Early Cambrian microfossils preserve introvert musculature of cycloneuralians

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Prof. ZHANG Huaqiao from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (NIGPAS), leading an international research team, announced a groundbreaking discovery of early Cambrian microfossils, approximately 535 million years old, revealing the preserved introvert musculature of cycloneuralians.

This animal group includes various creatures like roundworms, horsehair worms, and mud dragons. The findings offer crucial insights into the early Cambrian cycloneuralians, closely linked to arthropods, the planet’s most successful animals.

Published in Proceedings of the Royal Society B: Biological Sciences on Oct. 11, the study sheds light on the Ecdysozoa superphylum, encompassing diverse bilaterally symmetric animals. This group includes Scalidophora (Kinorhyncha, Loricifera, Priapulida), Nematoida (Nematoda, Nematomorpha), and Panarthropoda (Tardigrada, Onychophora, Arthropoda), with Cycloneuralia comprising Scalidophora and Nematoida.

Unearthing three phosphatized specimens from China’s early Fortunian Kuanchuanpu Formation, the researchers detailed one specimen (NIGP179459) exhibiting interconnected rings and internal structures. These rings, showing signs of pliability, consist of radial and longitudinal structures, interpreted as fossilized muscles. The distinct muscle arrangement differs from basal animal body-wall musculatures like cnidarians or ctenophores, suggesting a bilaterian animal origin.

The hexaradial symmetry in the longitudinal muscles aligns with scalidophorans, indicating a similarity with the radial symmetry observed externally and internally in their introvert. Specimen NIGP179459 likely represents the anterior introvert musculature of priapulans, demonstrating a muscular grid reminiscent of this group. However, unlike modern scalidophorans, it lacked long introvert retractors, potentially limiting its retraction ability.

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These early millimeter-sized scalidophorans featured hexaradially arranged introvert structures, possibly facilitating locomotion and feeding. Their limited retraction capacity sets them apart from modern scalidophorans, reflecting evolutionary differences in these ancient organisms.

Header Image Credit : ZHANG Huaqiao

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