Material and repository

Material from the Indian Springs locality described herein was recovered in 1999 by L. E. Babcock and in 2002 by L. E. Babcock and L. E. Holmer. Supplementary samples were collected by M. Streng in 2009. The material comprises ~50 complete or almost complete specimens from various levels of the lower half of the investigated interval (Fig. 2). All specimens represent single valves, except one specimen that is preserved in a bivalved state. In addition to the macroscopic specimens, 29 carbonate horizons and lenses were sampled (Fig. 2) to obtain shell material of Mickwitzia occidens and other microfossils by the dissolution of the collected samples. Individual samples were dissolved in 10% acetic acid and acid residues were subsequently rinsed, dried, and examined under a binocular microscope to identify and pick Mickwitzia shell fragments and other microfossils. About two-thirds of the samples yielded indisputable specimens of Mickwitzia, which were most common in the samples originating from the interval 53–68 m above the base of the middle member (ca. 20–35 m below the base of the Dyeran Stage; Fig. 2). Recovered Mickwitzia specimens from the carbonate horizons are all fragmentary with individual fragments not exceeding 4 mm in size. The interval with the highest abundance of shell fragments also yielded the most common macrospecimens. The youngest specimens of Mickwitzia within the sampled sections were found ~119 m above the base of the middle member, i.e., 31 m above the base of the Dyeran Stage.

Besides Mickwitzia, a rich macrofauna, including trilobites, linguliform brachiopods, helicoplacoids, chancellorids, sponges, and hyolithids, has been reported from the locality (see English and Babcock, Reference English and Babcock2010; Hollingsworth and Babcock, Reference Hollingsworth and Babcock2011). Nonbiomineralized tissues found include mantle setae of brachiopods, demosponge skeletons, hyolithid gut tracts, trilobite appendages, algae, cyanobacteria, and body parts of nonbiomineralizing arthropods (English and Babcock, Reference English and Babcock2010). Associated with the Mickwitzia shell fragments in the carbonate horizons are common trilobite fragments and chancellorid sclerites, and internal molds of hyolithids and rare linguiliform brachiopods, as well as agglutinated protists (Streng et al., Reference Streng, Babcock and Hollingsworth2005).

Material described herein is deposited at the Paleontological Research Institution, Ithaca, New York, USA (macrospecimens; PRI 68586 through 68637) and the Palaeontological Collections of the Museum of Evolution, Uppsala University, Sweden (acid-etched material; collective number PMU 27409). It should be noted that portions of the acid-etched material were accidently damaged or destroyed after being photographed. This involves all acid-etched specimens with specimen numbers starting with 016–019, 021, 023, 024, 030, 032, and 033. Other institutional abbreviations used in the text refer to specimens deposited in the collections of the U.S. National Museum of Natural History, Washington, D.C. (USNM) and and the Sedgwick Museum, Cambridge (MP).

Photography and compositional analysis

Imaging of fossil macrospecimens was performed with a digital camera (Axiocam; Zeiss, Jena, Germany) attached to a dissection microscope (Leica, Heerbrugg, Switzerland). Brightness, contrast, and color values were, when needed, corrected using image-processing software (Adobe Photoshop CS6, Adobe Systems Inc., San Jose, California, USA).

Preparation for scanning electron microscopy consisted of mounting the selected specimens on aluminum stubs with adhesive carbon backing and subsequently sputter-coated with gold. Scanning electron microscopy of fossil material was performed at the Microscopy Unit at the Evolutionary Biology Centre, Uppsala University. Images were collected using a Zeiss Supra 35VP field emission and a Philips XL30 scanning electron microscope (SEM) operating at 5 kV.

Compositional data was obtained on a Zeiss field emission SEM at 23kV. EDS X-ray spectra and elemental maps were collected on an EDAX Apollo X silicon drift detector system using Genesis Spectrum software (version 6.45, EDAX Inc., Mahwah, New Jersey, USA). Elements mapped in transverse, polished shell sections were C, O, P, Ca, Al, Si, Fe, Mg, and F, with a total of 64 frames per element.

Morphometrics

A set of comparative geometric morphometric comparisons were made in order to evaluate the potential for this technique as a diagnostic tool in refining the species-level characters and taxonomy of the genus Mickwitzia. Measurements from the major and minor axes of the shell extent, position of apex, and juvenile growth zones were used as landmarks (see Fig. 15.1).

Landmark based geometric morphometrics were performed on complete dorsal and ventral valves from Indian Springs (N=7) and on M. monilifera from Lugnås, Västergötland, Sweden (N=31). The data was subdivided into partitioned datasets depending on species and valve position. Landmarks (designated 1–8) corresponded to: the widest points laterally on the outer shell margin (5, 8), the anterior and posteriormost points on outer shell margin (1, 4), the shell apex (2), and the respective widest (6, 7) and longest margins at the boundary of the juvenile and adult shell zones (3) (Fig. 15.1).

Digitization of the landmark data was performed with tpsDIG (ver, 2.22, Rohlf, F.J., 2015, Department of Ecology and Evolution, State University of New York at Stony Brook, New York, USA) and the resulting file analyzed in MorphoJ (Klingenberg, Reference Klingenberg2011). A Procrustes superimposition was employed to normalize size scaling and orientation of the specimens. We generated a covariance matrix and then calculated PCA values in MorphoJ (see Klingenberg, Reference Klingenberg2011). For distinguishing species, we also performed canonical variance and discriminant analyses on the covariance matrix to determine the certainty that individuals of a species would be recovered in the correct taxonomic grouping.

Preservation and taphonomy

The collection described falls into two distinct categories: (1) hand specimens of complete Mickwitzia, including dorsal, ventral, or both valves, and (2) acid-prepared shell fragments extracted from bulk, dissolved carbonate mudstone and wackestone layers and lenses. The mode of preservation of the shells of M. occidens differs greatly between specimens hosted in either limestone or shale units.

The hand specimens consist of weakly mineralized shells containing little original phosphate, with the most mineralized portion of the shell located at the umbonal region. Weathering of the source rocks could have contributed to this seeming lack of biomineralization by dissolution, and highlights the need for careful appraisal of taphonomic controls on the biomineralized components of shell and shell structures within organisms from this biota. The preservational style of the Indian Springs locality bears some overall similarity to the Chengjiang style of preservation (English and Babcock, Reference English and Babcock2010) in terms of the extent of lithology, weathering, and replication of shell and soft parts as pyrite.

In a small number of specimens, including three Mickwitzia and a few lingulid brachiopods, the marginal mantle setae are preserved. These structures are typically nonmineralized in analogous living and fossilized brachiopods and thus we can interpret this as an incidence of exceptional soft-tissue preservation of these structures.

Shale-hosted material contains numerous examples of complete specimens, retaining both valves in a few specimens although single valves comprise most of the recovered material. Specimens range in size from 10 to 47 mm in diameter, and include a range of ontogenetic stages interpreted as juvenile to adult forms. In carbonate-hosted material, the Mickwitzia shell fragments were easily recovered from acetic acid-treated limestone residues, identifiable by the characteristic pustulose ornament and presence of a distinctive, shell-penetrating pore system with inwardly pointing cones. Fragments collected range downward in length from a few mm to a few hundred microns. This shell hash includes pieces of individuals smaller than the macrospecimens collected from the shale facies.

General features and shape

Shale-hosted ventral valves are diagenetically flattened, being planar to slightly conical in lateral profile. The shell outline is ovoid in shape with the apex located posteriorly from the centroid of the shell and anterior to the posterior shell margin. A distinct, larval-shell growth area, lacking ornament is observed at the shell apex of the valve and abruptly ends at an ~100 µm radius from the apex point in adult specimens. Apices of ventral valves recovered from the carbonates (Fig. 6) are rather variable in shape, suggesting that ventral valves of M. occidens might also have been rather variable in lateral profile. Typically, the slopes of these conical fragments are moderately steep, more or less uniformly inclined in all directions, and have a straight to gently convex lateral profile. However, also rather flat apical shell fragments or fragments with distinct different inclinations between the anterior and posterior slope are known (Figs. 6, 7).

Dorsal valves appear to be more or less flat in lateral profile, but possess a thickly mineralized, marginal umbonal region that bends around on itself. More distal shell material is either nonbiomineralized, thus not present, or is weakly mineralized and has not survived preservation/acid processing. In dorsal valves, the position of the apex is typically located along the posterior edge of the shell margin, or is in close proximity to it. As in the ventral valve, the apex is free of ornamentation, indicating the larval shell.

All Mickwitzia shell material recovered by the dissolution of the limestone samples is fragmentary, with the individual fragments not exceeding 4 mm in maximum length (e.g., Fig. 5.16, 5.17). All fragments show the characteristic shell structure of the genus typified by a multilamellar fabric with inwardly pointing hollow cones and can therefore easily be distinguished from other phosphatic shell material in the acid residues (e.g., Fig. 6). Generally, three types of shell fragments can be distinguished: (1) common conical fragments, i e., apices of ventral valves (Figs. 6, 7, 13); (2) rare sickle-shaped fragments, i.e., apices of dorsal valves (Figs. 8–9); and (3) flat shell fragments, which represent the bulk of the recovered material and cannot be assigned to either valve (Fig. 5).

Ornamentation

Pustulose-reticulate ornament typical of mickwitziids is found on the outermost shell surfaces in well-preserved specimens (e.g., Figs. 5.10, 6.1, 6.4, 8.9, 11.14). Pustules form at the intersection of fine concentric and radial ridges, the latter of which multiply by intercalation. In exfoliated specimens, the reticulate pattern surface ornament becomes weaker depending on the degree of exfoliation (e.g., Figs. 5.7, 5.5, 5.15, 6.10, 8.10). There is seemingly no observed spatial connection between the surface pustules and the inwardly pointing cones, although this might not be the case.

Well-defined pustulose ornament is present in regular concentric bands, as observed in other species of Mickwitzia. Canals that contained tangential shell-penetrating setae are present in shell layers apart from the larval shell zones at the shell apices.

Whether the setae are retained throughout each growth stage or restricted to the shell margin with each previous generation being abandoned is unclear, although in exceptionally preserved specimens only the latter is observed, suggesting that the setae are restricted to the margin.

The ornamentation is not present within an area of ~100 µm radius from the nominal shell apex. We interpret this smooth area as the initial larval shell growth stage; it contrasts with the pustulose ornament on the rest of the shell. This feature is present to an equal degree in all observed shell apices, ventral and dorsal (Figs. 7.7–7.13, 9.10), suggesting it is not simply a function of ornament becoming abraded. There is, however, no clear discrimination between the smooth apex and the subsequent reticulately ornamented shell area; the transition between the areas is gradual.

Position of apex

The shell apex in ventral valves in all studied specimens is located along in a medial position lengthways that ranges in location from between the nominal centroid to close to the posterior shell margin. A scatterplot normalized for size and orientation differences with Procrustes superimposition shows the medial and distribution of apex locations, with standard deviations plotted (Fig. 15). In dorsal valves, the apex occurs at, or in very close proximity to, the posterior margin (Figs. 3.6, 8.3, 8.7, 9.6, 9.10).

Figure 3 Light photographs of shale-hosted macrospecimens of Mickwitzia occidens. (1) Exceptionally preserved, bivalved specimen with preserved mantle setae and remnants of the phosphatic shell (arrows) displaying characteristic cones; specimen PRI 68595 (level 102.5 m). (2) Preserved setae along right? posterolateral shell margin [detail of (1)]. (3) Preserved setae along left? posterolateral shell margin [detail of (1)]. (4) Exfoliated ventral valve with weakly mineralized adult shell, transversely elliptical in outline; specimen PRI 68632 (level 86.7 m). (5) Exfoliated apex [detail of (4)] showing four pairs of enigmatic, subradial grooves, which might correspond to ridges in the apical cavity. (6) Apical view of juvenile ventral valve showing preserved reticulate pattern of exterior shell surface around apex; specimen PRI 68597 (level 102.5 m; specimen coated with a sublimate of ammonium chloride). (7) Subspherical ventral valve, in which only phosphatic shell material of juvenile/proximal growth stages is preserved indicating a primarily poorly mineralized adult/distal shell; specimen PRI 68593 (level 69 m). (8) Left posterolateral margin of (7) with subradial short grooves interpreted as remnants of shell-penetrating setae (compare with Fig. 14.2). (9) Anterior margin of (7) lacking short grooves; fine pits indicative of characteristic shell structure of inwardly pointing cones. (10) Mold of elongate-elliptical ventral valve with original phosphatic shell almost entirely absent; specimen PRI 68605 (level 69 m). (11) Subspherical ventral valve with fine pitting, indicating remnants of original shell structure; specimen PRI 68621 (collected from talus, ca. level 104 m; specimen photographed submerged in water to increase contrast). (12) Subspherical dorsal valve with marginal apex and remnants of original shell structure; specimen PRI 68601 (level 102.5 m; specimen photographed submerged in water). (13) Posteriormost margin of (12) showing robust, well-mineralized shell zone (arrows); zone equivalent in size to the carbonate-hosted sickle-shaped shell fragments (Fig. 8). Scale bars=5 mm (1, 4, 7, 10–12), 2 mm (2, 3, 5, 8, 9), and 1 mm (6, 13).

Shell structure and composition

A degree of weathering and exfoliation affects most of the mineralized components of the shells in the shale facies with the original shell structure being significantly affected or absent as a result. The valves appear weakly phosphatized except for a distinctly thicker region that extends from the umbo of dorsal valves. In more distal portions of the shell, phosphate appears with a patchy distribution.

Exfoliated shell layer surfaces reveal the inwardly pointing cone structures. They manifest as a pustulose texture in the whole specimens (not to be confused with the external surface ornament within which they are interspersed; Fig. 3.2). These are equivalent to the inwardly pointing cones observed in the acid-recovered fragments.

The overall structure of the shell in cross section is dominated by a series of thinly laminated phosphatic sheets. These sheets are interrupted by the presence of regularly spaced, inwardly pointing phosphatic cones. At the interface of these cones with the parallel lamellae, there is a pronounced inward bending of the phosphatic lamellae around the cone structures (Figs. 4.6, 10.7, 10.13, 11.9). Thickness of the shell measured in a polished section of a shale-hosted specimen not affected by dissolution varies along the length from 150 to 300 µm but averages ~250 µm (Fig. 4.4). This contrasts with observations on the acid-etched material, where shell thickness does not exceed 100 µm (e.g., Figs. 5.12, 10.10–10.15). However, the shell of the polished shale-hosted specimen seems to be unusually thick also because the inwardly pointing cones barely elevate from the interior valve floor (Fig. 4.4). In the acid-etched material, a variety of fragments has been found that have irregular areas of similar shell thicknesses and barely elevated cones (Figs. 5.9, 5.13, 5.14, 13.6; potentially also Fig. 5.1, 5.4, 5.16). Inwardly pointing cones are commonly 200–250 µm apart (Figs. 5.6, 6.14). Arrangements of the cones in straight or bent rows have been observed (Figs. 3.2, 5.2), as well as denser groupings of cones and areas free of cones particularly in the ventral apical cavity (Figs. 5.3, 6.6, 6.8).

Figure 4 Backscatter electron (BSE) images of marginal setae and shell structural details in shale-hosted specimens of Mickwitzia occidens. (1–3) Specimen PRI 68595 with preserved mantle setae (see also Fig. 3.1); (1) anterolateral shell margin showing dispersed phosphate and setae; (2) close-up of exceptionally preserved mantle setae [detail of (1)]; bright spots correspond with iron pyrite framboids that comprise the exceptionally preserved setae; (3) detail of (2). (4–6) Polished shell cross sections of specimen PRI 68634 (collected from talus, ca. level 20 m); (4) micrograph showing compact shell lamellae bending around inwardly pointing cone structures (r=resin, sh=shell, mx=rock matrix); (5) internal structure of inwardly pointing cone showing faint transverse ridges (arrow), potentially equivalent to growth lines [detail of (4)]; (6) mapping of distribution of phosphorus (P), calcium (Ca), and silica (Si) in shell and rock matrix; note diagenetically formed silica deposits parallel to shell layers and phosphorus-free matrix. Scale bars=2 mm (1), 200 µm (2), 100 µm (4, 6), and 50 µm (3, 5).

Figure 5 SEM images of shell fragments of Mickwitzia occidens recovered by acid-dissolution of carbonate rocks. (1, 4, 9, 13, 14, 16) Shell fragments showing areas of thicker shell; (1, 4) specimen 062-10 (level 62 m); (9) specimen 024-01 (level 62 m); (13) specimen 032-03 (level 88.5 m); (14) specimen 039-05 (level 59 m); (16) specimen 021-03 (level 69 m). (2, 3, 6, 8) Shell fragments showing the inner shell surface with inwardly pointing cones; (2) specimen 062-11 (level 62 m) with aligned cones; (3) specimen 032-01 (level 88.5 m) with irregularly distributed cones; (6) specimen 019-03 (level 53 m) with regularly distributed cones; (8) specimen 019-04 (level 53 m) with worn cones. (5, 7, 10, 11, 15, 17) Shell fragments showing preservation of the outer shell surface; (5) exfoliated specimen 017-08 (level 69 m) showing no preserved ornamentation; (7) exfoliated specimen 024-08 (level 62 m) with radial sculpture still visible; (10) pristine shell surface without any pores, specimen 035-10 (level 62 m); (11) slightly exfoliated specimen 035-03 (level 62 m) with few pores; (15) specimen 018-08 (level 69 m) with exfoliated areas showing pores and pristine areas with reticulate ornament without pores; (17) large exfoliated fragment with weak reticulate pattern, specimen 030-04 (level 98 m). (12) Exfoliated shell fragment in cross section, specimen 017-09 (level 59 m). Scale bars=1 mm (16, 17), and 500 µm (remaining).

Figure 6 SEM images of apical fragments of ventral valves of Mickwitzia occidens recovered by acid-dissolution of carbonate rocks to show variability in shape of apices. (1, 2) Lateral and ventral views with commonly displayed shell breakage at shell edges, specimen 020-01 (level 59 m). (3, 4, 8–10, 13) Specimens displaying range of shape and preservation in recovered apices; note first-formed shell zone lacking ornamentation; (3, 8) slightly exfoliated conical specimen 018-01 (level 69 m) with few pores visible, revealing underlying inwardly pointing cone structure; apical (3) and profile (8) views; (4) rather flat specimen 024-11 (level 62 m) with pristine shell surface; (9, 10, 13) conical specimens with the lateral and posterior slopes evenly inclined and the anterior slope less so; pores only visible in exfoliated areas; (9) specimen 023-10 (level 62 m); (10) specimen 017-01 (level 59 m); (13) specimen 024-12 (level 62 m). (5–7, 11, 12, 14) Internal shell surfaces of preserved apices; note inwardly pointing cones present in all but absent in zone of first-formed shell in (5) and (7); (5) specimen 024-14 (level 62 m); (6) specimen 036-03 (level 62 m); (7) specimen 024-10 (level 62 m); (11) specimen 016-03 (level 55.5 m); (12) specimen 018-07 (level 69 m); (14) specimen 018-02 (level 69 m). Scale bar=500 µm.

Rather than penetrating the phosphatic laminations that run parallel to the shell surface, the cones are situated within a draped series of these laminations that consistently point inward away from the surface of the shell at both ends of the tube, bending to follow the direction of the associated perpendicular cone. The connection of lamellae between consecutive inwardly pointing cones resembles a series of arches in cross section (Fig. 4.4). By extension of this growth pattern to the shell surface, this explains how the pustulose ornamented texture observed on the surfaces of nonexfoliated shells develops, although an inward cone is not located at the ‘trough’ of every such depression. Thus inward cones, although closely allied to the pustulose ornament are not directly correlated in terms of the regularity of their appearance. More extensive surveying of thin sections is required to conclusively resolve the relationship between the inwardly pointing cones and surface ornament.

The core of the inwardly pointing cones appears to have no discernable structure, and there is no evidence of striated ornament or impressions on the wall of the canals that would point toward the presence of setae in these pores (Fig. 10.1–10.6). In some examples, the canals appear to change diameter along their length as they pass through the laminated shell layers.

In the carbonate-hosted specimens, the shell comprises a series of parallel-laminated phosphatic sheets penetrated by the hollow, infilled, phosphatic inwardly pointing cones. They only penetrate through to the surface when the outer original shell layers are exfoliated. The ornament of pustules is restricted to the outer shell layers and becomes weaker with increasing degree of exfoliation (e.g., Figs. 6.10, 11.1–11.3). Inner shell layers appear as massive sheets of phosphate with no conspicuous ornamentation (Figs. 5.5, 5.15, 7.4).

Figure 7 Additional SEM images of ventral valve preservation of Mickwitzia occidens. (1–3) Recrystallized specimen 023-09 (level 62 m) with corroded apex but with inwardly pointing cone structure preserved; (2) detail of (1); (3) detail of (2). (4) Exfoliated valve displaying pores corresponding with inwardly pointing cones, specimen 023-06 (level 62 m). (5, 6) Ventral valve showing depression at apex, specimen 023-01 (level 62 m); (6) detail of (5) showing circular pit of unknown origin [arrow in (5)]. (7–12) Well-preserved apices with little to no evidence of pores but with distinct, first-formed shell zone lacking ornament; (7) specimen 019-01 (level 53 m); (8, 9) same specimen as in Figure 6.1, 6.2; (10, 11) same specimen as in Figure 6.10; (12) same specimen as in Figure 6.4. (13–15) Specimen 018-03 (level 69 m) with ‘pore’ structures at apex; (14, 15) close-ups of ‘pores’ of (13). Scale bars=200 µm (1, 4, 7–9), 100 µm (2, 5, 10–13), 20 µm (14, 15), and 10 µm (3, 6).

The recovered shell fragments are generally well preserved, but many of them suffered recrystallization that has destroyed the original shell ultrastructure. In these specimens, shell cross sections appear massive, not displaying any original structural details (e.g., Figs. 7.2, 8.12, 10.11, 11.11). Most specimens, however, show the original multilamellar shell fabric, similar to that of Mickwitzia muralensis described by Balthasar (Reference Balthasar2004). Individual lamellae are 1–3 µm thick and separated by a narrow gap, most likely representing originally an organic membrane (Fig. 10.13). A silica infill in this location is noted in a number of specimens.

Figure 8 SEM images of dorsal valves of Mickwitzia occidens, acid-recovered specimens from carbonate samples. (1, 3, 5, 10) Robust, well-mineralized shell fragment with pustulose ornament, specimen 033-03 (level 62 m); (1) anterior view showing acrotretid-like laminated architecture and columns; (3) apical view; (5) oblique posterior view; (10) detail of (5) showing posteriormost shell margin with exfoliated area with pores and pristine area with reticulate ornamentation. (2, 4) Small, slightly exfoliated specimen 017-10 (level 59 m) showing inwardly pointing cones on interior valve surface; (2) anterior view; (4) apical view. (6) Oblique anterior view of robust specimen 023-08 (level 62 m) showing acrotretid-like shell structures. (7, 13) Remineralized dorsal valve fragment, specimen 023-11 (level 62 m); (7) obique posterior view of apical area; (13) apical view. (8, 9, 12, 14) Partly remineralized specimen 034-07 (level 59 m) with inwardly pointing cones (12, 14) and secondary phosphate growth over outer shell layers; (8) oblique ventral view; (9) posterior view; (12) shell cross section close to apex; (14) anterolateral view. (11) Fragment with poorly preserved apical surface area in oblique lateral vuew, specimen 037-03 (level 62 m). (15, 16) Exterior (15) and interior (16) of poorly preserved fragment still displaying pustulose ornament and inwardly pointing cones, specimen 037-04 (level 62 m); (15) posterior view; (16) anterior view. Scale bar=350 µm (16), 250 µm (8, 9, 14), 200 µm (10), 150 µm (12), and 500 µm (remaining).

Besides the hollow cones, straight canals that measure 40–50 µm in diameter pierce the shell at a low angle rather than perpendicularly (Fig. 12). Their function as setal canals is supported by the fine longitudinal striations of the canal walls (Fig. 12.8, 12.12), which are reminiscent of seta-forming microvilli, and the presence of phosphatized setal remains inside some of the canals (Fig. 12.11).

Exceptionally preserved structures

In a number of specimens (N=3), mantle setae are preserved in pyrite with a distinct framboidal texture. Mantle setae are composed of chitin secreted from setoblast cells in living brachiopods, thus the replication of the setae represents exceptional preservation of nonbiomineralized tissues.

The distribution of setal structures is restricted to the distalmost shell margin. Setae are typically oriented in a radial fashion originating from the centroid of the organism but with a posterior-facing tendency along the posterolateral margins (Figs. 3.1, 3.2, 14.1). Setae appear to be present around the whole circumference of the valve with the exception of the medial anterior margin. They are apparent as dark projections from the shell under stereoscopic light microscopy. Backscatter electron microscopy also delineates these structures from the surrounding matrix (Fig. 4.1–4.3). The distinction is facilitated by setal preservation as framboidal pyrite, which contrasts with the surrounding mudstone matrix. The observed direction and distribution of setae matches the distribution of small ridges on several macrospecimens (e.g., Fig. 14.2). These ridges measure 400–600 µm in length and most likely represent the setae-hosting canals that pierce the shell at a low angle. Their distribution indicates that such canals might have been an adult feature, because they only occur close to the shell margin and are absent from more juvenile growth stages of the shell.

Phosphatized setae contained within canals have previously been described in other fossilized linguliform and rhynchonelliform brachiopods (Jin et al., Reference Jin, Zhan, Copper and Caldwell2007; Popov et al., Reference Popov, Bassett, Holmer and Ghobadi Pour2009). Unlike in the siphonotretids described by Popov et al. (Reference Popov, Bassett, Holmer and Ghobadi Pour2009), in Mickwitzia occidens, the hollow canals containing the setae do not extend outward from the shell surface. The discovery of these shell-penetrating setae bearing canals in Mickwitzia further supports the view that the genera Micrina Laurie, Reference Laurie1986, Tannuolina Fonin and Smirnova, Reference Fonin and Smirnova1967, and Mickwitzia constitute members of a Cambrian stem group of Linguliformea because these structures are present in all of these taxa, although it should be pointed out that this is likely a symplesiomorphic character.

Acrotretoid-like shell structures

Within the apical part of both ventral and dorsal valves, structures are occasionally present that are reminiscent of columnar laminae of the acrotretoid shell structure (see, e.g., Williams and Holmer, Reference Williams and Holmer1992; Holmer et al., Reference Holmer, Popov and Streng2008a). These comprise hollow columns of calcium phosphate with an average diameter of ~15 µm bound by an inner and an outer lamella. Columns and lamellae define laminae of variable thickness (e.g., Figs. 9.13, 9.17, 13.13–13.15). The columns run perpendicular to the flat, phosphatic shell lamellae and are distributed in irregular fashion. The intralaminar space is typically filled with spherulitic phosphate granules (Figs. 9.13–9.17, 9.19, 13.7, 13.15) referred to as ‘cocci’ in previous descriptions (Holmer et al., Reference Holmer, Skovsted and Williams2002; Skovsted and Holmer Reference Skovsted and Holmer2003; Skovsted et al., Reference Skovsted, Brock, Holmer and Paterson2009a). These have been variably described as of bacterial, diagenetic, or decay-fabric origin. Cocci are present in both limestone- and shale-hosted examples and are often found clustered around phosphatic columns and lamellae alike. There seems to be a direct connection between the inwardly pointing cones and the columns of the laminae. The columns have about the same diameter as the cones and appear to be extensions of the cones (Fig. 13.9, 13.13). The ultrastructure of the columns is unclear; multilamellar (equivalent to cones; Fig. 13.14) and massive (potentially recrystallized; Fig. 9.14) structures have been observed.

Figure 9 SEM micrographs of additional dorsal valves of Mickwitzia occidens, acid-recovered specimens from carbonate samples. (1–3, 5) Heavily mineralized and recrystallized dorsal, sickle-shaped apex fragment; note pseudointerarea-like surface (2) and the W-shaped margin to the preserved shell area, specimen 039-02 (level 59 m). (4, 7, 9) Dorsal fragment with pseudointerarea-like surface, specimen 024-07 (level 62 m); (7) close-up of shell laminations showing an accretionary-like succession of phosphate layers with nick points presumably for setae canals, subsequently overgrown; (9) anterior view with poorly preserved interior surface. (6) Poorly preserved apical fragment with central apex, specimen 040-01 (level 62 m). (8, 11) Corroded specimen 037-01 (level 62 m) showing transition from ornamented posterior shell margin to pseudointerarea-like surface; (8) entire specimen in ventral view; (11) detail of (8). (10, 13, 17) Close-ups of specimen in Figure 8.6; (10) dorsal surface with unornamented first-formed shell; (13, 17) cross sections close to apex showing acrotretid-like columns interspersed between phosphatic lamellae; space between phosphatic layers is infilled with phosphatized cocci structures. (12) Inwardly pointing cone structures; same specimen as in Figure 8.2, 8.4. (14–16) Close-ups of specimen shown in Figure 8.1 showing cross section through acrotretid-like columns with surrounding cocci. (18) Detail of Figure 8.16 showing interior surface from near apex in a seemingly not-fully-grown specimen; note absence of acrotretid columns and poor development of inwardly pointing cones. (19) Details of phosphatic cocci of specimen 039-03 (level 59 m). Scale bar=500 µm (1–6, 8, 9), 200 µm (7, 10, 11, 13, 17), and 150 µm (18); thin scale bars=50 µm (12, 14–16) and 10 µm (19).

It is also currently unresolved during which ontogenetic stage the columnar layers would have formed. The columns might represent a juvenile feature that is lost or overgrown in subsequently deposited adult shell material, or they could be an adult/gerontic character that only would have been present in relatively large shells to provide extra stability.

The presence of columnar structures in members of the brachiopod stem group (Mickwitzia and Setatella) and in tommotiids (e.g., Micrina) strengthens the suggested link between one group of small shelly fossils and linguliform brachiopods (e.g., Holmer et al., Reference Holmer, Popov and Streng2008a; Skovsted et al., Reference Skovsted, Clausen, Álvaro and Ponlevé2014). We consider these as homologous structures in the stem Brachiopoda and closely allied tommotiids.

Growth patterns of valves

The conical ventral valve fragments with their centrally situated apices suggest a holoperipheral growth pattern in the ventral valve of Mickwitzia occidens. This is supported by complete ventral valves from the shale-hosted material that displays a subcentral apex. In contrast to the seemingly hemiperipheral growth of complete dorsal valves (Fig. 3), the sickle-shaped fragments tell a different story. The apex of the dorsal valve is not positioned on the margin itself but rather inset ~300 µm from the posterior margin. This implies that at up to a diameter of ~300 µm, dorsal valves grew in a holoperipheral fashion. Subsequently, a hemiperipheral growth pattern was then adopted. We interpret this as a shift in growth modes from a juvenile to adult pattern of shell growth.

Morphometric analyses

Although shell measurements and shape have been used as taxonomic criteria within stem Brachiopoda, no quantitative descriptive measure of such features has been yet utilized. The developmental basis of these features is also of interest in determining the character homologies of phosphatic-shelled stem lophotrochozoan taxa and possible links to tommotiid ancestors.

No discernible discriminant signal could be retrieved from the dorsal values, however, ventral valves from Västergötland and Nevada representing individuals of Mickwitzia monilifera and M. occidens, respectively, could be differentiated with principal component analysis, and discriminant function analysis. Based on these results, landmarks 1, 2, and 3 (Fig. 15.1), which indicate the rearward placement of the ventral shell apex, constitute strong signals for species diagnosis (recovering the separate taxonomic groups in 100% of cases). The outline landmarks also display a significant discriminant signal (length:width ratio) that can be more useful under an outline shape analysis. Cross-validation scores recovered the correct taxonomic groupings in 61.5% of Nevada specimens based upon comparisons of the ventral valve. Mickwitzia monilifera was recovered 66.67% of the time.

Comparison with the holotype

The holotype of Mickwitzia occidens (USNM 51518a) is an unevenly convex, irregularly shaped fragment of a variably exfoliated valve in exterior view, measuring ~6 mm in diameter. The unevenly convex shape of the fragment, with a steep and a gently inclined slope along its longest axis, suggests the position of an apex at the marginally situated culmination. No original shell margin or pristine shell surfaces are preserved. Instead, the fragment shows different degrees of exfoliation, with one-third of the shell area, which includes the presumed apex, being stronger affected than the other two-thirds. In the smaller area, pores related to the inwardly pointing cones are more distinct and no original shell ornament is evident (comparable with, e.g., Fig. 5.5 and exfoliated area in Fig. 5.15). In the remaining two-thirds, equivalent pores are smaller and remnants of the original external shell ornament are preserved as faint radial ridges (comparable with, e.g., Fig. 5. 7, 5.17). The convex shape of the fragment suggests the holotype to be a ventral valve, however, due to the small size of the fragment, an incomplete dorsal valve with a potentially marginal apex cannot readily be excluded.

The holotype was collected from sandstones at U. S. Geological Survey locality 174c on a small hill in the salt flat, 1.6 km northeast of Silver Peak Mill, Esmeralda County, Nevada (Silver Peak Quadrangle; Walcott, Reference Walcott1908, Reference Walcott1912), which is ~25 km west of Indian Springs Canyon. The type level is uncertain, but is likely to be within the Harkless Formation (Rowell, Reference Rowell1977; Mount, Reference Mount1980), which overlies the Poleta Formation and belongs to the Bonnia-Olenellus Biozone.

Based on the occurrence of the holotype and the Indian Springs specimens in the same general area and time interval, we consider them to be conspecific. However, the incomplete preservation of the holotype makes the comparison with the Indian Springs material not entirely unequivocal.

Remarks

Mickwitziids differ from linguliform brachiopods by the combination of a columnar shell structure with striated shell-penetrating setal tubes, poorly developed pseudointerareas, pustulose shell ornament, a shell apex in a submarginal, posterior-displaced position, and the shell margin round in shape and much less elongate. However, the taxonomic composition and phylogenetic position of this informal grouping need to be further investigated.

Genus Mickwitzia Schmidt, Reference Schmidt1888

Type species

Lingula? monilifera Linnarsson, Reference Linnarsson1869, p. 344–345, pl. 7, figs. 1, 2 from the lower Cambrian (Series 2, Stage 4) Mickwitzia Sandstone Member of the File Haidar Formation near Lugnås, Västergötland (Sweden).

Diagnosis

Relatively large, ventribiconvex stem-group brachiopods, ovate to subcircular in outline, with organophosphatic shell composition. Shell ultrastructure multilamellar, typified by inwardly pointing cones perpendicular to shell surface containing hollow canals; shell can be penetrated by tangential canals bearing microvilli impressions and setae. Outer shell surface with a pustulose reticulate surface ornament. Both valves with rudimentarily developed pseudointerareas.

Occurrence

Mickwitzia has been reported from the Cambrian Series 2 of western Laurentia (e.g., Walcott, Reference Walcott1908; Mount, Reference Mount1976, Reference Mount1980; McMenamin, Reference McMenamin1992; Balthasar, Reference Balthasar2004), Baltica (e.g., Walcott, Reference Walcott1912; Jensen, Reference Jensen1993), and Australia (Skovsted et al., Reference Skovsted, Brock, Holmer and Paterson2009a). Material described as Mickwitzia from eastern Laurentia (Greenland; e.g., Skovsted and Holmer, Reference Skovsted and Holmer2003, Reference Skovsted and Holmer2005) is now considered to represent a distinct genus, Setatella (Skovsted et al., Reference Skovsted, Streng, Knight and Holmer2010).

Remarks

With the separation of the eastern Laurentian mickwitziids from Mickwitzia and their placement in the new genus Setatella by Skovsted et al. (Reference Skovsted, Streng, Knight and Holmer2010), a more precise and confined diagnosis of the genus Mickwiztia is now possible (compare Holmer and Popov, Reference Holmer and Popov2007). Mickwitzia is clearly defined and separated from Setatella and the crown linguliform brachiopods by its diagnostic shell ultrastructure of inwardly pointing cones. In this respect, Mickwitzia is only comparable to Heliomedusa, which in fact might represent a junior synonym of Mickwitzia (see Discussion and Interpretation below).

In addition to the type species, Mickwitzia occidens and M. muralensis are herein included in the genus. Mickwitzia lochmanae (nom. corr. herein, ex M. lochmana McMenamin, Reference McMenamin1992) from the Puerto Blanco Formation (Cambrian Series 2) of Sonora, Mexico is only known from a single, exfoliated macrospecimen, a supposed ventral valve (McMenamin, Reference McMenamin1992). Specimens described as M. multipunctata McMenamin, Reference McMenamin1992 were obtained by the dissolutions of limestone samples from the same formation. These specimens are nondiagnostic shell fragments, which are indistinguishable from any other Mickwitzia species and are most likely conspecific with M. lochmanae. Furthermore, the single known valve of M. lochmanae displays a central apex, which suggests reference to M. muralensis. Because of the unsatisfactory knowledge of both M. lochmanae and M. multipunctata, their likely synonymy, and the resemblance of the holotype of M. lochmanae with M. muralensis, both taxa described by McMenamin (Reference McMenamin1992) are herein considered nomina dubia.

Specimens described by Skovsted et al. (Reference Skovsted, Brock, Holmer and Paterson2009a) from Australia as Mickwitzia sp. have been compared with M. muralensis but are too fragmentary to allow a species determination. However, their ornamentation with smaller and seemingly wider-spaced pustules might suggest a new species.

Mickwitzia occidens Walcott, Reference Walcott1908

Figures 3–13

1908 Mickwitzia occidens Walcott, p. 54, pl. 7, fig. 1.

1912 Mickwitzia occidens; Walcott, p. 331, pl. 6, fig. 4.

?1976 Mickwitzia occidens; Mount, fig. 3.

1977 Mickwitzia occidens; Rowell, p. 79, pl. 1, fig. 12.

?1980 Mickwitzia occidens; Mount, fig. 3.

?1992 Mickwitzia muralensis Walcott, Reference Walcott1913; McMenamin, p. 181, figs. 4.1, 5.4–5.6.

2001 Mickwitzia, Babcock, fig. 2.

non 2002Mickwitzia cf. occidens; Holmer, Skovsted and Williams, text-figs. 2–3.

2003 Mickwitzia occidens; Skovsted and Holmer, fig. 2.

non 2003 Mickwitzia cf. occidens; Skovsted and Holmer, p. 3, figs. 3–5, 7–12.

non 2005 Mickwitzia cf. occidens; Skovsted and Holmer, p. 327–328, pl. 1, figs. 9–13.

2007 Mickwitzia occidens; Holmer and Popov, fig. 1711d–i.

non 2007 Mickwitzia sp. cf. occidens; Holmer and Popov, figs. 1712, 1713.

2010 Mickwitzia occidens; English and Babcock, fig. 4a, b.

2011 Mickwitzia occidens; Hollingsworth and Babcock, fig. 5.6, 5.7.

Holotype

Exfoliated valve (USNM 51518a) from the lower Cambrian (=Series 2) of the Silver Peak quadrangle, Esmeralda County, Nevada, U. S. Geological Survey locality 174c (Walcott, Reference Walcott1908, pl. 7, fig. 1; reillustrated by Rowell, Reference Rowell1977, pl. 1, fig. 12). According to Rowell (Reference Rowell1977) and Mount (Reference Mount1980), the holotype was collected from the Harkless Formation in the lower part of the Bonnia-Olenellus Biozone.

Diagnosis

Species of Mickwitzia with ventribiconvex shell and regularly distributed pustules. Marginal setae, where present, oriented in posterior facing direction on flanks. Position of apex variable but always posterior to shell centroid and anterior to posterior margin in ventral valve; in dorsal valves, apex typically coincident or in very close proximity to posterior shell margin.

Occurrence

Nevadia addyensis Biozone to lower? Bonnia-Olenellus Biozone (Cambrian Series 2, Stage 3–4), western Laurentia (northern Mexico, Nevada, and California; see synonymy; biozonation after Hollingsworth, Reference Hollingsworth2011).

Remarks

Balthasar (Reference Balthasar2004) interpreted the inwardly pointing cones with hollow canals to have hosted setae. However, no evidence for perpendicular shell-penetrating setae, such as striation in association with the canals, could be observed. We interpret this structure as a punctae-like outfolding of the mantle tissue.