The first animal that possessed the typical features of vertebrates, the "Arandaspis", was dated to have existed in the later Ordovician period. Thus few, if any, fossils of an intermediate type between invertebrates and vertebrates have been found, although likely candidates include the Burgess Shale animal, "Pikaia gracilens", and its Maotianshan shales relatives, "Myllokunmingia", "Yunnanozoon", "Haikouella lanceolata", and "Haikouichthys". Some of the reasons for the incompleteness of fossil records are:  Due to an almost-complete fossil record found in North American sedimentary deposits from the early Eocene to the present, the horse provides one of the best examples of evolutionary history (phylogeny). This evolutionary sequence starts with a small animal called "Hyracotherium" (commonly referred to as "Eohippus"), which lived in North America about 54 million years ago then spread across to Europe and Asia. Fossil remains of "Hyracotherium" show it to have differed from the modern horse in three important respects: it was a small animal (the size of a fox), lightly built and adapted for running; the limbs were short and slender, and the feet elongated so that the digits were almost vertical, with four digits in the forelimbs and three digits in the hindlimbs; and the incisors were small, the molars having low crowns with rounded cusps covered in enamel. The probable course of development of horses from "Hyracotherium" to "Equus" (the modern horse) involved at least 12 genera and several hundred species. The major trends seen in the development of the horse to changing environmental conditions may be summarized as follows:  Fossilized plants found in different strata show that the marshy, wooded country in which "Hyracotherium" lived became gradually drier. Survival now depended on the head being in an elevated position for gaining a good view of the surrounding countryside, and on a high turn of speed for escape from predators, hence the increase in size and the replacement of the splayed-out foot by the hoofed foot. The drier, harder ground would make the original splayed-out foot unnecessary for support. The changes in the teeth can be explained by assuming that the diet changed from soft vegetation to grass. A dominant genus from each geological period has been selected (see figure 3e) to show the slow alteration of the horse lineage from its ancestral to its modern form.