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Early Avian Evolution: Roundtable Report
Introduction
Few areas of vertebrate paleontology have advanced more
over the last few years than that of the early evolution of birds.
Recent findings of primitive, non-neomithine birds have been
so numerous that we have more than doubled the number of
valid taxa described between 1861, when the first early bird,
Archaeopteryx lithographica von Meyer, was reported, and
1990. Thus, to address the plethora of new ideas and discussions
that all these new findings have triggered, in the single
hour of roundtable discussion that I had been assigned to moderate,
was a daunting, if not impossible, task. With this in
mind, and after discussing possible topics of debate with other
colleagues, I decided to center the discussion on only three topics
within this new profusion of evidence.
The aim of this report is not to provide a review of the new
data on early bird evolution, nor is it to defend my own views
over those of others. Much of the new evidence has already
been reviewed, and a variety of choices are available for the interested
reader. Wellnhofer (1994) and Feduccia (1996) provide
reviews based on a traditional "evolutionary" approach,
whereas I have reviewed the new data from a strict cladistic
perspective (Chiappe, 1995a).
Discussion Topics
Before going into the actual debate at the roundtable, it
would be helpful to provide a general overview of the three
topics that were discussed.
1. PHYLOGENETIC RELATIONSHIPS AND SIGNIFICANCE OF
Mononykus.—Mononykus olecranus was first reported by A.
Perle, M. Norell, L. Chiappe, and J. Clark on the basis of a partial
specimen from the Late Cretaceous Nemegt Formation of
southern Mongolia (Perle et al., 1993). This flightless, turkeysized
animal, with short, stout forelimbs instead of wings, was
regarded as phylogenetically closer to modem birds than is Archaeopteryx,
and it was thus interpreted as a bird (Perle et al.,
Luis M. Chiappe, Department of Vertebrate Paleontology, Natural
History Museum of Los Angeles County, 900 Exposition Boulevard,
Los Angeles, California 90007, United States.
Luis M. Chiappe
335
1993, 1994; Norell et al., 1993; Chiappe, 1995a; Chiappe et al.,
1996a). Placement of the bizarre Mononykus within birds
raised significant debate. Opponents expressed their views both
in scientific journals and books (e.g., Patterson, 1993; Ostrom,
1994; Wellnhofer, 1994; Martin, 1995a; Zhou, 1995a; Feduccia,
1996) and in popular magazines and newspapers (e.g., Feduccia,
1994; Martin and Rinaldi, 1994; see also Norell et al.,
1993; Chiappe et al., 1995, 1996, 1997, for responses to these
criticisms), but with the exception of L. Martin, who regarded
Mononykus as a bizarre ornithomimid (Martin and Rinaldi,
1994; Martin, 1995a), critics of the avian hypothesis have not
proposed an alternative, specific hypothesis of relationships.
Moreover, proponents of the avian relationship of Mononykus
found additional support for their views in enlarged cladistic
analyses (Chiappe et al., 1996; Forster et al., 1996a) that include
data on new specimens (some preserving nearly complete
skulls) from the Mongolian Djadokhta-like beds of Ukhaa
Tolgod (Dashzeveg et al., 1995) and from close relatives of
Mononykus found in southern South America (Novas, 1996).
In addition, this hypothesis received support from the work
done by colleagues performing independent cladistic analyses
(e.g., Chatterjee, 1995; Novas, 1996).
Another topic of discussion surrounding Mononykus concerns
its life style, namely, whether its short, robust forelimbs
were used for digging (e.g., Ostrom, 1994; Zhou, 1995a) or for
other activities (Norell et al., 1993; Chiappe, 1995b). Although
this appears to be a more trivial issue, it has been used as an argument
against the hypothesis of avian relationships. For example,
Z. Zhou interpreted several of the characters used to
support the placement of Mononykus within birds as the result
of digging adaptations (Zhou, 1995a), concluding that a digging
animal cannot be a bird.
2. THE AGE OF Confuciusomis.—For more than a century,
and with the only exception being a "feather" of controversial
origin (see Bock, 1986), Archaeopteryx lithographica stood
alone as the oldest and only known Jurassic bird {Protoavis is
left outside this discussion because its avian nature still needs
to be confirmed; see Ostrom, 1987, 1996; Chiappe, 1995a). In
1995, L. HOU, Z. Zhou, L. Martin, and A. Feduccia reported on
a Chinese bird, Confuciusomis sanctus, from lacustrine deposits
of the Yixian Formation in the northwestern Liaoning Prov-