Vertebrates

1.1 Introduction

The vertebrate story unfolds over a span of more than half a billion years, an unimaginable depth of time. During this time, some of the largest and most complex animals ever known evolved among the vertebrates. Vertebrates occupy marine, freshwater, terrestrial, and aerial environments, and exhibit a vast array of lifestyles. Like amphioxus and tunicates, vertebrates are proper chordates and possess at some time during their lives all five defining chordate characteristics: notochord, pharyngeal slits, tubular and dorsal nerve tube, postanal tail, and endostyle. The diversity vertebrates enjoy might be attributed to opportunity.

They arose at a time when few large predators existed. Their success may be due to their great variety of innovations as well. Two of these innovations—the vertebral column and the cranium—provide names for this major taxon.

1.1.1 Vertebral Column

The vertebral column inspires the name vertebrates and is composed of vertebrae, a series of separate bones or cartilage blocks firmly joined as a backbone that defines the major body axis. Squeezed between successive vertebrae are thin compression pads, the intervertebral disks or bodies. A typical vertebra consists of a solid cylindrical body, or centrum, that often encloses the notochord, a dorsal neural arch enclosing the spinal cord, and a ventral hemal arch enclosing blood vessels. Extensions of these arches are neural and hemal spines, respectively.

The earliest vertebrates (Haikouella, Haikouichthys) relied upon a strengthened notochord to meet mechanical demands of body support and locomotion. But, these earliest vertebrates apparently also possessed rudimentary vertebrae. In these and other early fishes, the vertebral elements rode upon or surrounded a notochord that continued to serve as the major structural component of the animal’s body. In later fishes and terrestrial vertebrates, successive vertebrae take over the functions.

As the role of the vertebral column enlarged, that of the notochord was reduced. In adults of most vertebrates, the notochord is greatly reduced or completely absent. In mammals, it persists as part of the intervertebral disks called the nucleus pulposus, a spongy, jelly-like core within an intervertebral disk.

1.1.2 Head

The major innovation that evolved in vertebrates is the cranium, or skull. The cranium is the most prominent structure of bone and/or cartilage that supports and encases the brain. The head and cranium partially or fully encase sensory organs. The term cephalization applies to the anterior clustering of sensory organs. The anterior part of the neural tube enlarges to form a distinct brain that includes forebrain, midbrain, and hindbrain. The cranium, usually including specialized nervous tissue, constitutes the head.

Vertebrate evolution has been characterized by neural crest and placodes, which are embryonic structures found only in vertebrates. Neural crest cells and placodes are essential for the development of the vertebrate head and sensory structures.

1.2 Origin of Vertebrates

The origin and early distribution of vertebrates took place in marine waters. However, fossil and physiological evidence suggests that freshwater origins are also possible. Many early vertebrate fossils were recovered from what appeared to be freshwater deposits (Ordovician). Some of these fossils consisted of bony armor worn smooth, as if they were washed and tumbled in freshwater streams before being deposited in river deltas.

In the 1930s, the physiologist Homer Smith argued that the vertebrate kidney evolved to rid the body of osmotic influx of excess water, which is a problem in freshwater, but not among marine animals. However,the discovery of still older fish fossils (Cambrian) confirmed the earliest vertebrates in marine waters. It was shown from this find that the vertebrate kidney, while good at maintaining water balance, need not be interpreted as an innovation of freshwater forms. The kidneys of lobsters and squid work in similar ways, yet these invertebrates and their ancestors have always been marine.

Further, the Ordovician sediments first thought to be from fresh water, instead proved to be from shallow, near-shore parts of the sea. Today, few scientists insist that the very first vertebrates were products of freshwater environments.

1.2.2 Kidney Physiology and Early Vertebrate Evolution

Evolution of early vertebrates was characterized by increasingly active lifestyles hypothesized to proceed in three major steps. Step 1 comprised a suspension-feeding prevertebrate resembling amphioxus. The prevertebrate deployed only cilia to produce the food-bearing current that entered the pharynx. Step 2 comprised an agnathan, an early vertebrate lacking jaws but possessing a muscular pump to produce a food-bearing water current. Step 3 comprised a gnathostome, a vertebrate with jaws. Food collection was less random. This gnathostome fed on larger food items with a muscularized mouth and jaws that rapidly snatched and selected prey from the water. These three steps possibly unfolded as follows.

1.2.2.1 Step 1: Prevertebrate

This prevertebrate arose within the protochordates. The pre-vertebrates were a suspension feeder. Suspension feeding based on ciliary pumps is common to hemichordates, urochordates, and cephalochordates. The first prevertebrate deployed a similar method of suspension feeding. A non-burrowing, free-swimmer better able to tolerate estuary environments in which rivers enter and mix with the sea. The shift from such a prevertebrate to a vertebrate condition involved two mechanical changes in the pharynx that together produced a muscular pump. First, cartilage replaced the collagen in the pharyngeal bars. Second, strong and springy bands encircled the pharynx. Contraction of the muscle bands constricted the pharynx, squeezing water out of the pharyngeal slits. Upon muscle relaxation, the cartilaginous supports sprang back to expand the pharynx, restore its original shape, and draw in new water.

Initially, this new muscular pump merely supplemented the existing ciliary pumps in moving water through the pharynx. But in larger animals, surface ciliary pumps became less effective in supplying food for their greater body mass. Increased mass favored prominence of the muscular pump and loss of the ciliary mechism of moving water.