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Almost All of the Animal Kingdom, Given That Invertebrates Com- Prise Roughly 1,324,402 (96%) of the Approximately 7,382,402 Described, Living Animal Species

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Almost All of the Animal Kingdom, Given That Invertebrates Com- Prise Roughly 1,324,402 (96%) of the Approximately 7,382,402 Described, Living Animal Species y the time you have gotten to this chapter, you will have "toured" almost all of the Animal Kingdom, given that invertebrates com- prise roughly 1,324,402 (96%) of the approximately 7,382,402 described, living animal species. What an incredible diversity of form and function you have seen! Isn't it curious how unevenly these animals are distributed across the 32 metazoan phyla? Vastly unevenly distributed-81.5% of all described animals are arthropods, 5.8% are mol- luscs, and only 4.4'h are chordates. The other 29 phyla make up the re- maining 7%! Eight phyla have fewer than 100 described species in them, and half of those have fewer than a dozen known species-what are we to make of that? In fact, only 6 phyla comprise more than one percent each of the described animal species on Earth (Arthropoda, Mollusca, Annelida, Platyhelminthes, Nematoda, and Chordata). It would seem that the world belongs to insects,crustaceans, spiders, molluscs, worms, and vertebrates. Indeed, these are the only animals most humans ever see in their lifetimes (unless they happen to go diving on a tropical coral reef, in which case they are confronted by myriad sponges, cnidarians, and echinoderms). How fortunate that you have now been introduced to all 32 animal phyla! In reading the "animal chapters" of this text, you have learned a good deal about the evolution of these phyla, how they are related to one anoth- er, and what their internal relationships are. You have learned that phylo- genies can be constructed from a variety of different kinds of data, and that most recently the field of molecular phylogenetics has proliferated, giving us new ideas about how Earth's creatures are related to one another. The idea of phylogenetic trees should now be quite familiar to you. But let's take one last look, a "higher view" of animal phylogeny, before we close the cover on this edition of Inaertebrates. Molecular phylogenetics had only just begun to bear fruit in 2002, when the second edition of this book went to press. While some tantalizing sug- gestions of new and very different kinds of relationships among the animal phyla had been suggested, these new hypotheses had not yet been broadly tested. Since then, molecular phylogenetics has exploded onto the scene at a pace never imagined, and it now plays the primary role in reconstructing an- imal phylogeny. In just the past 15 years we have come from analyzing a few This chapter has been revised by Richard C. Brusca and Gonzalo Giribet 1048 ChapterTwenty"-Eight ribosomal genes to the point where new genome-level presence of high concentrations of the compound analyses are being published almost daily. Molecular 24-isopropyl cholestane are now disputed. Some of phylogenetics is now being incorporated into studies of the oldest metazoan fossils are from the Doushantuo ecology, oceanography, biogeography, conservation bi- Formation of southern China, dating to 600 million ology, medicine, archeology, and anthropology. And it years ago. Sponges, cnidarians, and other apparent is building a new frar-nework for the tree of life. diploblastic animals have been reported from these de- Unraveling the phylogehetic history of the Animal posits, as well as embryos ranging from two to thou- Kingdom has been one of biology's great challenges. sands of cells. However, many of these Doushantuo The "new phylogeny," built with molecular data, has metazoan fossils also have been disputed in one fash- many similarities to older trees built on morphological ion or another. It seems that interpreting ancient fossils and developmental data, but some big surprises have in rocks can be as challenging as inferring ancient phy- emerged and many uncertainties remain. The biggest logenies with morphology or molecules! challenge lies in the fact that life's deep lineages arose We discussed the origin of the Metazoa in several and began to diverge from one another so long ago, previous chapters. To reiterate, a large body of evi- over half a billion years ago for most phyla. So the traits dence, anatomical and molecular, has accumulated of animals, whether anatomical or genetic, that might since the 1960s that supports the view of multicellular be useful in revealing relationships among these an- animals sharing a common ancestor with the protist cient lineages are obscured by hundreds of millions of group Choanoflagellata. The flagellated collar cells years of evolutionary change. But, despite this, many of sponges and choanoflagellates have been viewed relationships are now well resolved. as nearly identical and unique to these two groups. A Early molecular phylogenetic studies relied heav- few interesting differences between them have been ily on the 18S ribosomal RNA gene (also known as the noted (e.g., Mah et aL.2014),but some divergence over nuclear small-subunit ribosomal RNA gene, or SSU a half billion years is to be expected. The Metazoa are rRNA). However, it quickly became apparent that un- defined by a number of synapomorphies, the most ob- derstanding deep-level metazoan phylogeny required vious being multicellularity arising through the embry- analysis of additional genes, particularly nuclear pro- onic layering process called gastrulation. Also, unlike tein-coding genes, and this led to an era of multigene coloniality, as seen in many protist groups (including trees that continues today as phylogeneticists add more choanoflagellates), in animals the epithelial cells are in and more genes (and taxa) to their datasets for analy- contact with each other through unique junction struc- sis (Chapter 2). Most recently phylogenomic stud- fures and molecules, some of which make transport of ies, using large parts of the genome (often using the nutrients between cells possible (e.g., septate or tight transcriptomel as a proxy for the whole genome) and junctions, desmosomes, zonula adherens). Additional analyzinghundreds or even thousands of genes, have metazoan synapomorphies include: striate myofibrils, begun to expand the scope of data available for phylo- actin-myosin contractile elements, the possession of genetic analysis, and these have added increased sta- animal (type IV) collagen (although collagen, or a col- bility to the structure of our phylogenetic framework. lagen homologue also occurs in some fungi), and a The advent of EvoDevo, or evolutionary developmen- basal lamina beneath the epidermis. In addition. sexual tal biology, has also begun to significantly impact our reproduction in animals involves a djstinct pattern of understanding of how genes relate to specific morphol- egg development from one of the four cells of meiosis, ogies, how they work, and what roles they might have whereas the other three cells degenerate. played in the unfolding of animal radiations. These Figure 28.1 presents a consensus tree of metazoan new techniques have provided answers to fundamen- phylogeny, based primarily upon the most recent tal questions, such as the identity of arthropod append- molecular phylogenetic research. Phylogenetic stud- ages and the nature of segmentation in animals. ies have largely been in agreement that the oldest liv- Estimates of divergence times of the animal phyla, ing animal phylum is Porifera, the sponges. However, based on molecular clock calculations, suggest that some recent molecular phylogenies have suggested the origin of the Metazoa was 875 to 650 million years Ctenophora might be the basalmost metazoans. The ago. Some trace fossils put the emergence of the bilat- draft genome of Pleurobrschia bachei,together with erians at around a billion years ago, although the na- other ctenophore transcriptomes, suggest that cteno- ture of those specimens has been disputed. Some re- phores may be rather distinct from other animal ge- cent datings that had assumed the presence of sponges nomes in their content of neurogenic, immune, and in Ediacaran (and even Cryogenian) rocks due to the developmental genes. However, a number of puta- tive synapomorphies link Cnidaria and Ctenophora with the Bilateria, a clade that has been called Neuralia 'Whereas a genome is the complete set of genes present in a celi (Figure 28.1). The "basal Ctenophora" hypothesis (or organism) and is sequenced from DNA, a transcriptome is a subset of those genes that are transcribed (or expressed) in a cell was challenged by J6kely et al. (2015) and Pisani et at any given time and is sequenced from RNA. al. (2015),both research groups suggesting it was an Metazoa Bilateria Deuterostomia Protostomia Chordata Ambulacraria Spiralia Ecdysozoa Lophophorata Nematoida Hemichordata Gnathifera .G r---,r.---Scalid?phor?-\Panarthropoda - sE Srd 6Flq =- P" o FY " s a* qdF6;:; X 6a o!so':.nxi: i!€gEggpf€sE E€.n.Eg c:1 NiFx3 Eg9;'dds ts!6A'Fhh, gg odduox# ! BEf;fi g FE€sg3EE!e SggFEgEEE"Fg Metazoa: (1) Gastrulation and embryonic tissue layering; (2) septate junctions, tight junctions, and/or zonula adher- ens present in epithelial tissues; (3) type lV collagen; (4) collagenous basal lamina/basement membrane beneath epidermis; (5) striated myofibrils and actin-myosin contractile elements. Metazoa beyond Porifera: (6) striated ciliary rootlets. Cnidaria (and Ctenophora) + Bilateria: (7) Sap junctions; (8) fixed, organized gonads; (9) synaptic nervous system; ('10) epithelium-lined gut (with digestive enzymes); (11) with primary larva, bearing apical organ (lost in Ecdysozoa); (12) presence of opsins. ('1 primary bilateral; (14) cephaliza- Figure 28.1 A phylogeny of Metazoa. This tree reflects a Bilateria: 3) symmetry tion, with concentration of
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