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.

Life cycle evolution: was the eumetazoan ancestor a holopelagic, planktotrophic gastraea?

“…A considerable number of Ediacaran (Vendian) fossils have been interpreted as bilaterians [42], but the metazoan nature of several forms has been questioned. Dickinsonia has been interpreted as a placozoan [25], and Kimberella as a mollusc [43].…”

Ancestral state reconstruction of ontogeny supports a bilaterian affinity for Dickinsonia

“…The hypothesis that D. costata grew like a bilaterian is consistent with (but does not necessitate) claims of resting/feeding traces (Ivantsov and Malakhovskaya ; Gehling et al ; Sperling and Vinther ; Ivantsov , ; Xiao and Knauth ; Evans et al ), muscular contraction (Runnegar ; Gehling ; Xiao and Laflamme ), and a digestive system (Ivantsov ). Additionally, if D. costata was mobile, then it appears to have moved in a direction opposite to the region of growth (Gehling et al ; Freeman ), which is further suggestive of a posterior growth zone. It is important to note that this study offers a conservative placement of Dickinsonia , and does not necessitate that the genus is an ancestral or stem member of the Bilateria; given that D. costata's metameric units transect the midline of the ectoderm (Fig.…”