Some of first animals were connected by networks of thread-like filaments, the earliest evidence yet found of life being connected in this way

Scientists from the Universities of Cambridge and Oxford discovered the fossilised threads -- some as long as four metres -- connecting organisms known as rangeomorphs, which dominated Earth's oceans half a billion years ago. The team found these filament networks -- which may have been used for nutrition, communication or reproduction -in seven species across nearly 40 different fossil sites in Newfoundland, Canada. Their results are reported in the journal Current Biology.

Towards the end of the Ediacaran period, between 571 and 541 million years ago, the first diverse communities of large and complex organisms began to appear: prior to this, almost all life on Earth had been microscopic in size1. Since rangeomorphs could not move and are preserved where they lived, it is possible to analyse whole populations from the fossil record. Earlier studies of rangeomorphs have looked at how these organisms managed to reproduce and be so successful in their time.

"These organisms seem to have been able to quickly colonise the sea floor, and we often see one dominant species on these fossil beds," said Dr Alex Liu from Cambridge's Department of Earth Sciences, and the paper's first author. "How this happens ecologically has been a longstanding question -- these filaments may explain how they were able to do that."

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  • 1. Fern-like rangeomorphs were some of the most successful life forms during this period, growing up to two metres in height and colonising large areas of the sea floor. Rangeomorphs may have been some of the first animals to exist, although their strange anatomies have puzzled palaeontologists for years; these organisms do not appear to have had mouths, organs or means of moving. One suggestion is that they absorbed nutrients from the water around them.
Fossilised threads - some as long as four metres - connecting organisms known as rangeomorphs, which dominated Earth's oceans half a billion years ago
Fossilised threads - some as long as four metres - connecting organisms known as rangeomorphs, which dominated Earth's oceans half a billion years ago © Alexander G. Liu and Frances S. Dunn

Alexander G. Liu, Frances S. Dunn. Filamentous Connections between Ediacaran Fronds. Current Biology, 2020;

DOI: 10.1016/j.cub.2020.01.052

Highlights:

  • Abundant filamentous structures are preserved on late Ediacaran bedding planes
  • Filaments terminate at, and can connect, individual members of frondose taxa
  • Filaments are interpreted as evidence for stolonic outgrowths
  • This implies a clonal reproductive strategy for many Ediacaran frondose taxa

Summary: Fossils of the Ediacaran macrobiota (∼571–539 mya) record phylogenetically diverse marine palaeocommunities, including early animals, which pre-date the “Cambrian Explosion”. Benthic forms with a frondose gross morphology, assigned to the morphogroups Rangeomorpha and Frondomorpha (see also Arboreomorpha), are among the most temporally wide-ranging and environmentally tolerant members of the Ediacaran macrobiota and dominated deep-marine ecosystems ∼571–560 mya. Investigations into the morphology, palaeoecology, reproductive strategies, feeding methods], and morphogenesis of frondose taxa together constrain their phylogenetic position to the metazoan (for Rangeomorpha) or eumetazoan (e.g.,  Arborea) total groups, but tighter constraint is currently lacking. Here, we describe fossils of abundant filamentous organic structures preserved among frond-dominated fossil assemblages in Newfoundland (Canada). The filaments constitute a prominent component of the ecosystems, and exhibit clear physical associations with at least seven frondose taxa. Individual specimens of one uniterminal rangeomorph taxon appear to be directly connected by filaments across distances of centimeters to meters. Such physical linkages are interpreted to reflect evidence for stolonic connections: a conclusion with potential implications for the phylogenetic placement and palaeoecology of frondose organisms. Consideration of extant stoloniferous organisms suggests that Ediacaran frondose taxa were likely clonal and resurrects the possibility that they may have been colonial.

Results: Fossilized macroscopic filamentous structures are here reported from 38 unique bedding plane horizons (out of 183 studied fossil-bearing horizons) on the Avalon and Bonavista peninsulas of Newfoundland. Filamentous structures manifest as low (<1 mm) positive epirelief impressions, with no visible cell walls, membranes, external ornamentation, or disarticulation. Filaments are typically 100–1000 μm in width and 2–40 cm in length, although the longest and thickest examples we have observed (on the LC6 surface) measure over 4 m in total length. Filament densities vary between different bedding planes, ranging from occasional individual strands to hundreds per square meter (extrapolated estimates suggest over 580 filaments/m2 from sections of the MUN Surface), but densities are largely uniform within individual bedding plane assemblages. Individual specimens possess broadly constant widths and traverse bedding planes in multiple directions. Where filaments meet, they are typically superimposed rather than cross-cutting, strongly suggesting that they are not trace fossils. Rarely, observed filamentous structures dichotomously bifurcate, while some examples are seemingly arranged into bundles from which individual filaments can radiate. Small bulges along the length of some filaments are also observed, often at triple junction branching points.