Jellyfish are primitive, tentacled, free-swimming invertebrate animals (Phylum Coelenterata, Class Scyphozoa) that have inhabited the marine environment for about 600 million years. Most of them are entirely soft-bodied and lack any kind of tissue capable of withstanding decay long enough to become fossilized.
The fossil record of soft-bodied jellyfish is scant, so scant that few taxonomic genera have been recognized. However, one unusual and extinct taxonomic order of jellyfish – the Conulariida – is represented fairly well in the fossil record. Conulariids existed from Paleozoic Middle Cambrian through Mesozoic Early Triassic, 510 to 250 million years ago. They are unique among jellyfish in having a thin but tough outer covering (periderm) composed of chitin hardened to some extent with phosphatic mineralization. As a result, they are prone to becoming fossilized. Indeed, the periderms of conulariids dominate the fossil record of jellyfish, with about 20 taxonomic genera recognized based on periderms. However, fossilized soft body parts of conulariids are not reserved and remain unknown.
Conulariid periderms have a four-sided pyramid shape that typically is elongated from apex to opposite end (aperture). In one unique genus, the pyramid is so squat as to be nearly flat. Periderm length ranges from 0.3 to 16 inches, though 1.5 to 4 inches is the norm. If you sliced across the periderms, they appear as squares or rectangles or even parallelograms. The outer surface usually is marked with grooves and ridges. When threatened the animal could close its aperture end by bending and folding the adjacent periderm inward. The aperture region of the periderm likely was more chitinous and less phosphatic to facilitate bending and folding. This would explain why fossil periderms are rarely found with the aperture region preserved.
Conulariids spent the first part of life with their apex of periderm attached to a solid object on the sea floor. Upon reaching adulthood, they detached themselves, secreted a rounded patch on the broken end and became freeswimming, presumably with the apex pointing up and tentacles spreading out from the aperture. Some scientists speculate that the periderm perhaps housed a small gas chamber for regulating buoyancy. As with all jellyfish, conulariids would have subdued prey by deploying stinging cells (nematocysts) that occur in great number on the tentacles of jellyfish.
Conulariid jellyfish and and other fossils are on display in our Ed Clark Museum of Missouri Geology.