PALAEONTOLOGICAL EVIDENCE BEARING ON GLOBAL ORDOVICIAN-SILURIAN CONTINENTAL RECONSTRUCTIONS

Abstract

The discreteness or otherwise of major Ordovician and Silurian terranes can be recognised by the shallow-water benthic faunas which lived upon them. Their borders are often indicated by the disposition of progressively shallow- to deep-water assemblages at the terrane edge as well as by structural features. Their positions relative to each other in the Early Palaeozoic can be best indicated by a combination of palaeomagnetic and faunal evaluation: the latter is the topic of this paper. Faunal evaluation is now possible quantitatively as well as quantitatively. Global palaeobiogeography is reviewed for the period as deduced from faunal evidence. There was one supercontinent, Gondwana, which stretched from West Gondwana (today's southern Europe and North Africa) at high latitudes to tropical East Gondwana (Australasia and adjacent areas), with intermediate palaeolatitudes in the Middle East and South America. Around Gondwana, especially to its north, were a large number of peri-Gondwanan terranes, particularly Avalonia, Perunica, parts of Turkey and Arabia and Sibumasu. In addition, there were the substantial independent continents of Laurentia, Baltica, Siberia, Annamia, North China and South China. Analysis of the shallow-water benthos, particularly trilobites and brachiopods, provides distinctive signatures for palaeo-position in most cases. Despite a large faunal turnover particularly corresponding with the latest Ordovician glacial event, the progressive evolution of the ecologies of benthic shelly faunas were also much influenced by changing geographies during the 80-Ma period. In the early Ordovician, oceans were at their widest, enabling Baltica and Laurentia to have different signatures from either East or West Gondwana. Siberia in early Ordovician times had faunal contact with Laurentia and East Gondwana, but in the mid-Ordovician, there were more endemics, and by the late Silurian, it was the only continent of substance in the northern hemisphere (hosting the Tuvaella Fauna). South China has varied faunal links but seems best treated as at the edge of the peri-Gondwanan collage for most of the period. We show how faunas document the early Ordovician rift of Avalonia from West Gondwana and its movement and subsequent collisions, first with Baltica in the end Ordovician and then with Laurentia in the early Silurian. Faunas also support the postulated movement of the Precordillera of South America from Laurentia in the early Ordovician to intermediate- to high-latitude Gondwana in the Silurian. We examine peripheral terranes bordering Iapetus to demonstrate their pre-collision positions. Analysis of some of the many terranes now forming Kazakhstan and adjacent areas in central Asia today reveals that the benthic faunas there have more affinity with Gondwanan and peri-Gondwanan faunas than with Baltica or Siberia, and thereby challenge structural models postulating an Early Palaeozoic Kipchak arc.