The functional performance of productidine brachiopods in relation to environmental variables

Abstract

Fossils are useful as palaeoenvironmental indicators when evaluated using large occurrence and lithology association data sets and/or functional morphology. However, when functional morphology is invoked on an ad hoc basis there exists a risk of circular reasoning. Performance spaces/landscapes constructed using biomechanical experiments can be used to tie performance to morphology quantitatively. They can be used to estimate fitness and function in specific environments by allowing multiple specific measures of function to be evaluated simultaneously. Absolute performance can be used to constrain palaeoenvironmental interpretations by detecting functional limits. We constructed a performance space with four variables (transport resistance, settling time, settling orientation and respiration potential) relating to hydrodynamic and metabolic variables. We combine new performance data with existing palaeoenvironmental interpretations and associated distributions of brachiopods of the suborder Productidina during the late Carboniferous and early Permian in the North American Midcontinent. Productidines were chosen because they lacked pedicles and relied on body and spine morphology to interact with their environment. Offshore dysoxia-associated taxa had increased resistance to sinking and proportionally larger lophophores, potentially improving survivorship in low-oxygen, soft substrate environments. Taxa associated with nearshore and oxygenated offshore environments display a range in performance consistent with a range of conditions, but all had proportionally smaller lophophores. Models that are less stable in high-velocity flows consistently settle in the correct convex down orientation if entrained, while more stable models did not. Our study suggests that as productidines radiated during the Carboniferous, they also differentiated within the performance space.