Introduction and conventions

Nearly all of the fossils described herein are internal molds that began as internal coatings (Fig. 1). They reflect the inner shell surface, and thus provide a negative image of the original inner shell microstructure that faced the interior of the shell. The imprints in these fossils often reflect fundamental microstructural units, such as laths (blades), tablets, and prisms.

Figure 1.

(1–11) Phosphatic steinkerns from the Upper Ordovician of Kentucky and Ohio, isolated (1–9) and in thin section (10, 11): (1–4, 10, 11) gastropods, (5, 6) cyrtonellid tergomyan, (7, 8) bivalves, and (9) bryozoan. (1) gastropod steinkern from the Kope Formation, AA Highway (KY 9) south of Alexandria Pike, LACMIP 14794 (LACMIP Locality 41957). (2) Gastropod steinkern, LACMIP 14795 (LACMIP Locality 41967). (3) Gastropod steinkern, LACMIP 14796 (LACMIP Locality 41967). (4) Gastropod steinkern, LACMIP 14797 (LACMIP Locality 41960). (5) Cyrtolites sp., LACMIP 14798 (LACMIP Locality 41960). (6) Cyrtolites sp., LACMIP 14799 (LACMIP Locality 41967). (7) Homilodonta sp., LACMIP 14800 (LACMIP Locality 41960). Arrow indicates muscle scar whose magnified views are shown in Figure 4.1–4.3. (8) Hinge-fill of Lyrodesma sp., LACMIP 14801 (LACMIP Locality 41958). (9) Bryozoan from the Kope Formation near Newport Kentucky, LACMIP 14802. (10) Thin section showing gastropod steinkern, LACMIP 14803 (LACMIP Locality 41968). (11) thin section showing gastropod steinkern, LACMIP 14804 (LACMIP Locality 41969). (12) Gastropod Cyclonema gracile, LACMIP 14805 (LACMIP Locality 41959). Thin section shows intersection of two whorls; upper whorl (left side) external surface is to the right. All scale bars are 200 μm.

Terms that indicate relative vertical position within the original shell (e.g., above, atop, below) are used with the up direction being from the inner shell surface (facing shell interior) toward the external (outer). Thus ‘smaller tablets below larger ones’ means there are smaller tablets on the very innermost shell surface, with larger ones closer to the external shell surface (i.e., following the sequence of secretion).

We utilize general terms for the layering within the shell, such as ‘inner shell layer’ or ‘outer layer’. We use these in place of the more specific terminology of layers defined in reference to the myostracum (e.g., m, m-1, m + 1) defined by MacClintock (Reference MacClintock1967) because in our specimens a complete vertical section of the shell microstructure is lacking, therefore it is impossible to know how many shell layers below the myostracum is the preserved inner shell layer. More importantly, the myostracum cuts diagonally through all other shell layers, so a system of layer identification based on the myostracum confuses the issue.

Guidelines for interpreting shell microstructure imprints in apatite are described in Runnegar (Reference Runnegar1985) and Vendrasco et al. (Reference Vendrasco, Rodríguez-Navarro, Checa, Devaere and Porter2015). A consideration of interfacial angles is important in the classification of the imprints wherever intersections of faces of the crystalline units (e.g., tablets and blades) are preserved. This method can distinguish, for example, between similar imprints produced by nacre and calcitic semi-nacre, two shell microstructures common in early mollusks and that have different structural properties. In rhombic calcitic semi-nacre, the calcite rhombs have expected angles of 78° and 102° (Taylor and Weedon, Reference Taylor and Weedon2000), whereas in nacre the typical expected angles are 64° (110/$ {\bar 1} $10), 90° (100/010), 116° (110/1$ {\bar 1} $0), and 122° (110/010) (Runnegar, Reference Runnegar1985). The two most common polygonal tablet shapes are diamond rhombuses showing 110, $ {\bar 1} $10, $ {\bar 1} $$ {\bar 1} $0 and 1$ {\bar 1} $0 faces, and hexagons showing all the rhombus faces plus added 010 and 0$ {\bar 1} $0 faces (Taylor et al., Reference Taylor, Kennedy and Hall1969, figs. 6, 7; Wada, Reference Wada1972, fig. 15, types ɑ1, ɑ2, and ɑ5). For rhomboidal nacre tablets (ɑ1) the expected interfacial angles are 64° and 116° (equally common); for hexagonal tablets (ɑ2, ɑ5) the expected angles are ~116° and 122° (with 122° twice as common). For type ɑ4 (Wada, Reference Wada1972) rectangular tablets, only 90° angles are expected. For the rare octagonal tablets (e.g., types ɑ3 and ɑ6 of Wada, Reference Wada1972), 122° (110/010) and 148° (100/110) are expected at equal frequency. Electronically inverted images of nacre in modern mollusks (Fig. 2) are useful in comparing with shell microstructure imprints in the phosphatic steinkerns described herein.

Figure 2.

Shell microstructure in modern mollusks, for comparison with fossil imprints. All images electronically inverted to allow direct comparison with the internal molds. (1) Aragonitic nacre-like tablets in the vetigastropod Ethminolia Iredale, Reference Iredale1924, LACMIP 14806; (2) foliated aragonite in the monoplacophoran Micropilina, LACMIP 14807; (3) nacre tablets in the bivalve Pteria hirundo, LACMIP 14808; (4) incipient nacre tablets in the bivalve Nucula, LACMIP 14809; (5) nacre tablets in the bivalve Pinctada martensii (Dunker, Reference Dunker and Küster1880), LACMIP 14810; (6, 7) nacre growth towers (6) and nacre tablets in the septum (7) of Nautilus pompilius Linnaeus, Reference Linnaeus1758, LACMIP 14811; (8) foliated calcite in the bivalve Anomia ephippium Linnaeus, Reference Linnaeus1758, LACMIP 14812. Scale bars are (1, 7) 10 μm; (2–6, 8) 3 μm.

Shell microstructure terms follow Carter (Reference Carter and Carter1990) and Carter et al. (Reference Carter, Harries, Malchus, Sartori, Anderson, Bieler, Bogan, Coan, Cope, Cragg, García-March, Hylleberg, Kelley, Kleemann, Kříž, McRoberts, Mikkelsen, Jojeta, Skelton, Tëmkin, Yancey and Zieritz2012). Higher-level molluscan taxonomy follows Carter et al. (Reference Carter, Altaba, Anderson, Araujo, Biakov, Bogan, Campbell, Campbell, Jin-hua, Cope, Delvene, Dijkstra, Zong-jie, Gardner, Gavrilova, Goncharova, Harries, Hartman, Hautmann, Hoeh, Hylleberg, Bao-yu, Johnston, Kirkendale, Kleemann, Koppka, Kříž, Machado, Malchus, Márquez-Aliaga, Masse, McRoberts, Middelfart, Mitchell, Nevesskaja, Özer, Pojeta, Polubotko, Pons, Popov, Sánchez, Sartori, Scott, Sey, Signorelli, Silantiev, Skelton, Steuber, Waterhouse, Wingard and Yancey2011) and Wagner (Reference Wagner2013).

Bivalves

Polygonal tablets merged into laminae were observed in internal molds of three species of bivalve in the assemblage: Homilodonta Cope, Reference Cope1997 (sub-class Protobranchia; order Nuculida), and Lyrodesma Conrad, Reference Conrad1841 (sub-class Autobranchia; sub-cohort Unioni), and a member of the Afghanodesmatida (sub-class Protobranchia) similar to Leiodysodonta Pojeta, Zhang, and Yang, Reference Pojeta, Zhang, Yang and Pojeta1986. The arrangement and morphologic varieties of the tablets vary among these bivalve taxa, but for each species the tablets tend to be more or less co-oriented within and between horizontal laminae. The ranges of size, shape, and interfacial angles are for each species consistent with nacre (see below). Tablet centers are offset in successive laminae, as is typical for bivalve nacre.

Some specimens of afghanodesmatid bivalves show good details of nacre, either regionally (e.g., in a comarginal band near the aperture in one specimen) or in patches at various places over the internal shell surface (Fig. 3). The nacre is preserved as overlapping laminae of merged and merging tablets. The entire outlines of tablets are preserved, indicating the nacreous laminae are horizontal. New tablets are not centered under overlying ones, but instead occur in various locations relative to the centers of slightly older tablets, typical of bivalve nacre. The extent of co-orientation of tablets within and between laminae varies somewhat from region to region on internal molds, but in many places the co-orientation is strong.

Figure 3.

Shell microstructure imprints on steinkerns of Afghanodesmatid bivalves from the Ordovician Fairview Formation of Kentucky. (1–6) LACMIP 14813 (LACMIP Locality 41961); photos from near the aperture, visible in upper right of (1); (7, 8) LACMIP 14814 (LACMIP Locality 41962); near umbo. Numbers show measurements of well-preserved interfacial angles. Scale bars are (1, 3–8) 2 μm; (2) 1 μm.

Prior to merging with neighbors, tablets in the Afghanodesmatid bivalves are pseudo-hexagonal. Some tablets are roughly equidimensional and many are elongate. The elongate varieties are similar to those seen in a few modern mollusks, such as near the aperture of modern specimens of Nucula and in the septa of modern Nautilus (Fig. 2.7). Merged tablets show a variety of roughly equidimensional outlines, as in the nacre of modern mollusks. Isolated (not yet merged) tablets are typically on the order of a few micrometers in width (i.e., the shorter dimension in elongate tablets), whereas merged tablets are on the order of 5–10 μm in width. Interfacial angles of some of the best preserved tablets are 109.6°, 112.5°, 115.2°, 116.5°, 116.7°, 117.7°, 118.8°, 122.0°, 123.0°, 123.1°, 124.1°, 124.1°, 124.7°, 125.4°, 126°, 126.7°, 127.3°, 127.8°, and 128.7°, close to the expected angles 116° and 122°. Growing tablets are typically ~5–10 μm in length (measured along longer axis of tablet).

Specimens of Homilodonta show detailed preservation of nacre (Fig. 4). In the best-preserved specimens, imprints of tablets consistent with nacre can be seen over the entire inner surface of the shell. The tablets overlap, with smaller ones below larger ones on the inner shell surface (Fig. 4.6, 4.8). New tablets are co-oriented within and between laminae, and overlapping tablets are offset from each other such that new tablets initiated at boundaries between tablets in the overlying layer (Fig. 4.5). These observations are consistent with bivalve sheet nacre. In most cases, the entire outlines of tablets are preserved, indicating that the laminae are horizontal. In the muscle scars, closely spaced ends of laminae are preserved (Fig. 4.1, 4.2), indicating a slight inclination of the inner laminae in this region.

Figure 4.

Shell microstructure imprints on phosphatic steinkerns of the bivalve Homilodonta sp. from the Ordovician Point Pleasant Formation, Kentucky (LACMIP Locality 41960). (1–5) LACMIP 14800: (1) region near the adductor muscle scar shown in the arrow in Figure 1.7; (2) magnified view of the same adductor muscle scar; (3) image from near same adductor muscle scar; (4) near umbo; (5) transitional zone from microstructure of the other adductor muscle scar (on left edge of image in Figure 1.7; without arrow) and rest of inner shell surface; (6–8) LACMIP 14815: (6) near middle/top (closer to umbo than aperture); (7) near umbo; (8) near aperture. Numbers show measurements of well-preserved interfacial angles. Scale bars are (1) 10 μm; (2) 1 μm; (3–8) 2 μm.

Tablets in Homilodonta range in shape from hexagonal to sub-circular to sub-oval. Tablets typically begin as polygonal and become more circular or oval as they begin to merge with neighbors. The hexagonal outline of isolated growing tablets and lack of distinct growth fronts of merged tablets are similar to what occurs in modern Nucula Lamarck, Reference Lamarck1799 (Fig. 2.4). Tablet imprints occur over a narrow size range from a few to nearly 10 μm in width/diameter, and they are ~400 nm in height, both of which are consistent with dimensions of nacre in modern mollusks. Interfacial angles of some of the better-preserved polygonal tablets are close to what is expected for nacre (112.9°, 115.0°, 115.3°, 120.2°, 124.1°, 124.8°, 126°, 126.4°, 126.6°, and 132.1°, compared with the expected 116° and 122°). Some large tablets appear to show spiral growth (Fig. 4.6), as is seen in the nacreous tablets of modern mollusks.

In and around the muscle scars is a laminar shell microstructure consisting of a broad stair-step texture. We interpret this as laminar aragonite because the ends of laminae are remarkably straight, as in the faces of nacre tablets and unlike foliated aragonite that should show the ends of blades. Also there is gradation between the stair-step pattern and nacre tablets along some borders of muscle scars (Fig. 4.5). The muscle scars of modern bivalves, reflecting the myostracum shell layer, typically display a prismatic shell microstructure. Nevertheless, a similar broad, shallow stair-step nacre-like shell microstructure can be seen in the muscle scars of modern abalone.

There are orientation patterns of apatite crystals within tablet imprints and exposed laminae of muscle scars, but these do not seem to reflect original tablet ultrastructure as described by Mutvei (Reference Mutvei1970, Reference Mutvei1978). There is a large variety of different orientations in the zones of co-oriented apatite crystals, in contrast to the 3–4 zones typical in modern nacre. Plus, adjacent regions do not show expected aragonite twin angles.

The fossil species we show here is characterized by an inner shell layer of nacre and muscle scars of a laminar aragonite texture. In Homilodonta the high occurrence of merging tablets over the entire internal surface indicates that active nacreous growth was occurring at a fast rate throughout the shell. Isolated tablets growing in a diffuse manner without active growth fronts of the nacreous laminae occur in modern bivalves such as Nucula (Fig. 2.4).

Fossils of Lyrodesma sp. occur as internal molds of segments of the ligament region of the shell (Dattilo et al., Reference Dattilo, Freeman, Peters, Heimbrock, Deline, Martin, Kallmeyer, Reeder and Argast2016). These hinge molds match the shape and size of spaces in the hinge of calcite-replaced shells of Lyrodesma. These molds reflect the shell microstructure expressed on the inner surface of the shell wherein the ligament had grown. Well-preserved tablets are preserved on the inner walls of the radiating hard hinge elements (Fig. 5.1–5.5). The tablets tend to be hexagonal, with angle matches close to expected typical angles of nacre, 106° and 122°: 113.0°, 113.1°, 115.4°, 118.4°, 120.7°, 132°. In one fragmentary specimen (Fig. 5.4), tablets have a more rounded, sub-polygonal shape, although rhomboidal tablets are also present in that individual.

Figure 5.

(1–5) Shell microstructure (nacre) imprints on internal molds of the hinge region of the bivalve Lyrodesma sp. from the Ordovician Fairview Formation of Kentucky (LACMIP Locality 41961). (1, 2) LACMIP 14816; (3) LACMIP 14817; (4) LACMIP 14818; (5) LACMIP 14819. (6–9) Shell microstructure (nacre and nacre-like) imprints on steinkerns of the bellerophontiform Cyrtolites sp. from the Ordovician Fairview (6, 7) and Point Pleasant (8, 9) formations. Numbers show measurements of well-preserved interfacial angles. (6, 7) LACMIP 14820 (LACMIP Locality 41961); (8, 9) LACMIP 14821 (LACMIP Locality 41960). Scale bars are (1, 6) 10 μm; (2, 7) 5 μm; (3) 2 μm; (4, 5) 3 μm; (8, 9) 1 μm.

Diamond-shaped tablets seen in other specimens of Lyrodesma are similar in outline to those of Pteria hirundo (Linnaeus, Reference Linnaeus1758) (Fig. 2.3) and Pinctada margaritifera (Linnaeus, Reference Linnaeus1758) (Taylor et al., Reference Taylor, Kennedy and Hall1969, fig. 10). The interfacial angles of rhomboidal tablets are close to what is expected for rhomboidal nacre (88°, 94.6°, 57.7°, and 73°, compared with the expected 90° and 64°), and those of hexagonal tablets are similar to that of hexagonal nacre (116°, 119°, 123°, and 126°, compared with the expected 116° and 122°).

Tergomyan monoplacophorans (cyrtonellids)

Tiny, symmetrically coiled cyrtonellid univalves that we tentatively identified as Cyrtolites Conrad, Reference Conrad1838 are relatively common in these beds. A few individuals preserve exceptional details of the inner shell microstructure of this species.

Imprints of subcircular to polygonal units were observed on the surface of internal molds of some individuals of Cyrtolites (Fig. 5.6–5.9). The units typically range from 5–10 μm in diameter and are isolated as well as partly merged into layers. The units show a high variation in outline (elongate-oval, elongate polygonal, sub-circular, diamond), even within the same individual. In many cases the impressions of smaller units are found within larger ones. The units are short, ~300 nm in height, and in some cases they show striking co-orientation. All these observations are consistent with the units being incipient nacre tablets, and the measured angles of the inferred tablets likewise are consistent with this interpretation. The best preserved angles are 109°, 112°, 115°, 117°, 117°, 118°, 118°, 120°, 121°, 121°, 122°, 122°, 124°, 124°, 127°, 128°, and 129°, which are close to the 116° and 122° expected for typical nacre.

One specimen showed highly inclined ends of presumably aragonitic tablets. Such semi-laminar, slightly inclined, aragonite tablet-rich types of shell microstructures are very common in the innermost shell layer of modern vetigastropods, typically occurring just below the nacreous layer (e.g., see Erben, Reference Erben1972, pl. 2).

Morphological gradation occurs among the symmetrically coiled fossils of the assemblage, and thus it is possible that they represent only one or a few species. The symmetrically coiled univalves in these phosphate-rich coquinas were considered to be Cyrtolites inornatum (Hall, Reference Hall1845) by Martin (Reference Martin1986). Hall (Reference Hall1845, p. 292) originally described this species as a member of the genus Microceras Hall, Reference Hall1845, and referred to it as one of the many tiny shells from what he referred to as the “decomposing marl slate of Cincinnati.” His description lines up with the fossils we observed, although external characters that diagnose Cyrtolites do not help in the classification of these internal molds. Wahlman (Reference Wahlman1992) classified Cyrtolites as a tergomyan monoplacophoran based on the presence of multiple sets of muscle scars in some species. The symmetrically coiled fossils from these shell beds were referred to as monoplacophorans by Dattilo et al. (Reference Dattilo, Freeman, Peters, Heimbrock, Deline, Martin, Kallmeyer, Reeder and Argast2016), but they did not classify them further.

Although nacre had been thought to be the characteristic inner shell layer of modern monoplacophorans, a closer analysis of modern representatives found that foliated aragonite is predominant in the group (Checa et al., Reference Checa, Ramírez-Rico, González-Segura and Sánchez-Navas2009a, b). In any case, the fossils described herein show that nacre had originated within at least one lineage of monoplacophorans at least by the Ordovician.

The nacre in the tergomyan monoplacophorans (Cyrtolites) of this assemblage show characteristics of both monoplacophoran and gastropod varieties. Overall, gastropods and cephalopods tend to show columnar nacre whereas monoplacophorans and bivalves show sheet nacre (Hedegaard, Reference Hedegaard1997). The tablets of nacre in the sheet form tend to be highly co-oriented, wherein some tablets have the b-axis pointing towards the local margin and other tablets are oriented with the b-axis ~120° and 240° to this direction, reflecting aragonitic twinning. In columnar nacre, the tablets tend to have less-horizontal co-orientation within any one sheet, although there are some exceptions (e.g., there is some degree of horizontal co-orientation near the aperture of Nautilus Linnaeus, Reference Linnaeus1758). The tablets of Cyrtolites are in some cases clearly stacked (Fig. 5.7), as in gastropods (Fig. 2.6), cephalopods (Fig. 2.7), and some bivalves near the margin.

Gastropods

The tiny gastropod fossils in these beds have been historically referred to as Cyclora, although most of them probably represent the juveniles of larger species (Dattilo et al., Reference Dattilo, Freeman, Peters, Heimbrock, Deline, Martin, Kallmeyer, Reeder and Argast2016), possibly in some cases Cyclonema Hall, Reference Hall1852. Identification to species is difficult with these internal molds, but there are high- and low-spired varieties that probably represent the apical region of multiple species (Fig. 1.1–1.4).

Some internal molds show isolated tablets of the same size (a few μm in width) and with the same angles (121°, 128°, 129°) as the common 122° of many forms of modern nacre (Fig. 6.9). Other specimens show an inclined laminar texture with straight or polygonal growth fronts, similar to what occurs in the innermost layer of modern gastropods (see description of textures in Cyrtolites above). In other instances a diffuse polygonal network is preserved in some regions of the internal mold, although it is unclear what this might represent. The clearest preservation of shell microstructure was observed in a specimen with a blunt top end (Fig. 6.4–6.10), possibly produced as the apex was sealed off in a manner similar to what was inferred by Dattilo et al. (Reference Dattilo, Freeman, Peters, Heimbrock, Deline, Martin, Kallmeyer, Reeder and Argast2016). At the blunt apical end of the specimen many sharply polygonal tablets typically ~5 μm in diameter can be observed (Fig. 6.5–6.10). These show some degree of co-orientation, and many of these tablet imprints directly overlie others in what must have been isolated stacks of tablets. Such stacks are reminiscent of the classic towered nacre of modern vetigastropods.

Figure 6.

Shell microstructure (3, 5–10), septation (1), and phosphate cast of the umbilicus (2) on steinkerns of gastropods from the Ordovician Fairview (2) and Kope (1, 3–10) formations of Kentucky. (1) LACMIP 14822 (LACMIP Locality 41963), apical region showing septation; (2) LACMIP 14823 (LACMIP Locality 41961), phosphate fill of the umbilical region; (3) LACMIP 14824 (LACMIP Locality 41963), portion of an external mold where the overlying apical part of the steinkern is missing; (4–10) LACMIP 14825 (LACMIP Locality 41964); (4) entire specimen, showing that the apical region is blunt and appears incomplete; (5–10) closeup views of the apical region, showing nacre-like tablets; numbers show measurements of well-preserved interfacial angles. Scale bars are (1, 3) 10 μm; (2) 40 μm; (4) 100 μm; (5) 4 μm; (6–10) 2 μm.

Thompson (Reference Thompson1970) noted the common co-occurrence of the large platyceratid gastropod Cyclonema with the small steinkern gastropod fossils identified as Cyclora. Earlier, Ulrich (in Ulrich and Scofield, Reference Ulrich and Scofield1897) suggested that Cyclora was either the young or a dwarfed form of larger gastropods, such as Holopea Hall, Reference Hall1847 or Cyclonema, with which it occurs. Thompson (Reference Thompson1970) in particular noted the co-occurrence of Cyclora minuta Hall, Reference Hall1845 with Cyclonema bilix Conrad, Reference Conrad1842 and Cyclonema humerosum Ulrich and Scofield Reference Ulrich and Scofield1897, and a striking resemblance between the whorl profile in cross section of Cyclora minuta and the apical region of Cyclonema in the specimens that preserve it. Wagner (Reference Wagner2013) reassigned Cyclora minuta to Cyclonema, suggesting that Cyclora may represent the juvenile form of Cyclonema. Given the broad area over which these micro-teilsteinkerns were drawn from and their morphological variation (Fig. 1.1–1.4), these fossils may represent a number of different species, genera, families, or clades.

The shell of Cyclonema is remarkably well preserved, showing original color patterns and the whole shell in calcite with prismatic outer layer and laminar inner shell layer (Thompson, Reference Thompson1970). New specimens examined by us show the same pattern. Based on the quality of preservation of the shell, Thompson (Reference Thompson1970) inferred the entire shell was originally calcite, based on the fact that this is the only genus of gastropod from the Cincinnatian where the shell is typically preserved. If so, the laminar inner shell layer could not be nacre, which is exclusively aragonitic. Earlier, Knight et al. (Reference Knight, Cox, Keen, Batten, Yochelson, Robertson and Moore1960, p. I240) noted the laminar inner layer and inferred that the inner shell layer of Cyclonema was “seemingly nacreous and aragonitic.” Batten (Reference Batten1984) also made this claim about Cyclonema bilix, but inferred the laminar inner layer was unaltered aragonite, which is unlikely. Dattilo et al. (Reference Dattilo, Freeman, Peters, Heimbrock, Deline, Martin, Kallmeyer, Reeder and Argast2016) mentioned that the inner shell layer of Cyclonema consists of calcite recrystallized from aragonite, but evidence was not shown. Based on the exceptional images in Thompson (Reference Thompson1970), Batten (Reference Batten1984), and our own observations of Cyclonema gracile Ulrich and Scofield, Reference Ulrich and Scofield1897 from the Fairview Formation (Fig. 1.12), the fine laminae of the inner shell layer of Cyclonema is so often preserved in such detail as to represent original calcitic mineralogy, as inferred by Thompson (Reference Thompson1970).

If Cyclonema had a calcitic shell, this indicates that at least some of the gastropod micro-steinkerns from these shell beds are not from juvenile shells of Cyclonema. The shell microstructure of at least one of the teilsteinkerns previously classified as Cyclora is clearly aragonitic and represents something like nacre (Fig. 6.4–6.10).

Bryozoans

Apatite internal molds of stenolematan bryozoan zooecia are common in these beds (Figs. 1.9, 7.1–7.4), with Dattilo et al. (Reference Dattilo, Freeman, Peters, Heimbrock, Deline, Martin, Kallmeyer, Reeder and Argast2016) noting the common occurrence of trepostomes and, to a lesser extent, cryptostomes. Laminar and bladed types of calcitic shell microstructures in general characterize stenolamatans (Williams, Reference Williams and Carter1990). Distinct shell microstructures were observed in many phosphate molds of trepostome bryozoans. In isolated, broadly expanding zoecia with a sub-triangular cross section, pristine imprints of foliated calcite are preserved wherein adjacent blades of calcite are oriented with the long axis parallel to long axis of zoecia (Fig. 7.1; Vendrasco et al., Reference Vendrasco, Checa, Heimbrock and Baumann2013, figs. 8.2, 8.4). The ends of these blades vary from straight to angular, and those with angular edges show angles hovering around 100°. Foliated calcite in Novocrania has the c-axis in plane and elongate along it. This may also be the case in some bryozoans. In bivalves, by comparison, the c-axis is at a higher angle to the lath plane (45° with 104 calcite rhombohedra or 64° with 108 rhombohedra).

Figure 7.

(1–4) Shell microstructure in phosphatic steinkerns of bryozoans from the Ordovician ‘Arnheim’ (1), Fairview (2, 3), and Kope (4) formations of Ohio (1) and Kentucky (2–4). (1) LACMIP 14449 (LACMIP Locality 41965); (2, 3) LACMIP 14826 (LACMIP Locality 41966); (4) LACMIP 14827 (LACMIP Locality 41963). (5–9) Shell microstructure in a phosphatic steinkern of the brachiopod Zygospira from the ‘Arnheim’ Formation of Ohio. (5–7) LACMIP 14828 (LACMIP Locality 41967); numbers in (5) show locations of photos (6, 9). (8, 9) LACMIP 14448 (LACMIP Locality 41967); (8) is from the a region similar to ‘6’ in the other specimen (5); (9) is from near the aperture. Scale bars are (1) 10 μm; (2–4) 4 μm; (5) 250 μm; (6–9) 50 μm.

The shell microstructures preserved in bryozoans, like those of brachiopods described below, are useful in that they provide a detailed record of the appearance of certain types of shell microstructure reflected on fine-grained internal molds. Unlike many early Paleozoic mollusks, the shell microstructures of these taxa are much better known, and therefore these images can serve to help in interpretation of problematic textures, such in those from Cambrian small skeletal fossils. In addition, shell microstructure seems to be under strong genetic control in Paleozoic stenolaemate bryozoans, and hence is of great use for classification (Boardman, Reference Boardman and Robison1983).

Brachiopods

Internal molds of the rhynchonellatan brachiopod Zygospira Hall, Reference Hall1862 show imprints of a remarkably well-organized calcitic fibrous prismatic shell microstructure (Fig. 7.5–7.9). The lateral regions of the inner shell layer show overlapping calcite blades, with only the tips preserved in most regions. These blade tips are slightly offset in subsequent layers, forming a scale-like pattern. In the middle-apical shell region, these calcite blades appear to have merged together to form a laminar type shell microstructure. Near the middle of the inner surface of the shell, elongate blades of calcite can be seen, and over the rest of the inner shell surface, the overlapping blade tip pattern predominates. Such a fibrous calcitic inner shell layer is characteristic of rhynchonellide brachiopods in general (Rowell and Grant, Reference Rowell, Grant, Boardman, Cheetham and Rowell1987). Other brachiopods from other beds of the Late Ordovician of the Cincinnati region show relatively unaltered calcitic bladed shell microstructures.