Haikouichthys ercaicunensis was one of the earliest known vertebrates, living in what is now southwestern China during the Cambrian, around 518 million years ago.
About 2.5cm long (~1″), it had a streamlined body with a long fin along its back and tail. It lacked limbs and jaws, but it did have a flexible “upper lip” that could potentially have been used to manipulate food particles, either engulfing them on the seafloor or stirring them up into the water for filter-feeding.
And a recent study revealed a big surprise — it had four eyes! Along with its already-known large sideways-facing pair of eyes, it also had another smaller upwards-facing pair in the middle of its face, giving it a much wider field of vision and the ability to watch out for predators from multiple angles at once.
The extra pair of eyes may be the evolutionary precursors to the pineal complex in later vertebrates — represented in modern forms by the pineal gland and the parietal eye.
References:
- Kasal, Krystal. “Four-eyed Cambrian fish fossils hint at origins of vertebrate pineal complex” Phys.org, 22 Jan. 2026, https://phys.org/news/2026-01-eyed-cambrian-fish-fossils-hint.html
- Lei, Xiangtong, et al. “Four camera-type eyes in the earliest vertebrates from the Cambrian Period.” Nature (2026): 1-6. https://doi.org/10.1038/s41586-025-09966-0
- Mallatt, Jon. “Vertebrate origins are informed by larval lampreys (ammocoetes): a response to Miyashita et al., 2021.” Zoological Journal of the Linnean Society 197.2 (2023): 287-321. https://doi.org/10.1093/zoolinnean/zlac086
- Wikipedia contributors. “Haikouichthys” Wikipedia, 16 Mar. 2026, https://en.wikipedia.org/wiki/Haikouichthys