早期生命形态演化研究中的定量方法

定量古生物学是现代古生物学的一个分支,提倡用定量的手段来研究地质历史时期生命的演化过程。我国从事定量古生物研究的群体较小,特别是对前寒武纪早期生命演化的定量研究还没有系统地展开。这篇文章将主要介绍如何利用定量手段来研究前寒武纪化石的形态演化。对于前寒武纪化石,由于大部分化石分类属性的不确定性,通常使用几何性状对化石的最基本形态结构进行分析,并用存在/缺失(1/0)这种离散变量对每个性状进行量化。非参数多维标量分析方法Non-parametric multidimensional scaling analysis(MDS)]可以将高维度的离散数据投影到二维或者三维的形态空间上,进而探讨生物群在形态空间中所占有的范围;由离散变量计算得出的生物群的表形分异度(morphological disparity)可以用MDS方差或者平均差异参数Mean dissimilarity coeffi-cient(MDC)]来计算。形态空间的范围(morphospace range)和表形分异度是相互联系的,如果形态空间范围是固定的,那么表形分异度实际上代表了生物群在形态空间中的分布密度。在解释数据之前,需要对可能存在的样本效应进行测试。常用的方法包括稀释法(rarefaction)、随机取样法(randomization)和自举法(bootstrapping)等。为了帮助读者进一步了解这些方法的使用,文中列举了三个实例:伊迪卡拉生物的形态演化,元古代宏观藻类的形态演化和元古代及寒武纪疑源类的演化。

QUANTITATIVE METHODS IN MORPHOMETRIC STUDY OF EARLY LIFE

Quantitative paleobiology is a well established field of paleobiology.One of its objectives is to resolve evolutionary patterns using quantitative methods.However,this field is still in its nascent stage in China;particularly,the application of quantitative methods in Precambrian paleobiology has been sparse but will become more important as the database of Precambrian fossils grows.The purpose of this article is to describe some quantitative methods that can be applied in the morphometric study of Precambrian organisms.The phylogenetic affinity of many Precambrian fossils remains obscure,and their morphological features are more objectively described by a series of presence/absence(1/0) characters,rather than multivariate continuous measurements.Non-parametric multidimensional scaling analysis can be used to ordinate multi-dimensional morphological data into two-dimensional or three-dimensional morphospace,so that the range of occupied morphospace can be visualized and quantified.Furthermore,the morphological disparity measured by discrete characters can be calculated as MDS variance or mean dissimilarity coefficient(MDC).The range of morphospace and morphological disparity,as the two most important parameters in quantifying morphological evolution,are related.Indeed,if morphospace range is fixed,then MDC can be understood as a measure of morphospace occupation density.Possible sampling bias must be considered and addressed before conclusions about morphological evolution can be drawn.Widely used methods in testing sampling bias and estimating confidence intervals include rarefaction,randomization,and bootstrapping.We use three case studies-morphological evolution of the Ediacara biota,Proterozoic macroalgae,and Proterozoic—Cambrian acritarchs-to illustrate the application of these methods.