News — Synovial joints, the lubricated cavity that lets the bones in our shoulders, knees, and elbows slide smoothly against each other, provide freedom of movement to all vertebrates. Scientists have known for decades that synovial joints were present in early tetrapods, and more recently also discovered them in bony fishes, but it wasn’t clear when they first appeared in the evolutionary tree. New research from the University of Chicago now shows that synovial joints first appeared much earlier than thought, in the common ancestors of all jawed vertebrates.
The study, published in , examined the skeletons of modern-day cartilaginous jawed fishes, like skates and sharks, along with jawless lampreys and hagfish. The jawed skates and sharks have articulated, synovial joint cavities, while the jawless lampreys and hagfish do not. Genetic analysis of modern skates showed that they also express proteins for genes known to assist with joint development, and that those joints require muscle movement to form properly.
The researchers also studied fossils of extinct fishes dating back almost 400 million years and saw evidence that some of the oldest jawed fishes had bones with joint cavities for articulated movements, suggesting that synovial joints first evolved as these creatures adapted to a mobile, 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,” said , the Robert R. Bensley Distinguished Service Professor of Organismal Biology and Anatomy at UChicago and senior author of the study.
Neelima Sharma, PhD, a postdoctoral researcher in Shubin’s lab and lead author of the new study, first got interested in the question of synovial joints in fishes because she had seen how skates “walk” by propelling themselves along the bottom of the ocean with their fins. She was convinced that they must have synovial joints but was surprised to find out that this hadn’t yet been explored in detail.
Working with Yara Haridy, PhD, another postdoc in Shubin’s lab and co-author of the paper, Sharma studied the skeletons of two cartilaginous fishes, the little skate (Leucoraja erinacea) and bamboo shark (Chiloscyllium plagiosum). They saw that both had bona fide synovial joints in the jaw and pelvis, with a clear cavity for synovial fluid and cartilage lining the adjoining bones. The skeletons of lampreys and hagfish, on the other hand, had joints filled with tissue instead of a synovial cavity.
The researchers also studied genetic expression in the skate joints to understand its similarity to other vertebrates during development. They found that skates express proteins from the TGF-β and Wnt genetic regulatory pathways, the same genes known to help with joint formation in zebrafish, mice, and chickens. The skate joints also require muscle contractions to develop properly; in an experiment where the muscles were paralyzed during development, the joints were fused instead of forming articulated cavities.
“This is such a conserved property that you see in animals which develop in eggs, they start twitching after they develop their muscles,” Sharma said. “The embryo has to move around for them to be able to develop joints properly, so when you remove muscle activity, that genetic expression disappears.”
Finally, the team ran a comparative analysis of fossils from extinct species of both jawed and jawless fishes. While a search of the scientific literature didn’t turn up evidence that early jawless fishes had synovial joint features, Sharma and team did find clear signs that one of the earliest jawed fishes had synovial-like joints. They CT scanned a fossil specimen of Bothriolepis canadensis, a placoderm fish with bony head plates that lived in the Late Devonian period about 380 million years ago. The placoderm skeleton had clearly defined articulating bone surfaces in its pelvic girdle and fins, with matching cavities to accommodate sliding movements. It’s not clear whether the joints were filled with cartilage or fluid, but the similarity to modern synovial joints is clear.
While jawless fishes have managed quite well without synovial joints for all these years, they still look more or less the same as their ancestors (although hagfish have developed some ). Smooth, flexible, articulating joints may be one of the evolutionary developments that helped jawed vertebrates spawn the incredible diversity of species we see today.
“Synovial joints have a set of features together that enable mobility, but they do so while retaining a stable joint,” Shubin said. “We couldn't walk if those joints didn't have a degree of stability, but also, we couldn't walk if they didn't have a degree of mobility either. So, this combination of stability and mobility is afforded by a very, very powerful biological invention.”
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