Bilateria

## Temporal Range:

Ediacaran - Today 546 - 0 Ma580 mya if Eolimax is a bilaterian

## Subphyla

* †Protobilateria
* ?†Eolimax
* †Infusidae
* Deuterostomia
	+ Thalamumsegmentia
	+ Echinoderma
	+ Chordata
* Ecdysozoa
	+ Panarthropoda
	+ Pancyclopsia

The Bilateria or Bilaterians, are animals with bilateral symmetry as an embryo and for most genera, as an adult, having a right and left side that are mirrors of each other. This also means that they have a head and a tail (anterior-posteriors axis) as well as a belly and back (ventral-dorsal axis). Nearly all Bilaterians have bilateral symmetry as an adult, but some are exceptions to this rule, most notably the Echinoderms.

Most animals are bilaterians such as Ecdysozoans and Deuterostomes. Bilaterians have a complex digestive track, possessing a separate anus and mouth.

## Body plan

The earliest known bilaterians were worm-like, allowing other bilaterians to have an anterior and posterior side. The head (anterior) end of the animal possesses a mouth and usually one or more sensory organs that allow the animal to do a variety of things depending on the sensory organ(s). The tail (posterior) end of the animal possesses an anus, that is connected to the mouth through a central gut, or internal cavity that allows anything consumed at the anterior end, to make it to the posterior end of the animal.

## Evolution

The first confirmed bilaterians existed during the later parts of the Ediacaran and the earliest Rimunian. However, it is possible that bilaterian body plans existed earlier or before the Ediacaran, however no fossils are known. It is hypothesized that the common ancestor of all bilaterians was worm-like, possessing a body plan that contained both a posterior and anterior end, as well as a ventral and dorsal side, however it is unclear whether it would've possessed many or any advanced sensory organs.

## Fossil Record

The earliest known bilaterian is from the Ediacaran period, known as *Protobilateria*. However, other known Ediacaran biota have a semi-bilateral body plan, with one side (either the left or right) being slightly higher than the other side, meaning both sides were identical, but the right side for example would be pushed upwards by one segment.

However, the lifestyle of Protobilateria and other Ediacaran biota is mostly unknown, so it is unclear whether these animals had both a separate anus and mouth, or if they had any sensory organs. Fossils from the Ediacaran are also poorly preserved due to them all being soft bodied animals.

## Phylogeny

Bilateria has been divided into two main phyla. The Deuterostomes, including Echinoderms and Chordates, and the Ecdysozoans, including the Cyclopsians, Mollusks, Triopsians and Arthropods.

Original cladogram of the Animal Kingdom, as suggested by Auvocan biologist Francesco Mancinelli (1996.)

*Cladogram of the Animal Kingdom (Mancinelli, 1996).*

The newly, more accepted cladogram of the Animal Kingdom (proposed by Nikoston biologist Luis Pérez in 2012) suggests that Vendiobiota are animals and that Infusids are not true Deuterostomes. It also theorizes, though not fully accepted, that Euarthropods are decedents of Vermopods.

*Cladogram of the Animal Kingdom (Pérez, 2012).*