Thrombolitic and stromatolitic mounds (pinch and swell structures).

Abstract ref. 2040:
Stromatolites have been recognized as opportunistic disaster forms following the end-Permian mass extinction. This occurrence was attributed to the partial relaxation of ecological constraints in the aftermath of the mass extinction. A similar pattern occurs in the Great Basin following the Late Ordovician mass extinction in association with a shift to low-diversity, generalized faunas.

Most Late Ordovician-Silurian faunal communities in the Great Basin are diverse and limited to relatively narrow environmental zones. Thin (10-20 cm thick) stromatolites occur throughout the section within inner-shelf tidal-flat facies. After the mass extinction, earliest Silurian (Rhuddanian) communities are broadly distributed with low diversity. Associated stromatolitic and thrombolitic mounds increase in abundance and reach a thickness of 2.5 meters. Bootstrapping analysis suggests the restriction of thick mounds to the post-extinction interval is non-random (probability <0.5%).

The widespread shelf stromatolites and thrombolites of the early Paleozoic declined, probably because their habitat was disrupted by the evolving Paleozoic Evolutionary Fauna. By mid-Ordovician, stromatolites and thrombolites were small and restricted to marginal, inner-shelf environments. They temporarily regained prominence in the aftermath of the Late Ordovician and end-Permian mass extinctions. In the earliest Silurian, shelf communities were undifferentiated in the long (3-4 MY) aftermath of the mass extinction. Thick stromatolites and thrombolites disappeared when diversity increased and differentiated communities were re-established. This patterns suggests animals that were suppressing stromatolites and thrombolites were reduced or absent after mass extinctions.