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
another ‘missing link’ found in the link from fish to us.
Or maybe not–note this line from Greg’s post: “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.”
It sounds to me as if the attached pelvic girdle developed in our ancestry may have evolved after our ancestors left the water.
Really interesting post. Thanks!
Way cool!
and the exception that proves (in the sense of “probes”) the rule is, as so often, the whales.
Actually, I wonder if this was an argument that early (say, 1800-1820) whalers used to bolster their classification of whales as being “fish”. I rather doubt it – I don’t think the whalers really felt a need for sophisticated justification of their business in those days. And much later, Cuivier and the comparative anatomists would have been looking in much more detail at the actual anatomy.
“An attached pelvis– “hips”– in a modern teleost, however, is a really neat, and not expected, finding.”
Hips don’t lie.
I wonder why this particular fish developed hips and other fish didn’t. Does this mean that there is a selective pressure that only applies to Cryptotora, but not to other fish?
I expect a load bearing issue drove this innovation, as these fish climb vertically out of water. Other fishes also climb (mudskippers), but for some reason have not come up with the trick, or did not need to.
That sounds plausible.
And this innovation was adaptive because it allowed these fish to exploit a resource not yet eaten by other fish.
Really cool.
Now, about those ray finned fishes. I had noticed that their pelvic fins can be far forward (but did not realize they could be anterior to their pectoral fins!). But this got me to wondering about their homeotic genes. I suppose the homeotic genes for rear limbs would be expressed in a more anterior position in these fishes.
Very cool! I remember well going, “Holy shit! There’s your tetrapod locomotion!” while watching Planet Earth – the caves episode. (Much to the consternation of my companions — until I explained.)
I was watching the cave fish climbing up the stream — and there it is!, the whole tetrapod motion, on pectoral and ventral fins!
isn’t from Planet Earth; but shows the motion well.
One more try on the link.
I like the rotating “hip” illustration at the end of that vid!
Fascinating. Thank you.
Actually the fashion among young women these days (at least in my neck of the woods) seems to be to leave the ilial crests exposed, no matter how bundled up they might otherwise be.
You mean love handles?
My non-medical understanding was that love handles are slightly superior to the ilial crests, not lateral. Think muffin top.
I suppose ‘muffin top’ does have a more provocative sound to it than ‘ilial crest’. If you’re beyond a certain age, you can at least imagine food.
Thanks for sharing this, Greg. Very interesting. I should have been more attentive in comparative vertebrate anatomy.
Another one of the endless forms most beautiful!
When I first glanced at the title I thought it was PCC reporting having fish and chips at an Indian restaurant. It was early…