One of our interests at the RVC is examining the early history of tetrapods (four-legged vertebrates) and how their ancestors made the transition from water to land. Some studies have examined species of living fishes that can move on land, such as mudskippers and lungfish, but although they are able to make do using their fins like crutches, to date it seems that none use walking patterns similar to those of salamanders or other living tetrapods with presumably ancient ways of moving. Now, researchers have discovered that the blind cavefish Cryptotora thamicola exhibits a kind of ‘walking’ behaviour only previously seen in true tetrapods such as salamanders: a diagonal-couplets lateral sequence gait. This means the right forelimb and left hindlimb step forwards almost exactly together, and vice versa, arching the body one way and the other with each step. Looking deeper at the anatomy of C. thamicola, Dr. Brooke Flammang and colleagues found that this remarkable fish also shows adaptations in its limb girdles which are associated with walking in modern tetrapods. The pelvic girdle, which articulates with the pelvic fins in fish, and the hind limbs in tetrapods, is more heavily ossified than in other fishes and, crucially, is joined to the axial skeleton as it is in walking vertebrates rather than being loosely connected by soft tissue as is typical in fish. This connection is important because it could allow the generation of large and vertical forces required to produce walking in a terrestrial environment without the help of buoyancy.  However, the cavefish is far from being a ‘missing link’ between ancestral fishes and tetrapods – it is clear from fossilised Devonian animals that the digits evolved before the modern-type limb girdles – but it does show that a similar transition has been made to (somewhat) tetrapod-like walking at least a second time in fishes, and could be instrumental in helping us understanding early fossils and fossilised tracks made by our own ancestors.

Image from Flammang et al. 2016, showing the robust structure of the pelvic girdle of C. thamicola and its fusion with the ribs and spine.

Read more about this work here. Photo courtesy of Danté Fenolio.

Reference: Flammang, Brooke E., Suvarnaraksha, Apinun, Markiewicz, Julie, Soares, Daphne. 2016. Tetrapod-like pelvic girdle in a walking cavefish. Scientific Reports, 6. 237111.

Axial skeleton: The section of the skeleton containing the head and the body trunk (e.g. chest, abdomen) but not the limbs.