Early Evolution of Vertebrates

Among the fossils of the Burgess Shale is a small, elongated animal named Pikaia gracilens. Two traits suggest that this is an early chordate: a notochord (precursor of the vertebrate spinal cord), and myotonies (muscles involved in swimming). Pikaia is reminiscent of the amphioxus or lancelet, Brachiostoma, a living chordate (Figure 2.6). Similar fossil chordates were discovered in China (Chen, Huang, & Li, 1999).

Chordates include three major groups: urochordates, cephalochordates, and vertebrates. It is widely accepted that this group is monophyletic, that is, derived from a common ancestor. Urochordates or tunicates (about 1,300 species) are all marine animals. The adults of most species live a sedentary life attached to a substrate and some form colonies, but free-swimming larvae possess a dorsal chord in their tails, called a urochord.

Cephalochordates or lancelets (about 28 species) are also marine animals that tend to live partially buried in the sea floor. Lancelets are characterized by a dorsal notochord and V-shaped myotones. They look like fish, but have no true bone and no brain in the neural tube—just a vesicle. It is widely assumed that cephalochordates and vertebrates are sister groups derived from a common ancestor resembling Pikaia.

The major vertebrate innovations relate to an active lifestyle: complex sense organs, motor ganglia, and a well-defined brain. Eight or nine classes of vertebrates are usually recognized in the literature. There are conservative and derived lineages in each of the vertebrate classes. Each vertebrate class represents a tree of radiating branches, of which only one gave rise to another class. The ancestral lineage is a fossil taxon whose identity is unknown. Therefore, it is inappropriate to consider a living species from one class as the ancestor of living species

Figure 2.6 (A) Pikaia fossil from the Cambrian preserved with soft tissue. Myotomes (muscles involved in swimming) and a dorsal notochord (precursor of the vertebrate column) are visible (photo courtesy of Derek Briggs). (B) Comparison with a living chordate, Brachiostoma floridae, displaying the same features (photo courtesy of Guan Li).

Figure 2.7 A cladogram depicting a classification of various animal phyla on the basis of rRNA data (modified from Valentine. 1995). Each node represents a divergence event. Major characters included in the cladogram apply to all the lineages located to their right: t (tissue), cell populations differentiated in structure and function: g (gastrulation). a stage of embryonic development characterized by the formation of an internal cavity from a hollow ball of cells; m (mesoderm), a third layer of cells in addition to ectoderm and endoderm; b-v (blood-vascular system), a circulatory system: h (hemocoel), a body cavity filled with blood: c (coelom), a body cavity lined with mesoderm cell. The numbers in parentheses reflect the approximate number of living species in each phylum.

from another class. For example, mammals evolved from a group of fossil reptiles known as the cynodonts; therefore, living reptiles (turtles, lizards, crocodiles) cannot be considered as mammalian ancestors because they represent other diverging branches of the reptilian tree.

Figure 2.8 summarizes vertebrate phylogeny, including their major evolutionary innovations, whereas Figure 2.9 shows the appearance of the various vertebrate groups in the fossil record. The brief description of each vertebrate class that follows is based on a variety of sources; a comprehensive view can be found in Benton (2014).