Vertebrate Relatives

All the taxa mentioned so far belong to the Deuteros­tomata, a major clade of coelomate triploblastic meta­zoans, multicellular animals that possess three primary body layers (ectoderm, mesoderm, and endoderm) and have a true body cavity that houses the viscera.

The synapomorphies (shared derived characters) of deuterostomes, at least among basal members, that indicate they are a clade are mainly similarities of early embryonic development. They include type of cleavage of the fertilized egg, pattern of mouth and anus forma­tion, and formation of the mesoderm and coelom (body cavity). The clades within the Deuterostomata share these features, but several of them have become modi­fied in some advanced members.

Next, we must consider the pattern of relationships, or phylogeny, among deuterostomes. For the most part, the phylogeny outlined here follows the traditionally recognized scheme based primarily on morphology. Be aware, however, that several recent analyses based mainly on mitochondrial or ribosomal gene sequences do not corroborate this scheme. Such discrepancies are noted appropriately below.

To review, the main clades of deuterostomes are the echinoderms, hemichordates, urochordates, cephalo- chordates, and craniates (Figure 1.1). It may seem sur­prising that the echinoderms, seemingly so different from what we usually think of as vertebrates, are closely related to vertebrates and included with them in the Deuterostomata. As noted above, however, they are clearly united by strong similarities in early develop­mental patterns.

One group traditionally considered important in verte­brate history is the Chordata, which includes the Uro­chordata, Cephalochordata, and Craniata. One reason the Chordata has been considered particularly impor­tant is that there are several easily recognizable charac­ters that are clearly shared by chordates.

At any rate, beginning with the Chordata is convenient. The chordates are united by the presence of the follow­ing synapomorphies: pharyngeal slits; an endostyle; a dorsal, hollow nerve cord; a notochord; and a postanal tail. These features are present at some point during the lives of all chordates, although they may be expressed to varying degrees and restricted to part of the life cycle in different vertebrate groups, or modified in advanced members. Humans, for example, do not possess a tail, notochord, or pharyngeal slits, but pharyngeal pouches, a notochord, and a tail are transient features that are present during embryonic development. The endostyle is represented by its homologue, the thyroid gland.

Pharyngeal slits are bilateral apertures that connect the pharynx (essentially the “neck” of the animal), which is the anterior part of the gut, with the outside. In forms that are familiar to us, such as fish, the slits are part of the respiratory system: The gills reside in the walls of the slits and perform gaseous exchange as water passes over them. In some fishes, like sharks, the slits open indi­vidually onto the surface of the body; in most other fishes, the slits open into a common chamber that then leads out to the surface of the body by a common opening. Originally the slits did not function in respi­ration. Ancestral vertebrates were suspension or filter feeders (as are urochordates and cephalochordates still), and the slits were the means for allowing water to exit the oral cavity and pharynx. As water passed out of the pharynx through the slits, food particles were filtered out and directed toward the digestive system.

The notochord is a relatively thin rod-like structure running dorsally along the length of the trunk and tail in less derived chordates. It is an important support structure, and the name Chordata is derived from notochord. It is a hydrostatic structure, consisting of a fibrous sheath that encloses a fluid-filled central core. It is flexible along its length, but, as it is filled with fluid, cannot easily be compressed anteroposteriorly (or tele­scoped). The notochord provides support for the body and allows the side-to-side locomotory movements characteristic of primitive vertebrates. In advanced ver­tebrates, the notochord is largely replaced functionally by the bone of the spinal column. It is present embry­ologically, and in adult humans, notochordal tissue may persist as part of the intervertebral disks that lie between adjacent vertebrae.

The presence of a tubular nerve cord enclosing a fluid- filled central canal occurs only in chordates. There are additional distinctive features about the chordate nerve cord. It is formed by an embryological process called invagination, a rolling and sinking into the body of ecto­dermal tissue. Further, it is dorsal to the digestive tract, whereas in most nonchordates the nerve cord is solid and ventral in position.

The postanal tail is a continuation past the anus of the trunk musculature and notochord. This extension is an important development that allows the locomotion par­ticular to vertebrates. Many chordates do not possess a postanal tail as adults, humans being an obvious example.

The Cephalochordata is usually considered the sister group to the craniates, a phylogenetic arrangement reflecting the idea that vertebrates and cephalochordates share a common ancestor. All five chordate characters are clearly present during the life of a cephalochordate. The name Cephalochordata is derived from the presence of a notochord extending from the tail nearly to the tip of the head (from the ancient Greek kephalos, head). The commonly studied cephalochordate is Branchios- toma. Cephalochordate species commonly are referred to as amphioxus (which means sharp at both ends) or lancelet (little spear). Given the fact that these little crea­tures essentially lack a head and so are pointed at both ends, amphioxus is an especially appropriate designa­tion. Although amphioxus has a fish-like body (see below), it is not an active swimmer as an adult. Instead, it burrows into the substrate, usually just out from sandy beaches, and assumes a position with only its mouth exposed. Its filter-feeding lifestyle is similar to that described above for ancestral vertebrates. Intake of water and its movement through the pharynx is accom­plished by ciliary action. The pharynx has numerous slits that collectively empty into a surrounding chamber, the atrium, before leaving the body though a common opening. The endostyle secretes mucus, which traps food particles suspended in the water.

The taxon including the Cephalochordata and Craniata is termed the Somitichordata (Figures 1.1,1.2). Although there are several differences between these sister groups, we are interested in their synapomorphies, for these fea­tures provide evidence of their shared ancestry. Among these characters are similarity in development of meso­derm, including the hypomere (or lateral plate meso­derm) and mesodermal somites, which develop into myomeres (segmented blocks of trunk musculature, which in amphioxus extends through to the anterior tip of the body); arrangement of the circulatory system, with dorsal and ventral aortae; and segmentally arranged spinal nerves.

The Urochordates are the next most related group, sharing a common ancestor with the Somitichordata.

FIGURE 1.2 Cladogram showing phylogeny of the Chordata. Some Synapomorphies of the main groups are provided in the boxes below the cladogram.

Urochordates are characterized by sea squirts or tuni­cates, which are sessile, sac-like organisms as adults. In the larval stage, however, all five chordate characters are present. Predictably, the three characters lost in adults— the tail, notochord, and nerve cord—are used in loco­motion by the free-swimming larva as it searches for a suitable place to anchor itself to metamorphose into the adult form. During this transformation, the tail is absorbed, along with the nerve cord and notochord, of which only small remnants remain in the adult. The name Urochordata is derived from the fact that the notochord is present in the tail (from the ancient Greek uron, tail). Conversely, the pharyngeal region expands dramatically into a barrel-shaped structure with numer­ous slits. Water and suspended food particles are drawn into this “barrel,” which is lined with mucus from the endostyle. Food particles are trapped by the mucus, and water leaves through the slits into the atrium, the chamber surrounding the pharynx.

This arrangement is the more commonly accepted phy­logenetic scheme. Some, however, reverse the positions of the Urochordata and Cephalochordata, with the former considered the sister group to the Craniata. One recent study removed cephalochordates from chordata, and considered them as a sister group to the Echino- dermata.

The phylogenetic position of the Hemichordata is partic­ularly uncertain. They were traditionally grouped with the chordates, as is done here, but this arrangement is far from stable. Molecular evidence has been mounting over the last decade that points to a monophyletic relationship of hemichordates with echinoderms; as well, morpholog­ical evidence suggests monophyly. Hemichordates com­prise two clades, the Enteropneusta (acorn worms) and the Pterobranchia, both of which are marine animals. The acorn worms are reasonably diversified and well known, but the pterobranchs are not as well understood. Some but not all pterobranchs have a single pair of pharyngeal apertures, whereas all acorn worms have several such openings. The presence of these slits and embryonic invagination of the nerve cord are about the only defini­tive evidence of a relationship with the chordates. On the other hand, evidence suggests that echinoderms originally had slits as well, although no living echinoderm possesses them. If this is true, then a monophyletic relationship between Hemichordata and Chordata becomes tenuous indeed.