Pharyngeal slits

In chordate embryos, one of the slits that form from the pharyngeal clefts and communicate to the outside, later developing into gill slits in many vertebrates.

lancelets

Member of the clade Cephalochordata, small blade-shaped marine chordates that lack a backbone.

Tunicates

Member of the clade Urochordata, sessile marine chordates that lack a backbone.

Craniates

A chordate with a head.

Neural crests

In vertebrates, a region located along the sides of the neural tube where it pinches off from the ectoderm. Neural crest cells migrate to various parts of the embryo and form pigment cells in the skin and parts of the skull, teeth, adrenal glands, and peripheral nervous system.

Conodonts

An early, soft-bodied vertebrate with prominent eyes and dental elements.

Gnathostomes

Member of the vertebrate subgroup possessing jaws.

lateral line system

A mechanoreceptor system consisting of a series of pores and receptor units along the sides of the body in fishes and aquatic amphibians; detects water movements made by the animal itself and by other moving objects.

Placoderms

A member of an extinct group of fishlike vertebrates that had jaws and were enclosed in a tough outer armor.

Acanthiodans

Any of a group of ancient jawed aquatic vertebrates from the Silurian and Devonian periods.

Chondrichthyans

Member of the class Chondrichthyes, vertebrates with skeletons made mostly of cartilage, such as sharks and rays.

Oviparous

Referring to a type of development in which young hatch from eggs laid outside the mother’s body.

Ovoviviparous

Referring to a type of development in which young hatch from eggs that are retained in the mother’s uterus.

Viviparous

Referring to a type of development in which the young are born alive after having been nourished in the uterus by blood from the placenta.

Cloaca

A common opening for the digestive, urinary, and reproductive tracts found in many nonmammalian vertebrates but in few mammals.

Osteichthyans

Member of a vertebrate clade with jaws and mostly bony skeletons.

Operculum

In aquatic osteichthyans, a protective bony flap that covers and protects the gills.

Swim bladder

In aquatic osteichthyans, an air sac that enables the animal to control its buoyancy in the water.

ray-finned fishes

Member of the class Actinopterygii, aquatic osteichthyans with fins supported by long, flexible rays, including tuna, bass, and herring.

lobe-fins

Member of the vertebrate clade Sarcopterygii, osteichthyans with rodshaped muscular fins, including coelacanths, lungfishes, and tetrapods.

Tetrapods

A vertebrate clade whose members have limbs with digits. Include mammals, amphibians, and birds and other reptiles.

Amphibians

Member of the tetrapod class Amphibia, including salamanders, frogs, and caecilians.

Amniotes

Member of a clade of tetrapods named for a key derived character, the amniotic egg, which contains specialized membranes, including the fluid-filled amnion, that protect the embryo. Include mammals as well as birds and other reptiles.

Amniotic egg

An egg that contains specialized membranes that function in protection, nourishment, and gas exchange. It was a major evolutionary innovation, allowing embryos to develop on land in a fluid-filled sac, thus reducing the dependence of tetrapods on water for reproduction.

Reptile

Member of the clade of amniotes that includes tuataras, lizards, snakes, turtles, crocodilians, and birds.

Ectothermic

Referring to organisms for which external sources provide most of the heat for temperature regulation.

Endothermic

Referring to organisms that are warmed by heat generated by their own metabolism. This heat usually maintains a relatively stable body temperature higher than that of the external environment.

Parareptiles

A basal group of reptiles, consisting mostly of large, stocky quadrupedal herbivores. They died out in the late Triassic period.

Diapsids

Member of an amniote clade distinguished by a pair of holes on each side of the skull. Diapsids include the lepidosaurs and archosaurs.

Lepidosaurs

Member of the reptilian group that includes lizards, snakes, and two species of New Zealand animals called tuataras.

Archosaurs

Member of the reptilian group that includes crocodiles, alligators and dinosaurs, including birds.

Pterosaurs

Winged reptile that lived during the Mesozoic era.

Dinosaurs

Member of an extremely diverse clade of reptiles varying in body shape, size, and habitat. Birds are the only extant dinosaurs.

Theropods

Member of a group of dinosaurs that were bipedal carnivores.

Ratites

Member of the group of flightless birds.

Mammals

Member of the class Mammalia, amniotes that have hair and mammary glands (glands that produce milk).

Synapsids

Member of an amniote clade distinguished by a single hole on each side of the skull. Include the mammals.

Monotremes

An egg-laying mammal, such as a platypus or echidna. Like all mammals, they have hair and produce milk, but they lack nipples.

Marsupials

A mammal, such as a koala, kangaroo, or opossum, whose young complete their embryonic development inside a maternal pouch called the marsupium.

Placenta

A structure in the pregnant uterus for nourishing a viviparous fetus with the mother’s blood supply; formed from the uterine lining and embryonic membranes.

Eutherians

Placental mammal; mammal whose young complete their embryonic development within the uterus, joined to the mother by the placenta.

Opposable thumb

A thumb that can touch the ventral surface of the fingertips of all four fingers.

Anthropoids

Member of a primate group made up of the monkeys and the apes (gibbons, orangutans, gorillas, chimpanzees, bonobos, and humans).

Paleoanthropology

The study of human origins and evolution.

Hominins

A member of the human branch of the evolutionary tree. Include Homo sapiens and our ancestors, a group of extinct species that are more closely related to us than to chimpanzees.

Cephalochordata (Chordates)

(lancelets)

Basal chordates; marine suspension feeders that exhibit four key derived characters of chordates

Urochordata (Chordates)

(tunicates)

Marine suspension feeders; larvae display the derived traits of chordates

Myxini (Chordates --> Craniates)

(hagfishes and relatives)

Jawless marine organisms; have head that includes a skull and brain, eyes, and other sensory organs

Petromyzontida (Chordates --> Craniates --> Vertebrates)

(lampreys)

Jawless vertebrates; typically feed by attaching to a live fish and ingesting its blood

Chondrichthyes (Chordates --> Craniates --> Vertebrate --> Gnathostomes)

(sharks, rays, skates, ratfishes)

Aquatic gnathostomes; have cartilaginous skeleton, a derived trait formed by the reduction of an ancestral mineralized skeleton

Actinopterygii (Chordates --> Craniates --> Vertebrate --> Gnathostomes --> Osteichthyans)

(ray-finned fishes)

Aquatic gnathostomes; have bony skeleton and maneuverable fins supported by rays

Actinistia (Chordates --> Craniates --> Vertebrate --> Gnathostomes --> Osteichthyans --> Lobe-fins)

(coelacanths)

Ancient lineage of aquatic lobe-fins still surviving in Indian Ocean

Dipnoi (Chordates --> Craniates --> Vertebrate --> Gnathostomes --> Osteichthyans --> Lobe-fins)

(lungfishes)

Freshwater lobe-fins with both lungs and gills; sister group of tetrapods

Amphibia (Chordates --> Craniates --> Vertebrate --> Gnathostomes --> Osteichthyans --> Lobe-fins --> Tetrapods)

(salamanders, frogs, caecilians)

Have four limbs descended from modified fins; most have moist skin that functions in gas exchange; many live both in water (as larvae) and on land (as adults)

Reptilia (Chordates --> Craniates --> Vertebrate --> Gnathostomes --> Osteichthyans --> Lobe-fins --> Tetrapods --> Amniotes)

(tuataras, lizards and snakes, turtles, crocodilians, birds)

One of two groups of living amniotes; have amniotic eggs and rib cage ventilation, key adaptations for life on land

Mammalia (Chordates --> Craniates --> Vertebrate --> Gnathostomes --> Osteichthyans --> Lobe-fins --> Tetrapods --> Amniotes)

(monotremes, marsupials, eutherians)

Evolved from synapsid ancestors; include egg-laying monotremes (echidnas, platypus); pouched marsupials (such as kangaroos, opossums); and eutherians (placental mammals, such as rodents, primates)