Journal of Paleontology, 91(6), 2017, p. 1123–1147 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2017.74
Early athyride brachiopod evolution through the Ordovician-Silurian mass extinction and recovery, Anticosti Island, eastern Canada
Paul Copper,1 and Jisuo Jin2
1Loupicoubas, 46220 Prayssac, France ⟨
pcopper@laurentian.ca⟩ 2Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada ⟨
jjin@uwo.ca⟩
Abstract.—The subfamily Hindellinae is an early group of athyride brachiopods, characterized by a simple jugum that connects the laterally directed spiralia, which are disjunct from the crura. Four genera (Hindella, Cryptothyrella, Koigia, and Hyattidina) are reexamined on the basis of their internal structures, such as the crura and their connection to the hinge, the jugum, and spiralia. The internal brachidium and shell of the Aeronian genus Cryptothyrella differ substantially from those of Hindella. Elkanathyris pallula n. gen. n. sp. is recognized as a posteriorly ribbed hindellide of Aeronian age. These genera are transferred from the Meristellinae to the subfamily Hindellinae (family Hindellidae). On Anticosti Island, Hindella is confined to the Hirnantian (latest Ordovician): it became extinct at the end Ordovician during the last of several mass extinction events that also extinguished the Laframboise reefs at the top of the Ellis Bay Formation. Post-extinction recovery of athyrides was pioneered by small-shelled Koigia, which are abundant in the basal Silurian Becscie Formation. Hyattidina, with a simple brachidium, is abundant in the Aeronian and Telychian of Antic- osti, but absent earlier. True meristellines, as envisioned here, first appeared in the Aeronian Gun River Formation. The revised taxonomy and stratigraphic ranges of these earliest athyrides shed light on the nature of the Ordovician–Silurian mass extinction and recovery, and help refine the biostratigraphy of the O-S boundary interval.
Introduction
The Ordovician culminated in one of the major Phanerozoic mass extinctions, ranked roughly fourth in severity, equivalent to the Cretaceous-Paleogene boundary mass extinction (Alroy, 2008, 2010a, b). Mass extinctions due to multiple glaciations in Gondwana severely affected the tropical coral-sponge reef ecosystem in the Late Ordovician (Copper, 2002, 2011; Webby, 2002), and its concomitant tropical shelly faunas, in which athyride brachiopods played a significant role. Several extinc- tion events mark the Ordovician-Silurian (O-S) boundary section on Anticosti, as evident within the Hirnantian Ellis Bay Formation (Copper et al., 2013). The Hirnantian carbonate- dominated succession, ~80m thick, was deposited over some two million years and marked the arrival of a rich and diverse suite of early spire-bearers (atrypides, hindellides, but no spiriferides), not seen in the Katian Vaureal Formation below. These all suffered losses at the end of the Hirnantian. The general recovery of brachiopod shelly faunas is recorded in the lower Silurian for Anticosti (Copper and Jin, 2012, 2014, 2015). The earliest shelly community of the Becscie Formation (Rhuddanian) was characterized by low diversity and small shells such as Koigia, described herein (Fig. 1). The upper Becscie Formation was marked by the appearance of the large-shelled pentameride Virgiana community, which became ubiquitous in Laurentia during the late Rhuddanian (Jin et al., 1996; Jin and Copper, 2000). Major diversification of Silurian-type athyrides, atrypides, and pentamerides began later in the Aeronian and Telychian.
Considerable confusion exists about the richly fossiliferous
transitional Ordovician-Silurian boundary interval on Anticosti, and where to draw the boundary itself (Copper et al., 2013). The drastic environmental changes were reflected by critical evolu- tion of the tropical marine faunas, such as those well preserved in the carbonate platforms of Baltica and Laurentia. Different species of spire-bearing athyrides and atrypides have, in the past, been variously assigned to the Late Ordovician or early Silurian, or sometimes to both. This study aims to clarify the morphology, evolution, and distribution of such key taxa in the Hirnantian and Rhuddanian, and provide an update and revision of the taxonomy proposed in the Treatise (Alvarez and Rong, 2002). Athyrides were late arrivals in the spire-bearing brachiopod
orders in Laurentia and Baltica during the Late Ordovician, and did not become major components of the benthic shelly fauna until the Hirnantian. On Anticosti Island, the genus Hindella (Figs. 2, 3) was an abundant component of the brachiopod fauna and locally formed shell beds in the Ellis Bay Formation. In their interpretation of athyride evolution, Alvarez et al. (1998, p. 834–835) regarded Dayia, with laterally directed spiralia and a simple jugum, as derived from Lissatrypa via lateral compression of the muscle field, thus regarding the orientation of the spiralia as insignificant, although the spiralia in these two genera have opposite directions (Copper, 1986; Copper and Gourvennec, 1996). They also viewed the laterally directed double spiralia of the Coelospirinae as compatible with atrypoid affinities (Alvarez et al., 1998, p. 836). These authors assigned Hindella to the Silurian Meristellinae (superfamily Mer- istelloidea), and considered Cryptothyrella as its junior
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