For most phrases beginning with directional words, e.g. "posterior," "dorsal," "external," etc., or some generic anatomical terms, e.g., "vena," look under the next word in the phrase. However, note that this convention is not used with complete consistency in this Glossary.
Eocene The second Epoch of the Cenozoic, 54.8-33.7 Mya. The Early Eocene (34.8-49 Mya) is the Ypressian Age. The Middle Eocene (49.0-37.0 Mya) includes the Lutetian and Bartonian Ages. The Late Eocene (37.0-33.7 Mya) is the Priabonian Age.
Epaxial the region dorsal to the lateral septum in fishes. Although tetrapods lack a lateral septum, this region remains developmentally distinct and gives rise to a special set of muscles, the epaxial musculature, which includes elements ultimately traceable to the dorsal fin musculature of fishes. The lateral septum in fishes extends laterally from the spinal cord. The spine tends to be somewhat centrally located, and the epaxial region is thus quite large. In tetrapods the epaxial region is more limited.
Epiblast see gastrulation.
Epicaudal lobe the part of the tail fin above the notochord.
Epicercalof the tail (caudal) fin of certain fishes, upwardly tapering. Generally, this signifies that the notochord or spine forms the upper margin of the tail fin. Same as heterocercal, according to some. But see the image at right, which makes more sense to us.
Epihyal this should be the same as the hyomandibular. That is, the hyomandibula is (very probably) the epal element of the hyoid arch. However, in many actinopterygian fishes, the ceratohyal (ceratal element of the hyoid arch) has two separate ossifications. The more proximal (or posterior or dorsal, depending on the fish and frame of reference) is frequently referred to as the epihyal.
Epimere See Early Development Terms.
Epiotic a ?median dorsal ossification of the otic capsule, somewhat like a second, more anterior, supraoccipital. This bone, if present, normally has no external exposure. Presumably, like the supraoccipital, it arises by ossification of dorsal soft tissues joining the otic capsules dorsally.
Epiphysis  the terminal, usually ossified sections of long bones, including the articulating surfaces. As opposed to the diaphysis and metaphyses.  a small, light-sensitive endocrine gland in the brain; situated beneath the back part of the corpus callosum which secretes elation.
Epipodiale (pl. epipodialia) one of the distal limb bones, i.e. ulna, radius tibia or fibula. See figure at metapodial.
EpipterygoidThe epipterygoid is a misnomer. Like the quadrates, the epipterygoids are ossified portions of the palatoquadrate (the original upper jaw which, like the hyomandibular, is homologous with an upper gill arch segment). The epipterygoids often appear to arise from the pterygoid, but do not. The epipterygoids are the true, old stuff of the palatoquadrate, while the pterygoid is but common dermal bone with pretensions. In fact, the epipterygoids are the original braincase articulations of the palatoquadrate. They demonstrate this ancient nobility by rising up in a graceful curve to reach the bones of the skull roof, like the last remaining columns of an abandoned temple.
Epitegum a shield element of jawless fishes, considered as arising from separate growth centers. The ventral shield is considered as a single epitegum.
Epithelium a tissue forming the boundary of an organ; a characteristic tissue type forming such boundaries.
Epitympanic recess the distal or upper end of the middle ear, opposite the Eustachian tube. The middle ear ossicles of mammals are lodged in this part of the middle ear.
Epoccipital one of the small bones along the margin of the parietal frill of many Ceratopsia.
Epural in fish tail anatomy, an elongate detached bone above the urostyle and behind the last neural spine supporting caudal fin rays. Apparently derived from neural spines or the urostylic centra; dorsal homologues of the hypurals. Vary in number between one in advanced fishes to three in primitive actinopterygians. From Dictionary of Ichthyology (site no longer available). See image at urodermal.
Eretmo- Greek root for oar.
Erythro- Greek root for red.
Escuminac Formation Frasnian? Late Devonian) of Canada. This is the formation containing the famous Miguasha fauna including many incomparable specimens of Eusthenopteron, possibly the best known Middle Paleozoic vertebrate. The sediments are probably coastal marine or possibly deltaic, although they are frequently reported as fresh water in the older literature. Schultze & Arsenault (1985).
Ethmoid Gr. ethmos = a sieve, and eidos = resemblance. A term describing the structure of the ethmoid bone of the skull, the most anterior of the four principal braincase regions. It is associated with the nasal capsules and chemosensation. Frequently it fuses with the sphenoid region and is referred to as the sphenethmoid. In mammals, the ethmoid is reduced to a series of turbinals: very thin bones in the nasal passage that help recover respiratory water vapor.
Ethmoid articulation of the palatoquadrate. An anterior articulation of the palatoquadrate in which the palatoquadrate articulates with a an anterior portion of the sphenethmoid portion of the braincase. See image at paratemporal articulation.
Ethmosphenoid same as sphenethmoid. The combined sphenoid and ethmoid regions of the braincase. The anterior half of the braincase, physically separate from the posterior, otoccipital unit in Sarcopterygii. See Bones: The Braincase.
Euautostylic jaw suspension in which the jaw is attached directly to the braincase as in placoderms.
Euryhaline tolerating a wide range of salinity; opposite of stenohaline.
Eustachian tube a passage joining the middle ear and pharynx.
Euxinic relating to a water layer or water column which is anoxic (little or no oxygen) and sulfidic (contains reduced sulfur species, e.g. H2S, FeS, etc., as opposed to sulfate). In Phanerozoic environments, euxinic waters are generally deep, stagnant, and cool, since any significant current or convection would tend to introduce oxygen. The best modern example is the bottom of the Black Sea. In fact, the term is derived from the Greek name for the Black Sea, Εὔξεινος Πόντος.
Evaporite a deposit created by the evaporation of sea water.
Exapt to adapt, by selection, to a different purpose. Examples: (1) the muscles homologous to the usual vertebrate eye muscles have become exapted to move the tentacles in caecilians; 2) the swim bladder, originally adapted for control of buoyancy, was exapted as a respiratory organ in various groups of fish.
Exoccipital The exoccipitals are paired bones of the occiput. They derive from the neural arch elements of embryonic vertebrae which have been incorporated into the braincase. Dorsally, the exoccipitals contact the supraoccipital and the foramen magnum. Ventrally, they contact the basioccipital. The exoccipitals often form part of the occipital condyle. See The Occiput for details.
Extensor muscles the muscles which extend the digits of a limb.
Extrinsic eye muscles the muscles which rotate the eyeball, as opposed to the intrinsic eye muscles which dilate the retina, etc. These are the various oblique and rectus muscles. See figure at rectus muscles; see also discussion and figures of the gnathostome orbit.