Animal Phyla

In biological taxonomy, a phylum (plural phyla) is a taxon in the rank below kingdom and above class. A Phylum represents the largest scientific grouping of life forms sharing evolutionary certain requirements of a common ancestor in evolution. Phyla can also be considered as a group sharing the same general body plan, which includes both the external appearance, but is more importantly dependent of the bodies internal organization.

There are 36 recognized animal phyla, of which but nine (Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Echinodermata, and Chordata) contain the vast majority of described, extant species. Debate continues regarding whether the different phyla appeared on earth prior to the Cambrian Explosion some 544 million years ago that roughly speaking marks the time when life forms had become large enough and many had acquired hardened body parts, so that a findable fossil record could begin to accumulate. The absence of a Precambrian fossil record greatly worried Darwin. The late Precambiran and Cambrian are believed to be times when some life forms that would otherwise deserve the rank of phylum both evolved only to meet extinction without leaving descendents.

The taxonomic rank of phylum is normally used with specificity as if science truly knows the placement of life in the tree of life. Therefore, remember not to put too sharp of a point on the term. Consequently the number of phyla, both extant and extinct, as well as what group constitutes a phylum varies from author to author, and changes over time. Modern molecular science (phylogenetics) has helped much to clarify the evolutionary descendancy of life forms, but can not help much to resolve mysteries within the fossil record.

Body plan is one approach to defining phyla. Development of the animal body plan was (and is) controlled by large and complex gene regulatory networks. It follows that evolution of body plans corresponded to adpative changes in the architecture of these developmental gene regulatory networks. The many components of these networks evolve at different rates and in different ways, and are very much dependent on selection pressures from changing environments. It is often hypothsized that animal evolution established essentially all phylum-level body plans by the early Cambrian. If so, it follows that certain components of the networks that were essential became immune to change. These highly conserved network components may well have originated in the Precambrian.

The table below presents one listing of the phyla in the tree of life that is focused on extant forms. Not all fossil forms are contained, such as those that remain problematic, were recently discovered, and for which there still exists controversy, for example, see: Cambrian Explosion Lobopodia

Extant Animal Phyla
Phylum	Fossil Record	Contained Within	Subphyla or Classes Contained	Estimated Number of Described Species
Acanthocephala	?	Kingdom Animalia Subkingdom Eumetazoa Superphylum Platyzoa	Classes: Archiacanthocephala, Eoacanthocephala, Palaeacanthocephala	some 750
Acoelomorpha	?	Kingdom Animalia Subkingdom Eumetazoa	Classes: Acoela, Nemertodermatida	?
Annelida	Cambrian (518 mya) to Present	Domain Eukaryota Kingdom Animalia Superphylum Lophotrochozoa	Classes: Polychaeta, Clitellata, Myzostomida, Archiannelida	some 17000 extant
Arthropoda	Cambrian (540 mya) to Present	Domain Eukaryota Kingdom Animalia Subkingdom Eumetazoa Superphylum Ecdysozoa	Trilobitomorpha, Chelicerata, Myriapoda, Hexapoda, Crustacea	more than 1.1 million
Brachiopoda	Lower Cambrian to Present	Domain Eukaryota Kingdom Animalia	Complex: Two main groups are Inarticulata and Articulata	Some 400 extant; prodigious fossil species
Bryozoa	Ordovician to Present	Domain Eukaryota Kingdom Animalia Superphylum Lophotrochozoa	Classes: Stenolaemata, Gymnolaemata, Phylactolaemata	some 5000 extant
Chaetognatha	Cambrian to Prsent	Domain Eukaryota Kingdom Animalia	Classes: Archisagittoidea, Sagittoidea	Some 100 extant
Chordata	Lower Cambrian (530 mya) to Present	Domain Eukaryota Kingdom Animalia Subkingdom Eumetazoa Superphylum Deuterostomia	Subphyla:Tunicata, Cephalochordata, Vertebrata	More than 60,000
Cnidaria	Precambrian (580 mya) to Present	Domain Eukaryota Kingdom Animalia	Subphyla: Anthozoa, Medusozoa	Some 11,000
Ctenophora	Cambrian to Prsent	Domain Eukaryota Subkingdom Eumetazoa	Classes:Tentaculata, Nuda	Some 100
Cycliophora	?	Domain Eukaryota Kingdom Animalia Subkingdom Eumetazoa Superphylum Platyzoa	Symbion species	3 or more
Echinodermata	Cambrian to Prsent	Domain Eukaryota Subkingdom Eumetazoa Superphylum Deuterostomia	Subphyla: Homalozoa, Crinozoa, Asterozoa, Pelmatozoa, Blastozoa	some 7,000 extant species and 13,000 extinct
Echiura (sometimes placed with Annelida)	Upper Carboniferous to Present	Domain Eukaryota Kingdom Animalia Subkingdom Metazoa Superphylum Lophotrochozoa	Echiuroidea, Heteromyota, Xenopneusta	130
Entoprocta	?	Domain Eukaryota Kingdom Animalia Superphylum: Lophotrochozoa	Four families	Some 150
Gastrotricha	?	Domain Eukaryota Kingdom Animalia Subkingdom Eumetazoa Superphylum Platyzoa	Orders: Macrodasyida, Chaetonotida	About 690
Gnathostomulida	?	Domain Eukaryota Kingdom Animalia Subkingdom Eumetazoa Superphylum Platyzoa	Orders: Filospermoidea, Bursovaginoidea	Some 100
Hemichordata	Cambrian to Prsent	Domain Eukaryota Kingdom Animalia Subkingdom Eumetazoa Superphylum Deuterostomia	Classes: Enteropneusta, Graptolithina (extinct), Pterobranchia, Planctosphaeroidea	Some 100 extant
Kinorhyncha	?	Domain Eukaryota Kingdom Animalia Superphylum Ecdysozoa	Orders: Cyclorhagida, Homalorhagida	Some 150
Loricifera	?	Domain Eukaryota Kingdom Animalia	Eight described genera	Some 120
Micrognathozoa	?	Domain Eukaryota Kingdom Animalia Subkingdom Eumetazoa	One Genus: Limnognathia	At least 1
Mollusca	Cambrian to Prsent	Domain Eukaryota Kingdom Animalia Superphylum: Lophotrochozoa	Classes: Aplacophora, Bivalvia, Caudofoveata, Cephalopoda, Gastropoda, Helcionelloida, Monoplacophora, Polyplacophora, Rostroconchia, Scaphopoda	Some 112,000
Nematoda	?	Domain Eukaryota Kingdom Animalia Subkingdom Eumetazoa	Classes: Adenophorea, Secernentea	Perhaps one million
Nematomorpha	?	Domain Eukaryota Kingdom Animalia Superphylum Ecdysozoa	Classes: Nectonematoida Gordioidea	About 300
Nemertea	?	Domain Eukaryota Kingdom Animalia Superphylum: Lophotrochozoa	Classes: Anopla, Enopla	More than 1000
Onychophora (Also see Lobopodia)		Domain Eukaryota Kingdom Animalia Superphylum Ecdysozoa	Extant families: Peripatidae, Peripatopsidae	Some 200 extant
Orthonectida	?	Domain Eukaryota Kingdom Animalia	Some 20 species	About 20
Phoronida	Putative Phoronids from Cambrian	Domain Eukaryota Kingdom Animalia Superphylum Lophotrochozoa	Extant genera: Phoronis, Phoronopsis Putative Phoronids occur in Maotianshan Shales	Some 20
Placozoa	?	Domain Eukaryota Kingdom Animalia	One so far - putative sponge ancestor	1
Platyhelminthes	?	Domain Eukaryota Kingdom Animalia Subkingdom Eumetazoa Superphylum Platyzoa	Classes: Monogenea, Trematoda, Cestoda, Turbellaria	Some 25,000
Porifera	Ediacara (Precambrian) to Present	Domain Eukaryota Kingdom Animalia	Groups: Calcarea, Hexactinellida, Demospongiae	Some 5000 extant
Priapulida	Cambrian to Present	Domain Eukaryota Kingdom Animalia	Extinct Genera:Ancalagon, Anningvermis, Corynetis, Ottoia Extant Classes: Priapulimorpha, Halicryptomorpha, Seticoronaria	Some 17
Rhombozoa		Domain Eukaryota Kingdom Animalia
Rotifera	Eocene to Present	Domain Eukaryota Kingdom Animalia Subkingdom Eumetazoa Superphylum Platyzoa	Classes: Monogononta, Digononta, Bdelloidea, Seisonidea
Sipuncula	Cambrian to Present	Domain Eukaryota Kingdom Animalia Superphylum Lophotrochozoa	Classes: Phascolosomatidea, Sipunculidea, Sipunculiformes
Tardigrada	Early Cambrian to Present	Domain Eukaryota Kingdom Animalia Subkingdom Eumetazoa Superphylum Ecdysozoa	Classes: Heterotardigrada, Mesotardigrada, Eutardigrada
Xenoturbellida	?	Domain Eukaryota Subkingdom Eumetazoa Superphylum Deuterostomia	?	2