Haikouichthys ercaicunensis was a small, primitive fish-like creature from the early Cambrian period. It possessed a streamlined body, a notochord, and paired sensory organs, suggesting early vertebrate features. Found in what is now China, it inhabited shallow marine environments.

Haikouichthys ercaicunensis is known from the fossil record and was discovered in the Chengjiang biota of Yunnan Province, China. This region represents the early Cambrian period, approximately 518 million years ago.

The Chengjiang formation, where Haikouichthys fossils have been found, indicates a marine environment. This area was part of a shallow sea with relatively warm waters during the early Cambrian period. The environmental conditions included sedimentary deposits that suggest a benthic zone with soft substrates.

Haikouichthys ercaicunensis is considered one of the earliest known vertebrates, occupying a niche as a small, likely free-swimming organism. It may have inhabited the water column near the seafloor, feeding on microscopic organisms or detritus. Its anatomical features suggest it was adapted to a life with some ability for directional swimming, possibly to evade predators or search for food.