There is increasing evidence that hydrodynamic conditions play a crucial role in shaping 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. In this study, we select and reconstruct three-dimensional (3D) life-size models of Calyxdendrum, a problematic graptolite interpreted in previous studies as an intermediate form between benthic Dendroidea and planktic Graptoloidea. By simulating the hydrodynamics of models with different nematode structures using computational fluid dynamics (CFD), we test the hypothesis that the morphological plasticity of nematode length may have played an important hydrodynamic role in acquiring the planktic lifestyle. The simulated hydrodynamic properties (velocity fields and mechanical forces) suggest that an elongated nemata at the proximal end was critical for the ecological transition from benthic to planktonic mode of life in Ordovician graptolites. A longer nemata would have provided better feeding efficiency and sufficient drag force to de-anchor the substrate, a possible pathway for initiating planktic life. Our study highlights that the transition from a benthic to a planktonic mode is a convergent adaptation that may have happened several times in the Ordovician. In addition, this study provides a more comprehensive perspective on the functional morphology of these ancient marine organisms, by integrating traditional palaeontological methods with modern computational techniques.