20世纪80年代在桑干河盆地一带考察泥河湾层时，在山西省天镇县南高崖乡的辛窑子沟一带发现了很多哺乳动物化石地点并出土了大量哺乳动物化石。最近的研究系统记述了马科化石的5个种，加上此前曾记述过的1个种，在辛窑子沟一带的早更新世地层中产出的马科化石至少有6个种。其中古马型马有4种：三门马(Equus sanmeniensis)、德氏马(E. teilhardi)、黄河马(E. huanghoensis)和古马(E. stenonis); 两种三趾马：中国长鼻三趾马(Hipparion (Proboscidipparion) sinense)和山西近三趾马(H. (Plesiohiparrion) shanxiense)。研究表明在早更新世期间新出现的古马型马的多样性是显著的，仅在辛窑子动物群中就有4种。而延续到早更新世的三趾马这些新近纪残留类群在辛窑子动物群中仍然有两个种。三门马和中国长鼻三趾马是早更新世期间真马与三趾马共存的最典型代表。它们在中国的产出地点很多，分布也较广，并有较大幅度的重叠。辛窑子动物群中马科化石种类的多样性表明它们所依赖摄食的植被也应存在较大的多样性。高冠的颊齿和发育的白垩质及完全臼齿化的前臼齿指示这些马科种类食物粗糙，很可能是单子叶草类。它们的栖息地应为草原草甸环境，且其范围较大，以便满足6个马科种类在那里摄食与活动。

Abundant mammalian fossils were uncovered during the field exploration for Nihewan beds at the beginning of the 1980s along Xinyaozi Ravine at Nangaoya Township of Tianzhen County, Shanxi Province in North China. But most equid material was not yet described except that of Equus stenonis. Six forms of Nihewanian equids were confirmed from the Xinyaozi specimens in the present study, five of which were described for the first time. They include four stenonids such as Equus sanmeniensis, E. teilhardi, E. huanghoensis and E. stenonis, and two hipparionines such as Hipparion (Proboscidipparion) sinense and H. (Plesiohiparrion) shanxiense. The diversification of stenonids in the Early Pleistocene was significant in North China with four taxa in Xinyaozi alone. The persistence of Neogene relics such as hipparionines was still present in the Early Pleistocene with two hipparionine taxa in Xinyaozi. Equus sanmeniensis and H. (Proboscidipparion) sinense were two representative equids not only coexisted in the Early Pleistocene but also widely distributed in China. The diversity of equids also implies the diversified vegetation on which they depended. The hypsodont dentitions and well developed cement, as well as completely molarized premolars of Xinyaozi equids indicate their abrasive diet mostly on monocotyledonous and grassland habitats with considerable scales enough to nourish six taxa of equids.