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.

Some of the earliest known pancrustaceans were the hymenocarines, a lineage of superficially shrimp-like mandibulates with a bivalved carapace covering their head and thorax. Although known only from the Cambrian, they were a diverse group during their existence and were some of the most abundant arthropods in some fossil sites.

Sometimes they’re considered to be early or stem-mandibulates, branching off before the euthycarcinoids and the myriapod lineage, but generally they’re placed as some of the earliest known pancrustaceans – and Ercaicunia multinodosa helps support that idea.

Ercaicunia multinodosa

Known from the Chinese Chengjiang fossil deposits (~518 million years ago), Ercaicunia was about 1.1cm long (~0.4″). Micro-CT scanning of some of its tiny fossils has revealed much of its anatomy in high detail, showing features that identify it as one of the earliest known pancrustacean fossils – and the earliest that isn’t microscopic.

It was either part of an early branch of the hymenocarines, or alternatively wasn’t quite an actual hymenocarine itself, possibly being a very close relative or stem-member of their lineage.

It had a pair of large oval valves forming a carapace around the front of its body, with its long limbless abdomen and tail fan extending out the back. There were 16 pairs of biramous limbs on its thorax and several pairs of small spines on its back, and its head bore two pairs of antennae – one large pair in front and another much smaller pair hidden under its carapace – along with a pair of mandibles and maxilluae.

It also doesn’t seem to have had any eyes, suggesting it had some sort of lifestyle where vision wasn’t much use. Its mouthparts also indicate it was probably feeding on something that required manipulation and chewing up, but details of its diet and ecology are still poorly understood.

Fibulacaris nereidis

Fibulacaris nereidis was one of the more unusual hymenocarines, known from the Canadian Burgess Shale deposits (~508 million years ago).

Up to 2cm long (0.8″), its narrow keeled carapace had a long backwards-pointing spine, with the front of its body curling over completely in a U-bend so that its head also faced backwards. It had stalked eyes, no obvious antennae or mandibles, and around 40 pairs of limbs running all the way along to its tail fan, indicating it had either a very short or a non-existent abdominal region.

It was probably a filter-feeder, using the shape of its carapace and the beating of its many legs to create a current drawing plankton and suspended organic particles towards its mouth. The combination of this lifestyle and its unusual anatomy suggests it probably swam around upside down, similar to some modern crustaceans like fairy shrimp.

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