Carrera et al.—Coralomorph in lowermost Ordovician reef mounds
size and dome shaped or encrusting laminar forms. They are widespread, but mainly well developed in North America and China, and less frequent in South America and Australia. Later, they became the main reef constructors in Silurian and Devonian carbonate platforms. Bryozoans have also been found associated with Ordovi-
cian reef mound structures, assuming a constructional role (Adachi et al., 2012b; Cuffey et al., 2013). The main framework of these reef structures was shared with demosponges, calci- microbes (Girvanella Nicholson and Etheridge, 1878), and the stromatoporoid Pulchrilamina. Corals were minor components of reefs structures during
the Early Ordovician. The Lichenaria specimens, recognized by Pratt and James (1982) in Newfoundland, are included in small thrombolite mounds or small bioherms composed of a microbe- Lichenaria-Renalcis boundstone (Pratt and James, 1982). The coralomorph Amsassia was recently reported from
north-central China as an important component of Middle Ordovician reef assemblages (Lee et al., 2014). The reef-mound structure is characterized by peloidal masses composed of aggregated peloids, micrites, and bundles of tubular calcimic- robes, with the subordinate Amsassia, the calcimicrobe Ortonella Garwood, 1914, green algae, and tetradiids. Amsassia argentina n. sp. is abundant, dome shaped, and
represents an important proportion, clearly >50%, of the Lower Ordovician reef framework in western Argentina. These char- acteristics suggest that this form had a dominant role in the skeletal framework construction. It can be considered a main frame builder associated with a complex microbial-algal con- sortium composed of renalciforms, Nuia Maslov, 1954, and a variety of thrombolites and small stromatolites. This structure differs slightly in composition and proportions from the younger Amsassia reef framework in north-central China (Lee et al., 2014).
Amsassia was reported from the western margin of Gond-
modifies the paleogeographic scenario envisaged by M. Lee et al. (2014, 2016). These authors suggested that Amsassia emerged in western Gondwana (Sino-Korean and Tarim blocks), where the genus appears to have originated in the Middle Ordovician, and might subsequently have spread into Kazakhstan and Siberia prior to the early Late Ordovician. With this new discovery in the Lower Ordovician of
wana around the Sino-Korean and Tarim blocks from the middle part of the Middle Ordovician (M. Lee et al., 2014, 2016, and references therein), and wider occurrences have been reported from the early Late Ordovician of Kazakhstan (e.g., Popov et al., 2002), Siberia (Bondarenko and Ulitina, 2009), and a single report from the Late Ordovician of Laurentia (Bolton, 2000). Discovery of Amsassia in the lowermost Ordovician
Argentina, the spatio-temporal distribution of Amsassia is sub- stantially modified: its origination and migration pathway is significantly changed. This new record shows that Amsassia emerged from the eastern margin of Laurentia, since the Argentine Precordillera rifted from this continent by the late Cambrian (Astini et al., 1995; Benedetto, 2004). Subsequent records occur in the Middle Ordovician of the Sino-Korean and Tarim block (Lee et al., 2014), and later the expansion continued in Siberia and Kazakhstan. The record in Canada (Bolton, 2000) by the early Late Ordovician can be explained by its proximity
83
with the Siberian block. The presence of Amsassia in the “Laurentian stage” of the Argentine Precordillera suggests that we should expect Lower or Middle Ordovician Laurentian records between the Lower Ordovician occurrence of Amsassia in the Argentine Precordillera and their subsequent presence in the Late Ordovician of Canada. This probably reflects a gap in the sampling record more than a true dispersion pattern. However, as Sun et al. (2014) pointed out, there is a critical need to re-examine the species previously referred to Lichenaria or Amsassia, which have been commonly misidentified. By the late Cambrian and Early Ordovician, a variety of
metazoan forms occupied the niches left by archaeocyaths with varying degrees of success. All these metazoan forms were aided in reef or biostrome construction by microbes. Large-sized organisms (orchoclad demosponges, calathids, stromatoporoids) could easily form the scaffold structure of the framework, and usually required a smaller proportion of microbes or other secondary constructors. In our example, it is clear that the high proportion of Amsassia colonies represented in the boundstone, the coalescent arrangements, and the stack- ing patterns they acquired, enabled them to be the primary scaffolding organism of the reef framestone. The new paleo- geographic scenario shows that Amsassia acquired a worldwide distribution with a significant participation in reef construction and represents a successful attempt of skeletal organisms to construct reefs during the Early to Middle Ordovician interval.
Acknowledgments
The authors acknowledge support from the National Research Council of Argentina. MGC acknowledges support from CONICET (Grant PIP 2009- 00861). RAA acknowledges sup- port from SECYT-UNC 213/14. Appreciation is extended to the reviewers, G.A. Young and D.-J. Lee, for their helpful com- ments and suggestions that significantly improved the final version of the manuscript. We would also like to thank the editorial staff of the Journal for their assistance. This is a con- tribution to the IGCP 653 project, “The onset of the Great Ordovician Biodiversity Event”.
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