In the Devonian period, vertebrates gained novel morphological and behavioural traits that facilitated life in a terrestrial environment. Among these, solid-substrate based locomotion, alternating gait by pelvic propulsors and digit-bearing limbs are considered to be critical adaptations for the emergence of tetrapods12,33,42. Recent works on terrestrial locomotion in fishes have shown that pelvic appendage driven locomotion on a hard surface was possible before the evolution of digited limbs12. Our work supports those findings and goes on to show that fish are capable of evolving a robust pelvic girdle that is firmly attached to the vertebral column in the absence of digited limbs; this finding is especially worth considering given the fossils trackways showing tetrapodal locomotion that predate the origin of digited limbs. While there have been multiple cases of secondarily aquatic vertebrates, we only have fossil evidence of one period of time in which vertebrates emerged from an aquatic lifestyle and evolved terrestrial walking behaviour19. Future studies of extant fishes that convergently evolved morphological and behavioural features typical of tetrapods will offer a window into understanding the biomechanical constraints that enhanced selection on this complex transformation.
Researchers from New Jersey Institute of Technology (NJIT) have identified unique anatomical features in a species of blind, walking cavefish in Thailand that enable the fish to walk and climb waterfalls in a manner comparable to tetrapods, or four-footed mammals and amphibians. The discovery of this capability, not seen in any other living fishes, also has implications for understanding how the anatomy that all species need to walk on land evolved after the transition from finned to limbed appendages in the Devonian period, which began some 420 million years ago.
This research is reported in a March 24 Nature article, “Tetrapod-like pelvic girdle in a walking cavefish,” by Brooke E. Flammang, Daphne Soares, Julie Markiewicz and Apinun Suvarnaraksha. Flammang and Soares, assistant professors in the NJIT Department of Biological Sciences, were assisted with the research by Markiewicz, an NJIT post-baccalaureate research volunteer in the Flammang lab. Investigator Suvarnaraksha is a member of the Faculty of Fisheries Technology and Aquatic Resources of Maejo University in Thailand. The full text of their article is available at www.nature.com/articles/srep23711.
Speaking of the unique anatomical structures seen in the cavefish, Cryptotora thamicola, Flaming says, “It possesses morphological features that have previously only been attributed to tetrapods. The pelvis and vertebral column of this fish allow it to support its body weight against gravity and provide large sites for muscle attachment for walking." With respect to evolutionary significance, she adds, “This research gives us insight into the plasticity of the fish body plan and the convergent morphological features that were seen in the evolution of tetrapods.”