Morphological plasticity of the outline and the internal structures of the shell of the Recent Terebratella tenuis sp. nov. (Brachiopoda, Terebratulida)

Abstract

The practice of classifying articulate brachiopods, including Terebratulida, from minor morphological features and internal structures has led to a proliferation of genera and species, especially when external morphology is quantified by conventional measurements. Morphological plasticity and variability of the internal structures have been studied in a large sample from a population of a species (described in this paper), Terebratella tenuis sp. nov. The shell outline is evaluated by using Fourier series while internal structures are measured by conventional methods. Both the external morphology and the internal structures of T. tenuis sp. nov. are highly variable and this study shows that allowance must be made for such variability when describing species. A number of internal structure indices commonly used in brachiopod species description are found to be subject to intense variation or even growth allometries. There are also asymmetric resorptions of the internal structures. These results demonstrate that morphological variability is not taken into account as much as it should be in the definitions of Recent or fossil species.