An osteohistological study of a Late Jurassic docodontan (Mammaliaformes) reveals its growth pattern and fossorial ecological habits

Docodontans are a clade of the Mesozoic mammaliaformes (Synapsida: Cynodonts) that were widely distributed across Laurasia from the Middle Jurassic to Early Cretaceous. Owing to their close phylogenetic relationship to crown mammals, docodontans have been the subject of extensive studies on morphology, systematics, and ecology. However, osteohistological investigations remain absent because of the scarcity of suitable material, leaving their bone microstructure and growth biology poorly understood. Here, we present the first detailed osteohistological study of the fibula of an indeterminate docodontan (SDUST-V0007) from the Middle-Late Jurassic Yanliao Biota of northern China. The specimen has well-preserved bone microstructure, providing valuable insights into its ontogeny and growth dynamics. Osteohistological evidence indicates that SDUST-V0007 represents a subadult individual that was approximately two years old at the time of death and experienced a juvenile period considerably longer than that of extant mammals of comparable body sizes. The occurrence of early lines of arrested growth (LAGs) suggest cyclical growth during early ontogeny, a pattern that is uncommon among extant small mammals but more typical of non-mammaliaform cynodonts and some Mesozoic mammaliaforms. These observations indicate that the growth strategy of SDUST-V0007 more closely resembled that of non-mammaliaform cynodonts than that of extant mammals. Furthermore, the combination of an unusual growth trajectory, characterized by an initial phase of slow growth followed by accelerated growth, together with exceptionally thickened cortical bone, may reflect adapations to a fossorial lifestyle. This study fills a major gap in the osteohistological record of docodontans and provides new evidence for the evolution of growth strategies in early mammaliaforms, as well as the relationship between bone microstructure and ecological adaptations.