Discovery of 3-dimensional algae and animal embryo fossils

A phosphorite is a sedimentary rock composed chiefly of phosphate minerals. The Proterozoic era (also called the Algonkian) is the geologic time between the Archean and the Paleozoic, with the Archean beginning about 3.9 billion years ago and involving the first appearance of sedimentary rocks and the first primitive organisms at the bottom of the oceans. In paleontology, the term "radiation" refers to a diverging and diversifying spread of animals or plants into new environments with a resultant production of new evolutionary forms, and the Ediacaran radiation refers to an assemblage (until now the oldest) of soft- bodied marine animals, the assemblage first discovered in the Ediacara Hills in Australia. The algae comprise a large mixed group of photosynthetic and essentially single-celled plants, and are considered ancestral to modern green plants. Thalli are primitive types of plant bodies not differentiated into stems, leaves, and roots; the term also refers to the gametophyte generation (the phase of the plant life cycle producing reproductive cells) of some ferns and lichens. The term "cleavage stages" refers to the early stages of embryo formation when the egg cell rapidly divides into smaller and smaller cells. The "Bilateria" are a major division of the animal kingdom comprising all forms with bilateral symmetry, and the term "bilaterians" refers to the first such forms appearing after the emergence of protozoa. The term "phylogeny" refers to the evolutionary history of an organism or group of organisms. ... ... Xiao et al (3 authors at 2 installations, US CN) report the discovery of phosphorites of the late Neoproterozoic (570 +- 20 million years ago) in the Doushantuo Formation, southern China, an apparent exceptional record of multicellular life from just before the Ediacaran radiation of macroscopic animals. Abundant thalli with cellular structures preserved in 3-dimensional detail show that late- Proterozoic algae already possessed many of the anatomical and reproductive features seen in modern marine flora. Embryos preserved in early cleavage stages indicate the divergence of lineages leading to bilaterians may have occurred well before their macroscopic traces or body fossils appear in the geological record. The authors suggest that discovery of these fossils shows that the early evolution of multicellular organisms is amenable to direct paleontological inquiry, and that paleontological observations, together with insights from molecular phylogeny and developmental genetics, can facilitate a modern integration of phylogeny, development, and paleontology that extends deeply into evolutionary history to address the early evolution of multicellular life.

QY: Andrew H. Knoll, Botanical Museum, Harvard University, 617-495-1000
Nature 5 Feb 98

Related Background:
The sponges (Porifera) are a phylum of primitive multi-cellular animals (Metazoa), always attached at one point to a substrate, usually without a definite symmetry, and usually marine. The sponge body is a loose aggregation of cells with little intercellular coordination, but the cells are specialized into various types with various functions important to the viability of the entire organism. There is also in sponges an internal "skeleton" of chalk, silica, or protein, and "calcareous sponges" are sponges containing a relatively large amount of calcium carbonate. In this report, "spicules" are the hard internal structures in sponges composed primarily of silica or calcium salts, and a "monoaxonal" spicule is a spicule having essentially only one dimension (e.g., needle-like). The term "epidermis" refers to the outermost layer of cells in any multicellular organism; "porocytes" are tubular cells that constitute the walls of certain sponges; "amoebocytes" are freely moving cells (sponge cells in this context) within a metazoan tissue; "sclerocytes" are cells involved in the formation of the sponge skeleton; "spongocoel" refers to the branching internal cavity of a sponge, the cavity having a connection in one place or another to the external aqueous environment; "flagella" are long and thin cellular organelles that protrude from the surfaces of cells and are specialized to produce locomotion. The Cambrian period extended from 545 to 505 million years ago, and was the time during which many multicellular organisms first arose, and the Vendian period is the Precambrian metazoan fossil period. The term "Cambrian explosion" refers to the apparent relatively sudden appearance of an enormous number of living forms during the Cambrian period. ... ... Li et al (3 authors at 2 install- ations, TW CN) report the identification of sponge remains in the Early Vendian Doushantuo phosphate deposit (cf. report #8, this issue) in south China. The skeletons consist of siliceous monoaxonal spicules, with preserved soft tissues including the epidermis, porocytes, amoebocytes, sclerocytes, and spongocoel, and among thousands of metazoan embryos a sponge larva having a shoe-shaped morphology and dense peripheral flagella. The authors suggest the data indicate the calcareous sponges may have an extended history in the Late Precambrian, and that animals lived 40 to 50 million years before the Cambrian explosion. They further suggest these Doushantuo rocks provide a potentially inexhaustible resource for understanding the early evolution of animal life.

QY: Chia-Wei Li, National Tsing Hua Univ., Hsinchu, Taiwan, China
(Science 6 Feb 98)

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