Growth variability in a tertiary brachiopod from antarctica; The significance for palaeoenvironmental reconstruction

Abstract

The growth histories of two populations of Bouchardia antarctica Buckman from the La Meseta Formation, Seymour Island, Antarctica, have been determined by means of growth-line analysis. Adult specimens in the first population were consistently smaller than those in a second population which were collected 120 metres higher up in the succession. The two populations had previously been subdivided into two species, but are now recognised as being conspecific. Adults of both populations consistently display up to 10 prominent, evenly-spaced growth-lines. On specimens from the lower population the mean growth-line spacing on the dorsal valve is 1.83 mm; the comparable figure for the larger population further up the sequence is 2.85 mm. This indicates that the life-span of the two populations was identical (on average 7–8 years with a maximum of 10 years), and that the difference in absolute size was due to a consistently greater growth-rate in the larger specimens. Therefore growth-line studies can be used as palaeoenvironmental indicators, as the growth-rate of cold-blooded organisms is directly related to prevailing environmental conditions. For brachiopods, an increased growth-rate can realistically be attributed to increases in prevailing temperature, food supply, dissolved oxygen concentration, a decrease in turbulence, or a combination of these and other factors.