One of the characteristic features of the crustacean lineage are their larval forms, passing through various tiny larval stages. They often look nothing like their eventual adult forms and historically weren’t even recognized as being the same species, with their complex lifecycles not being properly recognized until the late 1800s.
A lot of Cambrian crustaceans are only known from their larvae, preserved in exquisite microscopic detail in sites of “Orsten-type preservation”. Only disarticulated fragments of larger-bodied forms have been found in a few places, and it isn’t until much later in the Paleozoic that fossil crustaceans actually seem to become abundant in marine ecosystems.
It’s not clear why there’s such a bias in their early fossil record compared to most other arthropods, but possibly they were just very very rare animals early on. Adult forms may have mostly lived in places where they just didn’t fossilize, while their tiny larvae sometimes dispersed into different environments with a better chance of preservation.
One of the earliest known of these crustacean larvae, and one of the oldest known true crustaceans, is Wujicaris muelleri.
Found in the Chinese Chengjiang fossil deposits (~518 million years ago), this microscopic larvae was just 270μm long (0.01″). It had a wide shallow head shield with a backwards-pointing spine, a pair of eyes, and another long spine projecting from its underside at the front of its body.
Like similarly-shaped modern larvae it probably lived on or just above the seafloor as meiofauna, using its developed head appendages for both locomotion and catching food particles.
For such an early example of a crustacean it’s surprisingly similar to modern forms, resembling the “metanauplius” stages of some copepods and barnacles. Along with Yicaris, another larva from the same deposits, it was a probably a basal member of the major crustacean lineage that both those groups are part of: the altocrustaceans.
It suggests that some groups of crustaceans established their specific larval forms very early on in their evolution, and hit on a something that worked so well for them that they’ve barely needed to change it in over half a billion years.
The pentastomids, or “tongue worms”, are a very unusual group. Small worm-like animals that almost exclusive parasitise the respiratory tracts of vertebrate hosts, outwardly they don’t even look like they’re arthropods – but their true affinities are revealed by their chitinous “skin” and arthropod-like nervous system.
Their evolutionary affinities have been controversial, but they’re now generally considered to be highly specialized and modified crustaceans and very closely related to the parasitic fish lice. An alternate hypothesis proposes them instead as being surviving early panarthropods, related to tardigrades or lobopodians – but this is based purely on morphology, while the crustacean placement is also supported by genetic evidence.
Found in the Swedish Orsten Lagerstätte (~497 million years ago), this larva was about 0.5mm long (0.02″). Like other tongue worms it had four limbs on its head ending in hooked grasping claws, but unlike modern forms it also had two pairs of vestigial legs further down on its body.
It’s unclear if these very early pentastomids were actually parasitic, or what their hosts would have been at the time if they were. Conodonts have been proposed as potential hosts, but they also may have initially externally parasitized other types of arthropods – a Silurian-aged fossil shows an ancient tongue worm “caught in the act” of clinging on to an ostracod.