Morphometric redefinition of the component chronospecies of the Globorotalia conoidea - G. inflata lineage in DSDP Hole 29-284

Phylo-zonations (or lineage-zonations) are based upon morphological changes within individual evolutionary lineages. These zonations, although potentially of use for stratigraphic subdivision and correlation, often suffer from a lack of quantitative exactness in the definitions of chronospecies. Thus exact reproducibility is hindered for stratigraphic determinations.
The potential of morphometrically defined phylo-zonations is demonstrated on a temperate South Pacific Late Cenozoic lineage of planktonic foraminifera (Globorotalia conoidea through intermediate forms to Globorotalia inflata in DSDP Site 284) exhibiting phyletic gradualism. Our sampling interval is about 0.1 m.y. during the last 8 m.y. Changes in the number of chambers in the final whorl, test conicalness, percentage of keeled forms, and test roundness or inflatedness, are used to quantitatively define the following five chronospecies: G. conoidea (Late Miocene; 6.1->8.3 m.y.), G. conomiozea (latest Miocene ; 5.3-6.1 m.y.), G. puncticulata sphericomiozea (earliest Pliocene; 4.5-5.3 m.y.), G. puncticulata puncticulata (Early-Middle Pliocene; 2.9-4.5 m.y.), and G. inflata (Late Pliocene-Quaternary; 0-2.9 m.y.). This phylo-zonation is directly applicable to temperate cool subtropical Southern Hemisphere areas where the evolution took place (Kennett, 1967, 1973; Scott, 1979). It is still not known if the lineage occurs elsewhere; thus the applicability of the phylo-zonation over broader areas is still uncertain. Trends in general size and aperture shape seem to be climatically controlled, and thus may be only of local stratigraphic utility.
The practical applications of morphometric phylo-zonation for stratigraphy is to a large extent dependent upon the amount of time and effort required to statistically define the trends. Experiments with large numbers of subsamples from this lineage demonstrate that accurate stratigraphic determinations are possible from measurements on only 15 specimens per sample, except for those very close to chronospecies boundaries.

谱系分带（phylo-zonations，又称世系分带lineage-zonations）的建立基于单个演化世系内的形态变化。尽管此类分带可用于地层划分与对比，但在年代种的定义上往往缺乏定量精确性，因此地层鉴定的精确可复现性受到阻碍。

本次研究以南太平洋温带晚新生代浮游有孔虫世系（深海钻探计划DSDP 284站位中，从锥状球室虫Globorotalia conoidea经中间过渡类型演化为膨胀球室虫Globorotalia inflata，该世系呈现系谱渐变特征）为例，验证了基于形态计量学定义的谱系分带的应用潜力。本研究在过去8百万年的时间跨度内，采用约0.1百万年的采样间隔，通过最终壳圈的房室数量、壳体锥度、具龙骨壳占比、壳体圆度或膨凸度等指标，定量界定了以下5个年代种：锥状球室虫G. conoidea（晚中新世；6.1~8.3百万年）、近锥球室虫G. conomiozea（晚中新世末期；5.3~6.1百万年）、粒突球室虫球形亚种G. puncticulata sphericomiozea（早上新世；4.5~5.3百万年）、粒突球室虫指名亚种G. puncticulata puncticulata（早-中上新世；2.9~4.5百万年）以及膨胀球室虫G. inflata（晚上新世-第四纪；0~2.9百万年）。该谱系分带可直接应用于演化发生地所在的温带凉亚热带南半球区域（Kennett, 1967, 1973; Scott, 1979），目前尚不清楚该世系是否在其他区域存在，因此其在更大范围的适用性仍不明确。壳体总大小和壳口形状的演化趋势似乎受气候调控，因此仅具有局地地层学应用价值。

形态计量学谱系分带在地层学中的实际应用价值，在很大程度上取决于统计界定演化趋势所需的时间与人力成本。针对该世系开展的大量子采样实验表明，除年代种分界处附近的样品外，仅需对每个样品中的15个标本进行测量，即可实现精确的地层鉴定。