THE diversity of ochotonids and their EVOLUTIONARY DEVELOPMENT
The earliest ochotonids referred to the genus Sinolagomys are recognized from Oligocene deposits in China and Mongolia (Bohlin 1937,
Gureev 1964). The genus flourished in eastern Asia from the middle Oligocene through early Miocene (Gureev 1964;
Li and Qiu 1980;
Tong 1989). Oligocene taxa are characterized by having reduced roots, cheek teeth with closed crown bases, with rather high crown, hypostria of the upper cheek teeth nearly half of the tooth width, and talonids of lower teeth being much narrower trigonids (Erbajeva 1994, figure 2).
At the beginning of the Miocene ochotonids were widespread. A number of new genera of Sinolagomyinae appeared in Europe (Heterolagus), Africa (Kenyalagomys and Austrolagomys), North America (Oreolagus), and Asia (Bellatona and Bellatonoides). Sinolagomys, represented by new Miocene species, persisted in Asia. Heterolagus and Austrolagomys appeared at the end of early Miocene. All Miocene taxa differ from the Oligocene in their lack of cheek tooth roots, higher crowns, nearly equal widths of trigonids and talonids on P4-M2,, and deeper hypostria in the upper teeth.
Sinolagomys, Kenyalagomys, and Oreolagus became extinct the end of the Middle Miocene (Dawson 1965;
Janvier and Muizon 1975;
Li and Qiu 1980). Bellatona and Bellatonoides survived until the Late Miocene. By the end of the Late Miocene all genera in Sinolagomyinae became extinct.
The earliest record of subfamily Ochotoninae is from the early Miocene, after which members of both ochotonid subfamilies evolved in parallel throughout the Miocene (Erbajeva 1994).
It seems likely that Ochotoninae derived from primitive Bellatona-like ancestors described by
Qiu (1996). All Ochotoninae are characterized by rootless cheek teeth except Marcuinomys, which has teeth with closed crown bases similar to Sinolagomys kansuensis. However, Marcuinomys differs from the latter by having more a complex structure of the most diagnostic teeth (P3 and P3). By the end of Early Miocene all ochotonid cheek teeth are rootless and hypsodont.
Alloptox, of the early Miocene of Asia (Li 1978), has a tooth structure similar to European genera (Marcuinomys, Albertona, and Lagopsis;
Lopez Martinez 1986) suggesting that arid, continental climate and paleoenvironmental conditions of the Middle Miocene was favorable for pikas of the genera Alloptox and Lagopsis the diverse species of which is evident. They scattered in Asia and Europe, respectively, whereas the primitive specialized taxa Marcuinomys and Albertona disappeared.
In the Late Miocene, ochotonids underwent tremendous changes. All Sinolagomyinae and the Ochotoninae genera Alloptox and Lagopsis became extinct. Some new forms appeared, such as Paludotona in Europe and Proochotona in Eurasia, but they survived only to the end of Late Miocene and beginning of the Early Pliocene, respectively (Dawson 1959;
Chomenko 1914). At the end of Late Miocene three genera, Ochotonoides, Ochotonoma, and Ochotona, appeared in Asia, and Pliolagomys appeared in Eurasia to middle Pliocene (Kretzoi 1959;
Topacewski and Scorik 1977;
Erbajeva 1988). At the beginning of Pliocene the geographic range of ochotonids shrank, with all African forms becoming extinct. Concurrently, Ochotona migrated to North America (Ochotona spanglei
Shotwell 1956) and to Europe (Ochotona sp. from Maritsa) (de Bruijn et al. 1970).
During the Pliocene Ochotonoides (three species), Pliolagomys (three species) and Ochotona (36 species) flourished. They were represented by a number of species having wide distributions in Eurasia, from Hungary, Romania and Moldavia in the west, to North China in the east. They differed much in body size and tooth morphology. Ochotonoides and Pliolagomys are characterized by complicated structures of the P3 (Erbajeva 1988, figures 21-23). These genera became extinct in the Late Pliocene, and only Ochotona survives to the present. The latter had a wide distribution in Eurasia from the Late Miocene through Pleistocene and in North America from the Early Pleistocene through Recent (Erbajeva
1988). This genus with simple structure of P3 was represented by a number of species differing by the variations of teeth size (corresponding to body size), from large to medium and small among ochotonids (Erbajeva 1994, figure 6;
Erbajeva and Zheng 2005).
Ochotonid development and history in Eurasia
The genus Ochotona was represented in Eurasia by several species differing in size: large forms - Ochotona lagreli, O. guizhongensis, O. chowmincheni, O. gudrunae, O. ursui, O. gromovi, O. tologoica, O. transcaucasica, O. magna, O. zasuchini, O. zazhigini, O. zhangi; medium-sized forms - Ochotona antiqua, O. plicodenta, O. lingtaica, O. dodogolica, O. polonica, O. nihewanica, O. agadjianiani, O. tedfordi; and small-sized forms - Ochotona minor, O. sibirica, O. valerotae, O. horaceki, O. dehmi, O. youngi, O. Gracilis, and others.
Ochotona lagreli, O. guizhongensis, O. chowmincheni, and O. minor are Late Miocene forms distributed in China and Mongolia; they never appeared in the Pliocene (Schlosser 1924;
Erbajeva and Daxner-Hoeck 2001;
Erbajeva and Zheng 2005;
Erbajeva et al. 2006). During the Pliocene in Asia Ochotona was distributed from Transcaucasia in the west through Kazakhstan, Transbaikalia, and Mongolia to China in the east. Representative species include Ochotona agadjianiani, O. gromovi, O. intermedia, O. sibirica, O. nihewanica, O. plicodenta, O. lingtaica, O. youngi, O. gracilis, O. gudrunae, and O. zazhigini a.o. (Zheng 1982;
Agadjanian and Erbajeva 1983;
Zheng and Zhang 2000;
Erbajeva and Zheng, 2005;
Erbajeva et al. 2006). In the Europe, mainly in the east, Ochotona ursui (Dacic Basin, Romania), Ochotona sp. (Maritsa, Greece), Ochotona antiqua (Moldavia, Ukraine and the Russian Plain), and O. polonica (Zamkowa Dolna, Poland) are recognized (Simionescu 1930;
de Bruijn et al. 1970;
Erbajeva and Shushpanov 1988).
The Pleistocene is characterized by the development and diversity of small pikas of the "O. pusilla" group, the history of which began at the end of Pliocene-beginning of Pleistocene, approximately 2.00-1.67 Ma. The species attributed to this group are characterized by archaic (plesiomorphic) features of their cheek teeth such as wide confluence between the anteroconid and posteroconid in the P3, and by small size.
The distribution of the Early Pleistocene ochotonids in Eurasia expanded west and northeast relative to the Pliocene distribution. Fossil records of pikas in Europe are known from France (Ochotona valerotae, Valerots site), Germany (Ochotona dehmi, Schernfeld), Slovakia (Ochotona horaceki, Honce), Hungary (Ochotona sp., Ostramos 2,), and the Ukraine and Russian Plain (Ochotona pseudopusilla). Records in Asia are from west and east Siberia (Ochotona sp., Scorodum, Romanovo, Kizikha sites, O. filippovi, Podymakhino) and Yakutia (Ochotona cf. whartoni and Ochotona sp., Krestovka section). Ochotona continues to be rather abundant in Transbaikalia, Mongolia and Northern China (Dehm 1962;
Zheng and Zhang 2000;
Erbajeva et al. 2001;
Erbajeva and Zheng 2005;
Erbajeva 2005). During Middle and Late Pleistocene, successive glacial and interglacial phases resulted in episodic paleoenvironmental changes. With changes of climate towards cool and dry conditions, open landscapes became widespread. At that time vast territories in Eurasia were occupied by steppe-tundra (also called the Mammoth-Steppe). Throughout the Pleistocene, Asia was broadly connected with Europe, with no barriers to faunal interchange (e.g., ice sheets). This connection is illustrated by broad migrations of Asian-steppe faunal elements, such as ochotonids, jerboas, lagurids, and others to Europe. In Transcaucasia a geographically restricted population of large-sized pika, Ochotona transcaucasica, existed.
A wide geographic radiation of Ochotona pusilla occurred. A broad area of Eurasia was occupied by a number of taxa of the subspecies level distributed from southern England through the Netherlands, France, northern Italy in the west, Transcaucasia in the south, and to the Prebaikal region of Siberia in the east. From postglacial times through the Holocene, climate change led to moderately warm conditions and the reestablishment of forest and grasslands. Likely, as a result of these changing climate conditions, Ochotona pusilla became more restricted in its distribution. Holocene fossil remains are not known in Europe except in Crimea (Gromov 1961), the southern Urals (Smirnov 1993), and Hungary (Kordos 1978). The modern species (Fig. 1) is mainly steppe dweller, range of which includes a small part of Eastern Europe in the right side of Middle Volga river valley (approximately 54º N), through Northern Kazakhstan to the border of China. The modern range includes a small part of Eastern Europe, mainly steppe habitats of middle Volga river valley (approximately 54o N), through Northern Kazakhstan to the border of China.