Kb. Springer et Ma. Murphy, PUNCTUATED STASIS AND COLLATERAL EVOLUTION IN THE DEVONIAN LINEAGE OFMONOGRAPTUS-HERCYNICUS, Lethaia, 27(2), 1994, pp. 119-128
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.