by Greg Mayer

The four-legged land vertebrates– amphibians, reptiles, birds, and mammals, collectively known as tetrapods— get around, at least mostly, on their four legs. Their front and hind limbs are attached, respectively, to their pectoral and pelvic girdles, bones that attach or closely adhere to the axial skeleton. The pectoral girdle consists of the scapula, coracoid, clavicle, etc., while the pelvic girdle is comprised of the anteroventral pubis, the posteroventral ischium, and the dorsal ilium. The ilium is firmly attached to the vertebral column by a connection to the sacral ribs extending from the sacral vertebrae. Your “hip bones”, at the widest part of your lower body, are easily felt (or seen if you’re wearing the right– i.e. not much– clothing), and these are the crests of your ilia.

Most fish, by contrast, have an unattached pelvic girdle, floating more or less free in the ventral part of the body, and free to move, evolutionarily, along the venter. The primitive condition, as seen in the bowfin, for example, the pelvic girdle has a position akin to its place in tetrapods– toward the rear of the body:

In advanced ray-finned fishes, however, the pelvic fins may move far forward. In fact, the pelvic fins may move in front of the pectoral fins– the hind limbs are in front of the fore limbs! This is possible because the pelvic girdle is not attached to the vertebral column.

The reason for this disquisition on the pelvic girdles of tetrpaods and fish is that Brooke Flammang and colleagues have just published a description of a living fish with a pelvic girdle attached to the vertebral column. This is really astounding! The species of fish is Cryptotora, a rare cave-dwelling fish from Thailand, that climbs on the wet walls of the caves in which it lives. In the figure below, which is a a head-on view of the pelvic girdle skeleton, the vertebrae are green, the pubis and ischium brown, the fin itself blue, and the sacral ribs and ilium are dark purple. Note the complete bony ring encircling the body from backbone to belly.

Tiktaalik, the “fishapod”, had pubis and ilium, but no sacral attachment or ischium. Some early tetrapods only had a looser sacral attachment than later tetrapods. Cryptotora, an advanced ray-finned fish, is not at all close to the lobe-finned piscine ancestry of tetrapods, so this represents a quite independent evolutionary origin of an attached pelvic girdle.

Carl Zimmer in the NY Times notes the tetrapod-like way in which the fish “walks” up cave walls, with an alternating left-right motion, and also provides a brief gif of one of the fish walking. This motion is more tetrapod-like than those of other walking fish (e.g., walking catfish). The alternating left-right motion of primitive tetrapod limbs is exactly what you would expect from the lateral undulations of a swimming fish. An attached pelvis– “hips”– in a modern teleost, however, is a really neat, and not expected, finding.

Flammang, B.E., A. Suvarnaraksha, J. Markiewicz and D. Soares. 2016. Tetrapod-like pelvic girdle in a walking cavefish. Scientific  Reports 6(23711):1-8. pdf