Scientists Finally Solve Mystery of Tiny, Ancient Worm’s ‘Mismatched’ Head and Body

The head of the arrow worm, Parasagitta elegans. This group of animals are the closest living relatives to Amiskwia.

Credit: Rafael Martin Ledo/Consejería de Educación de Cantabría

A microscopic worm that lived about 505 million years ago was a “patchwork” species with body parts that matched up to different worm families, confounding experts who tried to classify the tiny creature.

As if this worm weren’t weird enough already, scientists recently discovered that it sports a set of hidden jaws that had gone undetected for more than 100 years.

However, rather than adding to the conundrum, these secret structures may have solved the century-old puzzle of where the worm belongs on the tree of life — between two ancient worm lineages that are still around today, scientists reported in a new study. [Cambrian Creatures Gallery: Photos of Primitive Sea Life]

Amiskwia, which was described in 1911, somewhat resembled its cousins in the arrow worm group, but it lacked some of their distinctive features, such as spines near the head that are used for grasping prey, lead study author Jakob Vinther, a senior lecturer with the School of Earth Sciences at the University of Bristol in the United Kingdom, told Live Science in an email.

The worm also shared some features with swimming ribbon worms — but again, it lacked certain other structures that are common in that group, Vinther explained.

The fossil Amiskwia sagittiformis from the Burgess Shale (508 million years old) preserves bilateral jaw elements inside its head.

Credit: Luke Alexander Parry/University of Bristol,Yale University

For decades, scientists have argued over where Amiskwia belongs: Should it be grouped with arrow worms, with ribbon worms or perhaps placed on its own branch of the worm family tree, representing a body plan that went extinct millions of years ago?

When the researchers examined fossils of the wormthey tried a technique that hadn’t been used before: coating the fossils with ammonium chloride. The chemical made internal structures stand out, and the scientists were able to discern “distinct robust elements in its head,” according to Vinther.

“It looked like a pair of jaws to me, and no other person had seen that before,” he said.

Worms known as gnathiferans have jaws like the ones the study authors saw in Amiskwia, Vinther explained. Since Amiskwia had a body like an arrow worm and jaws like a gnathiferan, the researchers suggest that arrow worms and gnathiferans are more closely related than once thought. In fact, arrow worms’ signature grasping spines could have evolved from Amiskwias jaw structures, which probably helped the worm gobble up zooplankton and small crustaceans, Vinther said.

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