Fossilized soft tissues in tentaculitids from the Upper Devonian of Armenia: Towards solving the mystery of their phylogenetic affinities

The tentaculitids represent a significant and diverse group of mysterious fossils found in Palaeozoic strata. Their biology and evolutionary relationships remain poorly understood due to limited information about their soft body anatomy. The discovery of fossilized soft tissues in tentaculitids from the Upper Devonian of Armenia helps to resolve the old mystery of tentaculitid phylogenetic affinities. The thick bundles of phosphatic bars in the tentaculitid interior closely resemble the musculature of several tubicolous invertebrates, such as phoronids and bryozoans. Based on this similarity, we interpret these structures as fossilized muscles of the tentaculitid animal. The attachment of tentaculitid body muscles in a honeycomb pattern to the body wall is unlike that of any mollusc. Based on this muscle arrangement, molluscan affinities of tentaculitoids can be refuted. The muscle arrangement in tentaculitids is more akin to that of bryozoans than to phoronids. Presumably, tentaculitids and bryozoans shared a common ancestor. This ancestor was probably a solitary animal, with coloniality emerging later in the bryozoan branch of the phylogenetic tree. Alternatively, tentaculitoids might have evolved from a bryozoan-like ancestor by losing their coloniality.