dubois@uwmacc.UUCP (Paul DuBois) (05/24/85)
> [Mike Huybensz] > A vestigial organ is one that is reduced from a previous state (induced > by comparison with homologous organs in other taxa.) What permits > natural selection to favor the reduction? The reduction in types or > quantity of function. The amount of that reduction of function determines > how much reduction of organ is adaptive. This looks to me like: Change is adaptive, because evolution is true. But that begs the question. > ... > The fact that an animal today is an obligate parasite in no way requires > its distant ancestors to have been obligate parasites. There are endless > examples (I'll list some on request.) Mesozoa are not really considered > likely ancestors anyhow. I'm interested, so: Requested. "With references, please." > Ancestors of today's multicellular animals are still hypothetical. > There are no good extant candidates, and fossil candidates will be > extremely hard to find (because of size, lack of preservable structures, > and difficulties in distinguishing cellular-level distinctions from > other organisms.) After all, how many fossil invertebrate embryos > are known? (They show comparable problems in fossilization.) Jellyfish don't fossilize well, either, but we have jellyfish fossils. Algae fossils are known, too, even though they aren't very rugged. -- | Paul DuBois {allegra,ihnp4,seismo}!uwvax!uwmacc!dubois --+-- | |
ethan@utastro.UUCP (Ethan Vishniac) (05/24/85)
> > > Ancestors of today's multicellular animals are still hypothetical. > > There are no good extant candidates, and fossil candidates will be > > extremely hard to find (because of size, lack of preservable structures, > > and difficulties in distinguishing cellular-level distinctions from > > other organisms.) After all, how many fossil invertebrate embryos > > are known? (They show comparable problems in fossilization.) > > Jellyfish don't fossilize well, either, but we have jellyfish fossils. > Algae fossils are known, too, even though they aren't very rugged. > -- > Paul DuBois {allegra,ihnp4,seismo}!uwvax!uwmacc!dubois --+-- > | But jellyfish fossils are certainly rare things. Intermediates between single and multiple cell organisms also have the disadvantage of size. If you had the fossil in your hand would you be able to recognize it? Finally, it might be very difficult to distinguish between fossils of mats of relatively independent cells and fossils of closely cooperating groups of cells. "Don't argue with a fool. Ethan Vishniac Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan Department of Astronomy University of Texas
mrh@cybvax0.UUCP (Mike Huybensz) (05/25/85)
In article <1135@uwmacc.UUCP> dubois@uwmacc.UUCP (Paul DuBois) writes: > > > [Mike Huybensz] > > The fact that an animal today is an obligate parasite in no way requires > > its distant ancestors to have been obligate parasites. There are endless > > examples (I'll list some on request.) Mesozoa are not really considered > > likely ancestors anyhow. > > I'm interested, so: Requested. "With references, please." Barnes: Invertebrate Zoology. Read about nematodes and flatworms. Oldroyd, Harold: The Natural History Of Flies. Read about the family Hippoboscidae and the tstse flies (two obligate parasitic groups) and what "primitive" flies are thought to have been like. As an interesting aside, just yesterday I was reading (in the manual for the Parasitic Hymenoptera V course) of what must be a transition from obligate parasite to phytophage. In some groups of parasitic wasps that normally parasitize gall-forming insects or leafminers, some species feed first on the host, then the plant; while others don't require a host at all. > > Ancestors of today's multicellular animals are still hypothetical. > > There are no good extant candidates, and fossil candidates will be > > extremely hard to find (because of size, lack of preservable structures, > > and difficulties in distinguishing cellular-level distinctions from > > other organisms.) After all, how many fossil invertebrate embryos > > are known? (They show comparable problems in fossilization.) > > Jellyfish don't fossilize well, either, but we have jellyfish fossils. > Algae fossils are known, too, even though they aren't very rugged. So what? Jellyfish are BIG compared to hypothetical early multicellular animals. Algae produce cell walls which might be less fragile than early animals. For example, where are the fossil protozoa of comparable size and durability? And finally, algae have nice habits of growing in large aggregations. That's why we have found them as stromatolites. Early animals might not have had that property, or might have it and have been mistaken for sponges. The fineness of detail preserved in a fossil is always less than we would like.... -- Mike Huybensz ...decvax!genrad!mit-eddie!cybvax0!mrh