The discovery of fossils with these elements associated with each other led to a jawless eel-like chordate (the major group of animals that includes vertebrates) affinity (Briggs et al 1983).
In a scientific first, the results suggest that bivalves, a class of animals that includes clams and oysters, are most closely related to gastropods such as snails and slugs.
Tetrapod (superclass Tetrapoda), a superclass of animals that includes all limbed vertebrates (backboned animals) constituting the classes Amphibia (amphibians), Reptilia (reptiles), Aves (birds), Mammalia (mammals), and their direct ancestors that emerged roughly 397 million years ago during the Devonian Period.
The newly discovered creature is a tiny arthropod - a group of animals that includes modern insects, spiders, shrimp, and crayfish.
At 55 million years old, it represents the earliest known member of this broad group of animals that includes humans.
This contrasts with most other cephalopods - the class of animals that includes octopuses, squid and cuttlefish - which expend vast amounts of energy to maintain their position.
The newly described species is one of the largest arthropods yet discovered, a class of animals that includes spiders and crabs.
The scientists say the presence of this sea slug, which belongs to a diverse group of animals that includes sea slugs and sea snails, is a significant concern.
Called 24-IPC, it is only produced by Demospongiae, a class of animals that includes most modern sponges and is thought to constitute the roots of the animal family tree.
The newfound species, Helicocystis moroccoensis, has "characteristics that place it as the most primitive echinoderm that has fivefold symmetry," said study co-author Andrew Smith, a paleontologist at the Natural History Museum in London, referring to the group of animals that includes starfish and sea urchins.
