Heldur Nestor

Late Ordovician and Early Silurian stromatoporoid sponges from Anticosti Island, eastern Canada: Crossing the O/S mass extinction

During Late Ordovician and Early Silurian time, from 450 to 428 million years ago, stromatoporoid sponges were some of the most common and abundant fossils in shallow water tropical settings of the Anticosti Basin (Gulf of St Lawrence). They formed dense, massive coralline skeletons of calcium carbonate, some up to a meter or more across, especially in reef environments, but also in deeper waters of the Anticosti shelf, down to the margins of the photic zone, where light faded. The Anticosti Basin reveals one of the most fossiliferous carbonate sequences worldwide for rocks of this age, straddling a global mass extinction boundary, and thus revealing not only those taxa that became extinct, but also how the seas were repopulated in an equatorial setting after the mass extinction.

Chapter 7 Biogeography of the Ordovician and Silurian Stromatoporoidea

Abstract Stromatoporoid sponges first appeared during the late Mid-Ordovician (mid–late Darriwilian) accompanying an important ‘Chazy’ reef-building episode. Representatives of the order Labechiida appeared first, initially splitting into two sister groups: those from North China (nine genera) and those from Laurentia (four genera). Two genera were common to the two regions, but others in North China and Siberia were endemic. This initial provincialism was not maintained into the Late Ordovician (Sandbian) as labechiids attained a wider dispersal, covering Laurentia with peripheral terranes in Northwest Scotland and Chukchi Peninsula, cratonic Siberia, the Urals (eastern margins of Baltica), and East Gondwanan blocks of Tarim North China, marginal Tasmania and the peri-Gondwanan New South Wales island-arc terrane. Only a few endemics remained present in the Sandbian – three in Laurentia, and one in Tarim. In the Katian, maximum diversification of labechiids occurred (19 genera, including a genus possibly transitional to first actinostromatids). Also four genera of the Clathrodictyida first appeared, and overall distribution continued to increase. The Hirnantian marked a dramatic decline in the global distribution (only Anticosti Island, Manitoba, Norway and Estonia) and marked diversity loss of labechiids. Altogether c. 70% of stromatoporoid species disappeared in response to end-Ordovician global cooling events. Llandovery stromatoporoids were widespread in Laurentia, Baltica and Siberia, with clathrodictyids as dominant and labechiids accessory (the latter group more common in Siberia and China). Gradual diversification and expansion of stromatoporoids followed in the late Llandovery, with appearances of the orders Actinostromatida and Stromatoporida. The stromatoporoids became most widespread and most diversified during the Wenlock, with clathrodictyids maintaining their leading position, and the appearances of the earliest Stromatoporellida and Syringostromatida. Rapid spread of new phylogenetic stocks indicates that widespread pandemism prevailed among Wenlock stromatoporoids. The Ludlow was characterized by final closure of the Iapetus Ocean, accompanying uplift of palaeocontinental regions, and declining prominence of stromatoporoid-bearing shallow carbonate shelves. However, clathrodictyids and actinostromatids remained dominant, whereas the incoming of Amphiporida in several regions (Somerset Island, Baltic area, West Ukraine, West and East Urals, Novaya Zemlya, Tien Shan, Japan and New South Wales) and restricted distribution of some other taxa suggest a certain provincialism developed. With regional regression and stratigraphic hiatuses, the stromatoporoids (mostly hangovers from the Ludlow) became less common in the Pridoli, and apparently even entirely absent from Gondwana and Siberia.

Biogeography of Silurian stromatoporoids

Abstract Stromatoporoids were widespread in the Ordovician, Silurian and Devonian shallow seas with carbonate types of sedimentation. Their occurrences in reefs and association with bahamitic type carbonate sediments, sometimes replaced by evaporites, allow them to be considered as stenothermal warm-water organisms which lived in the tropical to subtropical climatic belt between 40° N and S. The rapid eustatic deepening after the latest Ordovician glaciation gave rise to ecological stress in the shallow-water stromatoporoids and suppressed their adaptive radiation. Therefore a taxonomically monotonous cosmopolitan fauna of stromatoporoids formed at the beginning of the Llandovery. Few endemic genera existed during the late Llandovery and Wenlock. In Ludlow time a low degree of provincialism appeared when differences developed between the European and Asiatic faunas; these vanished again in the Pridoli. The data on stromatoporoid distribution are in good agreement with the current palaeogeographical reconstructions.