Benedetto—The strophomenide brachiopod Ahtiella in Gondwana
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septa), resulting in destabilization of the relationships among other taxa. Hesperonomiella was chosen as the outgroup for rooting the phylogenetic tree because it exhibits an ensemble of nonderived internal features, and lacks the majority of apo-
Figure 7. Generalized trend of morphological change through selected taxa from the Floian-Dapingian volcanosedimentary succession of Famatina. Ventral internal molds at left, dorsal internal molds at right. Hesperonomiella arcuata Benedetto, 2003b): left, CEGH-UNC 15740; right, CEGH-UNC 19078. Monorthis transversa Benedetto, 2003b: left, CEGH-UNC 19095; right, CEGH-UNC 19623. Ahtiella famatiniana n. sp.: left, CEGH-UNC 27140; right, CEGH-UNC 27135b. At the top of the succession, A. argentina Benedetto and Herrera, 1986 illustrates a more derived species of Ahtiella from the Darriwilian of Precordillera (left, CEGH-UNC 27111; right, CEGH- UNC 21118b).
and internal details along with gradation in some features, con- vergence seems highly improbable. To investigate whether the trends deduced from comparative morphology are phylogen- etically significant, a cladistic analysis of Ahtiella species was performed using TNT (Tree Analysis Using New Technology) version 1.5 (Goloboff and Catalano, 2016), selecting the heur- istic search option with multiple random addition sequences and the tree bisection reconnection (TBR) branch-swapping algo- rithm holding 10 trees in each addition sequence. A total of 23 characters comprising internal and external features were included within the Ahtiella analysis (Table 1). The 23-character matrix was analyzed for 10 taxa (Table 2). The Welsh species A. concava was not considered because it possesses some fea- tures atypical for the genus (e.g., apparently smooth exterior, elongate oval anterior adductor scars each flanked by prominent
morphies present in the group analyzed here. This, along with its first appearance in the middle Cambrian, suggests it as a potential ancestor of the taxa considered in this study. A heur- istic search of the data matrix in which all characters were unordered and equally weighted produced four minimal length trees 52 steps long, with consistency index of 0.692 and reten- tion index of 0.754. The strict consensus tree (Fig. 8) is pre- sented here with branch length calibrated to the age of the first appearance datum of each taxon. As the phylogenetic tree shows, the basal member of the Ahtiella clade is Monorthis transversa, which possesses ancestral features such as multi- costellate ornamentation, absence of a dorsal platform, an unthickened periphery of the ventral valve, an open delthyrium, and an unmodified notothyrium. Monorthis transversa has in common with the Ahtiella species a resupinate and alate shell (node 1), and cardinalia and muscle scars of the orthoid type. The Welsh species A. quadrata is expressed as the most basal member of the Ahtiella clade displaying vestigial or incipient pseudodeltidium and chilidium and a slightly thickened ventral valve margin (node 2). The apomorphy that defines the remaining Ahtiella species (node 3) is the presence of a dorsal platform, which varies from low and discontinuous in the older species to variably developed in the younger species. The Gondwanan species A. coloradoensis, A. zarelae, and A. fama- tiniana n. sp., which each retain ancestral uniform or nearly uniform radial ornamentation and an incipient apical pseudo- deltidium, appear as basal to the more derived species from Baltica, Cuyania, and Newfoundland. In addition, it should be noted that the presence of pseudopunctae has not been demon- strated in any of these Gondwanan species. Among them, A. famatiniana n. sp. is slightly more advanced by having a com- paratively larger pseudodeltidium and chilidium, a more pro- minent dorsal platform, a deeply impressed vascular system on the disc/trail deflection, and ornamentation tending to be inci- piently parvicostellate. The Famatinan species clusters with the group that includes A. tunaensis n. sp., A. lirata, A. jaanussoni, A. argentina,and A. paucirugosa, characterized by awell-defined disc and a dorsally directed trail in the interior of ventral valve (node 4). This group shares unequally parvicostellate ribbing, a well-developed pseudodeltidium and chilidium, and relatively large pseudopunctae (node 5). Ahtiella jaanussoni and A. lirata form a cluster based on a gibbous shell profile and prominent rugae or corrugations covering most of the valve surface, and likely reflect local radiation of the genus in the Baltica paleo- continent (node 6). This group of species served originally to define the genus Ahtiella. On the other hand, A. argentina and A. paucirugosa form a sister group of Baltic species by sharing deeply impressed mantle canals on the entire valve interior and curved ridges in the floor of dorsal valve (node 7). Our species-level phylogenetic tree shows that since its
origin in the Floian, the genus Ahtiella underwent successive speciation events along the Andean margin of Gondwana where the common ancestor would likely have inhabited, and subsequently dispersed and continued speciating as new geographic areas were colonized. The diversification of the
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