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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
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