Abstract
Hamelin Pool, Shark Bay, Western Australia, is home to the largest and most diverse assemblage of living marine stromatolites, with shapes and sizes comparable to ancient structures. A recent field-intensive program revealed seasonally ephemeral occurrences of modern dendrolitic microbial mats forming in intertidal, low energy settings. Dominated by filamentous cyanobacteria, dendrolitic microbial mats are formed when filaments provide a supporting framework as a result of gliding mobility, to build a shrubby morphology. Dendrolites, known throughout the rock record, refer to macroscopic microbialites with mesostuctures composed of unlaminated arborescent structures called shrubs. In these modern examples, thick filaments of Lyngbya aestuarii form the “trunk” of the bush, with finer filaments of Lyngbya fragilis, Phormidium sp. and Schizothrix sp. forming the “branches” These biologically-influenced dendrolitic structures provide insight into the complex interplay of microbial communities and the environment, broadening our understanding of shrub and dendrolite formation throughout the rock record.
Highlights
Complex life, complete with hard parts that could be fossilized and preserved in the rock record, did not appear until relatively late in the earth’s history
The maximum depth of Hamelin Pool is 11 m, resulting in a substantial annual temperature range and a nearly two-meter annual change in water level, with higher highs and lows leaving parts of the margin largely submerged in the Austral autumn months, and lower highs and lows exposing more of the margin during the Austral spring months [55,57,58]
Hamelin Pool shrubs are found in the form of irregularly spaced branching structures, composed of small clumps of cyanobacteria with entangled sediment grains, forming on the flat underlying microbial mat surface (Figure 2)
Summary
Complete with hard parts that could be fossilized and preserved in the rock record, did not appear until relatively late in the earth’s history. The fossilization of organisms within the rock record before the evolution of these skeletonized, complex organisms was rare, occurring only in environments with specific sedimentary conditions conducive to preservation [1]. Prior to the radiation of this higher, more complex life, the fossil record consists largely of three types of fossils: permineralized microfossils in chert, organic-walled microfossils preserved in siliciclastic rocks, and microbialites in limestone and dolostone. Topographic relief is the distinguishing characteristic of most microbialites, the complex interplay of biological versus environmental (e.g., physical and chemical) factors has resulted in many questions surrounding the genesis of particular structures [4,5,6,7,8,9,10]. Despite over 100 years of Geosciences 2018, 8, 212; doi:10.3390/geosciences8060212 www.mdpi.com/journal/geosciences
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