the chronology of the cercopithecoidea of east africa

17
HUMAN EVOLUTION Vol. 2 - N. 1 (1-17) - 1987 M. Pickford Palaeontology Dept., Johannes-Gutenberg-Universitat- Mainz, 21 Saarstrasse, D-6500 Mainz Federal Republic o/ Germany and Institut de PalSontologie, 8 Rue Buffon, Paris, 75005, France Key words: Cercopithecidae, Chronology, phylogeny East Africa The Chronology of the Cercopithecoidea of East Africa The East African fossil record of cercopithecoids spans nearly 20 m. y. Throughout the Miocene Epoch, the diversity of monkeys was low, although at some localities the numbers of individuals is rather high. During the Plio-Pleistocene in contrast, there was a major radiation, or radiations of monkeys, involving both colo- bines and cercopithecines. A late Pleistocene to Recent radiation within the genus Cercopithecus still seems to be under way. The history of diversity in the monkeys is in many ways a chronologi- cal inverted mirror image of the diversity history of the hominoid ~ rimates, which were highly diverse during the lower Miocene, ut became less diverse through time. The east african cercopithe- coid record is the only one which spans much of the Neogene, and it is consequently the main one by which detailed cercopithecoid cladogenetic and anagenetic events can be dated. In this respect, it provides constraints for interpreting branching schemes derived from neontological evidence. Most of the neontological estimates for monkey origins appear to be too old, the fossil evidence suggesting that the origin of the superfamily Cercopithecoidea and the origins of the Colobinae, Papionini and Cercopithecini are younger than usually suggested on neontological evidence. The superfamily is probably no older than 25 m. y., the colobines diverged as a distinctive group about 12-14 m. y. ago, the Papion- ines about 8-10 m. y. ago and the Cercopithecines perhaps as late as 7 m. y. However, since the Miocene fossil record is rather spotty, these fossil-based estimates may be revised downwards with new discoveries. They are unlikely to be revised upwards. The sequence and timing of cladogenetic events deduced from the east african evidence indicates that all the modern subfamilies arose in Africa, and snbsequently spread to Europe and Asia. The sequence of fossil events is in close agreement with neontological evidence such as karyology and molecular anthropology. It is only in the calibration of the sequence that there is disagreement between the fossil and neontological evidence. Strangely, the polarity of the differences in opinion are opposite to those con- cerning the hominoids, in which the neontological evidence has suggested much younger divergence dates than did the fossil evidence as perceived in the 1960's and 1970's. Geological Setting Virtually all the fossil cercopithecoid sites in East Africa occupy sedimentary succes- sion in which volcanic strata occur. Many of the volcanogenic levels can be, or have been dated by radioisotopic means (DRAKE et al., in prep; BROWN & FEIBEL, 1986; KALB et aL, 1982; NAKAJIMA & TORII, in prep.; MAcDOuGALL & WATKINS, 1985; BISHOP et aL, 1969). The east african record therefore is of crucial importance, in that it has become the yardstick by which sequences elsewhere in Africa and the Old World are compared. Such comparisons are usually of a biostratigraphic nature, utilising other mammalian groups,

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Page 1: The chronology of the cercopithecoidea of East Africa

HUMAN EVOLUTION Vol. 2 - N. 1 (1-17) - 1987

M . Pickford

Palaeontology Dept., Johannes-Gutenberg- Universitat- Mainz, 21 Saarstrasse, D-6500 Mainz Federal Republic o/ Germany and Institut de PalSontologie, 8 Rue Buffon, Paris, 75005, France

Key words: Cercopithecidae, Chronology, phylogeny East Africa

The Chronology of the Cercopithecoidea of East Africa

The East African fossil record of cercopithecoids spans nearly 20 m. y. Throughout the Miocene Epoch, the diversity of monkeys was low, although at some localities the numbers of individuals is rather high. During the Plio-Pleistocene in contrast, there was a major radiation, or radiations of monkeys, involving both colo- bines and cercopithecines. A late Pleistocene to Recent radiation within the genus Cercopithecus still seems to be under way. The history of diversity in the monkeys is in many ways a chronologi- cal inverted mirror image of the diversity history of the hominoid

~ rimates, which were highly diverse during the lower Miocene, ut became less diverse through time. The east african cercopithe-

coid record is the only one which spans much of the Neogene, and it is consequently the main one by which detailed cercopithecoid cladogenetic and anagenetic events can be dated. In this respect, it provides constraints for interpreting branching schemes derived from neontological evidence. Most of the neontological estimates for monkey origins appear to be too old, the fossil evidence suggesting that the origin of the superfamily Cercopithecoidea and the origins of the Colobinae, Papionini and Cercopithecini are younger than usually suggested on neontological evidence. The superfamily is probably no older than 25 m. y., the colobines diverged as a distinctive group about 12-14 m. y. ago, the Papion- ines about 8-10 m. y. ago and the Cercopithecines perhaps as late as 7 m. y. However, since the Miocene fossil record is rather spotty, these fossil-based estimates may be revised downwards with new discoveries. They are unlikely to be revised upwards. The sequence and timing of cladogenetic events deduced from the east african evidence indicates that all the modern subfamilies arose in Africa, and snbsequently spread to Europe and Asia. The sequence of fossil events is in close agreement with neontological evidence such as karyology and molecular anthropology. It is only in the calibration of the sequence that there is disagreement between the fossil and neontological evidence. Strangely, the polarity of the differences in opinion are opposite to those con- cerning the hominoids, in which the neontological evidence has suggested much younger divergence dates than did the fossil evidence as perceived in the 1960's and 1970's.

Geological Setting

Virtually all the fossil cercopi thecoid sites in East Afr ica occupy sedimentary succes- sion in which volcanic s t ra ta occur. Many of the volcanogenic levels can be, or have been da ted by radioisotopic means (DRAKE et al., in prep; BROWN & FEIBEL, 1986; KALB et aL, 1982; NAKAJIMA & TORII, in prep.; MAcDOuGALL & WATKINS, 1985; BISHOP et aL, 1969). The east african record therefore is of crucial importance, in that it has become the yardst ick by which sequences elsewhere in Afr ica and the Old W o r l d are compared. Such comparisons are usually of a biost ra t igraphic nature, uti l ising other mammalian groups,

Page 2: The chronology of the cercopithecoidea of East Africa

2 PICKFORD

particularly the Suidae and Elephantidae. There is no reason however, to prevent the cercopithecoids from being used in a like manner, except perhaps that monkeys tend to be rather rare in the Miocene. For the Plio-Pleistocene, monkeys are of potentially greater interest, and indeed cercopithecoid assemblages with chronological connotations have already been proposed as a means of correlating between sites in eastern and southern Africa (DELsON, 1984). The only place that these cercopitheoid assemblages can be dated by geophysical means is in East Africa, and this contribution summarises the evidence presented so far.

Some controversy has arisen during the past two decades, because there were some contradictions between the radioisotopic chronology and that worked out on the basis of fossils (BRowN & FEmEL, 1986). It is interesting to note that in every case so far identified as representing a clash between the two methods, the biostratigraphic method has yielded the more accurate initial assessment of the age of the deposits. Improvements in radioiso- topic dating techniques have been partly responsible for the rejection of age determina- tions made in earlier programmes of research, but the greatest changes have come with determination of the proper geological contexts of the date samples. The accurate mapping of strata and the correct determination of the geological context of the sample is an utterly crucial basic fact that has to be determined prior to dating a specimen. Without proper pedigree, radioisotopic dates are vulnerable, bordering, on meaningless. Fossils in contrast can yield evidence of age, even if they are on occasion collected with less than perfect control, although it is just as crucial for other reasons to be as accurate in determining the context of fossils as for date samples.

In one or two instances, it has been possible in retrospect to reconstruct the original context of key fossils by detailed mapping of the regions from which they were collected, especially if preservation characteristics vary from stratum to stratum within the confines of the locality. For example, it is clear from the geological map of Kanjera, and the preservation characteristics of fossils in the Kanjera Badlands (PIcKFORD, in press), that all the Kanjera Theropithecus oswaldi material including the holotype, came from a stratum in faulted contact with strata yielding mid-Pleistocene fauna and artefacts. The Kanjera Theropithecus is therefore not the same age as the mid-Pleistocene fauna, as was so often claimed by LEAKEY (1967, LEAKEY & WHITWORTH, 1958), and is considered by PICI(FORD (in press) to be appreciably older, probably lower Pleistocene or even upper Pliocene.

Lower and middle Miocene fossil cercopithecoids are known from a variety of geological settings. The earliest fossil monkeys known in East Africa come from volcano- genic sediments which accumulated on the flanks of a carbonatite]nephelinite volcano (Napak, Uganda) but such sediments are usually devoid of monkeys. In contrast, they usually yield rich and diverse assemblages of hominoids (PIC~FORB, 1981; PICKFORD et al., 1986) and records of the existence of monkeys in such deposits (eg LEAK~Y, 1946) have usually been based on misidentified fossils.

The richest assemblages of Miocene monkeys have been found in fluviatile sediments which accumulated in floodplain settings in tectonic basins. At Buluk, for example (LEAKEY, 1985) several fossil monkeys were found by the author in fluvial deposits of a large river system. At Maboko (PIcKFORB, 1985) more than 700 fossil monkey specimens have been recovered from clays and tuffaceous sediments which accumulated in a dry floodplain facies, in which calcrete pedogenesis was an important factor. A few monkey fossil have been found by the author in the Nachola Formation, northern Kenya, in shales which accumulated under very quiet depositional conditions. Other examples, although rarer, have been noted at Loperot (SZALAY & DELSON, 1979) and possibly Moruorot where a few postcranial bones have come to light.

Page 3: The chronology of the cercopithecoidea of East Africa

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Figure 1. - Fossil cercopithecoid localities of Eastern Africa. S = Suguta; C = Chemeron S.; Lainyam. = Lainyamok.

Page 4: The chronology of the cercopithecoidea of East Africa

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�9 / Fossil C RCOPITH COIO i. KE.YA

4 PICKFORD

Figure 2. - Fossil cercopithecoid localities in Kenya. The majority of sites occurs in the Gregory and Nyanza Rift Valleys. Marsabit has yielded a small Theropithecus and Nyeri a large species of the same genus. Ngeringerowa is close to Ngorora: Lukeino is close to Mpesida. Loboi is a latest Pleistocene site between Chesowanja and Kapthurin. Mf = Mfwangano.

In contrast, the number of Miocene monkey specimens found in palaeosols is very low. For example, in the richly fossiliferous deposits at Koru, Legetet, Chamtwara, Songhor, Rusinga, Uyoma and Fort Ternan, no fossil monkeys have been found (despite statements to the contrary e.g. by LEAI(EY (1946, 1967, 1968, 1969, 1970)).

Evidence from fossil gastropods suggest that the sites with the richest assemblages of Miocene monkeys accumulated under rather arid conditions, and that monkeys were

Page 5: The chronology of the cercopithecoidea of East Africa

THE CHRONOLOGY OF THE CERCOPITHECOIDEA 5

probably absent from areas in which forest was the dominant vegetation type. It is postulated that monkeys occupied a gallery niche in such arid to semi-arid countryside (PICKFORD & SENUT, in prep.).

The few fossil monkeys from upper Miocene sites such as Ngorora and Ngeringerowa (BENEFIT & PICKFORD, 1986) fit into the same broad pattern, the fossils having been found in fluviatile or marginal (littoral) lacustrine deposits. Indeed, to a great extent, most of the known Plio-Pleistocene monkey localities fall into the same depositional categories. There is therefore a major bias in depositional environment towards fluviatile and lake marginal facies. At Laetoli, Tanzania, however, the fossil monkeys come from palaeosol horizons and subaerially deposited tufts with little if any indication of deposition by surface waters (HARRIS, 1985).

The majority of fossil sites in East Africa accumulated in rift valleys (PICKFORD, 1984, in press). Their depositional settings therefore differ rather fundamentally from those monkeys found in southern Africa and Angola. They also contrast with the sites in North Africa such as Moghara, Gebel Zelten, Sahabi, Marceau and Wadi Natrun. Indeed, if it hadn't been for the tectonic and volcanic activity which so characterised Neogene geological events in East Africa, there probably wouldn't have been the rich fossil record that we have. The same applies to the fossil records of western Uganda and eastern Zaire, in the region of the western Rift Valley, and the Pliocene strata of Malawi.

Chronology of East African Cercopithecoid localities

Fossil monkeys from East Africa occur throughout the late Neogene period, albeit with gaps in the record here and there (see references in bibliography which are too numerous to list individually here). Figures 3 and 4 summarise current information concerning the ages of the various localities known to have yielded cercopithecids. Many of the sites have been dated by radioisotopic techniques, and these have in turn formed a calibrated basis from which to make biostratigraphic correlations for sites which can't for the moment be dated by geophysical means. PaXeomagnetic stratigraphy is an important additional method for Plio-Pleistocene periods but has had little success if not strongly supported by other techniques. It therefore provides confirmatory evidence where ages of strata are already known or perhaps reasonably estimated. In some cases, particularly if long sequences of strata are available as in the Siwaliks of Pakistan, palaeomagnetic stratigraphy can help to refine age estimates provided by other means.

During the Plio-Pleistocene period, monkeys were relatively common and diverse in East and Southern Africa, with the result that a sequence of cercopithecoid faunas can be erected. DELSON (1984) has suggested that comparble sequences occurred in East and Southern Africa. The chances seem to be high that the two sequences were contemporary or only slightly out of phase. He has therefore proposed that such sequences can be used for correlation purposes between sites in East and South Africa.

Succession of cercopithecoid faunas of East Africa

Three subfamilies of cercopithecoids occur in East African fossil deposits. The lower and middle Miocene monkeys are all assignable to Victoriapithecinae, although some authors ascribe these early remains to the two extant subfamilies (DELSON, 1975).

Page 6: The chronology of the cercopithecoidea of East Africa

6 PICKFORD

10

EAST AFRICAN CERCOPITHECOID SITES

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Figure 3. = Ages of East African cercopithecoid localities. Sites in the left hand column have radioisotopic age controls; those in the centre and right are positioned by biostratigraphic means. (Ongoliba, Zaire is omitted) For longer successions of Plio-Pleistocene strata see Figure 4.

Dentally it is difficult to sustain the subdivision of the Maboko Victoriapithecus sample, (BENEFIT & PICKFORD, 1986) but that at least two taxa exist in the sample, is evident from a perusal of the post-cranial bones (PIcKFORD & SENUT in press). Fossils from Buluk have recently been identified as Prohylobates a genus hitherto recorded only from North Africa (SIMONS, 1969; DELSON, 1975). However, the Buluk fossils are comparable in many ways to Victoriapithecus, and the two genera may be synonymous (LEAKEr, 1985). Undescribed fossils belonging to Victoriapithecus have recently been found in the Nachola area, north

Page 7: The chronology of the cercopithecoidea of East Africa

THE CHRONOLOGY OF THE cERcoPlTHECOIDEA 7

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Figure 4. - East African Plio-Pleistocene cercopithecoid localities. Summarised from BONNEFILLE et al. (1973); KALB et aL (1982); BROWN & FEIBEL (1986); DELSON (1984).

central Kenya, and they have been known for some time to occur at Loperot. The oldest known victoriapithecin~ is dated at about 19-20 m. y. on the basis of an isolated molar and a radio-ulna from Napak, Uganda (PILBEAM & WALKER, 1968; PICKFORD et al, 1986; SENUT, 1986). The youngest victoriapithecine known comes from Nachola, dated about 15.5 m.y . (NAKA~MA & TORII, in prep).

There is a gap of some 3-3.5 m. y. between the youngest victoriapithecine and the earliest known colobine, and a gap of 7.5 m. y. between victorapithecines and cercopithe- cines. The details of the transition between the extinct lower Miocene forms and the extant subfamilies are therefore unobtainable at present. In both cases, the earliest known colobines and cercopithecines are unequivocally modern in aspect, in contrast to victoria- pithecines which are primitive in a number of dental features (VON KOENIGSWALD, 1969).

Colobines are rare throughout the upper Miocene of East Africa being known by a mandible from Ngeringerowa, an isolated molar from Nakali and possibly a premolar from Ngorora (BENEFIT and PICKFORD, 1986). A few isolated teeth have also been collected from Lukeino and Mpesida, although the identification of these specimens is not clearcut. Colobines have also been found in late Miocene sediments of Ethiopia aged about 6 m. y. (KALR et al., 1982). It isn't until the Pliocene that colobines become common in the East African fossil record, and at the same time become very diverse (DELSON, 1984). Many of the Plio-Pleistocene colobines, such as Cercopithecoides were terrestrial or were more terrestrial than living Colobus. All these terrestrial to semi-terrestrial cotobines became extinct during the early to late Pleistocene, leaving only the fully arboreal genera Colobus and Procolobus to survive to the present day.

Page 8: The chronology of the cercopithecoidea of East Africa

8 HCKFORD

Cercopithecines are also rare throughout the upper Miocene of East Africa. The earliest record in East Africa is about 6-7 m. y. which is also approximately the age of the North African cercopithecine localities at Marceau (= Menacer), Sahabi and Wadi Natrun (ARAMBOURG, 1959; BOAZ et aL, 1979; STROMER, 1913, 1920). Possible cercopith- ecine teeth have been found at Mpesida and Lukeino, Kenya and Adu-Asa, Ethiopia (KALB et al., 1982) but they remain undescribed. From the P1iocene however, there are several described species, the earliest being Papionini such as Parapapio at Lothagam and Kalaloo. This genus ranges upwards to about 2.5 or perhaps even 2 m. y. ago. In these later Pliocene deposits Parapapio is joined by a large number of cercopithecines, almost all of them Papionini. The only other cercopithecine known in the late Pliocene is Cercopithe- cus which seems to be rare throughout the late Pliocene and the Pleistocene deposits of eastern Equatorial Africa, only becoming common in latest Pleistocene, Holocene and modern deposits such as Loboi (Baringo, Kenya).

Concordance between palaeontologically and neontologically derived sequences

It is interesting to note that the order in which each subgroup of monkeys appears in the fossil record of East Africa, is precisely the order in which anatomical, karyological and molecular anthropological studies suggest that they evolved phylogenetically. Thus Colobinae appeared before Cercopithecinae, and within the cercopithecines Papionini appeared prior to the Cercopithecini. Within the Papionini, the sequence of appearance as reconstructed from the fossil record was Macaca (in North Africa, but not known from East Africa), Parapapio (extinct), Theropithecus, and later Papio and Cercocebus (approxi- mately at the same time).

Given this concordance, it is tempting to suggest that the sequence and timing of events as reconstructed from the fossil record is similar to the actual sequence and timing of phylogenetic events that occurred within the superfamily. Although the Miocene cercopithecoid record of Africa is spotty, with significant gaps, monkeys were becoming fossilised (and later were found by palaeontologists) and the chances seem to be small that any particular group could have been present for long periods without entering the fossil state. If this is so, then one can estimate the times of divergence within the superfamily. It is suggested therefore that colobines became a distinctive subfamily of cerc0pithecoids about 12-14 m. y. ago, whereas cercopithecines diverged later, perhaps 8-10 m. y. ago. Macacines evolved by 8-9 m. y. ago, geladas by about 4-4.5 m. y. along with baboons and mangabeys. Vervets branched off later, perhaps about 4 m. y. ago and the genus Colobus about 3 m.y. ago.

A comparable sequence of appearances is reconstructed from the Eurasian fossil record suggesting that colobines evolved earlier than cercopithecines. Thus the lineages leading to Presbytis and other asian colobines diverged earlier than those leading to Macaca.

Coincidences between climatic changes and cercopithecoid evolution

An examination of the d180 curve for benthic foraminifera of the Atlantic and Pacific Oceans (MILLER & FAIRBANKS, 1985) reveals the existence of several rather abrupt changes or <<events~>. The more prominent of these may be described as megaevents, since the changes were abrupt and of large magnitude.

Page 9: The chronology of the cercopithecoidea of East Africa

THE CHRONOLOGY OF THE CERCOPITHECOIDEA 9

DISTRIBUTION

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Figure 5. - Chronological ranges of african cercopithecoids. Note that african cercopithecoids are here classified into three subfamilies. Note also the great increase in diversity which occurred during the Plio- Pleistocene.

Page 10: The chronology of the cercopithecoidea of East Africa

1 0 PICKFORD

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Figure 6. - Sequence and timing of events in cercopithecoid phylogeny. Major groups of cercopithecoids arranged according to their fossil records and divergence times estimated from fossil evidence. A = Colobine invasion of Europe from Africa; B = Major radiation of colobines in East and South Africa; C = Origin of the genus Colobus; D = Macaque invasion of Europe from Africa; E = major radiation of papionines (Parapapio, Papio, Theropithecus and Cercocebus) in East and South Africa; F = Origin of the genus Cercopithecus; G = major radiation of the genus Cercopithecus.

Recently, one of these megaevents, the mid-Miocene heating event has been correlat- ed with a continental event in East Africa (the Maboko event of PICKrORD, in prep). A comparably obvious event at the Eocene/Oligocene boundary has recently been correlated accurately with the <~grande coupure~> of Europe and the Plio-Pleistocene event has been correlated to the <<great american faunal exchange~ (MCKENNA, 1985). The mid-Miocene heating event, known in Kenya as the Maboko event, has been identified in other parts of the world, notably in the Mediterranean region where it is called the Langhian, in Japan where it is known as the Kurosedani event (KARYU et al., 1984) and in the Paratethys (CHEPALYGA, 1985). The evidence so far available suggests that these d180 events had effects which were global in scale, and that they affected both the oceans and the continents.

Comparison of the dlSO curve and the cercopithecoid fossil record reveals concor- dances between patterns of evolution in the monkeys and major d180 events. It is postulated that major continental climatic changes coincident with dlsO events had an impact on cercopithecoid distribution patterns and in the long term resulted in <<forced~ evolution of the family.

The richly fossiliferous lower Miocene strata of Western Kenya are devoid of fossil monkeys; none have been reliably recorded during more than 50 years of collecting during which hundreds of thousands of mammal fossils (including numerous hominoids and other primates) have been recovered. In contrast, as soon as we enter the lower part of the middle Miocene (17.5 m. y.) at the beginning of the Maboko heating event, monkeys occur in several sites, in some of which they occur in abundance (PIcKFORD, 1986). It is

Page 11: The chronology of the cercopithecoidea of East Africa

T H E CHRONOLOGY OF T H E C E R C O P I T H E C O I D E A 1 1

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Figure 7. - Correlations between a d~sO curve and cercopithecoid history. The dlsO curve is from MILLER FAmBANKS (1985) being determinations from benthic foraminifera of the Pacific and Atlantic Oceans. A - G are significant events in cercopithecoid history (and those of other mammals and gastropods). Major cercopithecid sites plotted in the diagram have provided most of the evidence concerning changes in cercopithecoid faunas. Numerous sites omitted from this diagram (PIcKFORD, 1984, in press) provide evidence for other periods during which monkeys are lacking in the east african fossil record.

postulated that victoriapithecines evolved elsewhere in Africa, perhaps during the late Oligocene (their existence during the lower Miocene is demonstrated by rare fossils at Napak, Uganda) but were only able to inhabit Western Kenya when suitably openl rather dry habitats became established there during the Maboko event (PICKFORD & SENUT, in prep) (A in Figure 7). Victoriapithecines survived in East Africa only until the mid- Miocene cooling event (15-14 m. y. ago). For example, they have not been found in the richly fossiliferous Fort Ternan site, nor in any of the late middle Miocene sites in the Gregory Rift Valley and it seems likely that they became extinct locally as a result of the cooling event (B in Figure 7).

The earliest modern cercopithecids (colobines) appear reasonably soon after the mid- Miocene cooling event, with possible records at Ngorora and definite ones at Ngerin- gerowa "and Nakali (BENEFIT & PICKFORD, 1986)(D in Figure 7).

Somewhat later, cercopithecines appear in the record (E in Figure 7) but both groups remain rare and have low diversity (as fossils) until the Messinian event (7-5 m. y.) (E in Figure 7). At this time both Subfamily groups begin to diversity, but the major period of radiation coincides with the beginning of the Plio:Pleistocene cooling events (F in Figure 7). Towards the end of the Pleistocene, the genus Cercopithecus began to experience a major burst of radiation (G in Figure 7).

Taken on their own the monkey evidences are seen to relate reasonably closely to the d180 curve. However, they are not the only groups the history of which coincides with the d180 curve. The Maboko event not only records the invasion of East Africa by victoria- pithecines, but also by such groups as listriodont suids (two genera), tetraconodont suids, tayassuids, horned bovids, ossiconed climacoceratids, and choerolophodont proboscide-

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ans. At the same time the local extinction of a number of groups such as Brachyodus and Sivameryx (two anthracotheres), some bunodont and zygolophodont proboscideans and two genera of suids took place. A number of lineages persist into Maboko times from earlier periods, but few of them remain unchanged. These lineages include the tragulids, sanitheres, and the suid genus Libycochoerus.

The primate fauna underwent similar changes. Not only did victoriapithecines appear at this time, but they dominate the primate fossil record at several of the sites, in contrast to their total absence in most prior sites. Victoriapithecines were accompanied by the hominoid Kenyapithecus, an early oreopithecid (PICKFORD, 1985) and a variety of Micropithecus. Other primate lineages such as Limnopithecus and Dendropithecus persisted, but are rare as fossils during the Maboko event.

The mid-Miocene cooling event not only witnessed the local demise of the victoria- pithecines, but also of a number of other mammalian lineages, including several rhinocero- rids, kubanocherine suids, geniohyid hyracoids, anthracotheres and possibly chalicotheres.

A number of new bovid lineages appeared at this time, as did a rhinocerotid (Paradiceros) and some giraffids. Faunally speaking, both the Maboko heating event and the mid-Miocene cooling event were traumatic. The changes greatly affected the mammals (PICKFORD, 1981, 1983) but also other groups such as terrestrial gastropods (PICKFORD, in prep).

During the Messinian event, several evolutionary trends occurred in tandem. For example, proboscideans, suids, rhinocerotids and equids developed byposodont cheek- teeth covered in cementum (equids were already hypsodont, but they became more hypsodont during the period) the trends more or less starting at the same time and comparable degrees of hypsodonty developing simultaneously. During this event, monkeys began to radiate, but it wasn't until the subsequent Plio-Pleistocene cooling events that rapid radiation took place. The major period of radiation (4-2 m. y.) was also the time that bovids in particular underwent major evolutionary changes (VRBA, 1985).

Later in the Pleistocene, numerous lineages of colobines and papionines became extinct, leaving relatively few genera. Subsequently, it appears that colobines have radiated a little, but Cercopithecini, in particular the genus Cercopithecus, has recently undergone (and is still undergoing) a major radiation.

The .question that is prompted by these coincidences between cercopithecoid history and the d180 curve is: <<Are the coincidences real or are they being imposed by the author on two unrelated histories?>> If the concordance is real, can it be taken as evidence of cause and effect? Were the changes in cercopithecoid distribution and diversity climatically <~forced>>?

Given that the changes in cercopithecoids coincide closely with changes in other mammalian groups as well as with distribution patterns of terrestrial gastropods (PIcK- FORD, in prep), then the continental fossil events seem to be real enough. Their correlation by radiometric and palaeomagnetic dating methods provides independent tie ups because the d180 curve is dated independently of the continental evidence. The concordance between the oceanic and continental histories is therefore probably evidence of concor- dance in changes of oceanic and atmospheric climates. It is therefore postulated that there was a strong element of cause and effect, and that changes in the oceanic atmosphere resulted in changes in the continental atmosphere which in turn led to changes in the distribution of plants and animals. Undoubtedly, major changes, or megaevents, resulted in comparably large changes in continental habitats that led to the <~forced>> evolution of numerous lineages. In this light, the effect of the changes on the monkeys were no different from those experienced by many other groups.

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Summary and conclusions

Of the four major areas in Africa where fossil cercopithecoids have been discovered, the only one which covers more than a limited time period is that of East Africa. Two factors, more than any others, have been responsible for the generation of the late Neogene sequence in East Africa. The first is that the region has been the scene of intermittent rift tectonic activity for the past 20 m. y. during which period numerous tectonic basins were formed, filled with sediments (and fossils) and subsequently upfauh- ed and subjected to erosion whereupon their fossil content is released for collection. The second factor, perhaps even more crucial than the rifting (although undoubtedly related to it) is the volcanic activity which characterised East Africa for the past 23 m. y.. Not only did the volcanic activity result in suitable geochemical environments for fossil preserva- tion, but it has permitted the calibration of the stratigraphic successions by various radioisotopic dating techniques performed on lavas and tufts intercalated in the fossilifer- ous sediments (PICKFORD, 1986).

The Cercopithecoidea is just one of the many mammalian groups represented as fossils virtually throughout the East African fossil record. Their abundance as fossils and their diversity is variable, partly as a result of inadequancies in the fossil record, but probably also as a result of fluctuations, both in population density and species diversity over time. The early part of the record in the lower Miocene, is sparce, a few Cercopithe- cid fragments having been recovered from Napak, Uganda. In the middle Miocene, Victoriapithecines become more common; indeed at some sites such as Maboko, they are more common than hominoids. However, the subfamily Victoriapithecinae disappears from the East African fossil record before the end of the middle Miocene, and its demise was possibly related to the mid-Miocene cooling event of 14 m. y. ago.

The earliest cercopithecoids identified with confidence as belonging to extant subfa- milies have been found in the early part of the upper Miocene, just prior to the earliest local records of the equid Hipparion.. These early cercopithecid fossils are all reasonably assigned to the subfamily Colobinae. A little later in time the earliest cercopithecines are recorded, and the fossils are all assigned to the tribe Papionini.

Throughout the upper Miocene of Africa, monkeys are rare as fossils. However, both subfamilies underwent major radiations during the Pliocene and the early Pleistocene. Not only did the diversity increase dramatically, but the number of fossils preserved suggests that population densities were considerably higher during the Plio-Pleistocene than during the upper Miocene.

The earliest Cercopithecini have been found in late Pliocene strata, but the tribe is rare until the latest Pleistocene and Holocene. The genus Cercopithecus is at present undergoing a major radiation, which may have started during the past few tens or hundreds of thousands of years.

There is some indication that climatic fluctuations may have precipitated changes in the cercopithecoid faunas, partly due to habitats (and their contained monkeys) shifting round the continent in response to climatic changes, but probably also partly due to major ecological changes throughout the continent. Such megaevents may well have acted as evolutionary <~forcing>> agents.

Climatic megaevents which appear to have had a marked effect on East African faunas occurred at about 16-17 m. y., at about 14 m. y., about 7 m. y. ago and from the Pliocene onwards. These megaevents affected the monkeys as much as they did other mammalian groups. There was, as a result, a remarkable tandem evolution in several lineages. For example,' the development of frontal appendages in mammals occurred for

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the first time in East Africa at about 16 m. y. in at least two groups (bovids, climacocera- rids) at the same time that lophodont dentitions became more common (listriodonts, tayassuids, monkeys). During the upper Miocene, comparable clumping of evolution occurred, with hypsodonty becoming well developed in several lineages at the same time (proboscideans, suids, equids, bovids, rhinocerotids). Major Plio-Pleistocene radiations in the monkeys occurred more or less synchronously with radiations in the bovids and suids.

These tandem developments were undoubtedly influenced (if not actually caused) by continent-wide climatic changes. Comparison of the continental record with the oceanic record indicates that the african continent and the Atlantic Ocean underwent compatible histories during the Neogene. Thus megaevents in dlSO curves for the Neogene appear to coincide quite closely with megaevents on the continent deduced from faunal evidence. Cercopithecoids were part of this history just as much as many other lineages of animals.

The East African evidence is complete enough to permit sequences and timing of events to be reconstructed within limits. For the monkeys, several key points in their evolution are identified in the fossil record, and it is noted that the sequence in which the extant subfamilies and tribes appeared as fossils is virtually the same as the phylogenetic sequence worked out on the basis of neontological evidence. This coincidence is probably not fortuitous; the earlier taxa would enter the fossil record prior to later taxa. Thus, even though there are gaps in the fossil sequences, the broad strokes of the evolutionary picture seem to be visible, and when they were painted can be determined within limits.

ACKNOWLEDGEMENTS - - I would like to thank NORBERT SCHMIDT-KITTLER for support and encour- agement while this report was in preparation. I am also anxious to thank Dr. HANS-JURG KUHN for help in organising this session within the scope of the Xlth Congress of the International Primatological Society held in Gottingen. This report was prepared while the author was funded by the Deutsch Forschungsge- meinschaft.

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Received: November 1986. Accepted: February 1987.