taxon_rank orohippus
Eohippus is an extinct ___MASK72___ 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 . Hyracotherium (; " hyrax - like beast ") is an extinct ___MASK72___ of very small ( about 60 cm 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 ". A ___MASK72___ ( genera ) is a taxonomic rank used in the biological classification of living and fossil organisms in biology . In the hierarchy of biological classification , ___MASK72___ comes above species and below family . In binomial nomenclature , the ___MASK72___ name forms the first part of the binomial species name for each species within the ___MASK72___ . The Eocene Epoch , lasting from , is a major ___MASK77___ 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 Eocene  Oligocene 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 . The Paleogene ( or ; also spelled Palaeogene or Palæogene ; 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 ___MASK84___ of diverse animals in the wake of the Cretaceous  Paleogene extinction event that ended the preceding Cretaceous Period . Carbon ( from " coal ") is a chemical element with symbol C and atomic number 6 . It is nonmetallic and tetravalent  making four electrons available to ___MASK17___ covalent chemical bonds . Three isotopes occur naturally , C and C being stable , while C is a radioactive isotope , decaying with a half - life of about 5 , 730 years . Carbon is one of the few elements known since antiquity . Ungulates ( pronounced ) are any members of a diverse ___MASK84___ 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 ___MASK94___ 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 . 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 . ___MASK94___ of Orohippus has also been referred to Protorohippus .