The tuzoiids were an enigmatic group of Cambrian invertebrates known mostly just from their spiny bivalved carapaces. Although hundreds of fossils of these arthropods were discovered over the last century or so, only vague fragments of the rest of their bodies have been found even in sites usually known for preserving soft tissue impressions.
…Until late 2022, when several new specimens from the Canadian Burgess Shale deposits (~508 million years ago) were described showing tuzoiid anatomy in exceptional detail, finally giving us an idea of what they looked like and where they fit into the early arthropod evolutionary tree.
Tuzoiids like Tuzoia burgessensis here would have grown up to about 23cm long (~9″). They had large eyes on short stalks, a pair of simple antennae, a horizontal fluke-like tail fan, and twelve pairs of appendages along their body – with the front two pairs at the head end being significantly spinier, and most (or all) of these limbs also bearing paddle-like exopods.
The large carapace enclosed most of the body, and was ornamented with protective spines and a net-like surface pattern that probably increased the strength of the relatively thin chitinous structure.
Together all these anatomical features now indicate that tuzoiids were early mandibulates (part of the lineage including modern myriapods, crustaceans, and insects), and were probably very closely related to the hymenocarines.
Tuzoiids seem to have been active swimmers that probably cruised around just above the seafloor, with their stout legs suggesting they could also walk around if they flexed their valves open. The arrangement of their spiny front limbs wasn’t suited to grabbing at fast-swimming prey, but instead may have been used to capture slower seafloor animals or to scavenge from carcasses.
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.