PUNCTUATED STASIS AND COLLATERAL EVOLUTION IN THE DEVONIAN LINEAGE OFMONOGRAPTUS-HERCYNICUS

Citation
Kb. Springer et Ma. Murphy, PUNCTUATED STASIS AND COLLATERAL EVOLUTION IN THE DEVONIAN LINEAGE OFMONOGRAPTUS-HERCYNICUS, Lethaia, 27(2), 1994, pp. 119-128
Citations number
55
Categorie Soggetti
Paleontology
Journal title
ISSN journal
00241164
Volume
27
Issue
2
Year of publication
1994
Pages
119 - 128
Database
ISI
SICI code
0024-1164(1994)27:2<119:PSACEI>2.0.ZU;2-L
Abstract
The changes in the sicula of Early Devonian monograptid graptolite pop ulations show a pattern that is described as punctuated stasis: two lo nger periods of stasis separated by a transitional population of short er duration between them. The pattern differs from punctuated equilibr ium in that anagenesis is invoked rather than cladogenesis. The patter n has been observed in several widely separated areas (Nevada, Thuring ia, Poland, Ural Mountains) and is an example of collateral evolution in graptolites. In this case, the collateral morphologic change occurs relatively rapidly in a large, globally distributed taxon. The study permits accurate correlation of the horizon of appearance of Monograpt us hercynicus Perner in the upper delta Zone (conodont zonation) of la te Lochkovian age (Devonian) on a global scale. The partial integratio n of these two biostratigra phies constrains correlations between the shelly, shallow-water biofacies and the deeper graptolitic biofacies. The pattern of punctuated stasis has been discovered in Lower Devonian strata of central Nevada, where 22 horizons in sequence were sampled. Statistical analyses were performed on 13 characters encompassing mea surements of the sicula, thecae, and rhabdosome. Most of these charact ers do not exhibit sustained trends or punctuated changes. However, si cular width exhibits an abrupt increase over a short stratigraphic int erval and is used to classify members of the group into four quantitat ively defined morphotypes. This natural perturbation in the pattern of change is used to divide the lineage into paleospecies. This makes th e makes the segments of the lineage easy to identify and imparts stabi lity to the classification and precision in biostratigraphy. The varia tion studies enable the ranges of the morphotypes to be used to furthe r subdivide the stratigraphic record of the late Lochkovian.