Ancient Shellfish Sailed The Seas On The Shells Of Turtles

Colonizing the shells of live sea turtles might have allowed mollusks to encounter previously unexplored niches, scientists say.

AsianScientist (Sep. 23, 2020) – Using detailed three-dimensional (3D) scans of fossilized shells, scientists in Japan have identified a species of mollusk that grew on living sea turtles about 100 million years ago. Their findings, which shed light on ancient parasitic or symbiotic relationships, have been published in Palaios.

Although paleontologists have managed to piece together a great deal about vertebrate life during the late cretaceous period from the many fossils that have been found, much less is known about how vertebrates interacted with parasites or symbionts, as those tend to be found in soft tissue and cartilage which do not fossilize well. By studying boreholes on fossilized turtle shells, however, scientists have discovered that certain species of mollusks grew on shells after the turtles had died and their remains sank to the bottom of the ocean. Nonetheless, there has not been any evidence that mollusks were parasites or symbionts while the turtles were still alive.

While studying the shell of an extinct basal leatherback marine turtle (Mesodermochelys sp.), a team led by Assistant Professor Kei Sato from Waseda University and Associate Professor Robert Jenkins from Kanazawa University found the first signs that mollusks could indeed have formed a parasitic or symbiotic relationship with live turtles.

The fossil, recovered from an Upper Cretaceous formation in Nio River, Japan, had a total of 43 boreholes left by boring bivalves, mollusks that have adapted to survive on hard surfaces. After observing the fossil up close and measuring the morphological characteristics of the boreholes, the team produced a 3D reconstruction of the shell and the cross-section of one of the boreholes, which allowed them to observe the intricate details left by the species.

“We saw that there were signs of healing around the mouth of boreholes, suggesting that the turtle was alive when the organisms settled on the carapace,” said Sato.

A cross section of the fossilized turtle shell with boreholes suggesting that the shell was colonized by boring bivalves while the turtle was still alive. Credit: Kei Sato/Waseda University and Robert Jenkins/Kanazawa University.

Based on the morphology and positioning of the boreholes, they determined that the likely culprits for these boreholes were bivalves from the superfamily Pholadoidea, creatures similar to the modern clams. However, the researchers were unable to match the characteristics of the boreholes they found with those made by any currently described species, leading them to conclude that they had discovered a completely new species, which they named Karethraichnus zaratan.

“This is the first study to report this unique behavior of boring bivalves as a symbiont of living marine vertebrate, which is a significant finding for the paleoecology and evolution of ancient boring bivalve clades,” Sato said.

By attaching themselves on a live, free-swimming substrate, such as the shell of a marine turtle, these pholadoid bivalves may have paved the way for a novel, yet-unknown evolutionary path of accessing previously unexplored niches and diversifying into new species, he concluded.

The article can be found at: Sato & Jenkins (2020) Mobile Home for Pholadoid Boring Bivalves: First Example From a Late Cretaceous Sea Turtle in Hokkaido Japan.


Source: Waseda University; Photo: Randall Ruiz/Unsplash.
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