Symmetrolestes

Symmetrolestes is an extinct genus of small spalacotheriid mammal[1] 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 Quarry in the Kitadani Formation, 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.

Symmetrolestes
Temporal range: Late Barremian-Late Aptian 130–113 Ma
Symmetrolestes parvus (NSM PV 20562, holotype). A. Lingual stereo view of the dentary. B. Drawing of the jaw in approximately the same position as in A.
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: †Symmetrodonta
Family: †Spalacotheriidae
Genus: †Symmetrolestes
Type species
†Symmetrolestes parvus
Tsubamoto and Rougier, 2004

Description

The type specimen (NSM PV 20562, holotype) is known from a fragmentary right jaw with the first incisor 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.[2]

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 mammals based on the dubious hunting habits of these of such mammals. The species name parvus means small, in reference to its small stature.[2]

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.[2]

Paleoecology
Koshisaurus and other large dinosaurs shared their habitat with Symmetrolestes and other mammals

The type specimen of Symmetrolestes was found in Barremian-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.[3] Animal fauna mainly consisted of dinosaurs such as the medium-sized theropod Fukuiraptor,[4] the small ornithopod Fukuisaurus,[5] and the giant sauropod Fukuititan. Smaller dinosaurs and basal birds like Fukuivenator and Fukuipteryx also coexisted with Symmetrolestes. The formation also preserves fossils of crocodilians belonging to the group Eusuchia, turtle shells[6][7] and the remains of prehistoric mollusks. Two other mammals from the Kitadani Formation remain undescribed.[8]

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References
  1. Han, Gang; Meng, Jin (May 24, 2016). "A new spalacolestine mammal from the Early Cretaceous Jehol Biota and implications for the morphology, phylogeny, and palaeobiology of Laurasian 'symmetrodontans'". Zoological Journal of the Linnean Society. 178 (2): 343–380. doi:10.1111/zoj.12416.
  2. 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
  3. Yabe, A., Terada, K. and Sekido, S., 2003: The Tetori-type flora, revisited: a review. Memoir of the Fukui Prefectural Dinosaur Museum, 2:23–42.
  4. Azuma, Yoichi; Xu, Xing; Shibata, Masateru; Kawabe, Soichiro; Miyata, Kazunori; Imai, Takuya (April 2016). "A bizarre theropod from the Early Cretaceous of Japan highlighting mosaic evolution among coelurosaurians". Scientific Reports. 6 (1): 20478. Bibcode:2016NatSR...620478A. doi:10.1038/srep20478. ISSN 2045-2322. PMC 4763874. PMID 26908367.
  5. Shibaba, Masateru; Azuma, Yoichi (2015). "New basal hadrosauroid (Dinosauria: Ornithopoda) from the Lower Cretaceous Kitadani Formation, Fukui, central Japan" (PDF). Zootaxa. 3914 (4): 421–40. doi:10.11646/zootaxa.3914.4.3. PMID 25661952.|
  6. Sonoda T, Azuma Y, Hirayama R, Ando H. (2015) New trionychoid specimens and turtle fauna from the Lower Cretaceous Kitadani Formation of the Tetori Group in central Japan. PeerJ PrePrints 3:e949v1 https://doi.org/10.7287/peerj.preprints.949v1
  7. Hirayama, R. (2002) Preliminary report of the fossil turtles from the Kitadani Formation (Early Cretaceous) of the Tetori Group of Katsuyama, Fukui Prefecture, Central Japan. Memoirs of the Fukui Prefectural Dinosaur Museum. 1: 29-40.
  8. Miyata, Kazunori; Azuma, Yoichi; Shibata, Masateru (2016-02-17). "New mammalian specimens from the Lower Cretaceous Kitadani Formation, Tetori Group, Fukui, Japan". Historical Biology. 28 (1–2): 139–150. doi:10.1080/08912963.2015.1012509. ISSN 0891-2963.
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