Benedetto—The strophomenide brachiopod Ahtiella in Gondwana


strophomenides, concluding that their phylogenetic links still remain unclear. Subsequently, Bassett et al. (2008) and Bassett and Popov (2017), based on a study of the ontogeny of the orthotetide Coolinia Bancroft, 1949, inferred an early diver- gence of strophomenate and rhynchonellate brachiopods. At the superfamily level, it has been assumed that Stro-


phomenoidea was derived from the Plectambonitoidea during the Early Ordovician. Spjeldnaes (1957) did not identify the group of plectambonitoideans that gave rise to the strophome- noideans and left open the possibility that the latter group is polyphyletic. According to Cocks and Rong (1989, 2000), strophomenoideans originated from plectambonitoideans by a transformation of the cardinal process from simple to bifid, suggesting as potential ancestor a leptellinid like Apatomorpha Cooper, 1956 or Toquimia Ulrich and Cooper, 1936. Recent discoveries demonstrated that the three basic types of stropho- menoid cardinalia were already differentiated in the oldest known members of the clade recorded in the Dapingian of southern China (Zhan et al., 2015), supporting that the Stro- phomenoidea originated in the Floian from an unknown ‘stro- phomenide stem group’ shortly after the first appearance of plectambonitoids. Dewing (2004) challenged the hypothesis of the plectambonitoid derivation of strophomenoids based on their different shell structure (laminar in the former and fibrous in the latter). Unlike the a priori assumption that shell structure is homoplastic (Cocks and Rong, 2000), i.e., evolved indepen- dently in different clades, Dewing (2004, fig. 3) proposed a phylogenetic scenario in which the Strophomenoidea arose from a Cambrian laminar-shelled billingselloid, whereas the common ancestor of both the fibrous-shelled Plectambonitoidea and Clitambonitoidea was left with interrogation. On the contrary, the parsimony analysis performed by Congreve et al. (2015) indicated that plectambonitoideans and strophomenoideans are phylogenetically related but, in contrast to previous inferences, plectambonitoideans do not constitute a monophyletic group but a paraphyletic grade of the strophomenoidean clade. Sig-


nificantly, in the phylogeny presented by Congreve et al. (2015, fig. 4), Taffia Butts, 1926, Railtonella Laurie, 1991, and Ahtiella, all currently included in the family Taffiidae, cluster as basal forms to all other Strophomenida, as Spjeldnaes (1957) intuitively depicted in his phylogenetic tree sixty years earlier. The absence or extreme paucity of undisputed plectambo-


775


alternatively, could be an homoplastic feature that appeared and became lost at different times in different clades. In fact, in certain basal plectambonitoids, e.g., Plectella Lamansky, 1905, Ingria Öpik, 1930, Aporthophyla Ulrich and Cooper, 1936, Tourmakeadia Williams and Curry, 1985, and Pelonomia Cooper, 1956, the pseudodeltidiumis rudimentary or lacking. In any cases, this structure is not as phylogenetically informative as previously supposed. Noteworthily, the widely splayed, rodlike socket ridges


running almost parallel to the hinge line of billingselloids are closely comparable to those of the family Plectambonitidae (e.g., Plectella, Plectambonites Pander, 1830, and Ingria). Such an arrangement is quite different from the typically orthoid cardinalia seen in taffiids. Therefore, it is not surprising that in the parsimony analysis carried out by Congreve et al. (2015), Plectambonites appears as monophyletic only if it is excluded from all other ‘plectambonitoids’ and placed in a separate superfamily. In this context, it is worth noting the close resem- blance between the billingsellide (?) Kozhuchinella Severgina, 1967 and the oldest known probable plectambonitoid Akelina Severgina, 1967, both from the upper Tremadocian Algan For- mation of Kuznetz-Altai, Altai Mountains, Siberia (Severgina, 1967). Despite the poor preservation of the latter (reillustrated by Cocks and Rong, 1989, figs. 13–17), both genera share a concavoconvex profile; parvicostellate ornamentation (typical billingsellides are multicostellate or ramicostellate); the absence of dental plates; a prominent dorsal median ridge; a simple knob-like cardinal process; long, widely divergent socket rid- ges; and a well-developed dorsal subperipheral rim. Accord- ingly, Akelina and Kozhuchinella are likely related forms, which could be considered either as early members of the ‘plectam- bonitoid’ clade or, alternatively, as derived billingselloids (the presence of pseudopunctae has not yet been demonstrated in these genera). This raises the possibility that only the plectam- bonitoid clade sensu stricto (the subfamily Plectambonitinae in the current classification) evolved from a billingselloid ancestor, and that ahtiellins (and probably other taffids) had a different ancestor, which should be sought among the Orthoidea, as discussed below.


Searching for the Ahtiella ancestors


nitoids in the Tremadocian, along with their sudden diversifi- cation around the Floian-Dapingian transition, suggests that this superfamily originated in the Early Ordovician rather than deep in the Cambrian. In my opinion, billingselloids are too derived morphologically to be considered direct ancestors of plectam- bonitoids (excepting the family Plectambonitidae, as discussed below). Since their first appearance in the middle Cambrian, billingsellides developed a proportionally high planar ventral interarea leading in the late Tremadocian to the hemipyramidal shells that characterize most polytoechioids, e.g., Protambo- nites Havlíček in Havlíček and Josopait, 1972 and Tritoechia Ulrich and Cooper, 1936, which form a consistent monophyletic clade (Benedetto, 2009; Topper et al., 2013). The apically per- forated pseudodeltidium—the main feature linking bill- ingsdelloids and plectambonitoids—could be a plesiomorphic condition of basal rhynchonelliforms already present in some of the earliest members of the clade (e.g., Nisusioidea) or,


In his outstanding morphological study of Middle Ordovician strophomenides from Norway, Spjeldnaes (1957, fig. 42) pre- sented a diagrammatic evolutionary tree of Strophomenida starting with two main branches, one of them lacking descen- dants including Plectambonites and allied forms, and the other including the ‘ahtiellinids’ (Ahtiella, Inversella Öpik, 1933, and Ukoa Öpik, 1932), which albeit with a question mark, were placed at the origin of the strophomenoid stock. A third, short- lived basal branch was represented by Taffia. In their compre- hensive revision of plectambonitoid classification, Cocks and Rong (1989, p. 83, fig. 5) also placed the Taffiidae at the base of the plectambonitoid tree “because we regard the absence of side septa as representing a more primitive state than their presence (as in the Plectambonitidae),” and also that the “oldest plec- tambonitacean[s] ... are essentially indistinguishable from their orthide (probably billingselloid) ancestors except by their pseudopunctate shell.” In fact, basal plectambonitoids share


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