Biology 1201: The Origin and Evolution of Vertebrates (Chapter 34)

Vertebrate

An chordate with a backbone. There are 65,000 species of vertebrates, thought to have colonizes land 365 MYA.

Chordates

Bilaterian animals that belong to the clade deuterostomia. The phylum Chordata is comprised of all vertebrates and two groups of invertebrates:

• urochordates

• cephalochordates

Shared Traits of Chordates

1. Notochord

2. Dorsal, hollow nerve cord

3. Pharyngeal slits/clefts

4. Muscular, post-anal tail.

Notochord

A longitudinal, flexible rod between the digestive and nerve cord. It provides skeletal support, but as the adult develops, a more complex skeleton develops and only remnants of the embryonic notochord remain.

Dorsal, hollow nerve cord

Developed from a plate of ectoderm that rolls into a tube above the notochord, it later develops into the central nervous system.

Central Nervous System

Brain and spinal cord.

Pharyngeal Slits/Clefts

Grooves along the outer surface of the pharynx, these develop into slits that open outside the body. This could be:

• suspension-feeding structures in invertebrate chordates

• gas exchange in vertebrates (except tetrapods)

• parts of the ear, head, and neck in tetrapods

Muscular, Post-Anal Tail

The tail contains skeletal elements and muscles, providing a propelling force in many aquatic species. In many species, it is greatly reduced during embryonic development.

Lancelets

From cephalochordata, they have a blade-like shape and are suspension feeders that retain many characteristics of the chordate body plan as adults.

Tunicates

Also called sea squirts, they belong to urochordata. While resembling chordates in their larval stage, they undergo metamorphosis and reabsorb the tail and notochord. They are more closely related to other chordates than to lancelets.

Cyclostomes

Living, jawless vertebrates. Made up of Myxini and Petromyzontida. They do not have a backbone but morphological structures and genetic analysis indicates they are still vertebrates.

Myxini

Also called hagfishes, they have a cartilaginous skull, reduced vertebrae, and a flexible rod of cartilage derived from the notochord. All are marine and most are bottom-dwelling scavengers. They produce slime to repel competitors and predators.

Petromyzontida

Also called lampreys, they inhabit both freshwater and saltwater. Some are parasites, clamping onto live fish/humans. They have a notochord and cartilaginous skeleton, and may feed as larvae for years before maturing, reproducing, and dying within days.

Gnathostomes

Jawed vertebrates, they include sharks and their relatives, ray/lobe-finned fishes, amphibians, reptiles, birds, and mammals. Their hinged jaws have teeth to grip and slice food. Their enlarged forebrain allows enhanced smell and vision.

Lateral line system

Rows of organs sensitive to vibrations, located along each side of the body of aquatic gnathostomes.

Early gnathostomes

Appeared in the fossil record 440 MYA, one of the first ones were armored marine vertebrates called placoderms.

Chondrichthyans

These animals have a skeleton composed primarily of cartilage. Sharks, rays, and skates are the most diverse group of chondrichthyans but there are also ratfishes and chimaeras.

Sharks

Most are carnivores, but the largest sharks are suspension feeders. Sharks have a short, spiral digestive tract to maximize surface area and slow the passage of food. Sharks have acute sight, smell, and can pick up on animal’s electrical fields.

Rays

Flat with enlarged pectoral fins that function like water wings. Some have barbs on their tail. Most are bottom dwellers that feed on molluscs and crabs.

Oviparous

Eggs mature and hatch outside of the mother’s body. Fertilization is internal or external and the embryo receives all its nutrition from the egg yolk. Ex.) birds, reptiles, amphibians.

Ovoviviparous

Eggs are retained within the oviduct; young are usually born after hatching within the uterus. Fertilization is internal and the embryo receives all of its nutrition from the yolk content of the egg sacs. Ex.) sharks, rays, snakes, and aquatic species.

Viviparous

The embryo develops within the uterus and is nourished from the mother’s blood through a yolk sac placenta, young are born alive. Fertilization is internal and the egg receives all its nutrition from the mother. Ex.) humans and most animals.

Osteichthyans

Ray-finned and lobe-finned fish, nearly all have a bony skeleton. They breathe by drawing water over gills, which are protected by an operculum. Fish can control their buoyancy with an air sac known as a swim bladder. Their skin secretes mucus and most fish are covered by bony scales.

Actinopterygii

Ray-finned fishes, they include nearly all familiar aquatic osteichthyans. They originated during the Silurian period, 444-419 MYA. Their fins are modified for maneuvering and defense, made up of long, flexible rays.

Sarcopterygii

Lobe-finned fishes, they have rod-shaped bones surrounded by muscle in their fins. Though gills are the main organs for gas exchange, they can also gulp surface air into their lungs. Three lineages survive, coaelacanths, lungfishes, and tetrapods.

Tetrapods

Gnasthostomes with limbs. Their specific adaptions are:

1. four limbs with feet and digits

2. a neck, allowing separate movement of the head

3. fusion of the pelvic girdle to the backvone

4. the absence of gills (except in some aquatic species)

5. ears for detecting airborne sounds

Tiktaalik

An important transition animal in the evolution of marine vertebrates to land vertebrates. It shares traits with fish (fins, gills, lungs, and scales) and tetrapods (ribs, neck, shoulders, tetrapod bone patterns in limbs, and a large pelvis). It could likely prop itself up and walk in water, but probably didn’t walk on land.

Amphibians

Represented by 6,150 species, consists of salamanders (urodela, “tailed ones”), frogs (anura, “tailess ones”), and caecilians (apoda, “legless ones”). Most have a moist skin that helps the lungs with gas exchange, external fertilization, and are typically found in damp habitats.

Salamanders

Urodela, amphibians with tails. Some are aquatic while others are terrestrial/both. Paedomorphism is common in aquatic salamanders.

Frogs

Anura, amphibians without tails but with powerful hind legs for locomotion on land and in water. Frogs with leathery skin are called toads.

Caecilians

Apoda, legless amphibians that are nearly blind and resemble earthworms. The absence of legs is a secondary adaptation.

Amniotes

Tetrapods who produce amniotic eggs. Consists of reptiles, birds, and mammals. They also have relatively impermeable skin and use the rib cage to ventilate the lungs.

Amniotic egg

A key adaptation to life on land, there are membranes surrounding the embryo. These are the amnion, chorion, yolk sac, and allantois and in some cases, a shell. The amnion encloses the embryo in a fluid filled sac that prevents eggs from drying out on land.

Reptiles

Mostly ectothermic, with the exception of birds, they have scales that create a waterproof barrier. Most lay shelled eggs on land after internal fertilization. Consists of tuataras, lizards, snakes, turtles, crocodilians, birds, and some extinct groups.

Diapsids

Diapsids have a pair of holes on each side of the skull behind the eye sockets where muscles pass through to attach to the jaw. Includes all living reptiles and early reptiles, specifically lepidosaurs and archosaurs.

Lepidosaurs

Tuataras, lizards, snakes, and extinct monosaurs.

Archosaurs

Turtles, crocodilians, pterosaurs, and dinosaurs.

Pterosaurs

The first tetrapods to exhibit flapping flight.

Theropods

Bipedal carnivores that were ancestors to birds.

Tuataras

Lizard-like reptiles, they live on small islands off the coast of New Zealand.

Snakes

Legless lepidosaurs that evolved from lizards with legs. They are carnivorous with certain adaptations to aid in capture and consumption of prey:

• chemical sensors

• heat detecting organs

• venom

• loosely articulated jawbones and elastic skin

Toxin

a chemical substance produced within living cells or organisms, ex. viruses, proteins, or peptides

Poison

a substance that can cause illness or death when abosrbed or ingested, ex. mercury, poison frogs, puffer fish

Venom

a substance produced by an animal for self defense and injected into another animal, ex. scorpion, stingers, fangs

Turtles

Consist of a boxlike shell made of upper and lower shields that are fused to the vertebrae, clavicles, and ribs. They lack holes in the skull behind the eye socket, although they once had them.

Crocodilians

Restricted to warm regions, they are from an archosaur lineage that dates back to the late Triassic.

Birds

Modified, flying reptiles belonging to the clade Neornithes. Their wings have keratin feathers and they have weight saving adaptions for flight, such as small gonads, no teeth, a lack of a urinary bladder, and a single ovary. Birds generally display complex behaviors, such as courtship rituals. Fertilization is internal and one or both parents keep the eggs warm.

Flight

While enhancing hunting, scavenging, migration, and escape from predators, it demands acute vision, high energy, and fine muscle control.

Archaeopteryx

The oldest known bird, it had feathered wings but retained teeth, claws, and a long tail.

Ratites

Flightless birds, like penguins, ostriches, and some species of ducks, pigeons, and rails.

Mammals

Represented by 5300+ species, they have shared characteristics of:

1. Mammary glands, which produce milk.

2. Hair and a fat layer under the skin for insulation.

3. Kidneys, conserving water from wastes.

4. Endothermy and a high metabolic rate.

5. Efficient respiratory and circulatory systems.

6. A large brain to body size ratio.

7. Extensive parental care.

8. Differentiated teeth.

Monotremes

A small group of egg-laying mammals consisting of echidnas and the platypus. Females secrete milk from glands on their bellies rather than nipples, and the baby sucks milk from the mother’s fur.

Marsupials

The babies are born very early in development, completing their development while nursing in a maternal pouch called a marsupium. In some species this pouch is on the front, in others on the back. Includes opossums, kangaroos, and koalas.

Eutherians

Placental mammals, the young complete their embryonic development within a uterus, joined by the placenta to their mother. Diversification of eutherians is spotty, with molecular data indicating it occurred 100 MYA and morphological data indicating 60 MYA.

Primates

Most have hands and feet adapted for grasping and flat nails instead of claws. Additionally, they have a large brain and short jaws, forward looking eyes (provides depth perception), and in monkeys/apes a fully opposable thumb. Includes lemurs, tarseirs, monkeys, and apes.

Humans

Humans and chimpanzees are 99% identical by genome, differing in the expression of 19 regulatory genes. Although we fall under the group Apes, we have several distinguishing characters:

1. Upright posture and bipedal locomotion

2. Larger brains capable of higher learning and language

3. Reduced jawbones and jaw muscles

4. Shorter digestive tract

Paleoanthropology

The study of human origins.

Hominins

Extinct species that are more closely related to humans than to chimpanzees. We know of about 20 species of extinct hominins, the oldest fossil evidence dating back to 6.5 MYA. Early hominins show evidence of small brains and increasing bipedalism, but were very short.

Misconception: early hominins were chimpanzees or evolved from chimpanzees

Correction: Hominins and chimpanzees shared a common ancestor.

Misconception: human evolution is like a ladder leading directly to Homo Sapiens

Correction: hominin evolution included many branches or coexisting species, though only human survived today.

Australopiths

A paraphyletic assemblage of hominins from 4 to 2 MYA. Some walked fully erect. Robust australopiths had sturdy skulls/powerful jaws while gracile australopiths were more slender/lighter jaws.

Early Homo

The earliest fossils of Homo are from about 2.4 to 1.6 MYA, of Homo habilis. Stone tools were also found with the bones, giving H. habilis its name that means handy man.

Homo ergaster

The first fully bipedal, large-brained hominid. This speces exiseted between 1.9 and 1.5 MYA. It shows a significant decrease in sexual dimorphism.

Homo erectus

Originated in Africa and was the first hominin to migrate out of Africa. The oldest hominin fossils OUTSIDE of Africa date back to 1.8 MYA.

Neanderthals

Lived in Europes and the Near East from 350,000 years ago to about 35,000 years ago. They were thickboned with a larger brain than modern humans, and buried their dead. We think they diverged from Homo sapiens around 400,000 years ago but continued having gene flow with us.

Homo sapiens

Oldest fossils of Homo sapiens are found in Ethiopia at around 195,000 years to 160,000 years ago. The oldest fossils outside of Africa date back 115,000 years ago in the Middle East. Humans arrived in the New World around 15,000 years ago.