Brachiopods in the Early Triassic and Their Morphological Adaptations to Environmental Changes

The Early Triassic fossil record is notoriously poor, and mostly composed of a few ecological opportunists and disaster taxa that are small marine invertebrates. Amongst them are inarticulate lingulid brachiopods that commonly appear in Early Triassic shallow-water sections worldwide, with high abundance. Lingulidae is a classic ‘living fossil’, normally considered as having unchanged ‘linguliform’ shape (elongate oval outline with subparallel or slightly curved lateral margins) since it first appeared in the Late Devonian. However, our morphometric analyses reveal subtle, but important, morphological adaptations of these opportunistic taxa during the Early Triassic, based on thousands of lingulid specimens from two shallow-water sections in South China. From the early Griesbachian to the early Smithian, lingulid shape changed to become slimmer, and by having narrower front ends and relatively wider posterior, and might more space to accommodate muscles posteriorly, which together, would make it easier to withdraw into, and protrude out from, their substrate shelters. These morphological changes could be related to increased predation intensity and sediment grain size in the studied sections, which would have negative impacts on the burrowing capability of lingulids. In addition, in the Griesbachian ocean, under warming climate conditions, lingulids in deeper water-depth environments might have been confronted with more intense competition for food and oxygen resources than in the shallower environment. This is reflected by reduced morphological disparity and shell size of lingulids in the Bozhou section, compared to increased morphological disparity and shell size of lingulids in Liuzhi section. In the Dienerian to early Smithian, as environmental conditions improved, the morphological disparity and shell size of lingulids increased. The greatly increased shell size of lingulids from both sections in the Dienerian may have been a response to the rising predation intensity.