29-1 A Paleontological Perspective on the Sister Group of Metazoans from Cambrian Sites of Exceptional Preservation

Session: Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts (Posters)

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The phylogenetic relationships among the earliest branching metazoans represent an ongoing and hotly contested debate. One of the prevailing hypotheses places the ctenophores as the sister group to all other Metazoa. Although largely substantiated by genomic evidence, it leaves open questions regarding the convergent evolution of complex traits such as muscles and the nervous system, as well as the ancestral ecology of the first animals. The more traditional Porifera-sister hypothesis is supported by morphological evidence, particularly comparisons with choanoflagellate protists, and the fossil record, but has fallen out of favor in the wake of genetic evidence. While molecular clocks estimate that metazoans and bilaterians originated between 700 and 900 million years ago, implying that poriferans and ctenophores diverged from other animals during the Cryogenian, the first recognizable body fossils of these early metazoan groups appear in late Ediacaran and Cambrian periods. In this context, Cambrian sites of exceptional preservation, such as the Burges Shale (Canada), Chengjiang (China) and Marjum (USA), have the potential to illuminate the early evolution of animals through the preservation of soft tissues and a high fidelity of morphological detail. While articulated fossil sponges have been known from the Burgess Shale since its discovery in 1909, the paleontological literature on fossil ctenophores and sponges has grown dramatically during the last decade, and new discoveries prompt a closer look into the exceptional fossil record around the globe. Cambrian sponges and ctenophores share aspects of their gross morphology, including visual similarities in tetraradial or rotational symmetry, stalked bodies or apical plates, and frilled oscular margins or oral skirts. However, proper comparisons across these phyla are largely lacking, and thus the affinities of problematic taxa in both groups remain unresolved despite their extraordinary morphological information. Reestudy of the enigmatic Cambrian sponges Takakkawia lineata and Petaloptyon danei prompts a reevaluation of their anatomy and possible affinities in the context of the early diversification of the first metazoans in the wake of the Cambrian Explosion.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6306

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