Proposed evolutionary event in the history of metazoa, producing the Ediacaran biota
Dickinsonia, an enigmatic quilted organism with glide symmetry which may have been an early animalCloudina may have been one of the first mineralized animals to appear.[1]Kimberella was originally interpreted as a cubozoancnidarian, although it is now believed it was an early mollusc.[2]The Ediacaran trace fossils are a sign of animal movement as well as sediment disturbance, they show possible signs of the earliest true animals.
Scientists are still unsure of the full extent behind the development of the Avalon explosion,[3] which resulted in a rapid increase in metazoanbiodiversity, including the first appearance of some extantinfrakingdoms/superphyla such as cnidarians and bilaterians. Many of the Avalon explosion animals are sessilesoft-bodied organisms living in deep marine environments,[4] and the first stages of the Avalon explosion were observed through comparatively minimal species.[3]
Charles Darwin predicted a time of ecological growth before the Cambrian Period, but there was no evidence to support it until the Avalon explosion was proposed in 2008 by Virginia Techpaleontologists after analysis of the morphological space change in several Ediacaran assemblages.[3][4] The discovery suggests that the early evolution of animals may have involved more than one explosive event.[5] The original analysis has been the subject of dispute in the literature.[6][7][8]
Trace fossils of these Avalon organisms have been found worldwide, with many found in Newfoundland, in Canada and the Charnwood Forest in England,[3] representing the earliest known complex multicellular organisms.[a] The Avalon explosion theoretically produced the Ediacaran biota.[4][3] The biota largely disappeared contemporaneously with the rapid increase in biodiversity known as the Cambrian explosion. At this time, all living animal groups were present in the Cambrian oceans.[3]
The Avalon explosion appears similar to the Cambrian explosion in the rapid increase in diversity of morphologies in a relatively small-time frame, followed by diversification within the established body plans,[3][9] a pattern similar to that observed in other evolutionary events.[9]
The Avalon explosion was a time of early evolution and low diversity in species. There were over 270 species defined,[10] with 50 different morphological characteristics categories, many of which the anatomical structure had to be inferred with fossils and casts.[4][10] These species were placed into 20 different genera.[10]
During this time, animals became bilateral and along with increasing complexity.[11] Many animals during this time fit into the annelid, arthropod, echinoderm, and cnidarianphyla.[11] Animals at this time developed bilateral symmetry with a clear anterior and posterior side, which included species like Spriggina, Charniodiscus and Yorgia.[10]
Many of the plants fit into a now-extinct phylum of Vendobionta.[11] The Vendobionta were radically arranged, with many tubular elements and a central stalk. Frondlets were a prominent aquatic plant during this time, with many different shapes, including Fractofusus which is a spindle shape, Fradgatia, a lettuce shape; and Rangea which was a leaf shape.[11]
^Freeman, Gary (30 June 2010). "Comment on Xiao et al. (2009), response to: the rise of bilaterians". Historical Biology. 22 (4): 430–432. doi:10.1080/08912960903562259. S2CID85339496.
