New species from the Pliocene of Tibet reveals origin of Ice Age mountain sheep
Modern wild sheep, Ovis, is widespread in the mountain ranges of the Caucasus through Himalaya, Tibetan Plateau, Tianshan-Altai, eastern Siberia, and the Rocky Mountains in North America. In Eurasia, fossil sheep are known by a few isolated records at a few Pleistocene sites in North China, eastern Siberia, and western Europe, but are so far absent from the Tibetan Plateau.
|Holotype of Protovis himalayensis, in frontal-lateral view (A) and dorsal view of horncores (B) , |
and cross-sectional shapes at four intervals along left horn
[Credit: WANG Xiaoming]
New fossil materials were collected from IVPP locality ZD0712 in Guanjingtai, Zanda County, Tibetan Autonomous Region in western Himalaya during the 2006 and 2007 field seasons. The holotype specimen (IVPP V18928), forming the main basis of this new species, is a nearly complete male left and right horncores. With a total horncore upper curve length of 443 mm, it is similar in size to some extant species of Ovis.
This new extinct sheep, Protovis himalayensis, has a combination of features distinguishable from other species such as Ovis, Pseudois and Tossunnoria. Smaller than the living argali, it shares with Ovis posterolaterally arched horncores and partially developed sinuses and possesses several transitional characters leading to Ovis.
|Map of extinct and extant species of Ovis in Eurasia and their evolutionary relationships |
[Credit: WANG Xiaoming]
Carbon isotopes on fossil mammalian herbivores from Zanda Basin indicate that C3 vegetation formed the dominant plant community during the Pliocene. It is likely that Protovis, too, had a C3 diet, as do the modern bovids within the Tibetan Plateau.
Ancestral sheep in the Tibetan Plateau, occupying a similar range as the extant argali, were adapted to high-elevation, cold environments during the Pliocene, when conditions elsewhere (including the high Arctic regions) were much warmer. These ancestral stocks evolved rapidly to morphological conditions similar to that of living Ovis. By the time the Ice Age arrives around 2.6 million years ago, Ovis possessed a competitive advantage for surviving in freezing environments and spread rapidly to regions surrounding the Plateau and beyond. Most sheep species survived along their Pleistocene route of dispersal, offering a highly consistent pattern of zoogeography.
|Artist reconstruction of a male Zanda sheep, Protovis, placed in a modern Zanda |
basement outcrop that was widely exposed during basin formation
[Credit: Julie Selan and WANG Xiaoming]
“Both this new fossil datum and the existing molecular phylogeny suggest that the Tibetan Plateau, possibly including Tianshan-Altai, represents the ancestral home range(s) of mountain sheep”, said study coauthor Dr. LI Qiang of the IVPP, “Fortunately, wild sheep were able to take refuge in mountain ranges, possibly an important contributing factor in protection against early human hunting, and they have largely survived the end-Pleistocene extinction that befell many of their megafaunal contemporaries”.
Source: Chinese Academy of Sciences [May 11, 2016]