壮鼠化石在中国的首次发现

在我国北京长辛店和内蒙古四子王旗发现了壮鼠亚科的化石：杨氏东方壮鼠Eosischyromysyoungi新属种。它的齿冠较低，齿脊亦较低，齿尖较明显等特点表明它比北美目前已知的壮鼠都原始。这是壮鼠亚科化石在亚洲首次发现，扩大了壮鼠亚科的分布范围。特别是由于它比较原始，可能它出现的时代较早，进一步确认长辛店组的时代为中始新世晚期。

DISCOVERY OF ISCHYROMYINAE (RODENTIA, MAMMALIA)FROM THE MIDDLE EOCEM OF NORTH CHINA

Members of the subfamily Ischyromyinae, the most derived of the Paleogene rodent family Ischyromyidae, have previously been reported only from North America,where they are relatively abundant in deposits ranging in age from lare Middle Eocene through Middle Oligocene. The two lower jaws from North China described here arethe first record of the subfamily in Asia. This discovery both provides another example of faunal exchange between Asia and North America during the Middle Eocene and assists us in determining the age of the Changxindian Formation.