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Enantiornithes: Earlier Birds than Archaeopteryx?
ABSTRACT
The oldest known remains of the Enantiornithes come from the
Early Cretaceous of Spain and northeast China. They represent
birds capable of flight, although it was not efficient enough to
enable them to fly over the Turgai Strait, which at that time separated
the eastern and western parts of the present-day Palaearctic.
A comparison of the coastlines of the continents in consecutive
epochs of the Jurassic and Cretaceous suggests that in order to
spread by land over all of Eurasia, both Americas, and Australia,
the Enantiornithes would have had to differentiate at the latest by
the Middle Jurassic (Bajocian), or about 25 million years before
the period from which Archaeopteryx is known (Tithonian).
Introduction
Remains of Cretaceous birds of the subclass Enantiornithes,
as described by Walker (1981), are known from many localities
in North and South America, Europe, Asia, and Australia
(Bocheriski, 1997) (Figure 1). The earliest remains may be
the European representative, Nogueromis gonzalezi Lacasa
from Spain, which has been referred to a period between the
upper Berriasian and lower Valanginian (Lacasa, 1989). The
remains of Sinornis santensis Sereno and Rao, Cathayomis
yandica Zhou, Jin, and Zhang, and Boluochia zhengi Zhou
from northeastern China come from the Valanginian (Sereno
and Rao, 1992; Zhou, 1995a, 1995b). Three other Spanish
species, Iberomesomis romerali Sanz and Bonaparte (1992),
Concomis lacustris Sanz and Buscalioni (1992), and Eoalulavis
hoyasi Sanz et al. (1996), are younger by several million
years. The remaining Enantiornithes, from Asia (Mongolia
and Uzbekistan), North and South America, and Australia
have been obtained from deposits representing a period between
the Albian and the Maastrichtian (Molnar, 1986; Chi-
Zygmunt Bocheriski, Institute of Systematics and Evolution of Animals,
Polish Academy of Sciences, Slawkowska 17, 31-016 Krakow,
Poland.
Zygmunt Bocheriski
285
appe, 1991, 1993; Dong, 1993; Lamb et al., 1993; Kurochkin,
1995b, 1996; Martin, 1995).
When describing particular forms included in this subclass,
authors have paid attention to the characters indicative of active
flying. Martin (1995) emphasized that, unlike the state observed
in Archaeopteryx, the stmcture of the shoulder girdle
and the possession of a keeled sternum indicate that these birds
were able to rise from flat surfaces. On the other hand, the sternum
and, especially, the carina stemi were proportionally very
small, pointing to restricted flight efficiency that did not permit
the birds to cover long distances. Poor powers of flight characterized
all Sauriurae, in contrast to contemporary Ornithurae
(Zhou, 1995c). The geographic situation of the earliest Early
Cretaceous localities (Figure 2) as seen against a background
of the paleocoastlines at that time (Smith et al., 1995) shows
that Enantiornithes occurred on both sides of the Turgai Strait,
which then was at least several hundred kilometers wide and
constituted a substantial obstacle for terrestrial vertebrates, as
pointed out earlier by Kurten (1967-1970). Naturally, some
cases of passive crossing of this barrier cannot be excluded, although
they seem unlikely. The Turgai Strait existed uninterruptedly
for many millions of years, from the Callovian to the
Aptian (Smith et al., 1995).
Protoavis texensis Chatterjee is considered to be a bird by
some authors (e.g., Chaterjee, 1991, 1994; Kurochkin, 1995b).
Its detection in the Upper Triassic layers (Chatterjee, 1991,
1994) and the presence of tme avian forms by the latest Jurassic
(Hou, 1995) indicate that, despite the lack of direct evidence,
the differentiation of birds occurred in the Jurassic. The
differentiation of the European and East-Asiatic Early Cretaceous
Enantiornithes on both sides of the Turgai Strait into
rather high systematic units (i.e., orders and families according
to Martin, 1995), or into various genera (Kurochkin, 1996), occurred
independently under relatively stabilized biotopic conditions,
and so they must have been the result of a long-lasting
evolutionary process. Thus, it seems plausible that the Enantiornithes
separated and spread in the Bajocian, 166-171 Ma BP
(Haq and Van Eysinga, 1987), when it would still have been
possible for them to spread over all the continents by land
(Smith et al., 1995). European sea straits at that time were narrow
and so could have been crossed much more easily than the