更新世代表性哺乳动物群的生物年代排序及与测年值的对比

BIOCHRONOLOGIC SEQUENCES OF THE PLEISTOCENE MAMMALIAN FAUNAS IN CHINA AND CORRELATIONS WITH NUMERIC DATING

哺乳动物随着环境的变化不断演化出与原来不同的特征,因此可以根据化石标本的形态特征判断它们在系统演化中的位置,并根据动物群中各个种类的演化程度判断不同动物群之间的生物年代顺序。本项研究在传统的绝灭率、共有属种、二元相似性系数等方法的基础上尝试拓展了二元共有度和排序梯度等共9种方法,对选自中国境内有代表性的15个更新世哺乳动物群进行了生物年代排序。自老至新的动物群以其产地为名的生物年代合意顺序及推测的绝对年龄为:泥河湾下沙沟(1.75~2.2 Ma)-繁昌人字洞(1.7~2.1 Ma)-巫山龙骨坡(1.5~1.9 Ma)-元谋人遗址(1.4~1.8 Ma)-建始龙骨洞(1.3~1.6 Ma)-柳城巨猿洞(1.15~1.5 Ma)-蓝田公王岭(1.1~1.3 Ma)-崇左三合大洞(0.6~1.2 Ma)-和县人遗址龙潭洞(490~800 ka)-周口店第一地点下部(460~770 ka)-周口店第一地点上部(230~450 ka)-海城小孤山(30~80 ka)-本溪山城子(约30 ka)-大连古龙山(27~30 ka)-周口店山顶洞(18~27 ka)。研究表明,使用不同的方法对不同的动物群进行生物年代排序的结果在大尺度范围内是一致的,但在小尺度范围内会出现一些差异,这与排序标准和统计计算方法有关。一旦化石标本的分类鉴定结果和生物年代排序的方法确定下来,动物群的生物年代序列就基本上是固定的,不会随研究者的更换而改变,因此可以用来检验绝对年龄的测年结果。由于组成各个动物群的种类可能会因鉴定人员的不同而有差别,造成生物年代排序结果与测年结果差异较大。因此有必要组织同一团队的研究人员核对各个地点的化石种类鉴定结果,并与测年工作配合进行。和县人遗址的龙潭洞动物群在本项研究的生物年代排序中早于周口店第一地点,这个动物群的化石标本以及产出地层值得重新观察和测年。根据本项研究的实践,这类数字化排序的方法还有细化的发展空间。

Morphological characters of mammals evolve by adapting environmental changes and that offers us key clues to understand their phylogenetic positions and thus to line up the chronological sequences of different faunas based on the phylogenetic positions of every taxon of a fauna. The present work selected 15 mammalian faunas of the Pleistocene from northern,southern,eastern and western China for biochronological sequencing,with nine methods such as traditional extinction rates,the new presence of modern taxa,binary similarity coefficients,as well as a series of binary co-possession indices developed by the present authors. The extinction rates and modern taxon rates are calculated with a single fauna,i.e. the percentages of the extinct taxa over total taxa of a fauna,or the percentage of the modern taxa over total taxa of a fauna. While binary similarity coefficients are based on at least two faunas. For example,if a taxon is present or absent in two faunas,it means that both faunas show unique similarity with this taxon and their binary matching coefficient with the taxon is defined as 1. But if the taxon is present in one of the two faunas only,their binary matching coefficient with this taxon is defined as 0. The sum of the matching coefficients of all taxa of the compared two faunas is defined as the binary similarity of the paired faunas. All compared faunas can then be sequenced according to Brainerd-Robinson’s rule based on the binary similarity coefficients of each pair of faunas. While binary co-possession coefficient of a pair of faunas developed by the authors defines binary matching coefficient as 1 only when a taxon is possessed by both compared faunas,and as0 when it possessed by only one of two faunas or absent in both faunas. The sum of the matching coefficients of all taxa of the compared two faunas is defined as the binary co-possession coefficient of the paired faunas. All taxa survived from the Neogene or the early Early Pleistocene are regarded as a presumed archaic fauna,and all extant taxa are regarded as a presumed modern fauna. The binary similarity coefficient between a selected fauna and the archaic fauna is defined as antiquity coefficient of the selected fauna,and that between a selected fauna and the modern fauna is defined as modern coefficient of the fauna. In the same way,the binary co-possession coefficient between a selected fauna and the archaic fauna is defined as shared antiquity coefficient of the selected fauna,and that between a selected fauna and the modern fauna is defined as shared modern coefficient of the fauna. All selected faunas can then be arranged in numerical order from the oldest to the youngest. The biochronological differences between sequenced faunas are not equal,but irregularly gradient. Such gradients are considered and all faunas are rearranged in consensus sequence.As long as the fossil specimens are taxonomically identified,the methods of biochronological sequence are selected,the biochronological sequence of the selected faunas will remain the same regardless who process the sequencing. And that can help to verify the results of numeric dating. The present work shows that the consensus biochronological sequence and estimated ages of the selected Early Pleistocene mammalian faunas named by their yielding localities from the oldest to the youngest is as follow: Xiashagou in Nihewan Basin( 1. 75 ~ 2. 2 Ma)-Renzidong in Fanchang( 1. 7 ~ 2. 1 Ma)-Longgupo in Wushan( 1. 5 ~ 1. 9 Ma)-Yuanmou Man Site( 1. 4 ~1. 8 Ma)-Longgudong in Jianshi( 1. 3 ~ 1. 6 Ma)-Gigantopithecus Cave in Liucheng( 1. 15 ~ 1. 5 Ma)-Gongwangling in Lantian( 1. 1 ~ 1. 3 Ma)-Sanhe Dadong in Chongzuo( 0. 6 ~ 1. 2 Ma)-Longtandong of Hexian Man Site( 490 ~800 ka)-Lower layers of Locality 1 of Zhoukoudian Site( 460 ~ 770 ka)-Upper layers of Locality 1 of Zhoukoudian Site( 230 ~ 450 ka)-Xiaogushan in Haicheng( 30 ~ 80 ka)-Shanchengzi in Benxi( ca. 30 ka)-Gulongshan in Dalian( 27~30 ka)-Upper Cave of Zhoukoudian Site( 18 ~ 27 ka). The fauna from Hexian Man Site,formerly dated as150 ~ 300 ka,is sequenced as older than that of Locality 1 of Zhoukoudian Site,formerly dated as 230 ~ 770 ka,in the present work and that is contradict to the traditional sequence which is in reversed one. The taxonomic determination of the fauna specimens from Hexian Man site,as well as the numeric ages of the specimens and the deposits yielding the fauna are worth being reexamined.