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Heinz Peter Bredow wrote-
"Contrary to one hypothesis, because of the primitive state of the
obturator
notch of the pubis in Siamotyrannus, tyrannosaurids cannot be derived from allosaurids, in which this character is more advanced. However, this would not preclude derivation from sinraptorids, in which the obturator notch is more primitive." So here Siamotyrannus is described both as a tyrannosaurid and a possible sinraptorid. This would of course mean that Tyrannosauridae would be the same as Neotetanurae (Allosauroidea + Coelurosauria). No, Buffetaut et al. are saying that allosaurids are too derived to be
ancestral to tyrannosauroids, as their absence of an obturator notch is more
advanced than the latter. However sinraptorids have obturator notches, so
they could hypothetically still be tyrannosauroid ancestors.
It's a horrible arguement though, that harkens back to Heilmann saying
birds cannot be derived from dinosaurs because the latter lack clavicles.
Reversals happen in evolution, so you cannot use such statements to prove
phylogenetic hypotheses. So they are still saying Siamotyrannus is a
tyrannosauroid, just stating that the latter clade could be derived from
sinraptorids. Luckily, Tyrannosauroidea is a stem-based group that doesn't
include Siamotyrannus in its definition, so we don't have to worry about being a
synonym of Neotetanurae/Avetheropoda.
Mickey Mortimer
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