The shieldless wonders: why these ancient jawless fish defy simple classification

Nature · Published 3 October 2026 Swipe more like this →

In short: Anaspida, or shieldless ones, were small marine fish that thrived from the early Silurian to the late Devonian. Unlike their armored contemporaries, they lacked heavy plating, opting instead for a flexible array of scales. Recent research suggests these mysterious creatures may be closer to modern hagfish than previously thought.

The Anaspida, a group of jawless vertebrates, earned their name from a lack of the heavy, bony armor that characterized many other ostracoderms of their era. Instead of a rigid shield, these fish were protected by small, weakly mineralized scales. In specific groups like the birkeniids, the body was covered in tile-like rows of aspidine, a type of acellular bony tissue. A defining physical feature of these animals was a large, tri-radiate spine located behind their gill openings, which typically appeared as a series of six to fifteen holes along each side of the body.

These fish were distinctively hypocercal, meaning their tails bent downward, and they possessed prominent, laterally placed eyes without scleral rings. While they were historically considered ancestors to modern lampreys, contemporary phylogenetic analyses have challenged this view. Some recent studies now propose that anaspids are stem-cyclostomes, placing them in a closer evolutionary relationship with hagfish and lampreys than with jawed fish. This shift in understanding highlights the complexity of early vertebrate evolution and the ongoing refinement of their taxonomic placement.

The classification of Anaspida has evolved significantly, particularly following the movement of genera like Jamoytius into the order Jamoytiiformes. Currently, the class is divided into two primary orders: the monogeneric Lasaniida, which includes the basal genus Lasanius, and the more diverse Birkeniida. The latter encompasses several families, such as Birkeniidae, Pterygolepididae, Rhyncholepididae, and Pharyngolepididae, which are identified through whole-body fossils. Other groups, like the family Septentrioniidae, are known primarily through their scale remains. As paleontologists continue to describe new genera, such as Kerreralepis and Cowielepis, the internal structure of these ancient fish continues to be re-evaluated.

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