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Journal of Paleontology 90(1):59–77
paratypes were collected near Batchtown, Calhoun County, Illinois, but other paratypes are from Glen Park, Jefferson County, Missouri. We were unable to gain access to the abandoned quarries at Glen Park, and could not locate the Batchtown site. New sclerites of A. katharina were collected from two
previously unstudied localities that will be described in detail elsewhere. Section M was measured and sampled at a large road cut through the uppermost Decorah and Kimmswick formations along County Road M, Jefferson County, Missouri, 200m southeast of the intersection with Old Lemay Ferry Road, about 6.5kmwest of the intersection with Interstate 55, and about 8.5km west of Bradley’s (1930)Glen Park
locality.Materialwas collected about 16meters fromthe base of the section. Section 79-Mwas at a low road cut along State Highway 79, Lincoln County, Missouri, approximately 5.25km south of the village of Elsbury, about 1.25km north of the intersection with County Road MO-M, and about 13km northwest of Bradley’s (1930) Batchtown locality. Sclerites of Achatella were collected 1m above the base of the section. At both of the new sections, the sampled horizons are likely near the base of the C1 sequence (Swisher, 2015).
Oklahoma.—Amati and Westrop (2006) reported Achatella from the Upper Ordovician (latest Sandbian–Katian) Viola Springs Formation as part of their Thaleops Biofacies, and further study demonstrates that it represents A. katharina. The species is present in the Katian reference section at State Highway 99, Pontotoc County (Goldman et al., 2007; see Amati, 2014, for a section log), ranging from 38 to 51m above the base of the Viola Springs, and probably falls in sequence M6 and, possibly C1 (Westrop et al., 2012; Swisher, 2015; see Young et al., 2005 for an alternative interpretation). As such, the occurrence of A. katharina probably overlaps with the range in Missouri, where it was sampled from what is likely the lower part of sequence C1.
New York, Ontario, and Quebec.—The type species, Achatella achates (Billings, 1860), was initially reported from the “Trenton Limestone” in Ottawa, Ontario, and subsequent work shows that this occurrence lies within upper Trenton Group strata (Ludvigsen and Chatterton, 1982) that are currently assigned to the Lindsay Formation (Dix et al., 2007, fig. 10); archival material at the Royal Ontario Museum from the other side of the Ottawa River in Hull, Quebec, is simply listed as from the “Trenton Group.” Achatella achates is also known from the underlying Verulam Formation (Wilson, 1947; Ludvigsen and Chatterton, 1982). Archival material from the Royal Ontario Museum included in this study is from the Verulam Formation at various quarries in the Lake Simcoe region, Ontario, about 285km southwest of Ottawa. These include the well-known quarries at Gambridge, and Lakefield (McFarland et al., 1999). The range of A. achates extends into New York as well, with occurrences reported from the Rust Formation (Brett et al. 1999). A second species, Achatella clivosa Lespérance and
Weissenberger, 1998, occurs in the Late Ordovician (late Katian; Ashgill) Pabos Formation and the informal, correlative Grande Coupe beds, of the Percé area, Quebec.
Laurentian Britain.—At least five species of Achatella have been reported from the Laurentian terranes (Chew and Strachan, 2013, fig. 2) of Scotland and Northern Ireland (Morris, 1988, p. 11), although only three of these are known well enough to merit discussion. In the Girvan area of Scotland, the mostly deep water, Middle to Upper Ordovician succession of the Midland Valley Terrane is a cover sequence above the Ballantrae ophiolite complex. The latter was emplaced on the Laurentian margin during Early-Middle Ordovician continent–arc collision (Chew and Strachan, 2013). Achatella consobrina (Tripp 1954) is from the Kiln Mudstone Member of the Craighead Limestone (Tripp, 1980b), which is assigned to the Dicranograptus clingani graptolite zone (Tripp, 1980a). This zone correlates into the lower part of the Katian Stage and the M5 or M6 depositional sequences of eastern Laurentia (Goldman et al., 2007, fig. 2). As such, A. consobrina is approximately the same age as A. achates and A. katharina from eastern and central Laurentia. Achatella retardata (Reed, 1914) is younger than A. consobrina, occurring in the Lady Burn Starfish Beds of the South Threave Formation (Morris and Tripp, 1984; see Harper, 1982 for a review of the stratigraphy). According to Harper (1982), the South Threave Formation falls in the Rawthyan Stage of the Ashgill Series of British nomenclature, which correlates with the upper Katian (Bergström et al., 2009, fig. 1). This means that A. retardata is closest in age to A. clivosa from Quebec. Even higher in the Girvan succession is a species from the latest Ordovician (Hirnantian) High Mains Formation, identified by Owen (1986) as A. cf. truncatocaudata (Portlock, 1843). This represents the youngest occurrence of the genus. Achatella truncatocaudata was originally described
(Portlock, 1843) from the Killey Bridge Beds in County Tyrone, Northern Ireland, which is also part of the Midland Valley Terrane (Chew and Strachan, 2013). This unit is assigned to the Cautleyan and, probably, Rawthyan stages of the Ashgill Series (Cripps, 1988; Harper and Parkes, 2000) and thus falls in the upper Katian. A single cephalon from the Knockerk House Sandstone Member of the Knockerk Formation, Gangreeth Terrane, was assigned questionably to A. truncatocaudata by Romano and Owen (1993, fig. 5A, F). As Romano and Owen recognized, this specimen is characterized by anteriorly positioned eyes and in our view records a distinct species. The Grangegeeth Terrane is currently interpreted as a peri-Laurentian volcanic arc (McConnell et al., 2010), and the Knockerk House Sandstone Member was correlated into the lower part of the Laurentian Mohawkian Series by both Romano and Owen (1993) and Harper and Parkes (2000). This indicates that “A. truncatocaudata” from the Grangegeeth Terrane is likely latest Sandbian in age (e.g., Bergström et al., 2009, fig. 1), and is therefore significantly older than the type material from County Tyrone. It is also older than A. achates and A. katharina, the earliest species from Laurentian North America.
Phylogenetic analysis
Taxon selection and coding sources.—All species of Achatella from Laurentian North America were coded fromexamination of
type or other archival material and sclerites from new field collections (Figs. 3–12), with the exception of A. clivosa, which is based on images published by Lespérance and
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