A

akinetic

In anatomy, this refers to a low level of flexibility in a structure due to a lack of moveable joints.

amniote

Those vertebrates with an amniotic egg. The extant clades are Testudines (turtles), Diapsida (lepidosaurians, crocodilians, and birds), and Synapsida (mammals).

anapsid

Skull possessing no temporal fenestrae (NB. an- = without).

Amniotes with this skull condition form a paraphyletic group including the Parareptilia (turtles and their extinct relatives), the extinct common ancestor of all amniotes, and basal eureptiles (the extinct precursors of diapsids).

Note that the Testudines (turtles and relatives) have modified the anapsid condition through a reduction (emargination) of the posterior region of the skull.

Apatite

Calcium phosphate: the crystalline component of bone.

apomorphy

A derived or specialised character.

Appendicular skeleton

The endoskeletal element of the fins or limbs of a vertebrate, and their associated girdles (pectoral or pelvic).

Axial skeleton

All parts of the vertebrate endoskeleton except the limbs or fins and their associated girdles. That is, the cranium, visceral skeleton, notochord,vertebrae, and ribs.

B

Basal

Of, relating to, located at, or forming a base.

Bicuspid

A tooth bearing two cusps.

C

Calcified cartilage

Cartilage strengthened with a scattering of apatite crystals (calcium phosphate), as seen in Chondrichthians.

Cartilage

A tough, elastic, fibrous connective tissue composed of collagen fibres. Used as skeletal tissue in vertebrates, it is non-mineralised and is often the developmental precursor of bone.

Clade

A phylogenetic lineage comprising a common ancestor and all its descendant species.

Note that the difference between a taxon and a clade is that a clade must include all descendant species from a common ancestor, whereas a taxon need not.

cladistic

Relating to the branching sequences of phylogeny.

cladogram

A branching tree-like diagram representing the phylogenetic relationships (evolutionary history) of a lineage.

cloaca

The common opening for the reproductive, urinary, and digestive tracts, seen in all vertebrates except therian mammals (marsupials and placental mammals).

The term comes from the Latin for sewer.

Cursorial

Adapted for running.

Cusp

The biting point of a tooth.

D

Dentary

The anterior bone of the lower jaw which bears the teeth. It forms the whole of the lower jaw in mammals.

Dentine

A bone-like substance, lacking cell bodies and consisting mainly of calcium phosphate (apatite) in a fibrous matrix.

Dermal bone

A type of bone forming within the dermis - the deep layer of vertebrate skin cells below the surface layer, the epidermis.

diapsid

Skull possessing both an upper and a lower temporal fenestra (NB. di- = two).

Amniotes with this skull condition form the monophyletic clade Diapsida, which includes the lepidosaurs (lizards, snakes, and tuatara), archosaurs (crocodilians, dinosaurs, and birds), and their other extinct relatives.

Note that some diapsids, such as lizards, have lost the temporal bar separating the fenestrae to form one large window. Others, such as the Aves (birds), have merged both fenestrae with the orbit.

E

Enamel

The crystalline material covering the crown of a tooth, or certain scales.

Endopterygota

A clade of insects charachterised by their undergoing complete metamorphosis (i.e. holometabolous).

See Insect Diversity WebBook for the clades within (from Neuroptera down).

Epidermal

Pertaining to, or originating from, the epidermis - the surface layer of skin cells in vertebrates

euryapsid

Skull possessing an upper temporal fenestra only.

However, animals with this skull condition do not represent an important amniote lineage, as they are likely to be a polyphyletic group, originating a least twice within the Diapsida. Euryapsids include the plesiosaurs and ichthyosaurs - Mesozoic marine reptiles.

extant

Not extinct.

F

fossorial

Specialised for burrowing.

furcula

The fused clavicle bones of a bird, also know as the wishbone.

H

Hemimetabolous

Refers to a type of insect development that is categorised by three distinct, progressive life stages: egg, nymph, imago (adult). Changes are gradual, with no pupal stage.

Some hemimetabolous insects include grasshoppers, cicadas, cockroaches, termites, earwigs, and dragonflies.

Also termed incomplete metamorphosis.

Holometabolous

Refers to a type of insect development that is categorised by four distinct, progressive life stages: embryo, larva, pupa, imago (adult).

Seen exlusively in the Endopterygota, which includes beetles, butterflies, wasps, bees, ants, and others.

Also termed complete metamorphosis.

Horny

Consisting of horn - a tough material composed mainly of keratin.

I

ilium

In tetrapods, the dorsal section of the pelvis, which articulates with one or more sacral vertebrae.

K

Kinetic

In anatomy, referring to a high level of flexibility afforded by numerous moveable joints.

L

Lymph heart

Muscular dilation in a lymph vessel, which pumps lymph (fluid containing white blood cells called lymphocytes important in immune response) around the body of some lungfishes, amphibians and reptiles.

M

Metacone

In mammals, the metacone is the distobuccal (rear-most and cheek side) cusp of an upper molar tooth.

monophyletic

Having a single evolutionary origin. A taxon is monophyletic if it contains all the descendants of a common ancestor.

For example, mammals are a monophyletic group, as all species descended from the first known mammal are considered mammals.

See paraphyletic and polyphyletic for alternative terms.

Myrmecophagy

Feeding behaviour categorised by an exclusive (or near exclusive) diet of ants ant termites.

O

Orbit

The bony socket of the eye.

Osteosclerosis

An increase in the density of bone.

P

Pachyostosis

A thickening of the bone, often associated with a reduction in the volume of marrow tissue contained within.

Paracone

In mammals, the paracone is the mesiobuccal (front-most and cheek side) cusp of an upper molar tooth.

Paraphyletic

A taxon including a common ancestor and some but not all of its descendants.

For example, the class Reptilia is paraphyletic, as it does not include birds, who are considered a separate class: Aves. However, birds evolved from theropod dinosaurs, and are therefore reptiles themselves. Similarly, all tetrapods are, evolutionarily speaking, lobe-finned fish.

Importantly, reptiles can be made monophyletic through the addition of birds to the taxon.

See monophyletic and polyphyletic for alternative terms.

Pectoral girdle

In vertebrates, the skeletal structure that provides support for the fore limbs or fins.

Pelvic girdle

In vertebrates, the skeletal structure that provides support for the hind limbs or fins, which also fuses with the sacral vertebrae.

phylogeny

The evolutionary history of organismal lineages as they develop through time.

plesiomorphy

An ancestral character.

polyphyletic

Referring to a group that does not contain the common ancestor of all the taxa within. Therefore, this is not a true taxonomic group, but is often a term used to categorise organisms with a similar ecology, such as insectivorious mammals, or marine mammals.

It is also used when the evolutionary origin of a group, such as snakes, is unsure, and characteristic species within may have originated separately.

Protocone

In mammals, the protocone is the mesiolingual cusp of an upper molar tooth.

Pulp cavity

The space within a tooth, or a dentine scale, occupied by blood vessels and nerves.

S

symplesiomorphy

A character that is shared between groups but was inherited from an ancestor prior to the last common ancestor.

These are characters that - at the level at which they are referred to as symplesiomorphies - are not used to form cladistic groupings, or clades.

synapomorphy

A derived or specialised character that is shared between two or more groups, and was inherited from the common ancestor in which it originated.

These are the characters that morphological systematists use to support the existence of particular clades, forming the basis of the field of cladistics.

synapsid

Skull possessing a lower temporal fenestra only.

Amniotes with this skull condition form the monophyletic clade Synapsida, which includes the mammals and their extinct ancestors, the non-mammalian reptile-like synapsids.

Note that in the Mammalia, the lower temporal fenestra has merged with the orbit.

T

taxon

A group of organisms sharing a common ancestry.

Note that the difference between a taxon and a clade is that a clade must include all descendant species from a common ancestor, whereas a taxon need not.

Pl. taxa.

temporal fenestra

An opening in the temporal region of the skull seen in amniotes, providing a flat edge for the attachment of strong lower jaw closing muscles to the skull.

Amniotes show four skull types, based on the position and number of these temporal fenestrae, two of which define two major lineages of the amniotes. The skull types and associated groups are as follows:

1) Synapsid - Skull possessing a lower temporal fenestra only. Amniotes with this skull condition form the monophyletic clade Synapsida, which includes the mammals and their extinct ancestors, the mammal-like reptiles. Note that in the Mammalia, the lower temporal fenestra has merged with the orbit.

2) Diapsid - Skull possessing both an upper and a lower temporal fenestra (NB. di- = two). Amniotes with this skull condition form themonophyletic clade Diapsida, which includes the lepidosaurs (lizards, snakes, and tuatara), archosaurs (crocodilians, dinosaurs, and birds), and their other extinct relatives. Note that some groups within the Diapsida, such as lizards, have lost the temporal bar separating the fenestrae to form one large window. Others, such as the Aves (birds), have merged both fenestrae with the orbit.

3) Anapsid - Skull possessing no temporal fenestrae (NB. an- = without). Amniotes with this skull condition form a paraphyletic group including the Parareptilia (turtles and their extinct relatives), the extinct common ancestor of all amniotes, and basal eureptiles (the extinct precursors ofdiapsids). Note that the Testudines (turtles and relatives) have modified the anapsid condition through a reduction (emargination) of the posteriorregion of the skull.

4) Euryapsid - Skull possessing an upper temporal fenestra only. However, animals with this skull condition do not represent an importantamniote lineage, as they are likely to be a polyphyletic group, originating a least twice within the Diapsida. Euryapsids include the plesiosaurs and ichthyosaurs - Mesozoic marine reptiles.

V

vertebrae

From anterior to posterior:

Cervical vertebrae: Facilitate the mobility of the head. The first two, the atlas and the axis are highly specialised, the former articulating with the occipital region of the skull.

Thoracic vertebrae: Articulate with the ribs that fuse with the sternum.

Lumbar vertebrae: Generally larger, with small ribs not attached to the sturnum, which support the posterior musculature.

Sacral vertebrae: Fused to the pelvic girdle, allowing the transfer of force from the appendicular skeleton (limbs) during locomotion.

Caudal vertebrae: Small and less specialised, forming the tail.

Vertebrate anatomical directions and axes

The image below illustrates the terms used for anatomical directions and axes in vertebrates.

Vestigial

Occurring as a structure that, once functional (whether during development or in earlier evolutionary forms), is now reduced or degenerate. An example is the vestigial pelvic girdle seen in many snakes, including the boas and pythons, which bears no function.

Z

Zygapophysis

Articular process of a vertebra that articulates with the corresponding process of an adjacent vertebra.

Plural = zygapophyses