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UNENLAGIA IS HERE!
After spending at least a few hours running round (like a headless theropod?)
in search of the new _Nature_, I finally have...
NOVAS, F.E. and PUERTA, P.F. 1997. New evidence concerning avian origins from
the Late Cretaceous of Patagonia. _Nature_ 387: 390-2.
..in my hands. The new maniraptoran taxon _Unenlagia comahuensis_ is described
from a partial skeleton of a medium-sized (approx. 2 m length) theropod. It is
from the Turonian-Coniacian Rio-Neuquen Fm of Neuquen Province, Argentina. The
name is from the Mapuche Indian words for 'half bird' ('unen' and 'lag').
Comahue, of course, is the Mapuche name for NW Patagonia.
The paper includes a life restoration in silhouette and a cladogram that places
_Unenlagia_ as between dromaeosaurids and Avialae (_Archaeopterx_ + Metornithes)
(and I thought this name had been officially abandoned in favour of Aves). The
pelvis is complete (I don't know whether to call it opisthopubic or propubic -
the pubis is just about vertical) and there is a scapula, humerus, femur,
tibia, some dorsal verts and caudal chevrons.
_Unenlagia_ has a laterally directed glenoid - the first time such a structure
has been seen outside of Aves - and 'forelimbs capable of excursions in the
avian manner' were therefore possible. The authors note that this possible
preadaptation for flapping renders it unlikely that avian flight evolved after
a passive gliding stage.
At this stage, I've only skimmed the paper once. However, I am somewhat
surprised to find that there is no suggestion that _Unenlagia_ may have been a
flightless descendant of volant forms. Witmer (in WITMER, L.M. 1997. A new
missing link. _Nature_ 387: 349-50.) interprets Novas and Puerta's position on
_Unenlagia_ as 'appropriately cautious'. Instead it is argued that the
laterally-directed glenoid and avian-style forelimb motions are preadaptations
that only became involved in the evolution of flight when _Unenlagia_-like
avialian ancestors changed their physical proportions and became smaller.
_Unenlagia_ does share lack of the fourth trochanter and five other listed
characters with avialians (no data matrix of course... never mind Tom;-)) but
is thought to be less derived than _Archaeopteryx_ because it lacks a
transversely reduced pubic apron, still has an obturator process on the ischium
and doesn't have a tibia that is 25% longer than the femur.
Hey, should we head messages like this 'spoiler' too?;-)
"It just wouldn't be proper"