You should have a working knowledge of the following terms:
Introduction and Goals
This last tutorial examines the deuterostomes in the phylum Chordata. This phylum contains three subphyla: Vertebrata, Cephalochordata, and Urochordata. We will explore the features that characterize this phylum and how these structures have evolved for many purposes. By the end of this tutorial you should have an understanding of:
- The characteristics of chordates
- The characteristics of organisms within the invertebrate subphyla of Phylum Chordata
- The characteristics of organisms within the vertebrate subphylum of Phylum Chordata
Chordates embody many sophisticated evolutionary adaptations. They have four major characters that distinguish them from other deuterostomes: a notochord, a dorsal nerve cord, pharyngeal slits, and a postanal tail.
Figure. 1 (Click image to enlarge)
The dorsal, hollow nerve cord is basically a sheet of ectoderm rolled into a tube. A chordate's nerve cord develops into the brain and spinal cord in adults.
Figure. 2(Click image to enlarge)
Just below the nerve cord is the notochord, which exists in all chordate embryos. The notochord is a long flexible rod that provides skeletal support. Typically the muscles surrounding the notochord are arranged in distinct segments.
The pharyngeal slits are openings in the pharynx (part of the throat) of an animal. In the most primitive state these slits allow water to enter the mouth and exit without passing through the digestive system. In the more derived state the pharyngeal slits (and the tissue between them, the pharyngeal arches) serve various functions, ranging from gas exchange to food collection.
At some point in their development, all chordates have a tail (a postanal tail) that extends beyond the anus. As with all of these basic chordate characteristics, the tail can be present in the developing embryo and absent in the adult (depending on the type of organism).
There are three subphyla within Phylum Chordata: Vertebrata, Urochordata, and Cephalochordata. All members of the subphyla Urochordata and Cephalochordata are invertebrates (animals without backbones), as were all animals discussed in the previous four tutorials. All members of the subphylum Vertebrata are vertebrates (animals with backbones). Subphylum Vertebrata will be discussed later in this tutorial.
Subphylum Urochordata includes the tunicates ("sea squirts"), which are marine animals that are typically sessile as adults. The lightbulb tunicates pictured here on the right are adhering to coral.
Figure. 3 (Click image to enlarge)
The adult tunicate, illustrated below left, does not appear to possess any of the standard chordate characteristics. However, the larval stage (rightmost figure below) clearly exhibits chordate characteristics; note the notochord, the hollow nerve cord, the pharyngeal gill slits, and the postanal tail. In adults the gill slits are modified for filter feeding. Molecular evidence supports the idea that the urochordates are the group most closely related to the vertebrates.
Figure. 4 (Click image to enlarge)
Figure. 5 (Click image to enlarge)
Subphylum Cephalochordata includes the cephalochordates (lancelets), which are small marine animals that burrow, tail first, into the sand. In this position they are able to draw food particles into their mouths via their waving tentacles. Examine this illustration of the anatomy of an adult lancelet, Branchiostoma, and be sure you can identify the common chordate characterisitcs retained in the adult cephalochordates.
Figure. 6 (Click image to enlarge)
Phylum Chordata: Subphylum Vertebrata
Subphylum Vertebrata includes organisms with several basic features, in addition to the typical chordate characteristics. Vertebrates exhibit the most pronounced cephalization, in which the anterior portion of the nervous system (the brain) is enclosed inside a protective case (the skull). These animals all have a vertebral column (backbone) that serves to support the head, anchor muscles, and protect internal organs. The dorsal nerve cord is protected by the vertebral column, and the anterior portion of this nerve cord is protected by a cranium.
This figure on the right illustrates the evolutionary relationships among members of Phylum Chordata. Focusing on the vertebrates, note that these all contain a vertebral column. The vertebral column is an axial endoskeletal system that forms dorsally to the notochord. (In the adult, the notochord may persist to some degree.) As you look at the basal characteristics, note the progression of the more derived features: jaws, teeth, lungs, lung derivatives, legs, amniotic eggs, and hair. We will explore these features (and their origins) in more detail in this tutorial.
Figure. 7 (Click image to enlarge)
Evolution of The Jaw
During vertebrate evolution, many modifications have occurred in the pharynx. If you have an opportunity to take a class in mammalian embryology, you will learn that many structures are derived from different regions of the embryonic pharynx (e.g., human nostrils originate from pharyngeal slits, which are present in the early human embryo). We will examine the evolution of the jaw so that you can appreciate how evolutionary biologists think these more derived structures evolved.
These jaw images are arranged left to right, with the more ancestral state on the left and the more derived state on the right. In the ancestral state, endoskeletal elements are seen in the pharyngeal arches between the pharyngeal slits. It is likely that in this configuration the endoskeleton evolved to keep the pharyngeal slits open. However, in a macroevolutionary event that probably involved a change in the allometric growth of the more anterior arches, the endoskeletal elements became thicker and the presence of primitive jaws appeared. These were adapted to capture food more effectively. In the most derived state (depicted on the right), further macroevolutionary events occurred and these anterior bones became thicker and new bony protuberances appeared in the form of teeth. Teeth made predation more efficient, and occur today in many vertebrates.
Figure. 8 (Click image to enlarge)
You should recognize this gradual adaptation of existing structures for new purposes as a reoccurring theme in evolution. The evolution of other pharyngeal structures, which gave rise to lungs and their derivatives, has also contributed significantly to vertebrate evolution.
Evolution of Legs and the Amniotic Egg
Legs and amniotic eggs are two additional derived characters that have played a significant role in the evolution of vertebrates. The significance of legs in the terrestrial environment is obvious. How do evolutionary biologists think they evolved?
The endoskeletal system of vertebrates consists of the axial elements that are associated with the vertebral column. In addition, more lateral elements, termed the appendicular skeletal system, also exist. The fossil record indicates that the appendicular skeletal system first appeared as small elements in association with fins of early vertebrates. However, a progressive thickening of these elements (again, involving changes in allometric growth) occurred. Some fish have extremely strong fins and are known as "lobefin" fishes. The fossil record (and other data) indicates that the amphibians arose from lobefin fish, and that legs arose from these highly modified lobe fins.
Figure. 9 (Click image to enlarge)
Early vertebrates arose in the oceans, where desiccation was not a problem. However, with the invasion of land about 365 million years ago, ancient vertebrates had another problem to face; namely, water loss. This was similar to the problem faced by land plants. The vertebrates solved this problem by evolving with a toughened egg that was resistant to drying.
In reptiles, birds and mammals, the amniotic egg has several unique features (e.g., a specialized membrane system that has many functions, including gas exchange and the transfer of vital nutrients to the developing embryo). The amniotic egg derived its name from the amnion, a membrane that houses amniotic fluid, which bathes and protects the embryo.
Figure. 10 (Click image to enlarge)
This armadillo (below left) belongs to the vertebrate class Mammalia, as does this panther (below right. Mammals can be distinguished from the other vertebrates on the basis of several characteristics. All mammals have hair that functions (to some degree) in insulation, mammary glands for milk production, and internal fertilization. Most mammals develop in the maternal uterus and are nourished by nutrients diffusing from mother to child across an organ called the placenta. Young marsupial mammals complete their development in a maternal pouch; marsupials include kangaroos, opossum, and tasmanian devils. A few animals (e.g., the echidna and the platypus) are monotremes, which are mammals that actually lay eggs and lack nipples.
Figure. 11 (Click image to enlarge)
Figure. 12 (Click image to enlarge)
Humans are vertebrates, and we are subject to the same evolutionary principles that govern all life on the planet. However, we are a relatively new species. Although our hominid ancestors might date back to 2 million years ago, modern humans (those that learned agriculture and demonstrated tool use and complex social structure) date back to a mere 100,000 years ago. While this may sound like a long time ago, consider that this represents only 5000 generations. This is not long on an evolutionary scale. This figure shows the major migration paths prior to 10,000 years ago, and all of us can trace our own ancestry back to one or more of these migrations.
Figure. 13 (Click image to enlarge)
This tutorial presented the phylum Chordata and the subphyla Urochordata, Cephalochordata, and Vertebrata. Individuals within the phylum all have a notochord, a dorsal nerve cord, pharyngeal slits, and a postanal tail. The notochord may or may not persist in the adult, and the pharyngeal slits are modified in various ways in the different groups. The Urochordates are the probable ancestors of vertebrates.
Vertebrates have a dorsal nerve cord that is protected by a vertebral column, and the anterior portion of this nerve cord is protected by a cranium. There are several basal features found in vertebrates, and they are used to distinguish the lineages. The presence or absence of jaws, teeth, lungs, legs, amniotic eggs, and hair are more derived features that arose in succession, and they give the vertebrates the diversity seen today.