Tracing the Changes of Life Strategy from Ordovician Dendroids to Graptoloids

Hydrodynamic conditions are increasingly recognized as an important factor in the morphological evolution of marine organisms. Understanding how Ordovician graptolites changed from a benthic to a planktic lifestyle is crucial for understanding their palaeoecology and palaeobiogeography. However, interpreting the exact process of these ecological shifts is difficult due to the lack of close modern analogues or preserved soft tissues. Here, we reconstructed life-size three-dimensional models of Calyxdendrum, a problematic graptolite previously regarded as a possible intermediate form between benthic dendroids and planktic graptoloids. Using computational fluid dynamics (CFD), we simulated models with different nematode structures to test whether variation in nematode length had hydrodynamic significance during the acquisition of a planktic lifestyle. The simulations show that elongation at the nematode could improve feeding efficiency and generate sufficient drag to facilitate detachment from the substrate. These results suggest that such elongation may have been a key functional innovation in the ecological transition from benthic to planktic life in Ordovician graptolites. Our study highlights the role of hydrodynamic constraints in this life-strategy shift and supports the view that the transition to planktonic life may have evolved convergently more than once during the Ordovician. In addition, this study pro-vides a more comprehensive perspective on the functional morphology of these ancient marine organisms, by integrating traditional palaeontological methods with modern computational techniques.