The oldest (lowermost Ordovician) tabulate coral, a distinctive sessile metazoan component of carbonate mounds from the Argentine Precordillera.

CARRERA, MARCELO G.; ASTINI, RICARDO

Mendoza

Congreso; 4th International Paleontological Congress; 2014

THE OLDEST (LOWERMOST ORDOVICIAN) TABULATE CORAL, A DISTINCTIVE SESSILE METAZOAN COMPONENT OF CARBONATE MOUNDS FROM THE ARGENTINE PRECORDILLERA Marcelo G. Carrera1 and Ricardo A. Astini1 1. CICTERRA-CONICET, Facultad Cs Exactas Físicas y Naturales, Universidad Nacional de Córdoba, Córdoba X5016GCA, Argentina. mcarrera@efn.uncor.edu The earliest putative corals occur in the early Cambrian, although many of these generally coralline forms (the ?coralomorpha?), are of uncertain affinities, among them the Order Tabuloconida are undoubted corals. The oldest definite tabulate corals occurred in the Lower Ordovician (e.g. Lichenaria and other selected early tabulate corals such us: Eofletcheria, Nictopora, Saffordophylum, Foerstephyllum and Lyopora). The earliest favositid Paleofavosites come from the Middle Ordovician of the Urals and spread through Eurasia, North America and Australia by the Upper Ordovician. The aim of this contribution is to report and describe a favositid tabulate coral found in the lowermost Ordovician rocks of the Precordillera, Western Argentina. The studied material comes from the Tremadocian La Silla Formation, a carbonate unit located in the Precordillera basin, Western Argentina. The fossiliferous levels are within the Cordylodus angulatus conodont zone (lower Tremadoc) slightly over the Cambrian-Ordovician boundary, given by the presence of the Clavohamulus hintzei and C. lindstromi zones. The new tabulate is included in patchy mud-rich microbial mounds that have grown above a major flooding surface that initiates the second shallowing upward grand cycle within the La Silla Formation at ~95 m from its base. This unit has been considered a typical Bahamian shallow-water carbonate environment and, within the Precordillera, it represents the transition from the late Cambrian largely barren cyclic dolomites into the fossil-rich Ordovician limestones. These levels yield abundant gastropods and few trilobites intimately related to domal thrombolites. These facies association may be compared with the thrombolites-Lichenaria mounds reported in Newfoundland. The new form shows cerioid massive colonies, corallites polygonal, septal structure developed as septal spines, double wall with epitheca, common lateral increase, and mural pores specially corner pores. These main characteristics can be found in the favositids and in particular the diagnosis of Paleofavosites. However, tabulae and septal spines are scarce in our material and overall structure looks irregular with undulated walls. Although similar in general shape and structure, Lichenaria, shows corallites separated by fibrous, continuously fused common walls, lacking back-to-back epithecae. The septal structures are absent and it has also abundant tabulae. The new taxon represents the oldest record of a tabulate coral, probably older than the Lichenarid reports, and certainly the oldest occurrence of a favositid coral, whose oldest records come from the Middle Ordovician. Lichenaria is considered the basal genus in tabulates early radiation. This must be revised in the light of the new findings.