Athena Review Vol. 5, no. 1
Records of Life: Fossils as Original Sources
7. Precambrian Life: Late Proterozoic
The next step in the complexity of life was the association of eukaryotes into colonies, and the development of specialized structures and functions within a larger grouping of similar cells. There is an almost total lack of fossil evidence of any such organisms until the last part of the Precambrian, the Latest Proterozoic (dated at 600-542 mya), when evidence of complex marine organisms from about the same time frame of 570-550 mya appears in various places around the world. This stage is named Ediacaran after a fossil site in Australia which has yielded numerous fossils from this time frame. Prior to that, however, similar fossils had been reported from the Mistaken Point Formation in Novia Scotia in the late 19th century, which remains a major source for this period. Many contemporary fossil taxa have been also been reported during the past 20 years from the Duoshantuo Formation of southern China, and at various sites in
Russia
including along the White Sea near Archangelsk. Some of these formations
which predate the Edicarian deposits in Australia by 10-30 million
years are grouped in the Vendian biota.Fig.1: Dickinsonia costata, a marine invertebrate fossil from Ediacara, Australia dated at 558-555 mya (Spriggs 1947).
Ediacaran stage of the Precambrian (600-542 million years ago).
Significant evidence of Late Proterozoic biota came with the 1946 discovery of late Precambrian fossils at Ediacara in the Flinders Mountain Range in southern Australia. Reginald Spriggs (1947), a mining geologist, made the initial discoveries of fossils in a sandstone and quartzite formation dating from the Late Precambrian. Subsequently, Martin Glaessner (Glaessner and Daly 1959) and Mary Wade (1968; Glaessner and Wade 1966) made detailed analyses of the fossils, providing an initial definition of the Ediacaran biota.
The fossils from Ediacara, dating from about 560-550 mya, constitute among the first complex, non-microscopic organisms known in the fossil record. They, and many other examples now known from Novia Scotia, Ukraine, Russia, and China, show a diverse range of forms not easily classified. While these precede the better known, and much more diverse Cambrian biota by only about 20-30 million years, there is a discontinuity between most of the Ediacaran and Cambrian forms, suggesting at least partial extinction of many Ediacaran species by about 550 mya..
In some cases,the Ediacaran fossils resemble marine invertebrates including jellyfish, sponges, or sea pens who mainly stayed attached to the marine substrate, an example being Dickinsonia (fig.1). In other cases, such as the segmented form Spriggina (fig.2), they appear as more complex animals which may have been predators.
Fig.2: The Ediacaran fossil Spriggina, a segmented marine animal from 560-550 mya.
Like many of the Ediacara biota, the relationship of Spriggina to other groups is unclear. It bears some similarity to extant segmented forms such as polychaetes or sea worms. The bottom of Spriggina is covered with two rows of tough interlocking plates, while one row of plates covered its top, whose front two segments fused to form a head. The frontmost segment was in the shape of a horseshoe (fig.2), with a pair of depressions on its upper surface which may represent eyes. The second segment may have borne antennae.
Vendian stage: Duoshantuo Formation in China (590-565 mya).
In central Guizhou province, near Weng’an, late Proterozoic sedimentary rocks are represented by the Nantuo, Doushantuo, and Dengying formations. The lowermost Nantuo Formation (620 to 590 mya) is overlain by the early Vendian Doushantuo Formation (590-565 mya). The Duoshantuo is represented by a 33-55 m thick formation at Weng’an, consisting of dark phosphate, cherty phosphate, chert, and gray dolomite. Preservation in the phosphate layers is excellent, with soft tissues sometimes preserved to the cellular level (Li et al 1998; Xuan et al 2000). The overlying Dengying Formation is a 180 m thick dolomite sequence extending from Ediacaran to basal Cambrian deposits (565 to 544 mya).
Fig.3: Detail of preserved thallus tissue of Thallophyca corrugata, a marine algae or seaweed from the Duoshantuo Formation. The arrow points to cells showing nuclei and chloroplasts (after Li et al. 1998, fig.3).
The Duoshantuo biota includes a diverse, well-preserved floral assemblage, including multicellular thallophytes, acritarchs, and cyanophytes. The best-preserved specimens include thallus tissues that resemble those of modern seaweed (fig.3; Li et al. 1998). Also present are marine fauna that closely resemble sea pens (Cnidarians) or sponges (Xiao et al. 1998, 2000).
Thallus tissues from the Duoshantuo algae lack specialized cell structures but have differing structural forms in the same individual species. A present-day analogy is provided by kelp (fig.4), a well known brown algae of the order Laminariales. While kelp has various parts that in someways resemble those of land plants, their thallus tissue lacks the specialized, distinctive vascular structures of roots, branches, or leaves found in land plants. Kelp has rootlike tendrils called holdfasts, a central stem or stipes, and leaf-like blades, all composed of similar thallus cells which are homogeno
us in
structure. In spite of its having these various structures,
all cells from all parts have chloroplasts and perform photosynthesis.Fig.4: Kelp of the genus Laminaria, a familiar brown algae or seaweed.
The thallus tissue of algae may be contrasted to the more specialized vascular tissues of land plants, where only the leaf cells have chloroplasts. Root cells of land plants are specialized to absorb water, and their stem or trunk cells are designed for both structural strength and the transmision of liquids. Vascular plant structures did not begin to develop until the Silurian and Devonian periods (440-360 mya), about 40-50 million years after green algae of the Chlorophyta division (fig.5) started to colonize shorelines and estuaries, and evolved into land plants.
Fig.5: Sea Lettuce (Ulvo sp.), a green algae found in both sea water and fresh water.
References:
Glaessner, M.F. and B. Daly 1959. "The Geology and Late Cambrian Fauna of the Ediacara Fossil Reserve." Records of the South Australian Museum 13, pp. 369-401.
Glaessner, M.F.and M. Wade, 1966. "The late Precambrian fossils from Ediacara, South Australia". Palaeontology 9 (4).
Li, C-W, J-Y Chen, and T-E Hua 1998: "Precambrian Sponges with Cellular Structures." Science 279, pp. 879 - 882.
Spriggs, R.J. 1947. "Early Cambrian (?) Jellyfishes from the Flinders Ranges, South Australia." Transactions of the Royal Society of South Australia 71, pp. 212-224.
Taylor, E.L., T.N. Taylor, and M. Krings 2009. Paleobotany: The Biology and Evolution of Fossil Plants. New York, Academic Press.
Wade, M.J. 1968. "Preservation of soft-bodied animals in Precambrian sandstones at Ediacara in South Australia." Letheia 1, pp. 238-267.
Xiao, S., Y. Zhang, and A.H. Knoll 1998. "Three-Dimensional Preservation of Algae and Animal Embryos in a Neoproterozoic Phosphorite." Nature 391, pp. 553-558.
Xiao, S., X.Yuan, and A.H. Knoll 2000: "Eumetazoan Fossils in Terminal Proterozoic Phosphorites?" Proceedings of the National Academy of Sciences of the United States, 97 (25), pp. 13684-13689.
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