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You've seen the cartoon before: a fish hoisting itself up on land with its front fins, being greeted with some snarky sign like, "Evolve at your own risk," or something similar. This fish has become a meme, so much so that when the discovery of Tiktaalik roseae, a 375-million year old fossil that displays a mix of fish and tetrapod characters, was announced in 2006 by the lab I was working in at the University of Chicago, I thought I'd start a movement with the geekiest version of the Darwin fish ever:
Well, wouldn't ya know it, that pesky thing called progress has unseated the "front-wheel drive" version of life invading land and replaced it with something much more... well, hipster. More evidence is in, and Tiktaalik, it seems, was built like an old Volvo - with rear-wheel drive.
A new paper in the January 13th early edition of PNAS describes Tiktaalik's newly discovered tail end, complete with a beautifully preserved pelvic bone and partial hind fin from the same specimen that made headlines in 2006. Finally, a near-complete image of Tiktaalik is emerging:
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What is so incredible about this find is a delightful detail of the pelvis. The hip socket is smooth and round - a ball and socket joint - that suggests the hind fin had quite a wide range of motion. This is unlike the shoulder joint which has an oblong shape with an angled facet that would have prevented the front fin from rotating up and around. Judging by fossil data alone, the front fins were robust and capable of supporting weight but they were probably not helping Tiktaalik walk like an amphibian on land without some serious body drag. The hind fins were also quite strong and clearly played a part in holding up the beast. But what of the ball and socket hip joint?
The trouble with fossils is that they're terribly static. We can learn a lot about what was physically possible from looking at the anatomy, but we can never hot-wire a fossil into moving again, and we certainly can't observe their behaviors in their natural environment. But this is where interdisciplinary studies can be so powerful.
Upstairs from Neil Shubin's paleontology lab at the University of Chicago in 2010 was a graduate student named Heather King. King was studying the biomechanics of African lungfish and had rigged up a long, thin aquarium where she could film the lungfish as they did what lungfish do (spoiler alert: not much). Very occasionally, she noticed a bizarre behavior: the lungfish would use their whip-like hind fins to walk along the bottom of the tank.
It was at a presentation to her advisory committee that King first revealed this behavior, and Shubin and another adviser familiar with Tiktaalik's pelvis, Michael Coates, nearly fainted. Lungfish, after all, are one of Tiktaalik's closest living relatives, being part of a group of fish known as lobe-finned fish (most of the fish you can think of - tuna, goldfish, blobfish, chub - belong to a different group, the ray-fins). Suddenly, the round hip socket of Tiktaalik came sharply into focus. Would Tiktaalik have been capable of walking along the bottom of a stream with its hind fins like its cousin the lungfish? Was Tiktaalik a rear-wheel drive beast?
Articles cited:
Neil H. Shubin, Edward B. Daeschler, & Farish A. Jenkins, Jr. (2014). The Pelvic Girdle and Fin of Tiktaalik roseae PNAS : 10.1073/pnas.1322559111
King HM, Shubin NH, Coates MI, & Hale ME (2011). Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes. Proceedings of the National Academy of Sciences of the United States of America, 108 (52), 21146-51 PMID: 22160688
