You’ve Heard of Water Bears, but How About These Ancient Mold Pigs?

A mold pig in amber, seen through a microscope.
Image: G. Poinar et al., 2019

An analysis of 30-million-year-old amber has resulted in the discovery of a previously unknown microscopic creature from the Cenozoic period. Bearing a resemblance to tardigrades (aka water bears), these now-extinct “mold pigs,” as they’ve been dubbed, are unlike anything seen before.

Introducing Sialomorpha dominicana, a newly discovered microinvertebrate found locked in amber from the Dominican Republic. Its discoverers, paleobiologist George Poinar Jr. from Oregon State University and invertebrate zoologist Diane Nelson from East Tennessee State University, have dubbed the creature a “mold pig” in honor of its portly, porcine appearance and its diet, which consisted primarily of fungi. Details of the discovery were recently published in Invertebrate Biology.

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The 83-year-old Poinar is no stranger to working with fossils trapped in amber. His 1982 research paper gave sci-fi author Michael Crichton the idea of extracting dinosaur DNA from insects trapped in amber, as portrayed in the film Jurassic Park. Poinar has made a career working with amber, finding fossilized flies, bees, bats, and ancient flowers.

This time around, however, Poinar, along with Nelson, discovered a creature that’s invisible to the human eye—a microinvertebrate measuring no more than 100 micrometers long.

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Adult mold pigs in amber.
Photo: G. Poinar et al., 2019

“It took me many days, weeks, and months to examine [the specimens], and then under the compound microscope, “ Poinar wrote in an email to Gizmodo. “They are as small as the smallest tardigrades, and they have eight legs like tardigrades. However they possess mandibles but no claws, whereas tardigrades have claws and stylet mouthparts,” meaning a sharp, piercing mouthpiece.

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The amber fossil analyzed by Poinar and Nelson contained literally hundreds of mold pig specimens, allowing them to study a host of different biological aspects, including their anatomy, reproductive behavior, growth, development, and diet. For example, the mold pigs featured flexible heads and they grew by molting their exoskeleton.

The researchers also discovered other creatures locked inside the amber, including pseudoscorpions, nematode worms, fungi, and various protozoa. The mold pigs preferred warm, moist environments, where they fed on fungi, and sometimes other small invertebrates, the researchers found.

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Because nothing comparable exists in the scientific record, whether extinct or extant, the mold pigs were assigned to an entirely new family, genus, and species. The “fossil shares characteristics with both tardigrades and mites, but clearly belongs to neither group,” wrote the authors in their paper. Its major distinguishing features “are its mouthparts in combination with the lack of claws, four pairs of legs, terminal anus, and reproductive openings,” Poinar told Gizmodo.

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The researchers don’t know when this family of invertebrates originated, how long it lasted, or if any descendants are still around today. The discovery shows that extremely tiny animals were able to live in Cenozoic microhabitats, and that these creatures could use fungi as a food source, according to Poinar.

Tardigrades, nicknamed water bears, are known for their extreme resiliency, as they’re able to survive long-term deep-freezing and the vacuum of space. We can only guess as to whether these mold pigs would have possessed similar powers.

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About the author

George Dvorsky

George is a senior staff reporter at Gizmodo.