Abstract
Uppermost Cretaceous (Campanian?-Maastrichtian) to lowermost Paleogene (Paleocene) rocks of New Zealand contain an exceptional record of Bivalvia, Gastropoda and Scaphopoda. The composition of these faunas is closely linked with the separation of New
Zealand from the East Antarctic-Australian sector of Gondwana, development of open oceanic conditions by Early Campanian time (c. 80-85 Ma), and Cretaceous-Tertiary boundary extinctions. This work aims to: 1) document the changes in paleobiogeographic elements (endemic, paleoaustral, Indo-Pacific/Tethyan, and cosmopolitan) of molluscs across the K-T interval; 2) provide a quantitative analysis of the Weddellian Biotic Province hypothesis of Zinsmeister (1979), which has resulted in a refined biogeographic model for the Gondwana Realm; and 3) examine the extent and effect of the terminal Cretaceous extinction event on the Mollusca of New Zealand.
This study recognises 329 species of latest Cretaceous to earliest Paleogene molluscs from New Zealand. Of these, 118 new species are described in the systematic catalogue together with comments on their inferred feeding habits. Previously established species are refigured and where justified, redescribed and taxonomically updated. Of 329 species, 159 are of latest Cretaceous age (Piripauan to Haumurian stages) and 170 are Paleocene ("Wangaloan" or Teurian). A total of 79 bivalve, 47 gastropod and 2 scaphopod genera/subgenera are recognised in the Late Cretaceous record, with 35 bivalve, 76 gastropod and 3 scaphopod comparable taxa from the Paleocene. New genera/subgenera proposed in the systematics section are Brookula (Paleobrookula n. subgen.) (Skeneidae), Kaiparomphalus n. gen. (Trochidae), Saulopsis n. gen. (Tudiclidae), Wangaluta n. gen. (Volutidae), Wangacteon n. gen. (Acteonidae), and Kaurueon n. gen. (Acteonidae). Thirty-four new combinations are proposed. Zelandiella Finlay, 1926, is synonymised with Austrocominella von Ihering, 1907. Conchothyra marshalli (Trechmann, 1917) is considered as distinct from C. parasitica Hutton, 1877. The following species-group synonymies are proposed: Bittiscala
communis Finlay and Marwick, 1937, with~- simplex (Marshall,
1917); Conchothyra expedita Finlay and Marwick, 1937, with C. australis (Marshall, 1916); Euspira lateaperta (Marwick, 1924) with Euspira fyfei (Marwick, 1924); and Campylacrum debile Finlay and Marwick, 1937,with C. sanum Finlay and Marwick, 1937.
Though the biostratigraphic resolution of uppermost Cretaceous and Paleocene New Zealand rocks has improved recently, problems still exist in correlating some macrofossil localities because of rarity of index microfossils, facies faunas, and incomplete knowledge of stratigraphic ranges of some taxa. Thus, Piripauan (Campanian? Maastrichtian?) and Haumurian (Maastrichtian) stages are virtually inseparable in the field and laboratory. At Wangaloa, southeastern Otago, a K-T boundary section appears to be present c.10 m below the shell beds. Reintroduction of the abandoned Wangaloan Stage may be appropriate, pending further microfossil work, as the relationship of this stage to the very long (c. 8 m. y.) Teurian Stage could be established. Molluscan faunas at Wangaloa and elsewhere in South Island can be correlated to the lower part of the dinoflagellate Palaeocystodinium qolzowense Zone, indicating a late Early Paleocene age. A macrofossil zone, Zeacolpus
(Leptocolpus) semiconcavus Assemblage-zone, is proposed for widespread molluscan-rich Paleocene faunas of South Island.
Compared to other Austral sequences, New Zealand has an excellent record of latest Cretaceous to earliest Paleogene molluscs. Dramatic compositional changes across this interval ultimately reflect the break-up of Gondwana and environmental perturbations on shallow shelf faunas. Endemism in New Zealand latest Cretaceous molluscs was weak (c. 8% of genera/subgenera), but increased substantially (c. 32%) by Paleocene time, partially reflecting the geographic and genetic isolation of "greater New Zealand" at this time. The paleoaustral component, including endemics herein, was also relatively weak (c. 21%) during the Cretaceous but marked (c. 60%) by the Paleocene. The IndoPacific/ Tethyan and cosmopolitan components decreased from c. 48% to c. 26% and c. 26% to c. 12% across the interval, respectively. Few cosmopolitan Late Cretaceous species and endemic genera/subgenera of this age have been recorded. Most Late Cretaceous taxa are endemic species of cosmopolitan or wide-ranging genera/subgenera (c. 92%). The Paleocene fauna is entirely endemic at species-level, apart from possible records of Cucullaea (Cucullona) inarata Finlay and Marwick, 1937, from the Paleocene of southeastern Australia and Taieria allani Finlay and Marwick, 1937, from the Paleocene of Chatham Islands.
High similarity coefficient values (≤ 0.76; Simple, Dice and Simple Matching) of latest Cretaceous and Paleocene molluscan genera/subgenera from New Zealand, Chatham Islands, Australia, New Caledonia, Antarctic Peninsula and southern South America, indicate strong faunal links between these areas during the latest Cretaceous. Such links corroborate the Weddellian Province hypothesis. The marked homogeneity of these widespread faunas probably reflects large shelf area, a low sea-surface temperature gradient, and favourable oceanic circulation including perhaps a trans-Antarctic seaway during the latest Cretaceous. Sea-surface temperatures across the K-T boundary were probably subtropical to at least warm temperate in New Zealand. The New Zealand Subprovince of the Austral Province
(Kauffman, 1973) should be abandoned because of the low endemic component of New Zealand molluscs during the Late Cretaceous. For the Paleocene only New Zealand and southeastern Australia have significantly similar faunas (Simpson similarity indices: 0.36-
bivalves, 0.91-gastropods). In the Paleocene, the Weddellian Province probably was reduced to the New Zealand-southeastern Australia and possibly Chatham Islands region along the newly opened Tasman Sea.
New Zealand data are consistent with the idea that gradual environmental changes resulting from the final fragmentation of Gondwana, coupled with a probable bolide impact(s) in lower latitudes, caused mass extinction and suppressed biomass productivity, but was followed by faunal rebound during the Early Paleocene. In this study the latest Maastrichtian – earliest Danian time interval could not be resolved enough to identify any low diversity "disaster fauna" above the K-T boundary. The magnitude, however, of the extinction can be gauged along with the extent of the inferred initial radiation phase. In total, about 39% and 36% of bivalve genera/subgenera and gastropods became extinct at the end of the Cretaceous in New Zealand. All New Zealand species recorded in uppermost Cretaceous rocks became extinct sometime during the late Maastrichtian and earliest Danian. The extinction at genus-level was not as severe as in other areas including North Africa, Atlantic Coastal Plain, and northern Europe. The extinction particularly affected infaunal and epifaunal suspension feeders (c. 89%), fewer epifaunal herbivores and browsers (c. 65%), and negligible carnivores and deposit feeders. Data support the idea that the K-T event was selective against suspension feeders and that there was diminished productivity of phytoplankton at the base of the food chain.
About 50% of latest Cretaceous molluscs, including Lazarus taxa, crossed the K-T boundary. Most survivors were seemingly eurybathyal with recorded species in a spectrum of shelf settings. Approximately half of Paleocene survivors were infaunal suspension feeders, but few were epifaunal. Most gastropod survivors were carnivores and deposit feeders. Significantly, Paleocene survivors and presumed new taxa became extinct during the Paleocene (c. 27%). Gamma (total) species diversity changed little across the K-T boundary indicating that diversity quickly recovered in the first few million years after the extinction. Bivalve diversity decreased substantially over the K-T interval, whereas gastropod diversity climbed. This “flip-flop” in diversity indicates that gastropods outpaced bivalves during the early Tertiary and could help explain the veritable burst of radiation in inferred "bloom" families (e. g. N.e.ticidae, Turridae, Acteonidae). The "Wangaloan" fauna probably characterises rapid evolution in isolation and an initial phase of radiation reflecting high speciation in which many new groups filled the ecological vacuum left by the extinction, followed by a further less severe extinction pulse in late Danian time.