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

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

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 inhabit ...

-like odd-toed ungulate. 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 Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

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

Wyoming Wyoming () is a U.S. state, state in the Mountain states, 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 south ...

. The finds date to the Lower

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

between 56 and 48 million years ago. In general, ''Homogalax'' was very small, only reaching the weight of today's peccaries, with a maximum of 15 kg. Phylogenetic analysis 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 o ...

and tapirs (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 Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

Perissodactyla, such as '' Hyracotherium'' 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'', w ...

, 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 b ...

, which rose steeply and came into contact with the

nasal bone The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Eac ...

, 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 zygomati ...

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 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 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

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a roun ...

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 with t ...

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 Shin may refer to: Biology * The front part of the human leg below the knee * Shinbone, the tibia, the larger of the two bones in the leg below the knee in vertebrates Names * Shin (given name) (Katakana: シン, Hiragana: しん), a Japanese ...

became as long as the thigh bone. The forefoot consisted of four digits with a very pronounced central toe (

metacarpus In human anatomy, the metacarpal bones or metacarpus form the intermediate part of the skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist, which forms the connection to the forearm. The metacarpal bones ...

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 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 tapirs.

Fossil Finds

Finds of ''Homogalax'' are largely only known from

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

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 of the Bighorn Basin 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 skeletal finds, such as individual long bones, a complete foot and hand skeleton and parts of the

pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...

and

shoulder blades The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either ...

. Other remains are from the Washakie Basin, as well as from the Powder River Basin; both sites, however, contained mostly only isolated teeth. Remnants of the

Golden Valley Formation The Golden Valley Formation is a stratigraphic unit of Late Paleocene to Early Eocene age in the Williston Basin of North Dakota.Hickey, 1977 It is present in western North Dakota and was named for the city of Golden Valley by W.E. Benson and W.M ...

North Dakota North Dakota () is a U.S. state in the Upper Midwest, named after the Native Americans in the United States, indigenous Dakota people, Dakota Sioux. North Dakota is bordered by the Canadian provinces of Saskatchewan and Manitoba to the north a ...

have been reported outside Wyoming. Far away from these sites, the northernmost finds of ''Homogalax'' come from the

Margaret Formation The Margaret Formation is a geologic formation of the Eureka Sound Group in the Sverdrup Basin in Northwest Territories and Nunavut, Canada. The unit belonging to the Eureka Sound Group which crops out at Ellesmere Island preserves fossils dating ...

on the

Ellesmere Island Ellesmere Island ( iu, script=Latn, Umingmak Nuna, lit=land of muskoxen; french: île d'Ellesmere) is Canada's northernmost and List of Canadian islands by area, third largest island, and the List of islands by area, tenth largest in the world. ...

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 u ...

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 The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...

Taxonomy

''Homogalax'' is a

from the

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

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

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

Isectolophidae Isectolophidae is a potentially paraphyletic family of browsing, herbivorous, mammals in the Perissodactyla suborder Ancylopoda that show long, curved and cleft claws A claw is a curved, pointed appendage found at the end of a toe or fin ...

. 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 A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are all ...

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 Chalicotheres (from Greek language, Greek ''wikt:χάλιξ, chalix'', "gravel" and ''wikt:θηρίον, therion'', "beast") are an extinct clade of herbivorous, odd-toed ungulate (perissodactyl) mammals that lived in North America, Eurasia, and ...

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