Swisher et al.—Achatella Delo, 1935
Diagnosis.—Eye extending from L3 to S2 or anterior tip of L2; located away from glabella anteriorly, with fixigena traversed by short palpebral ridge (e.g., Fig. 8.4, 8.11). Conspicuous glabellar tubercles on each side of anterior glabellar embayment. Lateral cephalic margin curved outward behind intersection with posterior branch of facial suture. Pygidium with pointed posterior margin.
Holotype.—A cephalon (UC 20685; Fig. 8.5–8.7) from the Kimmswick Limestone near Batchtown, Calhoun County, Illinois.
Other material.—Paratypes: three cephala (UC 33780a, 33780c, 33780e), a hypostome (UC 28975a) and two pygidia (UC 33780b, UC 28977 [latter specimen assigned questionably to the species]); additional sclerites from Missouri: three cephala (OU 222760, OU 222761, OU 222763,), a cranidium (OU 222762), a librigena (OU 222764), and three pygidia (OU 222766, OU 222767, OU 222768); sclerites from Oklahoma: three cephala (OU 12121, 12123, OU 222765), a cranidium (OU 12124) and a pygidium (OU 12125)
Description.—Cephalon semielliptical in outline, excluding genal
spines.Cranidial outline subtriangular to subtrapezoidial, narrowing toward a gently rounded anterior margin. Cranidial length approximately 48%of maximumcranidial width. Axial furrows are shallow grooves, with conspicuous apodemal pit at intersection with
S3.Glabellamoderately convex, expands evenly forward from minimum width at S1 to maximum width across the anterior lobe; maximum glabellar width approximately 45% of maximum
cranidial width. Median embayment of anterior glabellar margin flanked by conspicuous tubercles. SO nearly transverse, deepest abaxially and curved slightly forward, but shallows medially. S1 also transverse, expands adaxially becoming weakly bifurcate with very shallow, short (exsag.) branch in some specimens (e.g., Fig. 8.7). S2 strongly incised, proceeds obliquely forward fromaxial furrow; width (tr.) equal to 27%of glabellarwidth at L3. S3 shallower than S2; extends obliquely backward from prominent apodemal pit at axial furrow; terminates adaxially at second shallower pit. LO curved gently backward, increasing in length (exsag.) toward midline; maximumlength approximately 17.5%of glabella length; dorsal surface slopes upward and backward, becoming raised above the rest of the glabella. L1 approximately transverse, expands forward weakly near axial furrow. L2 with straight posterior and oblique anterior margins; expands adaxially. L3 subtriangular in outline with adaxially convergent margins so that length (exsag.) becomes reduced by approximately 50% away fromaxial furrow. Anterior lobe suboval in outline. Anterior border very short, separated from glabella by shallow preglabellar furrow; narrows abaxially and eventually cut out entirely by suture. Palpebral lobe forms narrow (exsag.), steeply inclined, tapering band (Fig. 8.10); eye short (exsag.), extending from L3 to S2 or anterior tip of L2; located away from glabella anteriorly, with steeply inclined palpebral area of fixigena traversed anteriorly by short palpebral ridge. Palpebral furrow well defined, finely etched groove. Anterior branch of the facial suture runs approximately parallel to margin of glabella before curving inward sharply along the anterior margin of the anterior lobe. Preoccular fixigena is narrow (tr.), tapers anteriorly. Posterior branch of the facial
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suture curves gently backward toward genal spine. Posterolateral projection of fixigena is wide (tr.) with strong posterior border furrow that gently curves toward genal spine, fading beyond intersection with shallow lateral border furrow. Lateral cephalic margin curved outward behind intersection with posterior branch of facial suture. Genal spine relatively long, slightly greater than 50% of cephalon length. Sculpture of rounded tubercles developed moderately across glabella and fixigena. Conspicuous median tubercle on posterior margin of LO. Lateral librigenal border widest (tr.) opposite librigenal
slope between the librigena field and lateral border. Librigena field strongly convex; suboccular furrow well defined. Eye raised above level of glabellar crest; strongly curved and subconical; basal width in lateral view equal to approximately 85% of height. Visual surface with 20 columns of domical lenses, with up to nine lenses per column. Pygidium relatively narrow, long and triangular in outline,
field, but becomes greatly reduced anteriorly along sutural margin. Border furrowweak and expressed largely by change in
with maximum width approximately 90% of pygidial length. Lateral and posterior profiles strongly convex. Axial furrows well defined, shallow toward posterior. Axis moderately convex and tapers backwards toward a bluntly pointed posterior tip; width approximately 39% of pygidial width across the anteriormost ring; includes 12 well-defined axial rings and short (sag.) terminal piece that comprises at least two segments. Axial ring furrows strongly impressed anteriorly, becoming more faintly impressed posteriorly; deepest abaxially, shallowing and becoming slightly longer (sag.) medially. Pleural field strongly convex with steeply sloping lateral margin. Faint interpleural furrows oblique, becoming curved backward distally, and defining up to 12 pairs of pleurae. Pleural furrows deep and oblique; define subequal anterior and posterior bands. Slight dorsal flexure present along pygidial axis, readily apparent in lateral view (e.g., Fig. 12.2), raising posterior tip. External surface generally smooth and lacking large tubercles. Doublure strongly concave in dorsal view and lacks terrace ridges; extends to level of abaxial termination of pleural and interpleural furrows.
Remarks.—The short (exsag.), tall eye (e.g., Fig. 8.8–8.11) separates A. katharina from all other members of the genus apart from A. achates; these species were compared in detail under the discussion of the latter. One of Bradley’s paratype pygidia (Fig. 11.1–11.3) is relatively shorter than all other pygidia (e.g., Fig. 11.4–11.9) and is assigned only questionably to the species.
Acknowledgments
This research was supported by NSF grant EAR 0819715. D.M. Rudkin and P. Fenton at the Royal Ontario Museum, B. Hunda and J. MacKnight at the Cincinnati Museum,M. Coyne at the Geological Survey of Canada, and P. Meyer at the Field Museum arranged loans of type material. T.N. Barker,
D.K.Moss, and A. Thomas assisted in fieldwork in Missouri; R. Burkhalter provided invaluable help in the lab and in fieldwork in Oklahoma and Missouri; C.E. Brett guided us to the Blue Licks locality in northern Kentucky. Alan Owen and an anonymous reviewer provided useful comments that improved the paper.
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