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Details on Microraptor



Microraptor Xu, Zhou and Wang 2000
M. zhaoianus Xu, Zhou and Wang 2000
Etymology- "Zhao's small thief", after renouned paleontologist Zhao Xijin.
Barremian, Early Cretaceous
Yixian Formation, Liaoning, China
Holotype- (IVPP V 12330) (48 cm) partial skull (45 mm), lower jaws, four dorsal vertebrae, several dorsal ribs, sternal ribs or uncinate processes, gastralia, sacrum (19 mm), caudal series (24-25 vertebrae) (246 mm), chevrons, radius, ulna (35 mm), radiale, ulnare, semilunate carpal, fourth carpal, proximal metacarpus, partial ilia, incomplete pubes, ischia, femora (one fragmentary) (53 mm), tibiae (one partial) (68 mm), fibula, metatarsi, pedes, feather impressions, keratin claw sheaths
Diagnosis- only posterior teeth have constricted roots; accessory crest on femur; very slender ischium; obturator process whose tip extends further distally than the dorsodistal corner of the ischium; near-arctometatarsalian metatarsus; metatarsal IV most robust.
Description-
This is the "dromaeosaur" part of Archaeoraptor liaoningensis.  The other part is a pygostylian that remains to be described.  This species is claimed to be the smallest non-avian theropod, a distinction it shares with Parvicursor.  Using Archaeopteryx as a guide, but adjusting for the longer tail of Microraptor (both had similarily proportioned heads, presacral vertebrae are poorly known for Microraptor), gives an estimate of 48 cm for its total length.  Several features attest to its maturity, including the fused sacrals, pubic symphysis, last dorsal partially fused to the sacrum and well-ossified cortical bone.
The skull is badly preserved, except for the snout.  The premaxilla is intermediate between the pointed rostrum of Archaeopteryx and Sinornithosaurus and the blunt snout of Deinonychus and Velociraptor.  The subnarial process is shorter than any of these, suggesting the naris contacted the maxilla.  The external naris itself was placed in a position similar to dromaeosaurids, not further back as in Archaeopteryx.  Premaxillae were unfused.  The maxilla and nasals show no useful information, except that the top of the snout was fairly straight and the external naris did not extend posteriorly much past the subnarial process.  The only other identifiable cranial element is the quadrate, which shows no useful details.  The lower jaws are preserved, although distorted.  The dentary is slender and straight laterally, but bows medially in dorsal view.  There are at least fifteen teeth, which are placed in separate alveoli.  The posterior lower jaw appears to be preserved in medial(?) view, as the internal mandibular fenestra is shown.  It is said to be different than the large, triangular fenestra of dromaeosaurids.  Teeth vary with position.  The premaxillary and anterior dentary teeth are flattened, recurved and lack serrations.  Their roots are of normal theropod type.  Posterior teeth possess posterior serrations, but no anterior serrations.  Their crowns are not as compressed and they have constricted roots.  Serrations are not enlarged, numbering 13-14 per mm on a 2 mm tooth crown.
The four preserved dorsal vertebrae are apparently from the anterior end of the series, as they possess large hypapophyses.  Structures that are either sternal ribs or uncinate processes are preserved with the dorsal ribs.  Gastralia are also present.  The sacrum contains five incompletely fused vertebrae, the first three of which are expanded transversely.  The ventral surfaces of the first three seem to lack prominent grooves or keels.  Twenty-four or twenty-five caudal vertebrae are present.  Transverse processes end before caudal eleven, but most caudals possess dromaeosaurid-like elongate prezygopophyses and chevrons.  The distal caudal centra are elongate (to at least 130% of proximal centrum length).
The ulna is strongly bowed and the radius is less than 70% of ulnar width.  The ulnofemoral ratio is .66, which is comparable to dromaeosaurids, Protarchaeopteryx and some oviraptorosaurs, while it is much smaller than Archaeopteryx and other volant forms (>.8).  This suggests Microraptor was flightless.  Four carpals are present, including the semilunate.  The semilunate carpal has a larger contact with metacarpal II than metacarpal I, which differs from dromaeosaurids, but agrees with avians and more basal taxa.  The unfused bases of three metacarpals are preserved, with mc II being most robust and mc III being least.
Only portions of the acetabular area and the postacetabular processes are visible in the ilia, but these don't reveal much information.  The supracetabular crest is probably absent and the postacetabular processes flare outward and taper sharply, suggesting a reduced brevis shelf.  The pubes are preserved in anterior view. The authors code this taxon as having a mesopubic pelvis (similar to Unenlagia, Rahonavis and Archaeopteryx).  The symphysis is 49% of pubic length and the pubic apron is wider than dromaeosaurids.  It tapers abruptly following the symphysis and has parallel sides further distally until it ends bluntly.  The ischium is merely 47% of pubic length, but is much more slender than other eumaniraptorans.  There is a narrow, triangular obturator process whose tip extends further distally than the dorsodistal corner of the ischium.  A mediodorsal process is present, but there is no proximodorsal process.  Despite its slenderness, the ischium is said to be plate-like in section.
An accessory trochantor is present at the base of the lesser trochantor.  The tibia is 1.28 times femoral length and the fibula reaches the calcaneum.  The metatarsus is unfused and subequal to the radius in length.  Metatarsal I is reversed and placed distally.  The second digit has phalanx II-2 >85% of phalanx II-1, as in dromaeosaurids and avians.  Although the ungual is enlarged and possesses a larger flexor tubercle than the other unguals, phalanx II-2 is only slightly expanded proximally.  It lacks the proximoventral heel of dromaeosaurids and troodontids, and the proximoventral tubercle of Rahonavis.  Metatarsal III tapers proximally to the point that it may be arctometatarsalian.  Metatarsal IV is the most robust, similar to troodontids, but exhibits a posteromedial flange similar to dromaeosaurids.  The fifth metatarsal is elongate and laterally bowed.
The feathers are preserved around the ulna, manus, pelvis, femur and tibia.  They have an outline similar to those of Yixian birds.  Femoral impressions have an outline similar to feathers and are 25-30 mm long.  Other impressions near the pelvis and tibia are shorter.  Rachis-like impressions are preserved near the femur and manus.  Keratinous sheaths are preserved on the pedal unguals, making the claws half again as long as the unguals.  Studies of arc measurements show Microraptor plots with climbers, while Archaeopteryx, Confuciusornis and Sinornis plot with perchers, Sinornithosaurus plots between perchers and runners, and Compsognathus and Caudipteryx plot with runners.
Relationships-
The authors performed an 89 character analysis on 13 taxa (using the characters from the Sinornithosaurus paper, plus three new ones) and found two most parsimonious trees.  These show Allosaurus, Compsognathus, ornithomimids, oviraptorids, Caudipteryx, Protarchaeopteryx and troodontids branching off in that order.  At the top are an Archaeopteryx + Rahonavis group and a revised Dromaeosauridae (Microraptor (Sinornithosaurus + Dromaeosaurus + Velociraptorinae)).  This analysis shows several faults including the absence of relevent taxa (Pygostylia, Alvarezsauridae, Unenlagia, Achillobator, Segnosauria), the absence of any proposed characters to support some clades (Oviraptorosauria) and the untested assumption Velociraptorinae as used (Velociraptor + Deinonychus) is monophyletic.  Thus, its conclusions should be seen as preliminary at best.
I performed an analysis with 48 taxa and 310 characters, including most of those used in the authors' analysis.  One thousand eight most parsimonious trees result (CI .36, HI .64, RI .61), which show polytomies in various parts of the dromaeosaurid and non-maniraptoriform coelurosaur areas.  The paravian portion of the tree shows Bagaraatan branching off first, then alvarezsaurids + Avimimus, troodontids, dromaeosaurids, Unenlagia, Rahonavis, Microraptor, Archaeopteryx, Yandangornis and finally pygostylians.  As a side note to those who have read my posts on the extent of the Dromaeosauridae within the Deinonychosauria and the structure of the family in general, Sinornithosaurus and Bambiraptor are the most basal members (and as such, are not dromaeosaurids sensu stricto).  Velociraptor is next, followed by a clade with the remaining members (Dromaeosaurus, Adasaurus, Utahraptor, Deinonychus, Saurornitholestes, Achillobator).  According to this, the latter taxa are dromaeosaurines and have secondarily plesiomorphic character states.  The latter is a conclusion also reached by the authors, although for different reasons.  In any case, my study would suggest Microraptor is not a dromaeosaurid, but instead a basal avialan.  An interesting thing happened while testing the removal of Utahraptor to increase resolution in the dromaeosaurid tree.  In addition to some minor restructuring of non-maniraptorans, the eumaniraptoran tree was changed to support (cue drum roll) secondary flightlessness of dromaeosaurids.  The structure above Troodontidae was ((Yandangornis + Pygostylia) (Archaeopteryx (Rahonavis (Unenlagia, Microraptor, Sinornithosaurus (Bambiraptor + Dromaeosauridae sensu stricto)))))).  In this case, all of the above taxa besides Yandangornis and pygostylians are deinonychosaurs.  This has been suggested by Paul (for Archaeopteryx) and several others on the list (for Rahonavis), and is quite an intriguing possibility.  Holtz's recent talk at SVP on the the effect including taxa has on phylogenetic analyses would seem to be quite accurate.  Obviously, this section of the tree is rather unstable.  This makes accuracy in future studies all that more important.  The identification of new characters, accurate description and coding of characters in known taxa, and discovery of new specimens and species of Eumaniraptora will be integral to identifying the exact relationships of birds closest relatives.  A number of other taxa were removed to test for other potential phylogenies.  Dromaeosaurids were usually primarily flightless, Microraptor was never in the Dromaeosauridae sensu stricto and occasionally a Microraptor + Archaeopteryx + Rahonavis group formed.  Because of the uncertainty surrounding the placement of basal eumaniraptorans, I recommend Microraptor be classified as Eumaniraptora incertae sedis and choose to refrain from listing synapomorphies supporting the unstable subgroups.  Microraptor is most closely related to Archaeopteryx, Rahonavis, Sinornithosaurus and Bambiraptor among eumaniraptorans.
 
That didn't end quite as neaty as I wanted it to, but it shows the need for more study in this area (if that's possible :-) ).  You can bet I'll be busily adding characters and taxa.  Those of you who want figures from the paper, details on my analysis, or just have questions, feel free to ask.  Eoenantiornis, Protopteryx and Fukuiraptor are coming up this week, so stay tuned!
 
Mickey Mortimer