Copper and Jin—Ordovidian–Silurian brachiopod evolution, extinction, and recovery Koigia occurs in the basal Becscie Formation, within ~1m


of thin- to medium-bedded, hard, micritic mudstone and wack- estone that overlie the Laframboise reefs at the O-S boundary (Copper and Jin, 2014). It is a much smaller-shelled athyride compared to Hindella in the Ellis Bay Formation, although their internal structures are similar. Koigia is locally abundant, alongside other Rhuddanian brachiopods, such as Zygospir- aella, Becscia, and Viridita. In Estonia, the type species of Koigia also occurs in the basal Rhuddanian (Rubel, 1970 initi- ally cited it as Hindella). So far, Cryptothyrella has not been found on Anticosti


Island, although some athyride specimens from the Aeronian Gun River and Menier formations (Copper and Jin, 2012; Copper et al., 2012) may be assignable to the genus, pending further study. The type species, Cryptothyrella quadrangularis (Foerste, 1906), as seen in Ohio, is characterized by an unu- sually large and prominently elongate shell (30–40mm in length). The Rhuddanian ‘Cryptothyrella benthic community’, recognized by Cocks and McKerrow (1973, p. 293) for the platform setting in Laurentia and Baltica, has not been observed on Anticosti Island, nor has its presence been confirmed in the Rhuddanian of Laurentia. Hyattidina is very abundant in the upper Gun River,


Menier, and Jupiter formations, ranging from mid-Aeronian to mid-Telychian. The genus is absent from the older Becscie, Merrimack, and lower Gun River formations (Fig. 1). It retained the simple jugum and brachidium of Hindella, but has a well- developed fold and sulcus.


The early athyride shelly community on Anticosti Island


The carbonate sediments of Anticosti were deposited in the northern paleotropical latitudes, on a platform to ramp flanking the southeast side of Laurentia (Copper, 2002; Cocks and Torsvik, 2011). Strata are undeformed with dips <2° today. During the Late Ordovician and early Silurian, Baltica was directly to the east at a similar paleolatitude, with an ocean ~1000–1500km wide separating it from Laurentia. To the northeast, Siberia was mostly north of the paleoequator. In the Anticosti Basin, siliciclastic sediments were rare, and consisted of episodic storm-generated or seismic deposits, marked by slumped beds, mostly during the late Katian and Hirnanian. Wet coastal climates created an epeiric sea of mixed salinities (similar to the Java and Arafura epicontinental seas today) that affected the distribution of shelly and coral faunas in the early Silurian (Edinger et al., 2002). Facies differences between the east and west ends of Anticosti reflect a curving shoreline along the 200km long outcrop belt. In such a carbonate-dominated depositional setting of the


Anticosti Basin, athyrides formed common shell clusters, or extensive shell beds. Their minute pedicle, as indicated by the small apical to trans-apical foramen, and common co- occurrences with small and delicate bryozoans or broken shells suggest that they anchored on skeletal clasts in the sedi- ments. In the reefal Laframboise Member, Hindella was gen- erally rare or only locally abundant (such as locality A1161), but in the reefal East Point Member (Aeronian) athyrides are


1125


generally common. This may indicate a later adaptation to shallow-water, higher-energy, reefal settings. The athyrides were more common in mid-shelf settings,


and rare in deeper waters of the Clorinda-Dicoelosia commu- nity found in the Menier and Jupiter formations. Hindella habitats varied from deeper muddy seafloors (alongside solitary rugosans or bryozoans), to somewhat shallower, but still low- energy, carbonate substrates (where it was commonly mono- specific), and extending into shallow and high-energy reefal settings. In the Parastro Member, a relatively small form of Hindella (H. bulbusa n. sp.) occurs as a common component of the Parastrophinella pentameride association (Jin and Copper, 2008). The small shells (<7mm in width) of Koigia occur


commonly in higher energy, storm-influenced settings represented by the Becscie Formation, especially in the lower Fox Point Member, associated with other small-shelled taxa (e.g., Becscia and Viridita; see Jin and Copper, 2010). The small shells usually show various degrees of distortion, damage, and disarticulation, with common geopedal structures in conjoined shells, indicating rapid burial by micritic mud during storms (for more discussions on the depositional environments, see Copper and Jin, 2014). Koigia may be regarded as an opportu- nist that thrived immediately after the Late Ordovician mass extinction events, but athyrides became scarce in the upper Becscie Formation (Chabot Member) when the monotypic, large-shelled Virgiana brachiopod community became dominant. On Anticosti, Hyattidiina and Elkanathyris n. gen. occur


as common components of the Pentamerus community, indi- cating a mid-shelf depositional environment (Jin, 2008). They may also be associated with rich and diverse atrypides. This agrees with the treatment of the Hyattidina community as equivalent to the Pentamerus or Stricklandia communities in the Welsh Borderlands (Cocks and McKerrow, 1973). It is rela- tively rare in the Eocoelia community.


Materials and methods


The basis of the paper is the stratigraphic work that covers ~2000 Anticosti localities, located on metric grid maps and with GPS coordinates (Copper et al., 2013; Copper and Jin, 2014, 2015). The large brachiopod collection is stored at the Geological Sur- vey of Canada, Ottawa. Well-preserved, pristine, calcitic speci- mens were serially sectioned with a Croft Parallel Grinder. Acetate peels were taken at 0.1mm intervals, mounted between 35mm glass slides, and examined and photographed under microscope. To prepare the serial section drawings, the peels were projected to a scale of x16 or x20, with the main internal features traced in ink and then scanned. The technique for reconstruction of the brachidia uses peels transposed into a view of the dorsal shell interior, employing the plane of symmetry as orientation (technique described in papers from 1967 and earlier, accurate to within a millimeter, and available from first author).


Repositories and institutional abbreviations.—Figured speci- mens are housed in the Type Collections of the Geological Survey of Canada (GSC), Ottawa, the Cincinnati Museum


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