Abstract

The taphonomy, sedimentology, and paleoecology of a rare peritidal hardground in the Lower Devonian (Lochkovian) Thacher Limestone Member of the Manlius Formation in east-central New York State provides insights into sediment dynamics and modes of life of ancient hard substrate-inhabiting organisms. The hardground occurs near the top of a shallowing-upward carbonate cycle, slightly below desiccation-cracked micritic ribbon limestones. The hard surface developed on a partially exhumed tentaculitid pavement and overlying pelletal calcisiltite; the hardground has a relief of about 2–3 cm and is overlain by a thin siliciclastic mud layer. Tentative correlation of the hardground across a lateral distance of about 35 km perpendicular to depositional strike suggests a very subdued topographic profile for this region and development of an extensive peritidal hardground pavement. Tentaculites gyracanthus occurs in a dense pavement of strongly bimodally aligned shells with an ENE to WSW orientation, parallel to the inferred paleoshoreline. Some specimens also occur vertically embedded, suggesting possible life orientations within firm substrates. The presence of pits around the apertures of these shells suggests scouring effects around the shells burrowed into the substrate. Upper portions of the irregular hardground surface were preferentially colonized by an undescribed small postibulinid edrioasteroid, which may have lived somewhat like acorn barnacles on wave-swept rock platforms. The latter shows a population structure with few juveniles. Although fauna within the Thacher limestone includes leperditian ostracodes, ramose and encrusting bryozoans, Howellella vanuxemi brachiopods, and rare pterioid bivalves, the hardground community is rather limited. This low-diversity assemblage represents an unusual edrioasteroid-dominated hardground community type that persisted from at least Early Ordovician to Late Devonian in peritidal hardgrounds and rockgrounds.

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