Affinities of Successive Faunas 



The genera of New Zealand moUusca can be readily grouped in four 

 ■categories depending on whether they are : (1) endemic, (2) common 

 to Australia and New Zealand, (3) distributed through the subantarctic 

 zone of surface water, (4) common to New Zealand and to some part 

 of the great Indopacific Region. More categories would be desirable, 

 but have not proved feasible. In particular, it would be useful to 

 subdivide the group showing Indopacific affinity. Some genera classed 

 as Indopacific are present in Australia and seem to have come to 

 New Zealand from there ; others are unknown in Australia and seem 

 to have reached New Zealand by a more direct route from the north. 

 Marwick (1926) long ago stated that " Australian and Indopacific 

 elements have undoubtedly reached New Zealand during the Tertiary." 

 When successive Pliocene and Pleistocene faunas are analysed (Fig. • 3) 

 it is apparent that no catastrophic change in the percentage composition 

 of the fauna occurred during the period. The Australian element 

 increased by 5-5 per cent, and the subantarctic element by T5 per cent, 

 at the expense of the Endemic and Indopacific. This relative stability 

 of faunal composition is the result of a balance between the opposing 

 factors of extinction and invasion, and suggests that throughout the 

 period the New Zealand fauna shared the same faunal influences, with 

 a slight improvement in contacts with the subantarctic and with 

 Australia. 



Derivation of Immigrant Genera 



The " first appearances " of mollusc genera since the Miocene are 

 analysed in Fig. 4. The graphs for the four groups of genera fluctuate 

 in sympathy ; there is no simple alternation in the affinities and implied 

 derivation of newcomers to the fauna ; no period, for instance, when 

 Indopacific " invasion " was replaced by Australian " invasion." 

 (The number of subantarctic genera is too small to warrant generaliza- 

 tion.) On the contrary, the several " lean periods " affected immigrants 

 of all groups. There is, however, a marked (if fluctuating) decrease in 

 the number of endemic genera that appear cryptogenetically, and the 

 Indopacific influence, strong at first, is considerably reduced in the middle 

 of the period studied, recovering temporarily in the Putikian substage. 



The high number of endemic genera that appear for the first time 

 in the Pliocene deserves comment. For the vast majority of such endemic 

 genera no immediate ancestors are known in New Zealand ; our 

 ignorance of some may be due to the imperfection of the record, but, 

 for most, we must infer an origin in some other area, and later 

 immigration to New Zealand seas. The places of origin of such genera 

 may not have been far distant, in the many areas which have supplied 

 no adequate record of their Pliocene fauna, such as eastern Australia, 

 New Caledonia, and other islands to the north, and the Subantarctic. 



OCEANOGRAPHIC CHANGES 



That the periods of faunal deterioration noted in the above sections 

 may have been periods of climatic deterioration is a plausible hypothesis 

 and has been invoked by many students to account for post-Miocene 

 extinction of genera. Many of the extinct genera are stenothermal 

 in warm seas where they persist, and many of those which " invaded " 

 New Zealand between the " lean periods " are also warm-water ones. 



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