Thoughts On The Evolution And Natural History Of Scaphopods.
Why Here And Why Now?
This post is, obvously, the continuation of a series dealing with scaphopods and some of the data I will be posting subsequently are also to be found on one or another of my website’s scaphopod pages. However, these blog entries are not strictly duplicative; I have added a number of new data and I have altered some of the information to reflect my present thoughts. Some of the ideas and data to be presented here are somewhat iconoclastic, and contrary to what some authorities have proposed. It is unlikely I will get the opportunity to publish these ideas in more formal, peer-reviewed, jounals, and as a result I thought this is an appropriate place to let the ideas see some glimmer of the light of day, albeit dimly and through some wet mud. To the questions of “Why Here and Why Now?” I think the reasonable answers are, “Because I think this is an appropriate place and it is time.” Or phrased another way, “Why Not?”
One of Three Groups…
The scaphopods are the last of the classical molluscan classes to show up in the fossil record, with arguably the first unequivocal scaphopod being Rhytiodentalium kentuckyesnis Pojeta & Runnegar, 1979. However, this unequivocality is not likely the case; the specimens of Rhytiodentalium are all significantly altered fossils, and from personal examination, it is impossible to tell exactly what they are. Although some of them match the general shape of modern – and presumably – highly derived scaphopod shells, these “shells” appear to be comprised of small pelletized material. It is unclear if these pellets are the result of significant or minor diagenesis. In the first case, the shells could considered as scaphopods. In the second, they would have to be something else, perhaps, some sort of worm tube. I think the latter is much more likely than the former.
The term “armchair quarterback” has been coined to describe those individuals who after watching a football game at home on the “aptly-named” boob tube, dissect a quarterback’s performance and describe, a posteriori, what he should have done. Of course, such a critique, if that’s what it may be called, is done without the experience of being under the tremendous pressure of the momnet on the field of play, without the sport’s equivalent of the “fog of war” clouding information input and, most importantly, it is done with the precision vision of hindsight. Of course, in the armchair experience, errors made on the field become glaringly obvious. One of the prime theories of scaphopod evolution is that scaphopods arose from an ancestor that either was in the extinct class, Rostrochonchia, or pehaps in its ancestral group, is the malacological equivalent of such airchair quarterbacking, however, with one glaring exception. It is undoubtedly wrong, most likely as a result of being proposed by individuals who have had no experience examining or studying live scaphopods or, indeed, live animals of any sort..
There are a number of very serious problems with the Scaphopods from Rostroconchs derivation, not the least of which is that the scaphopod shell is univalved and tubular, while the rostroconch shell is bivalved of various non-cylindrical shapes. Additioanally, the scaphopods are all predators or scavenger/predators; as a result, they must move; no predator on infauna waits for the prey to come to it. Then, the scaphopod radula, the structure used to macerate, break, crush or smash prey is the largest radula relative to the adult body size in all the mollusca. On the scale of the organisms, it is a truly massive structure. This massive radula is presumed to have been derived from an ancestor in the same group that is supposed to have given rise to the bivalves. However, not only do the bivalves lack the radua, but also any remnant of the head it is found in. While the scaphopod head is reduced and kept within the shell, it is present, and has a relatively large brain, also a structure missing in the bivalves – and presumably their rostroconch ancestor. The rostroconch shapes vary quite a bit, but one thing that is evident in all of them is that they are not streamlined and capable of easy movement through sediments. Indeed, with the shapes typically found in rostroconchs, it is quite likely, that like some oddly shaped infaunal bivalves today, they did not move at all as adults. Scaphopods, on the other hand, are all mobile and many of them, given the appropriate stimulus, are capable of bursts of relatively rapid motion, after which they often stop, construct a feeding cavity and feed. Given the sizes of the adult scaphopods, the number of body lengths that they are able to move in any given amount of time, and the media that they move through, it is quite reasonable to consider many of them to be “high speed” predators. Finally, recent molecular genetic work shows them to be grouped with the cephalopods, not the bivalves.
I think it is likely that one of the first branchings of the ancestral molluscan stock gave rise to a predatory organism that had a tendency to develop or elongate in a dorso-ventral direction. In turn, this ancestor, over time, gave rise to three successful clades, eventually leading to the crown groups of the cephalopods, gastropods, and scaphopods. All of these groups are all characterized by dorso-vental flexing in the visceral region, a well-developed radula, and elaborations of the cephalic tentacles.
Each of the three dorso-ventrally flexed groups shows particular adaptations and modifications for its primary habitat. The cephalopods are highly successful predators in the pelagic enviroment. Gastropods have radiated into virtually every possible niche except aerial flight, and are found in all terrestrial, fresh-water, and marine environments, although their ancestral habitat was the marine benthic epifaunal environment. Scaphopods have become highly adapted for predation on organisms living in unconsolidated marine benthic sediments.
Cadulus tolmiei in situ, modified from Poon, 1987.
The above image shows Cadulus tolmiei feeding in sediment, cb= captacular bulb, dd= digestive diverticula, fc = foot cavity, g = gonad, m= mantle, pa = posterior aperture, s = shell,
References:
Pojeta Jr., J. et. al. 1972. Rostroconchia: A New Class of Bivalved Mollusks. Science. 177: 264-267.
Poon, Perry A. 1987. The diet and feeding behavior of Cadulus tolmiei Dall, 1897 (Scaphopoda: Siphonodentalioida). The Nautilus: 101: 88-92.
Steiner, G. and H. Dreyer. 2003. Molecular phylogeny of Scaphopoda (Mollusca) inferred from 18S rDNA sequences: support for a Scaphopoda–Cephalopoda clade. Zoologica Scripta. 32:343-356.
More to come…
Until then,
Cheers!!!
