Amber preserves unique look into evolutionary history of land snails

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Ninety-nine million years ago, a snail had some babies and they all got preserved in a unique amber fossil, shedding light on the evolutionary behavior of these gastropods. (Pixabay/Kathy Büscher)

Ninety-nine million years ago, in the tropical forest of what is now Myanmar, a snail gave birth to five children near the site of what could have been their first meal. Except the possible food source, tree resin, ended up swallowing them all, leaving behind a unique amber fossil that captures not only a snail, still soft inside, but a rare glimpse into its life history.

A paper published May 18 in Gondwana Research describes the fossil and how it changes the evolutionary history of land snails: Unlike many modern snails, this one carried its young inside its body, pushing back the known incidence of this life strategy by 80 million years. The specimen, which contains a newly categorized snail species, is only the second amber fossil of a soft-bodied cyclophorid from the Cretaceous on record. 

It is particularly remarkable because snail behaviors aren't usually preserved in amber, while amber fossils of other invertebrates, such as insects shown mating, and laying or carrying eggs, are more common. 

"Behavior is so difficult to to capture, and the rare preservation — this exceptional preservation — in amber is phenomenal," said lead author Adrienne Jochum, a malacologist at the Naturhistorisches Museum Bern and the University of Bern. "It's a time capsule, and to get a glimpse into this window into the past is exciting."

Jochum communicates with researchers at the Nanjing Institute of Geology and Palaeontology, where her co-author Tingting Yu, who contributed equally to the paper, works. Yu sent high-resolution images of the amber fossil to Jochum for interpretation. The Chinese paleontologists could tell they had a snail shell, but not much else without the help of Jochum.

"I said, 'Oh wow, that's a birth,' and I zoomed in, and I saw the mucus and I saw the babies, and then it was clear and I said, 'Here's a narrative. This is what I can build on. This is cool stuff,'" Jochum said in an interview with The Academic Times

The mucus Jochum saw linked the mother's "marshmallow-like" body to the largest and likely most recently born neonate, according to the study, likely signaling a birth inside the amber.

After analyzing the initial images, Jochum asked the Nanjing researchers for micro-CT scans of the fossil from specified angles to help get a better look at the snail, and verify if her original interpretations were correct. 

The micro-CT scans revealed key diagnostic features of the snail that helped distinguish the new species, Cretatortulosa gignens, from other species. For example, the Cretatortulosa gignens shell had five keels, which are band-like features that cross the outside of its body whorl, the largest whorl where most of a snail's non-shell body is contained; whorls signify each 360-degree revolution in the spiral of a snail's shell. A similar species found in the same amber bed named by Yu, Cretatortulosa multilinea, only had one to two keels. 

Yu named the group Cretatortulosa because they resemble snails from India and Sri Lanka from the Tortulosa genus. She added the "Creta'' prefix to signify their Cretaceous age. Jochum, on the other hand, says that they look more like those from the Vargapupa genus, due to their number of keels, among other features. 

The finding is also significant because the start of the evolutionary record of viviparity, or ability to develop an embryo inside a parent, in gastropods was originally 80 million years before this Cretatortulosa gignens became encased in amber. According to Jochum, this species may have been viviparous, rather than egg-laying, due to the dangerous environment it inhabited where eggs could have been vulnerable to predation, but the exact reason is not certain, because so few mid-Cretaceous snails have been found.

"In the fossil record, we're trying to find where the ancestors branch off," Jochum said. "It's one incidence, but looking at the bigger picture of the tree for land snails, then this is significant, because it shows how far back viviparity is in land snails, which we didn't have before." 

Records of viviparity are very rare in snails, but more common in snakes and reptiles, according to Jochum. Today, snails from the Cerithioidea superfamily, mostly found in Southeast Asia, display viviparity. 

In North America, other recent research on trace fossils, fossils that evidence animal behavior, has shown that mammals started entering water habitats 9 million years earlier than the fossil record previously indicated. 

Jochum will continue her research on southeast Asian snails, and in a future paper will describe a group of 50 new snail species found in caves in Vietnam and Laos. According to Jochum, these are the smallest snails in the world, each roughly the size of the point on a mechanical pencil, and seemingly competing to be the most diminutive. 

In 2015, she described what was then the smallest group of snails, but now she says that one may be the fourth-smallest, due to discoveries within the last six years. 

"We're hearing a lot about biodiversity dying off," Jochum said. "Well, there's a lot of biodiversity we haven't discovered yet. There is still a lot out there."

The study, "Mother snail labors for posterity in bed of mid-Cretaceous amber," published May 18 in Gondwana Research, was authored by Adrienne Jochum, Naturhistorisches Museum Bern, University of Bern and Senckenberg Research Institute and Natural History Museum; Tingting Yu, Chinese Academy of Sciences and University of Science and Technology of China; and Thomas A. Neubauer, Justus Liebig University and Naturalis Biodiversity Center. 

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