Sanctacaris was an arthropod. Its mouth appendages were clearly jointed, and it had a hard exoskeleton and many other characteristics of arthropods.
In fact, Sanctacaris was probably a chelicerate — the arthropod group that includes spiders, scorpions, and horseshoe crabs. This group got its start in the Precambrian seas, invaded the land more than 400 million years ago, and still thrives today.
The now extinct eurypterids were also chelicerates. These ancient “water scorpions” lived in the ocean and grew to gargantuan sizes — up to three meters (about 10 feet) long!!