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Craniata and Vertebrata

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Craniata and Vertebrata CHAPTER 1 Craniata and Vertebrata The vertebrates or Vertebrata form an ancient group of descent. As hypotheses, they are testable and thus with a history spanning some 545 million years. On the open to falsifi cation when new data become available. If one hand, they include the organisms most familiar to a hypothesis is falsifi ed, then another one may be us, such as fi sh, birds, cats and dogs, as well as humans; proposed — for example, new evidence might show that on the other, few people are aware of the great diversity humans share a recent common ancestor with a different in their form, structure, and habits. Indeed, they include great ape than chimpanzees, such as orangutans. some of the largest and more complex organisms ever evolved. But vertebrates are part of a larger grouping of animals, and to understand their history and the devel- PHYLOGENY AND CLASSIFICATION opment of their structure, they must be placed in phy- logenetic context. For most of the past 250 years, the classifi cation of organ- In discussing vertebrates, several other groups of isms has followed the Linnean system, which uses ranks organisms are usually considered. A monophyletic or to designate levels of organization of the organisms being natural group (see later) of organisms is referred to as a classifi ed. Most readers will be familiar with the main taxon (plur., taxa ). The taxa related (in terms of recent formal Linnean ranks, ordered hierarchically from most common ancestry) to vertebrates include Echinodermata to least inclusive: Kingdom, Phylum, Class, Order, (sand dollars, sea lilies, star fi sh, sea cucumbers, urchins), Family, Genus, and Species. Researchers have differed in Hemichordata (acorn worms and pterobranchs), Uro- assigning rank to the vertebrates and their relatives. For chordata (tunicates or sea squirts), and Cephalochordata example, some authors have recognized three phyla: (amphioxus). These are the typical nonvertebrate (or Phylum Echinodermata, Phylum Hemichordata, and “ invertebrate” ) relatives of the group in which we are Phylum Chordata. Others consider Urochordata and mainly interested. The vertebrates themselves, or Verte- Cephalochordata as phyla on their own, separate from brata, are included in a larger taxon termed Craniata. Chordata. Still others have viewed Urochordata as a Within Craniata and Vertebrata are many taxa. These separate phylum, but Cephalochordata as a subphylum taxa and the evolutionary relationships among them (see of the Phylum Chordata. If you fi nd this confusing, Figure 1.1 ) are briefl y outlined here to provide an orga- you ’ re not alone! The different designations did — or at nizational framework for undertaking the dissection of any rate were meant to— have some grounding in bio- the vertebrates discussed in this manual. Before this, logical reality. They refl ected a particular researcher ’ s however, it is necessary to present an explanation of perception of the magnitude of the difference in the several important terms used in discussions of phylogeny . levels of organization (a quality that may be referred to Phylogeny refers to the pattern of descent among taxa. It as a grade ) among the taxa under consideration. Thus, if describes, in other words, the evolutionary or genealogi- a taxon was considered a phylum, it mainly implied that cal relationships among them. Evolution is a historical its members had a fundamentally different basic body (and on-going) process. Therefore, the evolution of plan than if it was considered only a subphylum of a organisms occurred in only one way —for example, larger taxon. As you have probably already realized, humans and chimpanzees are, among living creatures, researchers ’ perceptions along these lines are subjective. either each other ’s closest relatives (they share a common In recent years, however, the formal Linnean ranking ancestor not shared by other organisms) or they are not. system has fallen increasingly into disuse as systematists Only one of these possibilities is correct. Evolutionary have become aware that there is no intrinsic or special biologists try to recover the pattern of descent based on value of any particular taxon that would justify its recog- the data they have available to them. Phylogenies are nition as a higher or lower rank, compared to other taxa. therefore hypotheses that approximate the true pattern In other words, there is no special reason for “ elevating ” The Dissection of Vertebrates, Second Edition DOI: 10.116/B978-0-12-375060-0.00001-2 1 © 2010 Elsevier Inc. 2 1. CRANIATA AND VERTEBRATA PROTOSTOMATA DEUTEROSTOMATA ECHINODERMATA PHARYNGOTREMATA HEMICHORDATA CHORDATA UROCHORDATA SOMITICHORDATA ENTEROPNEUSTA PTEROBRANCHIA CEPHALOCHORDATA CRANIATA DEUTEROSTOMATA PHARYNGOTREMATA CHORDATA • first embryonic opening becomes anus; • pharyngeal skeleton • endostyle second opening becomes mouth • pharyngeal slits • dorsal, hollow nerve cord • basally, mesoderm forms bilaterally as • blood vascular system • notochord out-pocketing of embryonic gut; coelom, • postanal tail which develops within mesoderm, thus originally has connection with gut • basally, a ciliated looped band is present on surface of larva FIGURE 1.1 Cladogram showing phylogeny of Deuterostomata. Some synapomorphies of the main groups are provided in the boxes below the cladogram. birds or Aves to the rank of Class, equal and thereby that they share this feature because they have inherited it excluded from, the Class Reptilia. In fact, it is improper from a common ancestor, rather than each having evolved to do so, because the birds are properly part of the taxon the character independently, and so infer that the taxa are named Reptilia. Here, formal ranks are not used, and descendants of the same ancestor (which we are not able taxa are referred to simply by their name (except for the to actually recognize, and thus refer to as hypothetical). preceding paragraph, in which ranks were used to refl ect Biologists use many characters in trying to reconstruct the historical understanding of the groups). phylogeny. The practice is complicated by the fact that Formal names are applied to natural or monophyletic organisms can and do evolve very similar characters groups. A monophyletic group includes an ancestor and independently of each other, an occurrence referred to as all of its descendants (provided that the phylogeny has homoplasy . In reconstructing phylogeny, a researcher con- been carefully reconstructed). Such groups are termed siders the totality of evidence. It is rare that only a single clades . Clades are recognized based on common ancestry. character can be used to reconstruct phylogeny. If two taxa are in a clade, it is because they are linked by The pattern of relationships among taxa is depicted a common ancestor. Biologists infer such ancestral rela- visually by a cladogram , which is essentially a diagram tionships through the presence of shared derived charac- of nodes and branches, with the nodes representing ters or synapomorphies (see later). If two (or more) taxa ancestors and the branches that diverge from a node share a character that is exclusive to them, then we assume representing the descendant taxa of the ancestor. 1 The 1 Technically, nodes are speciation events of one ancestor into two or more descendant or daughter species. The root and internodes (segments between the nodes) represent the common ancestors and loci of character transformations. THE DISSECTION OF VERTEBRATES PHYLOGENY AND CLASSIFICATION 3 node, then, may be thought of as representing the hypo- derived trait, it is described as synapomorphic . Synapo- thetical ancestor of the two taxa that diverge from it. The morphies do indicate phylogenetic relationship. In the pattern of branching represents the pattern of relation- most basic sense, sharing a derived trait is a shorthand ship. Examine the cladogram in Figure 1.1 . Note the way of saying that the organisms under consideration node from which the Hemichordata and Chordata possess a modifi ed trait because it was inherited from an diverge. This node represents ancestor species that split ancestor that fi rst acquired or evolved the modifi cation. to produce two lineages, one that evolved into Chordata An assortment of organisms united by synapomorphies and the other into Hemichordata. The two branches that forms a natural group or clade; that is, the clade is a real diverge from this ancestor represent the evolutionary entity in evolutionary terms. It means that all the organ- paths to the divergent taxa. isms included within the clade were ultimately derived Only the branching pattern is of concern. The length from the same ancestor. All vertebrates that possess jaws of the branches is immaterial in terms of absolute time, do so because this character was inherited from an ances- but relative time is implied by branching sequence. tor that had evolved jaws as a modifi cation of the man- Clearly, the divergence of Cephalochordata and Crani- dibular arch (see later). If we wish to understand the ata occurred after the divergence of Hemichordata and relationships among a lamprey, a fi sh, and a dog, the Somitichordata. presence of jaws is a character state that indicates that Informal names, set between quotation marks, are the dog and fi sh are more closely related to each other used to designate a group of organisms that do not than either is to a lamprey. When two groups are each descend from the same common ancestor, but that do other ’ s closest relatives, they are said to be sister groups . possess (or lack) some of the features of the taxon in A natural or monophyletic group may be recognized which we are interested. Many of these terms were con- formally by a name. The only restriction imposed is that sidered formal names in earlier classifi cations. For a monophyletic group include the ancestor and all example, the term “ protochordates ” is commonly used descendants of the ancestor, even though the ancestor to refer to the hemichordates, urochordates, and cepha- cannot be identifi ed. A monophyletic group may also be lochordates.
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