A THALLOPHYTIC-ALGA-DOMINATED BIOTA FROM THE SILURIAN ERAMOSA FORMATION, ONTARIO, CANADA

A "thallophytic-alga-dominated biota" is a distinctive and recurrent fossil assemblage in which noncalcified thallophytic algae ("seaweeds") and worms (primarily annelids) are the principal, and often only, constituents. This biota provides a remarkably complete view of a community characteristic of restricted, shallow marine settings since at least the Cambrian and ranks among the most common of Konservat-Lagerstätten, a biased representation that stems from habitat parameters particularly conducive to preservation. Here, we describe a thallophytic-alga-dominated biota from the Silurian (late Wenlockian or early Ludlovian) Eramosa Formation near the town of Wiarton in the Bruce Peninsula region of Ontario, Canada. The biota is preserved within thinly laminated, organic-rich dolostone; worms (polychaetes) are represented as articulated scolecodont assemblages, whereas algae are preserved as carbonized compressions. The taxonomic affiliations of most of the latter cannot be determined, even at division-level, owing to thallus morphologies which are ubiquitous among the algae and to an overall lack of preserved cellular-level detail; the most abundant alga taxon within the biota, however, exhibits a thallus architecture diagnostic of the green alga order Dasycladales. Specimens of this taxon have an upright, cylindrical, noncalcified thallus, 2-5 cm in height, composed of a narrow main axis encircled by numerous unbranched laterals in whorls (euspondyl). The laterals are distinctly heteromorphic: verticils of hairlike, distally-tapered (trichophore) deciduous laterals alternate along most of the length of the main axis with verticils of club-shaped (phloiophore) laterals bearing a vesicular termination suggestive of a gametophore (cladospore). This dasycladalean taxon is sufficiently distinct to warrant designation as a new genus and is among the earliest with heteromorphic laterals known in the fossil record. Along with other recently described noncalcified dasycladaleans from early Paleozoic thallophytic-alga-dominated biotas, it points to the attainment of a high level of morphologic disparity among this clade by the Silurian, considerably earlier than indicated by previous studies focused solely on calcified taxa.