Sediments across the Earth contain numerous macrofossils (fossils of big things that can be found with the naked eye, as opposed to smaller microfossils which are found by sieving sediments or examining thin slices of rock under a microscope) from the Cambrian Explosion (542 million years ago) onwards, with numerous such fossils also now known from the Ediacaran Period (635-542 million years ago) and even some from the later part of the Cryogenian (720-635 million years ago). Earlier than this the fossil record is harder to interpret, with several disputed claims of multicellular organisms, as well as uncertainty as to whicj are the earliest cells that can be described as tuly Eukaryotic (having cells with a nulceus like animals and plants, as opposed to Prokaryotic cells which lack such nuclei, as in Bacteria and Archeans).
In a paper published in the journal Nature Communications on 17 May 2016, Shixing Zhu of the Tianjin Institute of Geology and Mineral Resources of the China Geological Survey and the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, Maoyan Zhu of the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, Andrew Knoll of the Department of Organismic and Evolutionary Biology at Harvard University, Zongjun Yin and Fangchen Zhao, also of the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, Shufen Sun, aslo of the Tianjin Institute of Geology and Mineral Resources of the China Geological Survey, Yuangao Qu of the Centre for Geobiology at the University of Bergen, Min Shi, also of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences and Huan Liu, again of the Tianjin Institute of Geology and Mineral Resources of the China Geological Survey, describe a series of Multicellular Eukaryotic fossils from the 1.56-billion-year-old Gaoyuzhuang Formation of North China.
At 1.56-billion years old the Gaoyuzhuang Formation is considered to be Calymmian, or Early Mesoproterozoic, in age, though these are definations based upon age rather fossil content as with later geologic periods. It comprises a series of calcarous shales and mudstones that outcrop across a wide area of North China, though fossils were found only at two locations, one in Qianxi County in Guizhou Province and the other in Kuancheng County in Hebei Province.
Zhu et al. found four different fossil forms, linear forms with parallel sides but no preserved ends, leaf-shaped forms which taper at one end, oblong forms with rounded ends and parallel sides, and tounge-shaped forms. All forms show well defined margins on at least two sides, unlike algal mats which spread amorphously, and one small example of a leaf-shaped form was found to have a stipe (stem) and holdfast similar to that seen in modern seaweeds.
Macroscopic fossils from the Gaoyuzhuang Formation. (a) Linear fossil without preservation of either end (a(1)) and fragment of tongueshaped fossil (a(2)), Qg98017. (b) Linear fossil without preservation of either end (b(1)) and tongue-shaped fossil with longitudinal striations (b(2)), Qg20011; (c,d) Cuneate fossils, 07kg1332 (c), Qg20017 (d). (e) Oblong fossil with possible holdfast, 07 kg1331. (f) Cuneate fossil preserved with differentiated holdfast, Qg98021; (g) linear fossil without preservation of either end. Scale bars, 5 cm (in a,b,g), 20mm (in c), 40mm (in d) and 5mm (in e,f). Zhu et al. (2016).
These morphologies strongly support the idea that these were photosynthetic multicellular organisms with pre-determined growth patterns, albeit simple ones, similar to moder marine Macro-Algae (Seaweeds). Of the three modern groups of Macro-Algae (Red, Green and Brown) two (Red and Green) have been predicted by some studdies to have shared a common ancestor in the Early Mesoproterozoic (Brown Algae are thought to be quite unrelated and much younger). The Gaoyuzhuang fossils could potentially be representatives of this ancestral Algal group, though the presence of similar morphologies in the unrelated Brown Algae indicates that this simple bodyplan has evolved at least twice in Eukaryotes, raising the possibility that similar forms could have arisen quite separately in an extinct group of Eukaryotes in the Early Mesoproterozoic.
In addition to examening macrofossils Zhu et al. took sediment samples from the Gaoyuzhuang Formation and treated them by acid maceration (mashing up and disolving in acid). This yielded a number of fragments of cellular material, up to 1 mm across, comprised of tightly packed masses of polyhedral cells 6–18 μm in diameter. These were found to be made of altered carbon-based material, strongly supporting the idea that these were remains of living material, though evidence of internal structure could not be found. These cannot be directly linked to the macrofossils, but they are large enough to be clearly Eukaryotic in origin, and in the absense of any other structures from which they might have come, are thought to provide further evidence for the idea that the macrofossils represent Multicellular Eukaryotic Algae.
Polygonal cells forming a multi-layered network from the Gaoyuzhuang Formation. Scale bar is 20 μm. Zhu et al. (2016).
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