Patterns and implications of stasis in trilobites

[摘要] Stasis may be operationally defined as the occurrence of little or no evolutionary changeduring an interval of geological time, and is an important consequence of punctuatedequilibria. Studies of stasis in the fossil record of necessity address only morphologicalstasis, and that only in the subset of phenotypic characters preservable in the fossilrecord. Stasis in single characters may be recognised in fossil taxa by lack of significantchange in mean value through an interval of geological time; stasis in multiple charactersmay be recognised by overlap in morpho space occupation by taxa where morpho spaceoccupation is calculated by multivariate techniques. No quantitative definition is placedon 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: developmentaland genetic constraints; environment fidelity; selection of generalist phenotypes influctuating 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 thestratigraphical 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 andspecific durations are highly positively skewed. Study of the species composition of thelongest duration genera (those whose durations exceed the 90% quantile value for thesystem in which they originated) suggests that species stasis played an important role inthe Cambrian and Carboniferous; no clear pattern is revealed for the interval SilurianDevonianinclusive. Chronostratigraphical range charts are presented for species andgenera from England, Scotland and Wales. Study of the durations of OrdovicianLaurentian genera in relation to their position on the palaeoslope shows that longestduration genera are eurytopic; their wide geographical and environmental dispersalenabled them to avoid localised factors which caused extinction in more endemic genera.Taxonomy-independent phylogenetic and morphometric analysis of selected longduration shape conservative genera from the middle to upper Ordovician and Silurianshows 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 ofabout 22 myr. (upper Llanvirn - Hirnantian, time scale of Tucker et al. 1990). Ninespecies (three new) and one form under open nomenclature have been diagnosed.Calyptaulax Cooper, 1930 had a duration of about 25 myr. (lower Llanvirn - upperRawtheyan, time scale of Tucker et al. 1990). Two subgenera are diagnosed, each ofduration about 20 myr. (time scale of Tucker et al. 1990). The nominate subgenus iswell resolved on the cladogram, and five species have been diagnosed. CalyptaulaxAbstract. 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 speciescould not be assigned to a subgenus. Acernaspis Campbell, 1967 had a duration ofabout 11 myr. (lower Llandovery - Wenlock, time scale of Harland et al. 1989).Eighteen species have been diagnosed, three of them new. Several stratigraphicalsamples of Ananaspis Campbell, 1967 have been studied and an hypothesis that thisgenus arose through neoteny from Acernaspis has been confirmed, although not afurther hypothesis that progressive neoteny continued throughout the existence ofAnanaspis. Four Ananaspis species have been diagnosed, one of which is new. Thisdoes not constitute a complete survey of Ananaspis.The disassociated mosaic pattern of peripheral character states probably reflects differingdegrees of developmental canalisation at different levels of phenotypic organisation. Thebasic body plan is strongly canalised, whereas at "peripheral" levels, less strongcanalisation allows emergence of superficial characteristics. This, combined witheurytopic distribution, may keep the taxa adapted to their (various) environments withoutneed for more major evolutionary change.

[发布机构] University:University of Glasgow;Department:School of Geographical and Earth Sciences

[关键词] QE Geology