Haikouichthys

Haikouichthys is an extinct genus of early chordates known from the Early Cambrian Maotianshan Shale deposits of Yunnan Province, China. The organism lived about 525 million years ago and is among the oldest described chordates. The genus includes the type species Haikouichthys ercaicunensis. Fossils show a small, fish-like animal about 2–3 centimeters long.

Morphology: The fossils reveal a head with a well-defined, possibly cartilaginous skull and eyes, a notochord extending along the body, a dorsal nerve cord, and segmented body musculature (myomeres). It possesses pharyngeal slits and a tail with fin-like elements. It lacks jaws and paired fins, features that place it outside modern fishes and toward primitive chordates.

Discovery and naming: The fossils were described in 1999 from the Maotianshan Shale. The genus name Haikouichthys combines Haikou, the locality, with ichthys, Greek for fish. The species epithet ercaicunensis refers to the Ercai Cun village near the site.

Significance: Haikouichthys is often cited as evidence for an early chordate body plan in the Cambrian explosion and is considered one of the earliest fish-like animals in the fossil record. Its exact placement within Chordata remains debated; some analyses suggest it is a primitive vertebrate or near-vertebrate, while others classify it as a non-vertebrate chordate.