Patterns and implications of stasis in trilobites

• Main Author: McCormick, Timothy
• Published: University of Glasgow 1995
• Subjects: 565; QE Geology
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.637480

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.