The rise of bilaterians

Abstract

Recently Shen et al. have argued that the Ediacaran faunas from Avalon-Charnwood [580–560 million years ago (MA)], the White Sea-Flinders Range (560–550 MA) and Namibia (550–543 MA) occupied the same morphospace even though these faunas differed in species composition, ecology, biogeography and age. The traits they used to characterise these faunas could not distinguish between important promorphological features such as radial vs. bilaterian and unitary vs. colonial animals. Their inappropriate assignment of symmetry properties led to the homogenisation of morphospace in these different faunas. Another way to examine shifts in morphospace involves sorting out radial vs. bilaterian and unitary vs. colonial genera in terms of their time of first appearance in the fossil record. While genera with both kinds of symmetry properties and unitary and colonial animals were present during the early Ediacaran, there was a large proportional increase in new bilaterian genera and a decrease in colonial genera beginning between 560–550 MA. The increase in bilaterians may have been associated with the ability to exploit food sources in and on the substrate of shallow water environments.