Molluscan Evidence of Pliocene Climatic Change in New Zealand.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 74, 1944-45, Page 207

Molluscan Evidence of Pliocene Climatic Change in New Zealand. By C. A. Fleming, Geological Survey, Wellington. [Read before Wellington Branch, June 6, 1944; received by the Editor, June 15, 1944; issued separately, December, 1944.] Abstract. Faunal changes in the New Zealand Pliocene suggest that late Tertiary lowering of sea temperatures reached its peak in the mid Pliocene, when sub-autarctic water extended northwards to Southern Hawke's Bay, bringing a characteristic cold water fauna. In the upper Pliocene, further advance of cold water isotherms was prevented by a new factor, the Notonectian subtropical current, which has continued to affect New Zealand coasts bringing a fauna of East Australian warm-water molluscs, and driving sub-antarctic waters and their fauna south to their present position. New Zealand mollusca did not suffer changes commensurate with the severity of the Pleistocene glaciation. This anomaly may be due to the persistence of the warm current influence through the Pleistocene, so that the low air temperatures of the Glacial Period failed to affect the marine fauna. Introduction. Students of the Tertiary mollusca of New Zealand have made a number of generalisations concerning the trends and events in the development of the present fauna. The chief of these are:— (1) The evolution of an autochthonous element, the ancestors of which were already in the New Zealand area in early Tertiary or even late Cretaceous times (Marshall, 1919, etc.; Marshall and Murdoch, 1920, p. 125; Finlay, 1925, p. 163). (2) The reinforcement of such a fauna by Indo-Pacific and possibly other elements during the Tertiary (Marwick, 1925, p. 376, 1926 A, p. 272) and particularly during the early half of the Tertiary (Marwick, 1929; Finlay and Marwick, 1940, p. 129). (3) The extinction of mid-Tertiary genera in the Miocene and early Pliocene (Marshall and Murdoch, 1920; Powell, 1931, p. 90; Laws, 1936; Finlay and Marwick, 1940). (4) The sudden immigration of species of East Australian origin in the latest Pliocene, which has continued in Recent times (Finlay, 1924, 1925, 1926, 1931, etc.; Finlay and Marwick, 1940; Powell, 1927). There has been general agreement (Marshall and Murdoch, 1920; Powell, 1931; Laws, 1936; Finlay and Marwick, 1940, p. 129) that the post mid-Miocene extinctions were due in large part to cooling seas, and that such cooling was part of the onset of world-wide refrigeration which culminated in the Pleistocene glaciation. Many of the genera and families which suffered extinction are now confined to tropical and subtropical seas (Cucullaea, Isognomon, Pteria, Spondylus, Pyrazus, Morum, Galeodea, Conospirus, Olivella, etc., etc.). Comparison of the Recent ranges of such molluscan groups with the distribution of the surface waters of the world to-day (Schott, 1935; Deacon, 1933, 1936) leads to the conclusion that New Zealand, in the mid-Miocene, lay wholly within seas with the thermal

and other characteristics of the present Sub-tropical Zone of surface waters. There is no latitudinal zoning in the Miocene faunas to suggest, for instance, that the Miocene seas at Clifden, Southland, were appreciably cooler than those at Kaipara, North Auckland, so that, if a hydrological regime comparable to the present existed in the Miocene, the Sub-tropical Convergence must have been well to the south of New Zealand. At the same time, the apparent absence of reef building corals throughout the New Zealand Tertiary would seem to place an upper limit in any assessment of Miocene sea temperatures. The progressive extinction of Miocene genera culminated in the Waitotaran Stage of the Lower Pliocene, which contains a number of lingerers from Miocene time, and they, with other features of the Waitotaran fauna of Taranaki (Marshall and Murdoch, 1920; Powell, 1931) point to hydrological conditions still warmer than at present. From the Nukumaruan until Recent times the generic extinctions are much fewer, and Hutton (1872) concluded that there was no difference other than what can be ascribed to passage of time between the Upper Pliocene and Recent mollusca. “He contended that the Wanganuian molluscan fauna … was so like that of present scas that no important lowering of temperature could have intervened” (Marwick, 1926 B, p. 1771). Hutton (1872; 1904, p. 18) therefore advocated a Lower Pliocene age for the glacial epoch in New Zealand. Subsequent study has confirmed the close relationship between Upper Pliocene and Recent faunas, so that it is inconceivable that a lowering of sea temperatures of as much as 4°–5° C., such as is suggested to have occurred during the Pleistocene in other parts of the Pacific (Davis, 1928, p. 219), can have taken place in the New Zealand Pleistocene. A change of such magnitude might reasonably be expected to show itself in more pronounced faunal differences than those between the Castlecliffian and Recent faunas. The abundant geomorphic evidence that the glacial epoch in New Zealand was Pleistocene (i.e., post Castlecliffian) and not Pliocene (as the above considerations tend to indicate) will not be summarised here. Some of the evidence relative to the problem was presented by Marwick (1926). The problem is complicated by the fact that in none of the heavily glaciated areas of New Zealand have Castlecliffian marine sediments been involved in the movements which initiated the geomorphic cycle in which glaciation was an important episode; on the other hand, mountain ranges, the elevation of which has been subsequent to the deposition of Castlecliffian sediments (Ruahine-Tararua-Kaikoura), show few evidences of glaciation, though it is probable that such occur and await fuller description (Adkin, 1912; Rose, 1933). The lengthy post-Waitotaran history postulated by Wellman and Willett (1942, pp. 304–5) from their studies of the structure and gcomorphology of the western Alpine region, makes it improbable that the glaciation could have been Nukumaruan or Castlecliffian, while, on the eastern flanks of the glaciated alps, Speight (1934, 1942) has shown that glacial deposits long post-date the Kowhai Series for which Mason (1941, p. 124) has quoted a Waitotaran-Nukumaruan fauna; the partly coeval

Bourne sandstones and conglomerates of the north (Fyfe and Healy, 1935, p. 10) contain faunas possibly as high as Castlecliffian. All told there is little doubt of the post-Castlecliffian age of the glaciation. In reconstructing Pliocene and Pleistocene climatic trends, the failure of the palaeontological and geomorphic evidence to synchronise constitutes an anomaly demanding explanation. It is the purpose of this paper to furnish data on the hydrological implication of some Pliocene faunal changes, and to put forward an hypothesis which might reconcile the anomalies stated above. A Nukumaruan Cold Water Faunal Immigration. In the Waitotaran and Lower Nukumaruan Stages of the New Zealand Pliocene the extinction of warm water mid-Tertiary genera was accompanied by a northward immigration of a number of molluscs of southern cold water origin into the middle North Island; in the Upper Nukumaruan some of the incomers had again retreated; a few remained till the Castlecliffian, but the descendants of all are in Recent times limited to areas directly under the influence of the Subantarctic Zone of surface waters. The implication is that subantarctic waters, which, in a pure state, now affect only Southern Otago, Stewart Island, the Chatham Islands, and islands to the south (Forsterian, Moriorian, and Rossian marine faunal provinces) advanced and then retreated, probably as a localised tongue or current, during the Pliocene. (a) Chlamys delicatula (Hutton). The most characteristic mollusc of this cold water faunule is the lineage group of Chlamys delicatula (Hutton). The earliest record of this group is in the Opoitian of the Chatham Islands, as the somewhat specialised C. seymouri Marwick, which is probably not the ancestor of later forms. There is no record of the lineage from Opoitian beds on the main islands of New Zealand. Chlamys delicatula, or a closely related ancestral form, first appears in New Zealand in certain beds of North Canterbury (N.Z.G.S. Locs. 2555, Lowry Peaks. SD., 3297, Stonyhurst S.D.) where the associated fauna includes Manaia manaiaensis (Marw.), Phialopecten triphooki (Hutton), Polinices waipipiensis (Marw.), and Austrofusus pagoda Finlay, all of which indicate Waitotaran age. There is, however, no trace of the species in the well-collected Waitotaran beds of Westland, Marlborough (King, 1934), South Taranaki (Powell, 1931; Laws, 1940) nor in the Waitotaran of the Eastern North Island from Palliser Bay to Hawke's Bay. In the Lower Nukumaruan, a zone recently recognised in southern Hawke's Bay (Lillie and Fleming, 1941) Chlamys delicatula is sufficiently widespread and strictly limited to be used as a key species to a zone intermediate between the Waitotaran and Petane (Upper Nukumaruan) beds throughout the area from Palliser Bay (King, 1933) to just north of Dannevirke (including Castlepoint, Powell, 1938). In the Amuri-Kaikoura area C. delicatula is abundant in beds which appear from associated forms to be Lower Nukumaruan. The species occurs in Lower Nukumaruan beds which cross the axial divide at Manawatu Gorge (G.S. Loc. 2779, Gorge S.D.; see also Wild and King, 1932, Pohangina) and in north-west Wellington between the

Manawatu and Rangitikei Rivers (G.S. Loc. 3096, Tiriraukawa S.D., collected by officers of Superior Oil Co.). In the areas where the species was so abundant in the Lower Nukumaruan, it had disappeared completely in the succeeding Upper Nukumaruan (“Petane”) zone (Lillie and Fleming, 1941). Figure 1. The distribution of the Chlamys delicatula-campbellica lineage in Pliocene and Recent times, showing its northward expansion in the early Pliocene, and its present restriction to southernmost New Zealand. Probable land in Pliocene shaded. In spite of its wide distribution as an abundant fossil in the middle region of New Zealand in the Lower Nukumaruan, Chlamys delicatula has never been collected from the Wanganui-South Taranaki Nukumaruan (e.g., Laws, 1940) nor from the richly fossiliferous “Petane” beds of Hawke's Bay north of about 40° S. Lat. so that the species reached its limit there in the Lower Nukumaruan. There is no record of the southward retreat of the northern limit of the species in Upper Nukumaruan and Castlecliffian beds in the South Island, but the distribution of the Recent C. campbellica Odhner, which is with little doubt “the direct Recent descendant

of the Pliocene delicatula” (Powell, 1939), shows that such a retreat took place. C. campbellica was described from Campbell Island, and has been identified from Auckland Island, off Macquarie Island, Stewart Island, and off Otago Heads (Powell, op. cit.). These localities are all within the Subantarctic Zone of surface water as described by Deacon (1936); to the north is an ill-defined belt of mixed water which is not truly of subantarctic nature and which limits the northward range of many subantarctic organisms (Fleming, 1939, 1941). C. campbellica is apparently one of such zonally limited (“stenozonal”) organisms, and the northern extension of the range of its Pliocene ancestor suggests a northward advance of subantarctic water in the Pliocene. Odhner (1924, p. 62) compared campbellica with C. patagonica King, from the coasts of Patagonia, Tierra del Fuego, and the Falkland Islands—all within the Subantarctic Zone of surface waters. In Patagonia, patagonica appears to have Pliocene and Miocene ancestors and has been associated with them by Ihering (1907, p. 250) within his subgenus Zygochlamys. Ortmann's figures (1902, Pl. XXIII, Fig. 2 a-e) of the subgenotype geminata Sow. show a shell superficially not unlike the New Zealand delicatula assemblage, and the group may prove to have had a common ancestry and to have developed in the Subantarctic on both sides of the Pacific. (b) Tawera subsulcata (Suter). This species has long been regarded as a reliable key species to the Nukumaruan (Finlay and Marwick, 1940, p. 127) and since the more persistant T. spissa lineage can be traced from Recent to Miocene times the sudden appearance and extinction of subsulcata suggests that it was an immigrant form. T. bollonsi Powell, from the Auckland Islands, has been compared with subsulcata by its author (1932, p. 69). The resemblance may be due to environmental effects or to genetic relationship, but in any case the restriction of the subsulcata type of sculpture to the Nukumaruan in New Zealand and the occurrence of a similar form in the Subantarctic Recent fauna backs up other evidence of the nature of New Zealand mid-Pliocene seas. (c) Stiracolpus aff. symmetricus (Hutton). The restriction of the Recent S. symmetricus (Hutton) to southern New Zealand suggests that it may be stenozonal. The occurrence of close relatives in abundance in the Waitotaran of North Canterbury (but not in the Waitotaran of Westland and of the North Island where occurs a different line, huttoni Cossmann, haweraensis Powell) and the wider distribution of symmetricus in the Nukumaruan and Castlecliffian of the middle North Island are hints that its ancestors may have been members of the cold water C. delicatula fauna which came north in the Pliocene. The case of S. symmetricus, however, is complicated by the occurrence of relatives in earlier stages (such as kaawaensis Laws). (d) Genus Zephos Fin. The only Recent species of the genus (otagoensis, Fin.) is so far only known from the Forsterian Province, so that the surprising abundance of the genus in the Waitotaran of North Canterbury

(purchasi Suter, cingulata Hutton) and in the Lower Nukumaruan of Palliser Bay (onokeana King) may be related to cooler seas. (e) Eucominia aff. nassoides (Reeve). The “superspecies” of E. nassoides (Rve.) is, like the preceding species, confined in the Recent fauna to areas directly affected by water of the Subantarctic Zone, races of the group being known from the Chatham Islands, Stewart Island, Foveaux Strait, Otago Heads, and the Subantarctic Islands. The group thus appears to be stenozonal and subantarctic, and its occurrence, as a close ally of nassoides, in places associated with Chlamys delicatula, in the Lower Nukumaruan of North Canterbury (G.S. Loc. 1590, 1592, Hawkeswood S.D., etc.) and of Dannevirke (2500, Tahoraite S.D.) substantiates the conclusions drawn in the previous section. Other species of Eucominia which are not within the lineage group of E. nassoides (elegantula Fin., verrucosa Fin., excoriata Fin.) were more widespread in the Nukumaruan and Castlecliffian and were possibly more tolerant, though the absence of the genus in North Island Recent seas suggests that the whole genus was to a certain extent stenozonal at least during the Pliocene* The Miocene species (marshalli Laws, media Hutton, nana Fin., intermedia Suter) are not typical and probably not ancestral.. (f) Ellicea aff. orbita (Hutton). Crass-shelled species of Ellicea such as orbita (Hutton) and coronata (Powell) appear to represent a somewhat different stock from the henchmani-conformata-perobtusa lineage (Upper Miocene to Waitotaran). A direct ancestor of orbita first appears in Opoitian beds in Southland (G.S. Loc. 2954, Rowallan S.D.); the species is present in Waitotaran beds in North Canterbury (e.g. 1589, Hawkswood S.D., 3297, Stonyhurst S.D.) and in Lower Nukumaruan beds in both Canterbury and South Wellington (King, 1933). The genus did not survive the Lower Nukumaruan, so that there is no proof that it had cold-water affinities, but its northward expansion with Chlamys delicatula and other forms is suggestive of pre-Pliocene origin somewhere in the south of the New Zealand region. Pliocene faunas contain a number of other puzzling Buccinulid forms which have no Miocene “roots” and which probably entered from the south with the cold water fauna here described. (g) Miscellaneous: New Zealand Recent species of Gaimardia Gould are entirely restricted to areas within the influence of subantarctic surface waters; a Nukumaruan species has been described by Laws (1940) from the Wanganui coast. The genera Lironoba Fin., Zeadmete Fin., and Monodilepas Fin. have their major development in the Subantarctic Zone of surface water in the New Zealand area, but Nukumaruan or Castlecliffian representatives have been recorded for all. The three genera are, however, present in the subtropical waters of the Aupourian marine province where they are members of an anomalous subantarctic element mentioned by Powell (1940, p. 205). Such an anomaly may weaken but does not invalidate the significance of such genera in the North Island Pliocene.

Summary: The occurrences listed above suggest that seas belonging to the Subantarctic Zone of surface water began to affect the North Canterbury area (but not Westland) in the Waitotaran; that such waters reached into Southern Hawke's Bay, and, through the Manawatu Strait, to the Rangitikei in the Lower Nukumaruan; and that the Upper Nukumaruan saw a withdrawal of Subantarctic waters from such latitudes towards their present position where, in a pure state, they affect only southernmost New Zealand. The conclusion that sea water isotherms reached their northernmost position in the New Zealand mid-Pliocene can only be reconciled with a glaciation in the Pleistocene on the assumption that some purely local cause operated to warm, or to prevent the further cooling of, New Zealand seas in post Nukumaruan times Evidence for such an assumption is to be found from examination of Castlecliffian (Uppermost Pliocene) molluscan faunas. Castlecliffian Seas: The Notonectian Immigration. The sudden appearance in the Castlecliffian beds of mollusca of East Australian origin which have no direct ancestors in earlier New Zealand beds has been commented on by Finlay (1925, 1926, etc.), and the list of species in the Recent fauna of late derivation from East Australian sources is now of imposing length. The characteristics of the Notonectian element in the New Zealand molluscan fauna are:— (a) Closer relationship with Peronian representative forms than with possible New Zealand Miocene ancestors. (b) Sudden appearance in the Castlecliffian or in the Recent fauna.* Such a definition excludes the genera Glycymerula F. and M., Gomphina Moerch and Zethalia Finlay, which arrived in New Zealand in the early Pliocene. They do not seem to be of Australian derivation and may represent a previously unrecognised immigration from the north. All have North-West Pacific affinities which in the first two genera have been noted by Marwick, 1927, and Finlay and Marwick, 1937. (c) Predominance of groups tending to be limited elsewhere to subtropical waters. (d) In the Recent fauna, the Notonectian element is predominant in the North Auckland (Aupourian) faunal province and progressively less important further south. (e) Of gasteropods, relatively large numbers have polygyrate protoconchs, sometimes of “Sinusigera” or “Agadina” form, which (Iredale, 1911) are associated with free-swimming, long-lived embryos. It has generally been assumed (Finlay, 1925, 1926, etc.; Powell, 1927, 1933, 1940, etc.) that such molluses owe their presence in New Zealand to the Notonectian Current, which, sweeping past New Caledonia as a southward branch of the South Equatorial Current, passes down the east coast of Australia and Bass Strait, swings eastward across the South Taman Sea and northward to affect chiefly northern New Zealand shores (Deacon, 1936). The fauna

which the current is believed to have transported to New Zealand began to arrive in the Castlecliffian, and Finlay (1931, p. 3) concluded that “such ocean current influence … does not seem to have antedated the Castlecliffian.” The Notonectian is a warm, subtropical current, and the fauna it brought is composed of chiefly subtropical stenozonal types. If the Notonectian commenced to operate in the uppermost Pliocene, it provides the clue, not only to the arrival of the warm-water Notonectian molluscs, but to the retreat southwards of the subantarctic fauna described in the preceding section. Put in another way, advance of cold waters northwards in the Pliocene would have culminated in the Pleistocene (when air temperatures reached their minimum) had not the Notonectian current begun to operate as a purely local South Tasman influence, and to blanket New Zealand seas from further cooling after the mid Pliocene. There is no reason why air temperatures should have shown precisely parallel changes with sea temperatures—many discrepancies occur in Recent geography—and the present hypothesis is put forward to reconcile the anomalies which have previously hindered interpretation of Pliocene-Pleistocene faunal development. Castlecliffian seas are believed to have been warmer than those of the Nukumaruan, and particularly Lower Nukumaruan, for the following reasons:— (1) Several of the cold-water forms of the Nukumaruan (Chlamys delicatula, Tawera subsulcata, Eucominia nassoides, Zephos, Ellicea) had retreated or become extinct by the Castlecliffian. (2) The incoming Notonectian element contains groups [Cymatiids, Cassids, Eunaticina, Heliacus, Agnewia (at Cape Runaway, Powell, 1934)], which elsewhere show, though to a varying extent, a preference for subtropical conditions. (3) In the Cassididae, it has been noted (Fleming, 1943, p. 98) that the Waitotaran saw the extinction of the subgenus Mauicassis which had developed in New Zealand in Mio-Pliocene times, and that the family is completely absent from the Nukumaruan. Its reappearance (as Xenophalium s. str.) in the Castlecliffian points to an alleviation of the cold conditions which had presumably caused its extinction. Allan (1937) has suggested that the brachiopod assemblage of the Castlecliffian is that living at present in Foveaux Strait, though his suggestion as to the nature of the faunal migration implied requires modification in the light of the contents of this paper. His suggestion that Castlecliffian seas were somewhat cooler than those of the same latitudes at present is further confirmed by the persistence in the Castlecliffian of North Wellington of Stiracolpus aff. symmetricus (Hutton); Eucominia elegantula Fin., and species of Monodilepas and Zeadmete (with the reservation made earlier concerning the latter two genera). The evidence indicates that Castlecliffian seas were decidedly warmer than those of at least Lower Nukumaruan times and that they were possibly slightly cooler than those of Recent seas in the area. It is suggested that the Notonectian current commenced to

affect New Zealand sometime between the Lower Nukumaruan and the Castlecliffian and that the current continued to operate during the Pleistocene and Recent Periods and thus effectively blanketed New Zealand seas during the Glacial Period when there was doubtless a marked northward advance of cold water isotherms in other parts of the world. Changes of the type suggested may well be connected with Pliocene orogenic movements in the South West Pacific which might have raised or lowered barriers across the paths of oceanic currents, deflecting them to new courses or allowing them passage where previously diverted. Molluscan Changes During the Pleistocene. Few of the post-Pliocene changes in benthic marine mollusca can be attributed to the Pleistocene glacial cooling: the extinction of Leucotina A.Ad. (s. str.), Eunatacina, Pterochelus, Barytellina might be attributed to such a cause, were it not for the persistence of a far greater number of apparently warm water types and the relative unimportance of other faunal changes. An extinction which can without doubt be attributed to Pleistocene cooling is that of Anadara trapezia (L.). Oliver (1923) has recorded dead shells of the species from northern New Zealand and Powell (1932) has given further details and noted that its extinction in New Zealand is paralleled by its occurrence in Tasmania and southern Australia as a Pleistocene fossil in raised beaches. The species now lives in subtropical mudflats along the East Coast of Australia, where it lies half-buried so that, as suggested by Hedley (1915), exposure to frost when the tide is out might prove fatal to its existence (see Powell, 1932, p. 71, for full discussion). The occurrences in New Zealand are (1) as worn shells on beaches and dunes formed partly by the erosion of extensive deposits of ancient sands of the Kaihu Series which Ferrar (1934, p. 44) places in the Older Pleistocene (Karekare, Muriwai, Waipu, Spirits Bay). (2) As pairs of valves in situ in ancient muds of the Hokianga Harbour (Powell, op. cit.). (3) From a baked mud beneath a basalt flow from the Pleistocene-Recent cone of Rangitoto, Auckland (specimen collected by Miss L. B. Moore). Turner and Bartrum (1928, p. 984) suggest that similar estuarine silts and volcanic eruptions in the Takapuna area, not far from Rangitoto, preceded the major submergence of Auckland Province and are Pleistocene. If the extinction of Anadara trapezia (L.) be attributed to the glacial cooling, its presence in the above beds confirms their age as Older Pleistocene. The Development of the Marine Faunal Provinces. The regional differences between the Recent mollusca of, say, North Auckland and Stewart Island are not paralleled by similar differences between the Miocene faunules of, for example, Pakaurangi Point and Clifden, Southland. Some time since the Miocene the faunal provinces have developed. The faunal provinces recognised in the New Zealand Region by Finlay (1925, 1926) and Powell (1937, 1940, etc.) owe their distinctness from each other to a combination of several factors:—

(a) To the development of representative forms in partial or complete isolation [e.g., the sub-species of Haliotis virginea, Gmelin, of Struthiolaria papulosa (Martyn), of Thoristella chathamensis (Hutton), and the sibling species Fractarmilla corrosa (A.Ad.) and subrostrata (Gray), etc.]. (b) To the ecological (chiefly hydrological) differences between various areas, which control the presence and absence of forms with a limited range of tolerance. (c) To the direction and nature of dispersal mechanisms (currents, coastlines) enabling interchange between populations. (d) To the complex of historical accidents in the past history of an area which may have upset the exact correlation between present conditions and faunas. Five provinces are at present recognised (Powell, 1937, etc.) (1) The Aupourian Province (northern part of North Island). The area may have experienced some isolation suggested by some of its endemic representative forms (Alcithoe depressa Suter, Venericardia reinga Powell, etc.) but owes its characteristics chiefly to its Figure 2. Marine Faunal Provinces in the New Zealand region, and their relation to the zones of surface water. Hydrology based on Deacon (1936) with modifications.

position well within the Subtropical Zone of surface water under the influence of ex-tropical warm currents including the Notonectian. To these factors it owes the presence of many Peronian and some Kermadecian elements (Powell, 1940). The currents have, in at least some cases, enabled such forms to colonize, and the subtropical conditions have enabled them to stay. In view of that, the presence of a distinct southern element in the fauna is an anomaly suggestive of a subantarctic influence sometime in the past. (2) The Cookian Province (middle regions of New Zealand). Largely in the belt of mixed waters and containing chiefly euryzonal forms of a fairly wide tolerance. Few limited subtropical and sub-antarctic forms occur. (3) The Forsterian Province. A good deal of isolation in the past is suggested by the number of endemic representative forms, but the chief factor is the predominant influence of subantarctic waters enabling the colonization and persistence of subantarctic limited (stenozonal) forms (Kerguelinia, Gaimardia, etc., Powell, 1939). (4) The Moriorian Province. The Chatham Islands attest their lengthy isolation by the number of endemic representative forms in their molluscan fauna, but their position on the Subtropical Convergence under direct influence both of warm currents from the N. and of the Subantarctic West Wind drift has resulted in the presence together of forms of diverse origins and affinities which have been remarked on by both reviewers of the fauna (Finlay, 1928; Powell, 1933). (5) The Rossian Province is entirely within the Subantarctic Zone of surface water and has an impoverished fauna containing only the most tolerant (euryzonal) of New Zealand genera, with the addition of stenozonal subantarctic types. Under present conditions communication by currents between the Rossian and other provinces must be limited to a one-way traffic towards New Zealand, so that any suggestions of reverse migration imply different conditions in the past. Since the differences between the provincial faunas cannot have arisen spontaneously in Recent times, the probability that past faunas, no less than living ones, showed regional differences, must be considered. As already stated, there is little evidence for such regional developments in the Miocene, but in the Pliocne there are indications that regional provinces were developing as a response to both the geographic and the hydrological evolution of modern New Zealand. The latter, hydrological, influence seems to have been most important: New Zealand lies athwart the junction (Convergence) of two contrasting zones of surface water (Deacon, 1936), which have divergent faunas elsewhere in the world, so that a northward movement of the Convergence across the New Zealand area in late Tertiary times or its fluctuation back and forth would have profound faunal consequences. In the Opoitian (Lowest Piocene) differences between the Kaawa Creek (Laws, 1936, etc), the Chatham Island, and other

New Zealand coeval faunas are possibly of such a regional type. In the Waitotaran, the faunas of Taranaki and Westland form a western province with many similarities; in the east, facies differences tend to obscure the issue, but the North Canterbury cold water faunule obviously represents a faunal province under distinct hydrological conditions. The same may be said of the Nukumaruan: the faunas with the cold water element have the same relationship to more northerly faunas lacking it as has the Forsterian fauna to the Cookian. Castlecliffian beds are not sufficiently widespread to show clear provincial differences which doubtless existed: the Cape Runaway fauna lacks Eucominia and Stiracolpus, and includes Agnewia, Austrosassia and other Cymatiids, Heliacus, and Nassarius, suggesting warmer water conditions than in the Wanganui Castlecliffian. Summary and Conclusions. Analysis of Pliocene faunal trends leads to the following, necessarily tentative, conclusions:— (1) In the Miocene, the New Zealand area lay wholly within the Subtropical Zone of surface waters, and there is no indication of faunal zoning due to hydrological differences. (2) In the early Pliocene (Opoitian-Waitotaran) subantarctic waters aproached New Zealand; a tongue of such waters had reached North Canterbury on the east coast by the Waitotaran, but did not affect other Waitotaran seas to the west and north. (3) By the Lower Nukumaruan (Mid Pliocene) subantarctic waters had extended north through North Canterbury, Marlborough, Wellington, and Southern Hawke's Bay to approximately 40° S. latitude. (4) Continued advance of southern waters was inhibited by the commencement, in the late Pliocene, of the Notonectian current of subtropical water which drove subantarctic waters south from their position in the Nukumaruan and prevented New Zealand seas from reflecting the further cooling which occurred in air temperatures and culminated in the Pleistocene glaciation. (5) Among the marine mollusca the only case of a Pleistocene extinction due to cooling climate is of an intertidal form vulnerable to low air temperatures. (6) The marine faunal provinces in New Zealand developed in the Pliocene as a response to the above hydrological changes and now reflect chiefly present conditions, though, to a lesser extent, conditions in the past may have left relict faunal elements. (7) The conception of New Zealand in the Pleistocene as a land of frigid meteorology set in relatively temperate seas may have important corrolaries in assessing the response of other organisms to the glacial period. Acknowledgments. The writer is indebted to the field officers of the Geological Survey who made many of the collections not so far reported on in detail which have been available for study, and to Dr J Marwick, Chief Palaeontologist, for helpful criticism and discussion.

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