Mammal diversity and environment evolution during the Plio-Pleistocene in East Africa

H U M A N E V O L U T I O N Vol . l l - N. 3 - 4 ( 1 9 3 - 2 0 4 ) - 1996

M. Gibernau CEFE (Centre d'Ecologie Fonctionelle et Evolutive) / CNRS BP 5051 34033 Montpellier Cedex 1, France

S. Montuire Laboratoire de PalOontologie-CC 64, ISEM (lnstitut des Sciences de l'Evolution) UMR 5554 CNRS Universit~ Montpellier 11 34095 Montpellier Cedex 5, France

Key words: mammals, hominids, faunas, cenograms, Plio-Pleistocene, climatic change, mosaic landscape

Mammal diversity and environment evolution during the Plio-Pleistocene in East Africa

Human evolution began in East Africa four million years ago, with a transition from an arboreal state to a more terrestrial one. This evolution seems to be correlated with a large environmental change in East Africa around 2.5 m.y. due to a major climatic change leading to drier and cooler conditions. Cenogram analysis (a graphical representation of community structure) can be used to reconstruct the vegetation cover at a regional scale, and to infer the changing climatic conditions. Using cenogram sequences of different sites along the Rift Valley, we were able to determine the regional ecological context in which mammals and hominids have evolved in East Africa during the last 3 million years. Between 3.5 and 2 m.y., during a general climatic change, suc- cessive faunas of South Tanzania reflect the progressive opening of their environment. In contrast around Lake Turkana a mosaic of isolated dry and wet habitats were present throughout this period. At this time, the Rift seems to have been spatially structured in several basins isolated from one other, and isolated faunas experienced separate speciation events (particularly with the appearance of Homo genus). After 2 m.y., the disappearance of the isolating barriers on one hand, and a regional increase in aridity, on the other hand, led to more homogenous faunas arising throughout the region. Replace- ments of mammal species occurred (especially Homo erectus re- placing Homo habilis) and several others mammal species, in- cluding australopithecines, disappeared during this same period.

Introduction

Hominid evolution began about four million years (m.y.) ago in East Africa with australopithecine species and, later " robust" australopithecines (Paranthropus) and Homo species (Susman, 1990; Stanley, 1992; White et at., 1994). This evolution seems to be correlated with a major environmental change that occurred around 2.5 m.y. due to cl imate change (Bonnefil le, 1983; Zagwijn, 1992). Thus, a transition is supposed to have occurred in hominids from an arboreal pr imit ive state to a more terrestrial one (Susman, 1990; Stanley, 1992; Wood, 1992; Turner, 1995). A similar adaptative change is also proposed for several other mammal groups during the same period. The large baboon, The1"opithecus brumpti, thought to have occupied gallery forest, became extinct between 2.5 and 2.0 m.y., and was replaced by Theropithecus oswaldi, which is supposed to have lived in open habitats (Turner, 1995). Moreover , throughout Afr ica south of the Sahara, there was an extinction of forest- dwell ing antelopes about 2.5-2.4 m.y., and soon thereafter, an appearance of numerous new grassland-dwell ing species (Vrba, 1988).

194 GIBERNAU and MONTUIRE

The cenogram method (Legendre, 1989) allows a description of mammalian community structure. By comparison with the structure of about 300 extant faunas whose environmental context is known (Legendre et al., in prep), it is possible to infer the cover and relative annual rainfall at a regional scale occupied by a given fossil fauna. Thus, a cenogram sequence of mammalian communities can provide evidence of climatic changes at a specific site. Geographical comparisons of environmental conditions are then possible using cenogram sequences of different sites. The purpose of our study was to investigate the ecological context in which mammals and hominids evolved between 3.5 and 0.5 m.y. along the Rift Valley in East Africa, using comparisons of community structure based on cenograms.

M a t e r i a l a n d M e t h o d

Selection o f faunas Twenty-six Upper Pliocene and Pleistocene faunas dated between 3.5 and 0.5 m.y. were

used in this study. Their stratigraphic position and age are listed in Table 1. The location of the studied sites are given in Figure 1. The East African Rift is divided by the Assouan Ridge that isolates a northern part from a southern one. In the northern part, faunas from Hadar and Omo (Ethiopia), and faunas around the Turkana Lake from Koobi Fora and Nachukui (Kenya) were analysed. In the southern part, Laetoli and OIduvai faunas (Tanzania) were studied.

The faunistic lists for the North Rift localities are taken from Feibel et al. (1991) and Kalb et al. (1982). Species lists for the South Rift faunas are from Leakey (1971) and Leakey & Harris (1987). The extant faunas used as reference cenograms are listed in Legendre (1989). The most complete faunas were used for the cenogram method, faunas in which medium and small species were missing, were eliminated.

The cenogram method A cenogram is built by putting on the X-axis the rank of the mammal species arranged

in decreasing order of body mass and on the Y-axis the logarithm of the body mass expressed in grams. Mammalian body masses, required for the construction of a cenogram, are estimated in this study by using the allometric relationship with the first lower molar area (Legendre, 1989). Missing values were completed with the measurements of the same species living in nearby sites (Leakey, 1969; Leakey & Savage, 1970; Leakey et al., 1973; Maglio & Cooke, 1978). In a few cases, when no measurements for the fossils were available, body mass was estimated by using the nearest modern species. Carnivora and Chiroptera are not included on the cenogram (for more explanations, see Legendre, 1989).

Four schematic types of cenogram can be obtained (Fig. 2) which allow to infer the vegetation cover (open or closed) and climatic conditions (arid or humid):

- A continuous distribution of body masses (numerous medium mass species) indicates a closed habitat (high cenograms).

- Rare or no medium mass species between 500 g and 8 kg (In(g): 6.2-9) suggests an open habitat (low cenograms).

- An abundance of large species with a body mass over 8 k g (an homogeneous slope along the cenogram) is a feature linked with moist conditions (left cenograms).

- The rarity of large species (a slope change between the left and right part of the cenogram) is associated with arid conditions (right cenograms).

Our reference cenograms are three extant faunas from East Africa. Turkana (Kenya) is a

MAMMAL COMMUNITY and ENVIRONMENT EVOLUTION 195

humid arboreal savannah; the corresponding cenogram has a shallow slope with some dis- continuity in the distribution (Fig. 3). Lokori (Kenya) is .a bare lava desert with sparsely vegetated alluvial terraces (Coe, 1972); the relevant cenogram is steeper than Turkana one and discontinuous (Fig. 4). Finally, Eastern Lake Kivu (Rwanda) is covered with humid forest, and characterised by a cenogram with a regular slope and no gap in body mass distribution (Fig. 4).

R e s u l t s

Between 3.5 and 2 m.y. - In the northern part of the Rift In northern Ethiopia, Lower and Upper Hadar formations and Matabaietu fauna are

described (Fig. 5). The cenogram of the Lower Hadar formation shows a large number of large species and a lack of medium mass species. The environment was apparently an open habitat more humid than modern-day Lake Turkana. The number of large species decreased in the Upper Hadar formation, compared to the Lower Hadar (the slope of the left part of the cenogram increases), and remained about the same in Matabaietu fauna (Fig. 5). These two cenograms are truncated, i.e., neither medium or small species were found. Accordingly, no information is available about the vegetation cover. However a more arid environment than that of Lower Hadar is clearly indicated.

In the area of Turkana Lake (Kenya), Shungura (B, C, F and G faunas), Koobi Fora (Tulu Bor fauna) and Nachukui (Lower and Upper Lomekwi faunas) were studied (Fig. 6). The slopes of the cenograms for the Shungura faunas (Fig. 6) are steep and the distribution of masses is discontinuous, indicating an open habitat and arid conditions similar to Lokori (Fig. 4) between 3.4 and 2.3 m.y. We have no complete faunas after 2.3 m.y. for the Shungura sequence. On the contrary, the cenograms of Koobi Fora and Nachukui (Fig. 6), with a shallow slope of the left part of the cenogram, indicate a humid environment, moister than that of Turkana today. The truncated cenograms for medium and small species do not allow inferences about the vegetation cover.

- In the southern part of the Rift This part of the Rift is studied with two faunas from Laetoli (Tanzania): Upper Laetoli

Beds and Upper Ndolanya Beds. Large species are numerous suggesting humid conditions similar to those at present-day Turkana (Fig. 5). The distribution of body masses was discontinuous in Upper Laetoli Beds fauna and less so in the Upper Ndolanya Beds fauna. The environment was therefore open and became slightly more closed similar to the present Turkana.

2 m.y. to recent time - In the northern part of the Rift In Ethiopia, the two faunas of Wehaitu (Andalee and Bodo) have few large species

suggesting a dry environment (Fig. 5). The low number of small species prevents making inferences about the vegetation cover. The same period around Turkana Lake is described by Koobi Fora (Upper Burgi, KBS and Okote) and Nachukui faunas (Kaitio, Natoo and Nariokotome).

Upper Burgi, KBS and Kaitio faunas (Fig. 6) are often incomplete for small species but the abundance of large species suggests that the environment was more humid between 2 and


1.6 m.y. than present-day Turkana. The KBS fauna (Fig. 6) is the most diverse fauna of our study. The abundance of large species there suggests that the environment was humid, and the discontinuous distribution of masses indicates that the environment was also open. After 1.6 m.y., for Okote, Natoo and Nariokotome faunas (Fig. 6), the slope on the left part of the cenogram becomes steeper than on the previous cenograms. The decrease of the number of large species indicates that the environment became drier around Turkana Lake, and it approached the number of species of the extant Turkana fauna (Fig. 3). This evolution seems to show that the environment became drier in the course of time around Turkana Lake, and was open at least during some periods.

- In the southern part of the Rift At Olduvai (Tanzania), for Lower and Upper Beds I (Fig. 5), the distribution of masses

on the cenogram is discontinuous. This indicates the presence and persistence of open habitats. The number of large species suggests less arid conditions than at Lokori today. Subse- quently, the number of large species decreases at Upper Beds i and Lower Beds II. This decrease corresponds to the transition to more arid conditions (Fig. 5). The distribution of masses is still discontinuous, so the vegetation cover was probably relatively open during this period. The number of large species increases at Middle and Upper Beds 11 suggesting a change to more humid conditions. The presence of medium mass species in Upper Beds I1 suggests that the habitat was still open at this time.

D i s c u s s i o n

A global climate change occurred during the Plio-Pleistocene which was reflected in the northern hemisphere by large-scale glaciations around 2.3 m.y. (Zagwijn, 1992). In Africa, this change resulted in as drier and cooler conditions around 2.8 m.y. (deMenocal, 1995), 2.6 m.y. (Bonnefille, 1983) or 2.35 m.y. (Rahman and Roth, 1989, 1990).

Between 3.5 and 2 m.y., a change in the structure of mammalian communities is observed between Lower and Upper Hadar in Ethiopian faunas, suggesting an environmental change between 3.4 and 2.6 m.y., and more precisely a progressive drying trend. During the same period, this change is not perceptible in Tanzanian cenograms, where an open and wet habitat is present as at in Turkana today. Likewise, around Turkana Lake (Tanzania), two habitats are present and stable between 3.4 and 2.3 m.y. A dry and open habitat is found in the northern part of the Lake at Shungura (Fig. 6), in contrast to contrary to the vicinity of the Lake (Koobi Fora and Nachukui, Fig. 6), where a wet habitat was present. These two habitats, separated by only fifty kilometres, are confirmed by palaeoecological data (Coppens, 1975). The cenograms associated with these two locations seem to be stable in spite of the above-mentioned climatic changes. Even though there is no change in the community structure, the faunistical composition can be modified. The cenogram method deals with ecological roles assumed by species according to their body mass in a community occupying one habitat and not with their taxonomic position. Therefore an ecological role in one habitat can be assumed by different species in different communities occupying the same kind of habitat. Moreover under constant environmental conditions, the community will be the same (cenogram constancy) whereas the species constituting it may changed according to events of speciation or migration. The cenogram Shungura sequence is stable whereas Wesselman (1984) described a faunistical change in the micromammals along this sequence, moreover 50% of large mammal species (Primates, Ungulates and Proboscides) have changed between each


studied fauna. Rodent faunas of Turkana Lake and Omo also changed between 2.8 and 2.4 m.y. (Denys, 1990). Finally, about 75 to 80 % of the large mammal species of Turkana (Koobi Fora and Nachukui) disappeared during this period.

Species changes underline a biogeographic dynamic that was not recorded at the regional level of the ecological community which depends on the climate and vegetation.

For at least 1.5 m.y., the cenograms illustrate that a mosaic of dry habitats in the northern part (Ethiopia, Fig. 6) and wet ones in the southern part (Kenya, Fig. 6) was present around Lake Turkana. This contrast is observed during all this period, according to the different cenograms in spite of the general climatic change. These environmental differences exist also between West and East Turkana (Kenya) faunas and those of low Omo Valley (Ethiopia). From 3.4 to 2.3 m.y., only 16% of the species of large mammals (Primates, Ungulates and Proboscides) are common between Shungura faunas and those of Koobi Fora and Nachukui, suggesting the presence of an ecological or geological barrier during all this time. The local diversity described in the Turkana Basin faunas (e.g. Homo rudolfensis, Equus koobiforensis, Rhinocolobus turkanensis, Pelorovis turkanensis, Suncus shungurensis, Aepyceros shungurae, etc.) may be explained by the isolation of the different sites leading to the persistence of various endemic faunas. The rodents also show a high rate of endemism in Omo and East Turkana (Denys et al., 1985). Similarly, "robust" australopithecine from southern Rift (Olduvai, Tanzania) and from South Africa are monospecific, while those from northern Rift (Turkana, Kenya, and Omo, Ethiopia) belong to two related taxa (Wood, 1992).

The Rift seems at this time spatially structured in isolated basins (Denys et al., 1985), constituting a mosaic of dry and wet savannahs as around Turkana Lake. In this context, faunas have been isolated as previously discussed and several speciation events seem to have occurred in rodents (Denys, 1990), antelopes (Stanley, 1992), molluscs (Wiltiamson, 1981, 1982) and hominids (Wood, 1992). In Ethiopia, the primitive australopithecus afarensis is found, whereas more evolved forms are present at Turkana (Paranthropus boisei), and at Shungura (Paranthropus boisei and Paranthropus robustus). The Homo genus appears later with Homo habilis at Shungura and Nachukui (West Turkana).

At Olduvai between 2 and 1.6 m.y., the wet and open environment present at the base of the Beds I formations becomes drier at the top of this Bed. This drying trend is also observed in the low Omo Valley (Coppens, 1975). Paleoclimate data of the KBS member of Koobi Fora shows a negative local water balance reflecting a regional increase of aridity (Feibel, 1988; Abell, 1982). This ecological change is also found in the variations of pollenic and faunistic records (Leakey, 1971; Jaeger, 1976, 1979; Gentry, 1978; Bonnefille and Riollet, 1980). On the contrary Lake Turkana cenograms are stable and illustrated humid conditions, suggesting that the recorded drying was not sufficient to modify the mammal community structure. This event coincides with the collapse of the southern part of the Rift at this time, and is correlated with a drying of the site, and the removal of the barriers in the Rift (Denys et al., 1985). Then faunas have migrated and the observed diversity is high in the studied faunas. Subsequently replacement of species have occurred, particularly for large mammals in West Turkana, at Koobi Fora (Harris et al., 1988), and Homo erectus appeared at Koobi Fora in replacement of Homo habilis (Susman, 1990). This situation leads to a homogenisa- tion of the faunas, for example rodents from the Omo Valley (Ethiopia) and East Turkana (Kenya) converge after 1.6 m.y. (Denys et al., 1990). The various species of Australopithecus, and also several other mammalian species, disappear at this time.

We also noted a very high diversity of very large mammals in the studied faunas that has no equivalent in any known present-day faunas (Legendre, 1989; Legendre et al., in prep). The micromammal assemblages of the Turkana Basin tend to confirm this observation


Table 1 - Stratigraphic positions and ages of the studied faunas.

AGE EPOCH

P P_i

-0.50"

I -1.00- o

w.

-2.00 L P ! t I PIACENZ

-3.00 0 C E N -4.00 E

-5.0(

STAGE

SICIUAN

Wehoitu-Bodc EMIUAN

Matabaietu PIACENZIAN Upper Hadar

Lower Hadar B

P ,,o 35 g v

Dkote

KBS Upp. Burgi

TuluBor

East A f r i ca

Natoo Lower Beds Kaitio Upper Beds I

[ Middle Beds I }uq | Lower Beds I

Upp Lome

Low. Lomel~

~or

%

3

Llpper Ndolanye Bed~

Upper Laeleli Beds

Fig. 1 - Map of the Horn of African showing the location of the studied faunas (after Pickford, 1987).

30 ~ 35 ~

c~

io

? Om o ,o

urkona

O~duvr ~ e

lS o

500 i o

50 ~

/ 07 "

a Ptio-Pleistocene sites


Fig. 2: Schematic representation of large categories of cenograms (from Legendre, 1989). In an open habitat, the medium mass species (between 5(1(I g and g kg) are rare or absent (low cenograms), whereas they are numerous in closed habitat (high cenograms). In humid conditions, the large species (mass over 8 kg) are abundant (left cenograms), on the opposite large species are rare in arid conditions (right cenograms).

Fig. 3 - Cenogram of the extant fauna around Lake Turkana.


15

~ 1 0 c-

. ~

0 _Q v 5 c-

0 0 0 0

I O O O

O i

0 0

0 0 0 0 0 0 0

0 0 0 O

. . . . i . . . . i . . . . i . . . . i . . . . i . . . . i ' ' �9

0 10 20 30 Rank of s p e c i e s

Lokori

15.

t ~ 1 0 r

. _ _

> , " o O

5 v r

Eastern Lake Kivu

0 � 9

0 � 9 O

� 9

0 0 0 0 0 0 O

� 9149149

000�9149

00000 � 9 1 4 9 1 4 9 1 4 9 1 4 9 0 �9 �9 0 0 � 9 1 4 9

000 0 O0 O

. . . . t . . . . i . . . . i . . . . i . . . . i . . . . i . . . . i . . . . i . . . . i . . . . i . . . . i . . . . i

0 10 20 30 40 50 60 Rank of species

Fig. 4: Cenograms ol the extant faunas at Lokori (Kenya) and in Rukwa Valley (Tanzania).


t ~ <

1 - -

2

3

Ethiopia

i ,.~. Afar

. . ," W e h a i t u - B o d o

I ~ ' ~ " Wehaitu-Andalee

l o ;P~ ~ 40 ~o

Rank of species

Matabaietu

Upper Hadar

Lower Hadar

Tanzan ia

I~ _,_ Olduvai

--.... " - ~ . ~ �9 . . Upper Beds II

~ ' . . .L~ ' , " * , ~ Middle Beds 1,

~-~..~._. " ~ Lower Beds II

[ ~" "% "% UpperBedsl

"..~..~ Middle Beds I

"" Lower Beds I

t * " " ~ * ~ ,o ,,,, Laetoli

Upper Ndo[anya Beds

Upper Laetoli Beds

Fig. 5: Cenogram sequences of the faunas in northern Ethiopia (Afar) and Tanzania (Olduvai and Laeloli), between 3.5

and 0.6 m.y.

202 GI BERNAU and MONTUI RE

A

v

1--

3--

Ethiopia

Koobi Fora

, , u n o u r a . . . . .

.--... t - . ~ ~,,o~u~

] Shungura C

. . Shungura B

R a n k o f s p e c i e s

Tulu Bor

Okote

'" KBS Ipper Burgi

Kenya Nachukui

..~. I L " ~ ' ~ Nariokotome

l " Upper Lomekwi

Lower Lomekwi

Fig. 6: Cenogram sequences of the faunas around Lake Turkana at Shungura (Ethiopia) and Koobi Fora and Nachukui (Kenya), between 3.4 and 1.2 m.y.


with a mixing of species l iving in mesic habitats and species inhabiting more xeric conditions (Feibel et al., 1991). This may be explained by the fact that the environment was unusual by been strongly structured in isolate basins or/and that the environment was perhaps be more productive in the past. An alternative explanation may be that some taxonomic treatments may have overest imated the species number.

During the Plio-Pleistocene, East-African Rift was occupied by a mosaic of habitats, which seem to have been stable during 1.5 m.y. through a general cl imate change. This structured and stable environment was recorded by a constancy of the mammal communit ies occupying the different habitats. But mammal species among which hominids shown a high biogeographic dynamic. Various species have appeared by speciation or migration, others have disappeared by extinction or competi t ion.

A drying trend is recorded after 1.6 m.y., resulting in a convergence of faunas and at some extant of communit ies in East-African Rift. This trend led progressively to the present- day situation.

ACKNOWLEnGEMENTS - - We wish to thank S. Legendre for making this study possible. We would also like to thank F. Kjellberg, A. Caizergues and M. Pickford for constructive comments on the manuscript; J. Aronson and J. Damuth for improving this work. This is contribution n. 96-106 from the "Institut des Sciences de I'Evolution" de Montpellier, France.

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Received April 7, 1996 Accepted May 20, 1996

Documents

Mammal diversity and environment evolution during the Plio-Pleistocene in East Africa