Mosura fentoni: a three-eyed radiodont with a divided trunk

Mosura fentoni is a small radiodont from the Wuliuan Burgess Shale, formally named in 2025 from material collected at several localities in British Columbia. It had three eyes, a pair of grasping frontal appendages and a trunk divided into regions with sharply different segment and flap sizes. The most striking region is the tightly packed rear trunk, where fossils preserve bands of lamellae and internal traces interpreted as part of a respiratory system. Mosura adds an unusual body plan to the Cambrian animal catalogue.

Quick facts

A newly named animal from long-studied quarries

Joseph Moysiuk and Jean-Bernard Caron described Mosura fentoni in 2025. The species is based on a large collection from the Burgess Shale, including Marble Canyon, Tokumm Creek and Raymond Quarry. Its holotype, ROMIP 67995, is a complete dorsal specimen preserved as part and counterpart. The authors examined 61 fossils, ranging from 15 to 61 millimetres in body length.

The name Mosura refers to Mothra, the fictional Japanese monster whose moth-like form reminded the researchers of their fossil. The species honours Peter E. Fenton, a long-serving technician in the Royal Ontario Museum's Invertebrate Palaeontology section. These names explain the taxon's history; they do not imply that its anatomy resembled a moth or that it was related to insects.

Three eyes and a pair of grasping appendages

The head is short and carries two lateral eyes on small stalks plus a median eye. Internal reflective traces associated with the eyes were interpreted as optic neuropils and nerves, while a central patch may represent part of the brain. These internal structures are preserved as mineralized traces, not as an intact nervous system, so their anatomical identity comes from position, composition and comparison with other radiodonts.

A pair of frontal appendages carried six elongate, inward-curving endites. Each endite ends in a forked tip, and the appendages could apparently flex forward in at least one specimen. The authors argued that the long endites were better suited to hook larger prey than to form a fine filtering mesh. This is a functional inference from shape: no captured prey or diagnostic gut meal has been reported for Mosura.

The rounded oral cone was interpreted as having a four-rayed arrangement, although individual plates are difficult to distinguish in most specimens. That combination of a grasping appendage and a circular mouth connects Mosura to other radiodonts such as Hurdia and Anomalocaris, while its particular trunk anatomy sets it apart.

Three trunk regions divide movement from the crowded rear

Behind the head are four short neck segments. The six mesotrunk segments carry broad, triangular flaps that project to the sides and provide the main visible swimming surfaces. Farther back, the posterotrunk is composed of much shorter, tightly packed segments with greatly reduced flaps. Some specimens preserve at least 16 segments in this posterior region, although the smallest posterior boundaries are difficult to count and the maximum could be higher.

This division is called tagmosis: neighbouring body segments become differentiated into regions with different anatomy. The fossils directly show changes in segment spacing and flap size. The interpretation that the mesotrunk provided propulsion is consistent with the broad flaps; the precise contribution of the posterotrunk to steering or stability remains uncertain. No tail fan or posterior blade is preserved as in some other radiodonts.

The smallest specimens seem to have fewer posterior segments than larger ones. That pattern could indicate that additional segments formed during growth, but the researchers note that tiny segments are harder to see in small, compressed fossils. A growth process that shifts segments between regions is possible, not settled. More clearly preserved juveniles would be needed to test it.

Why the rear is interpreted as respiratory

Each trunk segment carries a band of fine lamellae. Their bands continue across the underside, and individual lamellae are sometimes visible where mineralization is especially good. Reflective stains inside the body extend into triangular areas within the flaps. Moysiuk and Caron interpreted these as traces of lacunae, open spaces through which haemolymph circulated around organs and tissues.

The combination of posterior lamellae and internal traces supports a gill-rich rear region, but the function is not measured directly. The fossils do not preserve oxygen flow or an operating circulatory system. The study compared estimated gill length with body length across radiodonts; it could not reliably calculate actual gill surface area because the fossils are flattened and structures overlap. Its result is therefore a relative anatomical estimate, not a physiological reading.

The authors found Mosura had the highest ratio of estimated total gill length to body length among the radiodonts they compared. That may help explain the differentiated posterotrunk, especially in an active animal with broad swimming flaps. It also invites comparison with later arthropods that evolved specialized respiratory regions independently. Similar function does not prove that these structures were inherited from a common ancestor.

Relationships and behaviour remain hypotheses

The frontal appendages and oral cone include characters associated with Hurdiidae, the radiodont family that also includes Hurdia. Other features, such as the short head, long body and distinct neck, resemble non-hurdiid radiodonts. Phylogenetic analyses place Mosura near the base of the hurdiid grouping, but the exact position changes among alternative trees. It is not securely the direct ancestor of another named genus.

The overall body and appendages support a small swimming predator capable of pursuing mobile prey. The mesotrunk flaps likely supplied propulsion, while the front appendages could bring food toward the mouth. Yet the fossil record does not preserve a hunt, a specific prey species or the animal's speed. A reconstruction of Mosura as a fast hunter is plausible, not an observed event.

Exceptional preservation also records the limits of interpretation. Gut traces are incomplete, internal stains have been assigned to the circulatory lacunar system by comparison and chemistry, and extensive phosphate overgrowth obscures original tissues in some specimens. The species is remarkable because it preserves several anatomical systems together, but each trace still has to be distinguished from minerals formed during decay and burial.