Gradational benthic marine communities in the Early Carboniferous Rhipidomella fortimuscula Zone (late Visean), New South Wales

Four benthic marine fossil commumtles are recognised in faunas of the Rhipidomella fortimuscula brachiopod Zone of late Visean (Early Carboniferous) age using multivariate (cluster) analysis of bulk samples extracted from available fossiliferous horizons. As a consequence of their occurrence over a wide geographic area, the communities are considered to be representative collections of in situ invertebrates (largely brachiopods).  The number of species and genera in each community varies according to the favourability of water column and substrate conditions for habitation by such invertebrate filter-feeders.  Elements of the Balanoconcha elliptica community were present in marine shelf waters with high suspended-sediment concentrations in the water column, such conditions excluded all but a few species well adapted to them. The Rhipidomella fortimuscula community was present in turbulent nearshore conditions, but could also tolerate calmer conditions below wave-base. The Tylothyris planimedia and Marginicintus reticulatus communities inhabited quiet-water conditions, one closer to shore than the other, but differ in the number and variety of species present.  The Marginicintus reticulatus community is the most widespread and has the highest species diversity.  Sampling by bulk collection allows the communities to be identified by the distribution and abundance of all species. Clusters formed by multivariate analysis identify the recurrent species associations or communities. Examination of the communities show that several species are numerically important in more than one community. It is suggested from this evidence that the communities do not contain mutually exclusive species associations, but are abstractions from a continuum . As such the communities intergrade and are distinguishable on the basis of their total faunal content. Several species comprise the most abundant forms in more than one community: Balanoconcha elliptica, Rhipidomella fortimuscula, Marginicintus reticulatus, and Tylothyris planimedia. The community assemblages are gradational (Whittaker community concept) rather than forming fixed associations inhabiting specific depth zones (Petersen community concept).  Compared to the  time averaged nature of fossil communities, surveys of modern benthic communities are an  instantaneous view of biota and less likely to identify the long-term impact of periodic perturbations. Major periodic fluctuations in environmental parameters are more likely to be reflected over time in the fossil record where the populations of more than one generation are preserved. The inherently patchy nature of both fossil and modern benthic species populations is also a feature readily evident in th e fossil record, but more difficult to detect in modern surveys. Marine benthic communities of the Rhipidomella fortimuscula Zone provide an illustration of the cumulative effects of gradational faunal boundaries and the inherent patchiness of species populations.

Tags: Earth Sciences, GA Publication, Journal, NSW, Published_External, marine
Organization: Geoscience Australia Data