You can’t always judge a book by its cover. That is a lesson scientists have recently learned about the nectocaridid — a 500-million-year-old marine creature that looked like a squid but turned out to be something completely unexpected.
For the past 15 years, nectocaridids were believed to belong to the cephalopod family due to their tentacles and particularly large, squid-like head. Now, thanks to a new study published in Science Advances, they have been placed in their rightful home as ancestors to another recently-discovered marine animal: the arrow worm.
The new classification of the nectocaridids as ancient arrow worms both clears up a confusing evolutionary picture while also complicating another one.
“Around 15 years ago a research paper, based on fossils from the famous Burgess Shale, claimed nectocaridids were cephalopods. It never really made sense to me, as the hypothesis would upend everything we otherwise know about cephalopods and their anatomy didn’t closely match cephalopods when you looked carefully,” said Jakob Vinther, associate professor in Macroevolution at the University of Bristol, in a press release.
Read More: 518 Million Years Ago, the Biggest, Baddest Predators Were… Worms?
Solving A Marine Mystery
This exciting reclassification began when the team discovered new nectocaridid fossils during an excavation expedition at Sirius Passet in North Greenland. Due to exceptional preservation, the Sirius Passet is an archaeological gold mine for marine organisms from the Early Cambrian period (518 million years ago).
Researchers analyzed 25 nectocaridid fossils to determine whether this creature was indeed an ancient squid. Another collection of Sirius Passage fossils helped them achieve this goal — a small group of swimming worms called arrow worms.
Arrow worms have a very distinct feature known as a ventral ganglion. The ventral ganglion is a collection of nerves found on an arrow worm’s belly and is a biological feature unique only to them.
Although it may share physical characteristics with ancient squid, the presence of the ventral ganglion firmly places nectocaridids into the arrow worm family tree.
“We now had a smoking gun to resolve the nectocaridid controversy,” said Tae-Yoon Park, from the Korea Polar Institute, in the press release. “Nectocaridids share a number of features with some of the other fossils that also belong to the arrow worm stem lineage. Many of these features are superficially squid-like and reflect simple adaptations to an active swimming mode of life in invertebrates, just like whales and ancient marine reptiles end up looking like fish when they evolve such a mode of life.”
From Warrior to Worm
The addition of the nectocaridid to the arrow worm lineage is both exciting and revealing. Modern arrow worms are considered to be quite simple, non-predatory creatures. However, the nectocaridid was a formidable predator, existing high up on the food chain and stealthily hunting its prey.
One of the major differences between the arrow worm and its ancient ancestor is their eyes. Arrow worms can barely see, while nectocaridids have human-like eyesight capabilities.
“Nectocaridids have complex camera eyes just like ours. Living arrow worms can hardly form an image beyond working out roughly where the sun shines. So, the ancestors of arrow worms were really complex predators, just like the squids that only evolved about 400 million years later,” said Vinther in the press release.
Overall, nectocaridids were bigger, better swimmers than their arrow worm descendants, and their keen eyesight and long antennae suggest they were successful predators. So successful in fact that they may have been swimming carnivores, with the team finding several arthropod shells inside the nectocaridid’s digestive tract.
Read More: Once Thought Mythical, Colossal Squid Spotted Alive for the First Time
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
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Science Advances. A fossilized ventral ganglion reveals a chaetognath affinity for Cambrian nectocaridids
As the marketing coordinator at Discover Magazine, Stephanie Edwards interacts with readers across Discover’s social media channels and writes digital content. Offline, she is a contract lecturer in English & Cultural Studies at Lakehead University, teaching courses on everything from professional communication to Taylor Swift, and received her graduate degrees in the same department from McMaster University. You can find more of her science writing in Lab Manager and her short fiction in anthologies and literary magazine across the horror genre.
