The Rangeomorpha comprises ~20 frond-shaped species of Ediacaran organisms, and is defined by the presence of branching modules called 'rangeomorph units' (Narbonne et al., 2009). Rangeomorphs were some of the first macroscopic organisms to appear in the geological record around 571 million years ago. They dominated Ediacaran deep-water ecosystems for at least 10 million years, before experiencing a decline in diversity and eventually disappearing from the fossil record at the end of the Ediacaran Period. While most researchers agree that the rangeomorphs constitute an extinct clade, exactly where they fit in the Tree of Life remains the subject of debate. Some have argued that they are stem-group animals (Xiao and Laflamme, 2009), but others suggest they may sit even lower in the Tree of Life, closer to fungi (Sperling et al., 2007), or even as part of a (now extinct) kingdom, the Vendobionta (Seilacher, 1992). Recent studies based on their growth and development indicate a position within the animals is most likely (Dunn et al., 2017).
Communities of rangeomorphs exhibit different modes of life, as well as ecological tiering - the division of an ecospace in different niches (see Clapham & Narbonne, 2002). Some taxa, such as Fractofusus andersoni, would have been found lying flat on the sea floor, whereas others, such as Charnia masoni, projected higher into the water column, as they competed for nutrients or energy sources. Recent work applying spatial statistical analyses to populations of rangeomorph fossils in Newfoundland and the U.K. suggests that the observed tiering was not necessarily driven my competition for nutrients, but by reproductive factors (Mitchell and Kenchington, 2018). Such analyses are also revealing new insights into macroecology and reproductive strategies in these taxa (Mitchell et al., 2015, 2019).
The rangeomorph branching unit is a self-similar branching arrangement that is repeated throughout the frondose part of the organism, with the largest branches and smallest branches essentially looking exactly the same, just at different scales. This has led to the rangeomorphs being described as pseudo-fractal in the past. It is thought that their self-similar branching pattern would have increased the surface area of these organisms, helping them to efficiently extract nutrients from the water column (Laflamme et al., 2009).
The rangeomorphs are some of the most well-studied Ediacaran organisms, but they remain amongst the most puzzling. Whatever they were, they represent a hugely successful group of organisms that dominated their particular habitat within the Ediacaran oceans.
Mistaken Point, Newfoundland, Canada
Flinders Ranges, South Australia
Charnwood Forest, England
White Sea, Russia
Key research papers:
Pflug, 1972 - one of the earliest papers to describe Rangea and similar fossils
Narbonne, 2004 - the first paper to describe rangeomorph frondlets (later termed 'rangeomorph units')
Narbonne et al., 2009 - the earliest attempt to make sense of variation within the rangeomorphs
Brasier et al., 2012 - provides a taxonomic framework for rangeomorph taxa
Liu et al., 2015 - a review of rangeomorph taxa from Avalonian localities
Rangeomorph fossils from Newfoundland. Left: Beothukis plumosa; Top Right: A single primary branch in a specimen of Bradgatia sp.; Bottom Right: Charnia masoni.