''Ambolestes'' is an extinct

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 ...

eutherian Eutheria (from Greek , 'good, right' and , 'beast'; ), also called Pan-Placentalia, is the clade consisting of placental mammals and all therian mammals that are more closely related to placentals than to marsupials. Eutherians are distingu ...

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 ...

from the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

of China. It includes a single species, ''Ambolestes zhoui'', known from a single complete skeleton recovered from the

Yixian Formation The Yixian Formation (; formerly Romanization of Chinese, transcribed as Yihsien Formation or Yixiang Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the Barremian stage of the Early Cretaceous. I ...

(126 Ma), part of the fossiliferous

Jehol biota The Jehol Biota includes all the living organisms – the ecosystem – of northeastern China between 133 and 120 million years ago. This is the Lower Cretaceous ecosystem which left fossils in the Yixian Formation and Jiufotang Formation. These d ...

. ''Ambolestes'' is one of the most basal eutherians, presenting a combination of features from both early eutherians (

stem Stem or STEM most commonly refers to: * Plant stem, a structural axis of a vascular plant * Stem group * Science, technology, engineering, and mathematics Stem or STEM can also refer to: Language and writing * Word stem, part of a word respon ...

placentals 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 ...

) and early metatherians (stem-

marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...

s). This is responsible for the generic name of ''Ambolestes'': "''ambo''" is

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

for "both", while "-''lestes''" (

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

for "robber") is a popular suffix for fossil mammals. The species name honors influential Jehol paleontologist

Zhou Zhonghe Zhonghe Zhou (; born 19 January 1965 in Jiangdu, Jiangsu province) is a Chinese palaeontologist. He described the ancient bird '' Confuciusornis''.. Zhonghe graduated from Nanjing University and earned a Ph.D. in Biology in 1999 from the Univer ...

Description

''Ambolestes'' was a fairly small mammal, with an estimated mass of 34–44 g (about the size of a modern mouse opossum, ''

Marmosa The 27 species in the genus ''Marmosa'' are relatively small Neotropical members of the family Didelphidae. This genus is one of three that are known as mouse opossums. The others are '' Thylamys'' (the "fat-tailed mouse opossums") and '' Tlacu ...

''). It was likely similar in appearance and habits to other putative Yixian Formation therians, such as ''

Eomaia ''Eomaia'' ("dawn mother") is a genus of extinct fossil mammals containing the single species ''Eomaia scansoria'', discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower ...

'' and ''

Sinodelphys ''Sinodelphys'' is an extinct mammal from the Early Cretaceous, estimated to be 125 million years old. It was discovered and described in 2003 in rocks of the Yixian Formation in Liaoning Province, China, by a team of scientists including Zhe-X ...

''. There are several similarities between ''Ambolestes'' and ''Sinodelphys''. Both are interpreted to bear 8 upper postcanine teeth (5

premolar The premolars, also called premolar Tooth (human), teeth, or bicuspids, are transitional teeth located between the Canine tooth, canine and Molar (tooth), molar teeth. In humans, there are two premolars per dental terminology#Quadrant, quadrant in ...

s and 3

molars The molars or molar teeth are large, flat tooth, teeth at the back of the mouth. They are more developed in mammal, mammals. They are used primarily to comminution, grind food during mastication, chewing. The name ''molar'' derives from Latin, '' ...

) and 7 lower postcanine teeth (4 premolars and 3 molars) on each side of the skull. The rear premolars are similar to the tall, sharp tribosphenic molars (though the premolars lack a

protocone A cusp is a pointed, projecting, or elevated feature. In animals, it is usually used to refer to raised points on the crowns of teeth. The concept is also used with regard to the leaflets of the four heart valves. The mitral valve, which has two ...

). Earlier premolars are smaller, blade-shaped, and widely spaced. The shape and number of

incisor 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 ...

s are unknown in ''Ambolestes'', while the canines are distinctively double-rooted. The wrist has enlarged

scaphoid The scaphoid bone is one of the carpal bones of the wrist. It is situated between the hand and forearm on the thumb side of the wrist (also called the lateral or radial side). It forms the radial border of the carpal tunnel. The scaphoid bone ...

hamate The hamate bone (from Latin language, Latin wiktionary:hamatus, hamatus, "hooked"), or unciform bone (from Latin language, Latin ''wikt:uncus, uncus'', "hook"), Latin os hamatum and occasionally abbreviated as just hamatum, is a bone in the huma ...

, and triquetrum bones, similar to ''Sinodelphys'' and metatherians. Other traits are more similar to ''Eomaia'' and eutherians: the mandibular angle is not inturned, and the trapezium bone of the wrist is also large. ''Ambolestes'' preserves an

ectotympanic The ectotympanic, or tympanicum, is a bony structure found in all mammals, located on the tympanic part of the temporal bone, which holds the tympanic membrane (eardrum) in place. In catarrhine primates (including humans), it takes a tube-shape. ...

bone of the

middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles (malleus, incus, and stapes), which transfer the vibrations ...

, a delicate bone which is rarely preserved in Mesozoic mammal fossils. The ectotympanic is horseshoe-shaped and thickened at its lower half, similar to that of short-tailed opossums (''

Monodelphis ''Monodelphis'' is a genus of marsupials in the family Didelphidae, commonly referred to as short-tailed opossums. They are found throughout South America. , the most recently described species is ''M. vossi''. Species * Sepia short-tailed o ...

''). The lower part of the ectotympanic hosts a small groove, the meckelian sulcus. The sulcus is a vestige of the

meckel's cartilage In humans, the cartilaginous bar of the mandibular arch is formed by what are known as Meckel's cartilages (right and left; also known as Meckelian cartilages), above which the incus and malleus are located. Meckel's cartilage arises from the fir ...

, a thin plate which connected the middle ear ossicles to the jaw in earlier mammals. Like other therians, the front part of the

malleus The ''malleus'', or hammer, is a hammer-shaped small bone or ossicle of the middle ear. It connects with the incus, and is attached to the inner surface of the eardrum. The word is Latin for 'hammer' or 'mallet'. It transmits the sound vibra ...

is downcurved and confluent with the front edge of the ectotympanic, according to a facet on the latter bone. ''Ambolestes'' is also the first Mesozoic mammal to be discovered with a complete

hyoid apparatus The hyoid apparatus is the collective term used in veterinary anatomy for the bones which suspend the tongue and larynx. It consists of pairs of stylohyoid, thyrohyoid, epihyoid and ceratohyoid bones, and a single basihyoid bone. The hyoid appar ...

. The hyoid consists of seven linked bones, similar to some squirrels, though the thyrohyals (lower lateral prongs) are enlarged, more akin to the five-bone hyoids of marsupials.

Classification

According to a

phylogenetic analysis In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...

by Bi et al. (2018), ''Ambolestes'' forms a small

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

with ''

Montanalestes ''Montanalestes'' is an extinct mammal known from the Cretaceous in North America. Classification Known from only six fossil teeth, ''Montanalestes'' is an example of the less well-preserved early eutherian Eutheria (from Greek , 'good, ri ...

'', ''

Acristatherium ''Acristatherium yanensis'' is an extinct basal eutherian from the Early Cretaceous (early Aptian, about ) Lujiatun Bed of the Yixian Formation. It was described on the basis of a single specimen (holotype) from Beipiao, Liaoning, China, by Yaom ...

'', and ''Sinodelphys'' at the base of Eutheria. In 2022, the new Jehol eutherian '' Cokotherium'' was added to the clade, while ''Acristatherium'' shifted crownwards (closer to placentals). ''Ambolestes'' and its basal eutherian clade prompt a re-evaluation of ancestral conditions at the

common ancestor Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. According to modern evolutionary biology, all living beings could be descendants of a unique ancestor commonl ...

of Eutheria and Metatheria. ''Sinodelphys'', for example, was commonly considered to be the oldest known metatherian in most studies prior to 2018. If interpreted as eutherians, ''Sinodelphys'' and ''Ambolestes'' would indicate that early eutherians were more metatherian-like than previously considered. The removal of ''Sinodelphys'' from Metatheria would also expand the

ghost lineage A ghost lineage is a hypothesized ancestor in a species lineage that has left no fossil evidence, but can still be inferred to exist or have existed because of gaps in the fossil record or genomic evidence. The process of determining a ghost line ...

between the oldest eutherian (''

Juramaia ''Juramaia'' is an extinct genus of a therian mammal, possibly a very basal eutherian mammal, known from the Late Jurassic ( Oxfordian stage) or Early Cretaceous deposits of western Liaoning, China. It is a small shrew-like mammal weighing aro ...

'', 160 Ma) and the next oldest metatherians ( deltatheroids and marsupialiforms, 110 Ma).

References

{{Taxonbar, from=Q61893482 Prehistoric eutherians Early Cretaceous mammals of Asia Cretaceous mammals Fossil taxa described in 2018 Yixian fauna Monotypic prehistoric mammal genera