Mystery of the tusk shell's origins solved through genomics

The tusk shell, a tubular-shelled mollusk found in oceans worldwide, has long baffled scientists with its odd shape and uncertain evolutionary history. But a new genomic study published in PNAS has finally peeled back the layers on these enigmatic creatures.

The tusk shells exhibit a mosaic of morphological and developmental features resembling those of other mollusk lineages, such as tentacles resembling the arms of nautilus (cephalopods), a unique manner of shell development similar to that of clams (bivalves), and a partially degenerated head that seemingly represents an intermediate state between clams ("headless" bivalves) and snails (with a well-developed head, gastropods). These features have vexed scientists for decades, as they suggest different ideas about the evolutionary history of tusk shells.

Researchers sequenced the complete genomes of two tusk shell species and compared them to other mollusks like clams, snails and nautilus. The analysis reveals tusk shells are more closely related to bivalves like clams than to cephalopods like nautilus, or gastropods like snails, overturning previous assumptions.

The two lineages of tusk shells, Gadilida and Dentaliida (Image by IOCAS and Nick Roberts)

"This really reshuffles our understanding of how these unusual mollusks fit into the evolutionary tree," said Dr. Hao Song from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), first author of the study.

This discovery, together with careful molecular clock analysis, also prompts the re-interpretation of many important but controversial Cambrian fossils that show similarities to both tusk shells and clams. It also provides important insights into how the unique body plans of shells and clams evolved.

“By reading into their DNA, we can finally unravel how these odd ancient mollusks came to be,” said Dr. Hao Song. “More importantly, it helps us better understand the Cambrian Explosion itself.”

Tusk shells, Photo by Nick Roberts

The rapid development of next-generation sequencing technology and phylogenomics allows scientists to essentially decode the blueprints of bizarre Cambrian creatures that left few fossil traces. This provides an unparalleled insight into the evolutionary processes underlying this pivotal moment in the history of life. By reading and comparing the genomic code of Cambrian organisms and their descendants, scientists are gleaning new clues into the sudden diversity of new and complex animal forms that erupted over 500 million years ago.