Scientists from the University of Chicago have traced the evolutionary origins of synovial joints back to the common ancestors of all jawed vertebrates. The study, published in PLOS Biology, suggests that these joints appeared much earlier than previously thought.
Research on skeletons from modern-day cartilaginous fishes, like skates and sharks, alongside jawless species such as lampreys and hagfish, revealed significant differences. Skates and sharks exhibit articulated, synovial joint cavities, unlike their jawless counterparts. Gene expression in skates also aligns with known joint development processes, requiring muscle activity to form properly.
Fossil evidence indicates that early jawed fishes possessed bones with joint cavities, supporting articulated movements. This development likely arose as these creatures embraced a more predatory lifestyle. "This evidence suggests that these kinds of joints evolved at a time when creatures were becoming much more active predators with jaws and mobile skeletons so they could swim around and eat each other or avoid being eaten," stated Neil Shubin, a paleontologist at the University of Chicago.
Neelima Sharma, lead author of the study and postdoctoral researcher, got interested in synovial joints by observing skates' unique locomotion. Together with co-author Yara Haridy, they confirmed that the little skate and bamboo shark possess synovial joints, distinguishing them from jawless fish species.
The study also explored genetic expressions in skates, identifying the presence of TGF-β and Wnt genetic pathways, similar to those in zebrafish, mice, and chickens. Muscle contractions were necessary for proper joint development, as evidenced when paralyzing muscles led to fused joints instead of articulated ones.
Further analysis of extinct species' fossils revealed that early jawed fishes had synovial-like joints. A CT scan of Bothriolepis canadensis, a 380-million-year-old placoderm, showed articulating bone surfaces and cavities conducive to joint movement.
Despite the absence of synovial joints in jawless fishes, jawed vertebrates have seen remarkable diversity partly due to these joints. According to Shubin, "Synovial joints have a set of features together that enable mobility, but they do so while retaining a stable joint."
This study was funded by the Brinson Family Foundation, the University of Chicago Biological Sciences Foundation, and HFSP.