Cheers
Peter
Ka = thousands of years ago
Northern Province and Southern New Guinea: A strong relationship exists
between Fly River, New Guinea (as well as most of southern New Guinea) and the
Northern Province of Australia, with thirty-four out of seventy-five
freshwater fishes (45%) in common (Roberts, 1978; Allen, 1991). Furthermore,
four species, Nematalosa erebi, Amniataba percoides, Craterocephalus
stercusmuscarum, and Hephaestus carbo (and likely others, yet uninvestigated)
have apparent sister species in Fly River (increasing commonality to 51%).
Presently, Fly River drains to the Southern Pacific Ocean. However, it is
hypothesized to have been diverted from a southern route directly into Arafura
Sea (Blake & Ollier, 1969) between 35 and 40 Ka by upwarping (Torgersen et
al., 1988). The Arafura Sea between Southern New Guinea and the Northern
Province is shallow, hence regularly exposed during lowered sea-level, which
would potentially connect most drainages in Torres Strait west of Cape York
Peninsula (Fig. 14).
When did the fish faunas of Australia and New Guinea last meet? Allen & Hoese
(1980) suggested severing of the connection was 6.5-8 Ka during the last
sea-level rise. While likely correct for hydrological connectivity, it seems
unlikely for most freshwater species as a migration route. During Late
Pleistocene, a major lake known as Lake Carpentaria existed east of Groote
Eylandt during lowered sea-levels (Fig. 14) (Torgersen et al., 1983; Jones &
Torgersen, 1988). It is not known when the lake first formed, but, for much
of its examined history (up to 40 Ka), it was brackish or fresh-to-brackish
(Torgersen et al., 1988), which may have prevented migration. Furthermore,
during lowered sea-level it is thought this area was more arid than at present
(Webster & Streten, 1972), hence possibly decreasing available aquatic
habitats. Major faunal exchanges are more likely to have last occurred during
low sea-levels during Late Miocene (Haq et al., 1987) when global climate was
warmer and wetter than today (Partridge et al., 1995).
An interesting aside is the absence of five of the six most widespread
Australian fishes in New Guinea. Leiopotherapon unicolor and Neosilurus
hyrtlii have no known near relatives in New Guinea, Nematalosa erebi and
Amniataba percoides each have likely sister species (largely restricted to
Fly River), while Melanotaenia splendida has a widespread allopatric
subspecies in New Guinea (Allen, 1991). Given their widespread occurrence and
broad environmental tolerances ( Merrick & Schmida, 1984), these species are
most likely of all to have been able to migrate during the last low sea-level,
but they are absent. One could speculate many possible reasons for this, i.e.,
competitive exclusion or incorrect taxonomy, although insufficient evidence
exists to warrant further discussion. There are also species with extensive
southern New Guinea distributions that do not occur in Australia. Examples
include Arius carinatus Weber, A. latirostris Macleay, A. macrorhynchus
(Weber), the catfish genera Cochlefelis, Doiichthys, and Nedystoma,
Zenarchopterus novaegunineae (Weber), Melanotaenia goldiei (Macleay), and
Glossamia sandei (Weber). Furthermore, there are several species with more
limited ranges in central southern New Guinea in the middle and upper Fly
River that do not occur in Australia (Allen, 1991). Clearly, this region has a
long complex history that is poorly understood.