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The fossil record of the earliest animals has been enlivened in recent years by a series of spectacular discoveries, including embryos, from the Ediacaran to the Cambrian, but many issues, not least of dating and interpretation, remain controversial. In particular, aspects of taphonomy of the earliest fossils require careful consideration before pronouncements about their affinities. Nevertheless, a reasonable case can be now made for the extension of the fossil record of at least basal animals (sponges and perhaps cnidarians) to a period of time significantly before the beginning of the Cambrian. The Cambrian explosion itself still seems to represent the arrival of the bilaterians, and many new fossils in recent years have added significant data on the origin of the three major bilaterian clades. Why animals appear so late in the fossil record is still unclear, but the recent trend to embrace rising oxygen levels as being the proximate cause remains unproven and may even involve a degree of circularity.

Comparisons between completely sequenced metazoan genomes have generally emphasized how similar their encoded protein content is, even when the comparison is between phyla. Given the manifest differences between phyla and, in particular, intuitive notions that some animals are more complex than others, this creates something of a paradox. Simplistic explanations have included arguments such as increased numbers of genes; greater numbers of protein products produced through alternative splicing; increased numbers of regulatory non-coding RNAs and increased complexity of the cis-regulatory code. An obvious value of complete genome sequences lies in their ability to provide us with inventories of such components. This chapter examines progress being made in linking genome content to the pattern of animal evolution, and argues that the gap between genome and phenotypic complexity can only be understood through the totality of interacting components.

The advent of numerical methods for analyzing phylogenetic relationships, along with the study of morphology and molecular data, have driven our understanding of animal relationships for the past three decades. Within the protostome branch of the animal tree of life, these data have sufficed to establish two major clades; Ecdysozoa, a clade of animals that all moult, and Spiralia (often called Lophotrochozoa), a clade whose most recent common ancestor had spiral cleavage. In this chapter, we outline the current knowledge of protostome relationships and discuss future perspectives and strategies to increase our understanding of relationships within the main spiralian clades. Novel approaches to coding morphological characters are a pressing concern, best dealt with by scoring real observations on species selected as terminals. Methodological issues, such as the treatment of inapplicable characters and the coding of absences, may require novel algorithmic developments. Taxon sampling is another pressing issue, as terminals within phyla should include enough species to represent their span of anatomical disparity. Furthermore, key fossil taxa that can contribute novel character state combinations, such as the so-called 'stem-group lophotrochozoans', should not be neglected. In the molecular forum, EST-based phylogenomics is playing an increasingly important role in elucidating animal relationships. Large-scale sequencing has recently exploded for Spiralia, and phylogenomic data are lacking from only a few phyla, including the three most recently discovered animal phyla (Cycliophora, Loricifera, and Micrognathozoa). While the relationships between many groups now find strong support, others require additional information to be positioned with confidence. Novel morphological observations and phylogenomic data will be critical to resolving these remaining questions. Recent EST-based analyses underpin a new taxonomic proposal, Kryptrochozoa (the least inclusive clade containing the Brachiopoda and Nemertea).

Problematica are taxa that defy robust phylogenetic placement. Traditionally the term was restricted to fossil forms, but it is clear that extant taxa may be just as difficult to place, whether using morphological or molecular (nucleotide, gene, or genomic) markers for phylogeny reconstruction. This chapter discusses the kinds and causes of Problematica within the Metazoa, but particularly focussing on the invertyebrate taxa, as well as suggesting criteria for their recognition and possible solutions. The inclusive set of Problematica changes depending upon the nature and quality of (homologous) data available, the methods of phylogeny reconstruction and the sister taxa inferred by their placement or displacement. Rather than excluding Problematica from phylogeny reconstruction, as has often been preferred, this chapter concludes that the study of Problematica is crucial both for the resolution of metazoan phylogeny, and the proper inference of body plan evolution. This chapter provides an annotated list of key extant problematic taxa.

Understanding the evolution of a clade, from either a morphologic or genomic perspective, first and foremost requires a correct phylogenetic tree topology. This allows for the polarization of traits so that synapomorphies (innovations) can be distinguished from plesiomorphies and homoplasies. Metazoan phylogeny was originally formulated on the basis of morphological similarity, and in some areas of the tree was robustly supported by molecular analyses, whereas in others was strongly repudiated by molecular analyses. Nonetheless, some areas of the tree still remain largely unknown, despite decades, if not centuries, of research. This lack of consensus may be largely due to apomorphic body plans combined with apomorphic sequences. Here, the chapter proposes that microRNAs may represent a new dataset that can unequivocally resolve many relationships in metazoan phylogeny, ranging from the interrelationships among genera to the interrelationships among phyla. miRNAs, small non-coding regulatory genes, shows three properties that make them excellent candidates for phylogenetic markers: 1) new microRNA families are continually being incorporated into metazoan genomes through time; 2) they show very low homoplasy, with only rare instances of secondary loss, and only rare instances of substitutions occurring in the mature gene sequence; and 3) are almost impossible to evolve convergently. Because of these three properties, this chapter proposes that miRNAs are a novel type of data that can be applied to virtually any area of the metazoan tree, to test among competing hypotheses or to forge new ones, and to help finally resolve the correct topology of the metazoan tree.

This chapter focuses on ice shelves, supraglacial habitats (snow, supraglacial lakes, cryoconites holes), and other debris habitats including the ice margin. It discusses the nature of mat communities, and their photosynthetic and biogeochemical activity; the hydrology of glacier ice surfaces and the biology of supraglacial melt habitats; the formation of cryoconite and cryoconite holes, and their impact on albedo; and cryoconite hole communities, and their biological makeup, photosynthesis, bacterial production, metazoan activity, and viral dynamics for Arctic, Antarctic, and alpine glaciers.

In Systema Naturæ (1735, 1758), Carolus Linnaeus proposed a definition of the Kingdom Animalia: natural objects that grow, live, and sense. In contrast, plants grow and live but do not sense, while minerals grow but do not live or sense. In this definition of the animal kingdom, species are arranged in classes, families, and genera. This division of organisms into animals and plants was almost unchallenged for more than 100 years. In 1866, Ernst Haeckel came up with the first classification of living beings based on Charles Darwin’s ideas on evolution. He separated the Kingdom Animalia from the new Kingdom Protista based on the possession of tissues and organs. Today, the animal kingdom is restricted to multicellular animals, that is, the Metazoa. This chapter provides an overview of metazoans, including their apomorphies, sexual reproduction and life cycle, and genes involved in organising the metazoan body. It also describes some of the metazoan morphological characters, including cilia and flagella, choanocytes, cell junctions and epithelia.