Mei long (IVPP V12733, Xu and Norell 2004, Early Cretaceous, 130 mya, Figs. 1-3) is famous for its 3D preservation in a curled up sleeping posture. Originally considered a young juvenile, bird-like troodontid, Mei long instead nests in the large reptile tree between the Munich specimen of Archaeopteryx BSp 1999 I 50 and Scansoriopteryx along with other scansoriopterygid basal birds. A second specimen, DNHM D2154 (Gao et al. 2012), was also preserved in a sleeping posture.
Wikipedia reports, “There are multiple possibilities of the genera included in Troodontidae as well as how they are related.” Adding to this problem, in the large reptile tree several taxa sometimes included in the Troodontidae instead nest sequentially basal to birds (Archaeopteryx), not in a single offshoot clade.
From Xu and Norell (2004):
“Mei long is distinguishable from all other troodontids on the basis of extremely large nares extending posteriorly over one half of the maxillary tooth row*; closely packed middle maxillary teeth; maxillary tooth row extending posteriorly to the level of the preorbital bar”; a robust, sub-‘U’-shaped furcula*; presence of a lateral process on distal tarsal IV; and the most proximal end of the pubic shaft is significantly compressed anteroposteriorly* and extends laterally just ventral to the articulation with the ilium*.”
*These happen to be traits found in sister taxa, the Munich Archaeopteryx and/or Scansoriopteryx.
One branch of basal birds, the scansoriopterygids (Fig. 3), famous for their long third finger, now includes a new sister, Mei long.
The orbit is comparatively large in Mei long and several cranial and vertebral features are unfused. Gao et al. 2012 report, “Although the skeleton exhibits several juvenile-like features including free cervical ribs, unfused frontals and nasals, and a short snouted skull, other attributes, full fusion of all neurocentral synostoses and the sacrum, and dense exteriors to cortical bone, suggest a small, mature individual. Microscopic examination of tibia and fibula histology confirms maturity and suggests an individual greater than two years old with slowed growth. Despite being one of the smallest dinosaurs, Mei long exhibits multi-year growth and cortical bone consisting largely of fibro-lamellar tissue marked by lines of arrested growth as in much larger and more basal theropods.”
Distinct from its new sister taxa
Mei has shorter forelimbs and longer hind limbs. It is also a little larger even if not fully grown. Pedal digit 3 is much longer. The sacrum is much wider. The facial bones are much more gracile. The jugal may not have had an ascending process. Some of these are indeed juvenile traits that may have been retained into adulthood. Such fragility may have contributed to its general lack of fusion (less bone, lighter weight, but not for flying despite being (no doubt, but not preserved) fully feathered. Metatarsal 3 appears to be pinched between 2 and 4. Pedal 2.1 is less than half the length of p2.2 and pedal ungual 2 is quite long, but not tightly curved.
Mei to any node prior to Archaeopteryx currently and provisionally adds at least 12 steps.
Xu X and Norell MA 2004. A new troodontid dinosaur from China with avian-like sleeping posture. Nature 431:838-841.
Gao C, Morschhauser EM, Varricchio DJ, Liu J, Zhao B 2012. Farke AA ed. “A Second Soundly Sleeping Dragon: New Anatomical Details of the Chinese Troodontid Mei long with Implications for Phylogeny and Taphonomy”. PLoS ONE 7 (9): e45203. doi:10.1371/journal.pone.0045203. PMC 3459897. PMID 23028847.