Hettangian-Sinemurian (Early Jurassic) ammonites of New Zealand

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Stevens, G.R. 2004 Hettangian-Sinemurian (Early Jurassic) ammonites of New Zealand. Lower Hutt: Institute of Geological & Nuclear Sciences Limited. Institute of Geological & Nuclear Sciences monograph 23; New Zealand Geological Survey paleontological bulletin 76 107 p.

Abstract: All the occurrences of Hettangian-Sinemurian ammonites in New Zealand are in sediments of the Murihiku Supergroup. These sediments were originally deposited in a fore-arc basin in a convergent margin volcanic arc environment. The volcanic arc extended along the south west Pacific edge of Gondwanaland. In the Hettangian the following subzones are represented in New Zealand: (i) Early Hettangian: Planorbis (?and Pre-Planorbis Beds); Johnstoni (ii) Middle Hettangian: Portlocki (iii) Late Hettangian: Extranodosa: Complanata. The record of ammonites of Sinemurian age in New Zealand is rather fragmentary. Only some of the subzones of the Bucklandi and Semicostatum Zones are represented. The overall diversity of the New Zealand Hettangian-Sinemurian ammonite fauna is rather low when compared with that of Europe, for example – both in the numbers of taxa and the size of the populations. The most likely explanation for this low diversity is that in Hettangian-Sinemurian time New Zealand occupied an active intra-arc geosynclinal sedimentary environment, with facies conditions that were often unfavourable for ammonite life. In addition, New Zealand’s palaeolatitude position at the time placed it in a climate zone (warm-temperate) that was probably less than optimal for ammonite life, compared with the more favourable tropical and sub-tropical conditions then prevailing in Europe, for example. Particularly strong faunal links exist with NW Europe, and especially with the alpine regions of Bavaria, Austria and Slovenia. Very close links are also apparent with the ammonite sequences in British Columbia (western Canada) and Nevada (western USA). Links with South America are less clear and similarly those with SE Asia. The presence of these faunal links may be interpreted to indicate that very accessible faunal migration routes were available from Europe to western North America via the Hispanic Corridor and then via the embryonic Pacific Ocean to New Zealand and New Caledonia. (auth/DG)

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