THE GENERATION OF ANIMAL FORM: THE EDIACARAN MACROBIOTA AND PATTERNS OF CHARACTER ACQUISITION (Invited Presentation)

The origin of animals is one of the most profound events in the history of life on Earth, but our understanding of how and when animals evolved and diversified is patchy. Recent work has invigorated study of the Cambrian Explosion (~550–520 Ma) – a temporally constrained and unique event in Earth History, during which the bodyplans of all major living groups both appear and were fixed. The Ediacaran macrobiota are widely held to represent antecedents to modern animal groups, but their strange anatomies cannot easily be rationalised within the diversity of living animals, meaning that, for the most part, fossils from this time have been excluded from the debate.

We use the study of development to produce a phylogenetic bracket for members of the Ediacaran macrobiota, focusing initially on the longest-lived and most diverse of the Ediacaran macrobiota: the rangeomorphs. We then integrate these and other morphological data into a new phylogenetic analysis which samples the breadth of animal diversity and recover paraphyly of Ediacaran macrofossils. We constrain our results to produce phylogenetic trees conforming to the four major hypotheses for the inter-relationships of early-diverging animal lineages (Porosis, Ctenosis, Parahoxozoa, Coelenterata) and assess the position of Ediacaran macrofossils under these competing topologies. These data allow us to interpret the significance of the Ediacaran macrobiota in trait evolution of early animals under all major, competing scenarios. Our results suggest that rather than the unique Ediacaran quilted bodyplan being a dead end, the modular anatomy these fossils exhibit was plesiomorphic for much of the diversity of living animal life.