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Modes of evolution and their chronostratigraphic significance: evidence from Devonian invertebrates in the Michigan Basin
Published online by Cambridge University Press: 08 April 2016
Abstract
The origin and evolutionary histories of the two most abundant and diverse genera of invertebrates (Prosserella: Brachiopoda and Syringostroma: Stromatoporoidea) in the Detroit River Group and associated rocks in the vicinity of the Michigan Basin appear to support the models of either allopatric speciation and punctuated equilibria (Eldredge and Gould, 1972) or quantum evolution (Simpson, 1944; 1953). Four morphotypes of Prosserella arose almost simultaneously (a “burst”) just below the base of the Detroit River in sandstone deposited near the axis of the Findlay Arch and persisted without evidence of significant progressive evolutionary change (a “trend”) until their almost simultaneous extinction by lineage termination near the top of the Detroit River. Neither ancestors nor descendants of Prosserella have been recognized and even the familial placement of the genus is uncertain. The genus, its species and morphotypes probably arose by means of very profound genetic or chromosomal “revolutions” that probably took place in small allopatric populations; each population quickly increased in abundance and geographic range and persisted without further significant morphologic modification until its extinction.
S. ristigouchense from the Lower Devonian of New Brunswick is the probable ancestor to six (or seven) species of Syringostroma that appear almost simultaneously (another “burst”) in lower Detroit River carbonate rocks deposited near the eastern margin of the Michigan Basin. Two of these new species are known only from reefs of early Detroit River age (a “crash”), four species persist without significant morphological change to at least the end of Detroit River deposition, and one of these was the probable ancestor to yet another newly evolved species that is abundantly represented in the conformably overlying Columbus Limestone.
The “founding fathers” of chronostratigraphy were pre-Darwinian and based their concepts and methods on assemblages of co-occurring taxa of unknown phylogenetic relations (assemblage-zones, concurrent range-zones and Oppel-zones) rather than on the range-zones of successional species in the same phylogenetic lineage (lineage-zones). Assumptions of these early biostratigraphers concerning the temporal relations of taxa are in close accord with the premises of punctuated equilibria and quantum evolution. The development of the chronostratigraphic system during the 19th century attests to the success of these early methods which depend for their precision on the number of taxa used and determination of the significance of morphologic differences among these taxa and their geographic and stratigraphic distributions.
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