Z250 - Metazoa Clade (2005)

Zoology 250 Phylogenetic Trees (2005)

(demosponge)	(flatworm)	KINGDOM ANIMALIA (=Metazoa)*	(featherduster worm)	(sea urchin)
(octocoral)	(gastropod)	Modified from Tree of Life (after Ruppert & Barnes 1994 p. 1052; Brusca & Brusca 2003 p. 875)	(crab)	(sea squirts)

     ========================================== Porifera (sponges)
     |
     |      =================================== Cnidaria (jellyfish, anemones, corals, etc.)
     |      |
     |      |  ================================ Ctenophora (comb-jellies)
     |      |  |
     |      |  |       ======================== Platyhelminthes (flatworms, tapeworms,etc.)
     |      |  |       |
     |      |  |       |   =(branch uncertain)= Aschelminthes (nematodes, rotifers,etc.)
     |      |  |       |   |
<<=M=|      |  |   =P==|   |   ================ Nemertea (ribbon worms)
     |      |  |   |   |   |   |
     |      |  |   |   |   |   |        ======= Mollusca (snails, clams, squids, etc.)
     |  =E==|  |   |   =A==|   |  ======|
     |  |   |  |   |       |   |  |     ======= Sipuncula(peanut worms)
     |  |   |  |   |       =C==|  |
     |  |   |  |   |           |  |        ==== Annelida (segmented worms)
     |  |   |  |   |           ===|     ===|
     |  |   |  |   |              |  ===|  ==== Pogonophora (beard worms, vent worms; now put in the Annelida)
     |  |   ===|   |              |  |  |
     |  |      |   |              ===|  ======= Echiura (proboscis worms)
     |  |      |   |                 |
     ===|      |   |                 |  ======= Onychophora (velvet worms)
        |      |   |                 ===|
        |      |   |                    |  ==== Tardigrada (water bears)
        |      |   |  P-PROTOSTOMIA     ===|
        |      =B==|  D-DEUTEROSTOMIA      ==== Arthropoda (insects, spiders, crabs, etc.)
        |          |
        |          |      ===================== Chaetognatha (arrow worms)
        |          |      |
        |          |      |==================== lophophorates (bryozoans, brachiopods, etc.)
        |          |      |
        |          ==DA===|        ============ Echinodermata (starfish, urchins, etc.)
        |                 |     ===|
        |                 ======|  ============ Hemichordata (acorn worms, pterobranchs)
        |                       |
        |                       =============== Chordata (sea squirts, vertebrates, etc.)
        |
        ======(phyla of uncertain affinity)==== Placozoa, Monoblastozoa, Rhomobozoa, Orthonectida

TRAITS SUPPORTING EACH CLADE (** plesiomorphic- a primitive state, not unique to clade):

M= METAZOA: a) true multicellularity (no endoderm, no true gut)
b) radial symmetry**
c) monociliated cells**
d) radial, indeterminate cleavage
e) blastula stage larva
f) diplosome**
g) type IV collagen
E= EUMETAZOA: a) true tissues, basal lamina beneath epidermis
b) diploblastic organization
c) gastrula stage larva
d) blind gut (no anus)
e) true gonads
f) true nervous system with synapses)
B= BILATERIA: a) bilateral symmetry
b) cephalization
c) triploblastic organization (extensive mesoderm)
d) organ-system level of organization
e) protonephridia P= PROTOSTOMIA: a) spiral, determinate cleavage
b) mesoderm from single (mesentoblast) cell
c) fate of first germ cell set at 5th cleavage
d) mouth (and sometimes anus) from blastopore
e) sheets of subepidermal muscles
f) multiciliated cells
A: anus present (i.e., complete gut)
C= COELOMATE PROTOSTOMES: a) coelom via schizocoely
b) trochophore-like larva with apical cilia
c) larvae swim with compound cilia
D= DEUTEROSTOMIA: a) radial, indeterminate cleavage**
b) mesoderm from epithelial cells
c) coelom via enterocoely
d) blastopore yields anus
e) tripartite coelom & body plan
f) monociliated cells** (some multiciliated)

*NOTE: The tree above represents a very classical view of animal relationships. Recent molecular data are raising questions about many of these relationships, particularly among the bilaterian taxa. For an alternative view and discussion see the Metazoa systematics page of the University of California Museum of Paleontology.

Important differences include:

the Porifera may be paraphyletic
the Ctenophora may lie basal to the Cnidaria
the Platyhelminthes may be paraphyletic
the Protostomia is split into two major clades: Lophotrochozoa (composed of two clades: lophophore bearing animals and animals having a trochophore larva) and the Ecdysozoa (clade of molting animals). However, support for the Ecdysozoa clade is based largely on DNA sequence data from a few genes, and two recent papers using data from complete genome sequences do not support the Ecdysozoa clade:
- Wolf, Y. I. et al. (2004). Coelomata and not Ecdysozoa: Evidence from genome-wide phylogenetic analysis. Genome Research 14(1): 29-36.
- Hughes, A. L. and R. Friedman (2004). Shedding genomic ballast: Extensive parallel loss of ancestral gene families in animals. Journal of Molecular Evolution 59: 827-833.
lophophorates may group with other protostomes in the Lophotrochozoa instead of in the Deuterostomia
several aschelminth (pseudocoelomate) phyla may be placed in the Ecdysozoa.

For another detailed consideration of phylogenetic relations that includes fossil taxa and references to the original literature, see Mikko's Phylogeny Archive.

CONCLUSION: Experts continue to disagree about which tree is closest to the true phylogenetic tree of animal life, which means we have much yet to learn about how molecules and morphology evolve, and about which kind of evidence provides more valuable clues about animal relations. Beware trees that do not show well-defined synapomorphies.

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