Negative ontogenetic allometry of cardinal spines in the early Cambrian arthropod Isoxys volucris indicates their defensive function

The characteristic cardinal spines of Isoxys , a cosmopolitan bivalved arthropod, have been focal to understanding its role in Cambrian ecosystems. It has been proposed that the spines had either a hydrodynamic function, to aid buoyancy, or a defensive function, to protect against predators. Here, we demonstrate that the unusually elongated cardinal spines in Isoxys volucris (by far the most abundant taxon in the lower Cambrian Sirius Passet Lagerstätte, North Greenland) had a primary defensive function. Spine measurements of 85 specimens show a negative allometry during ontogeny, with the ratio of cardinal spine length to carapace length decreasing from >3.2 to 0.9. Negative allometric growth is inconsistent with a hydrodynamic function since larger carapaces would require spines which are proportionally at least as long (isometry or positive allometry). Instead, the negative allometry provides evidence for a defensive adaptation comparable to that seen in modern lower‐trophic organisms, in which elongated spines increase the overall size of juveniles to deter predators. Isoxys volucris was the dominant food source for higher‐trophic benthic and pelagic predators in the Sirius Passet biota, as revealed by the gut contents of arthropods, lobopods, palaeoscolecids and stem‐chaetognaths. Its long spines therefore indicate similar adaptive responses to extremely high predation pressures in both modern and early Cambrian ecosystems.