??? Never read of that one...
As Jaime already said, it's primarily
known from forelimb material (AMNH 6368, humerus, manual phalanx, manual
ungual) originally assigned to Alectrosaurus. New material has been
found (Currie and Eberth 1993) and may be synonymous with Erlikosaurus or
Segnosaurus.
Usually, the word "arctometatarsalian" is used
to describe a third metatarsal that is considerably constricted proximally
and a bit expanded distally. It is not necessary that it doesn't reach the
ankle. Since it does constrict in Harpymimus and
"Ginereemimus" (is this the to-be-correct spelling? Someone stated
that it is misspelled in the original and will be corrected, someone else
wrote it like here), AFAIK, the arctometatarsus can still be a
synapomorphy of Arctometatarsalia.
Note that the definition from Tom Holtz in his
1994 paper states that the third metatarsal must be "completely excluded
from anterior view". This is probably not the case in "Ginnareemimus"
and Harpymimus. As for the Arctometatarsalia, it only contains
ornithomimosaurs in my phylogenies. Tyrannosaurs may turn out to be
more closely related to ornithomimosaurs than birds, but the relationship
between these three groups has yet to be worked out in my cladograms.
Caenagnathids are oviraptorosaurs and troodontids, mononykines and Avimimus
are avialans in my analysis (although Avimimus may change position with the
new data I have available). So the arctometatarsus is not a
synapomorphy of Arctometatarsalia in my phylogeny.
Does this mean that basal dromaeosaurids
still have fourth trochanters?
Achillobator, Adasaurus, Velociraptor and
some Deinonychus individuals have fourth trochantors. Some Deinonychus
specimens, Unenlagia and Bambiraptor lack them.
What are the apomorphies of alvarezsaurid
caudal vertebrae? The pneumatic openings (shared with oviraptorosaurs and
many pygostylians)? (This is not intended as a rhethorical question, I just
don't know...)
Alvarezsaurid caudal centra lack
pleurocoels. The most obvious apomorphy (and reason Tugulusaurus
caudal vertebrae are not alvarezsaurid) is the presence of strongly
procoelous centra.
Mickey
Mortimer