Holmer et al.—Attachment strategies of Cambrian kutorginate brachiopods
(2014, 2015) studied the life strategies of the attached brachiopod fauna, which includes only 25 specimens (of a total of about 250) of Nisusia? burgessensis that are preserved directly attached to a hard substrate. As noted by Topper et al. (2015), this may suggest that the pedicle attachment was relatively weak; however, the pedicle itself cannot be observed in any of the specimens. Presumably the pedicle of Nisusia? burgessensis was very short, as the majority of attached specimens are in direct contact with the substrate. The posterior margins of some attached specimens are separated from the substrate by approximately 0.1 mm (Fig. 3.4, 3.5), a distance that may represent the true length of the pedicle. The orientation of complete individuals appears to be parallel, or close to par- allel, to the substrate (Fig. 3.4), and the short pedicle most likely emerged posteriorly between the valves. On one specimen, the pedicle foramen may be observed; however, with the valve’s orientation on the Wiwaxia sclerite, the foramen does not appear to be in a position conducive to attachment (Fig. 3.5).
Eichwaldia subtrigonalis.—Wright (1981) made the most recent detailed study of the unique attachment structures of the problematic rhynchonelliform Ordovician–Permian brachio- pods belonging to the Dictyonellida, which are now classified as the Chileata together with the equally problematic Cambrian– Silurian Chileida (Holmer et al., 2014). The chileates most likely had posteriorly fused mantle lobes, and there is no delthyrium through which a posterior pedicle could pass in any of the known chileates (Popov and Holmer, 2007). Instead, the single chileate attachment structure consists of an umbonal perforation (Fig. 3.1–3.3), which is usually covered posteriorly by a plate—the colleplax in the sense of Wright (1981).Asingle articulated silicified juvenile ventral valve (8 mm long and wide) of the type species Eichwaldia subtrigonalis Billings, 1858 has a 6 mm long and 2 mm wide silicified cylindroid emerging from the umbonal perforation, which is completely covered by the cylindroid. The cylindroid has closely spaced transverse grooves (Fig. 3.1, 3.2) and is more or less identical in morphology to the cylindroid in Nisusia sulcate; there is no indication that it represents “adventitious silica adhering to the silicified shell” as suggested by Wright (1981, p. 448).
Discussion and implications
The nature and origin of the brachiopod pedicle has for a long time been of critical importance for understanding the phylogeny and evolution within the Brachiopoda, and indeed some classificatory schemes, such as that of Beecher (1892), were almost entirely based on the variation in the nature of the pedicle opening. Features of the brachiopod pedicle remain important characters in more broadly based phylogenetic analyses, such as that carried out by Williams et al. (1996) and adopted in the revised brachiopod volumes of the Treatise on Invertebrate Paleontology (Williams et al., 1997–2007), which still is the most up-to-date phylogenetic and classificatory framework of the Brachiopoda. In the Treatise framework, the Kutorginata occupy an
unstable and unresolved basal position in the Rhynchonelli- formea (e.g., Popov et al., 1996; Harper et al., 2017), associated
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with the Chileata and the Obolellata, but the phylogenetic positions of these groups remain problematic (Harper et al., 2017) and we still have a poor understanding of the stem and crown group relationships within the earliest Cambrian rhynchonelliforms. Consequently, the new soft anatomy information concerning the extinct kutorginates is of great phylogenetic significance, and the new type of apical pedicle described here from Kutorgina chengjiangensis is closely comparable in all major features with the pedicles emerging from the apical foramen of both the chileate Longtancunella chengjiangensis (Zhang et al., 2011a) and the obolellate Alisina sp. (Zhang et al., 2011b), both also from the Chengjiang fauna. The pedicles of these species have an identical type of strong, three dimensionally preserved, distinctive annulations that
consist of densely stacked tabular discs (Zhang et al., 2011a, fig. 3; 2011b, fig. 2). Moreover, exceptionally preserved similar pedicles are also known to emerge apically from the Silurian chileate dictyonellid Eichwaldia subtrigonalis (Fig. 3.1–3.3; briefly described in the preceding) as well as from the recently described Silurian chileate Trifissura rigida, where the pedicle is preserved as a short series of phosphatized pads (Holmer et al., 2014, fig. 1). The earliest ontogeny of the strophomenates also includes
closely similar apical attachment structures, and in particular, the so-called pedicle sheath in, for example, Antigonambonites (Popov et al., 2007; Bassett et al., 2008) is long, hollow, and annulated and closely comparable to the pedicle of Kutorgina chengjiangensis; it is possible that the densely stacked tabular discs in Kutorgina were mineralized. The positions of the kutorginate, obolellate, chileate, and strophomenate pedicles indicate that they must have been secreted by a specialized modified region of the ventral mantle, rather than from a larval pedicle lobe as in all living rhynchonelliforms, and most likely represent homologous structures (Williams et al., 1996; Popov et al., 2007). By contrast, the adult pedicle of the nisusiids emerged
between the valves from the posterior gap as indicated by silicified pedicles in Nisusia (Rowell and Caruso, 1985) as well as the apparent posterior attachment of the Burgess specimens of Nisusia? (Fig. 3.4, 3.5); this type of pedicle is most likely homologous with the pedicle of extant rhynchonelliform brachiopods. Thus, the kutorginates had a dual system of attachment, but the detailed distributions and polarity of character transitions that are important for understanding the phylogeny are still far from clear. The fine details of the ontogeny in kutorginides unfortu-
nately remain unknown, but the morphology of the umbonal region of the ventral valve, which clearly lacks a pedicle sheath, suggests that their ontogeny may have been closely similar to that of the orthotetidoids. The ontogeny of this strophomenate superfamily has been recently described in detail by Bassett and Popov (2017). The ventral perforation in the orthotetidoid (Chilidiopsoidea) Coolinia shows that it has an apical plate that is closely similar to the chileate colleplax (Bassett and Popov, 2017), while at the same time the characters of the shell formation suggest that there is a substantial delay in the secre- tion of the ventral valve, possibly until the end of metamor- phosis; moreover, a similar ontogeny was described earlier for the problematic stem rhynchonelliform chileate Salanygolina
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