The precise reconstruction of phylogenetic trees is crucial for understanding the evolutionary relationships between organisms. However, the Cambrian Explosion, which saw the sudden appearance of diverse animal phyla and body plans, presents significant challenges in deciphering the deep phylogenetic relationships of metazoans.

In particular, the phylum Mollusca, the second-largest phylum in the animal kingdom, poses difficulties due to its diverse fossil record, morphological disparity among its eight living classes, and conflicting hypotheses based on paleontological, morphological, and molecular evidence.

To address this, a collaborative team of researchers from China, the United States, and the United Kingdom sequenced the genomes of spade shells, or Scaphopoda, which are enigmatic and understudied members of the phylum Mollusca. Their findings, published in the Proceedings of the National Academy of Sciences, reveal ancient incomplete lineage sorting (ILS) during the early evolution of mollusks.

By considering the impact of ancient ILS, the researchers resolved the long-standing debate on the placement of Scaphopoda in the mollusk tree. Their analysis confirms that Scaphopoda is the sister lineage to Bivalvia, supporting the concept of Diasoma proposed five decades ago based on morphology.

Furthermore, this discovery leads to a reinterpretation of important Cambrian fossils that exhibit characteristics of both bivalves and spade shells. The researchers suggest that certain fossils, such as Anabarella, Watsonella, and Mellopegma, are primitive diazomes, revealing information about the evolutionary origins of these unique body plans.

Dr. Song Hao from the Institute of Oceanology at the Chinese Academy of Sciences emphasizes the importance of their work in revolutionizing the understanding of early mollusk evolution.

This study highlights the underestimated impact of ancient ILS on the reconstruction of radiations during the Cambrian period and calls for further exploration of ILS in other challenging nodes of deep metazoan phylogeny.

– Song, Hao et al., Scaphopoda is the sister taxon to Bivalvia: Evidence of ancient incomplete lineage sorting, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2302361120