Chordates

Members of the Phylum Chordata include animals with which students are most likely familiar. Included in the phylum are the fish, birds, reptiles, amphibians and beasts like ourselves. Four structural characteristics set chordates apart from all other phyla: (1) a notochord, (2) a pharyngeal gill slits, (3) postanal tail, and (4) a hollow dorsal nerve cord. These attributes are always found in the larval forms or early embryo (although they may be absent in the adult). The notochord is mainly composed of fibrous connective tissue. For those animals in which it persists into the adult form, the notochord provides support (it acts like our backbone) and increases swimming efficiency. In animals like ourselves, bony structures called vertebrae develop near the notochord and eventually replace it during embryogenisis. Pharyngeal gill slits are cuts in the pharynx that connect to a cavity surrounding the pharynx. For organisms in which they remain in the adult, they are often elaborated into respiratory structures (and are sometimes involved in filtering food during feeding). The morphological equivalent of gill silts are seen briefly during our own development (weeks 4-5), but they usually close or develop into other structures. Occasionally, the slits do not close, resulting in the newborn having an opening in the neck area (a cervical fistula). Our nerve cord, like that of other chordates is hollow (even in the adult). In all the phyla we have studied up to this point, the anus was terminal (at the tip of the tail). Chordates, on the other hand, follow the anus with a tail of variable length (again, an adaptation for locomotion). In us, the tail is short and fused (the coccyx at the base of your spine). In this section of the laboratory you will study the invertebrate chordates (those in which the notochord is not replaced by vertebrae).

As unlikely as it seems, based on embryological evidence, the echinoderms appear to be the most-likely ancestors to the early chordates (See HERE and HERE). Primitive stemmed echinoderms are thought to have shifted from arm-feeding to filter-feeding. They then acquired a body plan similar to today's urochordates. Unfortunately, the fossil record is poor and intermediates are lacking. The rest of the evolutionary picture, however, better documented. The similarity between present-day tunicate larvae and adult cephalochordates (amphioxus) suggests that the larvae became paedogenic (capable of reproducing while retaining larval characteristics). Succeeding evolution and specialization of the larvae resulted in the cephalochordates.

The earliest vertebrates in the fossil record are the ostracoderms (extinct now, but within the class Agnatha). These fish had no vertebrae and some used their pharyngeal gill slits for filter feeding. A comparison of an ammocoete larva (class Agnatha) with adult cephalochordates and tunicate larvae indicates that the relationship among the three phyla must be extremely close.

Agnath-like fish continued to specialize, eventually evolving jaws. The most primitive of the jawed fish are the placoderms (named for the plate-like armor shields covering their bodies and heads). Placoderms later developed into the cartilaginous (Chondrichthyes) and bony fish (Osteichthyes). Although vertebrates most certainly evolved in the ocean, some primitive fish invaded freshwater habitats and adapted accordingly. It is these freshwater fish that served as the ancestral stock for the terrestrial vertebrates (Amphibians, and later the reptiles, birds, and mammals).