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Details on Yandangornis



Well, I have some free time this weekend to write a details segment.  Who would have thought?
 
Yandangornis Cai and Zhao 1999
Y. longicaudus Cai and Zhou 1999
Etymology- "long-tailed bird from Yandang Mountain", after where the fossil was found.
Santonian, Late Cretaceous
Tangshang Group, Zhejiang, China
Holotype- (Zhejiang Museum of Natural History, M1326) (588 mm) skull (47 mm), lower jaws, nine cervical vertebrae (80 mm), four dorsal vertebrae, dorsal ribs, gastralia, nineteen caudal vertebrae (305 mm), sternum (50 mm), sternal ribs, distal scapula, partial coracoid, partial furcula, humeri (80 mm), proximal radii, proximal ulnae, distal phalanx II-1, phalanx II-2, manual ungual II, distal phalanx III-2, phalanx III-3, manual ungual III, pubis (41 mm), femora (106 mm), tibiae (132 mm), fibula (40 mm), tarsometatarsus (70 mm), pedal phalanges and unguals
Diagnosis- very large premaxillae (ventral edge equals 40% of skull length); premaxillae, maxillae and dentaries toothless; lower jaw less than three-fourths skull length; last cervical vertebra longer than others; no cervical ribs?; less than six caudal vertebrae with transverse processes; no chevrons?; sternal ribs subequal to sternum in length; sternum concave posteriorly; distal femoral condyles more than twice as wide as shaft; phalanx III-1 shortest in third pedal digit.
Description-
This specimen was discovered in 1986, associated with the pterosaur Zhejiangopterus linhaiensisi.
The skull is preserved in ventral view, with the lower jaws articulated.  It is elongate with a pointed rostral tip and subparallel lateral edges.  The premaxillae are toothless and large, even more extensive than confuciusornithids.  A broad premaxillary palatal shelf seems to be present.  The maxillae are also toothless and show a large antorbital fenestra.  Other preserved elements include jugals, quadrates, pterygoids and the basisphenoid, although no details are discernable.  The articulated (fused?) dentaries form an acute angle and end 7 mm behind the rostral premaxillary tip.  The dentaries are toothless as well and the retroarticular process appears short.
The nine cervical vertebrae are amphicoelous and seem to lack ribs.  They lengthen posteriorly from three to eleven millimeters.  No details are visible regarding the preserved dorsal vertebrae (two mid-dorsals, two posterior dorsals).  Dorsal ribs and gastralia are also preserved.  Nineteen caudal vertebrae are preserved, with at least one missing at the tip.  The centra are amphicoelous and the first five have transverse processes.  Vertebrae become very slender after the fifth.  There are no dromaeosaur-like elongate prezygopophyses and no pygostyle.  No chevrons are preserved and are claimed to have been originally absent.
The coracoid is preserved articulated to the sternum anteriorly and is described as being "similar to other Mesozoic birds, especially the Cretaceous birds".  This could suggest it was strut-like, but the figure is unclear regarding this point.  The distal scapula is said to be elongate and straight.  A furcular fragment is preserved, possibly the left distal end.  The sternum is fused and twice as long as wide.  It is convex anteriorly, with paired triangular lateral processes, subparallel lateral margins behind them and concave posteriorly with a short median process.  It lacks a keel, but has three pairs of elongate sternal ribs attached behind the lateral processes.
The humerus is sigmoidal, with a low proximally placed deltopectoral crest and no pneumatic fossa.  The distal condyles are distinct and a shallow olecranal fossa is present on the ulnar condyle.  The radius is two-thirds as wide as the bowed ulna.  There is a slight olecranon.  The phalanges are slender, with digit III reaching to the tip of II-2.  Manual unguals are reduced.
The pubis is preserved in an opisthopubic position and tapers distally.  It was not fused to the other pelvic elements.  The authors state it lacks an "anterior process", perhaps the anterior pubic foot?
The femur is slender and straight, with a trochanteric crest and horizontal or slightly inclined head.  There is a slight neck and no fourth trochantor or posterior trochantor.  The distal end is greatly expanded.  The tibia is fused to the astragalus and calcaneum.  No fibular crest is visible, the distal end is expanded more than the proximal end.  The fibula is only 30% of the tibial length.  The tarsometatarsus is non-arctometatarsalian, with the proximal end of metatarsal III actually wider than the distal end.  Fusion is extensive, but incomplete in the distal tenth.  It is elongate and slender, close to Tochisaurus in these regards.  Metatarsal II is more robust than metatarsal IV, and is slightly shorter.  Proximally, the tarsometatarsus has two shallow cotylae.  Digit I is set 20% up the shaft of metatarsal II and is not reversed.  The ungual, like the others, is small and almost straight.  The ungual on digit II is slightly longer than the others.  Digit II is not modified for predatory use in any way.  Phalanx II-1 is much shorter than III-1, like most paravians.  Phalanx II-2 is longer than II-1, as in some eumaniraptorans.
The humeral length suggests this species was flightless, while the elongate hindlimbs and small sightly curved pedal unguals suggest it was cursorial, but not a good percher.
Relationships-
The authors refer Yandangornis to its own family (Yandangithidae) and order (Yandangithiformes) within the Sauriurae.  It should be noted that the family and order names are formed incorrectly (should be Yandangornithidae and Yandangornithiformes), but ammendment is uneccessary under the new ICZN rules.  As the Sauriurae is based on symplesiomorphies, its relationships must be reconsidered.
Yandangornis' relations are obviously to be found within the Eumaniraptora.  Unfortunately, the topology of this clade is unstable, as several taxa (Archaeopteryx, Sinornithosaurus, Unenlagia, Microraptor, Rahonavis) could be basal deinonychosaurs or basal avialans.
It is more derived than Archaeopteryx based on- less than eight caudal vertebrae with transverse processes; trochanteric crest; fibula does not reach calcaneum; astragalocalcaneum indistinguishably fused with tibia; proximal end of metatarsal III greater than 75% as wide as distal end.  Rahonavis also has characters 2 and 3 of that list, but lacks 1, 4 and 5.  Microraptor is known to lack 3 and 5, while Sinornithosaurus lacks 3, 4 and 5.  It seems clear Yandangornis is closer to pygostylians than any of the aforementioned controversial eumaniraptorans.  It is less derived than pygostylians based on- cervical centra not heterocoelous; more than twenty caudal vertebrae; absence of pygostyle; absence of sternal keel.  On the other hand, it is more derived than confuciusornithids based on- olecranal fossa; phalanx II-2 longer than II-1?; reduced manual unguals.  I consider the former possibility more likely not only because it is supported by more characters, but also because the phalangeal lengths are based on the skeletal reconstruction and the digit is broken proximally.  However, I don't think a placement above confuciusornithids can be ruled out at this point.  There are no features indicating it is more derived than ornithothoracines.  It is less derived based on- gastralia present; humeral pneumatic fossa absent; pelvis unfused.  In conclusion, I think Yandangornis is an avialan more closely related to pygostylians than Archaeopteryx, Rahonavis and Microraptor, but is not an ornithothoracine.  It is probably less closely related to ornithothoracines than confuciusornithids are.
reference- Cai Zhengquan and Zhao Lijun (1999).  A long-tailed bird from the Late Cretaceous of Zhejiang.  Science in China (Scientia Sinica) Series D: 42(4): 434-441.
 
If you want scanned figures of Yandangornis (skeleton, skull/neck, sternum, humerus, femur, tibiotarsus, pes; 2 pages total), just ask.  Some great details segments are coming in the future (Jeholosaurus, Kuszholia, Chuandongocoelurus, Abrosaurus, Gasosaurus, Lingyuanornis, etc.), so stay tuned!
 
Mickey Mortimer