T A N E 20 1974 THE PALAEOECOLOGY OF THE RAPANU1 SHELL BEDS

by Anne B. Grace*

SUMMARY

Eight fossil molluscan communities are recognised from collections made at three localities of the Pleistocene Rapanui shell beds on the Wanganui coast, New Zealand. A brief attempt is made at synthesis of the palaeoenvironment.

INTRODUCTION

The Rapanui sequence is Oturian in age 1 0 and consists of basal conglomeratic shell beds, marine sands, lignitic sands and dune sands. The beds lie unconformably on the eroded surface of rocks belonging to the Wanganui series.

Three localities on the Wanganui coastline were visited and material was collected from the Rapanui shell beds by staff and students of the Geology Department, University of Auckland, in April, 1973.

The fossils studied were generally transported before deposition, so that a mixture of life assemblages, or communities, are represented in the shell beds. Interpretation of the fauna relies strongly on a comparison of the fossil assemblage with the benthic communities recognized for Recent organisms. Soft-bodied animals which do not fossilize may have formed part of the ancient communities, and it is only through application of the principle of Uniformitarianism ("the present is the key to the past") that the complete dynamics of fossil assemblages may be inferred. Loss of information through non-preservation is an important aspect of studies of fossil benthic communities. Transportation of some species of a community and not others also contributes to the information loss, so that benthic communities are considerably altered after death, and only the hardiest remnants are fossilized.

RESULTS Description of Sampling Localities N129/f558

Fossils were collected from a shell band at the end of Rifle Range Road, Hawera. At the locality the Rapanui Formation overlies the Tangahoe Formation unconformably. N129/f563

Samples were collected from two shell bands in the Rapanui Marine Sands and from the upper surface of the Tangahoe siltstone which underlies the Rapanui Formation, unconformably. Note was also made of species in a boulder which had tumbled from the basal beds of the Rapanui at Waihi beach.

•Department of Geology, University of Auckland.

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N137/f692 One sample was collected from a 10cm shell band on the coast near

Waverley. Here the Rapanui Marine Sands were overlying the Lower Waipipi shell beds. Species lists

Species lists for each inferred community are recorded in Tables 1. to 8. An indication of relative abundance is made for each species at each locality in the tables. A = abundant, C = common, F = frequent, 0 = occasional, and R = rare. No quantitative values have been assigned to each of these degrees of abundance as only rough estimates were made in the field.

DISCUSSION The Communities Represented 1. Tawera spissa / Glycymeris modesta Community

Grace2 in his study of the benthic communities of the Whangateau Harbour recognised a Tawera/Glycymeris modesta association in Little Omaha Bay. The community was observed in clean sand at depths from 2m to 5m. In 1972, Grace3 modified the analysis and records the Tawera/ Glycymeris modesta community as being a variation of the Tawera/ Dosinia maoriana community. The variation was noted to occur in a coarse sand and shell substrate. Gari stangeri is found locally in the association, whilst Glycymeris laticostata is rare.

Species which would not be preserved as fossils but were likely to form a part of the community represented in the Rapanui shell beds are: Lanice conchilega, Magelomma suspiciens (polychaetes) and the cephalochordate Epigonichthys hectori, which are characteristic of Grace's Tawera/ Glycymeris modesta community.

2. Paphies australe community The number of Paphies australe valves found is sufficient evidence for

raising the species from the status of a secondary species in the Austrovenus community to the status of a dominant species in its own community. Paul 6

analyzed samples from Ohiwa Harbour, Bay of Plenty, and concluded that Paphies australe formed a separate community.

Grace's3 subtidal variation of the Paphies australe community best fits the fauna from the Rapanui as all subdominants are echinoderms, the hard parts of which are less likely to be detected.

The pipi is found in modern sediments composed of fine to medium sand3. Paul located pipis in clean find sand.

3. Ostrea lutaria/ Aulacomya maoriana community There is no doubt that these species occurred together in the life

assemblage, for the base of two specimens of Ostrea lutaria had impressions of Aulacomya maoriana which implies that the oyster spat settled on valves of the mussel. Since the mussel required a hard substrate for attachment it is likely that these lived in an oyster bed, or possibly on a local reef formed of eroding concretionary horizons in the Tangahoe Formation, the further erosion of which provided the boulders in the basal Rapanui (Grant — Mackie, pers. comm.).

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Fleming1 described a Foveaux Strait oyster bed and lists A. maoriana as a secondary species. He further suggests that it is distributed locally, in a similar manner to Barbatia novaezelandiae, Perna canaliculus, Modiolus aerolatus, Modiolaria impacta, Terebratella inconspicua, barnacles, and the tunicate, Boltenia. AH of these species, apart from Boltenia (which would not fossilize), are found in the Rapanui fauna. Ostrea sinuata of the modern community is parallelled by Ostrea lutaria in the Pleistocene one. Species also mentioned by Fleming, such as Evechinus chloroticus and Modelia granosa were recorded for the Rapanui shell beds.

Herbivores, such as Trochus viridis and Maurea punctulata, may have browsed over algae growing in the oyster bed.

Borings in A. maoriana valves may have been made by Lepsia sp., the oyster borer. Buccinulum sp. is also a carnivorous gastropod which may have lived in the ancient oyster bed.

Fleming deduced that the substrate from which his samples were taken was dominantly shelly and coarse. In the sediment he notes that an infauna exists composed of Tawera spissa, V. purpurata, and G. laticostata. The Buccinulum sp. fossils contained G. modesta valves (double), so that it is likely that the Tawera/Glycymeris and Ostrea/Aulacomya communities lived in close proximity.

The presence of A. maoriana in the Rapanui shell beds indicates that the waters in which the assemblage lived were colder than those of North Island, New Zealand. Powell, 8, notes that the species is distributed throughout the Cookian, Forsterian and Morioran marine provinces.

4. Austrovenus stutchburyi community Ralph and Yaldwyn 9 recognised an Austrovenus community in the Otago

Harbour. Important in the community were Macomona liliana and Solemya parkinsoni, two species not found in the present study. Grace2 ' 3 recorded an Austrovenus/ Nucula / Macomona community in the Whangateau Harbour. None of the variations he recognized Fit exactly the fauna of the Rapanui. The subdominants in both Grace's and Ralph and Yaldwyri's studies are delicate in comparison with the thick-shelled Austrovenus stutchburyi so that it is concluded that the delicate valves of the subdominants were decimated during transport to the site of deposition, whilst A. stutchburyi valves could withstand the rigours of transportation.

The sediments in which the Austrovenus communities occur are described by Grace3 as fine muddy sands.

The cockle occurs both intertidally and subtidally, so that the Rapanui community could have come from either environment. It is suggested that the life assemblage was subtidal as the cockles in the Rapanui collection are large and must have grown in optimum conditions. It must also be noted that several species of molluscs grow to larger sizes in cooler waters, e.g. Haliotis iris and Pecten novaezelandiae are considerably larger in the South Island than the North Island specimens. It is possible that cooler temperatures are responsible for the larger A. stutchburyi specimens.

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5. Anchomasa similis community Anchomasa similis was one of the few species recorded in living position at

Waihi Beach, having bored into the rock of the Tangahoe Formation. A tumbled boulder revealed other species occupying the specialized infaunal habitat. Zelithophaga truncata and A. similis were in place in their borings in the boulder, whilst other vacant borings were occupied by a nestling and crevice fauna, made up of Hiatella australis, Leptomya retiaria, Marikellia rotundata, Modiolaria impacta, and Notirus reflexus. The fauna is comparable with that described by Morton and Miller 5 .

Accessory species of barnacles and bryozoa encrusted the boulder.

6. Scalpomactra scalpellum community McKnight 4 described a Scalpomactra/ Maorimactra community containing

Scalpomactra scalpellum and Maorimactra ordinaria, as well as Tellinella huttoni, Nucula nitidula, Divaricella huttoniana, Nemocardium pulchellum, Pleuromeris zelandica, Nuculana bellula, and Gari lineolata.

The absence of the subdominant M. ordinaria implies that the assemblage is a variation of McKnight's community. Some of the species not recorded for the Rapanui shell beds in the present study, e.g. T. huttoni may have remained at the site of the life assemblage as they are deeper burrowers.

7. Paphies subtriangulatum community A P. subtriangulatum community was recognized intertidally at Takapuna

Beach by Morton and Miller 5 . Grace3 also recorded the occurrence of a similar community at Whangateau Harbour. In the association he lists Baryspira australis and Cominella adspersa as subdominants, with T. spissa as a secondary species. The absence of the two subdominant species is difficult to explain, for the Rapanui fauna. Fleming 1 0 records Baryspira australis as present in the Rapanui Formation but not C. adspersa. It is possible that hermit crabs occupied the shells after the molluscs' death, preventing transportation from the area inhabited by the life assemblage.

8. Paphirus largillierti / Maoricolpus roseus community Grace3 recorded a Paphirus/ Atrina / Maoricolpus community in the

Whangateau Harbour. He also noted that there are very few empty M. roseus shells in the harbour as these provide excellent homes for the hermit crab, Stratiotes setosum. If a similar phenomena occurred in the ancient community then almost all M. roseus shells would have remained in the vicinity of the life assemblage, and transportation to other localities would be rare.

Atrina zelandica rests in the sediment to such a depth that dislodgement is uncommon. The valves are delicate and not designed to withstand the effects of transportation. A. zelandica, therefore, is not expected to form part of the Pleistocene shell beds.

Grace3 also recognized a specialised sand-bank dwelling community which consisted of P. largillierti, and lacked A. zelandica and M. roseus. It is possible that the community represented in the Rapanui shell beds was of this type.

Powell 7 describes the sediment of the Paphirus/ Maoricolpus community as 'mud, with a strong admixture of shell', whilst Grace3 recorded medium to

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fine muddy sand in samples containing/0, largillierti andM. roseus.

Environment of Deposition 1. AtN129/f558

Few communities were represented at the locality, and of these there were predominantly inner shelf faunas rather than immediately subtidal.

The environment of deposition was, therefore, probably similar to that of the inner shelf, today.

2. AtN129/f563 The presence of A. similis in the underlying Tangahoe Formation implies

that the rock surface was swept clear of sand for some time before the deposition of the Rapanui Marine Sand. It is likely that the Tangahoe Formation formed a subtidal shore platform which was wave-beaten. The presence of tumbled boulders implies that the rock surface may have been strewn with boulders derived from concretionary horizons in the Tangahoe Formation.

The diversity of communities coupled with the number of intertidal, or in this case immediately subtidal, communities and species implies that the sands were deposited in, or near, an immediately subtidal situation.

3. AtN137/f692 Only two communities were represented at this locality implying that

environmental conditions tended towards uniformity. It is concluded that the site of deposition was deeper water than the other localities. A more diverse fauna characteristic of a stony seabed (as well as an oyster bed) implies that the fossils may have been transported from stony ground.

4. Overall Conclusions There appears to be a trend from the immediately subtidal fauna at

N129/f563 through an inner shelf environment at N129/f558 to an offshore environment at N137/f692.

The temperature of the sea in which the fossil faunas lived is similar to the Forsterian temperatures of todays marine provinces.

ACKNOWLEDGMENTS I am grateful to Messrs J.A. Grant-Mackie, L . Wakefield and R.F. Whitten

for helpful advice and comments and for identification of some specimens.

REFERENCES

'Fleming, C.A. 1952: A Foveaux Strait oyster bed. N.Z.J.Sci.Tech. 34/2):73-85. 2Graee, R.V. 1966: The bottom communities of the entrance to the Whangateau Harbour.

Tane 72:63-70. 3Grace, R.V. 1972: The benthic ecology of the entrance to the Whangateau Harbour,

Northland, New Zealand. Unpublished PhD thesis, University of Auckland. 4 McKnight« D.G. 1969: Infaunal benthic communities of the New Zealand continental shelf

N.Z.J. mar. Freshwat Res. 3(3) :409-444.

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5Morton, J.E.; Miller, M.C. 1968: "The New Zealand Sea Shore" Collins, London. 638pp. 6Paul L.J . 1966: Observations on past and present distribution of mollusc beds in Ohiwa

Harbour, Bay of Plenty. N.Z.J. Sci. 9(1): 30-40. 7Powell, A.W.B. 1937: Animal communities of the sea-bottom in Auckland and Manukau

Harbours. Trans. Roy. Soc. N.Z. 66(4)-.354401. Lowell, A.W.B. 1957: "Shells of New Zealand" Whitcombe and Tombs. 202pp. 'Ralph, P.M.; Yaldwyn, J.C. 1956: Seafloor animals from the region of Portobello Marine

Biological Station, Otago Harbour. Tuatara 6(2):51-&5. 1 0Fleming, C.A. 1953: The geology of Wanganui Subdivision. N.Z. G. S. Bull.52: 362pp.

Table 1. Species from the Tawera/'Glycymeris community recognized in the Rapanui shell beds.

SPECIES

Tawera spissa Glycymeris modesta Glycymeris laticostata Gari stangeri Venericardia purpurata

Notocorbula zelandica Longimactra elongata Marginella pygmaea Amalda mucronata

ABUNDANCE N129/f5S8 N129/f563 Nl37/f692

A A

() 0

A A O

O () O

()

COMMENTS

Specimens with indented margins at N129/f563 implying high den­sities in the life assemblage. Colour markings preserved

Table 2. Species from the Paphies australe community recognized in the Rapanui shell beds.

SPECIES ABUNDANCE COMMENTS

Paphies australe - V -

Table 3. Species from the Ostrea/'Aulacomya community recognised in the Rapanui shell beds.

SPECIES Ostrea lutaria

Aulacomya maoriana

Barbatia novaezelandiae Chlamys dieffenbachi Modiolus aerolatus Maurea punctulata Buccinulum sp. Trochus viridis Micrelenchus rufozonus

O R

R R

ABUNDANCE

F

R 0

C () () O

()

COMMENTS Some specimens with ornament impressions ol Aulacomya maori­ana n left valves implying that the Aulacomya valves formed at least part of the substrate for the community. One specimen stunted on pos­terior region implying crowding in the life assemblage

Specimens contained G. modesta (some with double valves)

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Sigapatella novaezelandiae 0 0 Coelotrochus tiaratus - o Modelia granosa - R Tugali elegans - 0 May also be from rocky ground

Evechinus chloroticus - R Spine only Terebratella inconspicua - O Notosaria nigricans - 0 Magasella sp. - 0 Balanus decorus 0 O 0 Plates only

Table 4. Species from the Austrovenus community recognised in the Rapanui shell beds.

SPECIES ABUNDANCE COMMENTS Austrovenus stutchburyi 0 C Generally large, encrusted, and Myadora striata 0 — _ bored single valves Pervicacia tristis o Xymene plebeius R -Table 5. Species from xhe Anchomasa community recognised from the basal Rapanui beds.

SPECIES ABUNDANCE COMMENTS

Anchomasa similis R C In place atN129/f563 Zelithophaga truncata - F In place Leptomya retiaria - O In place Marikellya rotundata - 0 In place Hiatella australis - F In place Modiolaria impacta - O In place Balanus trigonus — 0 On tumbled boulder containing

A. similis community Bryozoa — F On tumbled boulder containing/I

similis community

Table 6. Species from the Scalpomactra community recognized in the Rapanui shell beds.

SPECIES ABUNDANCE COMMENTS Scalpomactra scalpellum F — _

Pec ten novaezelandiae R O R Divaricella huttoniana 0 — _

Nucula nitidula o -Table 7. Species from the Paphies subtriangulatum community recognized in the Rapanui shell beds.

SPECIES ABUNDANCE COMMENTS Paphies subtriangulatum o F

Table 8. Species from the Paphirus .Maoricolpus community recognized in the Rapanui! shell beds.

SPECIES ABUNDANCE COMMENTS

Paphirus langillierti - O

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