Horodyskia is one of the oldest multicellular macroorganisms: Study

Horodyskiacharacterized by strands of beads of uniform size and spacing, is a macroscopic fossil with records spanning from the early Mesoproterozoic Era (~1.48 Ga) to the late Ediacaran Period (~550 Ma).

Now, researchers led by Dr. LI Guangjin and Prof. PANG Ke of the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences (NIGPAS) and Prof. CHEN Lei of Shandong University of Science and Technology has revealed that Horodyskia is one of the oldest multicellular macroorganisms and may have achieved macroscopic size through a combination of coenocytism and simple clonal colonization.

The study was published in Communication Biology on April 12th.

Proterozoic macroscopic fossils are of great importance for understanding the early evolution of eukaryotes, especially how and when early eukaryotes developed complex levels of multicellular organization and acquired body sizes visible to the naked eye.

One of the salient features of Horodyskia in the previously reported specimens is preservation of mold and molding in fine-grained siliciclastic rock or in chert (silicified), without evidence of preservation of organic walls. This feature further hinders biological and phylogenetic interpretation Horodyskia.

The new one Horodyskia the specimens in this study, from the Tonian Shiwangzhuang Formation (~850–720 Ma) in western Shandong and the Jiuliqiao Formation (~950–720 Ma) in the Huainan region, North China, are unique in their diverse preservation styles, including carbon compression, three – Dimensionally preserved organic walled fossils, shallow impressions, and casts and impressions.

Carbon properties of Horodyskia the fossils, similar to carbon-compressed macrofossil representatives from the Shiwangzhuang Formation, suggest an underlying biological origin. Importantly, the carbon-compression specimens and the preserved three-dimensional organic walled specimens are similar in morphology and size, strongly debating the possibility of sedimentary, mud-flock, or intraclastic structures.

Raman spectroscopy revealed that the carbon material from the Shiwangzhuang Horodyskia specimen has similar spectral characteristics to the coexisting multicellular fossils, indicating that both underwent low-level metamorphosis with similar peak metamorphic temperatures. Given the possibility of compressional folding and three-dimensional preservation, the Shiwangzhuang and Jiuliqiao specimens show that the Horodyskia beads are bona fide fossils with organic-walled vesicles.

Two groups of fossils with different bead sizes were observed in the same bedding plane in the Shiwangzhuang Formation. They share a common preservation style, show similar positive correlations between diameter and bead spacing and between diameter and bead spacing, and share a largely overlapping chemospace distribution, indicating that they are not only biogenic, but also likely congeneric organisms. Their bead diameter range overlaps with that Horodyskia minor from the Liuchapo Formation and from H. ring shape from the Appekunny Formation, respectively, supports the assignment of specimens with submillimetric beads to the genus Horodyskia. This finding shows that Horodyskia has a stratigraphic range from early Mesoproterozoic to late Ediacaran, with outstanding long-term morphological stability over 900 Myr.

Whole, Horodyskia can be reconstructed as chain-like organisms or individual colonies consisting of several to dozens of vesicles or cells with recalcitrant organic walls and possibly embedded in an amorphous agar matrix. The division observed in paired beads and beads most likely resulted from binary fission and indicates that the beads are submillimetric down to millimeters of Horodyskia very likely represents a giant cell. Horodyskia The cell is an order of magnitude larger than a typical prokaryotic cell, and the large recalcitrant cell wall is concluded to be unusual for a prokaryotic cell. Because of that, Horodyskia cannot be a prokaryotic organism.

“Big cell size Horodyskia indicating that it is not only a eukaryote, but also possibly a coenocytic or multinucleated one,” said Prof. PANG. “Cells are submillimeter to millimeter in size Horodyskia apparently it takes multiple nuclei to organize the gigantic masses of cytoplasm.”

Disconnected cellular nature and features, but occasionally splits Horodyskia beads, indicating that these fossils were less likely to be complex multicellular organisms, but more likely to be protists whose clone cells formed simple colonies and were not fully integrated (i.e., simple clone colonies). Protists with simple clonal colony construction, submillimetric to millimeter cell sizes (or inferred coenocytes), and recalcitrant cell walls—defining features Horodyskia—limited to a small number of eukaryotic clades. This study evaluated the three most likely potential analogues: arclinid testat amoebae, foraminifera, and several groups of algae, all of which have been proposed in previous studies.

The most plausible phylogenetic interpretation for Horodyskia is that it represents a multicellular, giant-celled protist, which has acquired a large body size and shares some similarities with living coenocytic algae and monothalamid foraminifera, although the latter are usually unicellular and therefore less comparable. Along with other possible early-mid Proterozoic coenocytic fossils, Horodyskia provide an important temporal limitation on the origin of coenocytic eukaryotes.