Animalia comprises, the metazoans, that are multicellular
organisms having eukaryotic cells with
a nucleus, a cell membrane, and organelles such as mitochondria.
They are heterotrophic (obtain food from external sources)
utilize oxygen for energy catabolism.
Some of the oldest known fossils of these organisms are found
in rocks ranging from 540-650 million years old corresponding
to the Vendian (also
called the Ediacaran Period), but the paucity of these fossils
make their association with those of the Cambrian almost impossible,
and the possibility exists that they are not even animals.
for the Ediacaran biota, most animal phyla (some that persist
and some that did not) appear in the fossil record rapidy and
essentially simultaneously at the base of the Cambrian period,
some 570 million years ago, an event known as the Cambrian
Explosion. Still in dispute is whether the Cambrian Explosion
really was a rapid divergence between different groups or was
instead a change in size and the emergence of protective armor
such as exoskeletons and shells that facilitated fossilization.
is widely accepted that animals evolved from flagellate protozoa.
If so, the closest living relatives are the choanoflagellates,
which are heterotrophic,
having the same structure as some sponge cells. Phylogenetic studies
place them in a supergroup called the opisthokonts,
which also include the fungi and a few small parasitic protists.
The name comes from the posterior location of the flagellum in
motile cells, such as most animal sperm, whereas other eukaryotes
tend to have anterior flagella.
(cells truly coorperate - see note 1)
symmetry with 2 layers of cells (diblastic)
Cnidaria (corals, jellyfish, Hydra)
Ctenophora (comb jellies)
appears second - see note 3)
Echinodermata (crinoids, starfish, sea urchins, sea cucumbers,
(acorn worms, graptolites)
Chordata (Vertebrates and closely related invertebrates
all having a notochord)
Vetulicolia (note 4 - Extinct)
Ecdysozoa (molting animals)
Arthropoda (crabs, spiders, insects, etc.)
Onychophora (velvet worms) - see Phylum
Lobopodia (note 5)
Annelida (segmented worms)
Echiura (Spoon worms note 6)
Brachiopoda (lamp shells)
Bryozoa ("moss animals")
Mollusca (snails, clams, squid, etc.)
phylogenists believe sponges and eumetazoans evolved separately
from single-celled organisms, which would mean that the
entire animal kingdom does not form a distinct clade descended
from a common ancestor. However, some genetic studies
and some morphological characteristics support a common
origin and ancestor.
The Bilaterians are distinguished by bilateral morphological
symmetry, and are a major group of animals that contains
the majority of phyla. Sponges and cnidarians are notable
exceptions. Most Bilaterian bodies develop from three
different germ layers, called the endoderm, mesoderm,
and ectoderm, thus earning the developmental term triploblastic.
Nearly all Bilaterians are bilaterally symmetrical, or
approximately so. The most notable exception is the echinodermata
that are bilaterally symmetrical as larvae, but become
radially symmetric as adults.
(taxonomic term: Deuterostomia; from the Greek: "other
mouth") are a superphylum of animals. They are a
subtaxon of the Bilateria branch of the subregnum Eumetazoa,
and are opposed to the protostomes. Deuterostomes are
differentaiated by their embryonic development where the
first opening (the blastopore) becomes the anus, while
in protostomes it becomes the mouth.
is a recently-erected phylum comprising several early
Cambrian fossils from Chengjiang, China that are suggested
to have been among the earliest deuterostomes. See - Shu,
D.-G., et. al. 2001. Primitive Deuterostomes from the
(Lower Cambrian, China), Nature, 414:419-424.
are worms with legs that occur in the early Cambrian,
and are mainly known from the Burgess Shale and Chengjiang
Biota. Most theories hold they are related to arthropods,
and like here they are often placed with the predatory
Onychophora (or Velvet Worms) that are extant. Some theories
place lobopods as a stem group of
arthropods, or posit that arthropods arose from lobopods.
Clearly, they are a poorly understood
group of animals.
were initially regarded as an annelid group, were then
excluded, but newer evidence suggests they are in fact
annelids (Hessling and Westheide, 2002), albeit this placement
remains unresolved. The Echiura fossilize poorly and the
earliest known specimen is from Mazon
Creek dating to the Pennsylvanian. see - Hessling,
R., and W. Westheide. 2002. Are Echiura derived from a
segmented ancestor? Immunohistochemical analysis of the
nervous system in developmental stages of Bonellia viridis.
J. Morph. 252:100-113.