The cart before the horse?

Recent arguments for the origins of horse domestication have centred on Botai sites in northern Kazakhstan dated to c. 3500 BC. These sites have yielded overwhelming quantities of equid remains, with some Equus przewalskii specimens exhibiting apparent damage to the dentition linked to the use of a bridle mouthpiece, or ‘bit’ (Outram et al. Reference Outram2009; Gaunitz et al. Reference Gaunitz2018). The earliest definitively domestic horses, however, are associated with the southern Trans-Uralian Sintashta Culture, dated to c. 2000 BC (Fages et al. Reference Fages2019). Sintashta horses are found in paired burials and are sometimes accompanied by chariot wheel remains and bridle components (Chechushkov & Epimakhov Reference Chechushkov and Epimakhov2018). Historical records and archaeological finds from the late second millennium BC reveal the proliferation of chariots across much of western Eurasia and North Africa, with only sparse reference to horse riding (Littauer & Crouwel Reference Littauer and Crouwel1979). Ridden horses only appear in western Eurasian historical records in the early first millennium BC (Littauer & Crouwel Reference Littauer and Crouwel1979). Nonetheless, many scholars (e.g. Littauer et al. Reference Littauer, Crouwel and Raulwing2002; Olsen Reference Olsen, Olsen, Grant, Choyke and Bartosiewicz2006a; Anthony Reference Anthony2007) believe that mounted horseback riding must have occurred long before this time in the steppes, where historical records are rare or absent and horse equipment is scarce.

Understanding early horse transport in East Asia

The chronology of chariotry and mounted riding is crucial to understanding pastoral prehistory in Mongolia, where horses historically underpinned highly mobile herding economies and one of the world's oldest ‘horse cultures’. Recent research suggests that the tending of pastoral livestock and dairy production using sheep, cattle and goats dates as far back as c. 3000 BC in the Eastern Steppes (Kovalev & Erdenebaatar Reference Kovalev, Erdenebaatar and Bemmann2009; Wilkin et al. Reference Wilkin2020). The earliest domestic horses so far identified in Mongolia, however, belong to the Bronze Age Deer Stone-Khirigsuur Complex (DSK), which emerged towards the end of the second millennium BC. This culture is named for its ritual standing stones (deer stones) and burial mounds (khirigsuurs) that are often surrounded by multiple horse burial features (Fitzhugh Reference Fitzhugh and Bemmann2009). DSK sites have yielded evidence for the ritual sacrifice and dietary exploitation of horses (Fitzhugh Reference Fitzhugh and Bemmann2009), while skeletal changes indicate that DSK horses were bridled and heavily exerted, and received sophisticated veterinary care (Taylor et al. Reference Taylor, Bayarsaikhan and Tuvshinjargal2015, Reference Taylor2018). The co-occurrence of the first DSK horses with the appearance of domestic horses at late Shang Dynasty sites in China suggests that these two processes were linked (Honeychurch Reference Honeychurch2015).

The role of horseback riding in DSK culture is less clear. Although we have previously hypothesised that mounted horseback riding may have stimulated the spread of DSK culture (Taylor & Tuvshinjargal Reference Taylor, Tuvshinjargal, Bartosiewicz and Gál2018), only chariots, not riders, are depicted on Mongolian deer stone carvings (Figure 1). Images of DSK horse-drawn vehicles depict a light, two-horse chariot with a platform situated over a central axle, with two animals under yoke (Figure 1: right) and, in one case, additional animals along the outside (Figure 1: middle). Archaeological discoveries from adjoining regions, such as trousers and horse burials from Xinjiang and horse skeletons exhibiting transport-related pathologies from Arzhan (to the north, in Tuva) (Benecke Reference Benecke2007; Wagner et al. Reference Wagner2011), suggest the emergence of horseback riding in the Altai region by the early first millennium BC. To date, however, the earliest unequivocal evidence for horseback riding in Mongolia comes from Pazyryk Culture burials in the western Altai Mountains (c. 400 BC), which have yielded horses equipped with pad saddles and elaborate regalia (Rudenko Reference Rudenko1970). Based on the current evidence, it is unclear how DSK horses were used for transport. Beyond the head, neck and hoof bones of sacrificed horses found at DSK sites, no other datasets are available to assess the use of horses in early East Asia.

Figure 1. Chariot images on central Mongolian Deer Stone-Khirigsuur Complex (DSK) monuments, including a two-horse (right) and a four-horse vehicle (middle); left) a typical DSK ritual head, neck and hoof burial (chariot drawings by W.T.T. Taylor modified after Volkov (Reference Volkov2002 [1981]: 93 & 218) and Nyambat and Odbaatar (Reference Nyambat and Odbaatar2010: 64)).

Horse transport and the equine skeleton

As our knowledge of the equine skeleton and its responses to human activity has expanded, new tools and approaches have emerged to reconstruct the use of horses in the past. Pioneering work by Clutton-Brock (Reference Clutton-Brock1974) and Anthony et al. (Reference Anthony, Telegin and Brown1991) revealed that bridle mouthpieces can cause recognisable damage to horse dentition in the form of uneven wear to the occlusal surface of the mandibular second premolar—otherwise known as ‘bit-wear’. Although bit-wear became a favoured tool to argue for early horse domestication, Olsen (Reference Olsen, Levine, Renfrew and Boyle2003) demonstrated that natural occlusion problems can also produce bevelling of the tooth surface in wild equids. Intentional modification of horse teeth through dentistry can also induce premolar bevelling (Cross Reference Cross, Bartosiewicz and Gál2018). Thus, individual teeth must be considered alongside opposing dentition when seeking to identify anthropogenic tooth modifications.

Several scholars (e.g. Bendrey Reference Bendrey2007; Cook Reference Cook2011; Taylor & Tuvshinjargal Reference Taylor, Tuvshinjargal, Bartosiewicz and Gál2018) have identified a suite of other changes to the dentition caused by bridle mouthpieces, including erosion of the anterior surface of the lower second premolar, bone spur development or bone loss along the diastema, chipping of the enamel and remodelling of the bone at the alveolar margin of the premolar, and even damage to the bones of the palate. These forms of damage are influenced by the type of bit and bridle used, the behaviour and technique of the horse and of the rider, the type and duration of activity performed, the shape of the horse's head and mouth and the age and sex of the horse (Bennett Reference Bennett, Baker and Easley2005; Manfredi et al. Reference Manfredi, Clayton and Rosenstein2005; Bendrey Reference Bendrey2007; Cook Reference Cook2011). This variability enables archaeologists to reconstruct important details about how horses were used, even in the absence of tack or other direct evidence.

Both riding and cart traction typically employ a bridle and bit. Hence, the presence of bit-wear alone cannot distinguish ridden horses from those used to pull carts (Dietz Reference Dietz, Renfrew and Levine2003), although other skeletal criteria can help differentiate these activities. Before the invention of the frame saddle in the first millennium BC, mounted horseback riding caused characteristic changes to the horse's lower back, which are typically more extensive/severe on the animal's left side (Levine et al. Reference Levine, Whitwell and Jeffcott2005; Li et al. Reference Li2020). In contrast, a sample of chariot horses from ancient China (Levine Reference Levine2005), along with early donkey/onager hybrids used in cart traction in the ancient Near East (Weber Reference Weber2008), shows higher pathological frequencies affecting the front shoulders and lower limbs, and less damage to the vertebrae. Schrader et al. (Reference Schrader, Smith, Olsen and Buzon2018) also report osseous changes, linked to chariotry, in the final cervical and first thoracic vertebrae, situated where the strain of a yoke is borne. Unfortunately, horse remains from DSK sites and many others from across the Eurasian Steppes comprise so-called ‘head and hoof’ features, either lacking neck vertebrae entirely or containing only the cervical vertebrae (Allard et al. Reference Allard, Popova, Hartley and Smith2007; Outram Reference Outram2011).

More commonly recovered from steppe archaeological contexts, horse crania may also yield useful osteological differences between chariotry and riding. The use of a bridle noseband often causes deformation of the nasal bones, which, in contemporary Mongolian horses, is biased towards the left side due to the predominance of left-handed reining (Taylor & Tuvshinjargal Reference Taylor, Tuvshinjargal, Bartosiewicz and Gál2018). Ossification of the nuchal ligament attachment site at the rear of the skull occurs in both ridden and driven animals—a feature that apparently reaches greater levels of ossification in ridden than in draught animals (Bendrey Reference Bendrey2008), and is generally absent in wild animals (Taylor et al. Reference Taylor, Bayarsaikhan and Tuvshinjargal2015). The degree of discrimination provided by these features in distinguishing transport types, however, remains unclear.

In summary, while chariotry and riding produce differential impacts on the equine skeleton—particularly the vertebral column—the most useful diagnostic differences often occur on skeletal elements not typically recovered from early domestic horse burials in Mongolia or elsewhere. To date, these approaches have therefore made little headway in clarifying the early trajectory of horse transport in prehistoric Central Asia. Here, we assess the possibility that damage patterns to the dentition, which frequently survive in the archaeological record, may help distinguish between prehistoric equine transport strategies.

Bit-wear, riding and chariotry

To assess whether meaningful osteological differences in dental wear may be found between chariot and riding horses, we analysed a sample of modern and ancient Chinese and Mongolian horses used for both types of transport. For each horse, we assigned a categorical score of 0–4 based on observed wear to the anterior and occlusal portions of both the upper and lower second premolars. A score of 0 represents no visible wear; 1 represents cementum abrasion but no enamel exposure; 2 corresponds to enamel exposure; 3 indicates visible erosion of exposed enamel; and 4 represents visible erosion of anterior enamel, paired with occlusal bevelling (see Anthony & Brown Reference Anthony and Brown1998). As we know that Bronze and Early Iron Age Mongolian horses were controlled using an organic (e.g. wood, leather, fibre, or bone) bit (Taylor et al. Reference Taylor, Tuvshinjargal and Bayarsaikhan2016), we did not attempt to distinguish ‘natural’ wear from ‘bit’ wear a priori based on the morphology of anterior enamel exposure (c.f. Bendrey Reference Bendrey2007). Examples of teeth in each of the four categories are provided in online supplementary material (OSM) 1.

Our results show that in East Asia, mounted riding differentially impacts the lower dentition compared with the upper. In a sample of 23 analysed riding horse specimens, more than half exhibit exposed enamel on the lower premolars, with 4 being the most commonly assigned score for lower dentition. In contrast, more than half of the same analysed group showed limited or no damage to the upper dentition, with the most common score in this group being 0. Among those riding specimens that display damage to the upper dentition, this is always paired with more severe damage or erosion to the lower premolars (Figure 2: left).

Figure 2. Bit-wear to the upper and lower premolars in two riding horses from Turkic (fifth to eighth centuries AD; NMM 011) (left) and Pazyryk (fourth century BC; NMM 013) (right). Images show the pairing of upper premolar wear with severe lower premolar wear (figure by W.T.T. Taylor).

In contrast, our sample of chariot horses from first-millennium BC Chinese archaeological contexts lacks the clear patterning of damage seen in the modern and archaeological riding assemblage, suggesting that chariotry could produce more evenly-spread bit damage across both the upper and lower dentition. A large number of chariot horses have been recovered from Eastern Zhou Dynasty (770–256 BC) sites in Xinzheng City, near modern Zhengzhou (see OSM1). All of these horses were adult males, and archaeological research indicates that, as with our modern/archaeological riding assemblage, horses of this period were typically controlled using a jointed bronze/iron snaffle bit (Figure 3) in tandem with rein-rings (Wu Reference Wu2013: 81). Most of the 42 Xinzheng horses, however, exhibit scores of 2 or less, and few show any signs of occlusal bevelling. The Xinzheng chariot horses, however, exhibit meaningful levels of enamel erosion in neither their upper nor lower dentition. This could imply that the horses’ role as property of the social elite did not entail intense activity. Age may also have had a minor effect on these patterns; in both samples, horses over 10 years of age have a higher average wear score (~3.2 and 2.2 in the riding and chariot groups, respectively) than those younger than 10 years (~2.3 and 1.5, respectively). Our sample of riding horses exhibits a slightly higher average estimated age (~10.4 years) than that of the Chinese chariot sample (~8.1 years). Together, these osteological comparisons of East Asian riding and chariot horses suggest that mounted riding may produce more damage to the lower jaw, although limited pathology in the Chinese sample and age differences between sample groups limit our confidence in this assertion.

Figure 3. Left) bit damage to both lower and upper premolars caused by the use of a metal bit while pulling a chariot, from the Eastern Zhou Dynasty site of Cuiyuanxiaoqu (photograph by J. Cao); right) bronze bit from tomb 62, Xincun site (modified by W.T.T. Taylor after Guo (Reference Guo1998: 61)).

Comparative data in hand, we carefully reanalysed all adult DSK horses at the National Museum of Mongolia that retain complete (upper and lower) dentition (n = 19) for evidence of incongruous wear to the upper premolars that, upon refitting with the opposing jaw, could not be explained by natural malocclusion. Although a previous study (Taylor et al. Reference Taylor, Tuvshinjargal and Bayarsaikhan2016) found no evidence of bit-wear on the lower premolars, our reanalysis shows that eight of 19 specimens display bevelling of the upper second premolars (Table S1), most of which exhibit uneven wearing of the enamel and cementum—a feature sometimes referred to as the “Greaves effect” and taken as a sign of natural wear (Olsen Reference Olsen, Zeder and Smith2006b). Two specimens in the sample, however, have worn upper second premolars with even wear of the enamel and cementum, which contrasts with the wear seen elsewhere in the dentition (Figure 4). This implies that a material of similar hardness to tooth enamel, such as a bone bit, was responsible for the wear. In one case, wear on the upper second premolar is so severe that it also affects the upper third premolar (Figure 4: lower left).

Figure 4. Abnormal wear to the upper premolars in Deer Stone-Khirigsuur horses: top) 14–15-year-old female (NMM 002); lower left) >16 year-old female (NMM 095); lower right) >16-year-old of indeterminate sex (NMM 001) (figure by W.T.T. Taylor).

Five of the 19 DSK specimens also display an unusual type of wear, first noticed by Bartosiewicz and Gal (Reference Bartosiewicz and Gal2013: 134) in early medieval horses from Hungary, wherein the intercuspid space between the first and second cusp of the second or third lower premolar develops a rounded, scooped shape that is attributed to firm grasping of the bit between the teeth (Figure 5). On these five specimens, the location of this intercuspid, lower premolar wear corresponds with wear observed on the upper premolars. In the horse from the site of Morin Mort that exhibits wear extending deep into the third premolar, intercuspid wear is also found on the lower second premolar (Figure 5). These patterns suggest a bit being lifted over the lower teeth and ground upwards into the upper premolars. One horse even exhibits polishing around the intercuspid space, and nowhere else, which appears to have been caused by the bit (Figure 5: top left). Three of the five DSK horse specimens with mandibular intercuspid wear also exhibit deformations of the alveolar margin, where it appears that downwards pressure in gripping the bit has caused the tooth to bow laterally outwards (Figure 5: lower right; see OSM2).

Figure 5. Intercuspid wear of the lower jaw in Deer Stone-Khirigsuur horses: top left) polishing of the occlusal surface of a lower right second premolar (~6–7-year-old male; NMM 008); top right) wear to the lower third premolar (>16-year-old female; NMM 095). Several instances of bulging deformation of the mandibular margin of the lateral alveolar exterior surface near the affected teeth (>20-year-old of indeterminate sex (NMM 051, lower left) and male, 6–7 years (NMM 008, lower right)) (figure by W.T.T. Taylor).

Our analysis of DSK horses reveals a high frequency of damage to the upper premolars, paired with an absence of classic forms of lower premolar damage, along with an unusual form of intercuspid wear of the second premolar and deformation of the alveolar margins of the lower premolar row. These patterns appear to have resulted from chronic gripping of a bridle mouthpiece or bit, which was ground against the upper premolars by the lower jaw as the horse took the ‘bit between the teeth’. The uneven nature of wear caused by DSK bits across enamel and cementum (e.g. Figure 4: lower left) suggests that soft organic materials—perhaps fibre or leather—were often used for mouthpiece construction. Other instances of upper premolar wear to both enamel and cementum in our analysed sample imply the use of a hard bit (perhaps bone, antler or even metal), similar to equipment recovered from later first-millennium BC contexts in Mongolia (Chan Reference Chan2018: 87).