Abstract
Sediments of the Torridonian sequence of the Northwest Scottish Highlands contain a wide array of microfossils, documenting life in a non-marine setting a billion years ago (1 Ga).1-4 Phosphate nodules from the Diabaig Formation at Loch Torridon preserve microorganisms with cellular-level fidelity,5,6 allowing for partial reconstruction of the developmental stages of a new organism, Bicellum brasieri gen. et sp. nov. The mature form of Bicellum consists of a solid, spherical ball of tightly packed cells (a stereoblast) of isodiametric cells enclosed in a monolayer of elongated, sausage-shaped cells. However, two populations of naked stereoblasts show mixed cell shapes, which we infer to indicate incipient development of elongated cells that were migrating to the periphery of the cell mass. These simple morphogenetic movements could be explained by differential cell-cell adhesion.7,8 In fact, the basic morphology of Bicellum is topologically similar to that of experimentally produced cell masses that were shown to spontaneously segregate into two distinct domains based on differential cadherin-based cell adhesion.9 The lack of rigid cell walls in the stereoblast renders an algal affinity for Bicellum unlikely: its overall morphology is more consistent with a holozoan origin. Unicellular holozoans are known today to form multicellular stages within complex life cycles,10-13 so the occurrence of such simple levels of transient multicellularity seen here is consistent with a holozoan affinity. Regardless of precise phylogenetic placement, these fossils demonstrate simple cell differentiation and morphogenic processes that are similar to those seen in some metazoans today.
Highlights
The morphology of the new multicellular organism consists of a spheroidal mass of mutually adpressed cells enclosed by a peripheral layer of elongate, sausage-shaped cells
Further investigation revealed a second set of cell clusters that appeared very similar in size and form but that lacked the fully differentiated second cell type. We describe these interesting fossils and show intermediate morphologies that are consistent with an ontogenetic series driven by a differential cell-adhesion model
There is no evidence that these interior cells possessed rigid cell walls, because the shape of each cell is established by mutual compression with adjacent cells
Summary
The morphology of the new multicellular organism consists of a spheroidal mass of mutually adpressed cells enclosed by a peripheral layer of elongate, sausage-shaped cells. In well-preserved specimens, such inclusions occur in about half of the interior cells. These might represent preserved nuclei, but we consider that, more likely, they are the condensed remains of the entirety of the cytoplasmic cell content.[6] The cells of the stereoblast retain mutually compressed walls, so that the original multicellular topology, including Y-shaped junctions[17] (Figures 2B and 2C, circles), is retained. There is no evidence that these interior cells possessed rigid cell walls, because the shape of each cell is established by mutual compression with adjacent cells. This indicates the likelihood that individual cells were bounded by just a cell membrane or a thin, non-rigid cell wall.
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