How and why did the Lingulidae (Brachiopoda) not only survive the end-Permian mass extinction but also thrive in its aftermath?

The end-Permian mass extinction devastated most of the organisms in the sea and on land. However, a few Late Permian taxa survived the mass extinction and also the subsequent Early Triassic post-extinction catastrophic environments. Among them, the Lingulidae brachiopods were perhaps one of the most noted conquerors. Not only had they survived the mass extinction, but also they thrived in the Early Triassic marine realm, forming a nearly globally distributed Lingulidae fauna characterized by low taxonomic diversity but high abundance of individuals. The reasons that this group of brachiopods was able to achieve this remain unclear, and could possibly include a number of interacting factors. Among them are their wide ecological adaptations and their shell composition and appearance. The long-term evolutionary trends in the shell microstructure and exterior of the Lingulidae since the Late Devonian are thought to have advanced considerably by the end-Permian to the extent that the shells of the brachiopods were able to sustain stressful (e.g. anoxic) environments. In addition, a variety of short-term adaptive changes in the shell size (reducing shell size), shape (shells became more pointed and flatter anteriorly), and thickness (reducing shell thickness) are believed to have also aided the survival of the brachiopods through the highly toxic marine environments from the latest Permian to the Early Triassic. The long-term unchanged organophosphatic shell composition of Lingulidae brachiopods is considered to be another key feature that must have aided the survival of this group in the end-Permian to the Early Triassic Period, for organophosphatic shell composition would have prevented the brachiopods from dissolution in the highly acidic marine environment prevailing globally during the Permian–Triassic transition. The ability that the lingulids had to live in a wide range of environmental conditions, from high-latitude to low-latitude and from the shoreface to the relatively deep sea environments, is also considered vital for the group to have managed to survive the catastrophic event in the latest Permian and then thrive in the vacated ecological space in the ensuing Early Triassic disaster period.