Sunday, January 31, 2016

[PaleoMammalogy • 2016] Ptilocercus kylin • An Early Oligocene Fossil Demonstrates Treeshrews (Scandentia) are Slowly Evolving “Living Fossils”

Ptilocercus kylin 
 Li & Ni, 2016

Treeshrews are widely considered a “living model” of an ancestral primate, and have long been called “living fossils”. Actual fossils of treeshrews, however, are extremely rare. We report a new fossil species of Ptilocercus treeshrew recovered from the early Oligocene (~34 Ma) of China that represents the oldest definitive fossil record of the crown group of treeshrews and nearly doubles the temporal length of their fossil record. The fossil species is strikingly similar to the living Ptilocercus lowii, a species generally recognized as the most plesiomorphic extant treeshrew. It demonstrates that Ptilocercus treeshrews have undergone little evolutionary change in their morphology since the early Oligocene. Morphological comparisons and phylogenetic analysis support the long-standing idea that Ptilocercus treeshrews are morphologically conservative and have probably retained many characters present in the common stock that gave rise to archontans, which include primates, flying lemurs, plesiadapiforms and treeshrews. This discovery provides an exceptional example of slow morphological evolution in a mammalian group over a period of 34 million years. The persistent and stable tropical environment in Southeast Asia through the Cenozoic likely played a critical role in the survival of such a morphologically conservative lineage.

Figure 1: Upper and lower dentition (in color) of Ptilocercus kylin sp. nov., compared with P. lowii (USNM 32409, in gray-scale).
(A) Crown view of the upper dentition. Fossils include the lingual half of a left M1 (IVPP V20689, reversed), the buccal half of a right M1 (IVPP V20690), a complete right M2 (IVPP V20691), and the buccal half of a left M3 (IVPP V20692, reversed). I1-2, C, P2-4, and M1-3 are shown for P. lowii. (B) Crown view of the lower dentition. Fossils include an isolated right lower canine (IVPP V20693), a right jaw fragment preserving p3-4 and the alveoli for i2-3, c, and p2 (IVPP V20694), an isolated right m1 (IVPP V20695), and a right jaw fragment preserving m2-3 (IVPP V20696, holotype). The alignment of the fossils is based on a left lower jaw fragment retaining a small portion of the i2-3 alveoli, alveoli and roots of c and p2, and p3-m3 (IVPP V20699, Fig. 2 and Supplementary Information). The i1-3, c, p2-4, and m1-3 are shown for P. lowii. (C), Lingual view of the lower dentition of P. kylin. (D), Buccal view of the dentition of P. kylin. Scale bar equals 5 mm.

Ptilocercus kylin sp. nov.

Etymology: Specific epithet is derived from the name of Qilin District, in Qujing City. Qilin is the pinyin for kylin, a hoofed dragon-like beast of Chinese myth.

Holotype: IVPP V20696 (Fig. 1), a right mandibular fragment preserving m2 and m3.

Locality and horizon: Lijiawa Mammalian Fossil locality, Yunnan Province, China. Earliest Oligocene, ~ 34 Ma.

Figure 3: Summary phylogeny of treeshrews.

Figure 4: Ptilocercus treeshrew distribution in the context of southern Asia’s modern geography and early Oligocene palaeogeography.
(A) Fossil locality of Ptilocercus kylin sp. nov. (blue dot) and the distribution of the living species Ptilocercus lowii (pale reddish shading). The background map is from: (under the Creative Commons Share Alike license: (B) Fossil locality (blue dot) and reconstructed palaeogeographic distribution of the closed canopy of tropical rain forest and monsoonal forest (pale reddish shading) in the early Oligocene. The palaeogeographic reconstruction is from ref. 46 (Nature Publishing Group License: 3646200322068). The position of the fossil locality on the palaeogeographic reconstruction was estimated based on its distance from the Tibetan Plateau and the Sino-Burman Ranges (SBR).

Qiang Li and Xijun Ni. 2016. An Early Oligocene Fossil Demonstrates Treeshrews are Slowly Evolving “Living Fossils”. Scientific Reports. 6; 18627. DOI: 10.1038/srep18627

Earliest-known treeshrew fossil found in Yunnan, China via @physorg_com

No comments:

Post a Comment