QH 1 UGUST 1975 VOLUME 69 | S67X p ne D ISSN 0303-2515 INSTRUCTIONS TO AUTHORS 1. MATERIAL should be original and not published elsewhere, in whole or in part. When accepted, copyright becomes the property of the Trustees of the South African Museum. 2. LAYOUT should be as follows: (a) Masthead to consist of Title: informative but concise, without abbreviations and not includingthe names of new genera or species Author’s (s’) name(s) Address(es) of author(s) (institution where work was carried out) Number of illustrations (figures, enumerated maps and tables, in this order) (b) Abstract of not more than 200 words, intelligible to the reader without reference to the text (c) Table of contents giving hierarchy of headings and subheadings (d) Introduction (e) Subject-matter of the paper, divided into sections to correspond with those given in table of contents (f) Summary, if paper is lengthy (g) Acknowledgements (h) References (i) Abbreviations, where these are numerous 3. 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REFERENCES cited in text and synonymies should all be included in the list at the end of the paper, using the Harvard System (ibid., idem., loc. cit., op. cit. are not acceptable): (a) Author’s name and year of publication given in text, e.g.: ‘Smith (1969) describes .. .’ ‘Smith (1969: 36, fig. 16) describes .. .” “As described (Smith 1969a, 19696; Jones 1971)...’ ‘As described (Haughton & Broom 1927)...’ ‘As described (Haughton et al. 1927)...’ Note: no comma separating name and year pagination indicated by colon, not p. names of joint authors connected by ampersand et al. in text for more than two joint authors, but names of all authors given in list of references (b) Full references at the end of the paper, arranged alphabetically by names, chronologically within each name, with suffixes a, b, etc. to the year for more than one paper by the same author in that year, e.g. Smith (1969a, 196965) and not Smith (1969, 1969a). For books give title in italics, edition, volume number, place of publication, publisher. For journal article give title of article, title of journal in italics (abbreviated according to the World list of Scientific periodicals. 4th ed. London: Butterworths, 1963), series in parentheses, volume number, part number (only if independently paged) in parentheses, pagination (first and last pages of article). Examples (note capitalization and punctuation) BULLOUGH, W. S. 1960. Practical invertebrate anatomy. 2nd ed. London: Macmillan. FISCHER, P.-H. 1948. Données sur la résistance et de le vitalité des mollusques.—J. Conch., Paris 88: 100-140. FISCHER, P.-H., DuvAL, M. & Rarry, A. 1933. Etudes sur les échanges respiratoires des littorines.—Archs Zool. exp. gén. 74: 627-634. Koun, A. J. 1960a. Ecological notes on Conus (Mollusca: Gastropoda) in the Trincomalee region of Ceylon.— Ann. Mag. nat. Hist. (13) 2: 309-320. Koun, A. J. 19605. Spawning behaviour, egg masses and larval development in Conus from the Indian Ocean.— Bull. Bingham oceanogr. Coll. 17 (4): 1-51. THIELE, J. 1910. Mollusca: B. Polyplacophora, Gastropoda marina, Bivalvia. In: SCHULTZE, L. Zoologische und anthropologische Ergebnisse einer Forschungsreise im westlichen und zentralen Siid-Afrika 4: 269-270. Jena: Fischer.— Denkschr. med.-naturw. Ges. Jena 16: 269-270. (continued inside back cover) ANNALS OF THE SOUTH AFRICAN MUSEUM ANNALE VAN DIE SUID-AFRIKAANSE MUSEUM Volume 69 Band August 1975 Augustus Part Zz Deel THERMALLY ANOMALOUS LATE PLEISTOCENE MOLLUSCS FROM THE SOUTH-WESTERN CAPE PROVINCE, SOUTH AFRICA By ANTHONY J. TANKARD Cape Town Kaapstad The ANNALS OF THE SOUTH AFRICAN MUSEUM are issued in parts at irregular intervals as material becomes available Obtainable from the South African Museum, P.O. Box 61, Cape Town Die ANNALE VAN DIE SUID-AFRIKAANSE MUSEUM word uitgegee in dele op ongereelde tye na beskikbaarheid van stof Verkrygbaar van die Suid-Afrikaanse Museum, Posbus 61, Kaapstad OUT OF PRINT/UIT DRUK 1, 21, 3, 5-8), 3(1-2, 4-5, t.-p.i.), 51-2, 5, 7-9), 6(1, t—p.i.), 711-3), 8, 91-2), 10(1), 11(1-2, 5, 7, t.—p.i.), 15(5), 24(2), 27, 31(1-3), 33 Price of this part/Prys van hierdie deel R2,90 Trustees of the South African Museum © Trustees van die Suid-Afrikaanse Museum 1975 ISBN 0 949940 70 4 Printed in South Africa by In Suid-Afrika gedruk deur The Rustica Press, Pty., Ltd., Die Rustica-pers, Edms., Bpk., Court Road, Wynberg, Cape Courtweg, Wynberg, Kaap THERMALLY ANOMALOUS LATE PLEISTOCENE MOLLUSCS FROM THE SOUTH-WESTERN CAPE PROVINCE, SOUTH AFRICA By ANTHONY J. TANKARD South African Museum, Cape Town (With 3 figures and 2 tables) [MS. accepted 29 May 1975] ABSTRACT Numerous exposures of Late Pleistocene marine sediments occur on the west and south coasts of the Cape Province. The mollusc fauna, which includes some 124 species, only 3 of which are extinct, lived in sediments deposited during the last interglacial. Two broad and contemporaneous facies are recognized: a cool-water open-coast facies, and a warm-water estuarine-lagoonal facies. Whereas the open-coast facies is characterized by molluscs which presently live on the adjacent coast, the estuarine-lagoonal facies regularly contains tropical species which lived far south of their known present-day geographic ranges. Differences in distribution between the fossil warm-water molluscs and their present-day temperature- sensitive counterparts have been used to interpret Late Pleistocene climatic change in the south-western Cape. These deposits are correlated with the Mediterranean Eutyrrhenian. CONTENTS PAGE Introduction : : : ‘ : : ; . 17 Geologic Setting . : : : : : : ; 18 Methods. : 5 ; : : : ; : 20 Palaeoecology. A : ; f , ; k P| Population Dynamics : : : ; : ; 25 Predation ‘ : : ‘ ‘ : i‘ 29 Teratological Specimens ; : : ; 4 ; 31 Depthy’ =. : 3 : j , ‘ : d 31 Temperature. , : : : ; 3 : 32 Summary : ; : : : 37 Palaeoclimatic Interpretation : 4 ; : 38 Review of Present Climate and Hydrology ; i ; 38 Late Pleistocene Climate . : : : ; : 40 Discussion , : ; ; : ‘ ; , 41 Acknowledgements : ; J i ; ; ; 43 References . ; : : ; 4 ; ; : 43 INTRODUCTION Isolated fossiliferous, marine sediments occur on the wave-cut platform and in the sheltered Late Pleistocene embayments from Elands Bay on the west coast to Knysna on the south coast. The fossils occur in unconsolidated quartzose and shelly quartzose sands. The invertebrate fauna is essentially modern in composition and comprises some 150 species which today live in shallow-water environments. Only three of the mollusc species from the west 17 Ann. S. Afr. Mus. 69 (2), 1975: 17-45, 3 figs., 2 tables 18 ANNALS OF THE SOUTH AFRICAN MUSEUM coast deposits are not known to be living today. In so far as most of these taxa are still living, and are inhabitants of shallow water, they provide ideal material for a palaeoecological study. The Late Pleistocene fauna of the west coast is broadly divisible into two distinct, but contemporaneous, ecologic zones. These are, firstly, a cool-water, open-coast facies characterized by rocky shore and sandy beach assemblages, and secondly, a warm-water, sheltered embayment facies (estuaries and lagoons). Whereas the open-coast facies is laterally continuous, the sheltered embayment facies is restricted in distribution. The open-coast facies is characterized by molluscs which commonly inhabit the present coast. A striking feature of the estuarine-lagoonal facies is the association of extant and extralimital thermo- philic species. (“Thermophilic’ and ‘extralimital’ imply species which occur outside their normal spawning range.) Examination of their present latitudinal ranges indicates that a significantly warmer hydroclimate prevailed when those species were common along the south-western Cape coast. In attempting to reconstruct the palaeoenvironment, a detailed examination of a south coast assemblage is necessary. The fossiliferous deposits at Knysna are therefore included in the study. Late Pleistocene faunas of open-coast and sheltered-embayment aspects from southern California are similarly distinctive. Nearly all the dominant species also inhabit the adjacent coast. But the sheltered embayments also contain a high proportion of thermophilic molluscs which are found far north of their present-day geographic range end-points. Several authors have used these anomalies as a key to Late Pleistocene climatic interpretation (Valentine 1955, 1957, 1961; Valentine & Meade 1961; Addicott & Emerson 1959; Emerson & Chase 1959; Kern 1971). Valentine (1955) explained the diverse nature of the fauna by changes in intensity of the oceanic circulation and upwelling, while the water of the embayments was heated by increased solar radiation. A recent alternative explanation suggests that the larvae of tropical molluscs were transported into the cooler areas by periodic local and temporary current changes, and that they became only temporary, non-breeding, members of the community (Zinsmeister 1974). The south-western Cape Province, with its extensive Late Pleistocene fossiliferous deposits, and its complex present-day ocean current systems, is well placed to make a contribution to the knowledge of Late Pleistocene hydroclimates. The purpose of this paper is to describe the mollusc fauna, particularly between Ysterfontein and Elands Bay, and to examine their palaeoenvironmental significance. The taxonomy of some of these molluscs appears elsewhere (Kilburn & Tankard, in press). GEOLOGIC SETTING The Pleistocene epoch was characterized by the waxing and waning of continental ice sheets with sea level oscillating in sympathy. Relative movement of sea level is shown by the emerged wave-cut platforms, stranded beaches, THERMALLY ANOMALOUS LATE PLEISTOCENE MOLLUSCS Dar-es- Salaam \ TANZANIA = St. Helena Bay 32°45 i) Paternoster 15 » Kruispad Vredenburg re % YASaldanha aldanha iers Bay lopefield Fig. 1. Locality map. The insert (partly after Boss 1969) shows the distribution of present-day coastal forms: black =_ tropical; stippled = warm-—temperate; area bounded by broken line on west coast = cold water fauna. 20 ANNALS OF THE SOUTH AFRICAN MUSEUM and marine terraces. Research on Barbados has suggested the existence of three transgressions in the last interglacial (Eem): +6 m (BI) at 122 ka (i.e. 122 000 years ago); —13 m (BIJ) at 103 ka; —13 m (BIII) at 82 ka (Broecker et al. 1968). These three peaks are confirmed by oxygen isotope studies of deep-sea cores (Shackleton & Opdyke 1973). Along the west coast of South Africa a thin mantle of Late Pleistocene marine sediments overlies earlier Pleistocene, Tertiary, and pre-Tertiary rocks. On the granitic headlands between St. Helena Bay and Ysterfontein a narrow seaward-sloping wave-eroded platform is encountered. St. Helena Bay has formed on Malmesbury rock which weathers more rapidly than the granite and gives a negative relief. The open-coast fossil assemblages were collected from marine deposits overlying the granite platforms, and from exposures in the coastal flats bordering St. Helena Bay. The last interglacial shorelines parallel the present coast very closely, with two exceptions. Whereas the present coastline is interrupted only by Saldanha Bay and its southerly offshoot, Langebaan Lagoon, the Late Pleistocene sea extended up the Berg River valley and along Verlorevlei to form two prominent estuaries. Verlorevlei is now separated from the sea by a bar of Palaeozoic sandstone at 1 m a.s.]. (above mean sea level). These sheltered Late Pleistocene estuaries and lagoons were of considerable palaeogeographic importance since all fossil sites within them contain a significant proportion of thermophilic molluse species. The greatest observed thickness of the open-coast facies exceeds 5 m at Velddrif; the greatest observed thickness of the estuarine facies is 2,2 m at Verlorevlei. The open-coast facies consists of medium-grained quartzose sand and comminuted shell. Frequently there are shell beds and banks with little detrital quartz, indicating a slow supply of detrital sediment. A series of emerged breaker-bars between Velddrif and Laaiplek parallel the modern coast. These are composed largely of shell material. In all the sheltered embayments the fossils are found in fine sands. At Verlorevlei the marine horizon is overlain by coarse, poorly-sorted colluvium. At Churchhaven the marine sediments are carbonate cemented and form an erosional bench just above high-water level. Most of the sites examined, both open-coast and sheltered embayment, are covered with a veneer of wind-blown sand. METHODS The absolute density of fossil specimens of each molluscan species at any particular site is difficult to determine because of the sampling problems inherent on the size of the shell. For instance, ‘Rissoa’ capensis is a small gastropod (usually less than 3 mm) and would frequently number in the hundreds from just 100 g of sediment, while the large (approximately 200 mm) Panopea glycymeris would occur at approximately 1 m intervals. Bearing in mind that the present study is a palaeoecological one, and that the larger THERMALLY ANOMALOUS LATE PLEISTOCENE MOLLUSCS Dh molluscs have been better studied with respect to taxonomy and ecology, emphasis has in all cases been placed on the macro-molluscs. A further source of error arises from the differential fragmentation of shells. Bivalve shells are more easily broken than gastropod shells, and in the living assem- blage they may have been far more abundant than the faunal list suggests. Table 1 should be taken only as an approximation to the original community structure. The procedures adopted in drafting Table 1 are as follows: 1. It was desirable to sample as small an area as possible to obtain not only the absolute density of each species, but also to obtain restricted samples for size-frequency analyses. In most cases a quadrat size of 1 square metre has been found adequate, although quadrat size may have to be adjusted up or down depending upon the relative abundance of specimens. 2. For a shell to be counted as an individual it must be nearly complete, or so nearly so that the remainder could not be identified and counted separately. Left and right valves of each bivalve species were counted separately, and the highest count taken as the total number of individuals of that species in that quadrat. 3. Each species has been recorded in Table | as percentage frequencies in the sample where more than 50 individuals were counted. Where a total of less than 50 individuals were counted they are recorded in Table | as ‘x’. No attempt has been made to relate species to sediment texture since every mollusc at some time must have been living among already dead and fragmented shells. The substrate would thus consist of quartzose sand and _bioclastic material ranging in size from complete shells to finely comminuted fragments. PALAEOECOLOGY In this paper past extensions of tropical and subtropical mollusc geographic ranges are used as a basis for interpreting Late Pleistocene palaeotemperature changes. These inferences are based only on fossils of still extant species, and the assumption (Durham 1950) that stenothermal organisms are in general more critically limited by minimum temperatures than by maximum tempera- tures. The validity of such palaeotemperature inferences depends on the fossils being preserved in the sediments in which they once lived. In describing the relationship between the fossils, after death, and the sedimentary environment, we define the following types of fossil assemblages: 1. Life assemblage: disturbance after death negligible (Hallam 1960). 2. Death assemblage: (i) Indigenous: organic remains disturbed after death but not trans- ported very far (Hallam 1960). (ii) Transported: organic remains introduced from a neighbouring contemporaneous or older environment. 3. Mixed assemblage: this comprises any combinations of the above ANNALS OF THE SOUTH AFRICAN MUSEUM 22 Le 67 OF I> O% I> SACOY YUONUAS DIDNT I> ABIQ) s1sua03jp pavidda prexuwy 2 WING yy QI L'ov x iS bsojnsnsavad Sjsuadva +> S Si x yorewmeyT ounjjaoi0d *D eI (uljewID) vivapnov vnpidaaD (snoeuury) Or [> x Cs x Sisuaulya vaoaddjoD & yosyieg apuuid nqujpy a AQIOMOS vibIINS_Aful DjDIG (MWS “Vv "A €OI unsojnpouofna wnpligoas “> L'€ ddymd 1420y wniys4ad x 401 SAVOY VaUuINsups * 7 yolewey oiafiuiavs +7 ssnery sisuadva Dpaj1dAny, AQIOMOG Sisuadva “Jd .vossiy, O[IYL, SIsua0sjp vossiy, (yos}reg) ae DIIDY “JO DlApunjAoD I> Yosyleg sisuapadfjo viunaypy tddyrmd 9p 07% 1G SISUBDUSAUY DUJAOI INT SnOeUUrT SnoaijoULADS * 7 iss ess UNOWUD Ssupplo Ogun y, (swepy ‘y) tS DIDINIIGAO DjJaJoUOJsopnasg OT ‘ds npjapanjos 8‘T (UIPWOL) OLpubxajp bajluojpy iC me LC V6 LST C= vl uojUYy) 21032}404 *O I> (zyTUWOYD) YUuj814) ajajsd{xOE I> ‘ds pyoulw 1> (exUIIA]) 42919 vINGGID 8°6 (AqioMog) juoynf SISUAZAADNS SnNp1ADYyIUDD eee? 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HL 07 snovuury D4Dqung ‘g CL eS iL ssnely Jayjiauasun vjjaIvg (ufyeuD) re snpnounjoad uolajayy Oi Gi Se ees AQIOMOG S1Iqvinu “J Is (AqIaM0S) Dj4adp DIJAANSSL (ayjeup) I> ss le pyjainas sodapiyaqjquy vdaodouLlsv9 Le 6r tT ¢* SUN aes x VVV er ve) I 6 SE co a x xn xX x x x xX xX x x VV pusduyy A00/T (44 1%4 0z 61 8T LI oT ST vl £I TI IT or 6 8 {t 9 g v 5 z I uoosvT SOSN][OJ OUSO}SIO[g 9}v’J JO (%) souUepunqy pue uoNNnqisiq T alav Lbs 23 THERMALLY ANOMALOUS LATE PLEISTOCENE MOLLUSCS x 9% AQIOMOG sisuaosjp "CE LES uosuey vuljUaIs °C Gas Wis “0S — 16/7 = iis Le WOMEN 12U0jS194I Da4ISE) AQIOMOS vpunjos “T | (2yruIEYD) s171504f viurT AQIOMOG DIDISOIIIINS UIjIag (PABByY) snjoury stuomjyD (ssneryy) SIDUOIPIAaU SNj1jAUOLOYD I x (snevuurq) vusad pusag 7 rs (eulOY]) 4270 vdwuoavjnp (pop) ze Djopidsnaig vuvjnonny VIATVAIG LT 91 OI I> PivUITeD 2 AONDH sisuadva ‘gs zs eI ST ssnely viadsp viupuoydis: AQGIOMOS vIpuydo sce (snoeuUury) vyndup viuojyng I> PINOD vsojnqni vuyoyd yD (snoeeuUry) vJ4adp auyiyg yosyieg 2uojing vyno1sury preyuer lL ZW UINGIDY isaavp vdng “ds pppiuoqgany, I> UISSO]D ISSNBAY VIPIUOGANT, (Yos}ieg) vauvsp vious AqIOeMOg I 9 SISUIOS]V SNIN[IS SNUOD > OT (u1Og) vJvnUIS vJJaU0ND I> (PINOD) vxajdiup vavyita I> (QAB9Y) ASuadvI winjjixa4, > > OT ee 8G x pee G:C O10), 101 ee O16T THATS, PSouuanian Daly ee spurpy Dvuadid “yy = x Xx ssnely sisuadvd vpyaulsiwjpyy > QADOY SMUT] VIVO = SULPY "W Susolgads "Ay xX SII OE x > xX Lp pavuieg snouvpndoos “Ny iz se ta 182 (suepy ‘W) smjjaiwayd “Ny x (qeyUN) suuvIssNDBAY “AT I> (aayUNG) snUvIYIOY “NV AQIOMOS T> x SNIIZOJVUD SNIADSSDNT (urawy) vudissMoavy "g UdYOSNeA] SOUTIP “J 161 > I> (qrginsnag) pou “dg iL xX 6'SE ie x (Suipgy) veova -ddod vaovaddod puadnuang I X96 serol x I> x I> AGT (Jorvweq) vsowovnbds v7 > (ssnery) Vigne VT fe st x (snovuury) BypnsU9 sy T (snevuury) l= MINLIDJOP WNYOWUAD (uyyeurD) I> me) WAY |e Me ce snsuy wnupoongosap is VV no a ae an I ik ihe OP 901 8 ANNALS OF THE SOUTH AFRICAN MUSEUM = NX v'09 c'se 187 6-2E 6Sb oe 6 007 61h O€E€ OLE s[enprArpur orrydourroyy % 8‘ZI 6S% 97 OTE 697 69 OEE O07 8ST TZ 19% OSL 009 soroods orrydoursey} % 76P I@l 9S% S8 86 €ST ESI 07 9SI Os Sol cL Shr O01 O07 S[ENPIAIPUL Joquinu [ej}O J, Il nnn ec eer nnn rrr In nn nS ES eT (u10g) s1uaudod]3 padoung I> Gu Wi xX 9 I> UNJaUDH DAN *A ‘i UTOWUD Wh €“€ OT x xX sZI xX ofss 2 fa Ss 1S sisuajp8auas sidniaua) (yorewey]) vajioday *q ‘6£ Or xX 981 0% x O05 OL9 OT8 shovuury snuidn) piuisog I uo YINVFY 11/0] VIIALT v'sI SE 88 eS LRG I> snovuUry vsoonddad snuaA o'r ee yosyeg sisuapasfjo vaoay T, AQIOMOG o'r OPE x x sysuadvo puyjajounUuDs gy pivyuey, 3W UING|ES wnidonjouvs *G T> Or x SG (zyTUWIOYD) DAdas xDUOg oT 0% (1oyunq) Majuny syaupsodaT pieyue eT wy UINGIDy vsougy *D < b (UITOULD) vopbioU DU_AISDH => (OS 07% (AOWIOY) VIDISOIIL "Ay I> (ssnery{) S1jD4011] “A > x KQioMog IpsofMn1a DULOODIy > Le AQAOMOG 1S1AYIII3 *T, 1> wal! UIJOUD YAIID/1AT “T, I> ee vet v6 TOL lie 8 OOL §3=0°9 «= UOWID Sisualpaspsppoiu * f, I> AQIOMOG 14 quosuod buljja J, = (Queuusg) snpianjad soxoyd ee > Tvl 0% x x PST O08 9% 007 06 (AqIOMOS) YINID.AY] SAdLAOT “0 non VV i TT a Gp, N io 5 = =o to x VR VV TOUS OOM MCC COG Ol St 21 9% St Pl cf, ch Tt! OF 6 8 L 9 ¢ v € z I (piu02) VINTWAIE uoosvyT SOSN]JOP, OUdIO}SIO[q 91" JO (%) souvpunqy pur voNNqiysid (‘piuor) J. aTav THERMALLY ANOMALOUS LATE PLEISTOCENE MOLLUSCS DP) possibilities, and is the general case (Hallam 1960). The status of this assemblage is Clarified by describing it, for instance, as a mixed life and indigenous death assemblage. In the present study this is the commonest case. POPULATION DYNAMICS Boucot (1953), Olson (1957), Craig & Hallam (1963), and Craig & Oertel (1966) have attempted to discriminate between life and indigenous death assemblages on the one hand and transported death assemblages on the other, by using size-frequency distributions. According to Boucot an indigenous death assemblage is characterized by a positively skewed distribution (large number of small forms), while negative skewness or a normal distribution characterizes the transported death assemblage. He suggested that negative skewness resulted from winnowing of the smaller shells. Craig & Oertel (1966) question this and maintain that the shape of the size-frequency distribution in a fossil population depends principally upon the growth-rate and mortality-rate of the relevant species. In this way negative skewness could arise from a decreasing growth-rate with constant mortality-rate which would concentrate the older age-classes in a few size-classes. Craig and Oertel suggest the following options: Growth-rate Mortality-rate Size-frequency distribution decreasing constant negative skewness constant decreasing positive skewness constant increasing flattening of curve, possibly negative skewness constant constant mirror image of living popul- ation by dead population Growth-rate and mortality-rate complement each other when one decreases and the other increases, but cancel each other if both increase or decrease. Higher mortality-rates which favour large populations may result from a fluctuating environment (Valentine 1971). Mortality is affected by nutrients, temperature, and salinity changes. In general, invertebrates have higher mortality-rates in the early stages of life, but the rate may be lower in some species than others (Craig & Oertel 1966). Environmental conditions in estuaries and lagoons would be expected to fluctuate widely and rapidly. They would be expected to show a variable salinity range due to evaporation and influx of fresh water, and a high diurnal temperature range. Furthermore, seasonal upwelling of cold water along the Cape west coast leads to marked instability of the environment on the open coast too. Although growth-rate is an important factor in this type of study, it is one of the attributes about which there is little information. There is evidence, however, that most bivalves maintain a slightly decreasing, but nearly linear, growth-rate throughout life (Craig & Oertel 1966). This has been shown. to be the case for Cardium edule, Tapes japonica, Dosinia exolata, and Venus striatula (Kristensen 1959; Wilbur & Yonge 1964). 26 ANNALS OF THE SOUTH AFRICAN MUSEUM The most likely effect of the unstable environmental characteristics of the west and south coasts of the Cape would be a high mortality-rate among the juvenile molluscs. Assuming a constant growth-rate for the bivalves, the inter- play of mortality-rate and growth-rate should lead to a positively skewed size-frequency distribution for a life assemblage, or an indigenous death assemblage. Besides the effect of growth-rate and mortality-rate on the shape of the histogram, the assemblage may be affected by post-death mechanical change such as sorting or winnowing by currents (Boucot 1953) and selective frag- mentation, and solution of the smaller or thinner shells. Experience with the west coast fossils shows that crushing and fracturing is of primary importance, and affects the bivalves more than the gastropods. The large number of fragmented shells in both the open-coast facies and estuarine-lagoonal facies sediments suggests death assemblages that have undergone considerable modification by wave-action. Whereas the open-coast bivalves are usually disarticulated, those in the estuarine-lagoonal sediments show a high degree of articulation. These sheltered embayment sites contain epifaunal and infaunal molluscs, some of which are preserved in their living positions. Size-frequency distributions for some of the bivalves are shown in Figure 2. Since the warm-water element inhabited the Late Pleistocene lagoons and estuaries, samples from those environments have been analysed in most detail. A single sample from a known high-energy open-coast site was examined in detail. Figure 2A shows the size-frequency distribution for Venerupis senegalensis from the Velddrif site. The field-setting suggests a transported death assemblage in which vigorous wave-action piled shell debris up to form a breaker-bar. In these deposits the thinner shells have generally been fragmented. The V. senegalensis population is composed of thick-shelled forms. Its estuarine ecomorph, on the other hand, has a thinner shell and is of more constant morphology. The Velddrif V. senegalensis shows marked negative skewness (—0,82). It has an articulation ratio less than 0,05. Articulation ratio is defined as the ratio of complete shells/4 (RV+LYV). Size-frequency distributions of bivalves from the estuarine-lagoonal facies differ from the pattern of the Velddrif example. Tellina madagascariensis from Verlorevlei (Fig. 2C) and Churchhaven (Fig. 2H) tends to have a flattened histogram. The Verlorevlei Tellina has a size-peak at 64-66 mm, and the Churchhaven specimens at 56-60 mm. While the narrower size range of the Verlorevlei material (50-86 mm) suggests a transported death assemblage, the greater range of the Churchhaven material (16-74 mm; more juveniles) suggests an indigenous death assemblage. That this argument can be misleading is shown by their articulation ratios of 1,46 and 0,47 respectively. At both of these localities T. madagascariensis is associated with comminuted shells and dis- articulated valves of other species. JT. madagascariensis at Verlorevlei was observed in a nearly horizontal attitude as were the other bivalves. But whereas THERMALLY ANOMALOUS LATE PLEISTOCENE MOLLUSCS 12 A 12 Dosinia lupinus RV 104 Venerupis senegalensis LV 104 Verloreviei (1) Art. rat. 040 Velddrif (9) 3 erat Art. rat.