Biogeography of the Ordovician and Silurian Stromatoporoidea
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.
Figures & Tables
The Early Palaeozoic was a critical interval in the evolution of marine life on our planet. Through a window of some 120 million years, the Cambrian Explosion, Great Ordovician Biodiversification Event, End Ordovician Extinction and the subsequent Silurian Recovery established a steep trajectory of increasing marine biodiversity that started in the Late Proterozoic and continued into the Devonian. Biogeography is a key property of virtually all organisms; their distributional ranges, mapped out on a mosaic of changing palaeogeography, have played important roles in modulating the diversity and evolution of marine life. This Memoir first introduces the content, some of the concepts involved in describing and interpreting palaeobiogeography, and the changing Early Palaeozoic geography is illustrated through a series of time slices. The subsequent 26 chapters, compiled by some 130 authors from over 20 countries, describe and analyse distributional and in many cases diversity data for all the major biotic groups plotted on current palaeogeographic maps. Nearly a quarter of a century after the publication of the ‘Green Book’ (Geological Society, London, Memoir 12, edited by McKerrow and Scotese), improved stratigraphic and taxonomic data together with more accurate, digitized palaeogeographic maps, have confirmed the central role of palaeobiogeography in understanding the evolution of Early Palaeozoic ecosystems and their biotas.