Mosura fentoni
Mosura fentoni is an extinct species of hurdiid radiodont from the Cambrian Burgess Shale in British Columbia, Canada. M. fentoni is the only species in the genus Mosura, and is known from sixty specimens collected between 1990 and 2022.
Discovery and naming
Mosura fentoni is known from sixty specimens. The holotype, ROMIP 67995 preserves a complete individual in dorsal view. Other notable specimens are ROMIP 66108, ROMIP 67998, ROMIP 68004, ROMIP 67999, and ROMIP 67979, which all preserve internal organs. The specimens were discovered in the Raymond Quarry and Marble Canyon localities within the Burgess Shale from 1990 to 2022, with the majority being placed in the invertebrate paleontology collection of the Royal Ontario Museum.In 2025, Moysiuk & Caron described Mosura fentoni as a new genus and species of radiodont based on these fossil remains. The generic name, Mosura, is named after Mothra a moth-like kaiju monster featured in films by the Japanese company Toho. The name was chosen for M. fentoni in reference to the animal's moth-like appearance. The name was romanized according to Hepburn style. The specific name, fentoni, honours Peter E. Fenton and his work as a technician in the Royal Ontario Museum Invertebrate Palaeontology section, and for his unwavering friendship to both authors.
Description
Mosura is known from specimens ranging in length from, making this taxon one of the smallest radiodonts known. The head has three eyes: a pair of eyes on short stalks and a median eye. The body is divided into 26 segments, the highest number found in any radiodont. The primary body sections are the head, the four-segmented neck, and the trunk. The trunk is divided into the anterior six-segmented mesotrunk, and posterior posterotrunk, which has at most 16 segments. Like other radiodonts, the body bears pairs of swimming flaps, which are considerably larger on the mesotrunk than on the posterotrunk. The gills are very large compared to body size.Mosura is one of four radiodont taxa with known juvenile fossils. The juveniles in Mosura are distinguished by fewer segments in the posterotrunk, and possibly mesotrunk. An increase in total segment number during ontogeny is consistent with previous observations in Stanleycaris. Moysiuk and Caron noted that an increase in mesotrunk segment number implies that some posterotrunk segments would have differentiated during ontogeny, incorporating with the mesotrunk. They note that this is uncommon, but not unprecedented, as this has been observed in other arthropod groups, such as the fuxianhuiid Fuxianhuia.
Classification
Mosura has a mixture of traits known from other basal hurdiids and non-hurdiids, traits apomorphic to Hurdiidae, traits shared by other hurdiids, and derived traits. It is not known whether the median eye in this taxon was compound or if it had a single lens, however Moysiuk and Caron favor the single lens theory, pointing to similarities between it, and the potentially homologous median eyes seen in crown-group euarthropods. Similar to Stanleycaris, this taxon seemingly lacks lateral sclerites, a trait only seen within hurdiidae between the two genera.In their phylogenetic analysis, Moysiuk and Caron recovered Mosura as a basal radiodont in the family Hurdiidae, finding it to be sister to all other hurdiids. Radiodonts are a diverse and long-lasting order of lower Paleozoic marine panarthropods. The results reproduced in the cladogram below are based on their pruned maximum clade credibility tree, they also produced a majority rule consensus tree where Mosura was found to be in a polytomy with Stanleycaris, Schinderhannes, Peytoia and the clade formed by Aegirocassis and Hurdiinae.
Mosura is one of three basal hurdiid genera that show a mixture of traits expected from hurdiids and non-hurdiids, the others being Stanleycaris and Schinderhannes. Regarding the specialised respiratory tagma, the authors speculate that they were—in conjunction with the reduced size—an adaptation for the oxygen-stressed environments that were proximal to the site of burial. However, while they find it unlikely, they could have been an adaptation to distinct behavioural traits such as the proposed nektobenthic sediment-sifting macropredatory ecology proposed for most hurdiids.