Modeling morphological diversity in the oldest large multicellular organisms.

Abstract

The terminal Neoproterozoic Ediacaran Period (635–541 Ma) is best described as a time of change. Following the end of the last global “Snowball” glaciation and a global rise in atmospheric oxygen levels, a biotic revolution began occurring in the oceans. The fossil record of this revolution showcases the transition from microscopic single cells into large, multicellular and morphologically complex organisms. Typifying this transition is the Ediacara biota, a group of globally distributed soft-bodied organisms whose affinities are fiercely debated and whose disappearance from the fossil record before the Cambrian explosion is equally perplexing (1, 2). Given the variation in shape, biological architecture, growth strategies, and body symmetries seen within the diverse Ediacara biota, it is most likely that these organisms represent an assortment of higher-level clades, many of which went extinct with the advent of bilaterian animals (2). Among the extinct clades, the Rangeomorpha (3) (Fig. 1) are particularly unusual in possessing repeating and apparently fractal branching architecture that is not known in any modern organisms (3). Rangeomorphs used a series of modular, millimeter-scale self-similar “frondlets” (Fig. 1, image 2) to construct a diverse array of larger, morphologically complex forms, including stalked fronds (Fig. 1, images 1–3), flat-lying mats (Fig. 1, image 4), lettuce-shaped bushes (Fig. 1, image 5), and erect fences (Fig. 1, image 6) (3). The remarkably high surface area-to-volume ratios generated as a result of this pseudofractal construction (4) suggest that these modules may represent the locus for passive diffusion-based (osmotrophic) (5) feeding, and could have aided oxygen uptake as well. Because of the uncertainty in their phylogenetic placement on the tree of life (6), many aspects of rangeomorph biology, including growth, development, reproductive strategies, and dispersal mechanisms remain elusive (although, see ref. 7). Now, innovative quantitative modeling of rangeomorph branching conducted by Cuthill and Conway Morris (8) is now shedding light on how these organisms came to dominate Ediacaran ecosystems.