Craniates and Vertebrates

Difference of opinion exists in precisely which chordates are to be regarded formally as Vertebrata (Figure 1.2). In part this is because one of the main characters of ver­tebrates is, of course, the presence of vertebrae, a repeat­ing series of articulating cartilaginous or bony elements forming the spinal column, which provides support for the body, muscular attachment, and protection for the nerve or spinal cord.

However, not all chordates traditionally included in the Vertebrata have complete vertebrae, as just noted above; and some have no trace of vertebrae at all. In large part, which chordates are actually recognized as vertebrates depends largely on the relationships of the most basal or primitive living vertebrates, the hagfishes (Myxi- noidea) and lampreys (Petromyzontoidea), both to each other and to unquestioned vertebrates. The hagfishes and lampreys are clearly more advanced than cephalo- chordates, sharing various characteristics with unques­tioned vertebrates (see below). However, they are undoubtedly less derived than the latter in the absence of jaws, the feature to which they owe their designation as “agnathans” (from the ancient Greek a-, without; and gnathos, jaw). Their mouths are circular, so they are also known as “cyclostomes” (round mouth). The undoubted vertebrates, united by the fact that they possess jaws, are grouped together as the Gnathosto- mata (jaw-mouthed). Traditionally, the hagfish and lamprey were considered to be each other’s closest rel­ative, and so grouped in the “Cyclostomata” as a formal taxon. As well, several groups of jawless extinct forms were considered more closely related to cyclostomes than to gnathostomes, and the whole lot of these jaw­less forms were included in the “Agnatha,” again, as a formal taxon.

“Agnathans” diversified into many different forms early in craniate history, but only two forms, the hagfish men­tioned above and the lamprey, represent the jawless condition among living craniates. Many of the early agnathans were excessively bony, but most of this bone was dermal and formed shields or plates that covered and protected the body. These forms are informally termed “ostracoderms,” and are not considered in the phylogenies presented here.

About 20 years ago, morphological analyses began to suggest that lampreys (and some extinct “agnathans”) are more closely related to gnathostomes than to hag­fishes. Following cladistic procedure, the lampreys were then grouped together with the gnathostomes. Because lampreys possess rudimentary vertebrae (or, at least, precursors of true vertebrae), termed arcualia, which are essentially cartilaginous blocks on either side of the spinal cord, most researchers began to restrict the Ver­tebrata to the lamprey + gnathostome assemblage, with the hagfishes considered the sister group to this Verte­brata. An important set of features shared by both hag­fishes and Vertebrata (lampreys + gnathostomes) is the development of a true head (see below), and so the term Craniata was applied to this clade. We note in passing that some researchers have continued to consider the Vertebrata as including the hagfishes (while recognizing the sister group relationship between lampreys and gnathostomes), and so consider Craniata and Vertebrata as synonyms. Most recently, however, molecular studies have not corroborated the morphological evidence. Instead, these studies suggest that the hagfishes and lampreys may indeed be each other’s closest relative, in which case the “Cyclostomata” would be monophyletic. The scheme followed here is based mainly on morpho­logical evidence and recognizes the Craniata and Verte­brata as successive clades (Figure 1.2).

As noted above, a major feature of craniates is the devel­opment of a “true” head. A distinct anterior enlargement of the nervous system, forming a brain, and of sensory organs occurs in craniates. The brain of craniates is tripartite, with three main primary subdivisions; and the specialized sense organs—eyes, ears, and nose—are complex. These structures are protected and supported by a bony or cartilaginous cranium or braincase. Closely associated with a head is the neural crest, a unique feature of craniates. This comprises embryonic tissue formed of cells assembled near the developing neural tube that migrate through the embryo to give rise to a great variety of structures. In the head region, neural crest cells initiate and largely form the vast array of cranial structures char­acteristic of craniates. Another unique feature associated with the head are neurogenic placodes. Placodes are thickenings of the ectoderm occurring early in embryonic development that differentiate and help form a variety of structures of the body. Neurogenic placodes occur only on the head and are involved in forming sensory receptors and neurons, and contribute to the cranial nerves. Thus, the presence of the neural crest and neurogenic pla­codes, transitory though they may be, are distinguishing feature of craniates. Pharyngeal slits are still present, of course, but in craniates they are associated with gills and are thus used for respiration rather than feeding. Yet another innovation of craniates is that moving water into the pharynx and out through the pharyngeal slits is accomplished by muscular, rather than ciliary, action (see Figure 1.2).