[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

The nearbird

That's what *Anchiornis* means. And, sitting behind a university proxy server here in Vienna, I have full online access.

Xu Xing, Zhao Qi, Mark Norell, Corwin Sullivan, David Hone, Gregory Erickson, Wang Xiaolin, Han Fenglu & Guo Yu: A new feathered maniraptoran dinosaur fossil that fills a morphological gap in avian origin, Chinese Science Bulletin, 6 pages, accepted November 15, 2008, published online sometime later


Recent fossil discoveries have substantially reduced the morphological gap between non-avian and avian dinosaurs, yet avians including *Archaeopteryx* differ from non-avian theropods in their limb proportions. In particular, avians have proportionally longer and more robust forelimbs that are capable of supporting a large aerodynamic surface. Here we report on a new maniraptoran dinosaur, *Anchiornis huxleyi* gen. et sp. nov., based on a specimen collected from lacustrine deposits of uncertain age in western Liaoning, China. With an estimated mass of 110 grams, *Anchiornis* is the smallest known non-avian theropod dinosaur. It exhibits some wrist features indicative of high mobility, presaging the wing-folding mechanisms seen in more derived birds and suggesting rapid evolution of the carpus. Otherwise, *Anchiornis* is intermediate in general morphology between non-avian and avian dinosaurs, particularly with regard to relative forelimb length and thickness, and represents a transitional step toward the avian condition. In contrast with some recent comprehensive phylogenetic analyses, our phylogenetic analysis incorporates subtle morphological variations and recovers a conventional result supporting the monophyly of Avialae.

"Locality and horizon" are given as "Yaolugou locality, Jianchang county, western Liaoning, China; lacustrine deposits of uncertain Jurassic-Cretaceous age." The first keyword is "Early Cretaceous", but that's it; there is no further information on stratigraphy or age. (Even though the paper is in the "Geology" section of the journal.) I suppose the locality was newly discovered and hasn't yet been correlated to anything.

The head, the end of the tail, and the right forearm & hand of the only known specimen are not preserved, but the rest of the skeleton is almost completely articulated, so the absence of a sternum (and uncinate processes, not mentioned in the paper) can probably be taken at face value and is compared to troodontids and *Archaeopteryx*. "Preserved soft tissues include horny sheaths around the pedal unguals, and patches of feathers. Specifically, extremely faint carbonized feather impressions are preserved dorsal to the presacral vertebral column and average about 20 mm in length, but their detailed structure is not clear."

Despite being tiny (total length estimated at 34 cm), the individual was almost or completely adult, as shown by its fused neurocentral sutures and by bone histology (yes, they cut up one of the femora). That makes it the smallest nonavian dinosaur so far, for some values of "nonavian" at least.

The forelimb is about 80 % as long as the hindlimb, the humerus is as long and thick as the femur, and the hand is about 130 % as long as the femur. The authors therefore speculate that *Anchiornis* might have been able to fly or perform some other activity that requires a "functional airfoil".

The tibiotarsus is 150 % the length of the femur. The first toe is preserved lying medial to metatarsal II. The metatarsals are not fused and are subarctometatarsal. The 2nd claw is considerably bigger than the others, but has the same shape; the 2nd toe is shorter than the 4th, even though their metatarsals end at the same level.

The semilunate carpal is laterally positioned so that it has little contact with metacarpal I. Metacarpals II and III seem to be fused at their proximal ends. "Manual phalanx II-1 is much thicker than all of the other manual phalanges and nearly as thick as the ulna."

Phylogenetic analysis from either the original *Microraptor paper* or the *Buitreraptor* paper -- the text contradicts the figure legend! -- plus 10 taxa and 48 characters:

 |  `--Aves [sensu Sereno]
 |       |--*Archaeopteryx*
 |       `--+--*Rahonavis*
 |          |--"*Jeholornis*"
 |          `--+--+--*Sapeornis*
 |             |  `--*Protopteryx*
 |             `--+--*Confuciusornis sanctus*
 |                `--+--*Apsaravis*
 |                   `--*Yixianornis*
    |  `--*Unenlagia*
    |--+--*Sinornithosaurus millennii*
    |  `--+--*Microraptor zhaoianus*
    |     `--*Graciliraptor*
    |  |  `--*Mei*
    |  `--+--"EK troodontid"
    |     |--*Jinfengopteryx*
    |     `--+--*Byronosaurus*
    |        |--*Sinornithoides*
    |        `--+--*Troodon*
    |           `--+--*Saurornithoides mongoliensis*
    |              `--*Saurornithoides junior*
       |  |--*Velociraptor mongoliensis*
       |  `--*Tsaagan*

Nice to see confirmation for the position of *Jinfengopteryx*. Strange to see that *Rahonavis* has jumped back (this is part of what the last sentence of the abstract means by "monophyly of Avialae"). Bizarre to see *Protopteryx* as the sister-group of *Sapeornis*. Frustrating to see the polytomy that is Deinonychosauria.

I have no idea what the outgroup was; the authors don't say. In the figure legend, the corresponding author (Xu) asks to be contacted "for detailed information of the phylogenetic analysis".

"Wrist characters account for a significantly higher percentage of synapomorphies diagnosing the Avialae than either the Paraves or Aves, suggesting rapid evolution of the carpal region at the base of Avialae". Avialae evidently means Aves + *Anchiornis*, and Paraves has to mean the entire shown tree...

Anyway, *Anchiornis* is a great way to begin the new year!