Early Stages in the Evolution of Vertebrates
Before continuing with the phylogenetic story of vertebrates, it is useful to examine the feeding habits of the earliest vertebrates, as evolutionary innovations of the feeding apparatus reflect the major transitions in vertebrate evolution.
It is generally hypothesized that the earliest step in becoming a vertebrate occurred in creatures considered to represent a prevertebrate stage, although such forms may have qualified as true vertebrates. Their precise phylogenetic position, in any event, is not of concern here, because we are interesting in feeding mode. The prevertebrate was probably a suspension or filter feeder (that is, it filtered food material that was suspended in water) and used ciliary action to generate a current of water into its mouth and out of the pharyngeal slits; in other words, a creature very much like amphioxus. It did not have jaws, and its pharyngeal bars were probably collagenous. The ciliary pump imposed limits on size, as it restricted the amount and type of food the animal was capable of ingesting.The next step involved a change from the ciliary pump with collagenous bars to a muscular pump with cartilaginous bars. This is the agnathan stage. The combination of these characteristics meant that the intake of water, and thus food, was controlled by active expansion and compression of the pharyngeal region, which allowed a diversification in size and type of food, and thus of the vertebrates themselves. The cartilaginous, rather than collagenous, bars were instrumental in this. Musculature could be used to compress the pharynx, including the bars, but once the muscles relaxed, the cartilaginous bars could spring back into shape, expanding the pharynx.
The third level of development is the gnathostome stage, which involved the development of jaws. Jaws conferred the ability to grasp prey and close the mouth to prevent its escape; hence, to seek and capture food. These features set the stage for the predaceous, active lifestyle of vertebrates, in sharp contrast to the sedentary lifestyles of protochordates.