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.... ALVAREZSAURID!
Or that's the conclusion I came across when
translating the original description by Lapparent (1960). This taxon comes
from the Aptian-Albian Continental Intercalaire of Niger. Here's the
material-
Holotype- cervical vertebra (80 mm), two dorsal
vertebrae (70 mm, 80 mm), posterior dorsal vertebra (50 mm), three sacral
vertebrae, three caudal vertebrae (80-85 mm), humerus (200 mm), distal pubis,
distal femur, partial tibia, partial fibula, four metatarsal ends, partial pedal
phalanx
Referred- ulna (300 mm) proximal metatarsal And here's why I think it's an
alvarezsaurid.
The cervical vertebrae are opisthocoelous, like a
wide variety of neotheropods, excluding most coelurosaurs (Compsognathus,
Calamosaurus and Mononykus being the exceptions). Alvarezsaurus is more
basal, as it has amphicoelous cervicals.
The dorsal neural canal is very deep. And I
mean VERY deep, over 50% of the centrum height. Neural canals over 40% of
the centrum height are only found in Patagonykus, mononykines and
pygostylians. The condition in Alvarezsaurus is unknown.
The caudal vertebrae are procoelous. This is
restricted to alvarezsaurids and some pygostylians. The vertebra is
obviously too long for a pygostylian.
The distal caudal centra are keeled
ventrally. This is only seen in alvarezsaurids and "Capita..... uh, USNM
3049 (must not get into another discussion about that :-) ).
The humerus, though missing its middle portion, is
quite short (10 cm wide proximally by 20 cm long). This is uniquely
alvarezsaurid, though unknown in Alvarezsaurus itself.
A few other comments- The posterior dorsal
doesn't look very procoelous to me, perhaps indicating "E." gautieri is more
primitive than Patagonykus and mononykines. The caudal centra have lateral
fossae or pleurocoels, resembling Patagonykus. The humerus lacks the
highly divergent internal tuberosity of Patagonykus and mononykines, also
suggesting it is more primitive than these taxa. Though the pubis could be
propubic as Lapparent presumedly interpreted it, reverse which way is anterior
and it is similar to Patagonykus'. Either way, it doesn't matter, as the
condition in basal alvarezsaurids is unknown and the boot otherwise resembles a
good plesiomorphic type such as ornithomimosaurs. The pubic symphysis is
said to be oval, perhaps indicating it's reduced as much as Patagonykus.
The presence of any symphysis shows it is not a mononykine. The hindlimb
elements are very slender, and the greatly expanded distal tibia seem to be an
apomorphy of the taxon.
Any comments?
Mickey Mortimer
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