Journal of Paleontology, 92(5), 2018, p. 751–767 Copyright © 2018, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2018.8


Linguliform brachiopods across a Cambrian–Ordovician (Furongian, Early Ordovician) biomere boundary: the Sunwaptan–Skullrockian North American Stage boundary in the Wilberns and Tanyard formations of central Texas


Rebecca L. Freeman,1 James F. Miller,2 and Benjamin F. Dattilo3


1Department of Earth & Environmental Science, University of Kentucky, Lexington, KY 40506, ⟨rebecca.freeman@uky.edu⟩ 2Geography, Geology, & Planning, Missouri State University, Springfield, MO 65897, ⟨jimmiller@missouristate.edu⟩ 3Department of Biology, Indiana University Purdue University Fort Wayne, Fort Wayne, IN 46805-1499, ⟨dattilob@ipfw.edu⟩


Abstract.—The Cambrian-Ordovician Diversity Plateau, between the Cambrian Explosion and the Ordovician Radia- tion, is punctuated by a series of well-documented Laurentian trilobite extinction events. These events define the bound- ing surfaces of trilobite ‘biomeres’ that correspond to North American stages, including those of the Sunwaptan and Skullrockian. Trilobites show a consistent pattern of recovery across these boundaries, and commonly each extinction and replacement of taxa is interpreted as a single event as changing environmental conditions spurred shoreward migra- tion of shelf or oceanic faunas that displaced established cratonic faunas. Linguliform brachiopods are also abundant in strata of this interval, and we investigate their stratigraphic distribution across the Sunwaptan–Skullrockian Stage bound- ary in Texas through high-resolution stratigraphic sampling of subtidal sediments. We document complete genus- and species-level turnover of the linguliform brachiopod fauna coincident with trilobite extinction events, suggesting that these brachiopods were affected by the same factors that affected trilobites. The Skullrockian replacement fauna was cosmopolitan, with ties to Gondwana and Kazakhstan and to the Laurentian shelf environment. The timing of appear- ances of taxa suggests that the faunal migration onto the Laurentian shelf came from elsewhere during a transgression. The disappearance of the Sunwaptan fauna and the arrival of the Skullrockian fauna are distinct events. We suggest that ‘biomere’ events may be complex, and the cause of the extinction is not necessarily the same event that facilitates the appearance of a replacement fauna.We describe one new species, Schizambon langei.


UUID: http://zoobank.org/f6a5c11f-cd3d-4c16-b184-d808d3a5285f Introduction


The late Cambrian–earliest Ordovician was a time of decreased to stagnant global diversity (e.g., Sepkoski, 1978, 1979, 1981, 1995, 1997; Bambach et al., 2004; Smith et al., 2012). This “Cambrian Diversity Plateau” followed the Cambrian Explosion and preceded the Great Ordovician Biodiversification Event (Servais et al., 2010). The cause of the global diversity plateau is unclear, but repeated trilobite extinctions that affected shelf communities punctuate this interval in Laurentia (e.g., Lochman and Duncan, 1944; Lochman-Balk and Wilson, 1958; Palmer, 1965, 1979, 1984; Longacre, 1970; Stitt, 1971a, b, 1975, 1977, 1983; Westrop and Ludvigsen, 1987; Westrop, 1988, 1989, 1990, 1996; Loch et al., 1993; Westrop and Cuggy, 1999; Taylor et al., 2004; Adrain et al., 2009; Saltzman et al., 2015). The term ‘biomere’ refers to the stratigraphic packages bounded by these extinctions (Palmer, 1965; review in Taylor, 2006). Directly above the strata recording the extinction event are thin ‘critical periods’ (Westrop and Ludvigsen, 1987) or ‘critical intervals’ (Taylor, 2006) dominated by one seemingly opportunistic trilobite species. These intervals


include taxa not related to genera found in strata below and are apparent migrants from deeper water environments (e.g., Palmer, 1979). The bases of these critical intervals are used to define North American stage boundaries (Ludvigsen and Westrop, 1983), although disagreement exists as to whether these intervals represent the last stage of one biomere (Stitt, 1971b, 1975, 1983; Taylor, 2006) or the first stage of the overlying biomere (Palmer, 1979). Further controversy has centered on whether the extinctions were caused by changing environmental conditions, such as water temperature or oxygen level (e.g., Lochman and Duncan, 1944; Stitt, 1971b, 1975, 1977; Palmer, 1984; Loch et al., 1993; Perfetta et al., 1999; Gill et al., 2011; Saltzman et al., 2015; Gerhardt and Gill, 2016) or were due to a changing distribution and merging of biofacies associated with sea level rise (e.g., Ludvigsen and Westrop, 1983; Westrop and Ludvigsen, 1987; Westrop, 1996; Westrop and Cuggy, 1999). Implicit in many of these hypotheses for the extinction events is the idea that the same changing environmental conditions that caused the extinction also facilitated the migration of trilobite species, giving rise to replacement faunas.


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