Користувач:AS/плани
Матеріал з Вікіпедії — вільної енциклопедії.
[ред.] ethmoid
Preliminary Note
The ethmoid is really a region, rather than a bone. In any case, it is somewhat too big a bite to swallow at once. Accordingly, after a brief overview, this first pass will be restricted to one of the easier elements of the ethmoid region, the cribiform plate. Overview
The ethmoid generally is the most anterior region of the braincase. In early development, two pairs of cartilaginous rods form parallel to the notochord in the cranium: the anterior trabeculae and the posterior parachordals. At the same time, the nasal placodes are developing within the cartilaginous nasal capsules formed by invading neural crest ectoderm. This ectodermal component also contributes at least to the anterior portion of the trabeculae. In some groups, including humans and most other mammals, the entire thing congeals into an ethmoid ossification, or ethmoid plate, incorporating the nasal capsules and the anterior portion of the trabeculae.
There is considerable variability on this general theme. For example, the mormyrid family of osteoglossomorph teleosts has members both with and without an ethmoid bone. Cichlids appear to have only the lateral ethmoids, which I take to be largely derived from the nasal capsules. By contrast, in eels, the fusion includes not only the elements listed above, but also the vomers and the premaxillae. See OceanLink Answers to Miscellaneous Fish Questions. In tetrapods other than mammals, the ethmoid region is not ossified at all. In general, it may be best to consider the ethmoid as containing three "bones": (a) the paired lateral ethmoids (presumably derived largely from the nasal capsules) and (b) the mesethmoid (presumably derived largely from the fused anterior ends of the trabeculae). For those with a perverse desire to see the whole thing in humans, see: II. Osteology. 5a. 6. Ethmoid bone. Gray, Henry. 1918. Anatomy of the Human Body. One relevant illustration from Gray's is reproduced as Figure 1.
The ethmoid articulates with the sphenoid(s) posteriorly and frequently co-ossifies in species (fish) in which these braincase elements ossify. It typically articulates with the vomers and may co-ossify with these palatal elements as well. Other palatal articulations are possible, particularly the ascending process of the palatine.
The ethmoid has a number of persistent structures:
(1) the cribiform plate: a sieve-like barrier between the nasal cavity and the brain through which olfactory neurons communicate between the two chambers.
(2) perpendicular plate: forms nasal septum with participation of cartilage, vomers, etc. The plate can be extremely deep, as in Homo. It splits the ethmoid in two lengthwise and descends all the to the palate, where it often articulates with the ascending processes of the vomers and palatines.
(3) the crista galli: continues the perpendicular plate dorsally, above the roof of the nasal cavity where it may partially separate the two hemispheres of the brain in mammals. The crista galli and perpendicular plate can be envisioned together as a thin sheet of bone oriented straight up and down.
(4) conchae (turbinals) which provide a sensory surface for olfaction and prevent water loss by recovering respiratory water.
(5) orbital plates: lateral processes of the ethmoid which, if present, may define part of the medial wall of the orbits.
There is a very good discussion and figures of the human ethmoid at Dept of Anth: Ethmoid Page, from which the image at left is derived. ATW 010622. The Cribiform Plate
The cribiform plate of the ethmoid is apparently a linguistic redundancy, since both terms refer to the sieve-like nature of the braincase at the roof of the nasal cavity. The structure seems to have been first noted by the Roman physician Galen who, like many physicians, was an acute observer but sometimes a defective reasoner. Galen believed that nasal mucous represented the drainage of excess matter from the brain through the cribiform.[1]
In fact, the flow of information is the other way. The sieve-like holes in the cribiform permit the axons of olfactory receptor neurons in the nasal mucosa to synapse directly with the mitral neurons in the olfactory bulbs. The axons of the mitral cells make up the first cranial ("olfactory") nerve. However, the olfactory bulb is somewhat more than a nerve terminus and contains a rather complex neural structure of its own. Limbic System.
How the other neural inhabitants of the olfactory bulb affect the sensory function is not clear, but certainly some substantial information processing occurs even at this level. The olfactory bulb is organized into perhaps two thousand small glomeruli. According to some sources, each glomerulus is believed to be specialized around one or a few specific smells. According to another, the glomeruli are presumed to be redundant, with the response of each encoded in a sort of 24-bit binary code, corresponding to some 16 million different smells. Both explanations seem somewhat simplistic. The nasal epithelium also contains more mundane receptors sensitive to temperature and pressure. However, these do not communicate with the brain via the specialized olfactory glomeruli, but rather through the trigeminal (Vth) nerve. ATW 010622. Turbinals, Turbinates, Conchae, or Whatever
This manifestation of the ethmoid has received considerable attention since John Ruben and co-workers argued that turbinals (to pick one of the three names by which these structures are known), or their functional equivalents, are more or less indispensable for endothermic animals. See, e.g., Ruben et al. (1997). Since non-avian dinosaurs are seem to lack ossified turbinals, or even the enlarged nasal cavities necessary to contain them, the inference was that these dinosaurs were not endothermic. Others have argued that the lack of ossified turbinals is meaningless and that the volume of the nasal cavity is larger than Ruben had supposed. Much has been written on this topic by both capable students of physiology and serious scholars of the Dinosauria. We cannot count ourselves among the number of either company. Consequently, we have nothing to contribute to this debate, although we will touch on some its parameters.
The turbinals are often described as "scroll-like," although that phrase does poor justice to their complexity and variety. See image at right from Will's Skull Page (used with permission). Some very high-quality images and notes on the mammalian system may be found at nasal. They are, in any event, very thin and delicate bones of great complexity. Frequently, they are not bones at all, but unossified cartilage covered with a thin sheet of epithelium. At least in mammals, turbinals come in at least two functional varieties. The ethmoturbinals are associated with the cribiform plate. They are covered with olfactory epithelium. Primarily, they serve to increase the surface area over which olfactory receptor neurons can come in contact with respiratory air. These olfactory receptor nerves then synapse with the mitral cells of the olfactory lobes through the cribiform plate in the manner described above. Some very high-quality images and notes on the mammalian system may be found at nasal.
What gets all the press are the maxilloturbinals, which are located anterior and ventral to the braincase and are covered with respiratory epithelium. These turbinals are in direct contact with the bulk respiratory air, rather than the small aliquot which is shunted aside for olfaction. In mammals, the maxilloturbinals are part of a complex countercurrent scheme which adjusts for the difference in temperature and humidity between the environment and the lungs. If colder and drier outside air were allowed to penetrate directly to the lungs, it would damage the very delicate alveolar membranes. Conversely, if warm, moist air from the lungs were allowed to escape directly, this might cause a devastating loss of heat and water. These effects are not particularly serious for a large-bodied ectotherm.. Its respiratory requirements are not ambitious, and some loss of body heat is generally only inconvenient. However, the problem is severe for an endotherm, particularly a small endotherm, which must maintain a steady internal temperature and has a considerably greater demand for respiratory oxygen. The presumed function of the maxilloturbinals is thus to create a large, vascularized surface area to recover water and heat from exhalant air, and to introduce water and heat into colder inhalant air. Note that, when an animal pants, it by-passes this nasal cavity system, deliberately increasing evaporative heat loss in order to recover from temporary overheating. Phylogeny of the Turbinals
In the following discussion, we generally follow Wittmer (1995). This is the most recent review of a topic which has frustrated anatomists since Gegenbaur in the 1870's. The principle difficulties are two. First, it is almost impossible to determine the form or even existence of turbinals in extinct species. Turbinals are only ossified in birds and derived synapsids. Even when ossified, they are so thin and delicate that they are very rarely preserved. Second, the position of turtles is unclear. Assuming (likely, but not certain) that turtles diverged from Eureptilia after synapsids, there remains the problem of whether turtles have turbinals.
Turbinals are probably an apomorphy of the Amniota. As mentioned, the problem is turtles. There are no obvious turbinals, nor is there the kind of nasal capsule in which the concha of lizards, Sphenodon or some archosaurs is found. There is, however, the laterale Grenzfelte, a laminar protuberance from the wall of the nasal capsule which seems very similar to the rostral concha of Sphenodon. These structures may be related. However, Wittmer finds nothing particularly close in any other living taxon.
What Wittmer does find is a very close and convincing homology between the conchae (turbinals, cristae, etc.) marked in red in the figures adapted from his review. That is concha of lizards, the caudal concha of tuataras and birds, the crista semicircularis of mammals, and the concha plus preconcha of crocodilians are all the same structure. For convenience, we will follow Wittmer and refer to this structure as the "primary concha." In order to follow the argument, we will need to look more deeply than usual into the fundamental issue of homology.
In drawing this conclusion, Wittmer sticks very close to the strict Patterson formalism for homology. That is, in order to be homologous, structures must satisfy the tests of similarity, congruence and conjunction. These tests can be summarized as follows: Similarity: Each homologue must have the same 1:1 topographical relationship with other structures. In practice, we may add "or a really good developmental explanation of why it differs." Congruence: The presence of the homologous character must be congruent with the cladogram. That is, the homolology must be a synapomorphy of some clade. Conjunction: If two structures are homologous, then both can never appear in the same organism.
This is, obviously, a rather strict definition, perhaps too strict in some respects. However, the primary concha generally satisfies these tests. In particular, the primary concha forms at or near the juncture of two major nasal cartilages (the paranasal and parietotectal); is the first conchal structure to form in development; is situated directly opposite the choana or its homologues (it starts and has its roots here in crocs); has a fixed relationship to the ophthalmic nerve; and contacts the lacrimal where present. In most cases, it also forms a capsular structure, the cavum conchae. Turtles present a problem for the congruence test which cannot be completely resolved. However, the general constriction of the nasal cavity in turtles probably results in secondary loss of the concha. Smelling Trouble
The problem with Wittmer's hypothesis -- and its only a small problem -- is really in the conjunction test. He essentially ignores this aspect of the equation and therein may have missed the forest for the trees. The fact is that there are a bunch of conchae in several derived forms. Even Sphenodon has two. Birds and crocs have three, and mammals may have six, or perhaps more. Significantly, as Wittmer notes, the primary concha in crocodilians splits into two parts during development, forming the concha and the pre-concha. Both in archosaurs and mammals, all of the conchae are structured in a similar way. Furthermore, when a particular species has conchae which are of a distinct form, it appears (from a very cursory review of a limited sample) that all of the conchae in that species have the same aberration.
So, its possible that a "concha" is not so much a particular structure as a rather general genetic program that can be duplicated and may operate at many points along the ethmoid and nasal capsule derivatives. This actually takes very little away from Wittmer's main argument. The similarities of the primary concha across many taxa are too strong to ignore. But, at the same time, we cannot fairly conclude that the other conchae are "neomorphs." The concha program may simply have been expressed more than once. In one case, the turtles, it may have been expressed only once, but not in the usual place. That is, it may be that the concha program has been activated in the laterale Grenzfelte, instead of the usual position. We see that both can also happen in Sphenodon. We might conclude that there is a fairly broad area, perhaps the entire ethmoid plus nasal capsule region, in which the concha program may be activated, with the most probable location being the primary concha.
Thus, this supposition has at least a bit more explanatory power than simple homology. However, it is virtually impossible to test without the techniques of genetic manipulation. Unfortunately, it may be some time before we have available "knock-out" turtles or crocodiles. In the mean time, strict homology is probably the best bet, so long as we don't take it too seriously. ATW021026.
[ред.] А
A/paracone: in symmetrodont upper molars, the central (lingual) cusp of the trigon. This is home-made terminology used to reflect the fact that cusp homologies are unclear in symmetrodonts so that some workers use only a letter designation. However, many others traditionally refer to this cusp as the paracone.
Abducens nerve: cranial nerve VI. The abducens enervates the lateral rectus muscle of the eye, which rotates the eyeball laterally. See figure at rectus muscles; see also discussion and figures of the gnathostome orbit.
Abduction: in tetrapod locomotion, rotation of a limb upward, in a vertical plane. Opposite of adduction; and in contrast to protraction or retraction (movement in a horizontal plane) or to rotation about the long axis of a limb. C.f. Humerus.
Abductor L. ab = away from, and ducere = to lead. These muscles (and nerves) are named for their function. For example, the abducens nerve is named because it turns the eyeball outward.
Abo-Cutler Formation: Late Carboniferous to Early Permian of New Mexico. The Abo Reef underlies the Early Permian Cutler fm of West Texas. One can only speculate that this is the same.
Abomasum: the fourth chamber of the ruminant digestive tract, the second of two chambers used for bacterial fermentation of cellulose. See Artiodactyla for figure.
Aboral: in a direction away from the mouth cavity. This slightly ambiguous term is used to mean the opposite of occlusal in some cases where the side of the tooth opposite the biting surface is not a root, as in the tooth plates of chimaeriforms, or other "bradydont" dentitions in which spent teeth are retained in the dentition as, e.g., part of a pavement dentition.
Abrahamskraal Formation: Late Permian of South Africa. Part of the Tapinocephalus Assemblage Zone of the Beaufort Group. Elliotsmithia, therapsids.
Acellular: without cells; e.g. "acellular bone" is bone that is not supported by or contain living cells.
Acetabulum: the socket in the pelvis for the head of the femur, normally at the junction of the pubis, ischium and ilium. See figure at antitrochanter.
Acinaciform: shaped like a scimitar (or cutlass, or even saber).
Acoelous: or "amphiplatynate" vertebral centra flat on both ends -- neither procoelous (anteriorly concave & posteriorly convex) or opisthocoelous (vice-versa). Same as platycoelous.
Acoustic meatus, internal: on posterior surface of petrosal (or petrous portion of temporal); transmits facial nerve, auditory nerve (vestibulocochlear or VIIIth nerve), & labyrinthine artery from brain to inner ear.
Acrocoracoid process: in neornithine birds, the distinctive hook-shaped process at the proximal end of the coracoid that forms a part of the triosseal canal. Actually, the process is not restricted to birds and isn't hook-shaped in, for example, ducks. But such details are tedious ...
Acrodont: Having teeth attached to the edge of the jawbone without sockets. See Tooth Implantation.
Acromioclavicular joint: The acromioclavicular joint is located between the acromion (a projection of the scapula that forms the point of the shoulder) and the clavicle (the collar bone). This is a gliding type of joint.
Acromion (or Acromial) (Process): the outer end of the spine of the scapula that forms the outer angle of the shoulder, and articulates with the clavicle. Etymology: Gr. ακρoν (akron) = tip or summit, and oμoς (omos) = shoulder. The word ακρoν gives the combing forms acra- or acro-. The Acropolis in Athens was built on a summit. See figure at supraspinous fossa.
Acuminate: tapering (usually gradually) to a sharp point.
Adduction: in tetrapod locomotion, rotation of a limb downward, in a vertical plane. Opposite of abduction; and in contrast to protraction or retraction (movement in a horizontal plane) or to rotation about the long axis of a limb. C.f. Humerus.
Adductor: L. ad = to, toward, and ducere = to lead. These muscles (and nerves) are named for their function. For example, the all-important jaw adductors are muscles that move the jaws together.
Adductor blade: longitudinal ridge on the femur of Elpistostegalia and basal Tetrapoda. "The term 'adductor blade' ... is used ... to distinguish the prominent ridge bearing fourth and internal trochanters, from the more acute, and what appears to be primitively short, adductor crest." Coates (1996). The adductor crest is continuous with the distal end of the blade. See image of Acanthostega hindlimb.
Adductor fossa: The opening in the palate which in life contained the adductor (jaw closing) muscles. Not to be confused with the interpterygoid vacuities, choanae or other holes in the palate. The image at right, from Damiani (2001), shows a temnospondyl palate with the various fossae labeled.
Adipose: fat.
Adsymphysial: same as parasymphysial, i.e. flanking the symphysis (the point at which the two halves of the jaw meet).
Aegithognathous: type of avian palate with vomer broad and truncate anteriorly. Maxillopalatines do not join but do touch basisphenoidal rostrum. e.g., Passeriformes.
Aeolian: of sediments, deposited by wind, as in desert sand dunes.
Aina Dal Formation: Famennian of East Greenland, Celsius Bjerg Group, above Elsa Dal and below Wimans Bjerg Fms. Ichthyostega (Acanthostega also present, but rare). "The Aina Dal Formation reaches a maximum thickness of 90 m at its type section on Stensiö Bjerg and towards Paralleldal (Olsen and Larsen 1993). The formation consists of dark red to grey or black fine-grained sandstones and siltstones, representing a variably active meandering river and flood basin environment. The Wimans Bjerg Formation conformably overlies the Aina Dal Formation, and is, apart from trace fossils, almost unfossiliferous." Blom (2005).
Alar process: generic term for a wing- or fan-like projection.
Albedo: a surface property of a material which measures the proportion of incident light is reflected, rather than absorbed or transmitted. Albedo varies with wavelength and angle of incidence. Unless these are specified, the term refers to an average reflectivity across the visible light spectrum and assumes normal incidence (i.e. that the incident light strikes at an angle of 90° with the surface). The effective albedo of a surface also depends on its texture and contour, since some incident light may be reflected onto other portions of the surface.
Albian: the last age of the Early Cretaceous (Early Cretaceous II), about 112-99 Mya.
Alisphenoid: epipterygoid bone in mammals.
Alisphenoid Canal: see dog_orbit.jpg. I'm a bit vague on whether this is really the "maxillary." In any case, it appears to be just below the middle ear. In somewhat more rational animal like insectivores, the alisphenoid canal consists of a foramen in the alisphenoid exposure on the basicranium, near the foramen ovale which communicates with the cavum epiptericum just behind its opening into the orbitotemporal region. Asher et al. (2002).
Allantois: one of the membranes of the amniotic egg, it provides a surface for gas exchange and waste removal.
Allotype: A term, not regulated by the ICZN Code, for a designated specimen of opposite sex to the holotype.
Alternating diplospondyly: see diplospondyly.
Alula: the small digit 1 (thumb) which emerges from the proximal base of the carpometacarpus in birds.
Alveolar shelf: the tooth-bearing palatal shelf of the marginal bones of the jaws.
Allometry: roughly speaking, proportionality at different sizes, but sometimes used with respect to age, temperature, or any other variable. A structure is said to exhibit "positive" or "negative" allometry if it becomes larger more or less than in direct proportion to size. For example the human head grows with age, but becomes a smaller proportion of overall body size with age and development. Therefore we might say (assuming that we wished to be unusually difficult to understand) that the "cranium exhibits negative allometry with ontogeny."
Ameloblast: a specialized epithelial cell type responsible for enamel deposition. See Teeth.
Amelogenin: the protein which appears to be largely responsible for forming the matrix in which enamel is formed.
Aminadav Formation: Late Cretaceous (Cenomanian) of Israel, Judea Group. Includes the 'Ein Yabrud limestone quarry, type locality of Haasiophis.
Amnion: the innermost layer of the amniotic egg, it retains a fluid which surrounds the embryo.
Amniotic egg: see Introduction to the Amniota.
Amphiarthrodial: slightly moveable joints, such as between vertebrae, as opposed to freely moveable (diarthrodial) or immoveable (synarthrodial), such as the knee and sutural joints, respectively.
Amphicoelous: vertebral centra concave at both ends. lab7 photos
Amphistylic: a form of jaw suspension (e.g. in basal gnathostomes, other than placoderms) in which jaw is suspended both by the hyomandibula and by a direct connection between the jaw and the braincase. Introduction to the skeletal system
Amphiplatinate: see acoelous.
Ampulla L. ampulla = a jug. Perhaps an onomatopoetic word whose sound suggests the object; in this case, fluid flowing from the jug. Applied in anatomy to a number or structures supposedly resembling a jug, such as the ampullae of the labyrinth in the inner ear. See The Ear.
Anacanthous: of fishes, lacking dorsal fin spines. Opposite of phalacanthous.
Anacleto Formation: Early Campanian of Argentina. Overlies the Bajo de la Carpa Fm. and underlies the Allen Fm. Part of the Río Colorado Subgroup, Neuquén Group, Neuquén Basin. Braided stream and floodplain deposits with soils. Fossiliferous with dinosaurs, fishes, crocs, birds, lizards & turtles. Apparently east, or forming part, of the Andes foreland Basin but also just west of the South Atlantic.
Analogy: see homology.
Anamestic: of fish bones, a dermal bone which forms from ossification of embryonic membranes without nucleating around a sensory canal. Anamestic bones tend to be small, irregularly shaped and show considerable variability between individuals.
Anapophysis: in some mammals, the articular surfaces of the vertebrae are on the neural arches, rather than on the centra. The anapophyses are the posterior articulations, analogous to the postzygapophyses. Frequently both structures are present and in close proximity. The corresponding anterior articulations are the metapophyses.
Anastomosis Gr. ana = up, and stoma = mouth; hence an opening up. (Don't take this etymology too seriously ...)
Anconal: [1] in medical terminology, relating to the elbow. [2] as a directional indication, looking towards the elbow. As far as we can make out, this is another antonym for palmar, and so synonymous with "dorsal" in the special sense of that word as it applies to limbs.
Anguilliform: [1] eel-like in shape. [2] eel-like in locomotion, in which the body subscribes more than 1 sine wave at any given time and the there is a perceptible "traveling wave" down the body during locomotion. As opposed to, e.g. thunniform or carangiform locomotion in which a limited posterior segment moves back & forth (or up & down in mammals). Some useful detail at DESIGNING AN UNDERWATER EEL-LIKE ROBOT AND DEVELOPING ....
Angular process: in mammals, the most ventral of the three proximal processes of the jaw. See image at coronoid process.
Angulated: bent, V-shaped. This useful term isn't used much. It generally refers to the shape of an otherwise plate-like bone with a relatively sharp bend in the middle, like the covers of a partly open book, or an open laptop computer.
Anisian Age: The first Age of the Middle Triassic (242-234 Mya).
Anisodactyl: In birds, the basic digital configuration in which digit 1 (the toe or hallux) points posteriorly.
Ankylosed thecodont: See Tooth Implantation.
Ankylosis: ontogenetic fusion of bones. That is, a bone fusion that occurs after birth. Often used of pathologically fused bones in medicine, but a normal biological process in many organisms including humans.
Annulus (pl. annuli). L. anus = ring + diminutive ulus. Hence a little asshole. Applied to any ring or ring-shaped structure, not merely to junior siblings.
Anocleithrum: a relict member of the supracleithral series (bones dorsal to the cleithrum in fishes), found in some early tetrapods. The anocleithrum is a small, oval bone, normally dorsal and medial to the cleithrum and articulating with it.
Ant: (abbr.) anterior
Anterior trochanter: [1] (of the femur) probably the same as the lesser trochanter. [2] (of the fibula) probably the same as the illiofibularis tubercle.
Antimere: the opposite member of a paired structure, e.g. flapping flight requires the coordinated exertion of each wing with its antimere.
Antimeric: relating to an antimere.
Antitrochanter: a tuberosity or ridge contained in the acetabulum. MUSEE. DINOSAURES - LES COLLECTIONS. See Figure. This definition is quite different from the usage of Romer, who states: "Above the acetabulum the upper margin of the ilium was thickened ... . In hadrosaurs and ceratopsians there develops a downward projection from this thickened area, the antitrochanter, from which the iliofemoralis muscle probably took origin." Romer (1956: 326). Romer's definition is probably the only correct usage.
Antorbital fenestra: a hole in the skull just in front of the orbit. See figure at maxillary fenestra.
Antorbital fossa: a depression in the skull anterior to the orbit. The fossa is frequently excavated all the way through the bone, at least in part, to form an antorbital fenestra (q.v.) as well as other fenestrations.
Antotic process: of the ethmoid or sphenethmoid. In sarcopterygians (and perhaps other fishes), a process of the braincase located posterodorsal to the orbit which articulates with the ascending process of the palatoquadrate (= epiptyerygoid) as part of the dorsal jaw articulation. See Diplocercides for gigantic image.
Antrum: Gr. antron = a cave. A cavity.
Apatite: A class of minerals including several incorporated in the teeth and/or scales of vertebrates. The basic unit of apatite has peculiar chemical formula sometimes given as: Ca5(PO4)3(OH, Cl, F). To understand what's going on, its best to start with a simple block of calcium phosphate: CaHPO4. This has a very simple structure in which the two positive charges of the calcium ion, Ca++ are balanced by the two negative charges of the phosphate ion, in the form at which it exists at physiologically relevant pH levels: HPO4¨¨. In apatite, some of the phosphate groups are replaced by another ion, one with only a single negative charge. This typically results from the inclusion of hydroxyl (OH ¨ ), chloride (Cl ¨ ), or fluoride (F ¨ ) ions. If the inclusion is a hydroxyl ion, the resulting material is hydroxyapatite (except in Europe, where it becomes hydroxylapatite), the mineral constituent of bone, including dentine and enamel. In order to maintain charge neutrality, two things happen. First, some of the charges are "shared" between adjacent cells of the crystal. Second, the phosphate is forced to behave as a trivalent ion (an ion with three negative charges), equivalent to phosphate at a much higher pH, i.e., PO4¨¨¨. The physical properties of the resulting material make it uniquely useful for living organisms. The cross-linking of crystal cells through charge-sharing make apatite extremely strong. The actual alignment of the ions in the crystal is partially planar, but adjacent planes are rotated at 60° angles to each other, which means that the bulk material is highly resistant to shearing -- a very important attribute for teeth. The substitution of a small, monovalent ion for phosphate also leaves a regular series of physical "holes" in the crystal structure, which contributes to three additional important properties. (1) It provides room for organic components of the matrix to approach the ions and bind closely to the mineral by polar interactions. (2) For the same reason, enzymes can "reach inside" the crystal to dismantle it, allowing bone to be reworked and reshaped. It also helps that the phosphate ions in the crystal are held in a trivalent state. All it takes to dismantle the structure is the introduction of hydrogen ions from water in exactly the right places to unlock the crystal structure. (3) Finally, the presence of "holes" in the crystal gives the bulk material some compressibility, so that it can adapt flexibly to compression rather than shattering. In short, apatite exhibits a remarkable combination of strength, hardness, flexibility, and biochemical reactivity.
Aphetohyoidean: a condition involving "the presence of a non-suspensory hyoid arch behind a full post-mandibular gill slit." Stahl (1988: 858). This is one presumed primitive condition for the jaw suspension of gnathostomes.
Apical: in mammalian dentition, toward the crown.
Aplesodic: of a fin, the condition in which the basals and radials do not reach to the distal margin of the fin. This may refer to very primitive fish which have no support for the distal fin, or to highly derived fish which may have ceratotrichia or other non-bony support for the distal fin. Opposite of plesodic.
Apocrine glands: sweat glands associated with hair cells which secrete sweat containing complex organic compounds often having a characteristic odor. In humans, these glands become active at puberty and the odors may reflect emotion, state of health, sexual identity and maturity, and diet. Compare eccrine glands.
Apomorphy: a character state which is unique to a single, terminal taxon. Example: among primates, complex grammar is an apomorphy of human beings. It is quite diagnostic of humans, but useless in determining phylogenetic relationships because it is not a shared, derived characteristic, or synapomorphy, of any larger group.
Aponeurosis: a sheet- or ribbon-like tendinous expansion, serving mainly to connect a muscle with the parts that it moves. The best example is perhaps "palmar aponeurosis," the dense sheet of tendons underlying the palm in humans and many other tetrapods.
Appress: to be in contact with.
Apteria: areas on the skin of the embryonic bird which do not develop feather primordia.
Aptian: an age of the Early Cretaceous (mid-Cretaceous) about 121-112 Mya.
Archenteron: the internal body cavity formed by gastrulation. The cells lining the archenteron develop into endoderm. See Early Development Terms.
Arcocentrum: in elasmobranchs, the cartilaginous arch and its base in the vertebrae. Dictionary of Ichthyology. I have also seen this used of actinopterygians. Poyato-Ariza & Wenz (2002).
Arctometatarsalian: condition in which proximal half of metatarsal III is thin, splint-like, or even absent and closely appressed by metatarsals II and IV. The distal portion of metatarsal III is hollow (as are II and IV) and typically longer than II and IV. The functional significance of this arrangement is not completely clear. Likely, the entire metatarsus was bound tightly and the condition served to transmit force evenly over the metatarsals and aid in running. Holtz (1995).
Arcual plate: oblong, denticulated palatal bones which covered the anterior end of the notochord in some sarcopterygian groups. Also referred to as parotic plates.
Arcualia: "There are primitively two pairs of [metamerically arranged endoskeletal] elements in each metamere and on each side [of the notochord]: the interdorsals and basidorsals. In the gnathostomes, there are two additional pairs ventrally to the notochord: the interventrals and basiventrals. These elements are called arcualia and can fuse to a notochordal calcification, the centrum. The ensemble of the arcualia + centrum is the vertebra, and the ensemble of the vertebrae is the vertebral column." See Vertebrata (Phillipe Janvier).
Arcuate: in the shape of a smooth arc; not straight or broken line.
Argillaceous: Describing rocks or sediments containing particles that are silt- or clay-sized, less than 0.625 mm in size; any sediment containing large amounts of clay.
Articulated: of a fossil, a condition in which the bones are still contacting the each other more or less as they would in life.
Arundel Clay: according to Cifelli et al. (1999), a member of the Patuxent Formation. According to the Maryland Geological Survey, it conformably overlies the Patuxent Formation in the Potomac Group. Early Cretaceous II (Albian) of Maryland, USA. Dark gray and maroon lignitic clays; abundant siderite concretions; present only in Baltimore-Washington area. Oxbow swamps. Deinonychus, Tenontosaurus, Acrocanthosaurus, and triconodonts.
Ascending process of the palatoquadrate: the epipterygoid -- termed ascending process when enough of the palatoquadrate is present to think of it as a process. Under either name, it forms an articulation between the upper jaw and the braincase (and/or the skull table). See pterygoid for more context and figure.
Aspect ratio: in wings, the ratio of length to mean width for wings of regular shape. For irregular wings, other formulae apply. Wings with high aspect ratio indicate high-speed, long distance fliers. Such wings are less useful for low-speed flying or high maneuverability because they generate relatively little lift. To back off from jargon for a moment, a wing keeps the critter in the air because it pushes down on the air. At high speed, even a narrow wing passes over (and so pushes down on) a lot of air during the power stroke. At low speed, it takes a lot more area to push on the same amount of air. Length of wing helps, since a long wing pushes down further for each degree of downstroke. However, a long wing creates other problems: (a) it takes a lot of muscle to move a wing-tip out at the end of a long lever arm; (b) a tiny change in attitude at the end of a long lever arm can make a big difference in exactly how the airstream is directed. The bottom line is that high aspect ratio wings are for strong animals flying fast and high who don't need to be troubled too much about precision maneuvers or the sudden changes in wind speed and direction found near the ground.
The same general principles apply to the caudal fins of fish and the expanded tails of aquatic tetrapods. However, these principles are less easy to correlate with design and function in water. As an empirical matter, caudal fins of low aspect ratio are usually found in "unsteady swimmers" -- such as ambush predators using very high acceleration lunges with no sustained swimming.
Aspidine (= aspidin): When all is said and done, aspidine is probably a generic term for acellular bone, as it occurs in early vertebrates. Janvier (1996: 84-85, 95). When fully elaborated, it is typically three layered. The outer layer is the dentine cap. The large middle layer is cancellous, with straight vertical walls enclosing large spaces. The thin inner layer is dense and lamellar with no included spaces. Janvier also states that aspidine is characterized by "incremental growth zones, as well as some peculiar fibre-like lineaments." As with most terms for bone-like substances, the word (in this sense) refers to a structural type, rather than a distinct chemical entity. Unfortunately, there is considerable structural variation in the materials referred to as "aspidine," making the foregoing definition -- or any definition -- somewhat suspect. Either the middle or outer layer may be missing or replaced by other material, as shown for psammosteidans in the figure. In psammosteidans, the cancellous middle layer is replaced by a thick layer of amorphous "trabecular aspidine" as shown in the figure.
Aspidospondyly: a condition in which all vertebral elements (centra, arches) remain as separate units. Opposite of holospondyly.
Aspondyly: the condition of having no vertebral centra.
Astragalus: One of the two proximal tarsals or upper ankle bones. It is the more medial of the two and usually articulates with the tibia. Compare calcaneum. See Figure at Tarsus.
Astragalus, ascending process: the ascending process of the astragalus is just like it sounds. The inner (medial) ankle bone (the astragalus) sends a process up the shaft of the tibia. The ascending process is normally fused to the tibia.
Astragalar foramen: the opening of a canal on the proximal surface of the astragalus through which a nerve and vessels pass in primitive mammals.
Atlantal: relating to the atlas.
Atlas: Gr. Atlas was the mythological Titan who supported the world on his shoulders. Vesalius, in the sixteenth century, gave this name to the first cervical vertebra of tetrapods, which articulates with the skull, normally via condyles which permit the skull to move dorsoventrally.
Auditory bulla: the "inflated"-looking osseous covering of the middle ear and the floor of the skull in that region.
Auditory meatus, external: the "ear" of conventional speech; the outer ear; a passage leading from the environment to the tympanic membrane, often shaped so as to gather and concentrate sound from a particular direction. See Ear
Auditory meatus, internal: in mammals, the common foramen for the VIIIth (auditory) and VIIth (facial) cranial nerves. Kermack et al. (1981: 97). "an opening on the posterior surface of the petrous portion of the temporal bone through which the auditory and facial nerves pass." EPIC Glossary
Auditory ossicle: a small bone used to conduct sound energy. The term is normally used to refer to the mammalian complement of malleus (articular), incus (quadrate) and stapes. It applies equally well to the columella (hyomandibula) of many other tetrapods and the weird assortment of bones in the Weberian organs of various teleost groups.
Aulacodont: See Tooth Implantation.
Autapomorphy: a character which is unique to a particular taxon.
Autodiastyly: a form of jaw suspension in which the palatoquadrate is suspended from two articulations with the braincase. This may be the original form of jaw suspension. See discussion at Holocephali.
Autogenous: of bones, separate, not fused.
Autopalatine: an endochondral bone consisting of the anterior portion of the palatoquadrate (primitive upper jaw). See image and additional information at pterygoid.
Autopodium (autopod): the manus or pes, including digits (phalanges), metacarpals or metatarsals (metapodium). May or may not include the carpals or tarsals (mesopodium).
Autostylic: a form of jaw suspension (e.g. in lungfishes and in stem tetrapods) in which the upper jaw (palatoquadrate) articulates or is fused with the chondrocranium, lower jaw forms from the mandibular cartilage, and the jaw remains unsupported by the hyomandibula.
Autotomy: in Lepidosaurs and related forms, an animal can frequently escape predators by allowing part of its tail to break off. This is referred to as autotomy.
Axial: toward an imaginary axis running antero-posteriorly through the middle of the organism or structure; central. Opposite of radial. In almost all cases, axially means the same as anteroposteriorly or longitudinally.
Axilla: arm pit.
Axillary foramen: in some antiarch placoderms, an opening in the anterior ventrolateral plate which presumably allowed nerves and blood vessels to communicate with the pectoral appendages ("arms"). See figure.
Axis: the second cervical vertebra of terrestrial vertebrates; rotary movements of the head occur between the atlas and axis. Fr L. axis = axle or pivot.
Axonost: pterygiophore (the cartilage or bone on the outer end of which sit the median fin rays or spines), sometimes the proximal pterygiophore