The Vertebrates J-L

Glossary: J-L

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.

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J The Jurassic Period.

Jacobson's Organ The vomeronasal organ; a chemosensory organ located in the roof of the mouth.

Javelina Formation Late Cretaceous (Campanian-Maastrichtian) of W. Texas. Pterosaurs.

Jobu Formation Early Cretaceous I or early Early Cretaceous II (Aptian) of Japan. Manabe 1999).

Jugal barIn birds, the bony process which supports the upper beak on the moveable quadrate.

Jugular canal in many sarcopterygians, on the side of the otoccipital (posterior) portion of the braincase, there is a sort of dorsoventrally oriented bone bridge bearing the articulations for the hyomandibula. This is the lateral commisure. Under this bridge is a tunnel that passes from the otic capsule, posteriorly, and opens onto the otic shelf, anteriorly. The jugular vein passes through this tunnel, and it is consequently known as the jugular canal. Note that this is not the same as the jugular foramen.

Jugular foramen An important landmark on the occiput. It is a small hole where the jugular vein exits the braincase. It generally marks the exit of the Xth and XIth nerves, as well. This jugular foramen is normally located along the articulation between the opisthotic (or petrosal) and the exoccipitals. In human and sometimes other mammals, the jugular foramen is referred to as the posterior lacerate foramen.

Jugular vein, internal the IJV drains the brain and internal structures of the head. At least in mammaliforms, it originates at the sigmoid sinus. The IJV exits the skull through the jugular (or posterior lacerate) foramen between whatever passes for the opisthotics and exoccipitals in the organism of interest.

Jurassic PeriodThe middle period of the Mesozoic, 206-144 Mya. The Early Jurassic 206-180 Mya) includes the Hettangian, Sinemurian, Pliensbachian and Toarcian Ages. The Middle Jurassic (180-159 Mya) takes in the Aalenian, Bajocian, Bathonian, and Callovian Ages. The Late Jurassic (159-144 Mya) is made up of the Oxfordian, Kimmeridgian and Tithonian Ages.


K the Cretaceous. Also the element Potassium. If you are likely to be confused and think that the dinosaurs expired at the end of the potassium, or that neuronal potentials are maintained by an ATP-dependent sodium-Cretaceous pump, then you may require more assistance than these Notes can provide.

Karaginskaya Formation Late Oligocene of western Kazakhstan. Squalodontids. Dubrovo & Sanders 2000).

Katian Age second age of the Late Ordovician, 456-446 Mya. See Katian.

Kayenta Formation Early Jurassic of Colorado.

Khashaat Formation Late Paleocene of Mongolia. Same as Gashato Formation.

Keratin, Keratinous keratin is a protein which comes in a variety of forms, all of which are waterproof and durable. It is the principle component of horn, skin, hair, feathers, hooves, and the rasping "teeth" of lampreys, among others.

Kimmeridgian Age The second age of the Late Jurassic Period (see also Late Jurassic), 154-151 Mya.

Kinesis ability of parts (e.g., of skull or jaw) to move or flex relative to each other. One of the critical design issues of the vertebrate skull and a frequent source of lineage-splitting, the degree of skull kinesis involves numerous trade-offs.

Kinetic line a feature of some sarcopterygian skulls in which there is a sharp angle between the skull table and the cheek region. The sutures along this line are typically not interdigitating and thought to be at least somewhat kinetic.

KinociliumNo different from any other cilium, it seems, but given a special name in mechanoreceptors for historical reasons. See The Ear.

Kota Formation Early Jurassic of India. Kotatherium.

Kuldana Formation Early to Middle Eocene of Pakistan. Shale, marl & bleached dolomite. Pakicetidae.


La Colonia Formation Late Cretaceous (Campanian - Maastrichtian) of Chubut State in South Central Argentina (Patagonia region). Known for snakes, turtles, fish, dinosaurs, plesiosaurs and some mammals.

La Victoria Formation Middle Miocene of Venezuela Cifelli & Villarroel (1997).

Labial (a) relating to the lips; (b) viewed from the "lip" or outside of the mouth; opposite of lingual & same as buccal c) of dentition, the "outside" surface.

Labial pleurodont   See Tooth Implantation.

Labyrinthsee "Semicircular Canal." See also, The Ear.

Labyrinthodont tooth (Eusthenepteron)

Labyrinthodont teeth (or having teeth) characterized by folded sheets of dentine. Such teeth are common in sarcopterygians and basal tetrapods. A pulp cavity is not always present, and the entire tooth may be filled with folded dentine. An outer enamel layer may also be present. See image: cross section of labyrinthodont tooth from Eusthenepteron.

Lacerate foramen, posterior same as the jugular foramen. Used in mammalian anatomy, where it is formed by the suture of the petrosal and basisphenoid..

Lachrymal an alternate spelling of lacrimal, a dermal bone of the facial series. Although, spelled this way, it may refer to a style of lute song invented by John Dowland -- a Seventeenth Century equivalent of the Blues, much emulated by later composers.

Lacrimal foramen Opening of the tear duct.

Ladinian Age The second and last age of the Middle Triassic 234-227 Mya).

Lafayette Bugt Formation Early Silurian (Llandovery) of Hall Land, north Greenland. Thelodonts. Blom (1999).

Lag deposit a layer of gravel or larger particles left behind when smaller particles have been removed by wind or water.

Lagena the region of the inner ear related to "normal" hearing. See The Ear.

Lagenar crest a crista in crocodylomorphs, birds, and lizards which walls off the lagena from the vestibular elements of the recessus scala vestibuli. Gower 2002).

Lambdoid relating to the articulation between the parietals and occipitals. The lambdoid crest is a transverse crest, typically formed by the parietal(s), that may serve as the attachment for neck muscles supporting the head. The squamosals, intertabular, tabulars, etc. may also get into the act.

Lamellar same as laminar.

Laminar layered.

Laminatein layers.

Lappet a projecting, flap-like structure. In osteology, "lappet" often, but not always, refers to a projection of one bone that overlaps another.

Lateral centrale One of the carpals. See Figure at carpus.

Lateral commisure in the braincase of sarcopterygians and basal tetrapods, a flange of bone on the lateral surface of the otoccipital region that folds over the jugular vein. See image at vestibular fontanelle.

Lateral condyle (of the tibia) the condyle at the proximal end of the tibia which is further from from the midline of the body. It contacts the lateral condyle of the femur and generally also has some contact (often soft tissue) with the proximal head of the fibula.

Lateral linea conspicuous sensory structure in aquatic vertebrates which usually appears as a system of lines on the surface of the animal made up of pores lines with mechanoreceptive and/or electroreceptive cells. Lines contain neuromast organs consisting of pairs of oppositely oriented hair cells embedded in gelatinous cupula. The electrosensitive elements of the lateral line system sense weak electric currents created by other fish (prey, school etc.). The mechanoreceptors detect current. Since electrosensory systems cannot function on land, and air currents are more effectively detected by other means, the presence of a lateral line system is an indicator of aquatic habits in a doubtful case.

Lateral otic fissure see metotic fissure.

Lateral plate mesoderm See Early Development Terms.

Lateral septum In fishes, this is an important structural entity, a sheet of tough, fibrous tissue extending straight laterally from the spine and dividing the body into dorsal (epaxial) and ventral (hypaxial) halves. The structure is easily visible, if you are ever the recipient of a well-presented salmon steak. In other vertebrates, the lateral septum per se is often lost, but the epaxial/hypaxial boundary still has enormous developmental significance, particularly in tracing the development of musculature. Caution the brain and limbs also have structures referred to as "lateral septa." Obviously, these aren't the same thing at all.

Lateral tubercle of the humerus, see Humerus.

Laterale Grenzfelte a possible homologue of the turbinals found in turtles. See Ethmoid.

Latissimus dorsi a group of (normally adductor or retractor) muscles which originate on the laterodorsal body wall and insert on the humerus.

Latissimus dorsi process see Humerus.

LCA (abbr.) last common ancestor (and all descendants of that ancestor).

Lectotype A syntype designated as the single name-bearing type specimen subsequent to the establishment of a nominal species or subspecies

Lenticular process (of incus) the long ventral process of the incus which articulates with the stapes. See The Incus.


Lepidomoriumthe basic unit of squamation and dentition, according to theories of Erik Stensiö 1891-1984) and other members of the Scandanavian School. However Stensiö's overall theory may stand, it is beyond dispute that (a) he accurately predicted the structure of the lepidomorial unit (since scales of this type are now known from early eugeneodontidsand (b) teeth and scales have a great deal in common, both structurally and developmentally. Scales and teeth both form at the level of the basement membrane (see Intgument) at the mesoderm - ectoderm boundary. In scale formation, the epidermal (ectoderm) cells form the base, which is invaded by mesenchyme recruited from the dermis.

Lepidotrichia Fin rays.  "Scale-like structures that form the segments of soft rays in bony fishes." Fishbase. They are usually described by anatomists as "modified scales," but the details are more complicated:

Teleost fins are composed of a species-specific number of branched rays. These are sustained by a segmented dermal bone called lepidotrichia, which is, in turn, composed of two concave, facing bones, named hemi-lepidotrichia. Lepidotrichia are immersed in a loose, vascularized, and innervated connective tissue, and are surrounded by a multilayered epidermis. At their distal tip, two palisades of actinotrichia parallel hemi-lepidotrichia inner faces. Actinotrichia are rigid, but non-mineralized, collagenic rods that not only support the ray tip, but are also though to have some morphogenetic role. Rays are connected to each other by the soft inter-ray region, which lacks skeletal elements.

Lepospondyly a condition in which the elements of the vertebra are fused into a single piece (i.e. holospondyly) and the centra have hollow core allowing for a continuous notochord to pass through the centra. Characteristic, not surprisingly, of the lepospondyls.

Lesser trochanter a trochanter located below the greater trochanter on the lateral or anterior face of the femoral shaft. See figures at femur and trochanter.

Levator arcus palatini, m. in fishes, a muscle involved in feeding. The origin and attachment(s) of this muscle are quite variable, but it always functions to expand the buchal cavity laterally. It may originate on the anterior neurocranium, the dermosphenotic, the parasphenoid, or any other similar location, usually at an antero-posterior position just behind the orbit. In fishes other than derived teleost groups, it runs posterolaterally (and somewhat ventrally) to insert on the hyomandibular. By retracting the hyomandibular, which is closely integrated with the dorsal rim of the palatoquadrate, it causes the ventral half of the palatoquadrate to rotate outward (laterally), expanding the mouth cavity. In advanced teleosts -- about which we disclaim any reliable knowledge -- it apparantly acts on the palatines directly, or even on the lower jaw.

Levator bulbi, m. this is a thin muscle in the floor of the orbit innervated by the 5th cranial nerve, that causes the eye to bulge outward and to enlarge the buccal cavity. This muscle is present in anurans and urodeles and in a modified form in caecilians.

Levator hyoidei, m. non-mammalian homologue of the stapedius muscle.

Lift The force opposing gravity in flight. For those who can envision air as moving in a laminar fashion, lift is generated by well-behaved laminar airflows. When air meets a well-designed airfoil, it splits into two streams. The lower surface of the airfoil is straight. By contrast, air flowing over the top is moving faster because it is magically attracted to the lamina it was previously associated with. This is a marvelous mathematical model but frankly makes very little physical sense. For those of us who insist on thinking of air as made up of molecules, rather than bed sheets, the math is difficult. However, one may intuit the proper result with some effort. Think of a moving airfoil as 3 surfaces. The bottom is flat. Molecules of air will hit this surface (and impart upward momentum) about as often as if it were sitting in a garage. The anterior surface is approximately parabolic. It will collide frequently with molecules, but will impart and absorb vertical momentum symmetrically, with no resultant force. The top is concave and in the air "shadow" of the moving anterior surface. As a result, fewer molecules will strike it (imparting downward momentum). The net result is that air molecules impart a net upward momentum on the moving airfoil. However, if the combination of speed and geometry is not optimal, the airfoil's anterior surface may absorb enough horizontal momentum per unit time to overcome the motive force (recall that F=dp/dt) of the bird, plane or superman, with unfortunate results. As a general rule, the more asymmetric the surface, the more lift, but also the more horizontal drag, since the front-to-back asymmetry also increases. In the laminar model, this drag is explained in terms of "turbulence," which is also treated by engineers as a magical property retarding progress. The turbulence is quite real, but is only mathematical proxy for the resultant of random collisions between air molecules and a moving surface of given shape and velocity.

General name


Hind limb





radius + ulna

tibia + fibula




proximal series



medial series

centralia (4)

centralia (4)

distal series

carpalia (5)

tarsalia (5)





metacarpals (5)

metatarsals (5)


fingers (5)

toes (5)

Phalanges (sing. = phalanx)



Limb the following table summarizes tetrapod limb nomenclature.

Limestone A carbonate sedimentary rock composed of calcite (CaCO3), which may or may not be biogenic. It is generally light colored. The composition of sediment in intermontane areas or alluvial plains may be classified as a limestone. This is not to imply the landform is rock but that it is primarily composed of sediments of that composition and may have rock fragments of other compositions and/or have windblown sediments from undetermined origins as well

Lingual (a) relating to the tongue; (b) viewed from the inside of the mouth, looking out (opposite of labial or buccal).

Lisbon Formation Middle Eocene (late Lutetian or early Bartonian) of Georgia, South Carolina and the Gulf Coast of the United States. Marine. Georgiacetus. Hulbert et al. (1998).

Lochkovian the first age of the Devonian Period, 417-412 Mya.

Lockhovian misspelling or alternate spelling of Lochkovian.

Lode Formation variously dated as Givetian or Frasnian of Latvia (probably Givetian). Overlies Sietini Fm unconformably and is probably contemporaneous with upper part of Gauja Fm. Light grey quartzitic sandstone. Low energy fresh water or deltaic environment.  Various sarcopterygians and antiarch placoderms. Comparable fauna to Escuminac Formation, but with the addition of psammosteids. Vorobyeva 1980), Ahlberg et al. 2000); Forey et al. (2000).

London Clay Early Eocene Ypresian) of England. Famous for any number of avian fossils, e.g. Anatalavis.

Longirostrine a suite of adaptations in crocs and others related to fish-eating. These include a long, thin rostrum and procumbent teeth.

Loph a ridge of enamel in a dentine surface (as opposed to a crista on an enameled surface). See Molars.

Lophodont (also called selenodont). Dentition characterized by lophs, as in ruminants. See Molars. The figure at right shows the nomenclature of lophs in the special case of notoungulate dentition.

Los Alamitos Formation Late Cretaceous of Argentina.

Los Chañares Formation Middle Triassic of Argentina, Agua de la Peña Group. Medium energy flood plain with seasonal drought. Tarjadia. Arcucci & Marsicano 1998).

Lossiemouth Sandstone Formation Late Triassic (Late Carnian) of Scotland.

Loxodontan extreme form of lophodont or selenodont dentition. See Molars.

Gracilisuchus Parrish (1993)

LTF abbreviation for lower temporal fenestra. See image.

Lujanian Age South American Land Mammal Age corresponding to the Late Pleistocene.

Lulworth Formation Late Jurassic to Early Cretaceous (mostly Early Cretaceous) of southern England. Purbeck Limestone Group. Symmetrodonts (Spalacotherium, Thereudon), unidentified dinosaur footprints. The fossiliferous Cherty Freshwater Member may have been a very large, anaerobic mud flat or marsh adjacent to a lake.

Lumachelle Fire marble - a brown limestone or marble "marble"?) with some areas that exhibit internal fire-like reflections from included fossil shells.

Lumbar The ribless region of the mammaliform body between the thorax and sacrum. Related to lumbus, Latin for loins, as in the often-used Biblical phrase succingere lumbos (to gird up [one's] loins). Somehow, between classical and medieval days, the meaning of the word seems to have shifted from front to back, so to speak.

Lumbos the lumbar region, esp. the lumbar region of the back.

Lumbricalesintrinsic muscles of the hand and foot originating in the palm and inserting on the radial side of the metacarpals. They are responsible for flexing the digits at the metacarpo-phalangeal joint and extending the digits at the interphalangeal joints for digits II-V.

Lunate [1] (L luna = moon) shaped like a (quarter) moon. [2] in mammalian osteology, one of the carpal bones, normally articulating distally with the magnum and proximally with the radius. See image at unciform.

Lutetian Age the first age of the Middle Eocene (mEc), 49.0-41.3 Ma.

Lystro- Greek root for spoon or shovel.

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