Wild dogs are medium- to large-sized canids that possess several hypercarnivorous craniodental features and complex social and predatory behaviours (i.e., social hierarchic groups and pack-hunting of large vertebrate prey typically as large as or larger than themselves). Two extant species of wild dogs survive in the Old World, the Indian dhole, Cuon alpinus (Pallas, 1811), and the African hunting dog, Lycaon pictus (Temminck, 1820). Both species are nowadays endangered or critically endangered according to the IUCN red list of threatened species1,2. The African hunting dog, known also as painted dog, and the dhole are among the top predators in their respective habitats3,4 thanks to the combination of a number of dental hypercarnivorous traits, skeletal adaptations to cursorial pack hunting and their social behaviour.
The evolution of these hypercarnivorous canids is still unknown and open to debate5,6.
Furthermore, there is also a great deal of confusion in the taxonomy of the extinct large-sized and hypercarnivorous canids, which use to be referred to different systematic denominations (see Supp. Inform.). Such names often hint implied or proposed affinities to extant taxa, yet seldomly based on phylogenetic analyses. Considering the results of molecular phylogenies7,8, from which it is evident that Lycaon and Cuon are sister taxa of the crown group of Canis, and that the large-sized members of the genus Xenocyon might be related to both Lycaon and Cuon, here we preferred to avoid names suggestive of a closer relationship to any of both genera, privileging the more parsimonious denomination Canis (Xenocyon) (see the Supp. Inform. for an in-depth discussion of the taxonomical issues).
The earliest record of a species of this group of hypercarnivorous canids corresponds to Canis (Xenocyon) cf. dubius Teilhard de Chardin (1940), which is represented by a single hemimandible6 from the Zanda Basin (3.81–3.42 Ma; Fig. 1). The species C. (Xenocyon) dubius is generally related to the lineage of Cuon6,9. A younger but more complete specimen from Fan Tsun (Taigu10) was ascribed to Canis (Xenocyon) antonii (ca. 2.5 Ma)11. The latter canid is large-sized and displays evident dental features hinting to an incipient adaptation to a hypercarnivorous diet. Other records of large-sized canids with hypercarnivorous features are rather scanty across Eurasia and are of difficult attribution, considering the presence of hypercarnivorous Canis s.s. in Asia during the Early Pleistocene, e.g., Canis chihliensis Zdansky, 1924; Canis teilhardi Qiu et al., 2004; or Canis yuanmoensis You & Qi, 1973 (like in North American Canis dirus Leidy, 1858, see10).
Around 2.0-1.8 Ma, different forms appeared in several parts of the Old World. These forms showed distinctive dental features (i.e., broad and stoutly-built carnassials with enlarged buccal cuspids), coupled with craniomandibular ones (robust mandibles and developed frontal sinuses). Their large size combined to these dental adaptations could have determined an advantage over the contemporaneous, medium-sized mesocarnivorous canids, as testified by the westward dispersion and radiation of Canis (Xenocyon) falconeri (Forsyth Major, 1877) in Western Europe and of Canis (Xenocyon) africanus (Pohle, 1928) from Olduvai Bed I in Africa. A close relationship between both taxa was suggested by5,11 who regarded them as the ancestor of modern L. pictus. However, such interpretation has not been shared by other researchers12. Recently, a new large-sized taxon was described as Lycaon sekowei Hartstone-Rose et al. (2010), with fragmented cranial material from Cooper’s Cave in South Africa (ca. 1.9 Ma) and an almost complete skeleton from Gladysvale (ca. 1.0 Ma)13. Some of the morphologies of the holotype from Cooper’s Cave (i.e., the high-crowned upper premolars, their mesial occlusal morphology, the lingual projection of P4 protocone, and the relative buccolingual length of the M1) cast doubts on its taxonomical attribution and its actual relation with Canis (Xenocyon)’s group. Moreover, the upper teeth resemble those of the Asian C. chihliensis, a large-sized canid possibly belonging to a hypercarnivorous lineage of Canis10.
During the late Early Pleistocene (i.e., Calabrian; 1.8–0.8 Ma), while other more primitive species remained in Africa [e.g., Canis (Xenocyon) atrox Broom14 in Kroomdrai A; Fig. 1] a more derived form of Canis (Xenocyon) appeared and became widespread across the whole Old World (Fig. 1). Canis (Xenocyon) lycaonoides (Kretzoi, 1938) was a large-sized canid that resembled C. (Xenocyon) gr. falconeri but with more derived craniodental features (e.g., the P4 protocone tends to attach to the tooth; the M1 metaconule is crest-like; the M1 talon is reduced; the m1 hypoconid is enlarged and tends to be centred in the talonid; the entoconid is reduced, being represented by a small crest-like cuspulid; and the m3 is single cusped). Its earliest record appears to be that of Tamagawa River near Tokyo (Japan, 15; Fig. 1). This locality is loosely dated to 2.1–1.6 Ma, based on the associated fauna and ichnofauna16. In spite of its uncertain chronology, this early occurrence suggests an eastern Asian origin for this hypercarnivorous species. Subsequently, during the late Early Pleistocene and the base of the Middle Pleistocene, from 1.8 to 0.7 Ma, C. (Xenocyon) lycaonoides became one of the most common and important members of the carnivoran palaeoguild of Eurasia (Fig. 1). Moreover, C. (Xenocyon) lycaonoides dispersed also in Africa, where it is documented in the northern and eastern part of the continent (e.g., Olduvai Bed II; Fig. 1). Considering the overall cranial morphology and its dental features, which confirm the original interpretation by Kretzoi17, Martínez-Navarro & Rook5 deemed C. (Xenocyon) lycaonoides as strictly related to extant L. pictus. Similar conclusions were shared by several other scholars10,18, who supported also a Eurasian origin for the living African hunting dog.
Among extant Carnivora, Lycaon pictus has one of the most complex, structured and unique social behaviours3. As one of the closest relatives to L. pictus, C. (Xenocyon) lycaonoides, the Eurasian hunting dog, might have had comparable complex sociality. Carbone and co-authors19 showed that the metabolic energy requirements for large-sized species (< 21.5 kg) force them to hunt prey larger than themselves and thus, in hypercarnivorous Canidae, cooperatively. As such, this element allows us to figure the social behaviour of extinct hypercarnivorous canids, even with limited direct evidence. Nevertheless, apart from indirect and inferred evidence, direct proof of social behaviour in the Eurasian hunting dog have been proposed20,21.
Here we report the first occurrence of wild dogs from the Georgian site of Dmanisi (Fig. 1; 1.78–1.76 Ma)22. This locality preserves an outstanding fossil record, both in terms of abundance, completeness of skeletal remains and preservation status, as testified by the recently described molecular phylogeny based on a fossil rhino tooth23. In this paper, we describe the newly discovered remains, identifying them taxonomically and interpreting in the frame of Early Pleistocene diversity of Canis (Xenocyon). Moreover, the site of Dmanisi yielded the earliest direct evidence of hominin presence out of Africa in their dispersal throughout Eurasia24,25 with also indication of complex sociality among individuals of this population26,27. The co-occurrence of two highly social species in the same site at 2.0-1.8 Ma, a time of extreme diversification and expansion of the two clades5,6 from their centre of origin, raises interest in the role played by social behaviour and by mutually-beneficial cooperation and reciprocity in the geographic expansion of these species. Questions to be explored in this paper.