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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:
Stem-based: This, as stated earlier, could create problems when abelisaurs
are taken into account.
Node-based: Normally, this would be a good solution. However, this is also
problematic with things like Piatnitzkysaurus, Cryolophosaurus, etc.
floating around out there (laughs at ridiculous thought of 25-foot theropods
floating around in inner tubes)
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.