Pelvic fin

a hallmark extension
that stretched evolutionary time
up, up, up – away

The pelvic fins are a pair of fins located on the bottom of fishes. Hey! My two legs are located on the bottom of me. Hmmm…

First there were lobe-finned fish with pectoral and pelvic fins – they lived 380-390 million years ago. Then the four-legged tetrapods, our ancestors, appeared about 30 million years later with no pectoral fins and no pelvic fins. So what happened in between?

Tiktaalik roseae happened.

Roaming this planet 375 million years ago was Tiktaalik the “fishapod”, part-fish part-tetrapod.

Tiktaalik‘s fossils were found on Ellesmere Island in Nunavut, Canada. First Tiktaalik’s frontal appendages were uncovered – a combination of pectoral fin and structures with the rotational abilities of a shoulder and elbow. Then Tiktaalik‘s hind appendages were unearthed –  a combination of pelvic fin and supportive pelvic bone. Tiktaalik likely lived in shallow, nearshore waters using its pectoral limbs to prop up and scope out the scene. Tiktaalik’s pelvic limbs were surprising – great for swimming and appeared strong enough to support walking underwater…and perhaps on land!

Fishes today retain paired pelvic fins and did not evolve legs. Pelvic fins likely have a role in steadying swimming fish. 

Time for an exception!

Gobies have a fused pelvic fin that resembles a suction cup. A goby uses its fused pelvic fin to perch and pose on rocky substrate, oh, and sometimes climb over waterfalls! The Nopoli rockclimbing goby (Sicyopterus stimpsoni) alternates between a mouth sucker and the pelvic fin sucker to ascend Hawaii’s waterfalls (a.k.a. “inching”). Less dramatically, this characteristic fin is used, among other traits, to identify invasive round goby (Neogobious melanostomus) in North America.

Thus concludes the fin series.