''Ukhaatherium'' is a now extinct species of

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

that lived during the upper

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

about 84 to 72 million years ago in today's

East Asia East Asia is the eastern region of Asia, which is defined in both geographical and ethno-cultural terms. The modern states of East Asia include China, Japan, Mongolia, North Korea, South Korea, and Taiwan. China, North Korea, South Korea and ...

. It is known above all from the fossil locality Ukhaa Tolgod, Mongolia. An adult ''Ukhaatherium'' has an estimated weight of about 32g and bears several similarities to lipotyphlan insectivorans such as the

tenrec A tenrec is any species of mammal within the afrotherian family Tenrecidae endemic to Madagascar. Tenrecs are wildly diverse; as a result of convergent evolution some resemble hedgehogs, shrews, opossums, rats, and mice. They occupy aquatic, a ...

Characteristics

''Ukhaatherium'' ''nessovi'', the type and only species for the genus, is known from multiple near-complete specimens. The most notable feature of the species is the presence of

epipubic bones Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials, monotremes and fossil mammals like multituberculates, and even basal eutherians (the ancestors of placental mammals, who lack them). They first occu ...

in the

pelvic girdle 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 p ...

, which have been lost in extant eutherian mammals. The loss of epipubic bones is associated with the evolution of prolonged

gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregna ...

in eutherian mammalian reproduction. This means that ''Ukhaatherium'' may have had a short gestation period resulting in the birth of

altricial In biology, altricial species are those in which the young are underdeveloped at the time of birth, but with the aid of their parents mature after birth. Precocial species are those in which the young are relatively mature and mobile from the mome ...

young, like

monotremes Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brains, ...

marsupials Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...

, and extinct Mesozoic mammals such as

multituberculates Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, ...

. This supports the hypothesis that the presence of epipubic bones is the primitive mammalian condition. Despite some primitive skull and dental traits, the skeletons of ''Ukhaatherium'' and two other asioryctitheres, ''Asioryctes'' and '' Kennalestes'', are classed as eutherians and show several similarities with the extant lipotyphlans. Derived eutherian characteristics include the restriction of the upper ankle joint to the parasagittal plane.

Fossil findings

Since the year 1990, joint expeditions of the

American Museum of Natural History The American Museum of Natural History (abbreviated as AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. In Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 26 inter ...

and the

Mongolian Academy of Sciences The Mongolian Academy of Sciences (, ''Mongol ulsyn Shinjlekh ukhaany Akademi'') is Mongolia's first centre of modern sciences. It came into being in 1921 when the government of newly independent Mongolia issued a resolution declaring the establi ...

have taken place in the

Gobi desert The Gobi Desert (Chinese: 戈壁 (沙漠), Mongolian: Говь (ᠭᠣᠪᠢ)) () is a large desert or brushland region in East Asia, and is the sixth largest desert in the world. Geography The Gobi measures from southwest to northeast an ...

of southern

Mongolia Mongolia; Mongolian script: , , ; lit. "Mongol Nation" or "State of Mongolia" () is a landlocked country in East Asia, bordered by Russia to the north and China to the south. It covers an area of , with a population of just 3.3 million, ...

in order to investigate upper

and

Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

vertebrates. More than 500 mammalian skulls (many of which with corresponding postcranial skeletons) were discovered at the Ukhaa Tolgod fossil field between 1993 and 1997, together with very well-preserved

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

skeletons, eggs, and embryos,

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

s, and

lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia alt ...

s. Fossils of two species of basal eutherian mammal were found, including the first known ''Ukhaatherium'' specimens and a new ''

Zalambdalestes ''Zalambdalestes'' (meaning ''much-like-lambda robber'') was a eutherian mammal, most likely not a placental due to the presence of an epipubic bone, living during the Upper Cretaceous in Mongolia. ''Zalambdalestes'' was a hopping animal with ...

'' specimen. These originate from the

Djadochta Formation The Djadochta Formation (sometimes transcribed and also known as Djadokhta, Djadokata, or Dzhadokhtskaya) is a highly fossiliferous geological formation situated in Central Asia, Gobi Desert, dating from the Late Cretaceous period, about 75 milli ...

Paleobiology

Systematics

''Ukhaatherium'' is a

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 ...

from the

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 ...

of the Asioryctitheria, which lived in the late Cretaceous. ''Ukhaatherium'' and ''Asioryctes'' form the subfamily Asioryctinae, while ''Kennalestes'' stands somewhat outside this group.

References

Late Cretaceous mammals Campanian life Cretaceous mammals of Asia Cretaceous Mongolia Fossils of Mongolia Fossil taxa described in 1997 Prehistoric mammal genera {{cretaceous-mammal-stub