Nectakarizids are enigmatic Paleozoic creatures with a debatable classification. These beings evolved for aquatic life, boasting fins, camera-like eyes on stalks, and paired tentacles. Earlier theories proposed they belonged to a unique crustacean-like phylum, notochord, cephalopods, or even radioyoloons. However, new fossil discoveries from North Greenland indicate that Nectocalids are actually the early relatives of Arrow worms, also known as Chaetognaths. This discovery suggests that these relatively simplistic marine arrow worms had ancestors that played a significant predatory role within much more intricate anatomical structures and food webs.

“About 15 years ago, a study based on the notable Burgess Shale fossil suggested that Nectochalidosis is a type of cephalopod,” said Jacob Vincer, a paleontologist at the University of Bristol.

“This argument seemed illogical to me as it contradicts much of what I understand about the taxonomy of these organisms.

In the recent research, Dr. Vinther and his team described Nektognathus evasmithae, a newly identified Nectocalid from the Sirius Passett Lagerstätte, dating back 519 million years in North Greenland.

By examining 25 fossil specimens of Nektognathus evasmithae, they successfully positioned the Nectocalid within the Tree of Life.

“We found that remnants of the nervous system manifested as paired mineralized structures, providing insight into these animals’ position in the evolutionary tree,” Dr. Winter remarked.

Nektognathus evasmithae holotype. Image credit: Vinther et al., doi: 10.1126/sciadv.adu6990.

Recently, paleontologists found a fossil from Sirius Passett, which belongs to a different branch of the evolutionary tree—a small group of swimming organisms known as arrow bugs and Chaetognaths.

“These fossils exhibit distinctive traits that set them apart from arrow worms, particularly the abdominal ganglia,” explains Dr. Tae Yoon Park, a paleontologist at the Korean Institute of Polar Research.

The abdominal ganglia consist of large nerve clusters situated above the abdomen in living arrow bugs, typical for this type of creature.

The distinctive anatomical features, coupled with unique preservation conditions, indicate that they may be replaced by phosphate minerals during the decomposition process.

“We now have a compelling piece of evidence to resolve the Nectkalido debate,” Dr. Park stated.

“Nectocaridids share numerous features with other fossils that are also part of the arrow worm lineage.”

“Many of these characteristics may superficially resemble squid and demonstrate a simple adaptation of invertebrates to a more dynamic swimming lifestyle, paralleling how whales and ancient marine reptiles evolved similar traits for their aquatic lifestyles.”

“Nectakalids possess complex camera-like eyes akin to ours,” Dr. Vincer elaborated.

“Contemporary arrow worms have a limited ability to form images, primarily detecting movement in well-lit environments.”

“Thus, the ancestors of arrow worms were indeed sophisticated predators, much like squids that appeared around 400 million years later.”

“Consequently, we can illustrate how arrow worms have a more significant role in the food chain than previously thought.”

“Our fossils are notably larger than the average living arrow worm and likely feature various adaptations for swimming, such as eyes and elongated antennae.

“To further substantiate the carnivorous nature of Nectkalizids, we discovered several specimens containing the remains of a swimming arthropod known as Isoxys in their gastrointestinal tract.

This study was published this week in the journal Advances in Science.

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Jacob Winter et al. 2025. Fossilized abdominal ganglia reveal the affinity of Chaetognaths for Cambrian calizids. Advances in science 11 (30); doi:10.1126/sciadv.adu6990