''Ukhaatherium'' is an extinct species of

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

that lived during the upper

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

about 84 to 72 million years ago in today's

East Asia East Asia is a geocultural region of Asia. It includes China, Japan, Mongolia, North Korea, South Korea, and Taiwan, plus two special administrative regions of China, Hong Kong and Macau. The economies of Economy of China, China, Economy of Ja ...

. 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 a mammal belonging to any species within the afrotherian family Tenrecidae, which is endemic to Madagascar. Tenrecs are a very diverse group, as a result of adaptive radiation, and exhibit convergent evolution, some resemble hed ...

Taxonomy

''Ukhaatherium'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

from the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

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

Description

''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 in the

pelvic girdle The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the Ilium (bone) ...

, 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 pregn ...

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

altricial Precocial species in birds and mammals are those in which the young are relatively mature and mobile from the moment of birth or hatching. They are normally nidifugous, meaning that they leave the nest shortly after birth or hatching. Altricial ...

young, like monotremes, marsupials, and extinct Mesozoic mammals such as multituberculates. 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 (AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. Located in Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 21 interconn ...

and the Mongolian Academy of Sciences have taken place in the

Gobi desert The Gobi Desert (, , ; ) is a large, cold desert and grassland region in North China and southern Mongolia. It is the sixth-largest desert in the world. The name of the desert comes from the Mongolian word ''gobi'', used to refer to all of th ...

of southern

Mongolia Mongolia is a landlocked country in East Asia, bordered by Russia to the north and China to the south and southeast. It covers an area of , with a population of 3.5 million, making it the world's List of countries and dependencies by po ...

in order to investigate upper

and

Tertiary Tertiary (from Latin, meaning 'third' or 'of the third degree/order..') may refer to: * Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago * Tertiary (chemistry), a term describing bonding patterns in organic ch ...

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 Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

skeletons, eggs, and embryos,

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s, and

lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

s. Fossils of two species of basal eutherian mammal were found, including the first known ''Ukhaatherium'' specimens and a new '' Zalambdalestes'' specimen. These originate from the Djadochta Formation.

Paleobiology

References

{{Taxonbar, from=Q4002998 Prehistoric eutherians Late Cretaceous mammals Campanian life Cretaceous mammals of Asia Cretaceous Mongolia Fossils of Mongolia Fossil taxa described in 1997 Prehistoric mammal genera