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evolved independently twice." Martin expanded on his thesis
that the structure of the metatarsals and distal tarsals was essentially
different in Enantiornithes and Ornithurae: "In Archaeopteryx
and the enantiornithine birds, the proximal end of
the tarsometatarsus fuses; the distal end, however, does not.
This is tme even of Maastrichtian Enantiornithes." Chiappe
pointed out that this was tme "except for Avisaurus gloriae
[see Varricchio and Chiappe, 1995] from the [Campanian]
Two Medicine Formation, which has some fusion [distally]."
Martin continued, "In all modem birds without exception, the
metatarsal bones begin to fuse distally, and this fusion then
moves forward to the proximal articulation. ... Modem birds
built an epiphysis; that epiphysis is created by one or more
distal tarsals,...but it makes a cap. ...The proximal end of
metatarsal III is wedge shaped.... In Archaeopteryx and Enantiornithes
the metatarsal bones are together in a row, and they
don't build a tarsal cap;.. .you can literally follow the metatarsals
up, look at the proximal end of the articulation and see the
ending.... So my argument is that indeed Archaeopteryx and
the enantiornithine birds and modern birds all have fused
metatarsal bones, but the way they put together the ontogenetic
constraints are different." He then added with emphasis, "I
would call this a fundamental way to discover convergence,"
and continued, "In all modern ornithurine birds...there is a
single prominence from the ischium.... In Archaeopteryx and
all the enantiornithine birds you get a double prominence,... a
little square thing that comes up... and then there is a little triangular
process behind that. ... The triangular process is homologous
with the stmcture in ornithurine birds, the other
structure is not found in ornithurine birds, it is found in all
enantiornithines."
After Martin's arguments, E. Kurochkin followed in the
same vein, reaffirming the metatarsal thesis as well as arguing
that the articulation between the coracoid and scapula is different
(reversed) in the Enantiornithes and Ornithurae. Unfortunately,
by that point time had mn out, and there was no possibility
of rebutting on morphological grounds. The interested
reader can find a specific analysis of the evidence in support
of and against the monophyly of the Sauriurae in Chiappe
(1995b), or can simply analyze the character distribution provided
in various cladistic analyses (e.g., Cracraft, 1986; Chiappe
and Calvo, 1994; Sanz et al., 1995; Chiappe et al., 1996).
The discussion was closed by C. Forster and S. Peters. Forster
presented a new, spectacular specimen from the Late Cretaceous
of Madagascar that combines an ulna with quill knobs,
a long tail, and a typical dromaeosaur/troodontid, sickleclawed
digit II of the foot (see Forster et al., 1996b). Peters
showed a specimen of Confuciusomis (recently acquired by the
Senckenberg Museum in Frankfurt) that proves that the tail of
this early bird was not long (as reconstmcted by Hou, Zhou,
Martin et al., 1995) but short, ending in a pygostyle (see Peters,
1996).
SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
Concluding Remarks
Although the roundtable discussion was played out in an
arena of cordiality, it was evident that the different methodological
approaches of the participants (cladists versus noncladists)
were clouding the debate on the interpretation of the actual
evidence.
The methodological miscommunication was more apparent
when analyzing the phylogenetic position of Mononykus and
the monophyletic status of the Sauriurae. A great many of these
misunderstandings appeared to center around the criteria used
toormulate and test homology and the way in which phylogenetic
statements are justified. For example, the hesitation in accepting
Mononykus as a bird appears to stem more from the
fact that its overall aspect (most notably its forelimbs) and its
presumed fossorial life style (e.g., Ostrom, 1994; Zhou, 1995a)
are at odds with the stereotypical view of a bird than from the
critical evaluation of the distribution of anatomical characteristics
among taxa. As has been shown by several researchers
(e.g., Perle et al., 1993; Chatterjee, 1995; Chiappe et al., 1996;
Forster et al., 1996a; Novas, 1996), cladistic analyses that have
used complete data sets have concluded that, in contrast to any
initial intuition, Mononykus is closer to modem birds than is
Archaeopteryx. Clearly, those arguing against the hypothesis of
avian affinities were understanding homology as being validated
by overall similarity (both morphological and functional)
rather than by congruence of derived characters (see Hall,
1994; Shubin, 1994, for a discussion of the homology concepts).
These different approaches to the concept of homology
were best portrayed by Martin. After remarking upon the different
ontogenetic pathways of Archaeopteryx and Enantiornithes
(proximal to distal metatarsal fusion) on the one hand and
the ornithurine birds on the other (distal to proximal metatarsal
fusion) in his defense of the monophyly of the Sauriurae, he
emphatically declared, "I would call this a fundamental way to
discover convergence." Again, the conflict between different
homology concepts ("biological homology" versus "phylogenetic
homology"; see Shubin, 1994) becomes apparent. Martin
prefers to assume the convergent evolution of the flight apparatus
(among other features) in the Sauriurae and Ornithurae over
the equally possible alternative of similar developmental constraints
evolving independently in Archaeopteryx and the
Enantiornithes.
We are living in an exceptional period of discovery. With
several early birds being described every year, new ideas are
being formulated at a pace that exceeds our ability to blend
them into a theory structured over this burst of new evidence.
Methodological miscommunication stands as another obstacle
in this process of assimilation. Clearly, fmitful discussions
such as this roundtable, along with a better understanding of
the methodological differences between us, can put us one
step closer to the most exciting goal of reaching a sound, comprehensive
theory of the early evolution of birds.