J. Anim. Breed. Genet. 105 (1988) 161-176 0 1988 Verlag Paul Parey, Hamburg und Berlin ISSN 0044-3581

Ms. received I . 7 . 1987

Zoological Institute, University of Helsinki

The small caballoid horse of the upper Pleistocene and Holocene By ANN FORSTEN

Introduction

SCHWARZ (1928), NOBIS (1955, 1971), SICKENBERG (1962), and MUSIL (1977, Fig. 3) gave fossil evidence for a decrease in size of the horses in the Pleistocene (but see AZZAROLI 1984, for a different opinion). Upper Pleistocene-Holocene dwarfing has been observed in many mammals (KURTEN 1965, DAVIS 1977, 1981); GUTHRIE (1984, 1985) discussed possible mechanisms for dwarfing in upper Pleistocene ruminant and non-ruminant herbivores. While in North America dwarfing culminated in the horses becoming extinct at the end of the Pleistocene or in the early Holocene, small, caballoid or true, horses survived until recently in the wild in Eurasia.

Since the middle Pleistocene most Eurasian horses have been caballoid forms, charac- terized by the enamel pattern of their cheek teeth which closely resembles that in domestic horse, Equus cuballus L., i.e. with a U-shaped entoflexid between the metaconid and metastylid of the lower cheek teeth. The history of the caballoid horses in Eurasia began in the early middle Pleistocene (according to HOPWOOD 1936 and SAMSON 1975, already in the lower Villafranchian) with the immigration of large forms from North America. In the middle Pleistocene the caballoids varied locally in size, but most remained large. Medium- sized horses first appeared at this time (PRAT 1968, Tables 60-105, VANGENGEIM 1961, RUSANOV 1968). There were no small caballoids in the middle Pleistocene, with the excep- tion of E. missi Pavlov, said to be Rissian in age (GROMOVA 1949) but possibly no older than the last interglacial (AZZAROLI 1966; PANIZKINA’S 1953 analysis of the flint tools from Missi also indicates a younger age than Rissian for this archeological site, the site of E. missi). Most finds from the upper Pleistocene pertain to medium-sized and small horses, of which the latter evolved at this time.

Because of transgressive size differences, the categories “small”, “medium-sized”, and ‘‘large’’ are necessarily arbitrary. I classify a horse as small if MC 111 mean length is less than 22.5 cm, MT 111 mean length less than 27 cm, and phalanx 1 mean total length less than 8.6 cm. I have earlier suggested (FORSTEN 1982) that samples in which metapodial mean distal articular width is less than 5.20 cm should be identified as E. cabullusprzewalskii Polj. (= small), if over 5.20 cm the sample should be considered as belonging to E. c. germanicus Nehring (= medium-sized). These length and breadth values approximately define the upper limits of metapodial size of the small caballoid horse of the upper Pleistocene- Holocene.

Length and breadth of the long bones and, to a lesser degree, of the teeth and skull, show that the caballoid horses decreased in size throughout their stratigraphic and geographic range, giving rise to successively smaller forms. Size decrease may have been speeded up in the upper Pleistocene-Holocene. At all times the horses varied locally in size (see, for instance, differences in limb bone size of the three local samples of upper Pleistocene E. urafensis Kuzm.; KUZMINA 1985, Tables 6 & 10). Size also seems to have varied irrespective of the prevailing climatic conditions (MUSIL 1977), although GROMOVA (1965) found the horses to have been predominantly large during interglacials, small during glacials (see also

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162 Ann Forsten

SICKENBERG 1962). PRAT (1968) observed differences mainly in the abundance of horses in faunas of different climatic phases. Since local size variation within an equid taxon at any one time tended to overshadow temporal size shifts, and since taxa of different size evidently occurred contemporaneously (see below), size cannot be utilized for relative dating of fossil equids, as NOBIS (1982) attempted to do, nor is detailed stratigraphic tracing of taxonomic changes, using size as a specific character, feasible. NOBIS (1971) traced taxonomic shifts within successive levels of the Rissian of Achenheim Cave, where in level 20a the medium- sized E. remagensis Skork. (= E. plicidens Owen, E. germanicus of most authors) alledgedly replaced the stratigraphically older, larger, E. achenheimensis Nobis, but in fact Nobis’ data (Tables LX-LXIX) show no decrease in size of the horses in Achenheim Cave: For instance the teeth were as a mean larger in the younger levels 18-11, although not significantly so. Neither was there a decrease in size of the two caballoid horses present in the successively younger levels BE-W2-W3 of Sveduv Stul, where some of the largest bones derive from the W3 upper Wurmian (MUSIL 1962, Tables 44-46). MUSIL (1969, Table 40) also traced dwarf- ing in the teeth from one level to the next within the Magdalenian of Pekarna Cave, but the size differences are contradictory.

In the early Holocene, due to climatic warming and encroachment by the forest on the former grass-lands, the horses decreased in number and/or their distribution became patch- ier. The large and medium-sized horses became extinct at the end of the Pleistocene (SCHWARZ 1928, NOBIS 1955, SICKENBERG 1962), but there are still quite a few, if poor,finds of small horse from the Mesolithic and Neolithic of both Europe and Asia (e. g. LUNDHOLM 1949, GROMOVA 1949, NOBIS 1955, PAAVER 1965, BIBIKOVA 8r BELAN 1981, TOMIRDIARO 1982). NOBIS (1955) compared and identified the small Holocene horse with E. przewalskii, the Recent Przewalski’s horse. NOBIS (1955, 1962) and BIBIKOVA (1967) derived the early domesticated horses in Europe autochthonously from such small wild forms (see also LUND-

Recent small caballoid horses

The alledged former presence of E . przewulskii in Europe (LEHMANN 1954, NOBIS 1955, RADULESCU 8r SAMSON 1962) has been denied on the grounds that equally small and slender, caballoid, horses are unknown here (HAY 1915, LUNDHOLM 1949, GROMOVA 1949, 1965, SICKENBERG 1962, MUSIL 1969, SAMSON 1975). In its range until recently, in SW Mongolia and adjacent parts of N. China, the species is now extinct (MOHR 8r VOLF 1984). However, the tarpan E. gmelini Antonius, a wild or feral horse present in Europe until 1879 (SOKOLOV 1959), may not have been specifically different from Przewalski’s horse. HERRE (1939) considered Przewalski’s horse and the tarpan conspecific geographic vicars and HEPTNER (1955) referred the tarpan to E. przewalskii as a subspecies gmelinz. On the other hand, SOKOLOV (1959), following GROMOVA (1949), believed them to be separate species and Przewalski’s horse to represent an endemic Asiatic branch of equid evolution.

GROMOVA (1949) believed Przewalski’s horse to have had a long endemic history in Asia because her data on its metapodials (long, slender) and skull (long snout and tooth row, narrow glenoid width) seemed to set it apart from other known forms (see also SICKENBERG 1962, MUSIL 1969, SAMSON 1975, Fig. 34). Recent studies (EISENMANN 1979, 1980, 1981) show that Przewulski’s horse is not aberrant, but closely resembles domestic horse, e. g. in over-all skull morphology. EISENMANN’S data (1980, Table 36) and my own measurements on the skull show a shorter mean tooth-row length, but greater glenoid width (20.05 cm k 0.16, N 8), than those given by GROMOVA (1949, Table I). These differences may partly be due to the proportion of zoo-animals, respectively wild-caught specimens, in each sample (see below). Przewalski’s horse does differ from most other small fossil o r subfossil caballoids in having a narrow snout but great forehead and glenoid width and a short cranium (Fig. 1-3). EISENMANN’S statistics (1979, Table 6) on the merapodials further show that these are not as exceptionally slender as GROMOVA’S data would indicate (Fig. 4-6).

HOLM 1949).

The small caballoid horse of the upper Pleistocene and Holocene 163

Przewalski’s horse is also cytologically and molecularly similar to domestic horse (RYDER, EPEL c BENIRSCHKE 1978, BOWLING e~ KYDER 1985) and fully interfertile with it.

The early appearance of Przewalski’s horse in Asia in unknown. Pleistocene fossils from China and Kazakhstan have been identified as E. przewalskiz (BOULE c TEILHARD 1928, CHOW et al. 1959, KO~AMKULOVA 1958), but they belong to larger horses or are inadequate for identification to species. Przewalski’s horse appears to be a late, dwarfed, descendant of earlier, larger, Eurasian forms, of which the one closest in size and geographic origin was the upper Pleistocene Tscherski’s horse (E . tscherskii Brauner = E. lenensis Rus.). The latter appeared in locally slightly different mean sizes, such as E. caballus subsp. (small) from Kolyma (SHER 1971), the small horse from Jana (TSCHERSKI 1892, Table p. 370) and Seleri- kan (VERESCHAGIN 8r LASAREV 1977), E. caballus ssp. B from Jakutia (VANGENGEIM 1961), the specimens named by RUSANOV (1968), the specimens from Kotelny and part of those from Kubekovo (GROMOVA 1949, Tables XV, XV1). All were more massive than Przewals- ki’s horse and had a broader snout and cranium (Fig. 2, 5). GROMOVA (1949, Tables XV, XVI) gave measurements on some small, slender, metapodials from TSikoi and Verholens- kaja Gora, which are within the size range of Przewalski’s horse. The age of these finds is said to be upper Pleistocene. However, since bones of domestic animals were found in Ver- holenskaja Gora, the status and age of the horse are uncertain (GROMOVA 1949). Very small horse metapodials were reported by Vangengeim (1961, Table 20,24) from N. Siberia, but their age and status are also uncertain. Alternatively, Przewalski’s horse may represent a late immigrant from North America, where the Yukon horse, E. lambei Hay, of the upper Pleistocene was closely similar in size and proportions (HARINGTON c CLULOW 1973, see also VERESHAGIN LASAREV 1977, FORSTEN 1986). More likely, however, E. lambei was the easternmost representative of a circumpolar species of small, caballoid, horse, of which Przewalski’s horse and the tarpan were the last wild Eurasian survivors.

The tarpan, believed to be the ancestor of at least some domestic breeds of horse (GROMOVA 1949), has variously been considered conspecific with Przewalski’s horse, an independent species, or even a feral domestic horse. It is said to have been stockier than

Fig. I . Simpson’s ratio diagram for the comparison of measurements on the skull of horses. Standard: means for Przewalski’s horse; grey field shows 95 % confidence limits of the mean (data EISENMANN 1980, Table 36, and own). Compared specimens: tarpan (0) (data GROMOVA 1949, Table 1) and tarpan (*) (data NITSCHE 1929)

0 2 0 0 2 0 4

basal length

I’-’-chwnae

,l-1 - 2-2

snout width

portorbital w.

glenoid w.

max. cranial w.

orbit post. rim 11-1 -

164 Ann Forsten

02 0 01 04 06

1 basal length

Ib’-chaana,

lw - 2-2

snout width

posorbitai w.

glenoid w.

max.cranial w.

l”-orbit post.rim

Fig. 2. Simpson’s ratio diagram for the comparison of measurements on the skull of horses: Standard: means of Przewals- ki’s horse; grey field shows 95% confi- dence limits of the mean (data as in Fig. 1). Compared samples from Siberia: Tschers- ki’s horse (0) (data VERESHACIN &

LASAREV 1977, Table 4) and five skulls (data RUSANOV 1968, Table 21)

Przewalski’s horse (GROMOVA 1949), but see BIBIKOVA (1972) for doubts about the general- ity of this character. The range of metric variation of the tarpan is unknown, because only one skeleton (of a castrated male) is preserved, but measurements on the bones of that skeleton and on the two extant skulls (GROMOVA 1949, 1959, 1963, NITSCHE 1929) fall mostly within the size range of Przewalski’s horse, although for several variables the obser- vation on the tarpan falls outside the 95 % confidence limits of the mean for Przewalski’s horse (Fig. 1,4). The skull of the tarpan is narrower than that of Przewalski’s horse, except for the snout, and the cranium is longer.

The tarpan lacks unequivocal fossil documentation. Fossil tarpan has been identified from the Mesolithic of Mirnoe, Mesolithic/Neolithic of Pogorelovka, and Neolithic of Floresti (GROMOVA 1949, ZALKIN 1962, BIBIKOVA 8r BELAN 1981, but see below), but these finds belong to larger and more massive horses than the tarpan. Other localities of uncertain age (e. g. in Bohemia, Diimmer, the Rhine Gravel, etc.) have yielded small bones, within the size range of the tarpan, and small, Metal Age, in many cases certainly domesticated, horses are closely similar in size and proportions to the tarpan proper, e. g. from Olmiitz UEIITELES 1872), Schlossberg (DURST 1904), and Novo Rosanovka (BIBIKOVA 1972).

Small fossil caballoid horses

Equus caballus gallicus Prat (a younger synonym of E. spelaeus Owen, according to SAMSON 1975, but see below), from the Wiirm 11-111 of Solutri, was among the largest of the small fossil caballoids. It was more massively built than Przewalski’s horse and had particularly large teeth. It was discussed by PRAT (1968, Tables 52-107, for data see also NOBIS 1971, Tables LXXIII-LXXXI). Equus gallicus has also been found in other upper Pleistocene localities in France (PRAT 1968, MOURER-CHAUVIRE 1980). Bones within the size range, and of the proportions, of those from Solutrk are from the upper Pleistocene of Freyburg, identified as E . c. cuballus (SCHWARZ 1928, Tables pp. 255-258) or as “Freyburg b” (SICKEN- BERG 1962, Abb. 2; note that Sickenberg gave a much narrower range of metapodial length

The small caballoid horse of the upper Pleistocene and Holocene 165 0 2 0 0 2 0 4

Fzg. 3 . Simpson’s ratio diagram for the comparison of mcasurements on the skull. Standard: means of Przewals- ki’s horse; grey field shows 95 % confidence limits of the mean (data as in Fig. 1). Compared samples from Eich (0) and Steinswort (*), the Rhine Gravel

basal length

I’-’-chaanae 1 Id

snout width

postorbital w.

glenoid w.

and shaft width for the sample from SolutrC than PRAT 1968 and NOBIS 1971), from the Aurignacian of Vogelherd, identified as E . przewafskii (LEHMANN 1954), from the Wiirm 11-111 of Sveduv Stul, identified as E . cf. gmefini (MUSIL 1962), from the Magdalenian of Pekarna, believed to belong to a horse related to E . germanicus (MUSIL 1969), from the middle and upper Wiirmian of Rumania, called E . spefaeus, E . s. cibinensis Samson, and E . sqythicus Rad. & Sams. (SAMSON 1975), from the upper Pleistocene of the Northern and Central Ural, described as E. urafensis (KUZMINA 1975, 1985, Fig. 6), and from Sirjanovsk, identified as E. cabaffus fossifis v. MEYER (KOZAMKULOVA 1969, Tables 8-13), and from the Solutrkan of Ofnet (FREUDENBERG 1914, Tables).

Horse bones transgressively shorter and more slender than those of E . galficus occurred in the upper Pleistocene and Holocene of Central Europe and Asia (STUDER 1883). PRAT (1968) mentioned a slender-built caballoid horse from the latestmost Wiirmian of France and D u c o s (1960) characterized E. cubaffus nehringi Durst from the Neolithic of Roucadour as smaller and more gracile than E. gufficus, as was E . juvilfucus Bravard from Tour de Juvillac (HOPWOOD 1936). GROMOVA (1949, Tables XV, XVI) reported small caballoid horses from Northern Siberia, although the common Siberian, upper Pleistocene, Tscherski’s horse was hroadly transgressive with the sturdy E. galficus (TSCHERSKI 1892, Table pp. 37C-373, VANGENGEIM 1961, RUSANOV 1968, SHER 1971, VERESHAGIN 8r LASAREV 1977, Figs. 17-18, see also SAMSON 1975, Figs. 35-36). Very small metapodials, which on their proportions evidently belong to true horse instead of to E . asinus as referred, were recently dredgedfrom the North Sea outside Holland (HOOIJER 1984,1985). Bones of such small horses overlap in size and proportions, on the one hand with the more massive bones of E . galficus, on the other hand with the exceedingly slender bones of the non-cahalloids E . hydruntinus Regalia and E. hemionus Pallas. Single specimens may, for this reason, be difficult to identify and have in some instances mistakenly been referred to as E . hydruntinus ( e . g. in STEHLIN ~r

GRAZIOSI 1935) and as E. hemionus (e.g. in NEHRING 1882, 1884, RAVEN 1935, DIETRICH 1959). In the metapodials of these caballoids, shaft-width may indicate great gracility, while the articular surfaces are relatively broad (see also SCHWARZ 1928). According to LUNDHOLM

166

humerus length

radius 1.

Mc 111 I.

proximal breadth

distol articular br.

mid-shaft br.

tibia I.

MT Ill 1.

proximal br.

distol articular br.

mid-shaft br.

calcaneum height

phalanx I 1.

mid-shaft br.

phalanx It 1.

mid-shaft br.

Ann Forsten

06 04 02 0 02

I

Fig. 4. Simpson’s ratio diagram for the comparison of measurements on the limb bones. Standard: means of Przewalski’s horse; grey field shows 95% confidence limits of the mean (data GROMOVA 1949, Tables; EISEN- MANN 1979, Table 6; DAVIS 1982, and own). Compared with tarpan (0) (data GROMOVA 1949) and sample from Eich (*)

(1949), this could be a mark of domestication, domestic horses having more slender bones than wild forms due to differences in nutrition. Since bone diameter increases under the influence of load bearing (LANYON RUBIN 1985), narrow mid-shaft diameter may indicate insufficient use, e. g. in stabled animals. AZZAROLI (1980) suggested that the observed discrepancy in proportions may be due to rickets, but Hooijer (pers. comm.) found no traces of rickets in the small, slender-shafted, metapodials which he studied (HOOIJER 1984,1985). O n the other hand, broad articular surfaces but a slender shaft of the long-bones is a juvenile character, which in the adult animal indicates neoteny. This is in keeping with the pedomor- phic reduction in size of these horses in the upper Pleistocene (see below).

Since the exact age of the small fossil horses in many cases is uncertain, especially the Holocene material may represent both wild and early domestic forms, e. g. the horse from Bielersee (STUDER 1883) and the smaller horse from Wauwyl (HESCHELER 1920), that from the Swedish early Holocene (LUNDHOLM 1949), from the Mesolithic/Neolithic of Pogorelovka (GROMOVA 1949), from the Neolithic of Kaapa (PAAVER 1965, Table 53), and from the Iberian Peninsula (UERPMAN 1976, note that UERPMAN’S E. hydr~ntinus is a

The smali caballoid horse of the upper Pletstocene and Holocene 167 caballoid horse). In the same size category belonged also the small horse from the “Diluvium” of Zuzlawitz (WOLDRICH 1881), from Schweizersbild and Thayingen (STUDER 1896, 1904), from the Magdalenian of Gonnersdorf (POPLIN 1976) and Saaleck (NOBIS 1982), from Oelinghofen, Rhein-Herne-Kana], Vucovar, Kahla, Klodebach, Lindenthaler Hohle bei Gera, Ziegenberge hei Kamsdorf, Pforten, Turmiz bei Kosten, Muhlhausen bei Ammern, Bad Berka, Jena, Weimar, Eich, Hamm bei Eich, Kastel, Nierstein, Steinswort, Biebesheim, and Erfelden (material in the Humboldt Museum, E. Berlin; Institut fur Quar- tarpalaontologie, Weimar; Hessisches Landesmuseum, Darmstadt; and Naturhistorisches Museum, Mainz). Of these samples the pooled one from the Rhine Gravel, i. e. from Eich, Erfelden, Biebesheim, and Steinswort, is the largest and most complete, comprising bones, teeth, and cranial material. The status of the small Rine Gravel horse, whether wild or domesticated, is unknown, as is its exact age. For its wild status speak the colour and preservation of the long bones which, with the exception of a few specimens, are similar to those of the rest of the Rhine Gravel, fossil, fauna. Its possibly domestic status is indicated by its close metric similarity with Bronze and Iron Age, undoubtedly domesticated, horses

Fig. 5 . Simpson’s ratio diagram tor the com- parison of measurements on the limb bones. Standard: means of Przewalski’s horse; grey field shows 95% confidence limits of the mean (data as in Fig. 4). Compared samples f rom Siberia: Jana (0) and specimen from Selerikan (0) (data VERESHAGIPI ~r LASAREV 1977, Table 3), Kolyma (data SHER 1971, Tables)

humerus length

radius 1.

MC 111 1.

proximal breadth

distol articular br.

mid-shaft br.

tibia 1.

M I 111 1.

proximal br.

distal articular br.

mid-shoft br.

calconeum height

phalanx I 1.

mid-shaft br.

phalonx II 1.

mid-shaft br.

168 Ann Forsten

(from Schlossberg, DURST 1904; Sweden, LUNDHOLM 1949; Vinzenta, Berlin/Spandau, and Hilsborngrund, data in SCHWARZ 1928, Tables pp. 472473. and Copper-Bronze Age horses from Spain, VON DEN DRIESCH ~r BOESSNECK 1969, VON DEN DRIESCH 1972), marks on the forehead of two of the skulls (Nos. 241 and 1968/320, both from Eich) indicating wounding with a weapon, possibly while the animal was tethered, and the presence in the RhineGravel of primitive domesticated animals, such as shorthorned cattle, small pig, dog, and sheep/ goat. However, some of the equally small horses in Eurasia seem too ancient to have been domesticated, e. g. the horse from the Rhein-Herne-Kana1 and E. graziosi Azzaroli, which alledgedly are early Wurmian in age (SCHWARZ 1928, AZZAROLI 1979, note that E. graziosi was described as an ass, but is clearly a horse on tooth morphology); possibly the age of these finds would have to be checked.

The skull of the small horse from Eich and Steinswort is similar to the skull of other caballoids of small size; basal length is 46-49.5 cm (Fig. 3). This corresponds with e. g. Przewalski’s horse, the mean basal length of which is 47.6 cm (range 44.7-50 cm, EISEN-

MANN 1980. Table 36), Tscher- ski’s horse (range 46.9-50.2 cm,

* 0 O2 O4 O6 08 VERESHAGIN ~r LASAREV 1977, humerus length

radius 1.

M C I I I 1.

proxlmoi breadth

distal articular br.

mid-shaft br.

tibia 1.

MTIII I.

proximal br.

distal articular br.

mid-shaft br.

calconeum height

phalanx I 1.

mid-shaft br.

phalanx I1 1.

mid-shaft br.

Table 4), Swedish Bronze Age horses (range 44.8-48.5 cm, LUNDHOLM 1949), and Norwe- gian Viking Age horses (range 45.1-50.7 cm, NOBIS 1962). The cheek teeth from Eich and Steins- wort are among the smallest known in any caballoid horse, comparable in size with those of E. graziosi and with E. cazurroi Cabr. from the upper Pleistocene of Spain (CABRERA LATORRE 1919). In the upper cheek teeth the protocone is short to very short; according to Schwarz (1928), this indicates domestica- tion. A short protocone also characterized the tarpan (GROMOVA 1949) and various fossil and subfossil horses (NOBIS 1955, Abb. 9, uppers from Bars- bek; RADULESCU 8r SAMSON 1962,

Fig. 6. Simpson’s ratio diagram for the comparison of measurements on the limb bones. Standard: means of Przewalski’s horse; grey field shows 95% confidence limits of the mean (data as in Fig. 4). Compared samples from Solutrk (*) (data PRAT 1968, Tables), from sis-Ural (0) and trans- Ural (0) (data KUZMINA 1985, Tables 6,101

The small caballoid horse o j the upper Pleistocene and Holocene 169

SAMSON 1975, Fig. 39, upper of E. scythicus; and BIBIKOVA ~r BELAN 1981, Table 1, upper PH from Mirnoe).

The great similarity in size, proportions, and morphology of the small fossil and subfossil horses on the one hand, and Przewalski’s horse and the tarpan on the other, supports the idea that the two latter represent the recent, terminal, forms in an evolutionary chain, the earlier links of which were generally larger. The presence in all of Eurasia in the upper Pleistocene and Holocene of a small horse, morphologically similar to early domesticated forms, supports the autochthonous origin of domestic horse from the wild form in various parts of the area of distribution of the latter (NOBIS 1955).

Taxonomy

Although in different samples the long bones differ in size and proportions, the enamel pattern of the cheek teeth of the caballoid horses is remarkably homogeneous throughout. Fossil caballoids have, accordingly, often been assigned as temporal and geographic sub- species of E. cuballus (SCHWARZ 1928, GROMOVA 1949, VANGENGEIM 1961, PRAT 1968; see AZZAROLI 1984 for a plea in favour of this usage). Other authors have refrained from using cuballus for non-domestic forms and instead applied various other specific names to the fossils (NOBIS 1971, MUSIL 1974, SAMSON 1975). NOBIS (1971) adopted the name ferus Boddaert, originally bestowed on the tarpan, for all small, upper Pleistocene-Holocene, caballoid horses. To E. ferus he also referred Przewalski’s horse as a subspecies.

The taxonomy of the upper Pleistocene caballoids and their relationship to domestic horse have receutly been discussed by NOBIS (1971, 1982), GROVES (1974), WILLOUGHBY (1974), and SAMSON (1975). NOBIS (1971) differentiated the West European E. ferus s o h - reensis Nobis (= E. cuballus gullicus = E. spekzeus) and its dwarfed Magdalenian, unnamed, form from the East European E.ferusferus and E . 5 gmelini (NOBIS’ combinations). As the type localities of E.f.f. and E.f. g. Nobis selected the upper Pleistocene fossil sites Mezin and Sungir, and suggested a fluctuation in size from a medium-sized, ancestral form ( E . remugensis lutipes Gromova = 1:. germunicus) to a small form (E . f . f . at Mezin), and back to a medium-sized form (E . g. 5 at Sungir). From Mezin he identified both E. Y . lutipes and E. 5 ferus, the latter on the basis of some small bones. However, BELAN (1985) pooled the small specimens from Mezin with the large ones in a single sample. The pooled sample from Mezin belongs to a horse smaller than the one from Sungir, but larger than the horse from Solutre.

SAMSON (1975) rightly criticized Nobis for nomenclatorial errors and for errors as regards the age of the fossil localities. Of the localities mentioned above, Sungir is in fact older than Mezin (age 25000, resp. 24200 years, VERESHAGIN ~r KUZMINA 1977). The horse from Sungir represented E. Y. lutipes, while the one from Mezin was a dwarfed form of the latter; BELAN (1985) referred it also to E. lutipes. Neither the Sungir, nor the Mezin, horse corresponded to the tarpan ( E . ferus, according to NOBIS), which was rather smaller than both. In spite of Samson’s criticism, Nobis is to be commended for attempting to tie together geographically disparate finds by comparing fossils from East and West Europe. In view of the great morphological homogeneity of and transgressive size differences between the caballoid horses, Nobis’ recognition of a single small species in the upper Pleistocene- Holocene-Recent is also praiseworthy.

GROVES (1974) adopted ideas on horse taxonomy expressed by LUNDHOLM (1949) on the basis of fossils and by EBHARD (1958) on the basis of the behaviour of different breeds of domestic horse. Lundholm (1949, Fig. 10) separated an eastern group of upper Pleistocene caballoids with M3 believed to be longer than MZ and a western group with MZ and M’nearly equally long. The latter group he further subdivided into “germunictrs”-horses and “Mi- nohippus”-horses. NOBIS (1955) and LIEPE (1958) showed that even within a single sample there is considerable variation in the length relations of MZ/M3. This is due to the change in the shape of the tooth crowns with wear: In the young animal the occlusal surface of MZ is

170 Ann Forsten

long, that of M3 short, but with increasing wear the occlusal surface of M2 becomes abso- lutely shorter and relatively even more so, because simultaneously the surface of M’ increases in length (see LUNDHOLM 1949, Fig. 7). Thus Lundholm’s “eastern horses” would tend to be senile ones.

GROVES (1974) elaborated LUNDHOLM’S three-group-sceme and differentiated four fossil species as the originators of the domestic breeds of horse. H e attempted to unite groups of horses differentiated on fossil evidence with Ebhard’s groups of domestic breeds differenti- ated on the basis of behaviour, implying that different breeds of domestic horse are descended from wild ancestors which differed specifically in behaviour. However, RUBEN- STEIN (1981) showed that the behaviour of feral horses in flexible and varies depending on the environment; this would contradict set behaviour patterns in different domestic breeds. Taxonomic unity of domestic horse is strongly indicated by the fact that although species of recent equids are noticeably different cytologically and molecularly (RYDER, EPEL BENIRSCHKE 1978), breeds of domestic horse so far studied have proved similar in these regards (BENIRSCHKE, MALOUF, Low HECK 1965, CLEGG 1974), and by the fact that representatives of Ebhard’s “groups” have been successfully intercrossed in various combi- nations.

SCHWARZ (1928) sceme, according to which there has been only one species of caballoid horse since the early middle Pleistocene, has been criticized as an oversimplification (AZZAROLI 1966, but see AZZAROLI 1984). The one-species-sceme, which differentiates the fossil caballoids as temporal subspecies of E. caballus, correctly reflects the morphological homogeneity of these horses, but is untenable in the light of sympatry. At certain, albeit rather few, fossil localities of the upper Pleistocene, two caballoid horses occur, evidently sympatrically. Such localities are, e. g. Cueto de la Mina (CONDE DE LA VEGA DEL SERRA 1916, according to CABRERA LATORRE 1919), Freyburg (SCHWARZ 1928, Tables pp. 255-258, SICKENBERG 1962, Fig. 2), Sveduv Stul (MUSIL 1962), Chiana Valley (AZZAROLI 1966, note that the alledged ass from China Valley is a horse, i.e. AZZAROLI 1979, Tables), Gusterita and levels 43-74 of the La Adam Cave (SAMSON 1975), Westeregeln and Willersin (material in the Humbold Museum and Dept. of Earth Sciences, University of Mainz); in Asia two caballoids occur in Gulongshan Cave (ZHOW, SUN, Xu LI 1985). RAKOVEC (1965) referred to two caballoids from Risovac, but the sample probably comprises only a single species. Gaal described two caballoids from Ohabaponor, but teeth referred to the smaller form may rather belong to E. bydruntinus (BOKONYI 1954). At some localities two caballoids occur, but in different stratigraphic levels, e. g. at Vogelherd (LEHMANN 1954), while at other localities the stratigraphic distribution of the two horses in unknown, e. g. at Zuzlawitz (WOLDRICH 1881, 1883), in the Jana Basin (TSCHERSKI 1892, but see VAN- GENGEIM 1961), and in the Rhine Gravel. The most common sympatric caballoids are the medium-sized E. germanicus (= E. remagensis, E. latipes, E. transsilvanicus Theod.) and the small horse (= E. ferus, E. spelaeus), which occur together, e. g. at Willersin, Freyburg, Westeregeln, Rhein-Herne-Kana], Rhine Gravel, Gusterita, La Adam Cave, Chiana Valley, and Gulongshan Cave; more seldom the large E. mosbacbensis Reich. occurs together with the small form. e. g. at Sveduv Stul, possibly also at Cueto de la Mina, La Adam, and Zuzlawitz. The sympatric occurence of a large and medium-sized caballoid is uncertain, but SAMSON (1975) identified the large Equus sp. from La Adam in the same levels with E. cf. transsilvanicus and E. spelaeus. MUSIL (1962) also identified E. germanicus from Sveduv Stul, but the material referred to germanicus may, in fact, be small specimens of the large E . mosbacbensis from that site. Sympatry of two morphologically similar forms which differ in size is the only objective evidence for their specific difference, the interpretation of allopatric samples is more subjective. As for the caballoid horses transgressive size change makes it difficult to differentiate in allopatric samples between the large E. mosbacbensis and medium-sized E. germanicus, and between E. germanicus and the small horse. Identification of allopatric samples of these species may in many cases remain arbitrary and subjective.

The small caballoid horse of the upper Pleistocene and Holocene 171

If the small caballoid horse comprises both domestic horse, recent wild forms (Przewufs- ki’s horse and tarpan), and the upper Pleistocene-Holocene ancestors of these, its earliest valid name would be E. cubuffur. AZZAROLI (1984) recently defended the use of cubuffus also for non-domestic horses. The large caballoids, although not clearly different in morphology or size, neither from one another, nor from the small horse, must be referred to different species, since they occasionally occur sympatrically with the latter. They did not contribute to the domestic stock of the horse.

Dwarfing

LUNDHOLM (1949) and MUSIL (1969) compared wild-caught and zoo specimens of Przewals- ki’s horse for skull and dental measurements. LUNDHOLM (1949) showed a statistically significant decrease in basal and tooth row length, cranial and snout width, in his zoo specimens as compared with his wild-caught specimens, whereas postorbital width did not differ. GROMOVA’S (1949, Table I) measurements on the skull of 5 Przewalski’s horses, of which 4 wild-caught (GROMOVA 1959), differ in a similar way from those, mainly zoo specimens, measured by EISENMANN (1980, Table 36), but in the latter sample postorbital width is greater at a lesser skull length. MUSIL (1969) compared the teeth of a single wild- caught specimen with those of 3 4 zoo specimens: The upper cheek teeth of the former are significantly larger, with a longer protocone, than those of the latter, but the lowers d o not differ in size. MUSIL’S wild-caught specimen also has larger teeth than those measured by GROMOVA (1949, Table Va, VI), while his zoo specimens do not differ in mean tooth length or protocone length from those measured by EISENMANN (1980, Table 46; 1981, Table 17), but show greater tooth breadths, probably because MUSIL measured breadth including, EISENMANN excluding, cement. LUNDHOLM’S (1949) and MUSIL’S (1969) comparisons show that zoo animals of Przewalski’s horse have decreased in skull and dental size, a decrease also observed in many domesticated animals as compared with their wild forebears, e. g. dog compared with wolf (DAVIS 1981).

Wild-caught and zoo specimens of Przewalski’s horse have not been compared for limb bone size and proportions. The largest sample of limb bones of this horse published is that of GROMOVA (1949, 1959, 1963), which includes 8 skeletons, of which some are wild-caught, some from Askania Nova, and some of unknown origin. GROMOVA’S (1949, Table XV, XVI) measurements of the metapodials differ from those of EISENMANN (1979, Table 6), especially in length. Whether this difference is due to a greater proportion of wild-caught (? larger) animals in Gromova’s sample than in Eisenmann’s, in unknown. The large size of GROMOVA’S specimens could .ilso be due to crossing with domestic horse, as is known to have happened with Askania Nova stock of Przewalski’s horse (RYDER et al. 1981,1983).

LUNDHOLM (1949) discussed the causes for dwarfing in upper Pleistocene-Holocene horses, in zoo animals of Przewulskz’s horse, and in island populations of domestic horse. The main reason for dwarfing he believed to be WRIGHT’S Effect (gene drift) in small, isolated populations. Theoretically the result of stochastic evolution in a small deme could as well be an increase, as a decrease, in size. At least regarding some of the dwarfing processes mentioned, the size of the population as such does not seem to be decisive for the direction of change, but rather the environmental conditions, particularly the supply and quality of the food. Size changes in island populations of mammals have recently been under renewed discussion (e. g. WASSERZUG et al. 1979, LAWLOR 1982, ANGEBJORN 1985, LOMOLIN 1985). These authors agree that for large herbivores on islands food shortage is common and decreased body size is the most economic way to deal with this. In accord with the rule that large herbivores isolated on islands tend to become dwarfed, horses confined to, e.g., Iceland, the Shetlands, and Gotland, are of pony size, although the present conditions on many islands with a native horse population appear superficially favorable in that the oceanic, humid climate guarantees a long, continuous growth season. However, GUTHRIE

172 Ann Forsten

(1984) makes a case for a long growth season resulting in vegetal growth of low nutritious value. O n the other hand, the size of the horses originally introduced on these islands is not with certainty known; Viking Age Icelandic horses seem to have been only slightly larger than the present stock (NOBIS 1962), and both Viking Age and present Icelandic horses correspond in size to the upper Pleistocene and Holocene small caballoids.

BOESSNECK and VON DEN DRIESCH (1978, cited in DAVIS 1981) also suggest nutritional deficiency as a cause for dwarfing of early domestic animals, due to man emphasizing the number of individuals, rather than the quality, of the stock, with consequent undernourish- ment in the winter. This mechanism, however, does not satisfactorily account for size decrease in regularly fed zoo animals. Also, it is unclear whether, in fact, the horses under- went dwarfing as a consequence of domestication, since late Pleistocene- early Holocene caballoids already were quite small.

Most authors believe that climatic warming was responsible for the decrease in size observed in many mammals at the turn of the upper Pleistocene-Holocene (KURTEN 1965, DAVIS 1977, 1981). A reversed BERGMAN’S Rule is implied. According to BERGMAN’S Rule, warm-blooded animals in cool areas tend to be larger than their counterparts in warm areas, but this interpretation is not tenable for the horses if GROMOVA’S (1965) observation that small forms dominated during glacials, large ones during interglacials, is correct. Neither is dwarfing of the horse as a result of climatic warming supported by the successive decrease in size from the middle Pleistocene horse giants to the upper Pleistocene dwarfs, when at the same time the vegetation of successive glaciations indicated increasingly cold and arid condi- tions (FRENZEL 1968, MOMIN 1984).

A decrease in the quality and/or amount of food, as a result of habitat patchiness or increased aridity, may, through “stunting”, lead to phenotypically small animals (LANDER 1978), small size later becoming translated by selection into genetic change (MARSHALL 8r

CORRUCHINI 1978). GUTHRIE (1985) stressed the nutritiously favorable, although winter- climatically harsh, conditions of the upper Pleistocene Mammoth Steppe fauna, conditions which he believed resulted in herbivore giants. However, the horse of the Mammoth Steppe fauna was not one of the large forms, but the small caballoid, the “loess horse” of authors (e. g. SICKENBERG 1962). In Asia this horse succeeded the larger caballoids, in southern and south-eastern Europe it occured sympatrically with them. It appears to have been the sole equid of the humid tundra and arid loess-steppe (in the latter area possibly sympatric with Asiatic half-ass) of north-western and northern Eurasia.

MARSHALL and CORRUCHINI (1978) believed that “stunted”, phenotypic, dwarfs can be differentiated from genotypic dwarfs on the basis of the proportions of tooth and body size, phenotypic dwarfs having larger teeth and skull in proportion to the rest of the body. Tooth size (e. g. occlusal length in cm or surface in cmZ) compared with metapodial length (= height at the withers) in the caballoids show no evident pattern; the small horses do not constantly have relatively smaller, nor larger, teeth than the larger forms. The small horse from Pekarna, Solutri, and Sveduv Stul had relatively and absolutely large teeth, while that from Eich and Valsgarde had small teeth. Among the medium-sized horses, those from Roter Berg, Quedlingburg, and Unkelstein had relatively and absolutely small teeth, while those from Moldavia (Brinseni, Starie Duruitory, and Vyhvatnitsu; data DAVID 1974) had large teeth. Small teeth, e. g. those from Valsgarde and possibly Eich, may be a consequence of domestication, but domestication hardly comes into question regarding the medium-sized, upper Pleistocene horses. The large-toothed and small-toothed forms appear to have been just extreme variants in a continuous size range within each species.

The small size of the small caballoid may have been an adaptation to the unpredictable, often fluctuating environment of the upper Pleistocene, with its seasonally rich and nutri- tious pasture (GUTHRIE 1985), but harsh glacial winter when food was of low quality. In such environments pedomorphic traits, e. g. small size, rapid physical and sexual maturity, are advantageous (GOULD 1976). The cold climate and the small size of this horse may have

The small cabalioid horse of the upper Pleistocene and Holocene 1 73

increased basal metabolism, thus reducing the gestation period, resulting in more rapid reproduction and generation turn-over (MCNAB 1980). In the periglacial areas during the growth season, the small caballoid competed favorably with the larger ones, since a small herbivore needs less food, but due t o relatively high energy requirements, the small horse may have been more selective, choosing more energy-rich plants and plant-parts (BELL 1971, JANIS 1975). Poor winter grazing may have forced the small horse t o move southward.

Post-glacial warming may, in its turn, have caused increased habitat patchiness due t o encroachment by forest o n former steppe and tundra, and by encroachment by man. Both processes may have had an effect similar to that of arid glacial climates, i. e. a decrease in the amount and quality of the food. GUTHRIE (1984) stressed a change t o a shorter growth season with associated detrimental changes in the type and zonation of the vegetation as the chief cause for dwarfing in upper Pleistocene-Holocene large herbivores. T h e horse followed the rule that reduced levels of resources may lead to communities with smaller individual size (PETERS 1983, citing THIEL 1975), rather than BERGMAN’S Rule.

Summary The small caballoid or true horse of the upper Pleistocene and Holocene and its relationships with Recent wild and domestic horse are reviewed. Dwarfing of the horse and its causes are discussed.

Zusammenfassung Das kleine caballoide Pferd des oberen Pleistocan und Holocan

Das kleine caballoide oder echte Pferd des oberen Pleistocan und Holocan und seine Verwandtschaft mit rezenten wilden und domestizierten Pferden werden besprochen. Verzwergung der Pferde und ihre Ursache werden diskutiert.

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459 pp.

pp. 1-99.

Author’s address: ANN FORSTEN, Zoological Institute, P. Rautatiekatu 13,001 00 Helsinki, Finland