| Ambolestes | |
|---|---|
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Clade: | Eutheria |
| Genus: | †Ambolestes Biet al., 2018 |
| Type species | |
| †Ambolestes zhoui Biet al., 2018 | |
Ambolestes is an extinctgenus ofeutherianmammal from theEarly Cretaceous ofChina. It includes a single species,Ambolestes zhoui, known from a single complete skeleton recovered from theYixian Formation (126 Ma), part of the fossiliferousJehol biota.Ambolestes is one of the mostbasal eutherians, presenting a combination of features from both early eutherians (stem-placentals) and early metatherians (stem-marsupials). This is responsible for thegeneric name ofAmbolestes: "ambo" isLatin for "both", while "-lestes" (Greek for "robber") is a popular suffix for fossil mammals. The species name honors influential Jehol paleontologistZhou Zhonghe.[1]
Ambolestes was a fairly small mammal, with an estimated mass of 34–44 g (about the size of a modern mouse opossum,Marmosa). It was likely similar in appearance and habits to other putative Yixian Formation therians, such asEomaia andSinodelphys.[1]
There are several similarities betweenAmbolestes andSinodelphys. Both are interpreted to bear 8 upperpostcanine teeth (5premolars and 3molars) and 7 lower postcanine teeth (4 premolars and 3 molars) on each side of the skull. The rear premolars are similar to the tall, sharptribosphenic molars (though the premolars lack aprotocone). Earlier premolars are smaller, blade-shaped, and widely spaced. The shape and number ofincisors are unknown inAmbolestes, while thecanines are distinctively double-rooted. The wrist has enlargedscaphoid,hamate, andtriquetrum bones, similar toSinodelphys and metatherians. Other traits are more similar toEomaia and eutherians: themandibular angle is not inturned, and thetrapezium bone of the wrist is also large.[1]
Ambolestes preserves anectotympanic bone of themiddle ear, 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). The lower part of the ectotympanic hosts a small groove, the meckelian sulcus. The sulcus is a vestige of themeckel's cartilage, a thin plate which connected the middle ear ossicles to the jaw in earlier mammals. Like other therians, the front part of themalleus 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 completehyoid apparatus. The hyoid consists of seven linked bones, similar to somesquirrels, though thethyrohyals (lower lateral prongs) are enlarged, more akin to the five-bone hyoids of marsupials.[1]
According to aphylogenetic analysis by Bi et al. (2018),Ambolestes forms a smallclade withMontanalestes,Acristatherium, andSinodelphys at the base of Eutheria.[1] In 2022, the new Jehol eutherianCokotherium was added to the clade, whileAcristatherium shifted crownwards (closer to placentals).[2]Ambolestes and its basal eutherian clade prompt a re-evaluation of ancestral conditions at thecommon ancestor 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 andAmbolestes would indicate that early eutherians were more metatherian-like than previously considered. The removal ofSinodelphys from Metatheria would also expand theghost lineage between the oldest eutherian (Juramaia, 160 Ma) and the next oldest metatherians (deltatheroids andmarsupialiforms, 110 Ma).[1]
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