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Jamie Stearns wrote:
It seems a lot of ways to define Tetanurae are problematic, and with no
easy way to resolve. The usual three options all have problems:
These problems apply to any and all clades, not just Tetanurae. It's a
consequence of the fact that topologies (the relative positions of taxa in a
cladogram) often change, especially when new characters or new taxa are
added to the matrix. Most researchers try to frame definitions such that
taxonomic content does not alter too much if there are relatively minor
changes in topology. (If, however, there are profound changes to topology,
then many affected clade names simply disappear.)
Stem-based: This, as stated earlier, could create problems when abelisaurs
are taken into account.
Not if _Carnotaurus_ is included as an external specifier, as Andreas
Node-based: Normally, this would be a good solution. However, this is also
problematic with things like Piatnitzkysaurus, Cryolophosaurus, etc.
floating around out there.
Yes, the lower depths of Tetanurae seem to be a very murky place, with
beasts like "megalosaurs" lurking about. For a number of reasons, the
relationships of basal tetanurans tend to be difficult to resolve. The
_Ceratosaurus_+Abelisauroidea clade, on the other hand, is well-supported
and many taxa are represented by excellent specimens. So it is better to
define Tetanurae as a stem-based clade, such that it excludes _Ceratosaurus_
Apomorphy-based: This may be more viable then the other two, as a popular
"rule of thumb" (no pun intended) places Tetanurae as "theropods with three
or fewer digits on hand." Unfortunately, as many of the big theropods have
dinky little arms that are rarely found, this comes with its own set of
problems. Nevertheless, a refined version of an apomorphy-based definition
that includes a few more characteristics may be the way to go.
Apomorphy-based definitions tend not be so popular these days. One problem
is that most anatomical characters are not cut from whole cloth. For
example, "theropods with three or fewer digits on hand" hinges upon what
qualifies a digit. If a theropod hand has three functional digits but the
fourth digit is represented only by a nubbin of a metacarpal, is the animal
four-fingered or three-fingered? A given character state may even vary
among specimens of the same species (or even within the same specimen, for
Having an apomorphy-based definition that incorporates more than one
character doesn't help too much. What happens if we use three characters
for an apomorphy-based definition, but we find a taxon that has only one or
two of the defining characters, but not the other(s)?
Then there's the problem you drew attention to: preservational artifacts, by
which specimens do not preserve a defining character for an apomorphy-based
clade. Ironically, this may not be such a problem given that the character
state may be inferred for the taxon based on its phylogenetic position. For
example, Gauthier and de Queiroz's (2001) apomorphy-based definition of
Avialae was based on "feathered wings used for powered flight". The
presence of this character can be inferred for many fossil birds that do not
preserve feathers, or (even more challenging) observing these taxa engaging
in powered flight. Gauthier and de Queiroz's definition poses even greater
problems for basal birds, given differing hypotheses and definitions
regarding 'powered flight' in _Archaeopteryx_ etc. Overall, apomorphy-based
definitions are just 'buying trouble'.
I hope that helped.
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