If confirmed as a genuine example of an early fish, the wormlike fossil would be older than the current oldest record of a fish, Guiyu oneiros. Discovered in 1999, Guiyu fossils found in China had been dated to about 530 million years ago, making them the earliest known examples of a fish that had all the main anatomical features that would characterize fishes throughout history — including a backbone, a head and a skeleton.
The newly described fossil, named Haikouichthys ercaicunensis, is part of a diverse group of wormlike creatures known as the stem group, meaning that they represent a common ancestor to what are now two distinct lineages: tunicates and craniates. Tunicates are filter-feeding sea animals, while craniates include vertebrates, such as humans, and many other animals with skulls and backbones.
Scientists have proposed various theories about the position of Haikouichthys in this evolutionary tree of life, but none of the hypotheses are definitive, and all have challenges.
Previous studies have proposed Haikouichthys is some sort of very primitive fish, while others, including a 2021 study, described the creature as a member of the tunicate lineage, though somewhat different from extant tunicates, making it a "stem-tunicate."
In the new paper, published May 19, 2022, in the journal Nature, Jian Han from Northwest University in China and colleagues re-examined the 2-inch (5.1 centimeters) long Haikouichthys fossil, using higher-resolution imaging techniques and newly collected specimens.
Their analysis concluded that Haikouichthys is "a primitive craniate," and thus may be an ancestor of all backboned animals, including humans.
The creature appears to have had structures called pharyngeal pouches, which in later fish help in breathing, and some evidence suggests it may have had some type of jaw, the researchers said.
The fact that Haikouichthys does not have a fully formed skeleton doesn't rule out the possibility that it is an early fish. Many lineages of vertebrates initially evolved with a cartilaginous skeleton, which is not fossilized readily, before developing a bony skeleton, the researchers noted.
"Therefore, the absence of a bony skeleton does not necessarily refute the possibility that Haikouichthys is an early fish," the authors wrote in the study.
The scientists hope that future studies, including more advanced chemical and microscopic analyses of the fossil, will help confirm the evolutionary relationship of Haikouichthys.
