Journal of Paleontology, 92(5), 2018, p. 768–793 Copyright © 2018, The Paleontological Society 0022-3360/15/0088-0906 doi: 10.1017/jpa.2018.9
The strophomenide brachiopod Ahtiella Öpik in the Ordovician of Gondwana and the early history of the plectambonitoids
Juan L. Benedetto
Centro de Investigaciones en Ciencias de la Tierra (CICTERRA), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and Universidad Nacional de Córdoba, Vélez Sarsfield 1611, X5016GCA Córdoba, Argentina ⟨
juan.benedetto@
unc.edu.ar⟩
Abstract.—The Precordilleran species Ahtiella argentina Benedetto and Herrera, 1986 is redescribed and illustrated and Monorthis coloradoensis Benedetto, 1998b from northwestern Argentina is reassigned to the genus Ahtiella Öpik, 1932. Ahtiella famatiniana new species from volcaniclastic rocks of the Famatina range (western Argentina) and Ahtiella tunaensis new species from the Precordillera basin (Cuyania terrane) are proposed. Paleogeographic and strati- graphic evidence strongly suggests that Ahtiella originated in the Andean region of Gondwana to further migrate to Avalonia, Baltica, and Cuyania. Contrary to previous assumptions, the fossil record from the Famatina volcaniclastic succession suggests that the plectambonitoid Ahtiella famatiniana n. sp. evolved from the hesperonomiid orthoid Monorthis transversa Benedetto, 2003 that always occurs in the underlying strata. Phylogenetic analysis of Ahtiella species shows that A. famatiniana n. sp. and the Peruvian A. zarelae Villas in Gutiérrez-Marco and Villas, 2007 are not only the earliest species of the genus but also are morphologically intermediate between Monorthis Bates, 1968 and the later and more derived species of Ahtiella from Baltica and Cuyania. If, as empirical evidence presented here shows, Ahtiella originated from Monorthis through a series of minor transformations, then the impressive morphological gap between orthides and strophomenides was bridged through short-time cladogenesis events, suggesting that it might not have a definite discontinuity between the species level evolution and the origin of higher taxa (macroevolution).
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experienced a significant speciation event encompassing at least five species. As Gutiérrez-Marco and Villas (2007) pointed out, the records of Ahtiella in the Floian of Peru and Dapingian of Famatina are the oldest known of the genus, strongly suggesting that it originated on the Andean margin of Gondwana and later migrated to other regions.
The genus Ahtiella Öpik, 1932 is a distinctive resupinate and variably geniculate plectambonitoid brachiopod described ori- ginally from the Baltic region (Öpik, 1932, 1933; Hessland, 1949) but subsequently recognized in Wales (Bates, 1968) and central Newfoundland (Neuman, 1976). In South America, Ahtiella is very common in the lower−middle Darriwilian carbonate-rampdeposits of the Precordillera basin ofwest-central Argentina where it defines the uppermost of the six brachiopod biozones recognized through the San Juan Formation (Herrera andBenedetto, 1991; Benedetto, 2002, 2007).Later, Ahtiella was reported from the Floian-Dapingian volcaniclastic succession of the Famatina Range (Benedetto et al., 2003; Benedetto 2003a) but these specimens remain undescribed. Its presence in southern Peru (Gutiérrez-Marco and Villas, 2007), together with its record in the central Andean Basin of northwestern Argentina (this paper) and probably Bolivia (described as Valcourea sp. by Havlíček and Branisa, 1980), indicate that this genus not only attained a wide geographic range in South America but also
One of the objectives of this study is to update the tax-
onomy of the genus Ahtiella from the three major Ordovician basins of Argentina: Precordillera, Famatina, and Central Andes. This includes: (1) the redescription of the Precordilleran species Ahtiella argentina Benedetto and Herrera, 1986, on the basis of extensive collections made in the past twenty years from the upper part of the San Juan Formation, as well as the proposal of a new species of Ahtiella from the somewhat younger Las Chacritas Formation; (2) the first description of the Ahtiella specimens from volcaniclastic rocks of the Famatina Range; and (3) the reassignment to the genus Ahtiella of Monorthis color- adoensis Benedetto, 1998b, from northwestern Argentina. Evidence presented here aims to shed light on the long-
standing and not yet resolved issue of the origin of plectambo- nitoid brachiopods. Although parsimony analysis constitutes an indispensable tool for unravelling the phylogeny of fossil groups, the most difficult task is to corroborate in the fossil record the phyletic lineages predicted in such analyses, and even more problematic is to detect thosemorphological transitions leading to the origin of new taxa. According to the punctuated equilibrium hypothesis (Eldredge and Gould, 1972; Gould and Eldredge, 1977; Benton and Pearson, 2001), this can be explained by the conjunction of the rapidity as cladogenesis events occur and the relatively small size and geographic restriction of populations undergoing phenotypic change. In this respect, the continuous and richly fossiliferous volcanosedimentary succession of the
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