Akidolestes is an extinct genus of mammals from the family spalacotheriid.
Although Akidolestes do not have any modern relatives, they are early mammals related to therians (the subclass containing marsupials and placentals). They belong to a group of theriiform mammals known as the Spalacotherioidea. However, unlike other members of this superfamily, Akidolestes have a few prototherian features.
The genus name, Akidolestes, is derived from akido, Greek for point, and lestes, Greek for thief. Akido- refers to the pointed snout and -lestes is a common suffix for fossil mammals. The specific epithet, cifelli, is in honor of Richard L. Cifelli, a prominent researcher in prehistoric mammals.[1]
An Akidolestes fossil preserved with a complete post-cranium and a partial skull was discovered in the Yixian Formation of Liaoning, China.[2][1] The holotype of Akidolestes cifellii, reserved in the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences,[2][1] has a complete skeleton with a partial skull and dentition.[2][1] It displays characteristics of monotremes but appears to be more related to modern therian mammals.[2]
Although it had some features similar to monotremes in the lumbar vertebrae, pelvis, and hindlimb, Akidolestes cifellii is still placed in the Spalacotheriidae family and close to Zhangheotherium and Maotherium.[1] Those convergent synapomorphies might derive from a shared early common ancestor.[1] Based on the analysis and comparison of anatomy and locomotory features of Akidolestes cifellii with its related taxa, there is a hypothesis that spalacotheroids might have evolved in Eurasia and then dispersed to North America, which is consistent with the geodispersal pattern common to several mammalian groups during the Early Cretaceous period.[1]
Most fossils of Mesozoic mammals exist as teeth or jaw fragments only. Akidolestes cifellii was the third spalacotheroid species discovered with a complete skeleton in the Yixian Formation, after Zhangheotherium and Maotherium.[2]
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The Zhangheotheriidae and Spalacotheriidae families form the superfamily Spalacotheroidea.[2]Akidolestes cifellii has acute triangulation of the molar cusp pattern, which is characteristic of Spalacotheroids.[3] However, unlike the Maotherium, which has symmetrical premolar and molar patterns,[3]Akidolestes‘s premolars and molars are gradually longer, respectively.[1] Also, Akidolestes has protocristid on its molars, which distinguish it other from Zhangheotherium and Maotherium.[1][3] The mandible of Akidolestes cifellii is similar to that of Zhangheotherium and Maotherium. They all have a coronoid process and dentary condyle.[1][3] At the same time, the structure and surface features of the teeth of Akidolestes are closer to spalacotheriids as compared with zhangheotheriids.[1] Based on these overall dental characteristics, Akidolestes has been classified as a member of Spalacotheriidae.[1]
There are several features in Akidolestes cifellii that are similar to monotremes rather than more common Mesozoic mammals.[1] In the original paper describing Akidolestes cifellii, the author compared Ornithorhynchus and Zhangheotherium with Akidolestes cifellii. Zhangheotherium, a Mesozoic mammal, belongs to the Spalacotheriidae family.[4]Ornithorhynchus is a living monotreme.[5]
On the pelvis, the epipubic bones of Akidolestes cifellii and Ornithorhynchus have a broad and triangular shape, but the epipubis of Zhangheotherium is a narrow bone.[1] Both Ornithorhynchus and Akidolestes have the tubercle for the M. psoas minor muscle on the pubis and tuber coxae on the ilium, but those are absent in Zhangheotherium.[1] On the femur, Zhangheotherium has a symmetrical distal medial condyle and a distal lateral condyle, but those condyles are more asymmetrical in both Akidolestes and Ornithorhynchus.[1] Moreover, compared to Zhangheotherium, both Akidolestes and Ornithorhynchus have a shorter neck on the femur.[1] On the fibula and tibia, Akidolestes and Ornithorhynchus have hypertrophied parafibular processes, proximolateral tuberosity of the tibia, and a distal tibial malleolus, all of which are absent in Zhangheotherium.[1]
Except the pelvic girdle and hindlimbs, Akidolestes shares several forelimb features with living monotremes as well.[1] Similar to its hindlimbs, Zhangheotherium has asymmetrical condyles on the humerus, but the condyles of the humerus on Akidolestes and Ornithorhynchus are asymmetrical.[1] Additionally, Zhangheotherium and other Mesozoic mammals have a straight tibia, but the tibia on Akidolestes and Ornithorhynchus are more curved.[1] Another striking feature of Akidolestes that is distinguishable from other Mesozoic mammals is the trochanter.[1] The trochanter on Zhangheotherium is bigger, tall, and vertical, but the trochanter on Akidolestes is smaller, broader, and triangular in shape, which is similar to Ornithorhynchus.[1]
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