Late Ordovician brachiopod endemism and faunal gradient along palaeotropical latitudes in Laurentia during a major sea level rise

Abstract

The palaeocontinent Laurentia experienced pulses of major marine transgression during the Late Ordovician (Katian) and was covered by extensive equatorial to subtropical epicontinental seas. Brachiopods that evolved in the newly created habitats showed increasingly strong endemism from the early to late Katian. At the faunal level, the Laurentian epicontinental brachiopod (LEB) fauna became ubiquitous in the epicontinental seas, with a high level of generic homogeneity but little in common with the coeval faunas in other palaeotropically located tectonic plates. At the level of species distribution, there was limited mixing between palaeoequatorial intracratonic basins and higher tropical to subtropical pericratonic shelves within Laurentia by late Katian. This was demonstrated by the palaeogeographical distribution of several species lineages and morphological clines along latitudinal and other ecological gradients. Such a high level of endemism suggests an extreme specialization of the LEB fauna in the tropical epicontinental seas, which made it most vulnerable to the drastic climate change and mass extinction during the Hirnantian glaciation.

Keywords::

• Brachiopoda
• faunal endemism
• latitudinal gradient
• Late Ordovician
• Laurentia

Acknowledgements

Research funding was provided through a Discovery Grant from the Natural Science and Engineering Research Council of Canada (JJ). The critical comments of Christian Rasmussen and an anonymous reviewer are greatly appreciated. This paper is a contribution to IGCP Project 591: The Early to Middle Palaeozoic Revolution.