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Avialian synapomorphies



As promised, here are the derived characters with which Gauthier (in his
1986 paper) unites Archaeopteryx with modern birds, outside of all other
theropods (with Gauthier's original clarifiers in parens) [and with my
editorial comments in brackets]:

Premaxillae elongate, narrow, and more pointed anteriorly, with longer nasal
processes [similar condition in bullatosaurs, and in the nondinosaurian
Megalancosaurus, but unique to Archie and later birds in Maniraptora]

Maxillary process of premaxilla reduced so that maxilla participates broadly
in external naris (also in troodontids)

Enlarged brain/basicranium (temporal musculature fails to extend origin onto
frontal bones [to a certain degree in bullatosaurs and therizinosauroids,
and perhaps in some of the newer, "brainier" dromaeosaurids]

Double-condyled quadrate displaced from distal position on opisthotic to
more anteromedial position in contact with prootic (Currie, pers. comm. and
Walker, pers. comm., disagree with Whetstone's interpretation of the
quadrate: Currie notes the anterior displacement of the quadrate in
troodontids [maybe Nino can comment on the condition in Pelecanimimus], and
Walker does not consider the quadrate to be double-condyled in Archaeopteryx)

Maxillary and dentary teeth reduced in size and number (or lost), with
unserrated crowns and enlarged roots that completely enclose replacement teeth
within them [many of the smallest theropod teeth are unserrated, which may
be a developmental constraint.  The pinched roots of bird teeth are similar
to those of troodontids, basal ornithomimosaurs, and (in at least a couple
of cases) dromaeosaurids].

Robust furcula for hypertrophied flight musculature [true to a certain
extent, although oviraptorids and (perhaps) some dromaeosaurids have
substantial furculae as well, and even the new Morrison ?allosaurid seems to
have a primitive furcula!]

Scapula with more or less prominent acromion process for ligamentous
connection to clavicle

Lenght/bredth ratio of scapula at midlength exceeds nine (not in penguins)
and scapula tapers distally

Acrocoracoid tuberosity larger than in other coelurosaurs

Coracoid enlaged and inflected posteromedially more so than in other
coelurosaurs

Very long forelimb and hands (e.g., in Archaeopteryx forelimb is 120-140% of
hindlimb length, and more than twice as long as distance between glenoid and
acetabulum), with forearm more than 87% of humerus length and mcII
approaching or exceeding one-half of humerus length

Ischium compressed and dorsoventrally deep

Compared to other theropods, tibia, fibula, and metatarsals relatively more
elongate with respect to femur, regardless of body size (mts short in
penguins and some other birds)  [Actually, as S. Gatsey and I have shown
elsewhere, what actually is happening is a difference in the allometry of
bird vs nonavian theropod hindlimbs.  In nonavian theropods, the
tibia/fibula and the metatarsus become relatively smaller as body size (and
femur size) increases; in birds, the tibia/fibula and the metarsus become
relatively longer as body size (and femur size) increases.  Unfortunately,
Archaeopteryx lies very close to the points at which the "bird" and
"nonbird" allometric lines cross, so it is not possible to say at present to
which of these curves the Urvogel belongs].

Fibula attenuate distally, and may not extend to end of tibia

Proximal tarsals fused to tibia/fibula and to one another in adults [also in
some ceratosaurs]

Distal tarsals and metatarsals fused at least distally in fully adult
individuals (convergent in some ceratosaurs, elmisaurids, and Hulsanpes)
[future work may show why this is no surprise for Hulsanpes...; also found
in Avimimus]

First pedal digit elongate and reversed (may be reversed in some extant
birds) [the first "reversed" he uses is "reverted"; the second is "reversed"
in the evolutinary sense]

Metatarsal I attached on the distal quarter of metatarsal II

Tail reduced to no more than 23 free caudal vertebrae

Feathers cover limbs and tail, feathers on lateral margin of tail and
posterior margin of arms enlarged, curved, and asymmetrically vaned,
indicating aerodynamic function [While it is true these features are
currently only known in Archaeopteryx and later birds, there is only
negative evidence with regards to this character in other theropods, as
discussed in the paragraph immediately following his character list]

Thus, there are derived features linking Archaeopteryx to modern birds. In
order to falsify this hypothesis, additional analyses must be conducted
which: a) add new characters, or demonstrate that the above are not coded
properly; and b) add new taxa, or demonstrate that the taxa previously used
were not coded properly.  Abundant new discoveries of Mesozoic birds and
new bird-like theropods will help on both these counts.  With several teams
working on just this problem (thankfully not me: nonavian theropods are a
problem enough!), we should get some interesting results.

Later,

Thomas R. Holtz, Jr.
Vertebrate Paleontologist
Dept. of Geology
University of Maryland
College Park, MD  20742
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Fax: 301-314-9661
Phone:301-405-4084