Haikouichthys ercaicunensis
- 演化之聲
- Mar 19
- 3 min read
Age
Cambrian（"Stage 3"）
525-520 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Superclass: Agnatha
Order: Myllokunmingiida
Family: Myllokunmingiidae
Genus: Haikouichthys
Species: Haikouichthys ercaicunensis
Morphological description
Haikouichthys ercaicunensis is an early Cambrian jawless fish from the Chengjiang biota of Yunnan, China, dating to approximately 525 million years ago. It exhibits a typical fish-like, streamlined body with well-defined V-shaped or chevron-like myomeres, indicating active swimming capability. One of its most distinctive features is a continuous median fin fold, including dorsal, caudal, and ventral fin folds. The dorsal and caudal fins are connected without a clear boundary, while the ventral fin fold is separated from the caudal fin by the position of the anus and extends anteriorly. Although early interpretations suggested the presence of paired fins, extensive examination of hundreds of specimens has failed to confirm this, supporting the view that it possessed only median fins, similar to modern lampreys and hagfish.
The dorsal fin displays fine, radiating or obliquely arranged filamentous structures. These were once interpreted as fin radials but are more likely dermal folds or collagenous supports, comparable to ceratotrichia in modern cartilaginous fishes. The tail is slightly expanded and leaf-like, with a short post-anal region.
At the anterior end of the head, a pair of dark, oval impressions is interpreted as eyes, possibly with a simple lens structure, positioned dorsally and anteriorly, suggesting limited mobility. Between the eyes lies a pair of darker structures that may represent nasal sacs, and near the anterior margin, a curved paired structure with a central notch may indicate the position of a single nostril.
Posteroventral to the head are seven to eight pairs of gill arches, curved posteriorly and composed of single skeletal elements, likely cartilaginous. The interbranchial regions appear coarse, and gill pouch boundaries are occasionally visible. Along the dorsal side of the anterior trunk, a series of relatively regular arcualia or vertebra-like elements are arranged along the notochord. These are typically semicircular or bifurcated in shape and were probably cartilaginous, with some evidence of partial mineralization. The digestive tract runs along the ventral side, with the anus located near the tail.
Etymology
Haikouichthys: from Haikou, the locality where the fossils were discovered, combined with the Greek ichthys ('fish')
ercaicunensis: referring to Ercaicun, the village near Haikou where the fossils were found
Biological description
Haikouichthys ercaicunensis belongs to the stem group of Craniata and represents one of the earliest known vertebrates. It is closely related to Myllokunmingia fengjiaoa, another Chengjiang taxon. Although it lacks jaws and is classified among jawless fishes, it already exhibits several advanced vertebrate features, including paired eyes, possible olfactory organs, a likely otic capsule, a notochord, segmented arcualia, and well-developed gill arches and filaments. Its gill arch structure is relatively simple compared to the complex branchial basket of modern lampreys and, in some respects, resembles the primitive condition seen in early jawed vertebrates.
This species inhabited shallow, well-lit marine environments and coexisted with other planktonic organisms such as Pomatrum. It is interpreted as a moderately active swimmer. Although the absence of paired fins limited maneuverability, the median fin folds likely contributed to stability during locomotion. The anteriorly positioned eyes would have been advantageous for detecting food and predators. Fossils are often found in clusters at the interface between fine sandstone or siltstone layers and overlying mudstone in graded bedding, suggesting rapid burial events such as storm-induced sediment flows.
The morphology of Haikouichthys ercaicunensis indicates that early vertebrates had already evolved relatively complex sensory systems and axial support structures during the Cambrian explosion, laying the foundation for later diversification. While it shares some features with modern lampreys and hagfish, those groups represent more derived conditions, and Haikouichthys likely approximates a more ancestral stage in vertebrate evolution.
（Author: Shui-Ye You）
References
Shu DG et al. (1999). Lower Cambrian vertebrates from south China. Nature.
Shu DG et al. (2003). Head and backbone of the Early Cambrian vertebrate Haikouichthys. Nature.
Zhang XG and Hou XG. (2004). Evidence for a single median fin-fold and tail in the Lower Cambrian vertebrate, Haikouichthys ercaicunensis. J. Evol. Biol.
Montgomery RM. (2024). Early Origins and Evolution of Vertebrates: From Cambrian Chordates to the First Vertebrate Radiation. BJSTR.
24.67, 102.977
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