections are part of the Threadgill Creek section measured and described by V.E. Barnes (Bridge et al., 1947) and
Welge Ranch
3371000mN QK COws K QK 3372000mN COws Cwp QK 7
Cwwm Cwp
Cr
Point Peak Member of Wilberns Formation
Welge and Morgan Creek members of Wilberns Formation
Riley Formation
Complexly faulted Cambrian strata
Strike and dip of Paleozoic strata
7 TEXAS
Study Area
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488000mE
489000mE
489000mE
489100mE
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Mormon
Langes Mill Road
Threadgill Creek
Creek
>>>>
Ott
Jackrabbit Road
Cwwm
Cwp
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