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Now that a fairly good illustration of the
pygostylian portion of Archaeraptor has been published, its relationships can be
examined.
undescribed Pygostylian (Archaeoraptor
liaoningensis sensu Sloan in partim)
Barremian, Early Cretaceous
Yixian Formation, Liaoning, China
Material- (IVPP collection) partial skull including
premaxilla, nasal, frontal and parietal, partial mandible including dentaries,
several cervical vertebrae, a few dorsal vertebrae, dorsal ribs, uncinate
processes(?), anterior synsacrum(?), scapulae, coracoid, furcula, sternum,
humeri, radii, ulnae, semilunate carpal,ulnare, proximal metacarpal I, phalanx
I-1, manual ungual I, incomplete metacarpal II, phalanx II-1, manual ungual II,
metacarpal III, partial ilium(?), partial ischium(?), femur, proximal
tibia
Diagnosis-
coracoid basal transverse width more than 70% of
proximodistal length
Several other characters could be added, such as
the strange acromial structure (see below), but this should wait until elements
are identified with certainty.
Description-
This specimen was originally part of the holotype
of Archaeoraptor liaoningensis, as controversially announced by National
Geographic (Sloan, 1999). However, several details let this pass as an
official description and the species was nomen nudum. This holotype was
later shown to be composed of several specimens artificially combined together,
one of which is the counterpart of the eumaniraptoran Microraptor
zhaoianus. Although there is further controversy regarding which specimen
will receive the name (Olson designated the tail, counterpart of Microraptor
zhaoianus, as the holotype), I doubt Archaeoraptor will be the final name for
the pygostylian when Czerkas describes it. Thus, it is referred to as
"Archaeoraptor" below.
The skull is low and pointed, with toothed
jaws. The premaxilla is acutely pointed with a slightly posteriorly
shifted naris (comparable to enantiornithines). The nasal is slender and
straight. A large posterior element may be a frontal. A smaller
element posterior to this may be a parietal. The dentaries are thin and
straight, some posterior mandibular elements are
also preserved.
Cervical vertebrae are elongate, but poorly
illustrated. An isolated dorsal vertebra has large pleurocoels, like many
basal pygostylians. Several dorsal ribs are preserved, with a possible
uncinate process. An irregular, elongate, tapering element in the pelvic
area may be a synsacrum.
An elongate element overlapping the coracoid may be
a scapula. It is curved, with a pointed distal end and an ?anterodorsal
process that may be the acromion. The only other possible identification
for this element is a second metacarpal. The amount of curvature and
?acromion process argue against this however. Another incomplete, but
similar, process with no obvious acromion is near certainly a scapula. The
coracoid is strut-like with concave lateral and medial margins. The
ventral margin is more expanded than most pygostylians. The furcula is a
narrow V-shape (~48 degree interclavicular angle) with gently curved rami and no
elongate hypocleidium. The sternum is very narrow, slightly expanded
posteriorly with a ventral keel. Two pairs of posterior processes are
present, an anterolateral one with a distal expansion and a small pointed
posteromedial one. Both anterior and posterior margins are pointed.
The humeri are both well preserved. They are
expanded proximally and distally in the same plane and lack the large
deltopectoral crest of confuciusornithids. The distal end lacks the
projected internal condyle of at least some enantiornithines (Alexornis,
Enantiornis, Kizylkumavis), the internal tubercle is pointed. The straight
radius is much more slender (~65%) than the bowed ulna. Oddly, the
semilunate carpal and ulnare are unfused to each other or the metacarpus.
A thin radiale may also be present. The metacarpal doesn't appear
fused. The first metacarpal is narrower than the second, the third is
narrower than both. Intermetacarpal space is limited. The third
metacarpal is subequal to the second in length. Digit I has a small curved
ungual and projects slightly past metacarpal II, if the digits are correctly
articulated (a break occurs through the metacarpus). Phalanx II-1 is
flattened and expanded mediolaterally. A small second ungual may be
present as well, subequal in size to the first.
Possible ilial and ischial remains are preserved
(the latter with a possible proximodorsal process). The femur is bowed,
but lacks recognizable details. A proximal tibial fragment is also
present.
Relationships-
Protopteryx+Jibeinia+Ornithothoraces
- tapered distal scapula?
- mobile scapulocoracoid joint
- strut-like coracoid
- interclavicular angle of furcula 70 degrees or
less
- ulna subequal or longer than ulna
- reduced manual unguals
basal to Jibeinia+Ornithothoraces
- manual digit I passes metacarpal II
Ornithothoraces
- interclavicular angle 50 degrees or
less
- sternal carina extends to anterior
edge
not enantiornithine
- coracoid not laterally convex
- hypocleidium not elongate
- metacarpal III does not extend past metacarpal
II
Thus, the "Archaeoraptor" specimen is obviously
more derived than confuciusornithids, but further relationships are difficult to
discern. There are a few characters that have a more complex distribution
than normally thought. The anteriorly extensive sternal carina is usually
thought to be a euornithine character, some enantiornithines exhibit this
condition (Largirostrornis, Neuquenornis), although most lack it (Cathayornis,
Concornis, Eoalulavis). Still, only ornithothoracines are known to have
it, so it is evidence "Archaeoraptor" was a member of that clade.
Similarily, the carpometacarpus has a very complex distribution, with
differences in which metacarpals and carpals are fused, and whether or not they
are fused distally. However, at least some enantiornithines (Sinornis,
Cathayornis? caudatus, Longchengornis?) have unfused metacarpals, indicating the
presence of the plesiomorphic condition in "Archaeoraptor" doesn't bar it from
the Ornithothoracines. The interclavicular angle is narrower than
Protopteryx and Jibeinia, which is another piece of evidence "Archaeoraptor"
might be an ornithothoracine. However, the elongate first manual digit
suggests it is more basal than Jibeinia or ornithothoracines. Whatever it
is, it does not appear to be enantiornithine. More detailed examination of
this specimen once Czerkas describes it will undoubtedly reveal further details
that can help determine its affinities.
Those who want a figure of the pygostylian section
of Archaeoraptor liaoningensis sensu Sloan can contact me offlist.
References
Sloan, C. P.,
1999. "Feathers for T. rex?" National Geographic 196(5): 98–107 [November
1999].
Rowe, Ketcham, Denison, Colbert, Xu and Currie,
2001. Forensic palaeontology: The Archaeoraptor forgery.
Nature 410, 539 - 540 (29 March
2001).
Mickey
Mortimer
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