Misplaced Pages

#755244

21-576: See text The Centroceratidae is the ancestral family of the Trigonoceratoidea and of the equivalent Centroceratina ; extinct shelled cephalopods belonging to the order Nautilida The Centroceratidae, which range from the Middle Devonian to the Lower Permian , are characterized by gyroconic, evolute tarphyceraconic, and involute nautiliconic shells with compressed whorls, typically with

42-560: A concave venter. Ventral shoulders are angular, umbilical shoulders broadly rounded. The suture is with broad, deep lateral lobes, the nature of the ventral and lateral lobes is unknown, as is the position of the siphuncle. Phacoceras as a moderately involute, highly compressed, smooth, lenticular shell with an acute venter and widest at the umbilical shoulder; suture with ventral saddle and broad shallow lobes on flanks; siphuncle slightly ventral from center. Genera in ascending stratigraphic order, descriptions from Kummel 1964. p K432 in

63-407: A quadrangular whorl section in which the flanks converge on venter that is much narrower than the dorsum and ventral and umbilical shoulders are sharply angular, or rarely rounded. In some, e.g. Centroceras , the flanks are divided by a ridge that runs along the middle. Sutures have ventral and lateral lobes but are transverse dorsally. The siphuncle is tubular and close to but not on in contact with

84-517: A variably positioned siphuncle. Whorl (mollusc) A spiral shell can be visualized as consisting of a long conical tube, the growth of which is coiled into an overall helical or planispiral shape, for reasons of both strength and compactness. The number of whorls which exist in an adult shell of a particular species depends on mathematical factors in the geometric growth, as described in D'Arcy Wentworth Thompson 's classic 1917 book On Growth and Form , and by David Raup . The main factor

105-401: Is quadrate in primitive forms; the venter typically narrow to acute, the dorsum broad. In some advanced forms, the venter may become concave or broad and rounded, and in some, the surfaces may be strongly lirate . The Trigonoceratoidea are based on the family Trigonoceratidae of Alpheus Hyatt , 1884, with which other phylogenetically related families are combined, and are equivalent to

126-573: Is how rapidly the conical tube expands (or flares-out) over time. When the rate of expansion is low, such that each subsequent whorl is not that much wider than the previous one, then the adult shell has numerous whorls. When the mathematical factors governing the pattern of growth are such that there is a very rapid expansion of the conical shape, of the shell tube, then the adult shell has very few whorls. The number of whorls present in an adult shell differs greatly in various taxa . The extant marine gastropod families Turritellidae and Terebridae , and

147-481: Is with ventral and dorsal lobes, the siphuncle very close to the venter. Carlloceras has a moderately involute shell with a compressed trapezoidal whorl section, nearly flat ventral and lateral areas, and slight dorsal impression. The suture has a ventral saddle and broad lateral lobe and the siphuncle is small and near the venter. Diorugoceras is very involute and smooth, with a compressed whorl section with broad, slightly convex flanks that converge toward

168-717: The Treatise Part K . Trigonoceratoidea Trigonocerataceae The Trigonoceratoidea are a superfamily within the Nautilida that ranged from the Devonian to the Triassic , thought to have contained the source for the Nautilaceae in which Nautilus is found. Trigonoceratoidea are characterized by open-spiraled, gyroconic , to closed, nautiliconic shells in which the Whorl section

189-400: The siphuncle small, subcentral, and orthochoanitic . In all the umbilicus is open and perforate. It includes some 17 genera. The Centroceratidae , ancestral stock, proposed by Hyatt in 1900, consist of gyroconic to evolute and involute shells that have a quadrangular cross section in which the venter is much narrower that the dorsum, the venereal and umbilical shoulders usually angular,

210-484: The Lower Carboniferous of Europe. Last is Phacoceras , found in the Lower Carboniferous of Europe and Lower Permian of Australia. (Kummel 1964) Homaloceras is the most primitive, with a laterally compressed, cyrtoconic to gyroconic shell with a narrow, concave venter, broadly convex flanks, and rounded dorsum, The suture is only slightly sinuous, the siphuncle tubular and near the venter. Centroceras ,

231-526: The Triassic Syringonautilidae. The Syringonautilidae, in turn, are the source for the Nautilidae (Nautilaceae) which contain the genus Nautilus . The Trigonoceratidae , type family, named by Hyatt, 1884, are loosely coiled to evolute , with oval to subquadrate , compressed to depressed whorl sections, and generally with longitudinal ridges or lirae . Sutures are typically slightly sinuous;

SECTION 10

#1732869972756

252-624: The Trigonoceratidae were first described. The Centroceratidae gave rise to the Trigonoceratidae and Grypoceratidae in the early Mississippian , while continuing until the very Early Permian . The Trigonoceratidae which ranged into the Permian left no descendants. The Grypoceratidae which ranged almost to the end of the Triassic gave rise to two small families, the Permian Permoceratidae and

273-745: The abandoned Centroceratida of Flower in Flower and Kümmel 1950, and to the Centroceratina of Shimanskiy 1957, revised to the Centrocerataceae, Shimanskiy 1962. The Trigonoceratoidea combine five families, the type, Trigonoceratidae, along with the Centroceratidae , Grypoceratidae , Permoceratidae , and Syringonautilidae . Phylogenetic study and age show that the Centroceratidae are the root stock in spite of having been first recognized 16 years after

294-412: The extinct Mesozoic family Nerineidae , have very high spired shells with a large number of whorls, and a relatively small aperture . The shells of a few genera of gastropods, and of the cephalopod genus Spirula , have whorls that are disconnected. To count exactly the number of whorls in gastropods a straight line is drawn to separate the semi-circular nucleus ( protoconch (1 - see image) from

315-408: The flanks flattened and converging on the venter. Sutures form lobes on the sides and venter but are transverse across the dorsum. The siphuncle is tubular, orthochoanitic , and close to but not on the ventral margin. It includes some six genera. The Grypoceratidae , predominant stock, established by Hyatt in 1900, are characterized by generally smooth, compressed, evolute to involute shells with

336-573: The genus Permoceras , are involute, smooth, with a compressed, higher than wide, whorl section, ventrally subcentral siphuncle, and a suture with a deep, narrow, pointed ventral lobe and large pointed lateral lobes, mimicking contemporary goniatites . The Syringonautilidae , Triassic offshoots named by Johann August Georg Edmund Mojsisovics von Mojsvar , 1902, gave rise to the Nautilaceae ( Nautilina ); containing five genera, they have generally smooth, involute shells with slightly sinuous sutures and

357-462: The shoulders; the siphuncle is tubular, near the venter; the living chamber half a volution in length. The surface is covered with alternating striae and lirae; fine, parallel "scratches" and "wires" Strophiceras has a probably gyroconic shell with a higher than wide, compressed, subrectangular whorl section with slightly arched dorsum and venter and flattened flanks. The venter has a median ridge aligned with diagonally elongated nodes. The suture

378-423: The type genus, is evolute with only a few, rapidly expanding whorls and wide, perforate umbilicus. The cross section may be faintly hexagonal from the effect of a median ridge on either flank, otherwise is tetragonal. Flanks converge from a broad dorsum to a narrow, barely convex venter. Ventral and umblical shoulders are sharp. The suture is with a shallow ventral lobe and broad lateral lobes with subacute saddles on

399-529: The venter (Kummel 1964). The Centroceratidae are thought probably to be derived from the rutoceratid stock (Flower 1950, 1988, Kummel 1964) Earliest are Centroceras , Homaloceras , and Strophiceras from the Middle Devonian; Centroceras and Homaloceras from North America, Strophiceras from Europe. Next is Carlloceras from the Upper Devonian of North America, followed by Diorugoceras from

420-744: The venter flattened to subangular . Ornamenation is not common, but some forms bear nodes or keels. Sutures have distinct ventral and lateral lobes. The siphuncle, which is narrow and tubular, is variable in position. It includes about 13 genera. Shimanskiy separated the Domatoceratidae, typified by the Permian Domatoceras , from the Grypoceratidae in the Treatise, leaving the Grypoceratidae for mostly earlier forms. The Permoceratidae , Permian offshoots named by Miller and Collinson in 1953 for

441-401: The younger part of the shell. An arrow placed at a 90° angle on this line, following the course of the whorl, reaches the end of the first whorl where it is parallel to its starting position. From that point all whorls are counted towards the margin of the shell, estimating the ultimate whorl with an accuracy of a quarter whorl. Other authors (Ehrmann 1933; Richter & Seapy 1999 ) applied

SECTION 20

#1732869972756
#755244