Patterns and implications of stasis in trilobites

McCormick, Timothy (1995) Patterns and implications of stasis in trilobites. PhD thesis, University of Glasgow.

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Stasis may be operationally defined as the occurrence of little or no evolutionary change
during an interval of geological time, and is an important consequence of punctuated
equilibria. Studies of stasis in the fossil record of necessity address only morphological
stasis, and that only in the subset of phenotypic characters preservable in the fossil
record. Stasis in single characters may be recognised in fossil taxa by lack of significant
change in mean value through an interval of geological time; stasis in multiple characters
may be recognised by overlap in morpho space occupation by taxa where morpho space
occupation is calculated by multivariate techniques. No quantitative definition is placed
on such stasis because of the lack of comparable data on non-static (i.e. rapidly evolving)
taxa to provide the alternative. Proposed explanations for stasis include: developmental
and genetic constraints; environment fidelity; selection of generalist phenotypes in
fluctuating environments; stabilising selection (including stabilising species selection);
developmental canalisation; effects due to population size and distribution.
Mean generic and specific durations (in myr.) of trilobites originating In the
stratigraphical systems Cambrian-Carboniferous of England, Scotland and Wales are,
respectively: Cambrian (4.42, 2.13); Ordovician (10.89, 2.06); Silurian (10.34, 3.54),
Devonian (4.19, 1.12), Carboniferous, (14.82,5.74). Distributions of both generic and
specific durations are highly positively skewed. Study of the species composition of the
longest duration genera (those whose durations exceed the 90% quantile value for the
system in which they originated) suggests that species stasis played an important role in
the Cambrian and Carboniferous; no clear pattern is revealed for the interval SilurianDevonian
inclusive. Chronostratigraphical range charts are presented for species and
genera from England, Scotland and Wales. Study of the durations of Ordovician
Laurentian genera in relation to their position on the palaeoslope shows that longest
duration genera are eurytopic; their wide geographical and environmental dispersal
enabled them to avoid localised factors which caused extinction in more endemic genera.
Taxonomy-independent phylogenetic and morphometric analysis of selected long
duration shape conservative genera from the middle to upper Ordovician and Silurian
shows that disassociated mosaic evolution in some characters is abundant in all three,
superimposed on an almost invariant body plan. Achatella Delo, 1935 had a duration of
about 22 myr. (upper Llanvirn - Hirnantian, time scale of Tucker et al. 1990). Nine
species (three new) and one form under open nomenclature have been diagnosed.
Calyptaulax Cooper, 1930 had a duration of about 25 myr. (lower Llanvirn - upper
Rawtheyan, time scale of Tucker et al. 1990). Two subgenera are diagnosed, each of
duration about 20 myr. (time scale of Tucker et al. 1990). The nominate subgenus is
well resolved on the cladogram, and five species have been diagnosed. Calyptaulax
Abstract. Page ii
(Calliops) is unresolved on the cladogram because of a disassociated mosaic pattern of
"peripheral" character evolution; ten species have been diagnosed. A sixteenth species
could not be assigned to a subgenus. Acernaspis Campbell, 1967 had a duration of
about 11 myr. (lower Llandovery - Wenlock, time scale of Harland et al. 1989).
Eighteen species have been diagnosed, three of them new. Several stratigraphical
samples of Ananaspis Campbell, 1967 have been studied and an hypothesis that this
genus arose through neoteny from Acernaspis has been confirmed, although not a
further hypothesis that progressive neoteny continued throughout the existence of
Ananaspis. Four Ananaspis species have been diagnosed, one of which is new. This
does not constitute a complete survey of Ananaspis.
The disassociated mosaic pattern of peripheral character states probably reflects differing
degrees of developmental canalisation at different levels of phenotypic organisation. The
basic body plan is strongly canalised, whereas at "peripheral" levels, less strong
canalisation allows emergence of superficial characteristics. This, combined with
eurytopic distribution, may keep the taxa adapted to their (various) environments without
need for more major evolutionary change.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: Q Science > QE Geology
Colleges/Schools: College of Science and Engineering > School of Geographical and Earth Sciences > Earth Sciences
Supervisor's Name: Owen, Dr. Alan
Date of Award: 1995
Depositing User: Ms Anikó Szilágyi
Unique ID: glathesis:1995-6336
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 11 May 2015 09:21
Last Modified: 11 May 2015 09:22

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