Abstract
While many skeletal biomineralized genera are described from Ediacaran (635–541 million years ago, Ma) strata, none have been suggested to have an affinity above the Porifera–Cnidaria metazoan grade. Here, we reinterpret the widespread terminal Ediacaran (approx. 550–541 Ma) sessile goblet-shaped Namacalathus as a triploblastic eumetazoan. Namacalathus has a stalked cup with radially symmetrical cross section, multiple lateral lumens and a central opening. We show that the skeleton of Namacalathus is composed of a calcareous foliated ultrastructure displaying regular concordant columnar inflections, with a possible inner organic-rich layer. These features point to an accretionary growth style of the skeleton and an affinity with the Lophotrochozoa, more specifically within the Lophophorata (Brachiopoda and Bryozoa). Additionally, we present evidence for asexual reproduction as expressed by regular budding in a bilateral pattern. The interpretation of Namacalathus as an Ediacaran total group lophophorate is consistent with an early radiation of the Lophophorata, as known early Cambrian representatives were sessile, mostly stalked forms, and in addition, the oldest known calcareous Brachiopoda (early Cambrian Obolellida) and Bryozoa (Ordovician Stenolaemata) possessed foliated ultrastructures.
Highlights
The Cambrian radiation records the seemingly abrupt appearance of biomineralized animals with highly sophisticated skeletons in the geological record
Namacalathus has been proposed to represent either a possible cnidarian on the basis of an overall goblet-shaped morphology and hexaradial cross section [4], a protozoan owing to small size and an apparent lack of accretionary growth [15], a stem-eumetazoan [16] or as a lophotrochozoan based on a microlamellar ultrastructure as manifest in limited petrographic thin section material [6]
The stem and outer cup surface can be covered with short, robust spines which previously have been identified in Namacalathus from the Ediacaran Byng Formation of the Canadian Rocky Mountains only [11]
Summary
The Cambrian radiation (starting at approx. 541 Ma) records the seemingly abrupt appearance of biomineralized animals with highly sophisticated skeletons in the geological record. 541 Ma) records the seemingly abrupt appearance of biomineralized animals with highly sophisticated skeletons in the geological record. These are represented by calcareous and phosphatic shelly fossils of triploblastic bilaterians, including various molluscs, brachiopods and diverse stem group lophotrochozoans in the early Cambrian Terreneuvian Epoch Namacalathus has been proposed to represent either a possible cnidarian on the basis of an overall goblet-shaped morphology and hexaradial cross section [4], a protozoan owing to small size and an apparent lack of accretionary growth [15], a stem-eumetazoan [16] or as a lophotrochozoan based on a microlamellar ultrastructure as manifest in limited petrographic thin section material [6]. This allows some potential constraints to be placed on original skeletal mineralogy, soft-tissue distribution and mode of biomineralization, as well as insights into the possible affinity of Namacalathus
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