Contrasting Late Miocene bryozoan faunas from the eastern Atlantic and western Mediterranean
Contrasting Late Miocene bryozoan faunas from the eastern Atlantic and western Mediterranean : implications for palaeoenvironmental reconstruction and the species concept in Bryozoa.
A comparative morphological study on bryozoan faunas from the Late Miocene of the eastern Atlantic and western Mediterranean has shown that there exist considerable differences in morphology within species. The presence of a great number of Mediterranean species in the eastern Atlantic Guadalquivir Basin (SW Spain) rendered it possible to compare directly morphological variation between these two regions. Measurements of zooid size yielded a general trend towards the existence of smaller zooids in specimens from the eastern Atlantic. Although it is well known that environmental factors can generate within-colony variations in morphology (e.g. there is an inverse relationship between zooid size and temperature), genotypic differences have as yet been found to be predominant in producing between-colony variations. However, the present study indicates that the environment might play a greater role in producing intraspecific morphological variation than previously recognised. This is exemplified by the extant, erect, cheilostome Myriapora truncata (Pallas) which shows intracolonial, within-site and between-site variability in zooid size and branch diameter in both fossil and Recent assemblages. These findings suggest yet again that species discrimination purely based upon zooidal or colonial dimensions should be avoided in bryozoans. Furthermore, if the extrinsic controls on bryozoan morphology can be qualitatively and quantitatively ascertained, (palaeo)environmental information may be gained at various spatial and temporal scales.