''Symmetrolestes'' is an extinct genus of small spalacotheriid mammal from the Early Cretaceous period of Japan. The genus contains one species known as ''S. parvus'', the type fossil (which is only fossil known) is from fluvial deposits located in the

Dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

Quarry in the

Kitadani Formation The Kitadani Formation ( ja, 北谷層 ''Kitadani-sō'') is a unit of Lower Cretaceous sedimentary rock which crops out near the city of Katsuyama in Fukui Prefecture, Japan, and it is the primary source of Cretaceous-aged non-marine vertebrate f ...

, near the city of Katsuyama which lies alongside valley of the Sugiyamagawa River. It was described by Tsubamoto and Rougier in 2004 keeping the Holotype at the National Science Museum, Tokyo, Japan.

Description

The type specimen (NSM PV 20562, holotype) is known from a fragmentary right

jaw The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serv ...

with the first

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 ...

and five postcanine teeth preserved. ''Symmetrolestes'' is more derived than zhangheotheriids as it had acute−angled molariform teeth with completely developed shearing surfaces, taller crowns on it teeth and more complete cingulids. It differs from other spalacotheriids due to the fact it had fewer molariform teeth, a higher number of premolariform teeth and gradual transition between premolariforms and molariforms. The jaw is gracile, slender, and never reaches more than 1.5 times the height of the teeth.T. Tsubamoto, G. W. Rougier, S. Isaji, M. Manabe, and A. M. Forasiepi. 2004. New Early Cretaceous spalacotheriid "symmetrodont" mammal from Japan. Acta Palaeontologica Polonica 49(3):329-346

Etymology

''Symmetrolestes'' means "symmetric hunter", The root Symmetro is in reference to the symmetric aspect of the molars and the root lestes meaning "hunter", a common ending of the taxonomic names of most

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

mammals based on the dubious hunting habits of these of such mammals. The species name ''parvus'' means small, in reference to its small stature.

Classification

A cladistic analysis that was made shows that ''Symmetrolestes'' is a sister group to other Spalacotheriidae. The scientists went on to say that the combination of the occurrences of the more primitive spalacotheriids and ''Symmetrolestes'', in Japan and of Zhangheotheriidae, which is the sister taxon of Spalacotheriidae, in China suggests a possibility the East Asian origins of the group Spalacotheriidae.

Paleoecology

The type specimen of ''Symmetrolestes'' was found in

Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 121.4 ± 1.0 Ma). It is a subdivision of the Early Cretaceous Epoch (or Lower Cretaceous Series). It is precede ...

- Aptian layers in the Kitadani Formation, the formation belongs to the Tetori Group in which is located in Central Japan. The formation shows a wide array of fauna, mainly many species of plants like cycads and conifers, which are mainly represented by cones and shoots. Animal fauna mainly consisted of dinosaurs such as the medium-sized

theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...

Fukuiraptor ''Fukuiraptor'' ("thief of Fukui") was a medium-sized megaraptoran theropod dinosaur of the Early Cretaceous epoch (either Barremian or Aptian) that lived in what is now Japan. ''Fukuiraptor'' is known from the Kitadani Formation and possibly ...

'', the small

ornithopod Ornithopoda () is a clade of ornithischian dinosaurs, called ornithopods (), that started out as small, bipedal running grazers and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous wo ...

'' Fukuisaurus'',, and the giant sauropod ''

Fukuititan ''Fukuititan'' (meaning " Fukui giant") is a genus of titanosauriform sauropod dinosaur that lived in the Early Cretaceous (either Barremian or Aptian age) in what is now Japan. It is known from FPDM-V8468, the associated partial skeleton of a ...

''. Smaller dinosaurs and basal birds like ''

Fukuivenator ''Fukuivenator'' ("hunter of Fukui Prefecture") is an extinct genus of therizinosaurian theropod dinosaur from the Early Cretaceous of Japan. Discovery and naming The type species ''Fukuivenator paradoxus'' was in 2016 named and described b ...

'' and ''

Fukuipteryx ''Fukuipteryx'' is an extinct genus of basal avialan dinosaurs found in Early Cretaceous deposits from Japan's Kitadani Formation. It contains one species, ''Fukuipteryx prima''.Imai, T., Azuma, Y., Kawabe, S., Shibata, M., Miyata, K., Wang, M. ...

'' also coexisted with ''Symmetrolestes''. The formation also preserves fossils of crocodilians belonging to the group

Eusuchia Eusuchia is a clade of crocodylomorphs that first appeared in the Early Cretaceous with '' Hylaeochampsa''. Along with Dyrosauridae and Sebecosuchia, they were the only crocodyliformes who survived the K-T extinction. Since the other two clades ...

, turtle shells and the remains of prehistoric

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

s. Two other mammals from the Kitadani Formation remain undescribed. The Kitadani Formation was likely not an arid environment, but one that was more wet with meandering rivers. Volcanic sediments have also been found in the formation in the form of tuffs.

References

{{Taxonbar, from=Q84053459 Symmetrodonta Cretaceous mammals Cretaceous mammals of Asia Early Cretaceous mammals of Asia Prehistoric mammal genera