Metaspriggina: a small Cambrian animal with a chordate body plan

Metaspriggina walcotti was a small Cambrian chordate known from fossils in British Columbia. The specimens preserve a flexible notochord, a postanal tail, repeated W-shaped muscle blocks, paired eyes and seven pairs of gill bars. This combination supports its interpretation as an early vertebrate relative, though its exact position on the vertebrate tree remains under study. The enlarged first gill-bar pair has prompted discussion about the origins of jaws, but Metaspriggina itself had no demonstrated jaw. The Cambrian animal catalogue separates these direct anatomical observations from the evolutionary questions they raise.

Quick facts

New specimens changed its significance

Metaspriggina walcotti was named from Cambrian material, but its evolutionary importance became clearer after new fossils were collected from the Burgess Shale and Marble Canyon in British Columbia. Simon Conway Morris and Jean-Bernard Caron described additional specimens in 2014, including a slab bearing dozens of individuals. The expanded record made it possible to identify head and trunk features that were difficult to interpret in earlier material.

A concentration of many animals on one slab can reflect transport, burial conditions or accumulation over time. It is not by itself proof that they swam together as a school. The locality helps explain how the fossils were preserved, while the anatomy provides the evidence for classification.

A notochord, tail and segmented muscles

The long notochord ran along the body and acted as a flexible support. A tail extended behind the anus, an arrangement known as a postanal tail. Along the trunk, repeated W-shaped myomeres formed the blocks of muscle. Together, these structures fit the chordate body plan and suggest that the animal could flex its body to move through water.

The fossils are compressed films rather than intact soft bodies in three dimensions. Researchers identify boundaries by following repeated patterns and comparing them with other chordates. This makes the overall organization persuasive while leaving some fine details uncertain. An illustration showing exact muscle depth or a specific swimming stroke goes beyond what the slab records.

Eyes, nasal sacs and a differentiated head

Paired camera-like eyes are among the striking features reported in the newer material. Nasal sacs and a distinct head region add to the evidence that Metaspriggina was not simply a featureless swimming strand. These structures support comparisons with early vertebrates, but their presence cannot tell us the animal’s visual acuity or whether it relied on sight to catch prey.

Describing the eyes as camera-like refers to their organization, not to a complete modern visual system. Fossils can preserve position and outline, while the neural connections and optical performance are unavailable. The most reliable conclusion is that paired visual organs were present and that the head had several differentiated sensory structures.

Seven pairs of gill bars and the question of jaws

The pharyngeal region contains seven pairs of bars. The first pair is thicker than the following bars, a feature that encouraged discussion of how structures in the throat might have changed during vertebrate evolution. These bars are not jaws. No jaw apparatus is preserved in Metaspriggina, and it should not be described as a jawed fish.

Some authors have proposed that the anterior bars could illuminate the evolutionary history of the jaw-bearing vertebrate skeleton. That is a hypothesis about homology and transformation across lineages. It compares anatomical positions and developmental possibilities; it does not show that a particular bar directly became a jaw in this individual animal. The distinction between an evolutionary clue and a structure actually present is important here.

Where does it sit among early chordates?

The notochord, myomeres and postanal tail support chordate affinity, while the paired eyes and gill bars have been used to argue for a position among early vertebrates. Researchers debate the exact branching order because Cambrian chordates preserve different combinations of characters. A phylogenetic tree is a testable model of relationships, not a sequence of fossils arranged as proven direct ancestors.

Its significance is therefore comparative. Metaspriggina preserves a combination of features that helps researchers ask which parts of the vertebrate body plan appeared early and which arose later. It does not close the gaps in the fossil record or identify one species as the ancestor of all living vertebrates.

What its fossils do and do not reveal

Several dozen specimens are known from the described localities, including material on a single slab. The fossils support a small, flexible, swimming animal with a chordate support and differentiated head. They do not directly preserve color, schooling behavior, exact diet or every aspect of its circulation and nervous system. A popular image of a shoal should be treated as a plausible reconstruction rather than an observation.

Exceptional preservation can retain soft outlines that ordinary rocks lose, but compression and decay still alter what is visible. The most useful account makes the evidence explicit: notochord, muscle blocks, eyes and branchial bars are observed; swimming mechanics and evolutionary transformations are inferred at different levels of confidence.