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5. Trans Tasman Migration
The earlier and mid-Tertiary successions on either side of the Tasman Sea are insufficiently complete to make any deductions as to faunal migrations within Australasia. Certain features of the echinoid time-ranges have suggested to me that possibly the Janjukian includes a middle Oligocene horizon; but the evidence of forams and molluses apparently does not support this view. The presence of the large fossil penguin Palaeeudyptes in supposed Miocene sediments of Australia does, however, invite comparison with the lower Oligocene (Duntroonian) of New Zealand (a point to which Dr. C. A. Fleming kindly drew my attention). At all events, the echinoid succession on either side of the Tasman is not yet adequately correlated in regard to finer details at or near the specific level. The evidence from the late Tertiary is clearer and, I think, susceptible to inductive treatment.
Pliocene and Post-Pliocene trans-Tasman echinoderm migrations have occurred, and they have been essentially from east to west. So far as Recent species are concerned, the movement has been, more precisely stated, from eastern Australia towards northern New Zealand, particularly to that northern extension of the North Island coastline now usually referred to by Finlay's term Aupourian. Common Australian Recent echinoids, for example Centrostephanus rodgersii, Holopneustes inflatus and Heliocidaris tuberculata, occur in small numbers at various points in North Auckland, notably at Cavalli Island. Phyllacanthus parvispinus occurs at the Kermadec Islands. On the other hand, correspondingly common New Zealand echinoids are completely unknown from any Australian fauna. Evechinus, for example, the commonest and most widespread New Zealand echinoid, would seem to be admirably equipped to take advantage of any reverse east to west dispersal mechanism if it existed. It is eurytopic (ranging through eighteen degrees of latitude), it has a large pelagic larva, it has already succeeded in traversing the deep water gap between New Zealand and the Kermadec Islands, and it has been present in New Zealand at least since the lower Nukumaruan (early middle Pliocene). Yet it is known only from the New Zealand region.
Another argument in favour of deducing that any trans-Tasman migration must have been from west to east, and not the reverse, is provided by the high proportion of endemic Recent species of New Zealand echinoderms (some 80 per cent. of species). It is recognised that this is indicative of Tertiary isolation, especially of late Tertiary and Recent isolation. But the isolation implied is rather that of an outpost with a one-way traffic highway, along which new im-
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migrants can occasionally travel from without, and along which few forms can return.
Before considering the probable generic content of the faunas which must have crossed the Tasman at successive stages since the Miocene, we may pause to discuss briefly the relative importance for echinoderm dispersal of ocean currents on the one hand, and former shallow ridges (such as the New Zealand Ridge, or Lord Howe Rise) on the other.
The East Australian current flows southward along the eastern coast of Australia to about latitude 40 degrees south, where it encounters the west wind drift. The junction results in a counter-clockwise swirl, which strikes towards New Zealand. According to Sverdrup, Johnson and Fleming (1942) the February surface isotherms along the 40 degrees parallel in the Tasman form east-west zones of water, the temperature of which is around 18 degrees Centigrade. Mortensen (1921) has provided some minimum figures for the life-span of the pelagic stage in some New Zealand and Australian echinoderms, from which it appears that a free-swimming larval stage of five or six weeks may well be common (for example, Evechinus larvae showed no sign of metamorphosis over the developmental period from December 18 to January 22; the pelagic stage of Heliocidaris tuberculata is over five weeks); the total pelagic life-span may well be longer. However, comparative embryology would lead one to deduce that the total pelagic life is not likely to exceed appreciably a tally of about eight weeks. On the evidence of Fleming (1952), this seems scarcely sufficient to enable a trans-Tasman crossing on the existing East Australian current of between four and a-half and nine miles per day (unless the pelagic stage were succeeded by an epiplanktonic phase on drifting algae, such as does occur in some starfish and ophiuroids). However, Fleming (1952) points out that it is possible that parts of the East Australian current may flow at speeds up to 30 miles per day, so that we cannot yet dismiss the possibility that genera like Heliocidaris may make the trans-Tasman crossing in the larval stage. Astropecten, with a larval period of a little over three weeks, could not; yet Astropecten polyacanthus seems undoubtedly to be a relatively recent immigrant to New Zealand from the Australian-Indo-Pacific area.
One cannot dismiss the possibility of migration along the present shallow-water ridge through the Lord Howe Rise during late Tertiary and Pleistocene lowerings of sea-level. In this connection the evidence from the sub-Antarctic is apposite. The present echinoderms faunas of Auckland and Campbell Islands have clearly been derived from New Zealand. These islands are linked with the New Zealand mainland by the shallow Campbell Plateau (Fleming and Brodie, 1951), which is nowhere deeper than 500 fathoms. Macquarie Island, in the same area, is separated from them by deep water of over 2,000 fathoms. Its echinoderm fauna is almost entirely Antarctic in character, and clearly has not been derived from New Zealand, though a single echinoid (Pseudechinus novaezelandiae) is shared; the latter has a pelagic development. This evidence seems to imply that a shallow route is more effective for echinoderm dispersal than is a much narrower, but deeper, gap—unless the gap is traversed by a favourable current. Thus, so far as the Tasman is concerned, I am inclined to attach some importance to the Lord Howe Rise as at least a former, if not a present, dispersal route.
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Considering now the generic content of the faunas concerned in trans-Tasman immigration:—Arachnoides, as already noted, had entered New Zealand by early Pliocene (Waitotaran) times, and no doubt came from eastern Australia. A mid-Pliocene New Zealand species much resembles a species still' found in Recent Queensland waters. The Recent New Zealand species is distinct, but it too occurs in east Queensland seas. It may well be a second, later immigrant to New Zealand. Peronella, an Indo-Pacific genus, had reached Australia by the Pliocene, and it may well have spread to New Zealand at about the same time, for the one Recent New Zealand species is endemic. Australia has five Recent species, three of them endemic. Perhaps Ctenamphiura was a Pliocene immigrant—the genus is confined to north-east Australia and New Zealand, with an endemic species at either extremity of its range; its fossil history is unknown.
Of genera which probably crossed the Tasman Sea later on, in Pleistocene or Recent times, the following may be noted as having one (or more) identical Recent representatives on both the New Zealand and east Australian coasts:—Araeosoma, Holopneustes, Heliocidaris, Clypeaster, Laganum, Stichopus, Lipotrapeza, Mensamaria, Chiridota, Asterodiscus, Astrobrachion, Ophiocreas, Ophiactis; and perhaps a second instalment of Arachnoides.
Some Indo-Pacific forms also arrived at a late date, perhaps by way of the Lord Howe Rise—for example, Brissopsis, Brissus, Astropecten and Coscinasterias all have identical, or very similar species both within and beyond New Zealand waters.
Apatopygus and Goniocidaris probably owe their present distribution on either side of the Tasman, not to any trans-Tasman migration of the type that has been considered above, but to the fact that their Recent species are a heritage from the former mid-Tertiary common fauna of Australasia. The ancestors of these forms may have arrived along shallow-water routes no longer existing. They represent an archaic element shared by Australia and New Zealand. Apatopygus has one abundant, endemic species in New Zealand, and one rare southern Australian species. The differences between the New Zealand and the Australian species seem at least as great as those between either of them and their Tertiary ancestors in the Nucleopygus lineage. The lineage is a conservative one, showing but little change over its long history; and the name Apatopygus, which is applied to the two Recent species, does not really cover any generic difference from the fossil genus—as Mortensen has shown (1948).