PROTOSTOMIA

Origins: 558 million years ago (Ediacaran period)

Extinction: Still extant

Protostomes were initially defined for being the group of animals characterized for the formation of the mouth before the anus, during embryonic development. This has since been challenged, as this is pretty variable among protostomes, even though in other animals so called deuterostomes (which might be a paraphyletic group) the anus typically forms before the mouth, during embryonic development. Together with vertebrates, echinoderms, and a few others, they form the Bilateria group, a group characterized for bilateral symmetry, an anteroposterior axis and three germ layers.

The main distinction between protostomes and deuterostomes lies in their embryos, where coeloms (secondary body cavities) are formed by schizocoely, in protostomes, with the coelom forming out of a solid mass of embryonic tissue that splits from the rest.

The common ancestor of protostomes and "deuterostomes" was a worm-like aquatic creature that lived in the early Ediacaran period. There is some consensus on the phylogeny of protostomes. They're typically divided in two groups, the Ecdysozoa (which includes arthropods and kin) and the Spiralia (which includes mollusks and kin), with these two groups likely splitting from each other some 558 million years ago, in the later Ediacaran period, radiating immensely soon since that point in time.

main source: Wikipedia

Phylogeny

Centroneuralia

- Protostomia

- Arthropoda

- Insecta

- Campterophlebia elegans

- Liassopsychops curvatus

- Multicrustacea

- Juracyclus posidoniae

- Uncina posidoniae

- Mollusca