The weird and wonderful of the Cambrian (Part 10): Myllokunmingia fengjiaoa – an early chordate on the threshold of vertebrates

Jon Trevelyan (UK)

This is the tenth in my series of short articles on fossils of the Cambrian. Among the richly preserved fossils of the Early Cambrian Chengjiang biota, Myllokunmingia fengjiaoa stands as one of the most revealing. Discovered alongside Haikouichthys, and dating to around 518 million years ago, this small, slender animal represents one of the earliest steps in the chordate-to-vertebrate transition.

Although only a few centimetres long, Myllokunmingia captures a crucial stage in the assembly of the vertebrate body plan, at a time when bones, jaws and complex fins had yet to appear.

Discovery and appearance

Myllokunmingia was first described in 1999 from beautifully preserved fossils in the Maotianshan Shales near Kunming, Yunnan. Like many Chengjiang organisms, its soft tissues are outlined with exceptional clarity, allowing a surprisingly detailed reconstruction of its anatomy.

Typically 2-3 cm in length, Myllokunmingia had a narrow, slightly flattened body with a tapering tail. A series of chevron-shaped myomeres – repeating blocks of muscle arranged in segments – run along the trunk, one of the clearest indicators that it belongs to the chordates, the group of animals that includes vertebrates and their closest relatives. Along the centre lies evidence for a notochord, a flexible internal rod that provided support and formed the main body axis in these early animals.

Near the head, the fossil reveals hints of pharyngeal structures interpreted as early gill pouches, which is a key innovation on the road toward vertebrate-style respiration. The head region itself is small and unspecialised, with no sign of jaws or armour, but shows a darker patch often interpreted as a primitive sensory area.

A continuous fin fold runs along the upper and lower margins of the body, providing stability in swimming long before differentiated fins evolved.

Interpretation and classification

The combination of a notochord, dorsal nerve cord, segmented myomeres, and possible paired gill elements places Myllokunmingia firmly among the earliest chordates and close to the base of the vertebrate lineage.

Compared with its close contemporary Haikouichthys, Myllokunmingia shows broadly similar organisation, although some details of the gill region and head structures differ. Together, the two highlight that early vertebrate-grade animals were already experimenting with a range of anatomical layouts.

Despite its primitive nature, Myllokunmingia was probably capable of active, undulatory swimming. Its fin fold would have helped stabilise the body, while the early gill structures hint at increasing respiratory efficiency, which is a major shift from the simpler filter-feeding strategies of pre-vertebrate animals.

Significance

Myllokunmingia is significant because it captures a very early stage in the development of features that define the vertebrate lineage. These include:

• a notochord;
• a dorsal nerve cord;
• V-shaped myomeres;
• possible gill pouches;
• a stabilising fin fold; and
• a simple, well-defined axial body plan.

These structures form part of the foundational architecture from which later fishes, and ultimately all vertebrates, would arise.

Perhaps most importantly, Myllokunmingia demonstrates that key vertebrate features evolved well before skeletons, jaws or complex fin systems. In the Early Cambrian seas, the chordate blueprint was already taking shape, but in a soft-bodied, minimally specialised form.

Conclusion

Myllokunmingia fengjiaoa may be small and subtle compared with some Cambrian oddities, but its evolutionary importance is unmistakable. As one of the oldest animals to show a recognisable vertebrate-grade organisation, it offers an invaluable snapshot of the earliest stages of our own distant lineage.

In this softly undulating Early Cambrian swimmer, preserved with remarkable fidelity for over half a billion years, we see the earliest outlines of the vertebrate story – a story that would eventually reshape life in the world’s oceans and far beyond.

Caption