Bilateria
Triploblasts, Bilaterally symmetrical animals with three germ layers
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This tree diagram shows the relationships between several groups of organisms.
The root of the current tree connects the organisms featured in this tree to their containing group and the rest of the Tree of Life. The basal branching point in the tree represents the ancestor of the other groups in the tree. This ancestor diversified over time into several descendent subgroups, which are represented as internal nodes and terminal taxa to the right.
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You can click on the root to travel down the Tree of Life all the way to the root of all Life, and you can click on the names of descendent subgroups to travel up the Tree of Life all the way to individual species.
For more information on ToL tree formatting, please see Interpreting the Tree or Classification. To learn more about phylogenetic trees, please visit our Phylogenetic Biology pages.
close boxThis tree represents the view of bilaterian relationships as it is currently emerging from analyses based on molecular data (mostly 18S rRNA sequences). For an alternative hypothesis of bilaterian relationships based on morphological data, see the Discussion of Phylogenetic Relationships.
Discussion of Phylogenetic Relationships
Due to new evidence from developmental biology and molecular phylogenetics, ideas about bilaterian relationships have undergone a major paradigm shift within the last decade. The new hypotheses shown in the tree above are now widely accepted, but there are also many sceptics who emphasize the pitfalls and inconsistencies associated with the new data. One of the most prominent alternative views based on morphological evidence is championed by Nielsen (2001):
References
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Hausdorf, B., M. Helmkampf, A. Meyer, A. Witek, H. Herlyn, I. Bruchhaus, T. Hankeln, T. H. Struck, and B. Lieb. 2007. Spiralian phylogenomics supports the resurrection of Bryozoa comprising Ectoprocta and Entoprocta. Molecular Biology and Evolution 24(12):2723-2729.
Hejnol, A. and M. Q. Martindale. 2008. Acoel development supports a simple planula-like urbilaterian. Philosophical Transactions of the Royal Society Series B 363(1496):1493-1501.
Helfenbein, K. G. and J. L. Boore. 2004. The mitochondrial genome of Phoronis architecta comparisons demonstrate that phoronids are lophotrochozoan protostomes. Molecular Biology and Evolution 21:153-57.
Helfenbein, K. G., H. M. Fourcade, R. G. Vanjani, and J. L. Boore. 2004. The mitochondrial genome of Paraspadella gotoi is highly reduced and reveals that chaetognaths are a sister group to protosotomes. Proceedings of the National Academy of Sciences (USA) 101:10639-43.
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Lüter, C. 1997. The phylogenetic position of Brachiopoda - a comparison of morphological and molecular data. Zoologica Scripta 26:245-253.
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Mallatt, J. and G. Giribet. 2006. Further use of nearly complete 28S and 18S rRNA genes to classify Ecdysozoa: 37 more arthropods and a kinorhynch. Molecular Phylogenetics and Evolution 40(3):772-794.
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Title Illustrations
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Scientific Name | Opisthocomus hoazin |
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Location | Sacha Lodge, Oriente, Ecuador |
Specimen Condition | Live Specimen |
Source | Hoatzin |
Source Collection | Flickr |
Copyright |
© 2007 Jim Scarff
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Scientific Name | Pisaster ochraceus |
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Location | Protection Island, British Columbia |
Specimen Condition | Live Specimen |
Source | Purple Starfish |
Source Collection | Flickr |
Image Use |
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Copyright | © 2006 Alanna@VanIsle |
Scientific Name | Argiope |
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Location | Dallas, Texas |
Comments | Argiope spider (Arthropoda) on web with stabilimentum |
Copyright |
© Greg and Marybeth Dimijian
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Scientific Name | Cyphoma gibbosum |
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Location | Curacao, Netherland Antilles |
Specimen Condition | Live Specimen |
Source | Flamingo Tongue Snail in Profile |
Source Collection | Flickr |
Image Use |
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Copyright | © 2006 LASZLO ILYES |
Scientific Name | Bipalium |
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Location | Dallas, Texas |
Comments | A planarian flatworm (Platyhelminthes) after emerging from wet soil after rain |
Copyright |
© Greg and Marybeth Dimijian
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Bilateria. Triploblasts, Bilaterally symmetrical animals with three germ layers.
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