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.
Wackestone carbonate rock which is matrix-supported; i.e., there are more than 10% grains, but the fine grain clay size matrix essentially surrounds the grains.
Waihao Greensand Middle? Eocene of New Zealand. Glauconitic sandstone, very fine-grained sediment from a low-energy marine environment. Köhler & Fordyce (1997). Zygorhiza (protocetid whale similar to Dorudon).
Weberian apparatus "Series of four or five modified vertebrae which connect the swim bladder to the inner ear of ostariphysian fishes; a set of structures, including a series of small bones, connecting the dorsal wall of the air bladder to the region of the ear; in carp, minnows, suckers, catfishes, and other ostariophysan fishes" See FishBase Glossary Searched Term. The apparatus is a chain of bony ossicles derived from vertebral processes) linking the swim bladder with the inner ear. A change in swim bladder volume as sound waves pass through the fish rocks the tripus. This movement is transferred through the intercalium and schaphium to the claustrum which abuts onto the perilymphatic sinus impar. The sinus, in turn, is linked to an endolymphatic transverse duct joining the saculi of either side. The system allows reception of an unusually wide range of frequencies and also seems capable of directional hearing. See also image at claustrum.
Weberian ossicles a chain of small bones which convey sound from the swim bladder to the inner ear in various teleost groups.
Wessex Formation Early Cretaceous (Hauterivian & Barremian) of England (Isle of Wight). Clayey sand containing considerable plant debris & wood fragments. Believed to be flood-born deposits. Hutt et al. (2001).
Westoll lines in some sarcopterygiansDipnoi and some others), "the cosmine of lungfishes shows more or less concentric lines of discontinuity, the Westoll lines ... which are interpreted as being due to cyclic resorption and redeposition during growth ... ." Janvier 1996: 209). Apparently, these occur both on the bases of individual scales and also as large structures between head shield plates.
Westphalian Epoch Normally subdivided into four stages, A through D. European Late Carboniferous epoch corresponding to the second half of the Bashkirian A&B) and the Moscovian C&D) epochs. That is, roughly 316 to 303 Mya.
Williston's Law another one of those paleontological "laws" which describe frequently -- but not invariably -- encountered trends. The "Law" is that, in any given lineage, the number of serially homologous elements tends to decrease while the individual elements tend to become divergently specialized. See Epibranchials for an interesting example and counter-example.
Wing loading ratio of weight to wing area. Lower wing loading permits slower flight without stalling.
Xia- Chinese (Mandarin?) prefix indicating lower? older? when attached to the name of a geological formation.
Xiphoid Process the most posterior bone of the sternum. See sternum.
Yarenskian Gorizont Early Triassic of Russia (Arkhangel'sk). Temnospondyls.
Yixian Formation Early Cretaceous (& perhaps uppermost Jurassic) of Liaoning Province, China. Fish, birds, theropods, iguanodonts, mammals. Famous for feathers on everything. Localities include the Jianshangou Beds (Zhangheotherium). See, generally, Luo (1999) for a non-technical discussion of age and biota.
Yolk saca membrane-bound compartment in the amniotic egg which contains stored food for the developing embryo.
Zalambdodont molar characterized by a V-shaped crest along the margin (ectoloph). At the apex of the V (on the lingual side of the tooth) is usually the paracone (fused with the metacone, with metacone lost). This looks odd, since the paracone is typically at the buccomesial corner of the molar. However, animals with zalambdodont molars have offset jaws, so that the ectoloph is occluding with the hypoflexid on the lower molar, just as the paracone typically does. In mammals with the usual cusp pattern, this occlusion is secondary to the protocone- talonid occlusion. In zalambdodont animals, the ectoloph- hypoflexid relationship is dominant. In fact, Asher et al. (2002) have defined zalambdodonty as incorporating three elements: a) primary occlusion between paracone & hypoflexid, (b) primary shear between preparacrista and protocristid, and (c) reduction or elimination of the metacone & talonid basin. The crests of the ectoloph run to stylar cusps on the labial side of the tooth. The protocone is typically absent. See Molars and The Diversity of Cheek Teeth at Animal Diversity Web.
Zap (abbr.) zygapophysis
Zeugopodium the lower part of the limb, i.e. radius + ulna or tibia + fibula. Compare stylopodium (upper limb) and autopodium (hand/foot).
Zygantrum see zygosphene
Zygapophyses articular processes that extend forward and backward of neural arches and help to strengthen union between vertebrae. The prezygapophyses of one vertebra articulate with the postzygapophyses of the next vertebra anterior to it. The angle of the zygapophyses commonly controls the degree to which the spinal column can bend laterally.
Zygodactyl a specialized digital configuration in birds in which both digits 1 and 4 are reversed. Some birds (e.g. musophagids, cuculiforms) are facultatively zygodactyl, i.e., digit 4 may be held in either an anterior anisodactyl) or reversed (zygodactyl) position.
Zygokrotaphic "When a skull is completely roofed (containing only openings for sense organs) it is called stegokrotaphic. A well-ossified skull with little kinesis is definitely an adaptation for a fossorial existence. In some rhinatrematids, scolecomorphids, and some caeciliids, the temporal region is partial open, is slightly kinetic, and is known as zygokrotaphic."
Zygomatic arch the arch running under the orbit and temporal fenestra in synapsids. Same as subtemporal arch, and roughly equivalent to the jugal arch in diapsids. Normally formed from jugal and squamosal (maxilla & squamosal in multituberculates). The presence of a zygomatic arch, at least when bowed outward, implies the presence of the masseter muscle which permits chewing of food.
Zygomatic plate The zygomatic process of the maxillary bone when this process is in the form of a thin plate.
Zygomatic process -A process of either the maxillary or squamosal bone that contributes to the formation of the zygomatic arch.
Zygomatic ridge a ridge which continues the zygomatic arch posteriorly. See image. Interesting trivia: the continuation of the ridge over the external auditory meatus, as in the figure suggests that the skeleton is male in anthropoids.
Zygosphene "A median process on the front part of the neural arch of the vertebrae of most snakes and some lizards [also mosasaurs -- ed.], which fits into a fossa, called the zygantrum, on the back part of the arch in front." OPTED v0.03 Letter Z.