The weird and wonderful of the Cambrian (Part 5): Haikouichthys ercaicunensis – one of the earliest steps on the vertebrate path
Jon Trevelyan (UK)
This is the fifth in my series of short articles on fossils of the Cambrian. Among the remarkable organisms preserved in the Early Cambrian Chengjiang biota, Haikouichthys ercaicunensis occupies a special place. Living around 518 million years ago, this small, fish-like animal is widely regarded as one of the earliest vertebrate-grade creatures known, which is far older than Pikaia and offering a crucial glimpse of the body plan from which true vertebrates would later emerge.
Although only a few centimetres long, Haikouichthys captures a formative stage in the evolution of the backbone-bearing animals that would come to dominate aquatic and terrestrial ecosystems.
Discovery and appearance
Haikouichthys was described in 1999 from specimens collected near Haikou in Yunnan, China, within the richly fossiliferous Maotianshan Shales. The Chengjiang deposits preserve animals in exquisite detail, and Haikouichthys is no exception.
Typically around 2.5-3cm long, this animal had a narrow, streamlined body, with a distinct head region, a tapering tail and a continuous fin fold running along the back and underside. A series of chevron-shaped muscle blocks are clearly visible along the trunk—these segmented muscles are typical of chordates, the group of animals that includes fish and, ultimately, all vertebrates.
Near the head, dark patches mark the likely positions of paired eyes, and the anterior region shows hints of simple gill structures. Running along the midline is a prominent rod-like structure interpreted as a notochord – a flexible internal support running the length of the body, giving it strength while still allowing it to bend during swimming.
Interpretation and classification
The combination of a notochord, a dorsal nerve cord, segmented musculature, and possible gill elements places Haikouichthys squarely within the chordates, and very close to the base of the vertebrate family tree.
Unlike later jawless fishes, Haikouichthys lacks mineralised armour or a hardened skeleton. Instead, its body was soft, flexible and adapted for undulatory swimming, which is an efficient way of moving through the water column at a time when complex fins and internal skeletons had not yet evolved.
The presence of simple gill structures suggests a shift toward more active respiration, while the fin fold would have stabilised the body in motion. Although its head was small and unspecialised, the presence of paired eyes marks a significant step in the development of vertebrate sensory systems.
Haikouichthys was probably a modest, agile swimmer that fed on small organic particles, plankton or soft-bodied prey. While not a predator in the mould of later vertebrates, it represents an early experiment in active, free-swimming lifestyles.
Significance
What makes Haikouichthys so significant is the clarity with which it displays the foundational features of vertebrate evolution. These include:
- a stiffening notochord;
- a dorsal nerve cord;
- V-shaped myomeres;
- a stabilising fin fold;
- possible gill pouches; and
- paired sensory organs.
Together, these form a remarkably advanced blueprint for such an early animal that is far more complete and vertebrate-like than the Middle Cambrian Pikaia.
Haikouichthys also illustrates how the transition toward vertebrates unfolded gradually, by adding and refining structures, rather than through sudden change. In its small, soft body lie the seeds of key vertebrate innovations.
Conclusion
Haikouichthys ercaicunensis may be tiny, but its place in evolutionary history is enormous. As one of the earliest animals to show a distinctly vertebrate-grade body plan, it provides an invaluable snapshot of the deep origins of the group that would one day give rise to fishes, amphibians, reptiles, birds and mammals. In this slender Early Cambrian swimmer, preserved with extraordinary clarity for more than half a billion years, we glimpse the very beginnings of the vertebrate journey, long before bones, jaws or armour ever evolved.