Ordovician conodont biogeography – reconsidered

Abstract

Review of the traditional separation of global Ordovician conodont distribution into the North American Midcontinent Province (NAMP) and North Atlantic Province (NAP) reveals a confusing variety of concepts and definitions that hinder biogeographic analysis. Use of this twofold scheme and its subsequent variants should be­discontinued in favour of the more detailed divisions proposed here. Major biogeographical entities of the Shallow‐Sea and Open‐Sea Realms, separated by the shelf‐slope break, are both further subdivisible into Tropical, Temperate and Cold Domains. In the Cold domains, faunal differences between the two Realms and their subdivisions are not easily discernible, since biofacies zones and different habitats were highly condensed. Faunal differences are amplified in the tropical regions, where the North American Midcontinent Province and North Atlantic Province were originally defined. Recognition of endemic taxa is essential for finer classification within domains of the Shallow‐Sea Realm (SSR). Our preliminary analysis of Early Ordovician conodont distribution identifies the Laurentian Province (in the Tropical Domain), Australian­(Tropical Domain), North China (Tropical Domain), South China (Temperate Domain), Argentine Precordillera (Temperate Domain) and Balto‐Scandian Province (in the Cold Domain). The Open‐Sea Realm (OSR) is dominated by cosmopolitan and widespread taxa, and formal subdivision at provincial level is yet to be achieved. The North Atlantic Province encompasses both the Open‐Sea Realm and the Temperate and Cold Domains of the Shallow‐Sea Realm. The North American Midcontinent Province sensu stricto is more or less equivalent to the Laurentian Province, representing shallow‐water regions fringing Laurentia; in a broader sense the North American Midcontinent Province includes all provinces of the Tropical Domain within the Shallow‐Sea Realm.

The conodont fauna in the Middle Ordovician Eoplacognathus pseudoplanus Zone of Baltoscandia

“…Lindström (1984) first applied these models to the distribution of conodont taxa in the Ordovician epicontinental sea of Baltoscandia, and Pohler (1994) distinguished Ordovician biofacies patterns in deeper shelf and slope environments in western Newfoundland. Zhen & * E-mail: anita.lofgren@geol.lu.se Percival (2003) recently explained the virtual lack of endemic conodont taxa in slightly older Ordovician strata of Baltoscandia as due to lack of warm, shallow environments there. Difficulties in differentiating separate biofacies in younger Baltoscandian rocks may arise for the same reason.…”

Trilobites and biofacies in the Early–Middle Ordovician of Baltica and a brief comparison with the Yangtze Plate

“…Consequently, the Early Ordovician faunas of the Yangtze Plate were more influenced by the Baltic fauna than vice versa, suggesting that these plates were rather distant from each other and the immigration was directed towards China. Zhen & Percival (2003) refined the division of conodont biogeographic realms, suggesting that the old division into only the North American Midcontinent Province and the North Atlantic Province should be abandoned. They suggested the division into Shallow-Sea and Open-Sea Realms, corresponding to shelf and slope areas, each divisible into Tropical, Temperate and Cold Domains.…”

“…The rest of the Urals instead has the conodont genera Drepanodus, Oistodus, Periodon, Amorphognathus and Keislognathus, i.e., a European-type fauna, or North Atlantic Province. According to the new concept on Ordovician conodont biogeography eliminating these terms (Zhen & Percival 2003), described a number of (relatively) deep-water conodont biofacies of the biogeographic Open-Ocean Realm of the tropical Domain in the South Urals. Similar to the situation with the conodonts, the trilobite faunas differ in the Darriwilian, and the Laurentian taxa, such as the cheirurids Kawina, Heliomera, etc., known from the reefal associations in western Newfoundland (Whittington 1963), western Ireland (Lane 1971), Nevada (Ross 1972) and eastern Spitsbergen (Fortey 1980), appear in the South Urals.…”

Lower to Middle Ordovician trilobite faunas along the Ural border of Baltica

“…Nasedkina 1981, . The most probable interpretation is that we are dealing with deep-water conditions, known to conodont specialists as the Tropical Domain of the Open-Sea Realm (Zhen & Percival 2003. The South Urals is thus distinguished from the rest of Baltica by having a Laurentian fauna equivalent to that conodont fauna.…”