Re: Hanson 2006, Mortimer, Baeker response

[mike@miketaylor.org.uk (Mike Taylor)]

Tim Williams writes:
>> I now find I don't need to reply to Tim because Mickey said
>> _exactly_ what I was going to say.  I think any disagreement
>> between Tim and me here is just cross-purposes.
> 
> And nothing a few pints couldn't solve.  :-)

[In Homer Simpson voice:] Ah, beer!  Is there _anything_ it can't do?

>> Of course in saying that _Tendaguruia_ and _Agustinia_ don't "fit
>> at all convincingly into any of the established clades less
>> inclusive than Neosauropoda" I wasn't proposing that there must be
>> other Neosauropod clades besides Diplocodoidea and Macronaria :-)
>> Just they they are morphologically so freaky that they're not "a
>> good fit" anywhere.
> 
> Yep, that's how I interpreted it.  But what I was driving at is that
> "morphologically freaky" is usually not the same as "not a good fit
> anywhere".  Weird oddball taxa that are given their own "families"
> because they are so different (morphologically speaking) usually end
> up being sunk into a pre-existing family.

Somehow we are still talking past each other here.  If a specimen is
morphologically very different from its closest known relative, then
that alone makes it interesting (to me, anyway).  So if we mean
anything at all by "family", then surely what we mean is something
like "group that is morphologically similar within itself, but
different to other things".  (And, yes, I know perfectly well that
this is not cladistically sound or any of that stuff.  That's fine.  I
am very fond of clades, and recognise their utility in discussing
evolution, but I see no reason why they should be the _only_ groups we
acknowledge).

>> So that if we accept that the notion of "families" is a useful one
>> (on which see below), then it does indeed make sense to accept
>> Bonaparte's erection of Tendaguriidae and Agustiniidae.
> 
> But if both _Agustinia_ and _Tendaguria_ turn out to be
> titanosaurids, then Tendaguriidae and Agustinidae cannot be used.
> Otherwise, Titanosauridae becomes paraphyletic.  The idea behind
> phylogenetic taxonomy is that common ancestry is used to define
> clades, not characters or degrees of 'different-ness', which was the
> basis for Linnaean ranks.

Ah, now I think you've confused yourself by tying together Linnaean
and phylogenetic principles.  In suggesting that Tenduguriidae
couldn't be considered a "family" because of orthographical accident
of falling inside another group with a name that ends "-idae", you are
taking us down a path that I'm sure neither of us wants to take!

>> Yes indeed.  But I don't have a problem with that.  Just because we
>> (mostly) all agree that families are not objectively real, it
>> doesn't follow that they have no utility.
> 
> The utility and validity of families should depend on the
> relationships of the taxon is question.  Agustiniidae and
> Tendaguriidae are only valid if _Agustinia_ and _Tendaguria_ are not
> contained within another "family-level" taxon, like Titanosauridae.

No, no, you've gone all ICZN-y on me again :-)

> But if _Agustinia_ and _Tengaguria_ are each demonstrated to be
> nested within the Titanosauridae, then the family-level distinction
> is no longer necessary (and is also misleading, from a phylogenetic
> standpoint).

OK, let's try again.  At best, I've not clearly communicated here; at
worst I may have misunderstood my own point, and it's helpful to have
the chance to sort that out.

The reason I don't feel the mandatory surge of outrage whenever I hear
the word "family" is that I want a way to say the following kind of
thing, which is actually pretty common:

        Other sauropod remains from the Hastings Beds Group
        represent basal Titanosauriformes, Titanosauria and
        Diplodocidae; the new taxon brings to four the number
        of sauropod 'families' represented in this unit.

Now everyone knows what we mean by this.  Basal titanosauriforms
(i.e. brachiosaur-like beasts), titanosaurs and diplocodoids are
different kinds of things, and finding all three in a single unit
tells you something interesting about diversity.  The new taxon's
difference from all three is also interesting.  Now, then: how do you
say the bit about "brings to four the number of XXX represented"?

The knee-jerk reaction seems to be to say "the number of clades", but
a moment's consideration shows that this is nonsense.  Since clades
nest, the number of clades can be equally legitimately considered to
be one (Sauropoda), two (Titanosauriformes and Diplocodoidea), or a
dozen (Chordata, Amniota, Reptilia, etc.)  Interestingly, the one
number that _does_ seem hard to support is three, since one of the
three groups we want to discuss ("basal titanosauriforms") is not a
clade at all, but a paraphyletic assemblage that is nevertheless worth
discussing as it is morphologically distinct.

What, then?  We could say "brings to four the number of groups", but
that is so vague as to tell you really nothing at all.

I suppose the bottom line of what this fragment is trying to say is
"brings to four the number of morphologically distinct sauropod groups
represented, where degree of morphological distinctiveness is
understood in the context of overall sauropod diversity and evaluated
as is typical for group".  That is what I am using "family" as
shorthand for.  I'd welcome a suggestion for a better term, but I
don't think there is one.  (Then again, given the confusion on this
list arising from my use of the term, and all the baggage of "-idae"
names and suchlike, maybe it's better to invent a new term or just
spell it all out in painful detail.)

 _/|_    ___________________________________________________________________
/o ) \/  Mike Taylor  <mike@miketaylor.org.uk>  http://www.miketaylor.org.uk
)_v__/\  "Observations on becoming a good writer: 1) The most important
         virtue in a writer is the will to write" -- Greg Gunther.