Chondroid bone in the cervical ribs of a new juvenile mamenchisaurid

As a well-known sauropod dinosaur clade dominant in the Middle–Late Jurassic of Asia, Mamenchisauridae exhibits unique body size evolution and ecological adaptability, making it a key focus in dinosaur biology. However, most previous studies have focused on macro-morphological descriptions, while microscopic histological data remain largely underexplored. Here, we conducted histological analyses of cervical ribs from a new mamenchisaurid specimen recovered from Sichuan, China, to characterize the histological features of cervical rib ossification and infer its potential ossification mechanism. The cervical rib comprises an anterior process, a shaft, a tuberculum, and a capitulum. The capitulum and its surrounding regions formed through the mode of normal bone matrix formation, whereas the others developed through the metaplasia of tendons into fibrocartilage, followed by ossification via a process analogous to endochondral ossification. We identified the widespread presence of chondroid bone (CB). Microscopically, CB is characterized by distinctive CB cell lacunae, which are irregularly distributed and resemble chondrocytes, accompanied by spheritic mineralization. Chondroid bone may significantly enhance skeletal growth efficiency during rapid growth phases, potentially explaining the extreme slenderness of cervical ribs in Mamenchisauridae. This special tissue may have been an adaptation to buffer mechanical stress during neck movements.