'1 , I

HIPPARION DATUM AND ITS CHRONOLOGIC EVIDENCE

IN THE MEDITERRANEAN AREA

Sevket Sen

Laboratoire de P~.leontologie des Vert~bres Univers~te Pierre et Marie Curie U.A. no DU CNRS 4 place Jussieu 752.52 Paris Cedex 05, France

INTRODUCTION

For mammalian biochronology in Europe, the immigation of hipparionine equids into the Mediterranean and central European areas constitutes a major event, which characterizes the Aragonian-Vallesian boundary (Fahlbusch, 1976; Ginsburg, 1975). Berggren and van Couvering (1974) popularized this event by introducing the concept of "Hipparion Datum."

During the late 60s and the 70s, the generally adopted opinIOn was that this group had a high rate of expansion, and that it invaded the whole of Eurasia and North Africa almost instantaneously with respect to resolution of geologic time. Berggren and van Couvering (1978, p. 49) restated this opinion as: "The real time difference in the observed first appearance datum in this vast region is probably less than 0.5 m.y.n However, hypotheses on the synchrony of Hipparion immigration throughout Eurasia and the Mediterranean area were founded on few chronological data. One can even wonder whether there exists anywhere suitable time data for such an hypoth­esis. The idea of synchrony is partly due to the limits of biochronological resolution of large mammal faunas. In fact, Hipparion is the unique American immigrant among the Eurasian large mammals of the early late Miocene.

Ages ranging between 12.5 and 10 Ma were proposed for the first occurrence of this group in Eurasia and the Mediterranean area. Berggren and his collaborators (Berggren and van Couvering, 1974, 1978; van Couvering and Berggren, 1977j Berggren et al., 1985) argued for an age of 12.5 Ma or between 12.5 and 12 Ma for this event, based on some radiometric data from How.enegg and Bou Hanifia. However, radio­metric datings and magnetostratigraphic correlations on the continental deposits of Turkey and the Indian subcontinent provided some younger ages for the oldest Hipparion faunas of these countries. It appears, therefore, that the synchrony of this event as Berggren and his collaborators Wlderstand can no longer be accepted in such a large area. Their assumption does not take into accoWlt ecological and paleogeo­graphical factors which influenced equid dispersal. The aim of this paper is not to insist more on this subject, but to present a review of available data on the Hipparion Datum, with some new results (K/ Ar dating and magnetostratigraphy) recently obtained from some Hipparion localities in the Mediterranean area.

European Neogene Mammal Chronology Edited by E.H. lindsay ef 01. Plenum Press, New York, 1990

495

Fig. 1. Some Vallesian mammal localities with Hipparion in the Mediterranean area. 1. Howenegg, Z. Hostalets de Pierola, 3. Bou Hanifia, 4. Kastellios Hill,S. Esme-Akcakoy, 6. Bayraktepe IT, 7. Yassioren. Some of these localities also contain small mammals.

lDPPARION IMMIGRATION

The earliest occurrence of the first North American hipparionine equid Cormohipparion goorisi was calibrated as being 15,* 0.5 Ma old (Woodburne et al., 1981j Flynn et al., 1984). It is at present obvious that this group emigrated into Eurasia much later. In several studies on the New World and the Old World hipparionines, a common interpretation is that the ancestor for the Eurasian species lived in North America. The potential ancestor is, according to Woodburne et ale (1981), Cormohipparion sphenodus (Cope, 1889), known in North American sequences between 13.6 and 11 Ma. On the other hand, Bemor and Hussain (I985) argued that Cormohipparion occidentale is particularly similar to the European species Hipparion primigenium in its morphological features. In any case, as demonstrated by Bemor and Hussain (I 985) (see also Bernor et al., 1987a,b), the Old World hipparionines probably have a monophyletic origin.

The generally accepted opinion was that this emigration happened some 12.5 Ma ago. Woodburne et al. (1981) explained this event as a result of a major drop in sea level to about 80 m below the present-day level, between 11 and 12 Ma, and this drop gave rise to a land connection across Beringia. I have reproduced in figure 3 the most recent curve of eustatic fluctuations (Haq et al., 1987; fig. 2) concerning the period from 16 to 9 Ma. It appears that sea level drop was progressive from a high level at about 15 Ma to a sharp fall a little before 10 Ma. This time interval also contains three successive fluctuations, with a general tendency toward decrease in sea level. The curve seems to speak in favor of a land connection at about 11 to 10 Ma between North America and Asia. For a reasonable correlation of the sea level drop with Hipparion emigration from North America to Eurasia, it would be better to have some calibrated key-localities in Asia. Unfortunately, no valid absolute ages have been published until now on the oldest Hipparion of Asian localities, except in the Indian subcontinent where the Hipparion occurrence is apparently much later than in the

496

other parts of Eurasia. Detailed magnetostratigraphic studies in the Potwar Plateau area (Pakistan) led to the calibration of the first occurrence of Hipparion in the Siwalik sequences as 9.5 Ma (Barry et al., 1985). It should be noted that this age was obtained by the correlation of magnetic polarity zones with the geomagnetic polarity time scale (GMPTS) of Mankinen and Dalrymple (I979). The correlation of the same polarity successions with more recent GMPTS will increase this age by some 0.3 Ma.

BIOCHRONOLOGIC BASIS OF HIPPARION DATUM

As initially defined at the 1975 Munich Symposium (Fahlbusch, 1976), European faunas with Anchitherium and without Hipparion belong to the Aragonian, and the Valles ian begins with the occurrence of Hipparion. Concerning small mammal associa­tions, the appearance of Progonomys is equivalent to that of Hipparion. However, it should be noted that Progonomys appears, in the Vallesian type area, somewhat later than Hipparion. Other rodent taxa have been more or less associated with this bound­ary, such as the Cricetodon-Hispanomys lineage, Cricetulodon and Megacricetodon (Agusti et al., 1984; Unay and de Bruijn, 1984).

Several localities in Europe and North Africa have yielded so-called early Vallesian Hipparion (figure 1). The aim of this paper is not to discuss what generic name should be employed for the Vallesian forms of the Old World nor which particu­lar horse association presents the more archaic characters than the other. These subjects were debated by several authors (Forst en, 1968; Sen et al., 1978; Steasche and Sondaar, 1979j Bernor et al., 1980, 1987b; Woodburne et al., 1981) who brought some light on the early Vallesian horses of the Old World. Woodburne and Bemor (I980) defined some superspecific groups for the Old World hipparioniones; this paper only concerns the Group 1 species. Bemor et al. (I987a, p. 44) noted that the Howenegg Hipparion primigenium is "the most primitive species" of the Old World hipparionines compared with H. catalaunicum from Spain and H. africanum from Algeria.

According to these studies, and taking into account chronologie data on the earliest Old World species, some localities present more interest than others for a biochronologic definition of the Aragonian-Vallesian boundary. These are Hostalets de Pierola in Spain, Howenegg in Germany, Bou Hanifia in Algeria, Kastellios Hill in Greece, Esme-Akcakoy, Yassioren, and Bayraktepe II in Turkey (figure 1).

In the type areas of Aragonian and Vallesian, this boundary is not characterized by any particular event, e.g. faunal extinctions at the end of the Aragonian. Moya Sohi and Agus.t( (I985, p. 403) note that in the Valles-Penedes Basin IIthis limit does not represent an important faunistic change and is only marked by the arrival of Hipparion ... from this point of view the other limits are qualitatively more important, especially the MN 9/MN 10 limit." Figure Z illustrates the time range of some char­acteristic rodent taxa and shows that no particular event occurred when Hipparion appeared in the Valles-Penedes Basin.

On the other hand, the Hipparion occurrence is apparently diachronous in differ­ent basins of the Iberian Peninsula. Lopez Martinez et al. (1986) and Alvarez Sierra et al. (I987) showed that Hipparion appears somewhat later in the Duero Basin than in the basin of Valles-Penedes and Calatayud-Daroca. This diachrony is stated on the correlation of rich rodent associations from these basins.

In another area, namely in Turkey, the faunal succession does not show any significant change in relation to the Hipparion arrival. According to the data pub­lished by Unay and de Bruijn (1984) on the Dardanelles localities and with my own observations on some other Turkish localities, changes in the composition of mammal faunas occurred either before the first occurrence of Hipparion (replacement of Turkomys by Byzantinia, Megacricetodon extinction, appearance of Cricetulodon) or following it (the first Progonomys).

497

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Fig. Z.

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Time ranges of some rodent genera associated with the Hipparion Datum in the Valh~s-Penedes Basin (Spain) and in Turkey.

Faunal composition for the Turkish localities mentioned in figure 2 are given by Unay (1981), Sickenberg et al. (1975), and Sen (1986). Yassioren and Esme-Akcakoy faunas possess several common genera and species including Hipparion senyureki Ozansoy, 1965. Common opinion among paleontologists who studied these faunas (Steasche and Sondaar, 1979, described the hipparionines) is that they are the oldest Vallesian localities of Turkey. The locality of Yassioren also yielded some remains of small mammals: Schizogalerix sp., Proochotona sp., Byzantinia dardanellensis l

Cricetulodon hartenbergeri, Atlantoxerus sp., and Myomimus dehmi (Sen, 1986). This assemblage is quite similar to that of Bayraktepe n, which also contains a still undescribed Hipparion. Unay (1981) and Unay and de Bruijn (1984) attributed an age of middle-late Vallesian to the Bayraktepe II fauna. If Progonomys is absent at Yassioren, its presence at Bayraktepe n is proven by a single first upper molar.

In figure Z, the time ranges of some rodent genera from Turkish localities are tentatively compared with their equivalents from the Valles-Penedes Basin. In both areas, Cricetulodon appears in the faunas without Hipparion, while Progonomys appears after the Hipparion arrival. However, the extinction of Megacricetodon is earlier in Turkey than in Spain. In the same way, the genus Turkomys in Turkey is replaced by Byzantinia far earlier than in Spain if we consider Cricetodon and Hispanomys as their western equivalents.

Unfortunately, none of the mentioned Turkish localities, nor the localities in the Valles-Penedes Basin and in other Spanish basins, have associated radiometric or paleomagnetic dates. Thus, there are no relevant absolute ages for the first Hipparion occurrence in Spain and in Turkey.

498

,

RADIOMETRIC DATA IN EUROPE AND NORTH AFRICA

I have to emphasize first the rarity of radiometric ages relevant to the Hipparion Datum. In spite of this situation, some scarce data help us to delimit the age of this event and to Wlderstand why different ages were proposed for the Hipparion Datum in Europe and North Africa. The age of 12.5 Ma (or between 12.5 and 12 Ma) proposed by Berggren and van Couvering (1974, 1978), and later repeated .by several authors, is radiometrically founded on data from two localities: Howenegg in West Germany and BOll Hanifia in Algeria.

The first important locality, Howenegg in southwest Germany, yielded two radiometric dates: 12.4 * 1 Ma on. hornblende of the fossiliferous level tuffs (Lippolt et al., 1963) and 10.8 ± 0.4 Ma on whole rock analysis of a basaltic bomb found within the fossiliferous bed (Baranyi et al., 1976). In a letter to J.D. Becker-Platen (Becker­Platen et al., 1977, p. 164), H.J. Lippolt acknowledges that, concerning the first date, "it is particularly difficult to prove if this age is true, because the genesis of horn­blende was not explained." H. Tobien (I982, 1986), an authority on Howenegg stratig­raphy and its fauna, preferred the second date as the age of this fauna.

The Bou Hanifia continental deposits in northwest Algeria yielded further determinations bearing on the Hipparion Datum. A simplified stratigraphy of these deposits in the vicinity of Bou Hanifia Dam - where are situated the BOll Hanifia mammal localities - is as follows:

1. Marine Anaseur Formation: Gray-green or blue marls near the top of this forma­tion contain planktonic foraminifera attributed to the zone N 13 (Ouda and Ameur, . 1978) and nannoplankton which indicates a correlation near the NN 6/,NN 7 bound­ary (Ameur et al., 1979).

2. Continental Bou Hanifia Formation: It overlies the Anaseur Formation with an erosional unconformity and develops a thickness of 300 m or more around the dam lake.

3. Unnamed upper marine deposits: The Bou Hanifia Formation is unconformably overlain by marine deposits from which Ouda and Ameur (1978) mentioned a plank­tonic foraminifera fauna of the late Tortonian.

Mammal localities as well as K/ Ar datings are known from the Bou Hanifia Formation. It is an alternation of conglomeratic, sandy, and marly sediments of grayish, pinkish, or reddish colors. In the lower part of this formation, Ouda and Ameur (I978, p. 415) mentioned the presence of marine intercalations with planktonic foraminifera that might indicate N 15 zone. During 1981 fieldwork, it was not possi­ble to relocate these levels and to confirm this observation.

Sediments in the lower half of this formation contain three thin volcanic ash levels. Chabbar-Ameur et al. (1976) published a K/Ar date from one of these levels without noting precisely its stratigraphic position within the formation. This date is 12.18"* 1.03 Ma. In 1981, I collected samples from the first volcanic ash level (thick­ness about 20 cm) observed 11 m above the base of this formation. New K/ Ar dating realized by P. Y. Gillot at the Centre des Faibles Radioactivites (Gif sur Yvette) on sorted biotites yields an age of 12.03 ± 0.25 Ma. This new date is in good agreement with the previous one, and it may show that the volcanic ash in the sediments of the Bou Hanifia Formation were issued from the same volcanic activity; its occurrence in different levels may be interpreted as a product of redeposition.

These two K/Ar dates indicate the oldest age that can be attributed to the Bou Hanifia Formation. However, they cannot be extrapolated to represent the age of the mammal-bearing horizons, which are situated near the top of the formation and some 100 m above the ash levels.

499

Several K/Ar dates from the middle and upper Miocene continental deposits in Turkey led Becker-Platen et al. (1977) to propose an age between 10.8 and 11.1 Ma for the Aragonian-Vallesian boundary, in other words for the Hipparion Datum. Among the mammal localities dated by these authors, Yenieskihisar 1 (13.1 ± 0.5 Ma) yielded a rich late Aragonian fauna, while the poor fauna of Yenieskihisar Z (11.1 ± 0.2. Ma) can belong to Aragonian as well as to Vallesian. At the locality of Yaylacilar, tlie mammal-bearing horizon without Hipparion is situated ZO m above a tuff layer which is dated at 11.6 ± 0.Z5 Ma. Localities with Hipparion yielded K/Ar dates younger than 10 Ma. Becker-Platen et al. (1977) proposed an age between 10.8 and 11.1 Ma for the Hipparion Datum, taking into account the fact that this group is always found when presen t in a fauna.

From all these K/ Ar dates, it can be concluded that no evidence exists on the presence of hipparionine horses in Europe and North Africa older than 12. Ma.

MAGNETOSTRATIGRAPHIC DATA

The best magnetostratigraphic calibration of the Hipparion Datum was done in the Siwaliks of Pakistan. The first Hipparion occurs, in the Nagri Formation, in the lower part of a long normal polarity zone which is accurately correlated with Chron 9 (Anomaly 5 equivalent). As mentioned above, Barry et al. (1985) attributed to this event an age of 9.5 Ma.

In the Eastern Paratethys area (Moldavia, Ukraine, and Caucasus), the earliest Hipparion occurs in the Middle Sarmatian. According to Chepalyga et al. (1985, p. 137) lithe Gritsevian complex with the oldest Hipparion primigenium ... is character-

500

HIPPARION DATUM

INDIAN SUBCONTINENT

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Fig. 3. Hipparion Datum in North America, Indian subcontinent, and Mediterranean area, and its relationships with the sea level fluctua­tions (after Haq et al., 1987). Chron numbers are from LaBrecq et al. (1977). The Chron 9 of this figure is equivalent to Chron 11 in the more recent terminology of Berggren et al. (1985) .

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Magnetostratigraphic study in the upper part of the Bou Hanifia Formation. From left to the right, lithology of the sampled sectioD, stratigraphic position of the mammalian localities BH 1 and BH 5, paleomagnetic sites, latitudes of the virtual geomagnetic pole (VGP) in each site, succession of the polarities in this section, and their correlation with the geomagnetic polarity time scale of Harland et ale (I982).

ized by the negative polarity of the upper 10 paleomagnetic epoch (10.8-10.3 Ma). So the first hipparions appeared near 11 Ma." In a more detailed paper, Pevzner et ale (1987) gave the results of their magnetostratigraphic studies in Moldavia, and proposed the correlation of the normal polarities obtained in the sections of Bujor 1, Kalfa, Lapusbna, and Atavaska with Chron 11. These localities yielded H. sarmaticum. Chepalyga et al. (1985) consider these localities correiative with Chron 9. In fact, in the sections studied by Pevzner et al. (I987), the polarity succession is poor {short sections and with only one polarity zone}, and these results cannot be correlated securely with the GMPTS.

501

The Kastellios Hill section in central Crete yielded, from several levels, remains of small and large mammals, and it also contains some marine intercalations with planktonic foraminifera (de Bruijn et al., 1971; de Bruijn and Zachariasse, 1979). Among the mammalian species, I have to mention Hipparion sp., compared by these authors to H. primigenium, and two species of Progonomys. The mammal associations of Kastellios Hill are attributed to the Vallesian, as being close to the MN 9/MN 10 zone boundary. The planktonic foraminifera indicate the base of N 16 zone (de Bruijn and Zachariasse, 1979).

Detailed magnetostratigraphic study of an 88 m thick section in Kastellios Hill (Sen et al., 1986) yielded reversed polarities which are interrupted by two sbort normal zones, each 1 m thick. Taking into account the biochronologic data from several levels (small and large mammals, planktonic foraminifera, ostracods, pollen), this polarity succession was correlated with a part of Chron 10. This correlation confers on these faunas an age situated between 10.3 and 11 Ma.

In the Bou Hanifia section in Algeria, I sampled 37 sites in the upper part of the Bou Hanifia Formation (see above regarding the stratigraphy). The sampling is not continuous and it is interrupted by two major gaps due to the poorness of the out­crops. The locality BH 1, which has yielded among other mammals Hipparion africanum and Progonomys cathalai (Jaeger et al., 1973), is situated in the lower gap. The stratigraphic position of the small mammal locality BH 5 is reported on figure 4. This figure also presents the stratigraphy of the sampled section, latitudes of the virtual geomagnetic pole (VGP) in each site, the succession of the magnetic polarities, and their correlation with the GMPTS of Harland et al . (1982) (see details of the paleomagnetic study in Sen, 1986). According to these results, the small mammal locality BH 5, which also yielded Progonomys cathalai, should be as old as 10.3 Ma, and the age of the Hipparion locality BH 1 should be a little older than 10.5 Ma, but certainly younger than 12 Ma.

In East Africa, the magnetostratigraphy of the Ngorora Formation has been studied at the type Kabarsero section (Tauxe et al., 1985). Bishop and Pickford (1975) have mentioned the Hipparion occurrence in Members D and E which are roughly correlated with Chrons 10 and 9. Contrary to Bishop and Pickford's observation, Hill et al. (1985) noted that "no equids are known from any of the sites that can be corre­lated to the Ngorora type section, suggesting an appearance later than 10 Ma. n How­ever, equids dated at between 10.5 Ma and 10. 7 Ma have been mentioned in the Chorora Formation of Ethiopia, associated with some large and small mammals (Jaeger et al., 1980; Kalb et al., 198Z).

CONCLUSIONS

Review of the available data shows that the synchrony of the Hipparion Datum throughout the Old World, as suggested by Berggren and van Couvering (1974, 1978), cannot be demonstrated. On the contrary, when biostratigraphical successions are well calibrated, some chronologie differences appear in this datum event from one area to another (figure 3).

Radiometric ages in relation to the Hipparion Datum are very rare and should be taken into account carefully. In fact, only a few number of Hipparion localities are associated with lava flows or volcanic ash deposits. No available radiometric dates can prove that the arrival of the hipparionine equids in the Mediterranean area is older than 11 Ma.

The magnetostratigraphic calibration of this event has displayed it~ diachrony in the Siwaliks of Pakistan and in some localities of the Mediterranean area. In the Siwaliks sequences of the Potwar Plateau, the first Hipparion-bearing horizons in the Nagri Formation are correlated with the lower part of Chron 9. In the Mediterranean are a sections (Kastellios Hill and Bou Hanifia), Hipparion occurs in Chron 10. Conse­quently, I proposed an age of about 11.5 Ma (Sen, 1986), which is roughly the age of

502

..,

the Chron 10-Chron 11 boundary, for a possible earliest occurrence of Hipparion in the Mediterranean area. In any case, a better calibration of this event and its correlation throughout the Old World require additional data.

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