Palaeos:		Glossary
The Vertebrates		B

Glossary: B

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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- B -

B/stylocone: see stylocone. 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 stylocone.

Baer's Law. General features common to all members of a group appear earlier in development than do the special features that distinguish the various subdivisions of the group. The use of phylogenetic systematics (cladistics) renders this synonymous with Haeckel's Law.

Baleen: in mysticete whales, a keratinous substance probably derived from mucous membrane which forms sheets, plates, or curtains used to trap small & micro- organisms for filter feeding.  Water is filtered either by pressing the enlarged tongue against the baleen plates, or by swallowing large quantities of water and squeezing the mouth shut, thus forcing the water out through the baleen sieve.

Balfour Formation: of the Beaufort Group, Early Triassic of South Africa.  A classic very Early Triassic exposure in the lowest part of the Lystrosaurus Assemblage Zone.  Apparently siltstone and mudstone derived from the floodplains of ephemeral, low-sinuosity rivers flowing through a generally arid region.  Sidor & Smith (2004).  

Barbs: the "branches" of a feather which emerge from the central shaft (rachis).

Barremian: an age of the Early Cretaceous (Early Cretaceous), about 127-121 Mya.

Baruun Goyot Formation: Late Cretaceous of Mongolia, probably Late Campanian, ~75 Mya. Inland arid & semi-arid environments with wind-borne deposits.

Basal: (a) viewed toward the base or root of the tooth (b) opposite of derived, i.e. "primitive" or plesiomorphic.

Basal Angle: In squaloid and certain other sharks, a thickened area of the basicranium between the orbits.  The basal angle bears a strong ventral process containing the foramina for the internal carotids.  It may also bear the articulation for the orbital process of the palatoquadrate. 

Basal tuber: (pl. tubera) posteroventral projections below the occipital condyle.  These may occur on the basioccipital, the basisphenoid, or both.  See also basioccipital tubera.

Basement membrane: The basal (inner) layer of an epithelial wall. Usually composed of an epithelial basal lamina (= layer) and a connective tissue reticular lamina.  See also, figure at integument

Basibranchial: the most ventral of the gill arch elements. See Gill Arches.  In sharks, the basibranchial elements are fused in a somewhat triangular structure referred to as the basibranchial copula.  

Basicranial articulation: See basipterygoid process for anatomical details.  This articulation staples the braincase to the palate.  In basal tetrapodomorphs, there may be mobility at this articulation, the nature and function of which is unclear and likely variable from one species to another.

Basicranial fenestra: a large vacuity in the oticoccipital region below the notochord in many sarcopterygians and some very basal tetrapods.  See vestibular fontanelle.

Basicranium: the base of the braincase, commonly involving the basioccipital, basisphenoid and elements of the otic capsule.

Basidorsal: "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).

Basihyal: the basibranchial of the hyoid arch (generally beloved to be the 2nd primitive gill arch).

Basilar membrane: the outer membrane of the cochlea (= lagena) facing the scala tympani. It vibrates in response to compression waves caused by the middle ear ossicles.  In mammals, the basilar membrane supports the Organ of Corti which sorts the vibrations according to frequency. See the Ear.  

Basioccipital: One of the bones of the occiput, the part of the skull which articulates with the spine. The basioccipital is located (if present) ventral to the foramen magnum. It often rests on or articulates with the basisphenoid, as well as meeting the exoccipitals. The basioccipital usually forms most (sometimes all) of the occipital condyle(s) and the ventral margin of the foramen magnum. See The Occiput for details.

Basioccipital recess: a large median recess on the ventral surface of the basioccipital, near or between the basal tubera of the basioccipital.  This structure is found in various Crurotarsi.  It may be found as a blind-ended structure.  However, it is also believed to be homologous with the crocodylian median eustachian foramen which communicates with the pharynx.  Gower (2002).  

Basioccipital tubera are a pair of ventrolaterally directed blobs descending from the basioccipital.  They are sometimes simply referred to as "basal tubera."  However, the basisphenoid may also bear tubera.  Presumably the basioccipital tubera act as attachment sites for ligaments stabilizing the head on the neck.

Basipterygium: in fishes, the pelvic bone or cartilage to which the pelvic fin is attached. FISHBASE

Basipterygoid articulation: of the palatoquadrate.  Same as palatobasal articulation (?). A ventral articulation of the palatoquadrate in which the palatoquadrate articulates with the basipterygoid process.  See image at paratemporal articulation.

Basipterygoid process: The basipterygoid processes are (despite the name) processes of the basisphenoid.  They act to join the braincase to the palate.  In many basal tetrapods and their ancestors, this was a moveable articulation.  In most derived tetrapods, they simply staple the braincase to the palatal bones. That is, this is the basicranial articulation, the main link between the braincase and palate in reptiles. The image shows the reconstructed braincase of the basal prosauropod Thecodontosaurus in left lateral view. The basipterygoid process is in red. From Benton et al. (2000).  The basipterygoid process may pass through a notch in the pterygoid, at the origin of the quadrate ramus.  In some texts, the resulting cavity is referred to as the conical recess.  

Basisphenoid: The basisphenoid forms the floor of the braincase anterior to the basioccipital.  Ventrally it is covered by a dermal bone, the parasphenoid.  The fusion between these two is so close that some workers refer to the complex as the "parabasisphenoid."  The basisphenoid gives rise to the basipterygoid process and other structures of the braincase.  

Basisphenoid pillar: a dorsal or anterodorsal process of the basisphenoid from near the base of the basipterygoid process into the orbit.  This process helps support the eyestalk, if present, and anchor the extrinsic eye muscles of the rectus series.  Also called the postorbital pila.  See figure at rectus muscles; see also discussion and figures of the gnathostome orbit.

Basitrabecular process: an embryonic skull cartilage derived from the polar cartilages at the junction of the trabecular and parachordal cartilages.  See diagram.  

Bathonian Age: The third Age of the Middle Jurassic, 169-164 Mya.

Baudelot's ligament: in fishes, a ligament, sometimes ossified, which joins the 1st centrum with the supracleithrum.  This ligament has been interpreted as homologous with a first rib.  Wilson & Veilleux (1982).

Bayn Shire Formation: Late Cretaceous (but very uncertainly dated) of Mongolia.  Holtz (2001a).

Bear Gulch: Lund Land or Flat Shark City.  Bear Gulch is an Early Carboniferous lägerstat in Montana featuring a high-diversity fish fauna, of which over half are chondrichthyans.  Details are provided at The Bear Gulch by Prof. Richard Lund, who is believed to have been digging there since at least the Late Permian.

Beaufort Group: Permian and Triassic of South Africa, including the famous Karoo Basin Permian exposures.

Belly River Formation: of the Montana Group. Late Cretaceous (Campanian to ?Maastrichtian) of southern Canada (Alberta, Saskatchewan & points East).  Probably continuous with the Judith River Fm. "The Belly River Formation comprises an eastward-thinning wedge of sandstones, siltstones, shales and minor coals, up to 900 m thick, which extends from the southern foothills to Saskatchewan, where correlative strata are assigned to the Judith River Formation. In the outer foothills, the Belly River Formation extends from the United States border north to the Bow River, where correlative strata are assigned to the Brazeau Formation of the Saunders Group."  from DinoData Fossilsites Belly River Fm.  The Belly River Fm. is overlain by the Bearpaw and itself overlies the Pakowki.  "Belly River Formation" is a name rarely used these days, with most of the territory previously ascribed to it now designated as the Dinosaur Park Formation.

Ben Nevis Formation: from the Red Bay Group of Spitsbergen.  Early Devonian (middle to late Lochkovian).  Many jawless fishes including thelodonts, psammosteids, and heterostracans.  Overlies the Fraenkelryggen Formation.  Blom & Goujet (2002).

Berriasian: The first age of the Cretaceous (Early Cretaceous), approximately 144-137 Mya.  See Berriasian.

Bi-: [1] Latin prefix meaning two; [2] Chinese combining form meaning wall.

Biogenic: having a biological origin.

Biramous: having two rami, i.e. two shafts or projections.

Biserial: of fish fins, bearing both preaxial and postaxial radials (long bones projecting from both the leading and trailing sides of the main fin axis, as in lungfishes).

Bissekty Formation: middle Cretaceous (Turonian) to Late Cretaceous (Santonian, perhaps Campanian) of Uzbekistan. Coastal and lowland environment. Mammals.  Archibald et al. (2001).

Bituminous: composed of partially decayed organic matter which has solidified with time/heat/pressure to form substance which crumbles or smears and will liquefy on heating under appropriate conditions.  Soft coal is the best-known bituminous deposit.  

Blastocoel: See blastula.

Blastoderm: See blastula.

Blastopore: the entrance to the archenteron where gastrulation begins. Among Deuterostomes, in contrast to Protostomes, the blastopore develops into the anus and, consequently, into a time-hallowed tradition of undergraduate humor.

Blastula: the hollow sphere of cells initially formed by repeated division of the fertilized egg in microlecithal development.  Alternatively, the stage of embryonic development characterized by formation of a blastula or its equivalent. The cells themselves, initially in a monolayer, are referred to as the blastoderm. The cavity is the blastocoel. In mesolecithal development, the process much is the same (i.e., holoblastic), except that the early cleavage pattern of the fertilized egg is unequal, leaving the blastocoel at the animal pole of the embryo. The opposite pole, the vegetal pole, is composed of a smaller number of large, yolk-filled cells. In macrolecithal development, cleavage is meroblastic, and the blastula stage embryo is not hollow, but generally consists of a sheet of blastoderm on the surface of the egg (exclusive, of course, of any shell, jelly coat, etc) at the animal pole. The blastula stage is followed by gastrulation.

Bob (the Basal Amniote): Bob is the proud possessor of the Standard Condition.   Bob is a (hypothetical) basal amniote.  He is our main point of reference for anatomical comparison among tetrapods.

Bodenaponeurosis: a tendinous mass that serves to attach the mandibular adductor to the coronoid process.

Bone: a rigid structural tissue primarily composed of apatite with varying amounts of structural protein. See Introduction to the skeletal system for discussion of some of the issues relating to formation, growth and morphology of bone.

Boss: a raised, thickened, normally round area of a dermal bone.

Bothrio-: Greek root = "pitted".

Brachiopatagium: The main flight membrane of pterosaurs (and bats?) .

Brachydont: cheek teeth having low crowns. Opposite of hypsodont.  

Bradydont: having slow tooth replacement.  This is inferred from retention of spent teeth in the dentition.  

Braincase: See Bones: Braincase

Branchial Arch: Vertebrates are unique in having the gills develop internally, as evaginations of the anterior gut. It is supposed that these internal gills were, primitively, filter-feeding structures which secondarily developed as respiratory organs. The branchial arches support the gills and have also been exapted for various other functions.

Branchiopercular: in fish anatomy, the last branchiostegal ray, conceived as a continuation of the opercular series (which it probably is).  

Branchiosaur: generic term for an aquatic, larval tetrapod, typically a temnospondyl. Presumably the reference is to external gills.

Breccia: rock containing pieces of angular gravel.

Brevis: L. brevis = short.

Brevis shelf: a ridge running along the inside surface of the ilium behind (i.e. posterior to) the acetabulum.

Britta Dal Formation: Famennian of East Greenland, Remigolepis or Celsius Bjerg Group, above Wimans Bjerg and below Obrutschew Bjerg Fms (both Late Famennian). "The thick Britta Dal Formation [is made up of] 550 m of red and gray siltstones and some red sandstones containing both Ichthyostega and Acanthostega. This unit has been interpreted as dominantly fluviatile and floodplain sediments. The taphonomy of these sites strongly suggests that Ichthyostega and Acanthostega inhabited the sedimentary basins in which they were buried and dwelled within large freshwater river systems."  Long & Gordon (2004) (citations omitted).  Also "Eusthenodon, plus the porolepiform Holoptychius, the dipnoan Soederberghia, the placoderm Remigolepis, and isolated ctenacanth fin spines."  Coates (1996).  Compare Blom (2005): "Recent stratigraphical and sedimentological studies may, however, challenge previous interpretations [i.e. river and flood plain deposits] against a new hypothesis of sheet flood origin for most of the main sandstone bodies (T. R. Astin and J. E. A. Marshall, pers. comm. 2002). The alternation of sandstone bodies and siltstones, nevertheless, characterizes the whole stratigraphical extension of the formation, which can reach about 500 m in thickness."

Bronchus (adj. bronchial) Gr. bronchos = windpipe; originally from Gr. brechein = to moisten. Plato believed that swallowed liquids went down the trachea into the bronchi. Remember that the next time someone mentions the old ...

Buccal L. bucca = the cheek. Of dentition, the "outside" of the teeth, toward the cheeks. Same as labial and opposite of lingual.

Buccinator L. buccinator = a trumpeter. Hence the muscles of the cheek.

Bucco-hypophyseal canal: perhaps a persistent remnant of Rathke's pouch.  This is a short canal which links the sella turcica with the mouth.  In other words, it provides a direct connection between the pituitary and the digestive tract.  

Bulla L. bulla = a bubble. The adjective is bullous; both are used of a lesion or structure which resembles a bubble. See Auditory bulla.

Bullion Creek Formation: Late Paleocene of North Dakota, USA.  Subtropical swamp, lake & river, & delta sediments. Formerly known as "Tongue River" Formation.  Presbyornis.  Benson (1999).

Bunodont: dentition of, for example, pigs (see Ungulate teeth, More on Artiodactyla) follows the basic tribosphenic pattern and consisting of low, rounded cusps. It is characteristic of fairly unspecialized omnivores. See Molars.  See also, your own molars, which are essentially bunodont.  

Burnt Bluff Group: Early Silurian of North America (Michigan Basin).  Carbonates.  Turner et al. (1999).

Bursa ML. bursa = a purse, hence any closed sac.

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