Journal of Paleontology, 90(1), 2016, p. 59–77 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2015.71
Systematics and paleobiogeographic significance of the Upper Ordovician pterygometopine trilobite Achatella Delo, 1935
Robert E. Swisher,1* Stephen R. Westrop,1 and Lisa Amati2†
1Oklahoma Museum of Natural History, and School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73072, USA ⟨
swestrop@ou.edu⟩ 2Department of Geology, SUNY Potsdam, Potsdam, New York 13676, USA
Abstract.—Study of type and new material of the pterygometopine Achatella Delo, 1935 demonstrates the presence of four species in Upper Ordovician (Katian) strata of Laurentian North America, A. achates (Billings, 1860) from the northeastern United States and the St. Lawrence lowlands of Canada, A. carleyi (Meek, 1872) from the Cincinnati region, Ohio and Kentucky, A. katharina (Bradley, 1930), from Missouri and Oklahoma, and A. clivosa Lespérance and Weissenberger, 1998 from the Gaspé Peninsula, Quebec. Perhaps as many as five additional species are present in Sandbian–Katian strata of the Laurentian terranes of Scotland and Northern Ireland, although only three of these are known well enough to code for phylogenetic analysis. The oldest pterygometopines, including species of Achatella,are known from Middle Ordovician strata of Baltica. Phylogenetic analysis supports a single migration event from Baltica from Laurentia, followed by a modest diversification in the latter region.
Introduction
Pterygometopine trilobites are well represented in Middle and Upper Ordovician strata of the western European countries of Baltica (Jaanusson and Ramsköld, 1993), but are restricted to species of the youngest genus, Achatella,Delo 1935, in Laurentian NorthAmerica. Achatella enters the succession in Laurentia during the Katian, but its record in Baltica extends down into Dariwillian strata (McNamara, 1980; Jaanusson andRamsköld, 1993)As such, the appearance of Achatella is generally viewed as a case of immigration (e.g., Ludvigsen and Chatterton, 1982) associated with the “Trenton transgression” (Shaw and Fortey, 1977; Ludvigsen, 1978b). Achatella becomes a persistent element in UpperOrdovician faunas of eastern and centralNorthAmerica, but the most recent revision (Ludvigsen and Chatterton, 1982) was based solely onmaterial fromthe Lake Simcoe andOttawa regions of Ontario and Quebec. In this paper, we offer a broader assessment of the genus from new and archival specimens from Oklahoma,Missouri, Illinois, Kentucky, and Ontario. Ludvigsen andChatterton’s (1982) study of Pterygometopidae
proposed a traditional, stratigraphic phylogeny in which Pterygometopinae was represented by Achatella, Pterygometopus, and Estoniops; Achatella was interpreted as a descendant of Pterygometopus. In this study, we assess the relationship between Baltic and Laurentian species attributed to Achatella with a computer-based parsimony analysis. This analysis will place hypotheses of immigration of Achatella on a firmer footing, and
* Present address: Department of Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, USA ⟨
rswisher@uchicago.edu⟩ † Present address: New York State Museum, Albany, New York 12230, USA ⟨
lisa.amati@
nysed.gov⟩
59
is a first step toward a more comprehensive revision of Family Pterygometopidae.
Stratigraphic setting
Achetella has a paleogeographic range that spans Laurentia and Baltica. Geographic and stratigraphic occurrences of Baltic species are reviewedbyJannusson andRamsköld(1993)and by Pärnaste et al. (2013). Distribution of Laurentian species is discussed below.
Ohio and Kentucky.—Achatella carleyi (Meek, 1872) was described fromKatian (Maysvillian) strata in theCincinnati, Ohio, region. The original collecting horizon was listed simply as “the Cincinnati group of the Lower Silurian,” but Foerste (1919) con- sidered A. carleyi to occur in the Fairmount Member of the Maysville Formation. In current nomenclature, the Fairmount is the younger of two members of the Fairview Formation and (Datillo et al., 2008, fig. 2) and lies in depositional sequence C2. New sclerites of A. carleyi were collected from older strata of the Fulton Submember at the base of the Kope Formation (Datillo et al., 2008, fig. 2) at the Blue Licks locality in northernKentucky (see Bulinski 2007). This new material is fragmentary and generally poorly preserved; it adds no new information on anatomy of the species and is not illustrated. However, they provide important data on stratigraphic occurrence and extend the range of A. carleyi down to the lower part of sequence C1.
Missouri and Illinois.—Achatella katharina (Bradley, 1930) is from the Upper Ordovician Kimmswick Limestone in this region and, contrary to Ludvigsen and Chatterton’s (1982) interpretation, it is a valid species rather than a junior synonym of A. achates (Billings, 1860). Bradley’s holotype and some
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