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Re: Romerograms... Some thoughts...



> Considering avicephalans "thecodonts" was made possible by
> the absence of tree-thinking. In precladistic times, all but maybe
> the real specialists on any group didn't imagine a group as a twig,
> they imagined it as a blob on a romerogram. Such blobs lack internal
> structure; the fact that no avicephalan fits into the "thecodont" tree
> didn't matter because people simply didn't think of it. At best, blobs
> on a romerogram consist of more blobs with stippled lines between
> them; adding one more little blob for *Megalancosaurus*,
> supported on a stippled line that originates from a free-floating
> question mark, is no big affair (if only you believe that
> *Megalancosaurus* has The Defining Feature, the antorbital
> fenestra).

I'm not sure if we can say 'tree-thinking' was entirely absent when romerograms were popular (ye olde precladistic times),

I didn't mean to say that. I meant that the BAND didn't use tree-thinking; that they imagined *Megalancosaurus* and *Cosesaurus* and *Longisquama* as points in an area (the thecodont blob of a suitable romerogram) rather than as points on a tree. That's what I did with all taxa on whose internal classification/phylogeny I had no idea.


So, I've always assumed that the authors of romerograms have always
been aware that there was a 'real tree' that had occurred [...]

Of course.

They didn't overstep the mark and say more than they felt they could
(BTW this should apply to both those of the precladistic and cladistic
eras!).

That's what all those support measures like bootstrapping are for. :-)

The 'blobs lacked internal structure' because that was the
resolution of their confidence in the pattern, not because they didn't think
that that internal structure was actually there (sitting beyond their
methodological grasp).

No, an internal structure was usually known, but romerograms were more or less always done at the level of a Linnaean rank. If it was done on the order level, Thecodontia was one blob (with Crocodylia, Pterosauria, Ornithischia and Saurischia as blobs that arose from it). If it was done on the family level, Thecodontia was a cluster of several blobs, with more or less stippled lines between them. If it was done on the class level, there was a Reptilia blob and no Thecodontia blob.


Sure, this may have lead to inadvertently 'shaping thought' (despite an
effort to avoid it)

This is what I'm talking about: those efforts didn't go anywhere near far enough.


And the width of a blob I'm sure was something they sweated over, knowing
it's limitations, inaccuracies, prejudices etc.

This is almost never how they portrayed it. (And I bet most authors didn't sweat over it at all, but I haven't asked any.)


The sorts of things they
wanted to indicate were (a) ecological importance* (which can involve body
size, which is what John Conway was getting at in his early post on this
thread when referring to biomass etc, as well as number of
individuals/species); or (b) diversity (no of species and possibly
problematic evaluations of degree morphological differences); based on (c)
data: amount of fossil evidence for a group (number of fossil species; or
possibly amount of fossil fragments/material in ecological works?).

Yes, but these all got conflagrated, and even the (c) part was almost never based on a real count. This is easy to see when a blob hits the Holocene, so that (b) and (c) are identical: the mammal blob is typically wider than the bird blob.


Cladistics is more about filling the gaps between the data with hypothetical
relationships.

It is about phylogeny alone, without trying to indicate diversity or any kind of importance.


I basically think
at this level of resolution either method would be as good as the other to
depict the data, depending on what you wanted show, and at this point
romerogram is probably showing more of what you do know than creating
patterns for what you don't

Together with support measures, cladistics can extract far more information out of the same data than is displayed in a typical romerogram. But that's all beside my point (as is the observation that the romerograms in Benton's book Vertebrate Palaeontology are based on cladograms). My point was just that romerograms impaired tree-thinking, which led to the idea that an animal could just be "a thecodont" without anyone needing to worry about "the details".


Reference:

Falsenstein

Felsenstein :^)