''Homogalax'' (from the Greek "ομογάιαξ") is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

tapir Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a pig, with a short, prehensile nose trunk. Tapirs inhabit jungle and forest regions of South and Central America, with one species inh ...

-like

odd-toed ungulate Odd-toed ungulates, mammals which constitute the taxonomic order Perissodactyla (, ), are animals—ungulates—who have reduced the weight-bearing toes to three (rhinoceroses and tapirs, with tapirs still using four toes on the front legs) o ...

. It was described on the basis of several

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

finds from the northwest of the

USA The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

, whereby the majority of the remains come from the state of

Wyoming Wyoming () is a state in the Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the southwest, and Colorado to the s ...

. The finds date to the Lower

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

between 56 and 48 million years ago. In general, ''Homogalax'' was very small, only reaching the weight of today's

peccaries A peccary (also javelina or skunk pig) is a medium-sized, pig-like hoofed mammal of the family Tayassuidae (New World pigs). They are found throughout Central and South America, Trinidad in the Caribbean, and in the southwestern area of North ...

, with a maximum of 15 kg.

Phylogenetic analysis In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

suggests the genus to be a basal member of the clade that includes today's

rhinoceros A rhinoceros (; ; ), commonly abbreviated to rhino, is a member of any of the five extant species (or numerous extinct species) of odd-toed ungulates in the family Rhinocerotidae. (It can also refer to a member of any of the extinct species ...

and

tapirs Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a pig, with a short, prehensile nose trunk. Tapirs inhabit jungle and forest regions of South and Central America, with one species inhabit ...

(collectively called

Ceratomorpha Odd-toed ungulates, mammals which constitute the taxonomic order Perissodactyla (, ), are animals—ungulates—who have reduced the weight-bearing toes to three (rhinoceroses and tapirs, with tapirs still using four toes on the front legs) ...

). In contrast to these, ''Homogalax'' was adapted to fast locomotion.Holbrook, LT; Lucas, SG; and Emry, RJ; "Skulls of the Eocene Perissodactyls (Mammalia) ''Homogalax'' and ''Isectolophus''." ''Journal of Vertebrate Paleontology 24''. (4) 951-956.

Description

''Homogalax'' was a small representative of the basal

odd-toed ungulates Odd-toed ungulates, mammals which constitute the taxonomic order Perissodactyla (, ), are animals—ungulates—who have reduced the weight-bearing toes to three (rhinoceroses and tapirs, with tapirs still using four toes on the front legs) ...

. In general, it resembled other, basal forms of the order Perissodactyla, such as ''

Hyracotherium ''Hyracotherium'' ( ; "hyrax-like beast") is an extinct genus of very small (about 60 cm in length) perissodactyl ungulates that was found in the London Clay formation. This small, fox-sized animal was once considered to be the earliest know ...

'' or ''

Sifrhippus ''Sifrhippus'' is an extinct genus of equid containing the species ''S. sandrae''. ''Sifrhippus'' is the oldest known equid, living during the early Eocene. Its fossils were discovered in the Bighorn Basin of Wyoming. Description ''Sifrhippus' ...

'', some of which were well-studied members of the early

equids Equidae (sometimes known as the horse family) is the taxonomic family of horses and related animals, including the extant horses, asses, and zebras, and many other species known only from fossils. All extant species are in the genus '' Equus'', ...

, what shows the very basal position within the odd ungulate. A body weight of 9.5 to 15.1 kg can be assumed on the basis of various skeletal elements.Rose, KD; 1996 "Skeleton of early Eocene ''Homogalax'' and the origin of Perissodactyla. Palaeovertebrata 25 (2-4).243-260. ''Homogalax '' is known from numerous fossil finds, which however only partially contain complete material. Underneath is a heavily weathered skull, which, however, shows few skeletal features. This is around 15 cm long and relatively flat, and overall it was similar to that of '' Hyracotherium ''. The short

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...

, which rose steeply and came into contact with the nasal bone, was typical, which is not the case with today's Ceratomorpha (tapirids and rhinocerotids). The ascending central jawbone formed the back of the interior of the nose, which is also unknown in today's odd-toed ungulates. As a result, the nasal bone had only a short extension to the front and only slightly exceeded the canine. The

occiput The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the cereb ...

had a distinctly short shape, while the

zygomatic arch In anatomy, the zygomatic arch, or cheek bone, is a part of the skull formed by the zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the opening of the ear) and the temporal process of the zygo ...

hardly protruded outwards, instead running parallel to the skull. Holbrook, Lt; 2001 "Comparative osteology of early Tertiary tapiromorphs (Mammalia, Perissodactyla)." ''Zoological Journal of the Linnean Society 132''. 1-54. The dentition includes the complete set of the early

placental mammals Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...

. The

incisors Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...

were chisel-shaped and small, but varied in size. In general, the third (outermost) incisor became the largest. The canine usually had a long and pointed shape and was somewhat pressed on the side. A short, less than 1 cm long

diastema A diastema (plural diastemata, from Greek διάστημα, space) is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars. More colloquially, the condition ...

occurred to the rear dentition only rarely; others could be observed between the last incisor and the canine as well as between the first two

premolars The premolars, also called premolar teeth, or bicuspids, are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mouth ...

. The premolars themselves were completely unmolarized, meaning they did not resemble the molars, and only had a raised enamel cusp on the chewing surface. The molars were characterized by low (brachyodont) tooth crowns and two tapir-like transverse tooth enamel ridges (bilophodont), which could also be inclined in the lower jaw. The length of the teeth increased towards the back. The first premolar was around 0.6 cm long, the last molar up to 1.7 cm long.Radinsky, LB; 1963 "Origin and Early Evolution of the North American Tapiroidea." ''Peabody Museum of Natural History Yale University Bulletin 17''. 1-106.Schoch, RM; 1989 "A review of the Tapiroids." In: Prothero, DR; Schoch, RM; "''The evolution of Perissodactyls''". ''New York and Oxford''." 298-320. A complete skeleton is not available, but skulls and some postcrania have been preserved. Of particular note among the latter are the

long bones The long bones are those that are longer than they are wide. They are one of five types of bones: long, short, flat, irregular and sesamoid. Long bones, especially the femur and tibia, are subjected to most of the load during daily activities a ...

. The humerus was built long and narrow and measured about 13 cm. The

thigh bone The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...

on the other hand reached a length of 15 cm and typically had a third trochanter, which is typical for odd-toed ungulates, but was not yet so prominent in "Homogalax". The shin became as long as the thigh bone. The forefoot consisted of four digits with a very pronounced central toe ( metacarpus III). The laterally attached digits (metacarpus II and IV), however, were somewhat reduced in length, the outermost digit (metacarpus V) was greatly reduced. The hind foot, on the other hand, had only three digits, but also had a strong central ray (

metatarsus The metatarsal bones, or metatarsus, are a group of five long bones in the foot, located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the me ...

III). Compared to the anterior feet, the posterior ones were around 30% longer (metacarpus III length is 5.1 cm to metatarsus III's 6.7 cm). The individual phalanges of the toes are extremely long. Four-toed front and three-toed hind feet are typical of primitive odd-toed ungulates and are now only found in the

Fossil Finds

Finds of ''Homogalax'' are largely only known from North America and come from the Lower

56 to 48 million years ago (locally stratigraphically called Lower Wastachian). They show that '' Homogalax '' was a regular but not frequent element of the fauna at that time. In the state of

in particular, numerous fossils have been recovered. The remains from the

Willwood Formation The Willwood Formation is a sedimentary sequence deposited during the late Paleocene to early Eocene, or Clarkforkian, Wasatchian and Bridgerian in the NALMA classification.Bighorn Basin The Bighorn Basin is a plateau region and intermontane basin, approximately 100 miles (160 km) wide, in north-central Wyoming in the United States. It is bounded by the Absaroka Range on the west, the Pryor Mountains on the north, the Bigho ...

are of great importance, including more than 100 fragments from several individuals, including a complete skull, which is one of the few well-preserved ''Homogalax'' fossils. This also includes the majority of known

postcranial Postcrania (postcranium, adjective: postcranial) in zoology and vertebrate paleontology is all or part of the skeleton apart from the skull. Frequently, fossil remains, e.g. of dinosaurs or other extinct tetrapods, consist of partial or isolated s ...

skeletal finds, such as individual long bones, a complete foot and hand skeleton and parts of the pelvis and shoulder blades. Other remains are from the

Washakie Basin Washakie (1804/1810 – February 20, 1900) was a prominent leader of the Shoshone people during the mid-19th century. He was first mentioned in 1840 in the written record of the American fur trapper, Osborne Russell. In 1851, at the urging o ...

, as well as from the

Powder River Basin The Powder River Basin is a geologic structural basin in southeast Montana and northeast Wyoming, about east to west and north to south, known for its extensive coal reserves. The former hunting grounds of the Oglala Lakota, the area is very s ...

; both sites, however, contained mostly only isolated teeth. Remnants of the Golden Valley Formation in

North Dakota North Dakota () is a U.S. state in the Upper Midwest, named after the indigenous Dakota Sioux. North Dakota is bordered by the Canadian provinces of Saskatchewan and Manitoba to the north and by the U.S. states of Minnesota to the east, So ...

have been reported outside Wyoming. Far away from these sites, the northernmost finds of ''Homogalax'' come from the Margaret Formation on the Ellesmere Island in the far north of

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

. They include, among other things, individual teeth.Eberle, JJ; Eberth, DA; 2015 "Additions to the Eocene Perissodactyla of the Margaret Formation, Eureka Sound GRroup, Ellesmere Island, Arctic Canada." ''Canadian Journal of Earth Sciences 52.'' 123-133.

Paleobiology

Based on the teeth, only a few differences in size and shape can be discerned in specimens of ''Homogalax''. However, the canine varies in size, which can be interpreted as sexual dimorphism. This is unconfirmed, though, as there are also intermediate-length canines. The lower legs, especially on the hindlimbs, show adaptations to a

cursorial A cursorial organism is one that is adapted specifically to run. An animal can be considered cursorial if it has the ability to run fast (e.g. cheetah) or if it can keep a constant speed for a long distance (high endurance). "Cursorial" is often us ...

lifestyle. Among other things, especially pronounced joint surfaces on the ankle bone ensured that the foot remained stable in the longitudinal direction and did not twist sideways at high walking speed. A permanent running gait within the

is generally believed to be an ancient characteristic and is only practised today by the highly specialized horses.

Taxonomy

''Homogalax'' is a

from the order Perissodactyla. It is a basal representative, usually placed in the potentially paraphyletic

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Isectolophidae. If the paraphyletic position is correct, ''Homogalax'' nests as more derived than most other isectolophids, with only ''Cardiolophus'' and ''Orientolophus'', and a clade including ''Karagalax'' and ''Gandheralophus'', being closer to the Ceratomorpha. Below is a phylogenetic tree based on Bai ''et al'' (2020), with particular focus on the Isectolophidae (recovered as paraphyletic). Today, one species of the genus ''Homogalax'' is recognized: ''H. protapirinus'' ( Wortman, 1896). Other species such as ''H. tapirinus'' and '' H. primaevus'' have been described in the past, but are considered to be synonyms of ''H. protapirinus''. Species in Asia were also named, such as ''H. namadicus '' and ''H. wutuensis'',Minchen, C; Chuan-kuei, L; 1965 "''Homogalax'' and ''Heptodon'' of Shantung." ''Vertebrata Palasiatica''. 9 (1), 15-22. but these are now to be regarded as representatives of basal chalicotheres such as '' Protomoropus''.Hooker, JJ; Dashzeveg, D; 2004 "The origin of chalicotheres (Perissodactyla, Mammalia)." ''Palaeontology.'' 47 (6): 1363-1386. Another species, ''H. reliquius'', was later identified as a specimen of ''Isectolophus''. Lucas, SG; Holbrook, T; Emry, RJ; 2003 "''Isectolophus'' (Mammalia, Perissodactyla) from the Eocene of the Zaysan Basin, Kazakhstan and its biochronological significance. ''Journal of Vertebrate Paleontology''. 23 (1): 238-243.

References

Eocene odd-toed ungulates Eocene mammals of Asia {{paleo-oddtoedungulate-stub