How did an ancient shark parasite end up fossilized in tree resin?

During its lifetime nearly 100 million years ago, a newfound parasitic worm likely made its home in the bellies of fish. So how one ended up preserved in amber, fossilized tree resin, has paleontologists scratching their heads.

Unearthed in northern Myanmar, the worm has several features that closely resemble those of modern tapeworms in shark intestines, paleontologist Cihang Luo and colleagues report March 22 in Geology.

Luo’s team had been examining amber collected from traders in Myanmar, finding mostly insects and roundworms trapped inside, when the researchers came across a “strange-looking fossil,” says Luo, of the Nanjing Institute of Geology and Paleontology in China. This 10-millimeter-long threadlike specimen appeared flatter than typical roundworms. Observations under a microscope revealed armor, tentacles and hooklets that looked bigger than, but still similar to, the tentacles of modern flatworms that infest sharks and rays.

Scientists have previously found flatworm eggs preserved in 270-million-year-old fossilized shark dung (SN: 6/5/01). Due to flatworms’ small, soft bodies and transient life cycles, “finding body fossils is exceedingly rare,” Luo says.

The fossil, says taphonomist Raymond Rogers of Macalester College in St. Paul, Minn., “is an exceptional preservation and a puzzle for people to solve.”

The strange finding is “very hard to explain because there are not a lot of sharks living in trees,” jokes paleontologist Kenneth De Baets of the University of Warsaw in Poland. “It’s like winning the lottery — one in a million.”

Perhaps a scavenger feasting on a beached shark carcass picked up the parasite and eventually somehow tossed it into a nearby tree, Luo and colleagues speculate.

Confirming this preservation scenario will require “complete specimens or host remains,” De Baets says.