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Re: Fossil Discovery Threatens Theory of Birds' Evolution



At 08:21 PM 25/06/2000 -0400, ELurio@aol.com wrote:

Just to show that convergance of the kind I've talked about is indeed a
possiblilty. That is relevent. BTW. If cuttlefish are squids, so are octopi.

No, they are not. Cuttlefish. octopuses (the correct plural, BTW - "octopi" is incorrect as the
Latin "pus" has the plural "podes", not "pi" [as in "Antipodes"]) and squid all belong to the Coleoidea, a clade of cephalopods with an internal, reduced or missing shell and two gills (the other living group, the Nautiloidea, has four gills). However, within the coleoids the octopuses belong to the Octopoda, which also includes the Vampire Squid Vampyroteuthis, while squids and cuttlefish belong to separate branches (Teuthida and Sepiida) of the Decapoda.


If they are the same class anyway, which they indeed are, and the convergence
involved seems "interesting but not all that remarkable," which is also the
case, then convergence between two anmiotes, which are the same superclass,
shouldn't be either.

This would be true if the convergence was as superficial, or involved as few characters, as that between the shell of Spirula and that of ammonites; as I mentioned earlier there are striking differences between the two. No modern teuthologist would mistake Spirula for an ammonite. However, what you are talking about with theropods would amount to massive convergence across a whole range of skeletal characters - so extreme, in fact, that no modern palaeontologist, including experts studying these animals in particular, has recognized it if it exists. Your analogy might work for something like (say) marsupial and placental moles, or snakes and eels, but it hardly applies here.


Um... I didn't say that either, and the term is "graptolite".  And what are
graptolites supposed to be convergent to?>>

Okay, it's a thingie that was found near New Guinea a decade or so back.
There was a possibility of a ghost lineage of four hundred million years if
it wasn't convergance.

The specific find you refer to was made off New Caledonia, and was described by Noel Dilly. It is a member of the group of hemichordates (acorn worms) known as pterobranchs. These are colonial animals very similar to what we know of graptolites; in fact Dr. Dilly (and others) are convinced that the two groups are closely related if not actually identical. The chief functional difference was that individual graptolites produced a long spine called a nema projecting fron their locations in the colony, but pterobranchs did not and, it was thought, were incapable of doing so. However, the new find was of a species, Cephalodiscus graptolitoides, that did produce a nema. Dr. Dilly announced this find as a "living graptolite", but he was speaking structurally rather than phylogenetically; C. graptolitoides is not the only species in its genus, and the others do not produce nemas. In effect he was arguing that there was now no reason to separate the two groups.


Once again, the "convergence" here is related to a single feature among closely-related animals, and is therefore not equivalent to the extensive multi-character convergences that would be necessary if theropods were polyphyletic (in particular if one of the lines you propose was closer to Longisquama than thjan to other theropods).
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