Ancient Fish Fossils Reveal Synovial Joints Evolved Millions of Years Earlier Than Thought

By Dr. Chinta SidharthanReviewed by Lily Ramsey, LLMMar 7 2025

Did you know that your knees, hips, and fingers all rely on the same type of joint that evolved hundreds of millions of years ago? In a recent study published in PLOS Biology, developmental biologists from the University of Chicago explored the origins of synovial joints, which are critical for movement in vertebrates, by examining fossil records and embryonic development in modern species.

By comparing ancient placoderm fossils with embryonic skates, the researchers uncovered evidence that these joints, once thought to be exclusive to bony vertebrates, actually date back to early jawed fish.

This discovery reshapes our understanding of skeletal evolution and provides important insights into the fundamental mechanisms that allow complex movement in animals.

​​​​​​​Study: Synovial joints were present in the common ancestor of jawed fish but lacking in jawless fish. Image Credit: HenadziPechan/Shutterstock.com

Evolution of Joints

Joints are essential structures that enable movement in vertebrates. Among them, synovial joints—characterized by a fluid-filled cavity that allows smooth articulation—are crucial for mobility in all tetrapods, including humans.

Fossil evidence shows that early jawless vertebrates lacked these joints, relying instead on rigid connections. However, while synovial joints are well-documented in bony vertebrates, their evolutionary origins remain debated.

Some scientists suggest that they evolved after the emergence of land-dwelling vertebrates, while others hypothesize that their development began in aquatic ancestors.

Furthermore, the developmental similarities in joint formation between cartilaginous fish, such as sharks and skates, and bony vertebrates also remain largely unexplored, as do the roles of molecular pathways in the early evolution of joints.

Investigating Synovial Joint Origins

To explore the origins of synovial joints, the researchers examined fossilized remains of the ancient placoderm fish Asterolepis ornata and analyzed embryonic development in modern cartilaginous fish, such as the little skate (Leucoraja erinacea).

Fossil samples were obtained from the Natural History Museum in Berlin and subjected to microstructural analysis at the paleo-histology lab at Chicago’s Field Museum. The fossils were embedded in epoxy, cut into thin sections, and analyzed using high-resolution microscopy to determine the presence of articulating joint structures.

In parallel, embryonic skates were raised in controlled laboratory conditions to observe joint development. The researchers chemically induced muscle paralysis in some embryos to assess the role of movement in joint formation. The embryos were then fixed, sectioned, and stained for histological examination, focusing on the expression of key molecular markers such as β-catenin and growth differentiation factor 5 (Gdf5), both associated with synovial joint development in tetrapods.

Further analysis included contrast-enhanced micro-computed tomography (CT) scanning to visualize internal cartilage structures and immunostaining of skates, Chiloscyllium plagiosum or bamboo shark, and chicken joints to detect the presence of specific proteins linked to joint formation.

The study also included in situ hybridization to examine gene expression patterns. By comparing fossilized joint structures with the developing joints in skates, the study aimed to trace the evolutionary origins of synovial joints and assess whether cartilaginous fish share developmental pathways with bony vertebrates.

Key Findings

The study found that synovial joints, which were once believed to be exclusive to bony vertebrates, actually trace their origins to early jawed fish.

Fossil analysis of Asterolepis ornata revealed reciprocally cavitated joint structures, indicating that these primitive fish had articulating joints similar to those seen in modern vertebrates. This suggested that the ability to form synovial joints existed long before the emergence of tetrapods.

In living species, embryonic analysis of little skates provided further support for this hypothesis. The researchers discovered that the developmental pathways governing joint formation in skates share key similarities with those in tetrapods.

Expression of β-catenin and Gdf5 in skate embryos mirrored patterns observed in synovial joint formation in mammals, highlighting a conserved genetic mechanism underlying joint development.

Furthermore, the muscle paralysis experiments revealed that movement plays a crucial role in joint cavitation, with paralyzed embryos failing to develop fully formed joint cavities. This aligned with findings from previous research on tetrapods and reinforced the idea that mechanical forces are essential for normal joint morphogenesis.

However, the study also revealed important differences. Unlike bony vertebrates, skates lack fully ossified bones, suggesting that while synovial-like joints may have originated in early jawed vertebrates, their structural composition has varied throughout evolution.

Additionally, no evidence of such joints was found in jawless fish, such as lampreys and hagfish, indicating that these features are unique to gnathostomes or jawed vertebrates.

Conclusions

Overall, these findings provided critical insight into the evolutionary history of joint formation, demonstrating that the mechanisms enabling complex movement in vertebrates evolved a lot earlier than previously thought. The study revealed that synovial joints originated much earlier in vertebrate evolution, dating back to primitive-jawed fish.

By combining fossil evidence with embryological research, the scientists uncovered developmental similarities between skates and tetrapods, providing new insights into how vertebrates evolved complex joint structures.

These findings not only reshape our understanding of skeletal evolution but also highlight the conserved role of ancient molecular pathways in modern joint development.