Journal of Paleontology, 91(1), 2017, p. 73–85 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2016.145


A lowermost Ordovician tabulate-like coralomorph from the Precordillera of western Argentina: a main component of a reef-framework consortium


Marcelo G. Carrera, Ricardo A. Astini and Fernando J. Gomez


CICTERRA-CONICET, Facultad Cs Exactas Físicas y Naturales, Universidad Nacional de Córdoba, Córdoba (X5016GCA), Argentina ⟨mcarrera@unc.edu.ar⟩, ⟨ricardo.astini@unc.edu.ar⟩, ⟨fjgomez@unc.edu.ar⟩


Abstract.—Although putative corals of uncertain affinities occur in the early Cambrian, the earliest definite tabulate corals have not been described prior to the Early Ordovician in North America. This paper reports a new finding of a tabulate-like coralomorph forming part of biostratigraphically well-constrained reef mounds in the latest Cambrian–Early Ordovician La Silla Formation in the Argentine Precordillera. The oldest record of the coralomorph genus Amsassia is reported and a new species, A. argentina, is erected. The discovery of this genus in the lowermost Ordovician modifies the previously proposed paleogeographic distribution and patterns of origination and migration routes of this coral-like organism. Amsassia argentina n. sp. constitutes a main framework builder together with a complex microbial consortium. This oldest occurrence of Amsassia as a reef builder represents a new record of a skeletal organism in the gap of metazoan reef constructors after the demise of archaeocyaths in the late early Cambrian.


Introduction


The earliest putative corals occur in the early Cambrian. Although many of these generally coralline forms (e.g., the Coralomorpha of Jell, 1984) are of uncertain affinities, among them are undoubted corals (Scrutton, 1997, 1999). The oldest definite tabulate coral, Lichenaria Winchell and Schuchert, 1895, occurs in the Early Ordovician (Scrutton 1979, 1999; Webby et al., 2004). The scarcity of coralomorphs reported from the Cambrian and Early Ordovician contrasts with the diversity of undoubted coral genera recorded after the Middle Ordovician. In this context any finding of coral-like forms from this critical interval significantly contributes to the global diversity, biogeography, and early evolutionary knowledge of corals and coral-like organisms. Previous works on Paleozoic corals in the Argentine Pre-


cordillera (Rodriguez et al., 2002; Fernandez-Martinez et al., 2004; Carrera et al., 2013, and references therein) show records of paleofavositids and sarcinulids from the Middle–Upper Ordovician units, and favositids, pleurodictyids, and rugosans from Silurian and Lower Devonian rocks. The aim of this con- tribution is to report and describe a tabulate-like coralomorph forming part of boundstones developed within the uppermost Cambrian–lowermost Ordovician that were found in the lower section of the La Silla Formation, Precordillera, western Argentina. This finding represents the oldest record of the genus Amsassia Sokolov and Mironova, 1959. Its association with boundstones and reef-mounds represents the earliest occurrence of this reef-related metazoan found in the gap of reef metazoan constructors after the demise of archaeocyaths in the late early Cambrian. The paleobiogeographic distribution and sig- nificance of this new record are discussed.


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Geological setting


The new tabulate-like coralomorph is included within shallow- water subtidal limestones of the La Silla Formation in the Argentine Precordillera (Fig. 1). The La Silla Formation (Keller et al., 1994) is considered to be an ancient tropical Bahamian-type platform (Cañas, 1999; Keller, 1999; Pratt et al., 2012) that accumulated on a uniformly subsiding passive margin within the exotic Precordillera terrane (Astini et al., 1995; Gomez and Astini, 2015). Paleogeographically, by this time, the Precordillera terrane was driftingwithin the southern Iapetus Ocean (Astini et al., 1995; Benedetto, 2004; Keller, 2012). From a stratigraphic viewpoint, it represents the transition from the late Cambrian largely fossil-barren cyclic peritidal dolomites (Zonda and La Flecha formations) into fossiliferous open-shelf pure limestones of the Ordovician San Juan Formation (Fig. 1.2). However, the high- frequency stacking pattern in the La Silla Formation is more similar to the underlying peritidal La Flecha Formation than to the overlying dominantly subtidal San Juan Formation. Although there is a clear faunal turnover starting from the


La Silla Formation onwards (Cañas and Carrera, 2003), the major change to truly open-marine settings seems to occur at the boundary between the La Silla and San Juan formations (Cañas, 1999; Buggisch et al., 2003; Thompson et al., 2012). Major type 1 sequence boundaries separate the La Silla Formation from the under- and overlying units (Cañas, 1999; Keller, 1999). Accommodation within this largely detached passive-margin setting is interpreted to relate to global sea-level fluctuations rather than with local subsidence (see Astini and Thomas, 1999; Thomas and Astini, 1999). The La Silla Formation is exposed in several thrust sheets over a distance of more than 350km along the Precordillera fold


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