USING EVO-DEVO TO UNDERSTAND EDIACARAN MACROFOSSILS (Invited Presentation)

Understanding the biology and relationships of most Ediacaran macrofossils has been hampered by difficulties in identifying reliable, phylogenetically informative morphological characters. One avenue to addressing this dilemma is examining developmental processes necessary to generate various Ediacaran organisms within the context of comparative studies of metazoan development. By establishing the needed developmental processes, we can provisionally assign phylogenetic affinities based on the wealth of recent comparative studies of development in living animals and their underlying control processes. Careful examination of Ediacaran specimens permits identification of anterior/posterior (A/P) and dorso/ventral (D/V) patterning, musculature, and a nervous system, among other features. In some cases, and with a bit less confidence, we can make estimates of the variety of different cell types required. A few caveats. While we can identify A/P and P/D patterning and infer the presence of a nervous system in some Ediacaran macrofossils, it is implausible to associate such developmental patterning with specific developmental control genes, even highly conserved systems such as Hox or distalless unless groups can be placed within metazoan crown clades. Second, comparative studies frequently show that the novelties associated with capacity to generate a range of morphological outcomes, or developmental capacity, originated well before the capacity is exploited morphologically. This decoupling may reflect the origin of these patterning novelties in cell and tissue-level processes, only later being co-opted for morphological patterning. Whether this lag between capacity and expression was also true during the Ediacaran is unclear. Finally, interpreting the extent of homology versus independent co-option among Bilateria continues to challenge inferences about the number of possible Ediacaran representatives.