Short answers, pulled from the story.
The first bona fide bilaterian fossil is Kimberella, dating to the 550 million year mark. This ancient creature represents a pivotal moment in evolutionary history, marking the transition from simple radial symmetry to a complex body plan that would eventually dominate the animal kingdom.
The Bilateria were named by the Austrian embryologist Berthold Hatschek in 1888. In his classification, the group included the Zygoneura, Ambulacraria, and Chordonii, the Chordata.
Bilaterian embryos are triploblastic, having three germ layers: endoderm, mesoderm, and ectoderm. With few exceptions, bilaterian embryos are triploblastic, having three germ layers, and have complete digestive tracts with a separate mouth and anus.
Bilaterians constitute one of the five main lineages of animals, the other four being Porifera, Cnidaria, Ctenophora, and Placozoa. They rapidly diversified in the late Ediacaran and the Cambrian, and are now by far the most successful animal lineage, with over 98% of known animal species.
In protostomes, the first opening of the embryo becomes the mouth, while in deuterostomes, it becomes the anus. The deuterostomes traditionally include the echinoderms, hemichordates, chordates, and the extinct Vetulicolia.
The phylum Xenacoelomorpha, once thought to be flatworms, was erected in 2011, and has provided an extra challenge to bilaterian taxonomy, as they likely do not belong to either group. The acoelomorph taxa had previously been considered flatworms with secondarily lost characteristics, but the new relationship suggested that the simple acoelomate worm form was the original bilaterian body plan.