A new Cambrian (Jiangshanian, Sunwaptan) trilobite fauna from Oklahoma and its biostratigraphic significance.

Non-technical summary. A newly discovered trilobite fauna from the Cambrian Honey Creek Formation marks a distinct interval that follows an extinction event. Dominated by the genus Monocheilus in association with Ptychaspis, it resembles faunas from Alberta, Canada, and the Upper Mississippi Valley region of the United States. Ptychaspis bullasa Lochman and Hu, 1959 is a species that has been reported widely in North America. However, restudy of various museum collections shows that the various occurrences record a set of more narrowly distributed species. The pattern of distribution is similar to groups of modern “pseudocryptic species” identified by a combination of genetic and anatomical data. Abstract. The Cambrian (Jiangshanian, Sunwaptan) Honey Creek Formation in the Wichita Mountains region of Oklahoma yielded a new fauna dominated by Monocheilus Resser, 1937 (senior synonym of Stigmacephalus Resser, 1937) in association with Ptychaspis Hall, 1863. It occupies the same stratigraphic position as similar faunas in the Upper Mississippi Valley and Alberta, lying a little above an interval characterized by species of Taenicephalus Ulrich and Resser in Walcott, 1924 and Orygmaspis Resser, 1937. Revision of Ptychaspis bullasa Lochman and Hu, 1959 from type material from Idaho and sclerites attributed to the species from Texas reveals a plexus of pseudocryptic species that share tuberculate sculpture on the cranidium. New species are Monocheilus reginae, Monocheilus richardi, Ptychaspis occulta, and Ptychaspis matuszaki.


Introduction
Cambrian trilobites of Oklahoma have been studied intensively for 70 years (e.g., Frederickson 1948Frederickson , 1949;;Stitt, 1971Stitt, , 1977;;Westrop et al., 2010), so it is surprising to discover an entirely new fauna in the succession.This fauna is of interest because it includes early representatives of two major clades (Ptychaspididae Raymond, 1924 andEurekiidae Hupé, 1953) that radiated in Laurentia during the Sunwaptan Stage.The composition of the fauna, which is dominated by Monocheilus Resser, 1937 and also contains Ptychaspis Hall, 1863, resembles assemblages that occur in the same homotaxial position in Minnesota and Wisconsin (e.g., Nelson, 1951) and in the southern Canadian Rocky Mountains (Westrop, 1986).As such, these assemblages may mark a distinct biostratigraphic interval in a transition from low-diversity faunas in the aftermath of the end-Steptoean extinction to morediverse Sunwaptan faunas (e.g., Westrop and Cuggy, 1999).
Monocheilus (considered here to be a senior synonym of Stigmacephalus Resser, 1937; see Systematic paleontology, which follows) is the earliest representative of the family Eurekiidae, and the occurrence of Ptychaspis is likely the oldest record of the family Ptychaspididae.Early representatives of Ptychaspis have often been assigned to a single species, P. bullasa Lochman andHu, 1959 (e.g., Bell andEllinwood, 1962;Stitt, 1971Stitt, , 1977)).Restudy of type material shows that there is in fact a plexus of pseudocryptic species (e.g., Westrop and Adrain, 2007;Westrop et al., 2018) that are differentiated readily by cranidial and pygidial anatomy (see Systematic paleontology).

Stratigraphy and study area
The study area lies in the Slick Hills, immediately north of the Wichita Mountains.The Honey Creek Formation and the underlying Reagan Sandstone compose the Timbered Hills Group (Stitt, 1971).The Timbered Hills onlaps the Carlton Rhyolite, which was exposed in late Cambrian Oklahoma as a volcanic archipelago with at least 300 m of relief (Donovan, 1986;Donovan and Bucheit, 2000;Donovan et al., 2000).The boundary between the Reagan and the Honey Creek is gradational through an interval of sandstone with bioclastic carbonate interbeds and lenses.Following Donovan and Ragland (1986), we place the base of the Honey Creek at the lowest occurrence of bioclastic carbonate, and this definition differs from the one offered by Stitt (1977, p. 5), who placed the boundary above the highest sandstone interbed.Lithologically, the formation is a pelmatzoan-rich, glauconitic, bioclastic grainstone and rudstone with thin siliciclastic drapes.Cross-bedding and ripple *Corresponding author marks are common, as are sandstone interbeds.Heterolithic intervals are composed of fineto medium-grained sandstone with recessively weathering bioclastic carbonate lenses (Westrop et al., 2010, fig. 1).Bucheit and Donovan (2000, fig. 16) interpreted the Honey Creek as a tidally influenced deposit that formed between rhylolite islands.
The Fort Sill Formation (lowest unit of the Arbuckle Group) succeeds the Honey Creek Formation and is composed of lime mudstone-wackestone (Stitt, 1971;Donovan and Ragland, 1986).Intraclastic rudstone is also present, and microbial buildups occur in the upper part of the formation (Stitt, 1971).Quartz sand and glauconite are minor components, suggesting that the archipelago was largely flooded during deposition of the Fort Sill.
The trilobites were collected from the succession in the Bally Mountain region (section BM) of Kiowa County, which was deposited near a rhyolite island (Donovan and Bucheit, 2000).The section (Fig. 1.1) was measured and logged on the west-facing slope of an unnamed ridge about one kilometer to the east of Bally Mountain (Fig. 1.2, 1.3; 34°57 ′ 39 ′′ N, 98°3 9 ′ 03 ′′ W).Elvinia Zone faunas are well represented in a carbonate unit about 52.5 m above the base of the section.An overlying 12 m sandstone unit did not yield trilobites, but Taenicephalus wichitaensis Resser, 1942 and Conaspis testudinata Ellinwood in Bell and Ellinwood, 1962 are present in the succeeding carbonate unit, between 70.5 m and 72 m above the base of the section (Fig. 1.1); the systematics of these species will be treated elsewhere.The newly discovered, low-diversity fauna dominated by new two species of Monocheilus, M. reginae and M. richardi (Figs.2-6), occurs about 8 m higher in the section, in the Honey Creek-Fort Sill boundary interval (Fig. 1.1).It is referred to here as the Monocheilus reginae fauna.
Ptychaspis is rare at the Bally Mountain section, and our study includes archival material from elsewhere in Oklahoma as well as Texas and Idaho.The type material of P. matuszaki n. sp. was collected from exposures of the Fort Sill Formation, along a section-line road 4.4 km southeast of Hennepin (SE Sec. 4, T1S, R1W; 34°29 ′ 46 ′′ N, 97°18.0′ 4 ′′ W) separating sections 3 and 4, Murray County, Oklahoma (Matuszak, 1957).This site is 450 m northeast of section DR of Westrop and Adrain (2007, p. 989, fig. 1c).Sclerites assigned to P. occulta n. sp. and Ptychaspis spp.are from the Morgan Creek Member, Wilberns Formation, central Texas, and were illustrated previously by Bell and Ellinwood (1962) and Longacre (1970) under the name Ptychaspis bullasa Lochman and Hu, 1959.Revision of P. bullasa is based on restudy of the type material from the Mink Creek region, near Preston, southern Idaho (Lochman and Hu, 1959).

Age and correlation of the Monocheilus reginae fauna
The Monocheilus reginae fauna occurs in two collections (Fig. 1.1).The lower of these (BM 79.4T) is a float sample of trilobite grain-to rudstone from a covered interval 1.6 m below the base of the Fort Sill Formation.It includes abundant sclerites of Monocheilus and rare specimens of Ptychaspis and Wilbernia Walcott, 1924. Collection BM 80.7 is lithologically similar to BM 79.4T but was recovered in place 1.3 m higher in the section and 30 cm below the base of the Fort Sill.
Monocheilus and Ptychaspis are both minor components of this collection, with Minkella Lochman and Hu, 1959 dominating.The latter genus is under study by S.R.W. and will not be treated in this paper.
Both species of Monocheilus described in this paper are related to M. oweni (Hall, 1863).However, that species is based on sandstone internal molds that provide no information on such features as sculpture of the exoskeleton and, as a result, is difficult to evaluate.It is best restricted to the types (see discussion of M. richardi that follows), and we use the shutter-mark convention (Wiley, 1979) in the following discussion to underscore uncertainty about reports of the species in various regions.Monocheilus "oweni" (under the name Stigmacephalus) has been reported from a collection (B57) from the Bison Creek Formation of Alberta (Westrop, 1986), which, as in Oklahoma, also includes Ptychaspis and a species of Wilbernia.Westrop (1986, p. 17) placed this collection in his Stigmacephalus oweni fauna, which encompassed a poorly fossiliferous interval whose base was defined broadly by the first occurrences of either the name-bearing species, Idahoia cf.I. lirae (Frederickson, 1949) or Taenicephalina sp. 1. Idahoia cf.I. lirae is distinct from Frederickson's types of I. lirae from Oklahoma (Westrop, 1986, pl. 16, figs. 5-7) by virtue of, among other characters, a long occipital spine (Westrop, 1986, p. 42).This species does not co-occur with M. "oweni" in any of Westrop's (1986) sections: Idahoia cf.I. lirae occurs in his section D, whereas M. "oweni" is from his sections B and S. As a result, there is some uncertainty about the relative ranges of these two species, although M. "oweni" seems to extend into younger strata (see Westrop, 1986, p. 18 for discussion).In Oklahoma, the M. reginae fauna lies above the I. lirae Zone, which is present in the Honey Creek Formation about 16 km to the southeast at section KR1 (see Westrop et al., 2010 for locality information), 3.1 m above the highest occurrence of Taenicephalus wichitaensis Resser, 1942 (unpublished data, Blackwell andWestrop, 2023).The evidence suggests that the ranges of M. reginae, M. richardi, and M. oweni likely overlap, and the M. oweni fauna and the M. reginae fauna may be broadly correlative.
In the Minnesota-Wisconsin border region of the St. Croix Valley (Nelson, 1951), M. "oweni" occurs above the Conaspis Zone (= Taenicephalus Zone; Grant, 1965), confirming that this species is among the older representatives of the genus.Sections farther to the south, along the bluffs of the Mississippi (Grant, 1962), apparently lie above the local range of M. "oweni" and instead contain M. anatinus (Hall, 1863) in association with Ellipsocephaloides curtus (Whitfield, 1878) and Idahoia wisconsinesis (Owen, 1852) (e.g., Grant, 1962, text-figs. 3, 6), among others.Both of the latter two species enter the succession in Alberta above the M. oweni fauna (Westrop, 1986).

Materials and methods
Unless indicated otherwise, treatment of each species is based on the figured specimens.Specimens were coated with a sublimate of ammonium chloride before photography.Depth of field was maximized by rendering digital images from stacks of images focused at 100 μm intervals using Helicon Focus 4.0 for the Macintosh <http://www.heliconsoft.com>.Proportions
There are enough similarities in characters to provisionally view Monocheilus as a basal member of Eurekiidae.Further evaluation of this hypothesis must await a computer-based phylogenetic analysis of Eurekiidae that will also need to include a revision of "Bayfieldia" simata Winston and Nicholls, 1967 (including their "var.A," which represents a distinct species), which combines a Monocheilus-like cranidium with a conventional eurekiid pygidium that includes five to six pairs of marginal spines and well-furrowed pleural fields (e.g., Winston and Nicholls, 1967, pl. 9, figs. 20, 23-26).
Diagnosis.-Eurekiidae with preglabellar field of cranidium separated from anterior border by weak, backwardly curved anterior border furrow in small cranidia that becomes effaced in larger holaspids, producing an undifferentiated frontal area.Glabella parallel-sided to gently tapered anteriorly with very weakly incised glabellar furrows barely perceptible in larger holaspids.Transversely subelliptical pygidium bearing one to four pairs of short, triangular marginal spines.Short, convex axis consists of one axial ring and terminal piece composed of at least two segments.Pygidial pleural field is nearly flat.
As will be discussed, there appear to be two pygidial morphotypes associated with cranidia of M. reginae and M. richardi in BM 79.4T that differ in outline (Fig. 7.1-7.5 and Fig. 7.6, 7.7, respectively; species assignments are uncertain), and both are very similar to those of M. anatinus (Hall, 1863;Bell et al., 1952, pl. 33, fig. 5a;Westrop, 1986, pl. 14, fig. 3) and M. micros (Walter, 1924;Westrop, 1986, pl. 15, fig. 4).In particular, all of these pygidia share triangular spines on the posterior corners and a single pair of broad, shallow pleural furrows on a relatively flat pleural field.The axis is short, occupying about half of pygidial length, with one well-defined axial ring and a second ring that is partly fused with the terminal piece.They lack a border and border furrow.All are distinct from the specimen attributed to S. oweni by Nelson (1951, pl. 109, fig. 9), which has a long axis that terminates close to the pygidial margin, a narrow border, and distinct border furrow and lacks marginal spines.It is almost certainly misassigned, and consequently, use of pygidial characters to separate Monocheilus and Stigmacephalus (e.g., Westrop, 1986, p. 87) can no longer be justified.(Hammer et al., 2001).For M. reginae, y = 0.39x + 0.37; for M. richardi, y = 0.275x + 0.32.Comparisons of slopes in PAST showed that they were significantly different (p < .0002).
Journal of Paleontology 97(4):865-890 Additional material.-Inaddition to the types, six cranidia were complete enough to provide morphometric data.
Remarks.-A comparison between cranidia of M. reginae and M. richardi, co-occurring species that differ in the size of the palpebral lobe (Fig. 4), is presented in the description of the latter.There appear to be two distinct pygidial morphotypes associated with the cranidia, raising the possibility that other characters may separate these species.However, more material is needed to confirm this.The most common morphotype (e.g., Fig. 7.1-7.5) is relatively long and narrow, with length (measured to the intersection of the anterior margin and the axis) equal to 46% of width, but one specimen (Fig. 7.6, 7.7) is distinctly shorter and wider, with length equal to 40% of width.

Monocheilus richardi new species
Figures 5, 6 Holotype.-Acranidium (Fig. Description.-Monocheilusrichardi is sufficiently similar to M. reginae that a comparison can be presented instead of a full description.Monocheilus reginae differs from co-occurring cranidia of M. richardi in the size of the palpebral lobe (Fig. 4).Compared with M. richardi (Figs. 5, 6), M. reginae is a relatively large-eyed species with a palpebral lobe that is equal to about 60% (59%; 42%-64%; lower values in larger specimens) of preoccipital glabellar length, whereas the palpebral lobe is noticeably smaller in similarly sized specimens of M. richardi, averaging slightly more than 40% (42%; 30%-50%; lower values in larger specimens).In other respects, the cranidia are similar.
Etymology.-ForRichard Blackwell, the name of both Sean Blackwell's father and Sean Blackwell's grandfather.
Additional material.-Inaddition to the types, three cranidia were complete enough to provide morphometric data.
The pitted sculpture is a diagnostic feature of both M. reginae and M. richardi, whereas other species, including M. micros and M. orestes, are smooth (e.g., Westrop, 1986, pl. 15, figs. 5, 6, 14, 15).Cranidia of M. richardi are similar to the types and other specimens of M. oweni (Hall, 1863) from the Upper Mississippi Valley region (e.g., Nelson, 1951, pl. 109, figs. 1, 2) in possessing a relatively small palpebral lobe, although M. richardi Blackwell and Westrop-Cambrian trilobite fauna differs in having a wider palpebral area of the fixigena so that the palpebral lobe is farther from the glabella.Unfortunately, all of the specimens from the Upper Mississippi Valley are preserved as sandstone internal molds, and the nature of the sculpture cannot be determined.It is impossible to compare M. oweni completely with other species that preserve the skeleton.As noted earlier, we recommend that the name M. oweni be restricted to the types, and the shutter-mark convention (M."oweni") should be used for internal molds from other collections in the Upper Mississippi Valley, as well as for specimens attributed to this species from Alberta (Westrop, 1986, pl. 15, figs. 10-13).
Remarks.-Monocheilus reginae and M. richardi are differentiated clearly by the sizes of their palpebral lobes.As noted in the preceding, there is sufficient variation in pygidia from collection BM 79.4T that we anticipate larger samples will demonstrate that these species are also differentiated by pygidial anatomy, although the correct sclerite associations cannot be made at present.Pygidial morphotypes are separated by their outlines.Most are relatively long and narrow (e.g., Fig. 7.1-7.5),with length (measured to the intersection of the anterior margin and the axis) equal to 46% of width.The other pygidial morphotype (Fig. 7.6, 7.7) is relatively shorter and wider, with length equal to 40% of width, and has less strongly curved lateral margins.In other respects, the morphotypes are similar, with a single pair of triangular marginal spines.The axis is short with one distinct axial ring bounded posteriorly by a complete ring furrow, and the terminal piece comprises at least two segments.The pleural field is nearly flat and crossed by a single pair of wide, shallow pleural furrows that curve sharply backward, separating narrow, convex anterior and posterior pleural bands.
Description.-Cranidium with length equal to 95% (90%-99%) of width.Frontal area is short and lacks border furrow.Anterior cranidial margin gently curved medially so that frontal area maintains nearly even length (sag., exsag.)across most of width.Glabella convex, long, and broad, occupying 89% (87%-91%) of cranidial length and half (50%; 47% Librigena with stout genal spine.Lateral border furrow is well defined and merges posteriorly with posterior border.Lateral border is narrow and descends steeply at cephalic margin.Librigenal field strongly inflated with shallow eye socle furrow and low eye socle.Borders, spine, and abaxial parts of librigenal field carry coarse striate ridges; adaxially, librigenal field with tubercles.
Hypostome with convex median body divided into short posterior lobe and much longer anterior lobe by oblique middle furrow that is effaced medially.Macula indistinct oval region on internal mold.Lateral and posterior border furrows form deep grooves; narrow, rim-like borders.Broad, triangular anterior wing.Patches of exoskeleton preserve terrace ridges on lateral border and smooth surface on anterior lobe of median body; internal mold smooth.
Pygidium semielliptical in outline, wider than long.Axis convex, extending almost entire pygidial length and narrow, occupying less than a third of pygidial width; composed of three axial rings and terminal piece on largest specimen (Fig. 12.1-12.3).Articulating and axial ring furrows transverse and deep; posteriormost shallower than others on smaller specimens, so that third ring poorly differentiated from terminal piece.Articulating half ring very short, less than half length of first axial ring.Deep pleural and shallower interpleural furrows extend nearly transversely from axis at no more than 10°-15°f rom transverse plane before curving backward more strongly near pygidial margin; posteriormost furrows more strongly deflected.Anterior pleural bands roughly equal to posterior bands.Posterior pleural bands carry row of faint tubercles.Pygidial border narrow, rising vertically from weakly convex pleural field.
Pygidia are incomplete, but the smallest (Fig. 11.8) is relatively narrower (tr.) than in the other specimens (Figs. 11.6,12.1),and the third axial ring is poorly defined.In all three specimens, the axis remains long and terminates close to the posterior border.
Remarks.-Restudy of the type material of Ptychaspis bullasa Lochman and Hu (1959;Figs. 9-12) shows that this species is characterized by an anteriorly positioned palpebral lobe that is centered opposite the S2 glabellar furrow, and pygidial pleural and interpleural furrows that are almost transverse over most of their widths.This allows several records of P. bullasa from outside the type area in southern Idaho  to be evaluated critically.
Cranidia that Bell and Ellinwood (1962) identified as P. bullasa from the Morgan Creek Member of the Wilberns Formation, Texas (Fig. 15), have palpebral lobes that are larger and more posteriorly located (see also Longacre, 1970, p. 44) than those of the types (e.g., Figs. 9, 10).There are other differences in the cranidia from Texas, including a longer frontal area with subtriangular anterior cranidial margin, more densely packed tuberculate sculpture, and striate ridges that are more closely spaced on the anterior border (Fig. 15).The Texas material is assigned to a new species, P. occulta (see the following).
Blackwell and Westrop-Cambrian trilobite fauna However, the two cranidia from the Morgan Creek Member that Longacre (1970) assigned to P. bullasa (Fig. 17) are distinct from both P. bullasa and P. occulta (see discussion of Ptychaspis spp. that follows).
The cranidium attributed to P. bullasa by Stitt (1977, pl. 2, fig. 4) from the Fort Sill Formation of Oklahoma has been damaged since it was photographed and can no longer be evaluated fully.However, sclerites from the same stratigraphic interval in the lower Fort Sill Formation (Figs. 13,14) represent a distinct species characterized by curved pleural and interpleural furrows on the pygidium, among other characters (see following discussion of P. matuszaki n. sp.).
The record of P. bullasa from the Deadwood Formation of South Dakota (Stitt and Straatmann (1997, fig. 7.16) is supported by a single illustrated librigena and is difficult to evaluate.This specimen has far greater development of striate ridges and a smaller area of tuberculate sculpture than either of the librigenae in the type lot of P. bullasa (Figs. 11.4,11.5,12.4),and the identification cannot be corroborated.
Rather than a single, widespread species, detailed comparisons of putative occurrences of P. bullasa indicate that there is in fact a geographically structured plexus of related species.Westrop et al. (2018) recently documented a similar pattern in the middle Cambrian trilobite Eodiscus, noting that such sets of pseudocryptic species are comparable to groups of modern species that are now recognized routinely in studies that integrate morphometric and genomic data.
Diagnosis.-Ptychaspis with closely spaced tuberculate cephalic sculpture yielding to sparse striate ridges along cranidial and librigenal margins.In front of transglabellar S2 furrow, glabella subcircular in outline, divided by barely perceptible S3 and S4 lateral furrows (most clearly visible in lateral view; Fig. 13.2).Palpebral lobe small and located opposite S2.Gently convex baccula present on fixigena opposite L1 (arrow on Fig. 13.1).Pygidium possesses gently inflated pleural field and axis composed of three axial rings and terminal piece.Anteriorly, pleural and interpleural furrows deflected obliquely backward at angle of at least 20°-30°from transverse plane.Pygidial border forms narrow upturned rim that thickens toward axis.
Remarks.-Cranidia of Ptychaspis matuszaki are similar to those of Ptychaspis bullasa Lochman and Hu, 1959 (Figs. 9-12).Both have tuberculate sculpture, but there are fewer, more widely spaced tubercles in P. bullasa ), which also has well-defined striate ridges on the frontal area.Both species have relatively small, anteriorly placed palpebral lobes that are centered near S2, and the overall proportions of the glabellar lobes are comparable.Ptychaspis matuszaki differs in having a distinct baccula next to L1 (seen most clearly in the best-preserved specimen; Fig. 13.1).In addition, cranidia of P. bullasa possess less rapidly diverging posterior suture branches, resulting in narrower (tr.) posterolateral projections.Pygidia share a raised, rim-like border, with those of P. matuszaki (e.g., Figs. 13.5,14.6,14.7)differing from P. bullasa 3) most clearly in having shorter axes and pleural and interpleural furrows that are deflected more strongly backward rather than being more transverse.

Ptychaspis occulta new species
Figure 15 1962 Ptychaspis bullasa Occurrence.-MorganCreek Member (figured specimens collected between 5.5 m and 6.7 m below the top), Wilberns Formation, central Texas (see caption for Fig. 15 for detailed locality information).
Description.-Ptychaspisocculta n. sp. is sufficiently similar to P. matuszaki n. sp. and P. bullasa Lochman and Hu, 1959 that comparisons can be presented in lieu of a full description.
Ptychaspis occulta and P. matuszaki are superficially similar yet differ in detail, and although sample sizes are small, there are clear apomorphic character states that separate them.Diagnostic states for P. occulta include more robust sculpture than P. matuszaki in both the size and packing density of tubercules, as well as the striate ridges on the frontal area (compare Fig. 15 with Fig. 13.1-13.3);note that the ridges are well defined on internal molds of P. occulta (Fig. 15.1-15.3)but are barely perceptible on molds of P. matuszaki.Ptychaspis occulta also lacks a baccula, and the anterior branches of the facial suture are more rapidly convergent so that the frontal area is subtriangular in anterior view (e.g., Fig. 15.6).By contrast, P. matuszaki is bacculate (Fig. 13.1), and the reduced divergence of the facial sutures produces a frontal area that is gently rounded and nearly transverse in anterior view (Figs. 13.3,14.3).
Blackwell and Westrop-Cambrian trilobite fauna Etymology.-Fromocculta (L), "having been hidden," in reference to the species having gone unrecognized within the literature since 1962.
The larger of the cranidia (Fig. 17.1-17.3)has sculpture of rounded tubercles that are more widely spaced than on cranidia of P. occulta (Fig. 15) and has fewer terrace ridges on the frontal area.The palpebral lobes are broken off but are almost certainly smaller and more anteriorly positioned than in P. occulta, and the portion of the glabella in front of S2 is relatively shorter.Finally, the anterior branches of the facial sutures of this cranidium are not as strongly convergent as in P. occulta so that the anterior cranidial margin in anterior view is more transversely oriented.Compared with similarly sized specimens of P. bullasa , tuberculate sculpture extends farther back over the fixigenae, reaching the posterior border furrow, and includes a row of conspicuous tubercles along the posterior border.Further comparisons are hindered by preservation, but palpebral lobes seem to have been in an anterior position, as in P. bullasa.
Remarks.-Two nearly complete cranidia possess large palpebral lobes that are equal to about 36% of glabellar length with firmly impressed palpebral furrows.The frontal area consists of a weakly inflated preglabellar field that is slightly shorter than the weakly convex anterior border.They are most like W. diademata (Hall, 1863;e.g., Nelson, 1951, pl. 109, figs. 8, 11) but have a shorter border and, consequently, a somewhat shorter frontal area.Cranidia identified as W. diademata by Bell and Ellinwood (1962, pl. 54, fig. 9) have shorter palpebral lobes than Wilbernia cf.W. diademata.Small cranidia attributed to the poorly known W. halli Resser, 1937 Blackwell and Westrop-Cambrian trilobite fauna expressed in percentages in descriptions and diagnoses are means, with the following pair of numbers indicating the range of values.All measurements were made on digital images to the nearest tenth of a millimeter using the Measure Tool of Adobe Photoshop.Repositories and institutional abbreviations.-Illustratedspecimens are housed at the Oklahoma Museum of Natural History, University of Oklahoma, Norman (OU), and at the National Museum of Natural History, Washington D.C. (USNM).

Figure 1 .
Figure 1.(1) Stratigraphic column and species range chart for the Honey Creek-Fort Sill boundary interval, Bally Mountain section (BM), Kiowa County, Oklahoma.(2) Map showing the location of the section.(3) Map showing location of the study area in Oklahoma.
Journal of Paleontology 97(4): Pygidium subelliptical in outline, width greater than length; gently arched in posterior view.Axis narrow, convex, occupies 67% of pygidial length.Three axial rings and terminal piece of at least two segments separated by well-defined, transverse ring furrows; shallower furrow present on terminal piece.Articulating half-ring short; articulating furrow nearly transverse.Pleural field crossed by at least four pairs of broad (exsag.),oblique pleural furrows deflected backward at angle of at least 20°-30°f rom transverse plane; interpleural furrows narrower but welldefined grooves.Anterior and posterior pleural bands subequal, convex.Lateral and posterior borders form short (sag., exsag.),weakly upturned rim.External surface smooth; internal molds with pits; closely spaced on pleural field and border but widely spaced on crest of axis.
occurred in the "basal Fort Sill Formation."Inflated librigenal field occupies 68% of width opposite eye; eye socle not preserved but eye socle furrow broad, shallow groove.Lateral border furrow well incised anteriorly but shallows near genal spine; lateral border narrow in dorsal view, descends nearly vertically from border furrow.Librigenal field with conspicuous tubercles that are lost near border furrow.Border with elongate, widely spaced striate ridges running parallel to lateral cranidial margin.
has a granulose surface but lacks striate ridges, but it has smaller and more anteriorly positioned palpebral lobes (opposite S2) than P. occulta.Occurrence.-UppermostHoneyCreekandbasalFortSill formations, section BM, Bally Mountain, Kiowa County, Oklahoma, collections BM 79.4T, BM 80.7, and BM 82.1, Monocheilus reginae fauna.Remarks.-PtychaspiscranidiafromsectionBMare incomplete and cannot be identified to the species level.However, they record what is likely the oldest occurrence of the genus, at the top of the Honey Creek Formation.Longacre, p. 44, pl. 2, figs.4,5.Occurrence.-UppermostMorganCreekMember (figured specimens collected within a meter of the top), Wilberns Formation, central Texas (see caption for Fig.17for detailed locality information).