Overview: Half a Billion Years of Backbones

Chapter 34

Vertebrates

Lecture Outline

Overview: Half a Billion Years of Backbones

In the early Cambrian period, 540 million years ago, slender 3-cm-long animals swam in the oceans.
These animals lacked armor and appendages, but they left behind a remarkable legacy.
They were the ancestors of the vertebrates, one of the most successful groups of animals ever to swim, walk, slither, or fly.
Vertebrates derive their name from vertebrae, the series of bones that make up the vertebral column, or backbone.
For nearly 200 million years, vertebrates were restricted to the oceans, but about 360 million years ago, the evolution of limbs in one lineage of vertebrates set the stage for these vertebrates to colonize land.

○On land, vertebrates diversified into amphibians, reptiles (including birds), and mammals.

There are about 52,000 species of vertebrates, far fewer than the 1 million insect species on Earth.
What vertebrates lack in species diversity, however, they make up for in disparity.

○Plant-eating dinosaurs, at 40,000 kg, were the heaviest animals to walk on land.

○The biggest animal that ever existed is the blue whale, at 100,000 kg.

○A fish discovered in 2004 is only 8.4 mm long and has a mass roughly 100 billion times smaller than that of a blue whale.

Concept 34.1 Chordates have a notochord and a dorsal, hollow nerve cord.

Vertebrates belong to one of the two major phyla in the Deuterostomia, the chordates.
Chordates are bilaterian animals belonging to the Deuterostomia clade.

○Two phyla of invertebrate deuterostomes—the urochordates and the cephalochordates—are most closely related to the vertebrates.

○Along with the hagfishes and the vertebrates, they make up the chordates.

Four derived characters define the phylum Chordata.

Although chordates vary widely in appearance, all share the presence of four key characteristics at some point in their lifetime: a notochord; a dorsal, hollow nerve cord; pharyngeal slits or clefts; and a muscular, post-anal tail.

The notochord, present in all chordate embryos, is a longitudinal, flexible rod located between the digestive tube and the nerve cord.

○The notochord is composed of large, fluid-filled cells encased in fairly stiff, fibrous tissue.

○The notochord provides skeletal support throughout most of the length of a chordate.

○In larvae or adults that retain the notochord, it also provides a firm but flexible structure against which muscles can work during swimming

○In most vertebrates, the notochord remains only as a remnant, surrounded by a more complex, jointed skeleton.

○For example, the notochord is the gelatinous material of the disks between the vertebrae in humans.

The dorsal, hollow nerve cord of a chordate embryo develops from a plate of ectoderm that rolls into a tube dorsal to the notochord.

○Other animal phyla have solid nerve cords, usually located ventrally.

○The nerve cord of the chordate embryo develops into the central nervous system: the brain and spinal cord.

The digestive tube of chordates extends from the mouth to the anus.

○The region posterior to the mouth is the pharynx.

○In all chordate embryos, a series of pouches separated by grooves forms along the sides of the pharynx.

○In most chordates, these grooves (known as pharyngeal clefts) develop into pharyngeal slits that allow water that enters the mouth to exit without continuing through the entire digestive tract.

○In many invertebrate chordates, the pharyngeal slits function as suspension-feeding devices.

○In vertebrates (with the exception of vertebrates that have limbs, the tetrapods), the slits and the structures that support them have become modified for gas exchange and are known as gill slits.

○In tetrapods, the pharyngeal clefts play an important role in the development of parts of the ear and other structures in the head and neck.

Most chordates have a muscular tail extending posterior to the anus.

○In contrast, nonchordates have a digestive tract that extends nearly the whole length of the body.

○The chordate tail contains skeletal elements and muscles, and provides propulsive force in many aquatic species.

Invertebrate chordates provide clues to the origin of vertebrates.

Lancelets (Cephalochordata) are bladelike in shape.

The notochord, dorsal hollow nerve cord, numerous gill slits, and post-anal tail all persist in the adult stage.
The larvae are suspension feeders, feeding on plankton in the water column.

Adult lancelets are up to 5 cm long.

Following metamorphosis, lancelets settle with their posterior end buried in the sand and their anterior end exposed for feeding.
Cilia draw seawater into the lancelet’s mouth, where a net of mucus secreted across the pharyngeal slits removes tiny food particles as the water passes through the slits, and the trapped food enters the intestine.
The pharynx and pharyngeal slits play a minor role in gas exchange, which occurs mainly across the external body surface.

A lancelet frequently leaves its burrow to swim to a new location.
Though feeble swimmers, their swimming mechanism resembles that of fishes through the coordinated contraction of serial muscle blocks.

○Contractions of chevron-shaped muscles flex the notochord and produce lateral undulations that thrust the body forward.

○The muscle segments develop from blocks of mesoderm, called somites, arranged serially along each side of the notochord of the embryo.

Data from a series of recent molecular studies suggest that tunicates (Urochordata) are more closely related to other chordates than are lancelets.

Tunicates most resemble chordates during their larval stage, which may last only a few minutes.
The tunicate larva uses its tail muscles and notochord to swim through the water in search of a suitable substrate on which it can settle, guided by cues from light- and gravity-sensitive cells.

Tunicates undergo a radical metamorphosis to form a sessile adult with few chordate characteristics.

○Its tail and notochord are resorbed, its nervous system degenerates, and its organs rotate 90°.

Tunicates are suspension feeders.

○Seawater passes inside the animal via an incurrent siphon, through the pharyngeal gill slits, and into a ciliated chamber, the atrium.

○Food filtered from the water is trapped by a mucous net and then passed by cilia to the esophagus.

○Filtered water and feces exit through an anus that empties into an excurrent siphon.

The degenerate adult stage of tunicates is a derived trait that evolved after the tunicate lineage branched off from other chordates.

Even the tunicate larva appears to be highly derived.

Studies of Hox gene expression suggest that the tunicate larva does not develop the posterior regions of its body axis.

The anterior region of a tunicate is elongated and contains a heart and digestive system.

Tunicates and lancelets may provide clues about the evolutionary origin of the vertebrate body plan.
Tunicates display a number of chordate characteristics only as larvae, whereas lancelets retain those characteristics as adults.

○The ancestral chordate may have looked something like a lancelet, with an anterior end with a mouth; a notochord; a dorsal, hollow nerve cord; pharyngeal slits; and a post-anal tail.

The genome of tunicates has been completely sequenced and can be used to identify genes likely to have been present in early chordates.

○Ancestral chordates likely had genes associated with vertebrate organs such as the heart and thyroid gland. Genes for these organs are found in tunicates and vertebrates but are absent from nonchordate invertebrates.

○In contrast, tunicates lack many genes that in vertebrates are associated with the long-range transmission of nerve impulses. This result suggests that such genes arose in an early vertebrate and are unique to the vertebrate evolutionary lineage.

Research on lancelets has revealed important clues about the evolution of the chordate brain.

○Rather than a full-fledged brain, lancelets have only a slightly swollen tip on the anterior end of the dorsal nerve cord.

○The Hox genes that organize major regions of the forebrain, midbrain, and hindbrain of vertebrates express themselves in a corresponding pattern in this small cluster of cells in the lancelet’s nerve cord.

○The vertebrate brain apparently is an elaboration of an ancestral structure similar to the lancelet’s simple nerve cord tip.

Concept 34.2 Craniates are chordates that have a head.

After the evolution of the basic chordate body plan, the next major transition was the appearance of a head.
Chordates with a head are known as craniates.
The origin of a head—with a brain at the anterior end of the dorsal nerve cord, eyes and other sensory organs, and a skull—enabled chordates to coordinate more complex movement and feeding behaviors.

Living craniates are distinguished from other chordates by a set of derived characters.

On the genetic level, craniates possess two clusters of Hox genes, whereas lancelets and chordates have only one.

Other important families of genes that produce signaling molecules and transcription factors are also duplicated in craniates.
This additional genetic complexity made a more complex morphology possible.

In craniates, a group of embryonic cells called the neural crest forms near the dorsal margins of the closing neural tube.

○Neural crest cells disperse throughout the body and contribute to the formation of various structures, such as teeth, some of the bones and cartilages of the skull, the dermis of the face, several types of neurons, and the sensory capsules of the eyes and other sense organs.

In aquatic craniates, the pharyngeal clefts evolved into gill slits.

○Unlike the pharyngeal slits of lancelets, which are used primarily for suspension feeding, gill slits are associated with muscles and nerves that allow water to be pumped through the slits.

○This pumping facilitates gas exchange and may also suck in food.

Craniates are more active than tunicates and lancelets and have a higher metabolism and a much more extensive muscular system.

○Muscles lining their digestive tract aid digestion by moving food through the tract.

Craniates have a heart with at least two chambers, red blood cells, and hemoglobin, as well as kidneys that remove waste products from the blood.

Cambrian fossils provide clues to craniate origins.

In China, several recent fossil finds of early chordates have provided information about the origin of craniates.

The early chordates appear to be “missing links” that straddle the transition to craniates, formed during the Cambrian explosion 530 million years ago.

The most primitive of these fossils is a 3-cm-long animal called Haikouella.

○This animal resembles a lancelet and was probably a suspension feeder.

○Haikouella also had a small but well-formed brain, eyes, and muscular segments.

○It had hardened structures (“denticles”) in the pharynx that may have functioned somewhat like teeth.

○Haikouella did not have a skull, however.

In other Cambrian rocks, paleontologists have found fossils of more advanced chordates, such as Myllokunmingia.

○Myllokunmingia had a skull composed of cartilage and is the oldest known true craniate.

○These fossils push craniate origins back to the Cambrian explosion.

Hagfishes are the least derived craniate lineage.

Hagfishes have a skull of cartilage but lack jaws and vertebrae.

Hagfishes swim in a snakelike fashion by using their segmental muscles to exert force against their notochord, which they retain in adulthood as a strong, flexible rod of cartilage.

Hagfishes have a small brain, eyes, ears, and a nasal opening that connects with the pharynx. They have tooth-like formations made of keratin.
All of the 30 or so species of hagfishes are marine scavengers, feeding on worms and sick or dead fish.

Rows of slime glands along a hagfish’s body produce large amounts of slime, perhaps to repulse other scavengers or deter a potential predator.

Concept 34.3 Vertebrates are craniates that have a backbone.

During the Cambrian period, a lineage of craniates evolved into vertebrates.
With a more complex nervous system and a more elaborate skeleton, vertebrates became more efficient at two essential tasks: capturing food and avoiding being eaten.
After vertebrates branched off from other craniates, they underwent another gene duplication, this one involving a group of transcription factor genes called the Dlx family.
This additional genetic complexity was associated with innovations in vertebrate nervous systems and skeletons, including a more extensive skull and a backbone composed of vertebrae.
In the majority of vertebrates, the vertebrae enclose the spinal cord and have taken over the mechanical roles of the notochord.
Aquatic vertebrates also have a number of adaptations associated with faster swimming, including fins stiffened by fin rays and a more efficient gas exchange system in the gills.

Lampreys represent the oldest living lineage of vertebrates.

Like hagfishes, lampreys offer clues to early chordate evolution but also have acquired unique characteristics.
About 35 species of lampreys inhabit both marine and freshwater environments.

Most lampreys are parasites that feed by clamping a round, jawless mouth onto a fish.

○They use their rasping tongues to penetrate the skin of their prey and to ingest the fish’s blood.

Lampreys live as suspension-feeding larvae in streams for years before migrating to the sea or lakes as they mature into adults.

○These larvae resemble lancelets and live partially buried in sediment.

○The sea lamprey (Petromyzon marinus) has invaded the Great Lakes over the past 170 years, where it has devastated a number of fisheries.

Some species of lampreys feed only as larvae.

○After metamorphosis, these lampreys attain sexual maturity, reproduce, and die within a few days.

The skeletons of lampreys are made of cartilage.

○Unlike most vertebrate cartilage, however, lamprey cartilage contains no collagen. Instead, it is a stiff protein matrix.

The notochord persists as the main axial skeleton in adult lampreys.

○Lampreys also have a cartilaginous pipe surrounding the rodlike notochord.

○Pairs of cartilaginous projections extend dorsally, partially enclosing the nerve cord.

Many vertebrate lineages emerged early.

Conodonts were slender, soft-bodied vertebrates with prominent eyes controlled by numerous muscles.

At the anterior end of their mouth, conodonts had a set of barbed hooks made of mineralized dental tissues.

Conodonts ranged in length from 3 to 30 cm.

Conodonts probably hunted with their large eyes and impaled their prey on hooks.
The food then passed to the pharynx, where a different set of dental elements crushed and sliced it.

Conodonts were very abundant for more than 300 million years.
Vertebrates with additional innovations emerged during the Ordovician, Silurian, and Devonian periods.

○These vertebrates had paired fins and an inner ear with two semicircular canals that provided a sense of balance.

○Although they lacked jaws, they had a muscular pharynx that may have sucked in detritus or bottom-dwelling organisms.

○They were armored with mineralized bone that offered protection from predators.

○These jawless, armored, swimming vertebrates went extinct by the end of the Devonian period.

The vertebrate skeleton evolved initially as a structure of unmineralized cartilage.

Its mineralization began only after lampreys diverged from other vertebrates.

What initiated the process of mineralization in vertebrates? Mineralization may have been associated with the transition to new feeding mechanisms.

○The earliest known mineralized structures in vertebrates were conodont dental elements.

○The armor in later jawless vertebrates was derived from dental mineralization.

○Thus, mineralization of the vertebrate body may have begun in the mouth.

○Only in more derived vertebrates did the endoskeleton begin to mineralize, starting with the skull.

Concept 34.4 Gnathostomes are vertebrates that have jaws.

The gnathostomes have true jaws, hinged structures that, especially with the help of teeth, enable vertebrates to grasp food firmly.

Living gnathostomes are a diverse group that includes sharks and their relatives, ray-finned fishes, lobe-fins, amphibians, reptiles (including birds), and mammals.

According to one hypothesis, gnathostome jaws evolved by modification of the skeletal rods that had previously supported the anterior pharyngeal gill slits.

The remaining gill slits were no longer required for suspension feeding and remained as the major sites of respiratory gas exchange.

Gnathostomes share a number of derived characters.

Gnathostomes share other derived characters besides jaws.
The common ancestors of all gnathostomes underwent an additional duplication of the Hox genes, so that the single cluster present in early chordates became four.

Other gene clusters also duplicated, allowing further complexity in the development of gnathostome embryos.

The gnathostome forebrain is enlarged, in association with enhanced senses of vision and smell.
The lateral line system evolved as a row of microscopic organs sensitive to vibrations in the surrounding water.
The common ancestor of living gnathostomes had a mineralized axial skeleton, a shoulder girdle, and two sets of paired appendages.
Gnathostomes appeared in the fossil record in the mid-Ordovician period, about 470 million years ago, and steadily diversified.
Gnathostome jaws and paired fins were major evolutionary breakthroughs.

○Jaws, with the help of teeth, enable the animal to grip food items firmly and slice them up.

○Paired fins, along with the tail, enable fishes to maneuver accurately while swimming.