Biogeography of Ordovician stromatoporoids

This paper reviews the distribution of stromatoporoids in relation to Ordovician geography. The labechiids were the first of the three Ordovician families to make their appearance. They became established in well-circulated, shallow, subtidal, equatorial waters of the open shelf during the Late Llanvirn-Llandeilo (North American Chazyan). Rapid generic diversification, and colonization of other shallow, near-equatorial habitats, such as the fringes of island arcs and continental interiors, followed in Early-Middle Caradoc (North American Blackriveran-Trentonian) times. The clathrodictyids and the cliefdenellids, in contrast, appeared first in an island-arc setting during the Middle Caradoc. The clathrodictyids spread later to shelf and platform habitats, taking until the end of the Ordovician to achieve an essentially circum-equatorial distribution. Cliefdenellids have a more limited distribution, being recorded only in New South Wales, southwest Siberia and Alaska. They became extinct in the Ashgill.

A limited amount of provinciality is discernible in the Caradoc and Early Ashgill. Two provinces can be recognized in the labechiid faunas of the Early Caradoc — a transitory North China Province (with 57% generic-level endemism) and the more persistent and widespread North American Province. A third, Australian-Eurasian Province (with its characteristic clathrodictyids and cliefdenellids) emerged during the Middle Caradoc. The maximum geographical spread of Ordovician stromatoporoids was attained during Middle Caradoc—Early Ashgill time. The Middle—Late Ashgill (North American Richmondian-Gamachian) fauna is an admixture of both North American and Australian-Eurasian elements. It has a restricted distribution. Of the faunal elements, the labechiids were in decline, with several genera, including Aulacera, becoming extinct. Clathrodictyids, in contrast, were emerging to become the dominant stromatoporoid group. Where dispersal patterns of Ordovician stromatoporoids can be determined, they appear to be mainly in the direction of west-flowing equatorial currents.

Ordovician stromatoporoids are inferred to be warm stenothermal, low-latitude-dependent organisms. Consequently, the lithospheric plates or the parts of them exhibiting occurrences of stromatoporoids are also constrained latitudinally. Recent palaeomagnetic-based Ordovician world-map reconstructions include some which show the North European plate in high—middle latitudes, and others, low latitudes. Occurrences of stromatoporoids provide support for inferring that this plate moved from middle to low latitudes between Middle Caradoc and Late Ashgill time. The northward move coincided with the narrowing of the Iapetus Ocean and the Middle—Late Ashgill mixing of North American and Australian-Eurosian faunal elements