Cambrian Explosion #58: Hymenocarina

The pancrustaceans are a grouping of mandibulates that contains all of the crustaceans and hexapods (insects and their closest relatives) along with their various stem-relatives.

They’re critical components of most ecosystems on the planet, and are major parts of the nutrient cycle. In aquatic environments the crustaceans dominate, with modern copepods and krill being some of the most abundant living animals and making up enormous amounts of biomass providing vital food sources for larger animals. On the land springtails and ants are especially numerous, and the air is full of flying insects, the only invertebrates to ever develop powered flight. Some groups of insects have also co-evolved complex mutualistic partnerships with flowering plants and fungi.

Hexapods and insects don’t appear in the fossil record until the early Devonian, but they’re estimated to have first diverged from the crustaceans* in the early Silurian (~440 million years ago), around the same time that vascular plants were colonizing the land.

(* Hexapods are crustaceans in the same sort of way that birds are dinosaurs. They originated from within one of the major crustacean lineages with their closest living relatives possibly being the enigmatic remipedes.)

But crustaceans and their pancrustacean ancestors go back much further into the Cambrian, and we’ll be finishing off this month and this series with some of those early representatives.

Continue reading “Cambrian Explosion #58: Hymenocarina”


Synophalos xynos, a shrimp-like arthropod from the Early Cambrian of China (~515 mya). Thought to be closely related to stem-crustaceans like Waptia, it was about 2cm long (0.75″) and had a bivalved carapace with a segmented body ending in a forked tail.

Unlike any other known arthropods, however, it formed long “conga line” chains of up to twenty individuals, with the tail of each animal locking securely into the shell of the next. The function of the these chains is unknown, although suggestions include some sort of mating behavior, migration, or defense against predators.

Only one specimen was found completely on its own, and its slightly longer carapace suggests it may represent a different solitary life stage of these strange little creatures.