Tabular Horn Research Articles

Depositional Environments of Devonian Cairn Formation, Rocky Mountain Foothills, Alberta, Canada: ABSTRACT

The Cairn Formation is a carbonate sequence of Frasnian (Late Devonian) age that is the stratigraphic equivalent of the major oil-producing Leduc Formation of southern Alberta. During the summers of 1982 and 1983, detailed stratigraphic sections were measured near Canmore where the Cairn Formation is a platform sequence of carbonates approximately 50 km (31 mi) from the nearest reef edge. The entire sequence consists of sediments deposited in a subtidal environment. No evidence of intertidal or supratidal deposition was found. The formation consists of very fine to medium crystalline, buff to dark gray dolomite, in which original textures (usually mudstones, grainstones, and stromatoporoid floatstones) often can be identified. The sequence contains fair to good intercrystalline, biomoldic, and vuggy porosity. Some of the vuggy porosity has been infilled by very late, coarse crystalline dolomite and calcite. Individual beds typically are composed of a consistent lithologic character throughout. These beds are usually 10 cm to 2 m (1 in. to 6 ft) thick, and are virtually always bounded by disconformities. Some individual beds can be seen to swell and pinch out. The stromatoporoid floatstones are composed of 10-40% fossils consisting of Amphipora, Euryamphipora, bulbous stromatoporoids (5-20 cm, 2-8 in.), tabular stromatoporoids, horn corals, and colonial corals. All organisms are not in place but have only moved a short distance. Some thin beds consist of a black bituminous dolomite and probably represent a restricted lagoon environment of deposition. Although most beds are horizontal, some constructional bioherms up to 5 m (16 ft) in relief have been recognized in the sequence. End_of_Article - Last_Page 490------------

The functional significance of nectridean tabular horns (Amphibia: Lepospondyli)

The order Nectridea of the subclass Lepospondyli is of Palaeozoic age. Within this order, later members of the Keraterpetontidae developed hyper-extended tabular horns, so that in plan view the skull is boomerang­ shaped. Many unsuccessful attempts have been made to explain this shape in functional terms. The two genera that show the greatest development of these horns,DiplocaulusCope andDiploceraspisRomer, are analysed experimentally in a low-speed wind-tunnel. Differences between the two genera include that of size: the former is about twice the size of the latter genus and it has a prominent, ventrally directed flange on the quadratojugal-squamosal region not seen to the same extent inDiploceraspis. InDiplocaulusthe otic notch, which has come to lie on the ventral side of the skull, is large and extends proportionally further towards the tip of the horn than inDiploceraspis. The otic notches may have supported pharyngeal pouches, developed as accessory respiratory organs. A full-scale model of theDiplocaulusskull was made from information obtained from published illustrations and casts. It was mounted in the wind-tunnel so that angles of incidence varying from -10 to +25° were possible, and a fixed body was modelled so as to account for interference effects. TheDiploceraspiscondition was simulated by removing the prominent quadratojugal flange. Four conditions were investigated: (1) theDiplocaulusmodel; (2) theDiploceraspismodel; (3) the investigation of the effect of roughness of the surface of the model, to simulate the labyrinthodont condition of the dermal bones; and (4) an investigation of the effect of mouth-opening on the behaviour of the model. In addition, a flow-visualization test was carried out. All the experiments were carried out at a speed corresponding to the animal moving at 1.65 m/s in water. Coefficients of lift, drag and pitching moment were measured, over the range -10 to +25°, at two degree intervals. The significance of the results lies in the behaviour of the lift and pitching moment coefficient curves. The position of the centre of pressure does not move with the change of these two parameters and therefore the point of action of the centre of lift is fixed with respect to the occipital condyles. Forces exerted on the head are proportional to the deflexion of the head. Small but significant differences are seen when the curves for the two genera are compared. InDiplocaulusthe lift and pitching moment curves cross the zero-line very close to the origin, but in the other genus they cross considerably to the left of the origin. Roughening of the surface in two stages causes lift coefficients to be diminished and drag increased. It is concluded that the surface of the living animals was smooth or almost so. With the smooth model, it was noted that when the mouth was opened that neither lift nor drag coeffi­cient was significantly altered. It is concluded that when taking prey these animals must have suffered little deceleration. The flow visualization tests show that two flow regimes operated on the upper surface of theDiplocaulusmodel. To begin with, the flow was streamlined and parallel to the midline over all the head area, but after a critical angle of incidence the central region of the head stalled and the flow over the horns became stabilized as laterally directed vortices. From all of the results it is believed that the Keraterpetontidae were active midwater feeders preying on small fish, larval amphibia, aquatic arthropods and gastropods. They used the unique physical properties of the head to effect steep climbing ascents from the lake or stream bed to attack their prey, before returning to the bottom. The differences between the two genera are related to their contrasting environments and the possible course of evolution giving rise to these extreme adaptations is considered. Dorso-ventral flattening is seen as an aid to an active mode of life in the two genera under discussion here, and the same possibly applies to the Labyrinthodontia as a whole.

On Anthracosaurus Russelli Huxley (Amphibia: Labyrinthodontia) and the family Anthracosauridae

A complete description is given of all the known material attributable to Anthracosaurus russelli Huxley. Apart from one skull specimen from Usworth Colliery, Washington, Tyne and Wear, England, all known specimens come, or are thought to come, from the Blackband Ironstone of Airdrie, near Glasgow, Scotland. Both horizons are of Coal Measure age, the former Westphalian A, the latter Westphalian B. Complete preparation of the holotype allows a reconstruction of the skull roof and occiput and, together with the Usworth specimen, the palate and a series of skull sections. Two lower jaw specimens are described for the first time. The cranial anatomy of Anthracosaurus is that of a massive embolomerous anthrocosaur which differs from eogyrinid embolomeres, notably Eogyrinus attheyi , in a number of important features. These include loss of the skull table/cheek kinetism and presence of an orbital lacrimal, biramous tabular horns and a wide dorsal exposure of thequadrate. Features of the palate are correlated with the distinctive dentition of relatively few very large marginal teeth and long palatal tusks: they include broad plate-like pterygoids, whose anterior rami meet in a median suture and extend laterally to the palatine tusks (thus largely concealing the palatine bones from ventral view) and whose quadrate rami lack a deep descending flange. The lower jaw is unique in having confluent meckelian fenestrae. Vertebrae attributed to A. russelli are of normal embolomerous type except that the notochordal canal is occluded by a bony plug in the centra. An interclavicle of distinctive shape is also attributed. Several North American embolomere specimens are presently placed within the family Anthracosauridae. Of these the most complete is Eobaphetes kansensis Moodie consisting of three blocks, the original holotype and paratype and a third block not before described. As was first demonstrated by Dr Donald Baird these comprise a single skull, here described in its entirety. Description of Eobaphetes is preceded by a discussion of its provenance. This was originally recorded as ‘Coal Measures’, Washington County, Kansas, which is improbable but not more so than its current attribution to the Namurian of Washington County, Arkansas. Coal and spore analysis, supported by palaeogeography, suggest a late Pennsylvanian, probably Stephanian, horizon. In most respects the anatomy of skull roof and lower jaw of Eobaphetes suggests relationships with Eogyrinus rather than Anthracosaurus . This is also the case with the closely related Leptophractus obsoletus from Linton, Ohio, to which species ‘ Anthracosaurus lancifer ' also from Linton, is referred. However, Eobaphetes and Leptophractus , together with Neopteroplax conemaughensis , share a number of distinctive features. These include a flat-topped surangular crest to the jaw and, importantly, a dentition of massive palatal tusks and anterior marginal teeth but numerous small posterior teeth in maxillary and dentary. Tooth shape is also distinctive. It is thus proposed to retain the family Anthracosauridae for A. russelli alone, while placing the American species within the Eogyrinidae, but as a new subfamily the Leptophractinae, distinct from the Eogyrinidae. A diagnosis of each of these taxa is given.

A new genus and species of anthracosaur amphibian from the Lower Carboniferous of Scotland and the status ofPholidogaster pisciformisHuxley

The isolated skull of a Lower Carboniferous anthracosaur labyrinthodont from the Midlothian coalfield was formerly but erroneously attributed toPholidogaster pisciformis. It is now redescribed after ‘Airbrasive’ development as the holotype of a new genus and species. The anthracosaur agrees in a number of diagnostic cranial characters with the embolomerous forms of the Coal Measures but other characters, particularly those of the braincase and lower jaw, suggest relationship to the Seymouriamorpha. Its cranial anatomy could well be antecedent to that of both these groups and the sparse evidence available suggests close relationship to the American Mississippian anthracosaurProterogyrinus. A new classification of the Batrachosauria is presented with a major subdivision into Anthracosauria and Seymouriamorpha. The former group comprises the newly proposed infraorder Herpetospondyli to include the Lower Carboniferous forms, together with the infraorders Embolomeri and Gephyrostegoidea. It is concluded that while no batrachosaur with an otic notch could be a reptile ancestor a primitive ‘notchless’ anthracosaur could be. The cranial anatomy ofPholidogaster pisciformisis then redescribed, after complete ‘Airbrasive’ preparation of the holotype skull. The holotype, like the anthracosaur, comes from the Gilmerton Ironstone of the Edinburgh region, but the earlier holotype skull ofOtocratia modesta, long considered to be an ichthyostegid, is also attributed toPholidogaster. After redescription of ‘Otocratia' a composite restoration of thePholidogasterskull is presented. The nature of the dermal ornament, the post-parietal-supratemporal suture, the absence of tabular horns and the nature of the post-cranial skeleton all demonstrate thatPholidogasteris an early temnospondyl labyrinthodont in no way closely related to anthracosaurs. It is closely similar to the American Mississippian formGreererpeton burkemoraniRomer, notably in the presence of large premaxillary fangs and the absence of otic notch and intertemporal. Both may be attributed to the family Colosteidae. It is concluded that there is no strong evidence of close relationship between the colosteids and the Devonian ichthyostegids.

The cranial anatomy of two coal measure Anthracosaurs

Amphibian material collected by Mr J. A. Coope from Swanwick Colliery, Derbyshire was developed mechanically and with pyridine. The described remains consist of imperfect skulls of two species of Coal Measure anthracosaurs. The nomenclature of the anthracosaur-type material in the Hancock Museum is discussed and on the basis of this discussion the two Derbyshire skulls are assigned to the speciesPalaeogyrinus decorusWatson andEogyrinus attheyi Watson. ThePalaeogyrinusspecimen is described and, with the aid of redescription of critical regions of the type skull, new restorations of the occiput, the braincase, the skull roof and the palate are produced.Palaeogyrinusis shown to have an open fenestra ovalis of normal tetrapod type, an external naris which cuts the jaw margin, rather like that ofIchthyostega, and an otic notch, which while dorsal in position, is in no sense a continuation of the squamosal-supratemporal joint. TheEogyrinusspecimen shows the articular surface of the squamosal and the posterior palatal structure. Removal of the right pterygoid and extensive epipterygoid revealed details of the braincase including a well-preserved basipterygoid process. The origin of the distinctive anthracosaur pattern of roofing bones is discussed and it is concluded that in this respect these animals are further removed from their fish ancestors than the temnospondyl labyrinthodonts, but that the kinetic separation between squamosal and supratemporal is primitive. The musculature of the jaws is discussed and it is concluded that the characteristic tabular horn was for the origin of the posterior depressor mandibulae rather than a link with the shoulder girdle. The tabular horn together with the absence of post-temporal fossae in the occiput are therefore seen as secondary characters.

The Amphibian Fauna from the South Joggins. Nova Scotia

The Amphibian Fauna from the South Joggins. Nova Scotia