Abstract
The Late Cretaceous echinoid genus Micraster (irregular echinoids, Spatangoida) is one of the most famous examples of a continuous evolutionary lineage in invertebrate palaeontology. The influence of the environment on the phenotype, however, was not tested so far. This study analyses differences in phenotypical variations within three populations of Micraster (Gibbaster) brevis from the early Coniacian, two from the Münsterland Cretaceous Basin (Germany) and one from the North Cantabrian Basin (Spain). The environments of the Spanish and the German sites differed by their sedimentary characteristics, which are generally a crucial factor for morphological adaptations in echinoids. Most of the major phenotypical variations (position of the ambitus, periproct and development of the subanal fasciole) among the populations can be linked to differences in their host sediments. These phenotypic variations are presumed to be an expression of phenotpic plasticiy, which has not been considered in Micraster in previous studies. Two populations (Erwitte area, Germany; Liencres area, Spain) were tested for stochastic variation (fluctuating asymmetry) due to developmental instability, which was present in all studied traits. However, differences in the amount of fluctuating asymmetry between both populations were recognised only in one trait (amount of pore pairs in the anterior paired petals). The results strengthen previous assumptions on ecophenotypic variations in Micraster.
Highlights
The environment plays a principal role in adaptive evolution by shaping through natural selection the means of population phenotypes and by evoking phenotypic variation without altering the genetic background, e.g. through uncovering cryptic genetic variation, or phenotypic plasticity [1,2,3,4]
PC 1 reveals the best separation between the German populations and the Spanish population (Figs 7 and 8); they reveal, some overlap, but a larger dispersion of the Spanish population is found along the positive direction of PC 1—both German populations disperse rather in a negative direction
It responded to an increase in the abundance of clasts in the sediment by an inflation of the test and by a higher positioned periproct. This pattern was observed in E. cordatum from northern France [34]. These observations correspond to the results found in M. brevis
Summary
The environment plays a principal role in adaptive evolution by shaping through natural selection the means of population phenotypes and by evoking phenotypic variation without altering the genetic background, e.g. through uncovering cryptic genetic variation (decanalization), or phenotypic plasticity [1,2,3,4]. Phenotypic plasticity, is a property of a genotype and, its norm of reaction is influenced by genetic variation as well [5]. The developmental origins of observed phenotypic variation are often open to debate in palaeontological studies, reasoned in the complex interplay of genes and environment. The fossil record provides, to some extent, only the factor environment in this equation. To decipher patterns in PLOS ONE | DOI:10.1371/journal.pone.0148341. To decipher patterns in PLOS ONE | DOI:10.1371/journal.pone.0148341 February 5, 2016
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