Blountia Walcott, 1916

Westrop, Madison Armstrong Stephen R. & Eoff, Jennifer D., 2020, Systematics of a survivor: the Cambrian kingstoniid trilobite Blountia Walcott, 1916 across the Marjuman-Steptoean (Guzhangian-Paibian) extinction interval in Laurentian North America, Zootaxa 4804 (1), pp. 1-79 : 8-10

Genus Blountia Walcott, 1916

Type species. Blountia mimula Walcott, 1916 , from the Nolichucky Formation , eastern Tennessee (by original designation) .

Diagnosis. Strongly convex glabella on cranidium and pygidium with well-defined, flat to concave pygidial bor- der.

Discussion. Assignment of Blountia to Kingstoniidae , rather than Asaphiscidae (e.g., Rasetti 1965), follows Westrop (1992), who drew attention to the structure of the occipital ring as a synapomorphy of the family. Maryvillia has been considered to be a junior synonym of Blountia by some authors (e.g., Pratt 1992, p. 66). However, a phylogenetic analysis ( Fig. 2 ) supports monophyly of both Maryvillia and Blountia . Maryvillia is relatively effaced, whereas Blountia is strongly furrowed. Only a single unambiguous apomorphic state of Blountia , the expression of the pygidial border (character 12, state 1 in Table 2) emerged from the phylogenetic analysis, and this is the basis for the diagnosis presented above. A more comprehensive analysis of Kingstoniidae may help clarify the status of Blountia , but this is beyond the scope of our study. Maryvillia is better supported ( Fig. 2 ), with synapomorphies including a relatively short (character 1, state 1), evenly tapered glabella with nearly straight sides (character 2, state 2), a concave anterior crandial border (character 6, state 1), and a pygidial axis that extends back beyond the anterior edge of the border (character 13, state 1).

Articulated specimens of B. mimula (Pl. 1, figs 1–4) and B. bristolensis (Pl. 3, figs 3–7) both have seven thoracic segments. Rasetti (1965, p. 58) reported an articulated exoskeleton of an older, undescribed species of Blountia from the Murphy’s Creek Formation in the Gaspé Peninsula, Quebec, with nine thoracic segments.

Figures

FIGURE 2. Phylogenetic analysis of Blountia and related taxa. 1. Strict consensus of six trees of length 30 (CI, 0.50; RI, 0.62; RC, 0.31). Names of species from Steptoean strata are identified by bold font. 2. Optimized character distribution plotted on one of the most parsimonious trees. Only unambiguous transformations (i.e., optimize to the same nodes under the assumptions of ACCTRAN and DELTRAN) are shown; open circles indicate characters that are homoplastic. Numbers above the circles refer to characters and those below identify particular states (Table 2).