Leptomitiids are some of the most iconic sponge fossils from Cambrian shale Lagerstätten, which had an effectively cosmopolitan distribution during the early and middle Cambrian. However, due to the slender and elongated shape, leptomitiids are rarely preserved as complete fossils, which, in turn, hindered the study of their ontogeny and ecological adaptive strategies. Based on 97 newly discovered and exceptionally preserved specimens of Leptomitus teretiusculus from western Hunan Province, we demonstrate for the first time that L. teretiusculus exhibits allometric growth in the size range from 1.47 to 12.49 mm in width. In addition, the stably repeated and completely preserved morphology in these fossils enables us to explore the potential hydrodynamic benefits of the body plan of L. teretiusculus through computational fluid dynamics simulations. The results indicate that the distinctive morphology of L. teretiusculus yields a lower drag coefficient and reduced fluctuations in lift force compared to a straight tube. Together with sufficient buoyancy, these factors enhance the structure's dynamic stability, minimizing fatigue at the junction between the holdfast and the substrate and thereby reducing the risk of holdfasting failure. In previous studies, leptomitiids have been interpreted as shallow sediment stickers with ineffective anchoring mechanisms, exhibiting limited adaptive capacity to colonize Phanerozoic-style soft substrates characterized by bioturbation-generated mixed layers (Dornbos et al., 2005). The distinctive morphological adaptations of leptomitiids partially compensated for their simple attachment systems. This hydrodynamic mechanism may represent a generalized adaptive strategy shared by many other Cambrian sponges exhibiting a tapered base, which significantly promoted their anchoring stability and ensured the survival in diverse dynamic environments, eventually enhancing their evolutionary persistance.
 

computational fluid dynamics,Cambrian,porifera