''Akidolestes'' 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 ...

of mammals of the family

Spalacotheriidae Spalacotheriidae is a family of extinct mammals belonging to the paraphyletic group ' Symmetrodonta'. They lasted from the Early Cretaceous to the Campanian in North America, Europe, Asia and North Africa. Spalacotheriids are characterised by ha ...

, a group of

mammals A mammal () is a vertebrate animal of the 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 middle e ...

related to therians (the subclass containing

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 and

placental 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 distinguished ...

s). The genus name, ''Akidolestes'', is derived from ''akido'', Greek for point, and ''lestes'', Greek for thief. Akido- refers to the pointed

snout A snout is the protruding portion of an animal's face, consisting of its nose, mouth, and jaw. In many animals, the structure is called a muzzle, Rostrum (anatomy), rostrum, beak or proboscis. The wet furless surface around the nostrils of the n ...

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. An ''Akidolestes'' fossil preserved with a complete post-cranium and a partial skull was discovered in 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 ...

Liaoning ) , image_skyline = , image_alt = , image_caption = Clockwise: Mukden Palace in Shenyang, Xinghai Square in Dalian, Dalian coast, Yalu River at Dandong , image_map = Liaoning in China (+all claims hatched).svg , ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

. The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

of ''Akidolestes cifellii'', reserved in the Nanjing Institute of Geology and Paleontology,

Chinese Academy of Sciences The Chinese Academy of Sciences (CAS; ) is the national academy for natural sciences and the highest consultancy for science and technology of the People's Republic of China. It is the world's largest research organization, with 106 research i ...

, has a complete skeleton with a partial skull and dentition. It displays characteristics of

monotremes Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

but appears to be more related to modern

theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...

n mammals. 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''. Those convergent synapomorphies might derive from a shared early common ancestor. 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 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 ...

period. 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''.

Description

Dentition

The Zhangheotheriidae and

families form the superfamily Spalacotheroidea. ''Akidolestes cifellii'' has acute triangulation of the molar cusp pattern, which is characteristic of Spalacotheroids. However, unlike the ''Maotherium'', which has symmetrical premolar and molar patterns, ''Akidolestes''s premolars and molars are gradually longer, respectively. Also, ''Akidolestes'' has ''protocristid'' on its molars, which distinguish it other from ''Zhangheotherium'' and ''Maotherium''. The

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

of ''Akidolestes cifellii'' is similar to that of ''Zhangheotherium'' and ''Maotherium''. They all have a coronoid process and dentary condyle. At the same time, the structure and surface features of the teeth of ''Akidolestes'' are closer to spalacotheriids as compared with zhangheotheriids. Based on these overall dental characteristics, ''Akidolestes'' has been classified as a member of Spalacotheriidae.

Post-cranial skeleton

There are several features in ''Akidolestes cifellii'' that are similar to monotremes rather than more common

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

mammals. 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. ''Ornithorhynchus'' is a living monotreme. 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. 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''. 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''. Moreover, compared to ''Zhangheotherium'', both ''Akidolestes'' and ''Ornithorhynchus'' have a shorter neck on the femur. 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''. Except 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) ...

and hindlimbs, ''Akidolestes'' shares several

forelimb A forelimb or front limb is one of the paired articulated appendages ( limbs) attached on the cranial (anterior) end of a terrestrial tetrapod vertebrate's torso. With reference to quadrupeds, the term foreleg or front leg is often used inst ...

features with living monotremes as well. Similar to its hindlimbs, ''Zhangheotherium'' has asymmetrical condyles on the humerus, but the condyles of the humerus on ''Akidolestes'' and ''Ornithorhynchus'' are asymmetrical. Additionally, ''Zhangheotherium'' and other Mesozoic mammals have a straight tibia, but the tibia on ''Akidolestes'' and ''Ornithorhynchus'' are more curved. Another striking feature of ''Akidolestes'' that is distinguishable from other Mesozoic mammals is the trochanter. 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''.

Paleobiology

Differences in the post-cranial skeleton between ''Akidolestes cifellii'' and related

taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

allow insights into ecological differentiation within early therian mammal evolution. Correlation between limb posture and locomotor function in ''Akidolestes cifellii'' indicates that the hypertrophied parafibular process on the fibula helped the flexed function of the knee joint. A short neck on the

femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...

and asymmetrical condyles on the humerus indicate a horizontal orientation of the femur. ''Akidolestes'' probably had a parasagittal forelimb posture and most likely a semi-erect or sprawling posture for both forelimbs and hindlimbs. ''Akidolestes'' is not considered a traditional terrestrial mammal like ''Zhangheotherium'' and ''Maotherium'', although there is debate about whether asymmetrical femoral condyles suggest that ''Akidolestes'' was a terrestrial mammal or an arboreal mammal. In "Postcranial Skeleton of the Cretaceous Mammal Akidolestes cifellii and Its Locomotor Adaptations", the author argued that both ''Zhangheotherium'' and ''Maotherium'' are considered as terrestrial mammals, and they both have symmetrical knee joints. The author noted that asymmetrical femoral condyles indicate that ''Akidolestes'' lived in

arboreal Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally (scansorial), but others are exclusively arboreal. The hab ...

habitats, but the hypertrophied parafibula on the fibula and medial malleolus on the tibia suggests ''Akidolestes'' was a terrestrial mammal. However, girdle is an important factor in inferring the habitat preference of ''Akidolestes'' as well. On the

pectoral girdle The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans, it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists o ...

, ''Akidolestes'' has glenoid fossa smaller than the humeral head, which offer a great range of rotation for the humerus. Moreover, ''Akidolestes'' has a scapula with a triangular outline, which is similar to ''Ornithorhynchus'' and ''Haldanodon'', but the scapula of ''Zhangheotherium'' is rectangular in shape. ''Ornithorhynchus'' is a semifossorial and semiaquatic monotreme which can swim and burrow. ''Haldanodon'' is a terrestrial mammal, and it is semifossorial and semiaquatic as well. At the same time, the hook-like, large

coracoid process The coracoid process (from Greek κόραξ, raven) is a small hook-like structure on the lateral edge of the superior anterior portion of the scapula (hence: coracoid, or "like a raven's beak"). Pointing laterally forward, it, together with the ...

on the scapula make ''Akidolestes'' closer to arboreal mammals than to terrestrial mammals. Overall, ''Akidolestes'' was likely a terrestrial mammal but not restricted to singular living habitats.

Geology and paleoenvironment

An ''Akidolestes cifellii'' fossil was found in the Yixian Formation. Yixian formation is one composition of Jehol Group. The Yixian Formation is dated to the

Berriasian In the geological timescale, the Berriasian is an age/ stage of the Early/Lower Cretaceous. It is the oldest subdivision in the entire Cretaceous. It has been taken to span the time between 143.1 ±0.6 Ma and 137.05 ± 0.2 (million years ago) ...

(145–140 mya). The stratum of the Yixian Formation is correlated with other localities including the Jingangshan Locality, Jianshangou Locality, and Lujiatun Locality. Other mammals found in the Yixian Formation include eutriconodontans, multituberculates, symmetrodonts, metatherians, and eutherians.

Classification

''Akidolestes cifellii'' and ''Spalacotherium'' are sister taxa and share a common ancestor with ''Zhangheotherium''. The clade that contains these three species is Spalacotheroidea, within the Trechnotherian group. The Theria clade includes Metatheria and Eutheria, which is the outgroup of Spalacotheroidea. Theriiformes includes Theria and Spalacotheroidea. (The cladogram below does not perfectly contain all important details due to a technical issue.) The cladogram is based on the content from "Evolution of the patellar sesamoid bone in mammals", "A cretaceous symmetrodont therian with some monotreme-like postcranial features", and ''Classification of Mammals Above the species Level''.

References

External links

Press release from Carnegie Museum
{{Taxonbar, from=Q608000 Spalacotheriidae Monotypic prehistoric mammal genera Mammal anatomy Terrestrial locomotion Berriasian life Cretaceous mammals of Asia Cretaceous China Fossils of China Fossil taxa described in 2006