Genomes of enigmatic tusk shells provide new insights into early Molluscan evolution

Accurate phylogenetic trees serve as essential tools in evolutionary and comparative biology.

However, the sudden emergence of major animal phyla and diverse body plans during the Cambrian Explosion presents significant challenges in constructing deep metazoan phylogenetic relationships.

This challenge is particularly pronounced in understanding the second-largest phylum, Mollusca, whose principal lineages originated during the Cambrian period. The extensive fossil record, vast morphological differences among the eight existing classes, and conflicting interpretations stemming from diverse paleontological, morphological, and molecular evidence impede a clear understanding of molluscan phylogeny.

Recently, a collaborative effort among researchers from China, the U.S.A., and the U.K. sequenced the initial genomes of Scaphopoda, commonly known as “tusk shells.” These creatures, among the rarest and least-studied members of Mollusca, revealed unprecedented ancient incomplete lineage sorting (ILS) that occurred during early molluscan evolutionary branching.

These revelations offer new insights into early molluscan evolution and shed light on why determining class-level relationships within Mollusca has remained a significant challenge for evolutionary biologists.

The researchers successfully resolved the long-standing debate regarding the phylogenetic placement of Scaphopoda. These organisms display a combination of morphological and developmental features akin to various other lineages. For instance, they possess tentacles resembling cephalopod arms, a shell formation process reminiscent of bivalves, and a somewhat degenerated head that appears to represent an intermediate state between bivalves (which lack a defined head) and gastropods (with a well-developed head).

Interpreting these features has led morphologists to propose conflicting phylogenetic hypotheses. Previous phylogenomic studies had failed to confidently position Scaphopoda within the molluscan tree, making its placement a major unresolved question.

However, the current study, by considering ancient incomplete lineage sorting, has resolved this debate. It confirms that Scaphopoda represents the sister lineage of Bivalvia (known as the Diasoma concept), a hypothesis initially suggested based on morphology five decades ago but lacking support from molecular phylogenetic studies until now.

This discovery, supported by meticulous molecular clock analysis, necessitates a reevaluation of several crucial yet contentious Cambrian fossils exhibiting characteristics shared by both bivalves and scaphopods. For instance, fossils like Anabarella, Watsonella, and Mellopegma, which were previously unclear, are now proposed as stem diasomes due to their lateral compression during the Early Cambrian.

Moreover, this study provides significant insights into the evolutionary development of the distinct body plans observed in scaphopods and bivalves.

Overall, the study emphasizes the underappreciated influence of ancient incomplete lineage sorting in the context of reconstructing Cambrian-age radiations. It urges further exploration of ancient incomplete lineage sorting concerning other challenging nodes in deep metazoan phylogeny.

Header Image Credit : IOCAS and Nick Roberts

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