Morphological evolution of respiratory structures in Brissopsis (Echinoidea, Spatangoida) over the past 40 million years, with new fossils from the Western Pacific

Abstract

Adaptations to diverse substrate environments and tiering behaviours are thought to have contributed to the evolutionary success of echinoids. However, the macroevolution of respiratory structures remains poorly understood because of the fragmentary fossil record. Here, we investigate the evolution of petaloid respiratory structures in Brissopsis , one of the most diverse and specialized genera of deep‐infaunal heart urchins. We describe new fossil specimens of Brissopsis luzonica from Taiwan and the Philippines, dating from the Early Pliocene to the Early Pleistocene, an interval during which fossils of the genus are globally rare. We compiled fossil and extant records of Brissopsis and analysed petal morphology using linear measurements and outline‐based geometric morphometrics. Morphospace analysis shows that petal morphology remained broadly stable through time, with only a slight expansion of morphospace occupation since the Eocene–Oligocene. Petal outline is strongly associated with relative petal size but shows little relationship with body size or pore pair number. Species with convergent posterior petals tend to have relatively smaller petals than divergent forms, suggesting alternative respiratory strategies potentially associated with different environmental conditions and tiering behaviours. Together, these results indicate that much of the petal morphological diversity observed in extant Brissopsis was established early in the evolutionary history of the genus and subsequently maintained throughout the Cenozoic, providing new insights into the evolution of respiratory structures and adaptation in infaunal echinoids.