Animalia

**Animals** are multicellular eukaryotic organisms that form the biological kingdom **Animalia**. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres (110 ft). They have complex interactions with each other and their environments, forming intricate food webs. The kingdom **Animalia** includes humans, but in colloquial use the term *animal* often refers only to non-human animals. The study of non-human animals is known as zoology.

Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan. The Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes, arthropods, and molluscs—and the deuterostomes, containing both the echinoderms as well as the chordates, the latter containing the vertebrates. Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago.

## Glide reflection

In 2-dimensional geometry, a **glide reflection** (or **transflection**) is a symmetry operation that consists of a reflection over a line and then translation along that line, combined into a single operation.

Glide symmetry can be observed in nature among certain fossils of the Ediacara biota; the machaeridians; and certain palaeoscolecid worms. It can also be seen in many extant groups of sea pens.

## Dickinsonia

Youtube: First life
***Dickinsonia*** is an extinct genus of fossils of the Ediacaran biota. The individual *Dickinsonia* typically resembles a bilaterally symmetrical ribbed oval. Its affinities are presently unknown; its mode of growth is consistent with a bilaterian affinity, though some have suggested that it belongs to the fungi, or even an "extinct kingdom". The discovery of cholesterol molecules in fossils of *Dickinsonia* lends support to the idea that *Dickinsonia* was an animal.

*Dickinsonia* fossils are known only in the form of imprints and casts in sandstone beds. The specimens found range from a few millimetres to about 1.4 metres (4 ft 7 in) in length, and from a fraction of a millimetre to a few millimetres thick.

They are nearly bilaterally symmetric, segmented, round or oval in outline, slightly expanded to one end (i.e. egg-shaped outline). The rib-like segments are radially inclined towards the wide and narrow ends, and the width and length of the segments increases towards the wide end of the fossil. The segments are separated by a thin ridge or groove along the axis of symmetry into right and left halves. The segments are organized in an alternating pattern according to glide reflection symmetry rather than bilateral symmetry; thus, these "segments" are isomers. This glide reflection is also found in *Spriggina*, another mysterious organism from approximately the same era.

The segments of *Dickinsonia* have been described by Adolf Seilacher as self-organising "pneu structures", chambers filled with a liquid at higher than ambient pressure, analogous to a quilted air mattress.

Some spectacular fossils attributed to *Dickinsonia* appear to preserve internal anatomy, believed to represent a tract that both digested food and distributed it throughout the organism.

## Charnia

***Charnia*** is a genus of frond-like Ediacaran lifeforms with segmented, leaf-like ridges branching alternately to the right and left from a zig-zag medial suture (thus exhibiting glide reflection, or opposite isometry). The genus *Charnia* was named after Charnwood Forest in Leicestershire, England, where the first fossilised specimen was found. *Charnia* is significant because it was the first Precambrian fossil to be recognized as such.

The living organism was a type of life form that grew on the sea floor and is believed to have fed on nutrients in the water. Despite *Charnia'*s fern-like appearance, it is not a photosynthetic plant or alga because the nature of the fossilbeds where specimens have been found implies that it originally lived in deep water, well below the photic zone where photosynthesis can occur.

Little is known about the ecology of *Charnia*. It was benthic and sessile, anchored to the sea floor. According to one currently popular hypothesis, it probably lived in deep waters, well below the wave base, thus placing it out of range of photosynthesis. Furthermore, it has no obvious feeding apparatus (mouth, gut, etc.) so its lifestyle remains enigmatic. Some have speculated that it survived either by filter feeding or directly absorbing nutrients, and this is currently the focus of considerable research.

The growth and development of the Ediacara biota is also a subject of continued research, and this has discredited the sea pen hypothesis. In contrast to sea pens, which grow by basal insertion, *Charnia* grew by the apical insertion of new buds.