Cambrian Explosion: late Ediacaran related (Introduction)

by David Turell , Saturday, January 02, 2021, 01:32 (1211 days ago) @ David Turell

Some forms seem early types of early Cambrian animals, as precursors:

https://advances.sciencemag.org/content/7/1/eabf2933?utm_campaign=toc_advances_2020-12-...

"Exceptional preservation of fossils from the Ediacaran-Cambrian, ca. 570 to 500 million years (Ma) ago, provides great insight into the first radiation of metazoans. While the oldest putative skeletal metazoans known are from the terminal Ediacaran, the general absence of both definitive skeletal characteristics and soft-tissue preservation has precluded clear assignment of affinity and, hence, an understanding of the origin of major metazoan groups. Here, we describe the first, three-dimensional, pyritized preservation of soft tissue in the Ediacaran skeletal metazoan Namacalathus hermanastes, from the Nama Group, Namibia, where new features support a bilaterian, lophotrochozoan affinity. In so doing, we also establish a strong evolutionary link between terminal Ediacaran and early Cambrian taxa.

***

"The confirmation of total group lophotrochozoans in the Ediacaran is supported by molecular phylogenies (25) and has implications for the earliest evolution of lophotrochozoans. The fossil record of biomineralized stem group lophotrochozoans is abundant from the early Cambrian onward where the possession of a calcareous skeleton or external sclerites likely represents the independent acquisition of skeletons of variable mineralogy in sessile, attached benthic fossil taxa, which have features shared with Namacalathus.

"The Entoprocta are an enigmatic monophyletic acoelomate group that occupies a phylogenetically basal position among lophotrochozoans, close to molluscs (30). Namacalathus displays some notable similarities with the entoprocts, including the goblet-shaped overall morphology (a body divided into distinct stalk and calyx), hexaradial symmetry, the position of both mouth and anus within the presumable tentacle collar, and bilateral buds emerging from frontal area of the parental individual. Hence, we are now able to reconstruct Namacalathus as a total group lophotrochozoan, capable of asexual budding with an organic-rich, foliated calcareous skeleton and an open, apical J- or U-shaped gastric cavity within the apical opening potentially accommodating a retractable collar of tentacles and with brood chambers around the lumens

"Sessile extant lophotrochozoan phyla such as annelids, molluscs, brachiopods, and phoronids have been suggested to have their origins in the earliest Cambrian small skeletal fauna [e.g., (26)], which are iconic representatives of the Cambrian Explosion. However, now, we can extend the origin of these modern lophotrochozoan phyla further back still into the terminal Ediacaran. In so doing, we establish a phylogenetic connection between Ediacaran and early Cambrian taxa, faunas that were previously thought distinct. We hence extend the roots of the Cambrian Explosion itself into the Ediacaran, where total group lophotrochozoans such as Namacalathus show a combination of features that became typical of both later lophophorates and representatives of the entoproctan-molluscan-annelidan branch."

Comment: What the article shows is that very late Edicarans and very simple early Cambrians are related. It doesn't solve the major gap posed by the full blown Cambrians of a little later time. This is a finding that is fully expected, at the boundary of the two periods. There certainly isn't an abrupt dividing line in evolution.