Remarks

Restudy of Pywackia and an appreciation that its stem-like zooaria* are better interpreted as a result of the close packing of autozooids* into a 14-hedron means that assignment of this bryozoan genus to the cryptostomes is more tentative than in Landing et al. (Reference Landing, English and Keppie2010). Pywackia is considered to be a very early ‘stony bryozoan,’ or a stem group stenolaemate, derived from unmineralized ctenostome-grade ancestors. The skeletalized autozooecia have a combination of hemiphragms and diaphragms and axial zooids as in some cryptostomes (e.g., Blake, Reference Blake1983a). Well-preserved autozooecia are deeper than Landing et al. (Reference Landing, English and Keppie2010) reported, but still relatively shallow, as in cryptostomes.

Family undetermined

Genus Pywackia Landing in Landing et al. (Reference Landing, English and Keppie2010)

Type species

Pywackia baileyi Landing in Landing et al. (Reference Landing, English and Keppie2010), upper Cambrian Cordylodus andresi Zone, type section of Tiñu Formation (lower Yudachica Member) near Río Salinas, Oaxaca State, Mexico.

Other species

The genus is presently monospecific.

Diagnosis

Emended from Landing in in Landing et al. (Reference Landing, English and Keppie2010). Probable cryptostome with stem-like, likely unjointed, roughly cylindrical zooaria with autozooecia, axial zooid, and probable nanozooecia; zooaria largely consist of closely packed autozooecia with hemiphragms and diaphragms; histology microgranular to rarely granular-prismatic. Proximal autozooecia* elongated roughly parallel to zooarium axis near proximal end*; autozooecia on medial stem* radiate away from zooarial longitudinal axis. Absence of extrazooecial structures means that the generally proximally–distally elongated autozooecia appear somewhat disordered as they form a 14-hedron; general longitudinal polarity or somewhat helical arrangement of autozooecia. Axial zooecia with widely spaced diaphragms and longitudinal crenulation occur at least in lower part of zooarium; possible nanozooids at intersections of autozooecial walls. Autozooecial walls trilamellar with thin, erosionally nonresistant inner layer. Proximal extremities range from spike-like forms with flat to low triangular cross-sections and low longitudinal ridges and furrows, but more commonly pointed to sharply rounded with autozooecia that extend to proximal extremity tip. Holdfast unknown, may be absent. Autozooecia form 6-sided zooarium; bud to form 12-sided zooarium with broadly elliptical to circular cross section. Apical end rounded. Autozooecia with roughly linear to helical arrangement and apertures that range from oval to 4–7-sided polygons with slightly convex, to straight, to weakly concave sides; autozooecial basal surface with internal zooecial ridge on midline; longitudinal crenulation on inner autozooecium walls; autozooecial walls imperforate, trilamellar with less resistant (possibly organic-rich) median lamina.

Occurrence

Presently known only from the type locality.

Description

Proximal extremity a trilobed, elongate spike with low expansion angle (5–16°), lacks autozooecia (Fig. 1.4), but more commonly sharply rounded or forms point with autozooecia appearing at base (Figs. 1.1, 1.2, 2.1, 2.2). Middle zooarium tubular, stem-like (Fig. 1.9), end zooarium a rounded apical tip (Fig. 1.5); zooarial cross-section generally circular to broadly elliptical (Fig. 1.13). No evidence of ball-and-socket joints, branching, or holdfast. On medial stem and at distal tip, autozooecia apertures rounded to prismatic (rectangular to 7-sided) and radiate away from zooarium axis (Figs. 1.9, 1.13–1.15, 2.3); at proximal end, autozooecia tube-like, elongated parallel to zooarium axis (Figs. 1.2, 2.1, 2.2); likely with weakly calcified frontal walls (Fig. 2.1, 2.2); series of hemiphragms and diaphragms in autozooecia (Figs. 1.6, 1.8, 2.1, 2.2). Interzooecial walls trilamellar with less resistant median lamina (Fig. 1.7) and granular/rarely granular-prismatic histology of outer and inner layers (Fig. 1.7, 1.11). Tiny polymorphs (possible nanozooecia) at wall intersections (Fig. 1.2, 1.3). Internal zooecial ridge, often bilobed, on basal surface of autozooecia (Fig. 1.9). Axial zooecium with widely spaced diaphragms (Figs. 1.15, 2.3), may not extend complete length of zooaria. Autozooids bud to form second-generation autozooids that lengthen and broaden zooarium (Figs. 1.12–1.15, 2.3).

Remarks

The diagnosis of Pywackia is emended from Landing et al. (Reference Landing, English and Keppie2010) to emphasize the close packing of autozooecia as the growth mode of the stem-like zooaria and the absence of interzooecial skeletal structures. The emendation emphasizes the tube-like autozooecia that are elongated parallel to the zooarial axis near the proximal extremity. By contrast, more distal autozooecia have a radial orientation along the zooarium.

Pywackia baileyi Landing in Landing et al., Reference Landing, English and Keppie2010

Figures 1, 2

2010 Pywackia baileyi Landing in Landing, Keppie and English, p. 549, 550, figs. 2A–L, 3A–D.

2013 Pywackia baileyi; Taylor, Berning, and Wilson, figs. 1.1–1.3, 2.1–2.11, 4.1

Types

Holotype NYSM 13515; paratypes NYSM 13508–13514 and13516–13520.

Remarks

Pywackia is presently monospecific; the emended diagnosis, description, and discussion of P. baileyi are the same as for the genus.

Pywackia and phosphatic taphofacies

Formic acid disaggregation shows that Pywackia-bearing samples are limited to limestones with higher concentrations of a granular, light to dark gray, porous (‘punky’ or ‘druzy’) phosphatic cement. The Pywackia-bearing samples yielded almost all of the phosphate-replaced or -infilled fossils at Río Salinas (from sponge hexaxons to crinozoan columnals; Table 1). Thus, the Pywackia zooaria are limited to a taphofacies with a distinctive diagenetic history. This history included the following: (1) phosphatic cementation of lower limestones in the Yudachica Member, and (2) the phosphatic replacement and infill of associated calcareous skeletons. The Pywackia type collection occurs in a phosphatic facies and with secondarily phosphatized calcareous skeletal debris (e.g., Landing et al., Reference Landing, Myrow, Benus and Narbonne1989; Brasier, Reference Brasier1990), features that are consistent with an interpretation that the Pywackia zooaria were originally calcareous (Landing et al., Reference Landing, English and Keppie2010). Other phosphate-replaced bryozoan zooaria occur in slowly deposited shelf successions, and are best known from the European lower Paleozoic (e.g., Dzik, Reference Dzik1992; Federov et al., Reference Federov, Krusanov and Federov1997).

Significance of pre-final burial taphonomy

Detailed study of colonial marine skeletal fossils traditionally relies on polished slabs or thin sections. However, thin-section preparation is largely unnecessary for the Pywackia zooaria as pre-final burial breakage and wave abrasion produced oblique, transverse, and longitudinal sections through the stem-like colonies. The preservation of the topotype Pywackia collection allows direct examination of the three-dimensional relationship of skeletal structures and shows the locations of asexual budding in some specimens (Fig. 1.12, 1.13). Wave abrasion destroyed many exterior walls and, consequently, exposed fine details on the interior walls of long, tube-like structures (e.g., Figs. 1.9, 2.2). Three-dimensional preservation reveals the external and interior surfaces of Pywackia specimens and allows determining just how wave-abrasion has modified individual specimens.

The outer edges of the walls of Pywackia zooaria and associated black phosphatic sand and granules are highly polished (e.g., Fig. 1.4, 1.7; Table 1). The shiny appearance of both types of clasts is regarded as evidence of a complex taphonomic history (detailed below). This history included: early burial, phosphatization, and manganese mineral staining/coating of Pywackia zooaria; excavation and wave abrasion with sclerite polishing and local removal of manganese coats; fracture of the phosphatized Pywackia zooaria; and final burial of the Pywackia zooaria, phosphatized sclerites, and phosphatic pebbles (Landing et al., Reference Landing, English and Keppie2010). This taphonomic history is recognizable through the lower 6.25 m of the Río Salinas section (Landing et al., Reference Landing, Westrop and Keppie2007a, p. 913), and compares closely with the taphonomic models for secondary phosphatization of calcareous sclerites in the early Cambrian (e.g., Brasier and Hewitt, Reference Brasier and Hewitt1979; Landing et al., Reference Landing, Myrow, Benus and Narbonne1989; Brasier, Reference Brasier1990, Reference Brasier1992). Post-phosphatization excavation and wave abrasion is also evident in the polishing and rounding of associated phosphatized trilobite, agnostoid arthropod, gastropod, and crinozoan debris.

Pywackia zooaria colors and taphonomy

Two dominant colors are represented by topotype Pywackia fragments. Most specimens are medium to dark brown and comparable to the thermal color alteration of associated, originally phosphatic conodont elements (Landing et al., Reference Landing, English and Keppie2010; e.g., Fig. 1.2, 1.14).

Other Pywackia zooaria are infused with a light purplish color (Figs. 1.12, 2.3), and a few have surfaces that are dark purple or almost black (Fig. 1.1, 1.10). This purplish to black color is reminiscent of the manganese coloration of shell fragments that have been buried in sediment at about the level of the transition from deeper anoxic/strongly dysoxic to shallower, oxic environments (e.g., van der Zee et al., Reference van der Zee, van Raaporst, Helder and de Heij2003).

The dark purple to black, manganese-based color is absent on the edges of higher-standing interzooecial walls of a few Pywackia specimens (Fig. 1.1, 1.10). This local absence of a dark coloration and the light brownish color of the top of the interzooid walls are consistent with exhumation and abrasion of a manganese coating from these specimens. The very low interzooecial walls of these multicolored (brown and purple/black) specimens suggests wave-induced abrasion of the interzooecium walls removed the dark surface coloration (Fig. 1.1, 1.10).

Taxonomic consequence of abrasion and fracturing

Post-phosphatization wave abrasion likely explains the wearing down of the upper edges of Pywackia zooarium walls. It also erased any original microtopography from the tops of the Pywackia autozooecia walls.

As discussed below, the wearing down of the autozooecia walls was so severe on most of the zooaria that the autozooecia appear to be shallow, open pits (Fig. 1.9). This abrasion was taxonomically useful, however, as it reveals the histology of the zooaria (Fig. 1.7), the internal, transverse skeletal structures (hemiphragms and diaphragms) on the proximal zooecia (Fig. 2.2), and shows how autozooecia budded to produce two generations that radiate away from an axial tube (Figs. 1.17, 2.3). This abrasion allowed observations that conclusively led to evaluation of Pywackia as Earth’s oldest-known bryozoan.

Hemiphragms

Some of the abraded Pywackia zooaria have short transverse ‘shelves’ that extend only part way across the interior of the autozooecia. These shelves occur in the lower and middle (older) parts of the tubes (Figs. 1.2, 2.2), and are comparable in form and position to autozooecial structures known as “hemiphragms” in stony bryozoans (Bassler, Reference Bassler1952, p. G11). Hemiphragms are a skeletal division in tube-like stenolaemate autozooecia that mark a position beyond which the zooid’s tentacles extend during feeding and behind which the feeding tentacles are retracted (Boardman, Reference Boardman1983, p. 83, fig. 40.6). As a bryozoan autozooecium grows and elongates, hemiphragms are formed as the aperture extends (Fig. 2.2) and mark successive positions of the base of the everted feeding tentacles (e.g., Boardman, Reference Boardman1983). Successive hemiphragms are common and, locally, regularly spaced along the interior of proximal Pywackia autozooecia (e.g., Landing et al., Reference Landing, English and Keppie2010, fig. 2G; Fig. 2.2).

This distribution of hemiphragms shows that Pywackia’s autozooecia are not only elongate, but are elongated roughly parallel to the longitudinal axis near the proximal end of the zooarium (Fig. 2.2). Earlier reconstructions of Pywackia autozooecia as shallow and cup-like are incorrect (Landing et al., Reference Landing, English and Keppie2010; Taylor et al., Reference Taylor, Berning and Wilson2013, p. 986, 988).

Diaphragms

Pywackia fragments help show that most specimens in the topotype collection are not representative of zooarial morphology because of wave abrasion. Most medial and apical stem fragments suggest a slender zooarium with a linear to slightly helicoid arrangement of autozooecia with apertures that open radially from the longitudinal axis of the zooarium (Fig. 1.9).

One specimen shows how wave abrasion changed the apparent morphologies of the zooaria. It has four autozooids with transverse partitions that overlie a ‘punky’ black phosphatic fill (Fig. 1.6). Such transverse partitions are common in stenolaemate and trepostome autozooecia and are termed “diaphragms.” In one part of this zooarium, so much of the colony has been worn away that a diaphragm sits on a pedestal of black phosphatic fill (Fig. 1.8).

In most medial and distal stem fragments of Pywackia, the outer zooarium and the distal parts of autozooecia were worn away so severely that only cup-like bases of the autozooecia remain; these are the “modules”* of Taylor et al. (Reference Taylor, Berning and Wilson2013; see Fig. 1.9, 1.12). However, the fortuitous preservation of a medial stem with diaphragms shows not only that autozooecia were not ‘cup-like,’ but were much longer. In addition, the presence of diaphragms in proximal and medial autozooecia is a strong argument for assigning Pywackia to the Bryozoa because many stony bryozoans have diaphragms (Landing et al., Reference Landing, English and Keppie2010). Of course, diaphragms are transverse skeletal structures that are not limited to bryozoans, and occur in tabulate corals and calcareous sponges. However, their association with other morphologic and histologic features also known in stony bryozoans (Table 2) is consistent with the assignment of Pywackia to the Bryozoa. Thus, the combination of successive hemiphragms and diaphragms in a calcareous zooarium with tube-like autozooecia (Figs. 1.2, 2.1, 2.2) is typical of trepostome and stenolaemate skeletons, and includes a number of homologies, or putative synapomorphies (Sober, Reference Sober1988, p. 116), between Pywackia and most bryozoan orders (Table 2).

Table 2

Homologies or putative synapomorphies (marked by *) and other characters common to Pywackia and non-phylactolaemate bryozoans. “Some cheilostomes¹” have trilamellar walls with a middle cuticle (organic layer) (e.g., Cheetham and Cook, Reference Cheetham and Cook1983, fig. 1b); “Some cheilostomes²” have “transverse partitions” that are otherwise comparable to diaphragms (e.g., Cheetham and Cook, Reference Cheetham and Cook1983, figs. 89, 90).

Pre-abrasion shape of Pywackia zooarium

The preservation of successive diaphragms (i.e., at least two) indicates not only long autozooecia on the medial stem of Pywackia, but demonstrates that the slender colony shape suggested by the available Pywackia zooaria is misleading. A wider zooarium existed before erosive beveling of the radially oriented autozooecia of the middle stem. Even if only one diaphragm was produced with autozooid growth along the zooarium (Landing et al., Reference Landing, English and Keppie2010; Fig. 1.6, 1.8), the pre-abrasion width of the medial stem was likely at least twice as wide as that of the abraded specimens.

Orientation of early and later budded autozooecia

As noted above, earlier produced autozooecia near the proximal extremity of the Pywackia zooarium are elongated roughly parallel to the longitudinal axis of the zooarium (Fig. 2.1, 2.2). Later produced autozooaria along the abraded stem and distal tip radiate away from the colony axis (Figs. 1.4, 1.5, 1.9, 1.14, 1.15, 2.3) and continue their elongation with the addition of diaphragms (Fig. 1.8).

The first generation of radiating autozooids includes six individuals (Landing et al., Reference Landing, English and Keppie2010), which can be counted when medial stem fragments are examined under the microscope and rolled around the longitudinal axis. This six-fold symmetry is also shown by transverse sections through the zooarium (Fig. 1.13). Larger medial stem fragments show twelve individuals when the zooaria are turned over. This bauplan of the Pywackia zooarium with chambered autozooecia and the replacement of long autozooecia near the proximal extremity by radially oriented zooaria along the medial stem is present in the growth of many stenolaemate bryozoans (e.g., Boardman, Reference Boardman1983, p. 118).

Bryozoans and substrate relationships

An expanded colony base (or holdfast) cemented to firm substrate is a common but not invariant feature in bryozoans (e.g., Boardman and Cheetham, Reference Boardman and Cheetham1987). Bryozoans range from sessile to mobile in habit, with sessile forms not restricted to attachment by holdfasts. Non-holdfast attachments in sessile bryozoans include (1) calcified or non-calcified runners on permanent substrates; (2) substrate bioimmuration (encrusting); (3) large, calcified ‘roots’ in soft substrates; (4) endolithic boring; (5) hair-like cuticular roots, which are unlikely to be fossilized, in ephemeral substrates; (6) calcareous plates or tubes, sometimes articulated by cuticle; and (7) large, sedentary or tumbled colonies. Habit (8) is the most peculiar benthic life mode—small mobile colonies that move by coordinated action of avicularia on their undersurface (e.g., Cook, Reference Cook1979; McKinney and Jackson, Reference McKinney and Jackson1989; Hageman et al., Reference Hageman, Bock, Bone and McGowran1998, Reference Hageman, James and Bone2000).

No holdfast is known in Pywackia. The genus may have had a holdfast that was not recovered either attached to or in association with the zooaria, or this early bryozoan lacked a holdfast. The Mexican collections show that Pywackia proximal extremities included two end members; a spike-like basal extremity with longitudinal ridges and furrows (Fig. 1.4) and a tapered end in which autozooecia appear near the proximal tip of the zooarium (Figs. 1.1, 1.2, 2.1. 2.2). The lower part of both proximal extremity end members is sharp and triangular in cross section, and there is no evidence that a holdfast was attached (Figs. 1.4, 2.1)

An absence of a known holdfast does not detract from assignment of Pywackia to the Bryozoa: the genus may or may not have had a holdfast, and, if not, bryozoans show a wide variety of substrate relationships. Blake (Reference Blake1976) documented that a holdfast may be unnecessary for some bryozoans. He illustrated Permian zooaria of the rhabdomesine Rhabdomeson Young and Young, 1874, that had been detached from a holdfast. The broken zooaria healed the break, and even show growth reversals in what must have been recumbent stem-like fragments. The possibility of a recumbent life habit opens up the life mode of Pywackia from an erect posture on the sea floor (Landing et al., Reference Landing, English and Keppie2010; Taylor et al., Reference Taylor, Berning and Wilson2013) to include a possible habit as a primarily recumbent form that could survive overturning and continued to live in a recumbent position (Fig. 4).

Figure 4

Reconstructions of Pywackia zooaria with erect (left) or recumbent (right) habits; dead zooarium in background. Black scale bar (lower right) is 2.0 mm long.

Tapered proximal ends in Bryozoa

Just as a cemented, expanded colony base is not invariant in bryozoans, the tapered proximal ends as in Pywackia are consistent with the morphology and life habit of Paleozoic bryozoans. Tapered proximal ends occur in Bryozoa, but have been regarded as exceptions, with occurrences limited to middle and late Paleozoic taxa with jointed, or supposedly jointed, colonies (Taylor et al., Reference Taylor, Berning and Wilson2013, p. 287). Indeed, jointed rhabdomesinad cryptostomates may have pointed proximal ends (e.g., Bactropora Hall and Simpson, Reference Hall and Simpson1887, [Devonian–lower Carboniferous] with the proximal end “extremely pointed” [Bassler, Reference Bassler1953, p. G132, fig. 91.4b] and Orthopora Hall, Reference Hall1886 [Lower Devonian]). Pointed proximal ends also occur in arthrostylid and ptylodictylid cryptostomates (e.g., Nematopora Ulrich, Reference Ulrich1888 [Blake, Reference Blake1983b, p. 583]; Coscinella Hall and Simpson, Reference Hall and Simpson1887 [Devonian, Bassler, Reference Bassler1953, fig. 98.5]; Escharopora Hall, Reference Hall1847 [Upper Ordovician, Bassler, Reference Bassler1953, figs. 98.4b, 98.5]; see Fig. 3). Thus, pointed proximal ends similar to that in Pywackia occur in a prominent Paleozoic bryozoan order.

There is no evidence that some proximally pointed ptylodictilines, as exemplified by curved specimens of Escharopora (Fig. 3.1), are always only in the upper part of a zooarium that had a broad attached base (e.g., Karklins, Reference Karklins1983, p. 499), even though Palmer and Palmer (Reference Palmer and Palmer1977) noted that some Escharatopora had a holdfast. If not jointed to a large base, then forms such as other Escharopora and the much smaller bryozoan Pywackia could, speculatively, have been comparable in their life habit to some solitary, erect to recumbent ceratiform corals (Fig. 4).

Unjointed zooaria from two other bryozoan orders also have pointed proximal ends. These include fenestellid cryptostomates (e.g., Protoretepora de Koninck, Reference de Koninck1876 [Carboniferous–Permian; Bassler, Reference Bassler1953, fig. 86.1a]) and sulcoretepirid cyclostomates (e.g., Ptilocella Simpson, Reference Simpson1897 [Devonian; Bassler, Reference Bassler1953, fig. 104.3a]).

Pywackia zooaria were likely unjointed because ball-and-socket joints have not been found (Landing et al., Reference Landing, English and Keppie2010), although wave-abrasion may have obscured evidence for jointing in the topotype collection. However, as noted above, proximally pointed zooaria may occur in bryozoans with unjointed zooaria. In addition, the ceratiform zooaria in genera traditionally assigned to bryozoan families with joints may not have been attached to a holdfast (Fig. 3.1). As bryozoan colonies do not have invariant attachment structures (discussed above), life habits in Pywackia, and possibly Escharopora, as epifaunal, detached colonies are possible (Fig. 4).

Zooids on tapered bryozoan proximal ends

Similarities of the proximally pointed zooaria of Escharopora and Pywackia include narrow longitudinal ridges and furrows on the proximal ends of Escharopora and the spike-like Pywackia proximal extremities (Figs. 1.4, 3.1). In well-preserved Escharopora specimens, the proximal extremities show that progressively more closely spaced partitions (likely diaphragms) appear distally in the furrows (Fig. 3.2). These diaphragms show, at the very least, that early-formed zooids have longer living chambers. In addition, the ridges and furrows are confluent distally with interzooecial walls and oval autozooecia (Fig. 3.2). Similarly, the spike-like apical ends of Pywackia show that the longitudinal ridges divide and become confluent with the walls of the earliest autozooecia (Landing et al., Reference Landing, English and Keppie2010, fig. 2B; Taylor et al., Reference Taylor, Berning and Wilson2013, fig. 1.3; Fig. 1.4).

A continuation of longitudinal furrows and ridges into the lowest autozooecia of Pywackia and Escharopora (Fig. 3.1, 3.2) suggests that the proximal ridge-and-furrow topography was taphonomically exposed, perhaps by abrasion of a weakly calcified, very elongate frontal wall in the proximal autozooecia. Indeed, frontal walls of Paleozoic stenolaemates may be thin and weakly calcified, and can be so long that they extend the length of the zooarium (Boardman, Reference Boardman1983, p. 66–70). Similarly, the sharply rounded to angular Pywackia proximal extremities with exposed hemiphragms and diaphragms in the early autozooecia (Fig. 2.1, 2.2) also suggest that a weakly/less-strongly calcified frontal wall was abraded away.

Regularity of zooid size, shape, and distribution in many Bryozoa

The somewhat irregular, oval to polygonal outlines, and variable sizes of the autozooid apertures in Pywackia are striking, and are dissimilar to the regular distribution and size of autozooecia in many stony bryozoans. This may seem to detract from an assignment of Pywackia to the Bryozoa.

However, the actual problem that exists is explaining the regularity, not irregularity, of autozooid and autozooecium size and distribution in most stony bryozoans. Close packing of autozooids in foliose, stem-like, or massive zooaria should lead to irregular distributions of autozooids with significant size and shape differences. These irregularities would result from space constraints with the close packing of autozooids with non-homogeneous internal structure as enhanced by interzooid competition.

The regular spacing and distribution and relatively uniform size of autozooids that is present and expected in most stony bryozoans is easily illustrated by specimens of Ordovician and younger stony bryozoans (e.g., Taylor et al., Reference Taylor, Berning and Wilson2013, fig. 3.1, 3.2). However, the latter illustrations feature Late Ordovician and middle Silurian rhabdomesine cryptostones that have a variety of extrazooidal skeletal features that are not present between the autozooecia in Pywackia. These skeletal features can be regarded as effectively serving as rigid structural units that help establish and maintain a uniform spacing and distribution of newly budded autozooecia.

For example, the linear arrangement and lateral and longitudinal separation of autozooecia in Nematopora Ulrich, Reference Ulrich1888 (Taylor et al., Reference Taylor, Berning and Wilson2013, fig. 3.1), mirrors the presence of significant amounts of granular/laminated dark matter at autozooecial boundaries and as wedge-shaped areas between the zooecia (e.g., Blake, Reference Blake1983b). Similar extrazooecial features occur between the thick-walled, quincuncially arranged autozooecia of Rhabdomeson progracile Wyse, Jackson, and Bancroft, 1995 (Taylor et al., Reference Taylor, Berning and Wilson2013, fig. 3.2), which has peg-like, calcareous rods (styles) at the corners of its autozooecia.

14-hedron Pywackia zooarium

Pywackia zooaria from the late Cambrian are dominated by autozooecia and lack any of the extrazooecial structures of Ordovician and younger stem-like zooaria that seemingly serve to establish and maintain regularity in zooecia size, shape, and distribution. The absence of these extrazooecial structures in the Pywackia zooarium permits it to be regarded as having a particularly simple morphology in the evolution of stony bryozoans. The stem-like arrangement of skeletalized zooecia may have served to elevate its feeding zooids above the sea floor (Landing et al., Reference Landing, English and Keppie2010; Fig. 3). However, Pywackia’s tiny size and consequent life in (probably) dominantly low-energy, laminar-flow conditions on the sea floor of the lower Tiñu Formation means its zooarium lacked the hydraulic efficiency of the more structurally complex zooaria of Ordovician and younger stony bryozoans. The relatively irregularly distributed and closely packed feeding individuals in Pywackia may have had a high degree of inter-individual competition and may not have functioned efficiently with the movement of water and food along the zooarium.

The stem-like Pywackia zooarium is a more-or-less tetrakaidekahedral (14-hedron) form that can be regarded as the product of close packing of soft spheres or bubbles (i.e., autozooids) with budding and zooarium growth. Thompson (Reference Thompson1917), in particular, provided a useful summary of several centuries of experimentation and showed that a 14-hedron with irregularly polygonal cells results from the close packing of soft spheres in a rod-shaped press.

By this interpretation, the initially soft (non-mineralized) autozooids of Pywackia are produced by axial zooid budding and are ‘wrapped’ into a stem. The natural tendency by closest packing is to organize the internally non-homogeneous cells (autozooids) into more-or-less irregular 14-hedra (Thompson, Reference Thompson1917), which, in the case of this colonial organism, has its cells (autozooids) open to the environment. The 14-hedron arrangement of open autozooids persists into generation 2 with budding of generation 1 autozooids. The result is a three-dimensional, stem-like structure that somewhat resembles the incomplete (i.e., also open-ended) cells in a two-dimensional, sheet-like, hexagonally close-packed honeycomb produced by bees.

The arrangement and form of autozooecia in the 14-hedron zooarium of Pywackia is somewhat irregular. Thompson (Reference Thompson1917) summarized experiments that showed that cells in a 14-hedron must be irregular, and range from roughly oval to polygons with four to seven sides. These shapes resemble the variable outlines of the autozooecia in Pywackia zooaria (Fig. 1.1, 1.2, 1.5, 1.9). The irregularly convex or concave wall outlines in Pywackia are surfaces of minimal curvature and maximum space partitioning (e.g., Thompson, Reference Thompson1917). The strong irregularity in autozooecial outlines in Pywackia zooaria reflect the close packing of two levels of cells (autozooids) that form the first and second generations of Pywackia autozooecia.

There is a clear polarity of elongate Pywackia autozooids parallel to the zooarium axis (Fig. 1.1, 1.2), although a tendency to a helicoid arrangement may be present (Fig. 1.9). All larger (first generation) autozooecia on the Pywackia proximal extremity, medial stem, and distal end (Figs. 1.5, 1.9, 2.1, 2.2) show this decided elongation parallel to the zooarium axis. The overall shape of stem-like structures and a visible polarity of their Pywackia cells do not provide evidence that strongly limits phylogenetic or taxonomic interpretation. However, given the fact that Pywackia zooaria lack the ‘spacers’ (dark matter, thickened walls, styles, etc.) that are considered responsible for the regular distribution of the autozooecia of later bryozoans with a derived morphology, the somewhat irregular distribution of autozooecia in Pywackia is not an argument against an assignment of this morphologically simple genus to the Bryozoa.

14-hedra and nonbranching Pywackia zooaria

Branching in Pywackia seems unlikely given how the zooarium grew. The autozooids were initially produced by axial zooid budding and later by autozooid budding. Once the 14-hedron-stem shape was initiated, the possibility of branching at the tip of the growing stem does not seem possible, as Pywackia lacked the extrazooecial deposits and structures that separate autozooecia in later bryozoans. Budding of initially soft autozooids likely meant that new zooids stayed in contact with adjacent zooids and could not diverge to form the bases of a branch. Both the somewhat peculiar appearance of the Pywackia zooarium and the absence of branching in its tiny zoorium (up to 7 mm long based on lengths of fragmentary proximal extremities, medial stem, and distal tips) reflect the absence of extrazooecial structures that occur in derived Ordovician and younger stony bryozoans.

Pywackia as an early stenolaemate bryozoan

The Tiñu Formation bryozoans provide an insight into the origin of mineralized (stony) bryozoans. The early occurrence and morphology of Pywackia in the late Cambrian are consistent with phylogenetic analyses (e.g., Taylor and Ernst, Reference Taylor and Ernst2001, Reference Taylor and Ernst2004, fig. 16.2, Taylor et al., 2010) that derive mineralized bryozoans from ctenostome-grade bryozoans, unmineralized taxa in which stolon budding forms a loosely integrated colony.

Despite Pywackia’s relatively early age, it has a variety of morphologic features that are concordant with a long list of homologies or putative synapomorphies (Sober, Reference Sober1988) that demonstrate it is a stenolaemate bryozoan (Table 2). Pywackia zooaria are regarded as having the morphology of a hypothetical stem-group stenolaemate with a very simply constructed zooarium. This zooarium consists of a stem-like 14-hedron composed of at least one or two generations of autozooids with an axial zooid and no extrazooecial structures. Pywackia shows that calcareous biomineralization and the specialized zooid polymorphs of later bryozoans, including the younger late Tremadocian trepostomes and cryptostomes of South China (Xia et al., Reference Xia, Zhang and Wang2007; Ma et al., Reference Ma, Taylor and Xia2013), appeared at least eight million years earlier in the history of bryozoan evolution (Fig. 5).

Figure 5

Ranges of fossil anthozoans and bryozoans, showing great range extension (dashed line) if Pywackia is considered a pennatulacean and modest range extension with determination that Pywackia is a bryozoan. Phylogeny of pennatulaceans after Williams (Reference Williams1997) as reanalyzed by Antcliffe and Brasier (Reference Antcliffe and Brasier2008). Figure modified from Antcliffe and Brasier (Reference Antcliffe and Brasier2008, fig. 8).

Calcareous biomineralization accompanied a key structural transition in bryozoan evolution. The muscles that everted and retracted the feeding tentacles changed from those that compressed and relaxed an unmineralized ctenostome-grade zooid’s flexible wall and fluid-filled body cavity. This rise of the stony bryozoans was apparently accompanied by the appearance of a muscle that attached to a newly inflexible (mineralized) body wall and retracted the feeding apparatus (Taylor and Ernst, Reference Taylor and Ernst2001). Pywackia appears to represent an early stage in stony bryozoan evolution because its mineralized, inflexible body wall would have been appropriate for attachment of a retractor muscle that pulled feeding tentacles below the level of the hemiphragms in its autozooecia (e.g., Boardman, Reference Boardman1983; Boardman and Cheetham, Reference Boardman and Cheetham1987). The loss of the frontal wall by wave abrasion in proximal Pywackia autozooecia (Figs. 1.2, 2.1, 2.2) suggests that the frontal wall was not strongly mineralized along the proximal extremity.

Autozooidal budding was an evolutionary advance from ancestral ctenostome-grade bryozoans (Taylor and Ernst, Reference Taylor and Ernst2001; Taylor et al., Reference Taylor, Berning and Wilson2013). By the interpretation herein (also Landing et al., Reference Landing, English and Keppie2010), it allowed production of an elongate, stem-like zooarium in which feeding could take place somewhat above the sediment-water interface. Thus, Pywackia zooarial growth was more complex than in its probable ctenostome-grade ancestors in that budding was no longer limited to a stolon, and its seeming homolog, the axial zooid. Pywackia colony growth possibly included an overall more rapid rate of zooarium size increase by the budding of numerous autozooecia that quickly lengthened and broadened the stem-like zooarium.

A wider, elongate, possibly erect zooarium (Fig. 4) was unlike that in Pywackia’s probable ctenostome-grade ancestors. Budding limited to a stolon in a recumbent or erect ctenostome-grade zooarium would produce an elongate, very thin, and fragile (brittle when mineralized) stem. One way to reach a higher level above the sea floor with a stronger and thicker mineralized zooarium would have been the adoption of autozooecia budding to form a second autozooecial generation (Figs. 1.13, 1.14, 2.3).

Without an ability to produce extrazooidal structures (e.g., calcareous laminae, stereom, styles), Pywackia’s budding formed 14-hedron stems composed of close-packed, open autozooecia in which branching was not possible in its tiny, disordered-appearing zooaria. This growth mode seems to make the genus distinct from Ordovician and younger taxa with branching, stem-like zooaria.

Other and earlier Cambrian bryozoans

Discovery of the late Cambrian stem group stenolaemate Pywackia is an artifact of preservation and sampling. There is no reason why other taphonomic windows will not expand the distribution of Cambrian bryozoans geographically and temporally. Pywackia has a number of features regarded as homologies or putative synapomorphies with later stenolaemates. While regarded as a stem group stenolaemate, it is morphologically too complex to be the ancestor of the Bryozoa. At the very least, Pywackia shows that a colonial habit and characteristic stenolaemate features (Table 2) had appeared by the late Cambrian. The early occurrence of colonial Pywackia also demonstrates that a solitary cornulitid-like organism that ultimately became an ancestrula, as suggested by Dzik (Reference Dzik1991), was not the ancestor of Ordovician bryozoans.

The discovery of Pywackia from low-diversity, dysoxic facies of the Tiñu Formation (discussed above) suggests Pywackia may be only one of a number of bryozoans that lived in other late Cambrian habitats. The fossil record of bryozoans may extend further back into the Cambrian and sampling of the correct (e.g., phosphatized) taphofacies may yield far older skeletalized bryozoans even in the early Cambrian part of the Cambrian Evolutionary Radiation.

The possibility of older Cambrian bryozoans exists because other skeletalized lophophorates such as brachiopods and possibly phoronids are widely distributed in sub-trilobitic successions of the upper part of the lowest Cambrian Terreneuvian Series (e.g., Landing et al., Reference Landing, Westrop and Keppie2007a, Reference Landing, Geyer, Brasier and Bowring2013a). Unillustrated, colonial like forms comprised of silicified, polygonal tubes that suggest bryozoan zooaria are reported from the middle Cambrian Burgess Formation of western Canada (Larwood and Taylor, Reference Larwood and Taylor1979, p. 211), and could prove to be a bridge between Pywackia and possibly older mineralized ancestors in the Cambrian Evolutionary Radiation.

Pywackia and the “oldest” bryozoans in China

Although repeatedly refered to as the oldest unequivocal bryozoans (Xia et al., Reference Xia, Zhang and Wang2007, p. 1308; Taylor et al., Reference Taylor, Berning and Wilson2013, Ma et al., Reference Ma, Taylor and Xia2013), early Ordovician bryozoans from South China are about 8 m.y. younger than Pywackia (Fig. 5). The Chinese bryozoans indicate a morphologically disparate stenolaemate fauna with at least two bryozoan orders had appeared by the late Tremadocian. This fauna includes two branching genera referable to the cryptostomid trepostomes and two species of massive (nodular and hemispherical or dome-shaped) trepostomes, which indicates an interval of unknown length that featured the evolutionary differentiation of the cyclostones and trepostomes (e.g., Xia et al., Reference Xia, Zhang and Wang2007, fig. 6). By comparison with Pywackia, the Chinese forms have zooecia that are much more complex (taxa with styles, thickened exozone walls, regular autozooecia spacing, branching, massive colonies), and this may imply a rapid evolution and morphologic differentiation from the late Cambrian to the late Tremadocian.

If Pywackia is regarded as an early cryptostome (Landing et al., Reference Landing, English and Keppie2010), the late Cambrian appearance of a cryptostome genus is older than the proposed Ordovician appearance of the cyclostomes and trepostones (Xia et al., Reference Xia, Zhang and Wang2007, fig. 6). The great age of Pywackia suggests that phylogeny of the bryozoan orders should be reevaluated, perhaps with cryptostomates at the base of the stony bryozoan radiation. Alternatively, an interpretation of the stenolaemate radiation with cryptostomes as an advanced clade (Taylor and Ernst, Reference Taylor and Ernst2004) requires much deeper/older roots of the radiation than the late Cambrian age of the likely cryptostome Pywackia.

Sparking the Ordovician radiation

The appearance of Pywackia in the late, but not latest, Cambrian was roughly coeval with appearance of the oldest known cephalopods, polyplacophorans, and the initial diversification of euconodonts (Landing et al., Reference Landing, English and Keppie2010, and sources therein). With the possible exception of euconodonts, these ‘new’ higher-level taxa were likely unimportant parts of late Cambrian communities. However, cephalopod macropredators and the reappearance of animal-produced reefs with bryozoans were key elements in the later Ordovician radiation (e.g., Webby, Reference Webby2002; Taylor and Ernst, Reference Taylor and Ernst2004). By the late Tremadocian, the South China bryozoans show that a key component of the animal-produced biostromes and reefs of the Ordovician radiation were at least locally abundant (e.g., Xia et al., Reference Xia, Zhang and Wang2007, fig. 5).

Morphologic distinctiveness of Lituaria sp. axial rod

The axial rod of Lituaria differs from that of other pennatulaceans in having cup-like autozooecia (e.g., d’Hondt, Reference d’Hondt1984), rather than being a smooth, solid, rod-like structure with rounded or quadrate cross section (e.g., Wilson et al., Reference Wilson, Andrews, Brown and Cordes2002). The Lituaria sp. axial rod has a long tube (central chord, which holds the budding zooid) that is contiguous with and opens into the midline of the autozooecia bases (Figs. 7.1, 6.2). This central chord originates in a furrow on the proximal extremity of the axial rod and continues under the autozooecia on each side of the Lituaria corallum (Fig. 7.2).

Figure 7

Axial rod of the modern pennatulacean Lituaria sp., scanning electron microphotography of specimen RMS ZRC.CNI.0935, peritidal sand flat near Changi Ferry Terminal, Singapore; (1) photomontage shows furrows of distal extremity confluent with hollows of central chord under midline of autozooecia; (2) autozooecium with linear opening to central chord (c. ch., or stolon) on basal midline; (3) characteristic longitudinal grain of microgranular calcite of pennatulacean axial rods is present in Lituaria sp. Specimen in Raffles Museum, Singapore (RMS), photographs modified with permission from Paleontological Society, with thanks to P.D. Taylor, B. Bering, and M.A. Wilson from Taylor et al. (Reference Taylor, Berning and Wilson2013, fig. 5.1, 5.5, 5.7).

Lituaria sp. differs from the type and other Lituaria species in having elongate, not rounded, autozooecia that are relatively large in terms of the axial rod diameter. Its autozooecia are not randomly or helically aligned, but are aligned and elongated parallel to the axis of the rod (compare Lituaria specimens in Kükenthal, Reference Kükenthal1915, and d’Hondt, Reference d’Hondt1984; Figs. 5.1, 5.3, 6.1, 6.2). In short, the morphology of the Lituaria sp. axial rod cannot be extrapolated as an example of a general physical similarity between Pywackia and the axial rods of other pennatulaceans.

Colony shape is not evidence of taxonomic relationship

One of the arguments used in a non-bryozoan assignment of Pywackia was that the “striking morphological similarities (of the Lituaria sp. skeleton) to Pywackia allows an alternative (phylogenetic and taxonomic) hypothesis to be proposed” (Taylor et al., Reference Taylor, Berning and Wilson2013, p. 987). Thus, the stem-like zooaria and somewhat irregular autozooecia in Pywackia and Lituaria sp. indicate homologies rather than a general similarity that result from the wrapping of autozooids of Pywackia into a 14-hedron and the production of a stem-like form by the sequential budding of autozooids in Lituaria sp. (Figs. 1.9, 6.1).

Other than being elongate, the two zooaria differ in their radial symmetry. The Lituaria sp. colony has the quadrate cross-section of many pennatulaceans. The Pywackia zooarium symmetry is more complex with a triangular proximal spike (Fig. 1.4) or proximal end tip (Figs. 1.1, 2.1) and a 6- or 12-sided medial stem depending on whether one or two autozooecia generations are present. Actually, the 14-hedron, stem-like shape of the nests of the Recent wasp Stenogaster Rohwer, Reference Rohwer1910 (Williams et al., 1919, fig. 103), also has a striking similarity to Pywackia zooaria, but shape/general topological similarity is no indication of homology or taxonomic relatedness, particularly when a number of homologies or putative synapomorphies exist between Pywackia and other stony bryozoans (Table 2).

Budding in Pywackia and Lituaria sp

Zooid budding and zooaria growth of Pywackia and Lituaria sp. are so distinct as to preclude assignment of Pywackia to the pennatulaceans. As noted above, budding in octocorals takes place from a central chord, or primary oözoid, in pennatulaceans, which remains in contact with autozooecia frequently by a complex system of tubules (solenia) that also connect adjacent autozooecia (Bayer, Reference Bayer1956, Reference Bayer1973).

The central chord of Lituaria sp. occupies a longitudinal, pipe-like cavity on the form’s axial rod. After budding, the central chord remains in continuity with daughter autozooecia along an open slot (Fig. 7.1, 7.2). A longitudinal ridge or two may lie to the left/right of the longitudinal slit at the base of Lituaria sp. autozooecium (Fig. 7.2). However, the Lituaria sp. adventitious ridge(s) do not lie almost precisely on the autozooecium midline and do not wall off the Lituaria sp. budding individual.

Examination of the 151 Pywackia zooaria from the Tiñu Formation (Table 1) indicates a fundamental developmental difference with Lituaria sp. specimens: no Pywackia autozooecium shows any holes or slots that indicate a continuing connection of the autozooids with the axial zooid. Thus, Pywackia and some other stony bryozoans are similar in the post-budding isolation of the axial zooid from the daughter autozooids. A thickened wall between the axial zooecium and daughter autozooecia is recorded by the median internal zooecial ridge on the basal surface of Pywackia zooaria (Fig. 1.9, discussed above). Taylor et al. (Reference Taylor, Berning and Wilson2013, p. 988) noted that the “longitudinal slots” in Lituaria sp. “recall some Pywackia specimens.” In answer, longitudinal slots in the inner or lower walls of Pywackia autozooecia have never been documented or illustrated. This fundamental difference in the relationship of the axial budding individual to the daughter zooids in Pywackia and Lituaria sp. reflects the higher-level taxonomic distance (bryozoan vs. cnidarian octocoral) of the genera.

Another fundamental difference exists between Pywackia and Lituaria sp. and other sessilifloraeds. Pywackia and other stony bryozoans have a second mode of asexual budding: autozooid budding that leads to the production of daughter autozooids. Bryozoan autozooid budding produces multiple generations of autozooids that overlie each other and enlarge the zooarium (Fig. 1.12, 1.13). An ability to increase colony size by budding of the increasingly numerous, large feeding individuals is not present in Lituaria sp. or other pennatulaceans, which rely on a specialized primary individual, or oözooid (Bayer, Reference Bayer1956, Reference Bayer1973), located on or close to the axial rod to produce a variety of daughter polymorphs (Fig. 7.1, 7.2).

Histologic differences

Pennatulacean axial rods are dominated by calcite, not phosphate minerals. The calcite occurs as very fine grains oriented in lines along the length of the axis (Fig. 7.3), a feature termed “woody fiber texture,” which also occurs in some gorgonians with organic holdfasts and axial structures. Reich and Kutscher (Reference Reich and Kutscher2011) illustrated wood fiber texture in axial rods of two Late Cretaceous species not assigned to a pennatulacean suborder and in two Recent subsellifloraed pennatulacean genera. Similarly, a calcitic axial rod with a woody texture occurs in the Recent sellifloraed Veretillum cynomorium Pallis, 1766 (Franc and Chassagne, Reference Franc and Chassagne1974; Ledger and Franc, Reference Ledger and Franc1978). This same linearly oriented, microgranular composition occurs in Lituaria sp., and is also evident in magnified autozooecial wall pictures (Taylor et al., Reference Taylor, Berning and Wilson2013, fig. 5.7) in which the granular texture is evident (Fig. 7.3). Although the mineralogic composition was not reported, the axial rod of Lutuaria sp. of Taylor et al. (Reference Taylor, Berning and Wilson2013), as other pennatulaceans, is likely to be dominantly a high-Mg calcite.

The granular/rarely granular-prismatic histology of Pywackia is unknown in Mesozoic–Recent pennatulaceans, but occurs in stenolaemate bryozoans (discussed above). This feature alone sets Pywackia apart from pennatulaceans.

More importantly, the granular/locally granular-prismatic histology of Pywackia, which also occurs in bryozoans, is best interpreted as phosphatic replacement of an original histology as discussed above. Even if Pywackia were originally phosphatic, this would imply that its assignment to the pennatulaceans is unlikely as all known pennatulaceans have dominantly calcitic axial rods and spicules.

Taylor et al. (Reference Taylor, Berning and Wilson2013) suggested that the present phosphatic composition of the Pywackia zooarium demonstrated an original phosphatic mineralogy. They followed Macintyre et al. (Reference Macintyre, Bayer, Logan and Skinner2000) in suggesting that this phosphatic composition reflected an original phosphatic skeleton in the earliest coelenterates.

Their argument is weakened by the fact that the primary phosphate minerals reported by Macintyre et al. (Reference Macintyre, Bayer, Logan and Skinner2000) are deposited on the dominantly organic axial structures of modern gorgonians, not the calcitic axes of pennatulaceans. Macintyre et al. (Reference Macintyre, Bayer, Logan and Skinner2000) followed the traditional and outmoded belief (discussed below) that earliest Cambrian metazoan skeletons were dominantly phosphatic, and that a phosphatic composition was essentially primordial in metazoan biomineralization.

Autozooecial walls and transverse structures

The trilamellar walls of Pywackia and a number of other stenolaemates are dissimilar to the Lituaria sp. autozooecial walls. The walls of the latter consist of microcrystalline, longitudinally oriented woody textured calcite (Fig. 7.3).

Transverse partitions (diaphragms and hemiphragms) are present in the axial zooecia and autozooecia of Pywackia and other stony bryozoans. However, to our knowledge no transverse, shelf-like skeletal structure similar to bryozoan diaphragms or hemiphragms has been described below the aperture of the pennatulacean autozooid, and such transverse structures are similarly not present in Liruaria sp. autozooecia (Fig. 7.2). The only transverse structures in octocorals are funnel-shaped tabulae composed of partly fused spicules in a Recent tubiporid stoloniferan genus (Bayer, Reference Bayer1956, p. F184).

The spinose/serrate outer edges of the autozooecia walls of the pennatulacean Lituaria sp. (Fig. 7.2) were noted to be a “notable difference” between Pywackia and Lituaria sp. by Taylor et al. (Reference Taylor, Berning and Wilson2013, p. 988; Fig. 3.2). This “difference” is merely a taphonomic feature without taxonomic significance because the upper autozooecial walls of Pywackia are abraded, and any original structures, except for possible nanozooecia, are absent.

Absence of associated spicules

Evidence against interpretation of the Pywackia zooarium as a phosphatized pennatulacean axial rod (Taylor et al., Reference Taylor, Berning and Wilson2013) is further suggested by the acid-disaggregation of samples from the Yudachica Member. The mineralized skeleton of pennatulaceans consists of a calcitic axial rod, and commonly includes calcitic spicules. The latter range from needles to rods to flat plates (Fig. 6.2). The spicules may occur in the axis or stalk, may support zooarial fronds or autozooids, or provide protection by forming an operculum-like dome over a retracted autozooid (e.g., Bayer, Reference Bayer1956).

Formic acid-disaggregated samples from Río Salinas were sieved on a Tyler Equivalent 400 (37 μm) mesh sieve that retained silt-sized particles, which might have been the size of a tiny pennatulacean’s spicules. Despite ~500 hours of picking samples from the lower Yudachica Member, no spicules were found. If Pywackia was an originally phosphatic pennatulacean (Taylor et al., Reference Taylor, Berning and Wilson2013), some of these spicules should have been recovered. Alternatively, if Pywackia was a calcitic pennatulacean known from phosphatized zooaria at Río Salinas, some of its spicules should have been phosphatized and recovered, just as the associated phosphatized, originally calcitic sclerites of other metazoans (Table 1).

Two caveats are associated with an absence of pennatulacean spicules of any composition from the Tiñu Formation as being consistent with Pywackia’s interpretation as a bryozoan. Indeed, not all pennatulaceans have spicules, just as not all Lituaria species have spicules (Köllicker, 1915). Spicules are absent to rare in the genotype species L. phalloides Pallas, 1766, but abundant in L. habereri Balss, Reference Balss1910 (Kükenthal, Reference Kükenthal1915, p. 8). Kükenthal’s (Reference Kükenthal1915) brief description of Lituaria noted flat, “bisquit-like” sclerites in species with spicules (Fig. 5.2). Unfortunately, the absence, presence, or possible form of spicules in Lituaria sp. were not discussed by Taylor et al. (Reference Taylor, Berning and Wilson2013).

The second caveat involving an absence of spicules in Pywackia-bearing samples could have a basis in pennatulacean evolution. Pennatulaceans probably lost spicules at least three times in their established Mesozoic–Recent phylogeny (Dolan et al., Reference Dolan, Tyler, Yesson and Rogers2013). Thus, an absence of pennatulacean spicules from the Tiñu Formation samples does not rule out a remarkable 420 million-year extension of the group’s range into the late Cambrian (Fig. 5). However, this absence of evidence is readily overcome by the stenolaemate homologies of Pywackia (Table 2).

Polymorphism

Polymorphism does not distinguish Pywackia from pennatulacean octocorals. The complex pennatulacean colony has a variety of polyps with separate functions that include budding (the oözooid), feeding (autozooids commonly with eight tentacles; Fig. 6.4), cleaning (siphonozooids with one tentacle), and burrowing (a proximal polyp) (Bayer, Reference Bayer1973). The distinction between pennatulaceans and bryozoans such as Pywackia is that, with exception of the Lituaria autozooecia, pennatulacean polymorphs do not occupy biomineralized and fossilizable theca-like structures. Rather, pennatulaceans rely on calcitic spicules and a cylindrical or quadrate axial rod for protection and support.

Proximal ends in pennatulaceans and Pywackia

Relatively little is known about pennatulacean axial rods, with their proximal ends poorly documented or illustrated (e.g., Reich and Kutscher, Reference Reich and Kutscher2011). Illustrated proximal ends show slightly expanded to laterally bent tips in axial rods with a circular cross-section (Wilson et al., Reference Wilson, Andrews, Brown and Cordes2002), and range into longitudinally ridged proximal ends in Lituaria sp. (Taylor et al., Reference Taylor, Berning and Wilson2013, fig. 5.1; Fig. 7.1). The Lituaria sp. proximal end seems to resemble the spike-like proximal ends in Pywackia (Fig. 1.4). However, the range of variation in Lituaria sp. proximal ends is presently undocumented, and it is unknown whether the variation in Lituaria sp. proximal ends includes forms similar to the more common, rapidly tapered proximal ends with autozooecia in Pywackia (Figs. 1.1, 2.1).

Establishing a range of variation in Lituaria sp. proximal ends is important as pennatulaceans burrow into soft substrates with a muscular basal polyp that envelops the proximal end (e.g., Antcliffe and Brasier, Reference Antcliffe and Brasier2008). Pywackia has been reconstructed to resemble Lituaria sp. as a pennatulacean that “lived with its proximal end buried in the sediment, and the distal polyp-bearing part oriented vertically in the water column” (Taylor et al., Reference Taylor, Berning and Wilson2013, p. 989). This reconstruction seems unlikely for several reasons. The reconstruction assumes that the spike-like proximal ends and abruptly tapered, autozooecia-bearing proximal ends of Pywackia (e.g., Figs. 1.1, 1.4, 2.1, 2.2) were somehow attached and confluent. However, most proximal ends of Pywackia zooaria have the polyp-bearing part extending to a pointed proximal tip (Fig. 1.10, 1.11) and, as a result, could not have been confluent/attached to a proximal spike (Fig. 1.4). Taylor et al.’s (Reference Taylor, Berning and Wilson2013) reconstruction of an erect life habit means that burial of a Pywackia zooarium with a sharply rounded/pointed proximal tip would consign the apical autozooids to a non-functional role/death, as they would be entombed in dysoxic mud that characterized the depositinal environment of the lower Tiñu Formation. With such a dearth of information, a life habit of Pywackia as a recumbent zooarium or as an erect colony attached by chitinoid rootlets at the sediment–water interface are equally speculative and satisfying (Fig. 4).