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Re: _Microraptor zhaoianus_



Hey everyone!  Here's the description of Microraptor, direct from Nature (am
I allowed to do this? Never seen it done before, but I can't see why not).
A very interesting animal indeed.  I'll write a post later tonight on the
issues of maturity and size in this specimen, and I'm just dying to write a
Details on segment.......  Enjoy.

The holotype of M. zhaoianus is an articulated skeleton missing the middle
portion of the body (Figs 1 and 2). The premaxilla is similar to that of
Archaeopteryx8 and Sinornithosaurus9 in that it has a sloping anterior
margin. As in Archaeopteryx and troodontids10, the maxilla contributes to
the border of the external naris ( Fig. 2b). There are at least 15 dentary
teeth, which are closely packed as in troodontids11. The anterior teeth,
including premaxillary teeth, are more recurved and laterally compressed
than the posterior teeth as in most theropods, but they lack serrations on
both the anterior and the posterior carinae as seen in some non-avian
theropods and birds ( Fig. 2c). The posterior teeth have posterior
serrations; however, they are bird-like in having a less compressed crown
and a constriction between the root and crown (Fig. 2d). These 'waisted'
teeth are reported here for the first time in a dromaeosaurid, although
among non-avian theropods12 they have already been reported in
therizinosauroids, troodontids and ornithomimisaurs. The heterodont
dentition provides information on the transition from the non-avian theropod
type of dentition to that of the typical avian type. It seems that the
reduction of serrations begins in the anterior teeth and with the anterior
margins of the teeth, however, the basal constriction of the tooth crown
first appears in the posterior teeth.

The body is extremely short, with an estimated trunk length of about 47 mm
(this length is inferred from the presumption that Microraptor had 13 dorsal
bones and that its cervicodorsals are relatively long as compared with the
remaining dorsals, as in other dromaeosaurids13). Five fused sacral bones
are preserved and the anterior three are transversely enlarged, although the
sacrals of the holotype are slightly crushed and flattened (Fig. 3a). The
tail is almost complete and articulated with the sacrum. There are 24 or 25
caudals, which is close to the caudal number of Archaeopteryx14. The
articulated tail has elongate rod-like extensions of the prezygapophyses and
chevrons characteristic of dromaeosaurid dinosaurs13, 15. These rod-like
extensions reach almost to the sacrum (Fig. 3a). The caudals are
significantly elongated. Gastralia and what appear to be sternal ribs and
uncinate processes are preserved.

The ulna is bowed posteriorly and when compared with the femur9 it is
proportionately shorter than in other dromaeosaurids; the radius is much
thinner than the ulna. There are at least three carpals and an additional
one may be present (Fig. 3b), a pattern that is similar to that of
Archaeopteryx. The semilunate carpal is relatively small as in
Sinornithosaurus (IVPP, V 12811) and in Archaeopteryx16, 17, and has more
contact with the proximal end of metacarpal II than with metacarpal I (Fig.
3b).

The pubis is probably retroverted as in other dromaeosaurids and birds (Fig.
3a), and its symphysis is about 49% the length of the pubis. The ischium is
plate-like and less than half the length of the pubis; it has a posterior
process as in Sinornithosaurus9, Unenlagia, Archaeopteryx and Rahonavis18. A
large distally positioned obturator process is present as in Rahonavis 18
and Sinornithosaurus (IVPP, V 12811).

The femur of M. zhaoianus has an accessory crest pendent at the base of the
lesser trochanter, as in the basal oviraptorosaurian Microvenator 19. The
tibia is about 128% the length of the femur. The fibula is slender and
reaches the calcaneum. Pedal digit I is preserved on the posterior surface
of metatarsals II-IV in both of the hindlimbs and has a more distal position
than in other known non-avian theropods. Metatarsals II, III and IV are
subequal in length. The proximal end of metatarsal III is significantly
compressed in posterior view, which suggests an arctometatarsalian
condition20. Metatarsal IV is much more robust than metatarsals II and III,
which is also the case in troodontids. Metatarsal V is long and bowed
laterally and its mid-shaft is expanded. Metatarsal IV bears a pronounced
flange on the posteromedial surface as in other dromaeosaurids9, 21. The
second pedal digit is a specialized raptorial tool13 as in other
dromaeosaurids, troodontids and Rahonavis. Like other dromaeosaurids22, the
ungual is more than one and a half times longer than phalanx II-1. All of
the pedal unguals are sharp and strongly recurved, similar to those of
trunk-climbing birds4-6. The horny sheath is well preserved, with a columnar
dorsal margin and a thin ventral margin.

Large patches of integuments are preserved in situ around the skeleton (Figs
1 and 2a), and the pattern of preservation is similar to that of early birds
from the same locality (IVPP, V 12325). Integumentary structures are best
preserved near the femur, where they run almost perpendicular to the bone
(Fig. 4). They are long (average 25-30 mm), narrow, and have a feather-like
contour, whereas those along the tibia and in the hip area are shorter. Some
impressions of the integuments contain a structure similar to that of a
rachis (Fig. 4b), suggesting that true feathers may have been present among
dromaeosaurids.

Mickey Mortimer