3 The New York Years (1931–1953)

178 4 Ornithologist and Zoogeographer
1972c, 1990b). Thus most of the Australian fauna developed through “continued
single origin colonization,” that is the steady and continuous colonization from the
northwest (1965q). Corresponding to their differentiation and relative age, Mayr
distinguished five major layers of colonists between which there are, of course, no
clear cut discontinuities (1944k):
(a) Strongly endemic families and subfamilies, about 15,
(b) Less isolated indigenous families and subfamilies, about 8,
(c) Endemic genera of non-endemic families, about 30,
(d) Endemic species of non-endemic genera, about 60,
(e) Recently immigrated species, not differentiated at all, or only subspecifically,
about 40.
The oldest layer comprising, besides emu and cassowary, the Megapodiidae,
Anseranas, Pedionomus and perhaps Stictonetta, probably represents Gondwana
elements, less than 3% of the bird fauna (Mayr 1990b). Australia split from Antarctica
about 50 million years ago. At that time Australia must have had a fairly rich
Gondwana bird fauna, but owing to circumstances that are not yet fully explained,
this older fauna became almost completely extinct and was replaced during the
Tertiary by immigrants from Asia. The ancestors of the Corvida immigrated into
Australia ca. 30 million years ago and radiated into the characteristic families of
Australian songbirds.
An island archipelago probably existed between eastern Asia and Australia
during the Mesozoic at least since the early Cretaceous and during the entire
Cenozoic (Tertiary-Quaternary). This archipelago probably included small drifting
continental plates (split off northern Gondwanaland) which today form portions
of Burma, Malaya, Borneo, Sumatra, Java and western Sulawesi; portions of what
later became Timor and New Guinea were located off northern Australia (Audley-
Charles 1987). The islands of this early Southeast Asian archipelago probably
served as stepping stones for Asian immigrants into Australia as well as an area of
intensive speciation and faunal differentiation during the Cretaceous and Tertiary.
Many animal groups originated here which today form part of the rich fauna of the
Malay Archipelago and the New Guinea region. The last faunal exchanges between
Asia, the Malay Archipelago and Australia took place during several periods of
low sea-level stand during the Pleistocene, when large portions of the shelf regions
of the world became dry. As an example, the distance Timor-Australia during
periods of lowered sea-level was only 50 miles instead of 300 miles today. This
greatly facilitated the exchange of certain members of the bird faunas, mostly
species of open savanna vegetation that was widespread on these shelf regions
during cold (glacial) periods of lowered sea-level (Mayr 1944e, 1944k).
There is no question about the Asian origin of nearly all of the ancestors of
Australian birds, as Mayr’s analysis had revealed. Recent authors speculate that
the ancestors of songbirds (Oscines) which reached Australia from Asia, also
had originated in Gondwana land. How they may have arrived in the northern
hemisphere (via contact of “Greater India” with Asia during the Eocene?) remains
open (Cracraft 2002).