Strength in numbers: High phenotypic variance in early Cambrian trilobites and its evolutionary implications.

Summary Analysis of the degree of intraspecific morphological polymorphism during the evolutionary history of trilo- bites using an informatic approach (1) provides striking evidence of a long-suspected but previously unsubstan- tiated pattern: degrees of polymorphism are markedly higher in phylogenetically basal, stratigraphically early species. This unequivocal pattern prompts further ex- ploration of the relationship between microevolutionary variance and macroevolutionary history. It demonstrates that the 'traditional' fossil record of skeletonized orga- nisms can provide unique insight into questions of major evolutionary interest. BioEssays 29:1081-1084, 2007. 2007 Wiley Periodicals, Inc. The delightful ''exceptionally preserved'' fossils that fre- quently grace the pages of the science weeklies are often spectacularbut itisunclearwhether such unusual-by-definition specimens are representative of the normal populations that they belong to. And in all the understandable excitement about ing species-level systematics for Cambrian trilobites. This is because, as Darwin recognized, species boundaries based on morphology are commonly indistinct. Indeed, it's always a challenge to distinguish intraspecific from interspecific dif- ferences, and especially so in fossils. Terms such as ''lineage crossing'' ''developmental flexibility'' and ''plasticity'' have been bandied about in studies of Cambrian trilobites to donate the observed large morphological variances, while those working with geologically younger, more evolutionarilyderived, trilobites have found species definition easier. This frustration in classi- fication of the early trilobites hints that there was something differentaboutthewayinwhichthesespeciesvariedphenotypi- callywhencomparedtolaterones,andpromptsthequestionof why this might be so. Trilobites appeared in the fossil record some 20 million years after the base of the Cambrian Period. While it's clear that the major differentiation both within and among animal phyla was already established by that time, it is stillpossiblethatpatternsofvariationinearlyCambrianspecies might reflect a ''warm afterglow'' of the Cambrian radiation. If such a pattern could be demonstrated convincingly, we can ask whether early variability differs from that in later species because of ''internal'' reasons related to the way in which development wascontrolled in these species, or whether it was due to ''external'' influences associated with the selective environment in which these species lived. (2) Furthermore, trilobites provide a striking example of the ''bottom heavy'' pattern of clade diversification, in which maximum morpho- logical and taxonomic diversity was generated early in clade history. This pattern is commonly documented in the fossil record and not only in association with the Cambrian radiation, hinting at a pattern of even broader significance. The first problem, and the one that Webster's study addresses, is to assess whether the impression of early species variability and later reduced species variability is truly valid.AlthoughdetailedstudiesofindividualCambrianspecies have claimed to support the trend, (3) such studies are subject to the general criticism of exceptionally preserved fossils: do such individual cases accurately reflect a general pattern? A similar problem applies to the classic trend from high to low variance in trunk segmentation specifically seen between early Cambrian and later trilobites: (4) it is only one character