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Journal of Paleontology 90(1):59–77
of values. All measurements were made on digital images to the nearest tenth of a millimeter using the Measure Tool of Adobe Photoshop. To maximize depth of field, all digital images were
rendered from stacks of images focused at 100- to 500-μm intervals using Helicon Focus 4.0 for the Macintosh <http://
www.heliconsoft.com>. All specimens were coated with a sublimate of ammonium chloride prior to photography.
Family Pterygometopidae Reed, 1905
Subfamily Pterygometopinae Reed, 1905 Genus Achatella Delo, 1935
Diagnosis.—Cephalon relatively flat with a weak anterior arch [1(1)]. L3 lobe subtriangular with conspicuous adaxial taper [7(1)], and longer (exsag.) than L1 and L2 [8(1)]. Posterior branch of facial suture nearly straight [16(1)]. Pygidium elon- gate, subtriangular in outline.
Figure 2. Results of a phylogenetic analysis of a character matrix (Table 1; see appendix for list of characters) for Achatella species. (1) Strict consensus of 42 trees discovered with a branch-and-bound search (implicit enumeration). Support metrics (calculated in TNT; Goloboff et al., 2008) show by numbers are Bremer support (boldface; only values >1 are shown), GC bootstrap support (italics) and conventional bootstrap support (roman font). Shaded region comprises species of Achatella as diagnosed in this study. B, species from Baltica; L, species from Laurentian North America and Britain; (2) optimized character distribution plotted on one of the most parsimonious trees. Numbers above circles refer to characters and those below identify particular states (Appendix). Only unambiguous transformations (i.e., optimize to the same nodes under the assumptions of both ACCTRAN and DELTRAN) are shown; filled circles show states that originate at a single node, and open circles indicate states that are homoplastic.
species possess a posterior branch of the facial suture that is nearly straight [16(1)]. Achatella kuckersianus (Schmidt, 1881), the type species of of A.(Vironiaspis) Jaanusson and Ramsköld, 1993, is part of a polytomy in our strict consensus tree and, accordingly, we leave Achatella undivided at the subgeneric level. The Laurentian species form a distinct, derived group within Achatella,with the representatives from Baltica lying in a basal position (Fig. 2.1). This result is consistent with a single invasion of Laurentia with subsequent radiation, rather than multiple invasions of species from Baltica.
Systematic paleontology
Repositories for figured material are indicated by the following acronyms:CM, Cincinnati Museumof NaturalHistory,Cincinnati, OH; UC, Field Museum of Natural History, Chicago, IL; GSC, Geological Survey of Canada, Ottawa; OU, Oklahoma Museum of Natural History, Norman; ROM, Royal Ontario Museum, Toronto. Morphological terminology follows Whittington et al. (1997);
however, the occipital ring is designated LO (lobus occipitalis) and the occipital furrow is designated SO (sulcus occipitalis). Proportions expressed as percentages in the descriptions and diag- noses are means, with numbers in parentheses indicating the range
Remarks.—In our analysis, Achatella is a clade that includes both Baltic and Laurentian species, with species from the former continent occupying a basal position in the cladogram (Fig. 2). Jaanusson and Ramsköld (1993) named a new subgenus of Achatella, A.(Vironiaspis), with Phacops (Pterygometopus) kuckersianus Schmidt, 1881, from Baltica (northern Estonia), as the type species. As this species, the only one assigned to A.(Vironiaspis)with any confidence (Jaanausson and Ramsköld, 1993, p. 766), forms part of a polytomy at the base of Achatella in the strict consensus tree (Fig. 2.1), subgenera are not used in this paper. Three characters states are unambiguous synapomorphies
of Achatella in the phylogenetic analysis (Fig. 2.2): a relatively flat cephalon with weak anterior arch [1(1)] (e.g., Figs. 5.1,
8.2, 8.5); a subtriangular L3 lobe that exhibits a conspicuous adaxial taper (7[1]), and which is longer (exsag.) than L1 and L2 (8[1]; e.g., Figs. 3.1, 8.1, 8.7). With the exception of A. truncatocaudata, which retains the plesiomorphic state seen in Pterygometopus (16[0] in the Appendix), all species of Achatella possess a posterior branch of the facial suture that is nearly straight (16[1]) (e.g., Fig. 8.1, 8.4). Where known, a long genal spine is present (e.g., Figs. 4.5, 8.1; A. schmidti [Warburg] is an exception that lacks a genal spine), and the pygidium is elongate and subtriangular in outline, with a gently rounded to pointed posterior terminus (e.g., Jaanusson and Ramsköld, 1993, pl. 5, fig. 2a; Figs. 5.5, 12.1, 12.4). Laurentian species share weak expansion (tr.) of the
anterior glabellar lobe (5[1]) and an apodemal pit at intersection of L3 and axial furrow (9[1]). A derived group of four Laurentian species, A. consobrina, A. carleyi, A. katharina, and A. achates, are characterized by a median embayment on the anterior glabellar margin (11[1]; e.g., Figs. 3.1, 6.4, 6.5, 8.7), and eyes that do not reach the axial furrows anteriorly (13[1]; e.g., Figs. 6.1, 6.6, 8.11). The median embayment appears to be associated with a weakly inflated, often barely perceptible, subtriangular region on the frontal lobe (e.g., Figs. 4.2, 9.5, 9.8) that corresponds to the pterygometopid cephalic muscle insertion scar as defined by Eldredge (1971). As this scar also occurs on chasmopine (Eldredge, 1971, fig. 2H) and eomonorachine (Eldredge, 1971, fig. 3) trilobites, it is
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