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Re: Earliest known Dromaeosaurid?
Mike Kessey wrote-
> It's important to distinguish between deinonychosaur (_Deinonychus_ <--
> Neornithes) and dromaeosaurid (_Dromaeosaurus_ + _Velociraptor_). I
> believe all of the forms listed above are potential Deinonychosauria, but
> probably not Dromaeosauridae proper. Correct me if I am wrong.
> _Deinonychus_ is indeed the earliest definite dromaeosaurid, although
> _Utahraptor_ occurs slightly earlier and may very well be a dromaeosaurid.
The issue of assigning taxa to the Dromaeosauridae or just to the
Deinonychosauria is very complex. First, we must have a dromaeosaurid
phylogeny to work with so we know what taxa fall into the (Dromaeosaurus +
Velociraptor) group. Due to forms like Adasaurus and Achillobator, this is
more difficult than it sounds. See my details on posts on the last two
genera for more information. Since no detailed phylogenetic study of
dromaeosaurs has been published, we can't be sure which taxa would fall into
a strictly defined Dromaeosauridae. The usual division between
dromaeosaurines and velociraptorines needs reexamined due to recent
discoveries (Deinonychus is less like Velociraptor than previously thought,
Achillobator has a dromaeosaurine maxilla and pes, but a velociraptorine
pelvis, etc.). My major analysis finds Achillobator and Dromaeosaurus at
the base, with Deinonychus, Velociraptor, Unenlagia, Bambiraptor and
Sinornithosaurus branching off later in that order. My restricted analysis
with more dromaeosaur-specific characters and less outgroups finds
Sinornithosaurus and Bambiraptor as sister taxa at the base, with Adasaurus,
Unenlagia and Achillobator at the top, and Deinonychus and Velociraptor
somewhere in between. If the latter phylogeny is correct, Sinornithosaurus,
Bambiraptor and possibly Deinonychus(!) would be excluded from the
Dromaeosauridae. If the former is correct, Adasaurus and Achillobator may
not be dromaeosaurids. The problem is readily apparent. Since no phylogeny
is the obvious best choice right now (and the only agreed upon clades in
both of my phylogenies are Bambiraptor + Sinornithosaurus and the
dromaeosaur group as a whole), the only taxa definately in the
(Dromaeosaurus + Velociraptor) group (and thus definately dromaeosaurids)
are Dromaeosaurus and Velociraptor! I suppose Unenlagia is always in the
(Dromaeosaurus + Velociraptor) group as well, how odd. The phylogenetic
definition was made assuming that the basics of dromaeosaurid phylogeny were
known, but future discoveries have proven that to be anything but the truth.
Because of this, I recommend we keep a loose definition of Dromaeosauridae
until a major phylogenetic study is published. I personally think a
stem-based definition is much better at this point (everything closer to
Dromaeosaurus than to Protarchaeopteryx, Troodon or Neornithes).
In any case, to address Mike's question, the Portuguese specimens can be
assigned to the Dromaeosaurinae and Velociraptorinae under the standard
phylogeny, so these would presumedly qualify as true dromaeosaurids. Also,
the Morrison specimens from Colorado described by Britt (1991) have
velociraptorine serration morphology. Finally, at least one of the
Bathonian English specimens has a velociraptorine serration morphology.
Although Utahraptor and Nuthetes both lack velociraptorine serrations,
Sinornithosaurus does possess them. Ornithodesmus cannot be compared to
potential dromaeosaurines. These four taxa would definately fit into a
stem-based Dromaeosauridae, but as mentioned above, only Dromaeosaurus and
Velociraptor would be certain members of the node-based Dromaeosauridae.