The Origin of Spines
BY ALPER ÖZKAN (MSN/PhD I)
d_ozkan@ug.bilkent.edu.tr
As you may remember, this week's column is supposed to detail how ancestral fish might have derived from ancestral starfish, for which I'll need quite a bit of space. This, in turn, severely limits the amount of idle banter I can get away with, so let's get down to business.
You'd be hard-pressed to find a habitat that displays greater diversity than the seafloor. Life itself may have originated in the abyssal depths, and from thence onward, the benthos was host to a fauna far more diverse than its pelagic and terrestrial counterparts, serving as the initial stronghold for many taxa that later conquered the latter environs. Yet, the seafloor offers an extra challenge for those trying to adapt to it: unlike the water column, where movement in all directions is possible, benthic animals not adapted to burrowing can only move in a two-dimensional plane, and sense organs must be concentrated on the top side in order to prevent them from sinking into the mud. Consequently, if such an animal swims or crawls with one side facing the floor, its sense organs will gradually move to the other side. This is seen in modern flatfish, where both eyes are on one side of the head due to an eye migration event that occurs as the planktonic flatfish larva metamorphoses into the benthic juvenile.
As it turns out, this is also the case with echinoderms. As mentioned, echinoderm larvae start off as bilaterally symmetrical embryos, but the right side eventually undergoes growth arrest and is absorbed by the left, which then develops into the complete animal. In a way, you could say that all starfish, brittle stars and sea lilies are left-handed (left-armed, rather), because they in fact have nothing but left hands! The fivefold symmetry of today's echinoderms is a later development, as is evidenced by their fossil record: the basal class Helicoplacoidea features three ambulacral grooves (the "slits" running along the underside of the arms in starfish; other echinoderms also possess these grooves, which are used in feeding) in a spiral along the body, while edrioasteroids, as a relatively more advanced group, have an almost-pentameral symmetry with five ambulacral grooves. Closer inspection, however, reveals that four of those form pairs of two and only three grooves actually meet at the animal's mouth (to which the ambulacral grooves are supposed to carry food). Later, those pairs are presumably separated in full, and we get modern echinoderms with their fivefold symmetry.
But what if an animal near the beginning of the echinoderm evolutionary pathway evolved bilateral symmetry again? Since it wouldn't have much of a right side remaining, it would have to partition its left side into a new "left" and "right," essentially rotating its body plan 90 degrees in the process. This would also mean that a great deal of structure migration should occur during embryonic development to redefine the left and right sides of the animal, which is incidentally what happens with the gill slit migration of lancelets. Given that lancelets are basal chordates and their development should give us a clue about how chordates as a whole came to be, and that there is an echinoderm group that just so happens to be almost bilaterally symmetrical (and with a canal that might have held a notochord to boot, not to mention the fact that they even had tails), it doesn't seem far-fetched to speculate that chordates may have derived from echinoderms after all! That is the gist of the Calcichordate Theory, and while it fares poorly against parsimony analysis, certain ideas about the lifestyle of these ancient echinoderms (which, by the way, are the order Mitrata) have found support in light of more recent evidence. But that's for another column!
By the way, in calling Pikaia the earliest known chordate last week, it seems that I had forgotten about Haikouella and Yunnanozoon from the Maotianshan Shales, which are likely chordates and definitely a good deal older than Pikaia.
My apologies.