Knee tissue diversity and evolution in different reptile taxa

The knee joint plays a critical role in tetrapod locomotion, yet its developmental trajectories and anatomical diversity remain underexplored outside of model taxa. Here, we examine knee joint development in three representative reptilian lineages, Phrynops hilarii (Testudines), Caiman latirostris (Crocodylia), and Columba livia (Aves), and compare them with adult knee morphology in two squamate species, Cercosaura parkerii and Hemidactylus mabouia. Using histological series spanning key developmental stages, we document patterns of ossification, meniscus formation, cartilage composition, and sesamoid presence. All taxa share delayed epiphyseal ossification and early ligament and meniscus differentiation. However, they differ in meniscal shape, tissue composition, tibial plateau morphology, and sesamoid expression. Notably, P. hilarii lacks a patella but seems to present both a fabella and a cyamella; C. latirostris and C. livia exhibit distinct patellar configurations, with C. livia displaying a multi-element patellar complex. Cartilage thickness and composition also vary, with thinner cartilage and a narrower interarticular zone observed in C. livia. Ancestral state reconstruction supports a single origin of the patella within Reptilia and highlights multiple instances of anatomical convergence. These findings underscore the functional diversity of the reptilian knee and provide a developmental framework for interpreting the evolution of limb joint morphology across tetrapods.