Zoology 250 Phylogenetic Trees (2006)


(demosponge)

(flatworm)
KINGDOM ANIMALIA
(=Metazoa)*

(featherduster worm)

(sea urchin)

(octocoral)

(gastropod)
Modified from
Tree of Life
(after
Ruppert & Barnes 2004 p. 220;
Brusca & Brusca 2003 p. 875
)

(crab)

(sea squirts)


     ========================================== Porifera (sponges)
     |
     |  ======================================= Placozoa (placozoans, Trichoplax)
     |  |
     |  |   =================================== Cnidaria (jellyfish, anemones, corals, etc.)
     |  |   |
<<=M=|  |   |  ================================ Ctenophora (comb-jellies)
     |  |   |  |
     |  |   |  |       ======================== Platyhelminthes (flatworms, tapeworms,etc.)
     |  |   |  |       |
     =T=|   |  |       |   =(branch uncertain)= Aschelminthes (nematodes, rotifers,etc.)
        |   |  |       |   |
        |   |  |   =P==|   |   ================ Nemertea (ribbon worms)
        |   |  |   |   |   |   |
        =E==|  |   |   =A==|   |        ======= Mollusca (snails, clams, squids, etc.)
            |  |   |       |   |  ======|
            |  |   |       =C==|  |     ======= Sipuncula(peanut worms)
            |  |   |           |  |
            |  |   |           |  |        ==== Annelida (segmented worms)
            |  |   |           ===|     ===|
            |  |   |              |  ===|  ==== Pogonophora (beard worms, vent worms; now put in the Annelida)
            ===|   |              |  |  |
               |   |              ===|  ======= Echiura (proboscis worms)
               |   |                 |
               |   |                 |  ======= Onychophora (velvet worms)
               |   |                 ===|
               |   |                    |  ==== Tardigrada (water bears)
               |   |                    ===|
               =B==|                       ==== 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.)


TRAITS SUPPORTING EACH CLADE (** plesiomorphic- a primitive state, not unique to clade):
M= METAZOA:
a) true multicellularity (no endoderm, no true gut)
b) sessile adult, superficial radial symmetry**
c) monociliated cells**
d) radial, indeterminate cleavage
e) blastula stage embryo
f) diplosome**
g) type IV collagen


T: motile adults; dorso-ventral body axis, true radial symmetry


E= EUMETAZOA:
a) true tissues; basal lamina underlies epithelia
b) diploblastic organization
c) gastrula stage embryo
d) blind gut (no anus)
e) true gonads
f) true nervous system with synapses
g) striated muscle


B= BILATERIA:
a) bilateral symmetry
b) cephalization
c) triploblastic organization (extensive mesoderm)
d) organ-system level of organization
e) protonephridia
P= PROTOSTOMIA:
a) spiral or modified spiral, determinate cleavage
b) mesoderm from single (mesentoblast) cell
c) mesoderm develops from mesenchyme cells
c) fate of first derm 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 and 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:

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.


Return to Zool 250 Home Page or try your skill at the Invertebrate Diversity Quiz.

Copyright (c) 2006 by A. Richard Palmer. All rights reserved.
(revised Jan. 7, 2006)