南海北部渐新世钙质超微化石楔石属定量形态变化与演化关系研究

OLIGOCENE NANNOFOSSIL SPHENOLITHUS MORPHOMETRIC VARIATION AND EVOLUTIONARY LINEAGE:RECORDS FROM THE NORTHERN SOUTH CHINA SEA,CHINA

钙质超微化石楔石属的三个分子:Sphenolithus predistentus,Sphenolithus distentus,Sphenolithus ciperoensis是渐新世生物地层重要的化石带标志分子。这三个物种属于该属渐新世的一个演化序列,该演化序列早期的S.predistentus-S.distentus和晚期的S.distentus-S.ciperoensis两对物种之间存在形态过渡类型,使得难以准确鉴定,导致渐新世生物地层划分一些问题。此外,对这三个物种演化的关系是渐进式或是间断平衡式也有不同看法。为进一步了解这三个物种形态演化特征并探讨它们的演化关系,我们选择南海北部ODP 1148站位及IODP U1435站位的渐新世沉积物样品,针对这三个物种个体的形态特征展开定量形态研究。使用奥林巴斯光学显微镜对两个站位19个样品里1 171个楔石个体进行六个形态参数度量,包括了个体总高、顶刺高、基座高、基座宽、基座夹角及消光线夹角。统计结果显示,S.predistentus-S.distentus-S.ciperoensis演化序列中,这三个种的六个形态学参数变化范围边缘相互叠复,缺乏可确切划分界限。三个种个体基座高与基座宽之比依三者的演化次序呈现逐渐增大的渐变关系。以上证据表明S.predistentus-S.distentus-S.ciperoensis为渐进式演化。

Three Oligocene species of genus Sphenolithus, S. predistentus, S. distentus and S. ciperoensis, are im- portant Oligocene biostratigraphic zonal markers. Morphological transitions among these three species commonly lead to difficulty in identification and result in inconsistencies for Oligocene biostratigraphic subdivision. Additionally, scientists have different opinions on the evolution of this lineage. In order to quantify their morphological variation and understand the evolution of this lineage better, we conducted a quantitative morphological study on Sphenolithus specimens from Oligocene sediments at Ocean Drilling Program Site 1148 and International Ocean Discovery Program Site U1435. We analyzed a total of 1 171 Sphenolithus specimens from 19 samples from these two sites. Observation and morphometric measurements were made on light microscope images captured at 1 000 × magnification using an Olympus BXS1 microscope equipped with an Axiocam digital camera. Measured morphological parameters include to- tal height, apical cone height, proximal cycle height, basal width, basal intersection angle and extinction line angle. The results show that the variation of these six parameters amongst the three species partially overlap with each oth- er, with no clear boundaries to separate them. The evolution of S. predistentus to S. distentus in the middle Oligo- cene, and then to S. ciperoensis in the late Oligocene was characterized by an increase in the ratio between basal height and basal width. The results of this study suggest anagenetic evolution for the S. predistentus-S, distentus-S. ciperoensis lineage.