Abstract

A diverse and abundant organic-walled microphytoplankton assemblage is reported from two measured sections of the Lower Devonian (Lochkovian) Ross Formation in Benton and Decatur counties, Tennessee, USA. The palynoflora comprises 24 genera and 34 species of acritarchs, including one new genus and species (Caulissoma gordonii); and five genera and 11 species of prasinophyte phycomata (excluding Leiosphaeridia and Tasmanites). Other components include chitinozoans, miospores, and scolecodonts. This is the first Early Devonian acritarch/prasinophyte assemblage to be described from Tennessee, and it is assigned a Lochkovian age based on marine invertebrate faunas, stratigraphic relationships, and the palynomorph assemblage. A high degree of similarity (67% commonality) exists between this palynoflora and that from the stratigraphic and age-equivalent Haragan and Bois d’Arc formations of Oklahoma. Appreciable numbers of the microphytoplankton taxa identified herein are common to the Haragan and Bois d’Arc formations and are, moreover, restricted to the Lochkovian and endemic to Laurentia. Additionally, a number of species are widely distributed and confined to the Early Devonian, and three species, Demorhethium lappaceum, Riculasphaera fissa, and Thysanoprobolus polykion – all cosmopolitan – are constrained to the Lochkovian. With increasing knowledge of Early Devonian palynofloras, the apparent degree of provincialism is progressively declining as seemingly endemic taxa are reported from more regions in both hemispheres. The Ross Formation sediments were deposited in a low-energy, offshore, normal marine environment, punctuated by intermittent episodes of shallow-water current and storm deposition.

摘要

美国田纳西州本顿县和迪凯特县下泥盆统（洛克科夫阶）罗斯组的两个测量剖面报告了一个多样化且丰富的有机壁微浮游植物组合。孢粉总目包括24属34种的无性动物，其中包括1个新属新种（Caulissoma gordonii）；和 5 属 11 种 prasinophyte phycomata （不包括Leiosphaeridia和Tasmanites）。其他成分包括几丁质动物、小孢子和scolecodonts。这是从田纳西州描述的第一个早泥盆纪 acritarch/prasinophyte 组合，它根据海洋无脊椎动物群、地层关系和孢粉组合被指定为 Lochkovian 时代。这种孢粉植物群与来自俄克拉荷马州的地层和年龄相当的 Haragan 和 Bois d'Arc 地层的孢粉菌群之间存在高度相似性（67% 的共性）。此处确定的大量微型浮游植物分类群在 Haragan 和 Bois d'Arc 地层中很常见，而且仅限于 Lochkovian 和 Laurentia 特有。此外，一些物种广泛分布并局限于早泥盆世，三个物种，Demorhethium lappaceum，Riculasphaera fissa和Thysanoprobolus polykion - 都是世界性的 - 被限制为 Lochkovian。随着对早期泥盆纪孢粉菌群认识的增加，地方主义的明显程度正在逐渐下降，因为在两个半球的更多地区都报告了看似地方性的分类群。罗斯组沉积物沉积在低能量、近海、正常的海洋环境中，不时出现浅水流和风暴沉积的间歇性事件。