Cambrian Explosion - Metazoan Architectures

According to the authors¹ the fossils recovered from latest Ediacaran strata, simple skeletal fossils, are a prelude to the explosive expansion of biodiversity that occurred in the Cambrian. The chemistry of the sea water during the latest Ediacaran was such that it permitted the secretion and preservation of mineralised skeletons, these durable structures being used in the organisms of the Cambrian as well as in the succeeding more than 500 My. Also, the first trace fossils that appeared in the Cambrian were an order of magnitude larger than most of their predecessors from the Ediacaran, and included burrows that penetrated relatively deep into the sediments of the seafloor. The animals at this time were apparently larger and more active than earlier forms. It was the beginning of the Phanerozoic Eon.

The internal architectures and phylogenetic relationships of the fossils from the Ediacaran are largely unknown, those of the Cambrian are mostly stem groups of clades of the Phanerozoic that are well-known. Though the fossils from the Cambrian are distinct from extant related forms, an understanding of their crown groups provides a foundation for their interpretation. There are representatives of at least 14 groups, nearly half of crown phyla, among the fauna of the Cambrian Explosion, though there still remain many fossils from the Cambrian that have not been connected to any living phyla. For some of these unconnected fossils not enough is known about them to allocate them to a known group, often because their remains have been found as isolated, often disarticulated, skeletal elements from which not much can be learned, while there is insufficient morphological evidence preserved to rule out membership among any extant phyla. The authors¹ suggest it is likely that most of these unusual forms are stems of superphyla to which crown phyla have been assigned. In that case they would represent early branches, with their own unique body plans, of groups that gave rise to living phyla later in evolution.

Sources & Further reading

1. Erwin, Douglas H., & Valentine, James W., 2013, The Cambrian Explosion: The Construction of Animal Biodiversity, Roberts & Co., Greenwood Village, Colorado