Guest Researcher

Abstract

The Cambrian Explosion is one of most important evolutionary event in the life history, during which almost all known animal phyla appeared explosively within 20 million years, efficiently established the major branches of the animal tree of life. As one of the most diversified lineages among metazoans, Lophotrochozoa includes various animal phyla such as brachiopods, bryozoans, mollusks, and annelids etc., emerged prominently during the Cambrian Explosion. Over the past two decades, Lophotrochozoa has been confirmed as a monophyletic group, and significant progresses have been made in their phylogeny. However, many controversies remain concerning the phylogenetic relationships among its various groups, and critical scientific questions, including the origin, early evolution, biodiversity, and co-evolutionary dynamics of Lophotrochozoa with their environment, remain unresolved. Resolving these issues requires not only molecular biology research but also empirical validation through paleontological evidence to reconstruct a more accurate picture of life and evolutionary processes in Earth's history.

Hyoliths, as one of the earliest biomineralized lophotrochozoans, are an important group for studying the origin and early evolution of this lineage. They first appeared during the Cambrian Fortunian, rapidly evolved and diversified, reached their peak during Cambrian Series 2, and declined sharply after the Ordovician before going extinct at the end of the Permian. Hyoliths possess calcareous exoskeletons composed of conical shells and opercula, with some taxa also featuring paired appendages extending from within the shell. It is widely agreed that they were globally distributed benthic primary consumers in Paleozoic marine ecosystems. They had evolved a series of living strategies to survive in the “arms race”. However, due to limited anatomical information, the phylogenetic affinity and evolutionary position of hyoliths have long been debated, forming a core research focus. Recent debates centered on whether hyoliths represent a stem group of Lophotrochozoa or a class within Mollusca or belong to Lophophorata.

The affinity of hyoliths is far from certain, raising new scientific questions: Did lophotrochozoan shells originate from a single shell or from tommotiid multipartite scleritomes? How did the two skeletal systems evolve? If hyoliths are ancestral lophotrochozoans, bearing a shell may represent a lophotrochozoan symplesiomorphy, with multipartite scleritomes being derived, and annelids lost shells secondarily (Smith, 2020). Conversely, if hyoliths belong to stem brachiopods, tommotiids could be ancestral lophotrochozoans, with shells arising from the fusion and mineralization of scleritomes, and tentacles evolving incrementally toward lophophores via hyoliths, tommotiids, and other intermediates (Smith, 2020). Therefore, in-depth studies of the phylogenetic position of hyoliths are crucial for resolving forementioned questions.

Reference

Smith, M. 2020. Finding a home for hyoliths. National Science Review, 7: 470-471.