Matilde Beresi

Chapter 8 Cambrian, Ordovician and Silurian non-stromatoporoid Porifera

Abstract The Cambrian, Ordovician and Silurian distributions of non-stromatoporoid sponges are reviewed. The earliest Cambrian faunas contain mostly hexactinellids, with protomonaxonids dominating middle Cambrian assemblages. There are no obvious palaeobiogeographical patterns, with many genera being found widely. Vauxiids, lithistids and heteractinids are apparently confined to low latitudes, but this may be due to a poor fossil record. Most known Ordovician faunas are from low latitudes, although some high-latitude faunas are known, which contain reticulosan hexactinellids and protomonaxonids. There is some division of faunas within Laurentia, into eastern and western provinces, with the western assemblage extending across low northern latitudes during the Late Ordovician. During the Silurian Period, sponge diversity was very low during the Llandovery Epoch, probably partly owing to lack of habitat for taxa restricted to carbonate facies, and also because of sampling bias. There was a dramatic increase in diversity through the Silurian Period, mostly owing to an apparent diversification in the demosponges; however, there are many ghost lineages, indicating that their fossil record is poorly known. Non-lithistid sponges are very poorly known, with few recorded outside Euramerica. The currently available data for Early Palaeozoic sponges are too incomplete to allow any reliable palaeobiogeographical inferences. Supplementary material: the compilation of Silurian sponge occurrences is available at: http://www.geolsoc.org.uk/SUP18666.

A new demosponge, Choiaella scotica, from the Caradoc (Ordovician) of Wallace's Cast, Southern Uplands

Synopsis Two specimens of a new choiid demosponge, Choiaella scotica sp. nov., have been identified in the collections of the National Museums of Scotland. This is one of very few post-Cambrian records for the family, and only the second from Scotland. Both Scottish records occur in fine sandstones of shallow-water origin, but in probably different depositional environments, with the new specimens being transported into deeper water. It is possible that this group was abundant and widespread in shallow-water siliciclastic environments during the Early Palaeozoic, but its record has been limited by the low preservation potential of these habitats. The taxonomy of Choiidae is discussed, with the emphasis on establishing potentially useful diagnostic features for future discoveries.

Lessons from the Past: Sponges and the Geological Record

Sponges have been a major part of marine ecosystems, in both shallow and deep water, from the time of the earliest animal communities. The great shifts in climate that have occurred over the past 541 million years have affected all organisms, including sponges. Although patchy knowledge of the sponge fossil record hinders recognition of trends, some general patterns are apparent. Shallow-water siliceous sponges were severely affected by glacial intervals, whereas deeper-water siliceous sponges appear to have flourished during these times. Some groups of hypercalcified sponges (such as stromatoporoids) were abundant during times of global warming and high sea level, but other groups (archaeocyathans and sphinctozoans) had their acme during times of low sea level and relatively cool climate. Overall, sponge diversity appears to have been controlled more by sea level than by climate: large-scale sponge biotas occurred at times of high sea level, when there were large areas of shallow sea.

Coral patch reef system and associated facies from southwestern Gondwana: paleoenvironmental evolution of the Oxfordian shallow-marine carbonate platform at Portada Covunco, Neuquén Basin, Argentina

During the Middle Oxfordian, the epicontinental shelf of the Neuquén Basin was a site of major coralline evolution and reef building. This work expounds the studies performed on the La Manga patch reefs at Portada Covunco locality, near Zapala city, Neuquén province. Based on the results of 12 facies/microfacies types and the vertical succession of coral morphotypes a shallowing-upward trend ranging from a shallow subtidal-lagoon- to intertidal settings is inferred. The microfacies model suggests an ooidal shoal area in the highest energy zone and various patch reefs on the shallow carbonate platform. Autochthonous reefal facies comprise a low diversity of platy coral and mixed coral-siliceous sponge framestone, ramose coral bafflestone, and microbial bindstone. Non-reefal facies are composed of ooidal packstone, bedded bioclastic wackestone-packstone, and marl levels. Several shallowing-upward episodes are evidenced by local erosional surfaces (main exposure surface-MES). The succession of platy corals (exclusively Australoseris) followed by ramose corals (Etallonasteria, Stelidioseris, and Stephanastrea rollieri) probably reflects local environmental changes. The upward change in reefal composition is best interpreted in response to extrinsic physical parameters (local relatively minor sea-level fluctuations). Siliceous sponges occur in low percentages. The La Manga reefal succession could be correlated with the “global carbonate reef event”. This event occurred in most basins associated with the Tethyan oceanic belt and the North Atlantic Ocean, in low paleolatitude. The Portada Covunco reefs grew at higher paleolatitudes (nearly 39° south), within an embayment of the Neuquén Basin, with open circulation to the paleo-Pacific Ocean on the southwestern margin of the Gondwana realm.