Information: - The transition between the end of the Eocene and the beginning of the Oligocene is marked by large-scale extinction and floral and faunal turnover (although minor in comparison to the largest mass extinctions). Most of the affected organisms were marine or aquatic in nature. They included the last of the ancient cetaceans, the Archaeoceti. - The Eocene Epoch, lasting from , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Paleocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene is marked by a brief period in which the concentration of the carbon isotope C in the atmosphere was exceptionally low in comparison with the more common isotope C. The end is set at a major extinction event called the "Grande Coupure" (the "Great Break" in continuity) or the EoceneOligocene extinction event, which may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay. As with other geologic periods, the strata that define the start and end of the epoch are well identified, though their exact dates are slightly uncertain. - Ungulates (pronounced ) are any members of a diverse group of primarily large mammals that includes odd-toed ungulates such as horses and rhinoceroses, and even-toed ungulates such as cattle, pigs, giraffes, camels, deer, and hippopotami. Most terrestrial ungulates use the tips of their toes, usually hoofed, to sustain their whole body weight while moving. The term means, roughly, "being hoofed" or "hoofed animal". As a descriptive term, "ungulate" normally excludes cetaceans (whales, dolphins, porpoises), as they do not possess most of the typical morphological characteristics of ungulates, but recent discoveries indicate that they are descended from early artiodactyls. Ungulates are typically herbivorous (though some species are omnivorous, such as pigs), and many employ specialized gut bacteria to allow them to digest cellulose, as in the case of ruminants. They inhabit a wide range of habitats, from jungles, plains and rivers. - The Paleogene (or ; also spelled Palaeogene or Palogene; informally Lower Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago (Mya) to the beginning of the Neogene Period Mya. It is the beginning of the Cenozoic Era of the present Phanerozoic Eon. The Paleogene is most notable for being the time during which mammals diversified from relatively small, simple forms into a large group of diverse animals in the wake of the CretaceousPaleogene extinction event that ended the preceding Cretaceous Period. - Hyracotherium (; "hyrax-like beast") is an extinct genus of very small (about 60cm in length) perissodactyl ungulates which was found in the London Clay formation. This small, dog-sized animal was once considered to be the earliest known member of Equidae before the type species, "H. leporinum", was reclassified as a palaeothere, a perissodactyl family basal to both horses and brontotheres. The remaining species are now thought to belong to different genera, such as "Eohippus", which had previously been synonymised with "Hyracotherium". - Orohippus ( Gr. , '' mountain horse '' ) is an extinct equid that lived in the Eocene ( about 50 million years ago ) . It is believed to have evolved from equids such as Eohippus , as the earliest evidence for Orohippus appears about 2 million years after the first appearance of Eohippus . The anatomical differences between the two are slight : they were the same size , but Orohippus had a slimmer body , a more elongated head , slimmer forelimbs and longer hind legs , all of which are characteristics of a good jumper . The upper premolars of Orohippus are more molariform ( flat - surfaced ) than in Eohippus , giving Orohippus more teeth for grinding , and the crests on the teeth are more pronounced , indicating that Orohippus probably fed on tougher plants . The outer toes of Eohippus are no longer present in Orohippus , hence on each forelimb there were four fingers ( toes ) and on each hind leg three toes . Species of Orohippus has also been referred to Protorohippus . - Eohippus is an extinct genus of small equid ungulates. The only species is "E. angustidens", which was long considered a species of "Hyracotherium". Its remains have been identified in North America and date to the Early Eocene (Ypresian) stage. What is the relationship between 'orohippus' and 'equidae'?
A:
parent taxon