Talk:Vernanimalcula
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I almost completely rewrote the article. It would be good to have some else's perspective. Dalbury 11:47, 13 October 2005 (UTC)
Well, since you ask; I think the changes look great. Now there is a picture and links and more information than before. The right way to use Wikipedia. It is interesting to see that the flatworms may have once have lost their coelom through progenesis. Vernanimalcula is the closest thing we can come to of how Urbilateria may have looked. Today we can find very small annelids with a very reduced coelom and reduced segmentation. And if I remember correctly, the only place to find a (pseudo?)coelom in Tardigrada is around their single gonad.
Some quotes from different articles:
"In any event, it is likely that all three platyhelminth clades (Acoelomorpha, Catenulida, and Rhabditophora) originated through a process of progenesis from a large-bodied, coelomate ancestor (Rieger, 1986, 1994; Tyler, 2001) even if those are independent origins. No other hypothesis of origins accounts for the histological structure of flatworms. Progenesis has played a major role in much of the evolution of the interstitial fauna (Westheide, 1987), and while this process can be readily recognized among a few taxa in which sufficient intermediates are known (the interstitial annelids, for instance), that for the flatworms seems to have produced such a drastic revamping of morphology that the origins have become obscurred."
"Taken together, these facts provide a basis for considering the acoelomate and pseudocoelomate worms as being derived from larvae or juveniles of coelomate ancestors with early maturation of gonads (progenesis) (Rieger, 1980, 1985). Acoelomate and pseudocoelomate organization may not arise by reduction of the coelom from coelomate ancestors, therefore, but rather by a forestalling of its development (cf. Smith & Tyler, 1985, p. 124, 137; Ax, 1985, p. 172)."
"Could the Acoela be the most basal group of the Bilateria? Yes, certainly, at least among extant animals. Did the ancestor of the Bilateria look or live like an acoel flatworm? Not likely. The Acoela are a specialized branch from that ancestor, and while they may be genetically more closely related to that ancestor than other living animals, that ancestor was probably a large-bodied coelomate organism. Present-day acoels are reduced in body size and have changed their mode of reproduction (and show all the morphological modifications those changes require) probably in adaptation to a benthic life in sediments. Perhaps other platyhelminths adapted similarly from another ancestor, but many of their similarities are rather difficult to explain as convergence."
If you have heard about Xenoturbella, you probably know they don't have a coelom either, and not even a brain or gut. But unlike flatworms and similar organisms, their larva seems to have these advanced structures and is lost in the adults. While flatworms may have evolved via progenesis, Xenoturbella could have evolved via reduction. Either way, it looks their ancestors had a body cavity. Xenoturbella is actually placed among the deuterostomes, and must then have evolved from a much more complicated animal, something like Vetulicolia, which had a coelom, gill slits and segmented tail. Quote: "However, the simple body plan of Xenoturbella is neither due to paedomorphy nor to plesiomorphy but to metamorphosis from a complex larva. No living embryos have been observed before. The larvae have alimentary tract, mantle cavity, gills and concentrated nervous system while the adults lack such structures."
Some links and theories about the early bilateral animals:
http://devbio.umesci.maine.edu/styler/globalworming/platyhelm.htm
http://www.findarticles.com/p/articles/mi_qa3746/is_199812/ai_n8818711
http://www.umesci.maine.edu/biology/labs/origin/default.htm
http://www.findarticles.com/p/articles/mi_qa4054/is_200302/ai_n9184843
- Very interesting! I was only vaguely aware, or not aware at all, of the arguments given above. Is this material in Wiki? The acoelomata article is quite short. I haven't pursued this very far (so much to read, so little time). - Dalbury 10:43, 18 October 2005 (UTC)
- Hi. No, I haven't found this on Wikipedia. I had it in my mailbox from one late evening when I was surfing and found much interesting stuff about early animals. And when I first was writing something on the discussion board about a related subject, it seemed like a good idea to post them here. Maybe I could write an article one day, but it could be a little difficult since English is not my mother language. So if anyone reads it and wish to write an article or two on the subject themselves, that would be just fine with me. And if not, at least it is fun to read what already is written.
Another comment about flatworms. They are all hermaphrodites with internal fertilization. If our common ancestors really had belonged to the flatworms, we should all be hermaphrodites with internal fertilization, which is not the case. Yet it is strange that internal fertilization has evolved quite often among those without coelom or pseudocoelom. The cells in the body of small animals like nematodes can get bigger, but the number is normally stable throughout their whole life, and is in a sense a dead end in evolution. These traits and their small size must because of this be secondary evolved. This becomes even more clear when we see other small animals as tardigrada and some mites. The old picture of bilaterian evolution as acoelomates, then pseudocoelomates and finally coelomates (and then segmented coelomates), seems artificial. All the new information about animal information is pretty exciting. Just too bad there is often too little information about them for the moment. But since everything is very new this will hopefully change in the near future.
Some more about one of the animals mentioned above: http://pharyngula.org/images/deuterostome_phylogeny.jpg http://pharyngula.org/index/weblog/comments/spectacular_echinoderms_from_the_lower_cambrian/
About Vetulicolia it was predicted many years ago such an animals once had excisted. "Many years ago, the famous American palaeontologist Alfred Sherwood Romer argued that the ancestor of the fish must be a composite animal, constructed of two sections: a large anterior with gill slits, and an elongated and segmented tail." http://cas.bellarmine.edu/tietjen/Evolution/once_we_were_worms.htm
It is very interested that the body was divided into two sections; a segmented tail with strong muscles and a nerve cord, and a soft "bag" in front that contained the inner organs and the gill slits. In the lancelet and the craniata, the tail continued to evolve and grow forward until the "bag" was enclosed in it. In fish, it is the "bag" we are throwing away while we eat the "tail" which is the rest of the muscular body. In tunicates the opposite happened. The "bag" became dominant while the tail disappeared in the adult (plus they have a lot of other adaptations, like the loss of segmentation, the cellulose-like material, the reduction of the coelom and the big hollow "room" inside them where they filter the water). This means that instead of vertebrates evolved from the larval form of tunicates, it is the larval form of tunicates (which indeed is divided into two parts) that most resembles the common ancestor of both vertebrates and tunicates, which from that origin have evolved into two opposite directions.
- With regard to tunicates, there are also the larvacea, which appear to be neotenous urochordates. I'm not sure what Vernanimalcula has to do with all this, however. The published microphotographs of Vernanimalcula show no signs of segmentation. In fact, the animal was little more than a bag with a mouth, a gut, a coelom, and an anus. We do not yet know if Vernanimalcula had a larval form. As the Doushantuo formation is reported to be rich in fossils of larvae and eggs, perhaps a larval form of Vernanimalcula will be identified. - Dalbury 17:07, 18 October 2005 (UTC)
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- I shouldn't have called larvacea neotenous, but they do retain the trunk-and-tail structure. - Dalbury 18:31, 18 October 2005 (UTC)
Holding on to the larval stage the rest of their life is as you know a secondary adaptation among the larvacea, and is an expection to the rule.
You are right, the Vernanimalcula don't have much to do with tunicates of Vetulicolia, and it shows no signs of segmentation. Segmentation and a bigger size came later in the evolustionary history it seems, but it still happened such a long time ago that it could be that most of or all the animals alive today are the descendants of a relatively big and segmented animal. Just as all the amniotes today came from an amphibian ancestor, even if the amphibians in their turn came from a fishy ancestor.
This only repeats what is already said, but it is still fascinating:
"Since fossils of larger bilaterians appear in the fossil record soon after Vernanimalcula's, it indicates that internal complexity in animals came before increased size."
http://www.uwm.edu/News/PR/05.09/fossils.html
The reason I mentioned them was because Vernanimalcula are older than the flatworms, and still have a coelom, which flatworms don't have. So they must have lost it, and I posted some theories about the loss of a coelom as an argument about why it must have been a primary feature among the first bilaterians (and I wonder why it evolved in the first place, what role it had). And obvious the last common ancestor must have looked very different from the modern animals, but how? The earliest known bilaterian and the last common ancestor to the modern forms (which evolved later) could very well be two different animals.
- Yes. Vernanimalcula pushes back the appearance of coelomate bilaterians, but leaves unanswered the questions of where did it come from, and how does it relate to the Cambrian phyla. - Dalbury 10:59, 19 October 2005 (UTC)
And so I tried to find information about how it could have looked. Bilateria can be divided into two groups; protostomes and deuterostomes. And according to newer research, deuterostomy is probably the ancestral condition of the modern Bilateria from which protostomy evolved (just once) a while after it branched off in its own direction (except from the arrow worms, who seems to come from animals that are from the same branch as the protostomes, but who split from it before the protostomian transformation). If this is true, the last common form must have been a deuterostomian. But how did the oldest deuterostomes look? The oldest finds of deuterostomes are the group Vetulicolia, relatively big and with segmentation. Is this the closest thing we can come to the ancient form that gave rise to us all, or are Vetulicolia an early specialised sidebranch? It looks like it has all the anatomical details from which the other deuterostomes could have evolved, indicating that it in any case is very primitive and have inherited these traits from an even older ancestor. Could it be from such an origin all the present bilaterian animals (or more precise, their ancestors) have evolved? Only more fossils can give the answer to that.
Maybe is the forum for Vernanimalcula not the best place to discuss this, but since it already is here...
Back to Vernanimalcula. With such very small, fragile and soft animals, no wonder it is so hard to find fossil evidence of early pre-cambrian creatures, especially with an advanced anatomy if this is how the first complicated animals looked like. And it takes time to evolve complex structures, the Vernanimalcula is only telling the minimum age where we can say for sure when they existed. I think it is safe to say that similar animals probably evolved and existed millions of years before these finds, the fossils are just not discovered yet. As you say, the first of them were little more than a small bag with a mouth, a gut, a coelom, and an anus. There are limits on how much variation this anatomy can allow, so its ancestors did probably not look that much different (but you never knows). The biggest mystery is why very small and tiny animals would evolve these advanced and complicated traits. Flatworms and jellyfish are much bigger and much simpler, and are doing just fine. They don't need to be more complicated than what they already are to survive and thrive. Perhaps is it all a result of some very early examples on exaptation?
- This is fascinating. I also think there is a lot of material that can be used in Wiki articles. I haven't explored this subject very far in Wiki, but I think a project to create and organize articles related to the early development of the animalia would be appropriate. Would you be interested in a collaboration? - Dalbury 10:59, 19 October 2005 (UTC)
It could be interesting to see some articles about the subject. But since all the information I have about this comes from other articles found on the net, it would have to be based on what already is available (even if it is not hear on wikipedia). I could try to find some info and come with some thoughts.
- The best sources will be the science journals, but many of them require a subscription to access on-line. I'll see if I can get to the library to do some research. - Dalbury 11:25, 20 October 2005 (UTC)
Sorry for late reply. I don't know how long articles you had in mind, but most of the information I had available are already posted. Further information would have to come from more free articles in the net.
- OK. Maybe I'll have a chance later to go to a library that keeps back issues of Science and Nature, and see what I can find. - Dalbury 20:13, 22 October 2005 (UTC)
I've added some somewhat deflating comments about the importance of this fossil, whilst still attempting to maintain NPOV. My PERSONAL opinion is that it is almost totally artefactual!
Grahbudd 18:56, 1 November 2006 (UTC)
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- I find it easier to believe that the object imaged at [1] is a fossil of an animal, than that it is an artefact. -- Donald Albury 12:17, 2 November 2006 (UTC)
