Whorl Section Research Articles

Describing Ammonite shape using Fourier analysis

A number of geometrical methods for comparing shapes have been developed recently. This paper explores two approaches for analyzing the morphological variation of some invertebrate fossil characteristics such as rib pattern and whorl section shape: (1) landmarks analysis (Procrustes methods), (2) mathematical modeling by Fourier analysis. The morphometric analysis has been applied to a faunal sequence of Graphoceratidae (Ammonitina) taken in the central High Atlas. In the first stage of analysis, we used landmarks to describe shapes. This calculation is done through the “Procrustes” program whose results generate phenetic trees with a typically morphological significance and whose nodes convey some degrees of morphological similarities among the different taxa analyzed. In the second stage of describing ammonite shape, a new approach will offer us a valuable morphologic descriptor by modeling the whorl section. It allows for transcription in the form and an equation will be used for descriptive variables which represent necessary data for an analysis in principal components. Factorial planes then correspond to morphological space within which the analyzed individuals are distributed. In this way, it is possible to determine the groups for which whorl section morphologies show similarities. These two morphometric techniques offer a valuable tool for the analysis and comparison of morphologies for both rib shape and whorl section. This allows one not only to analyze morphological diversity in Graphoceratidae with more reliability, but also to highlight the most important convergences among the analyzed taxa.

Relationship between mid-Cretaceous (upper Albian–Cenomanian) ammonoid facies and lithofacies in the Yezo forearc basin, Hokkaido, Japan

An examination of inner shelf, outer shelf, and slope deposits in the Yezo forearc basin, northern Japan, provides new insights into the relationship between mid-Cretaceous ammonoid facies and lithofacies. Although undergoing post-mortem transport to some degree, the ammonoids were not moved to areas outside of their original habitat. This assumption is based on the condition of the outer shell surface, general absence of fragmentation, and sedimentary structures. Desmoceras predominates in the upper Albian–Cenomanian succession regardless of lithofacies, the family Gaudryceratidae is the second-most dominant group in each lithofacies, the abundance of Zelandites decreases offshore, and other groups, including Acanthoceratidae, are uncommon but occur in both inshore and offshore facies. External shell ornamentation does not necessarily vary according to lithofacies differences, while the shape of the whorl section does vary with lithofacies as a reflection of ambient environments. The smooth, slender Zelandites and the compressed morph of the smooth Desmoceras predominate in high-energy regimes represented by frequent hummocky cross-stratification and current ripple marks of an inner shelf. In contrast, the depressed morph of Desmoceras predominates in low-energy, offshore, muddy sea-floor regimes.

New species of the genus <i>Douvilleiceras</i> (ammonoid) from the Lower Cretaceous Yezo Group in Hokkaido

Two new species of douvilleiceratid ammonite, Douvilleiceras compressum and D. kawashitai, are described from the Lower Cretaceous Yezo Group in the Pombetsu Valley of Hokkaido. D. compressum is unique in its compressed whorl section in the genus. D. kawashitai is fairly similar to D. orbignyi Hyatt in its morphological characters, but the former is distinctly characterized by six rows of tubercles, instead of seven for the latter, on each side of the whorl.(Communicated by Tatsuro MATSUMOTO, M. J. A., Oct. 14, 2003)

Devonohelicoceras hidaense, a new torticonic oncocerid (Cephalopoda) from the Lower Devonian of Central Japan.

Based on the four specimens from the Lower Devonian (Emsian?) D4 Member of the Fukuji Formation, and a specimen from an unknown locality, a new genus and species of the oncocerid cephalopod, Devonohelicoceras hidaense, is described as the second known oncocerid from Japan. The present genus is most similar to the Late Silurian genus Foersteoceras, but differs in its weakly depressed whorl section, submarginal siphuncle with positional shifting, and well-developed actinosiphonate deposits. It is likely that the actinosiphonate deposits functioned as a substratum of adhesive liquid which has a lower specific gravity than the sea water.(Communicated by Tatsuro MATSUMOTO, M. J. A., Sept. 12, 2003)

New documentationof the indicativ maastrichtian ammonite menuites fresevillrnsis

Menuites fresevillensis ( seunes, 1890a) is recorsded and described for the first time from the shiranish formation (late campanian - maastrichtian), NW iraq, these diagnosis were made on hte basis of planispiral involute shell and compressed whorl section (oxycone), with the mean ratio of whorl breadth to whorl height (wb:wh) attaining 0.4 the shell is moderately ornamented by numerous ribs which mostly arise on the umbilical wall becoming strengthened toward small bullate

Anomalies of embryonic shell growth in post-Triassic Nautilida

No egg of any fossil nautiloid has yet been discovered. However, anomalies of embryonic shell growth, described for the first time in several Mesozoic Nautilida, provide important clues on morphology, structure, and size of their egg capsules; on the physical characteristics where egg laying occurred; and on the hatching processes. Roughness inside the inner egg capsule–caused by hard and uneven egg-laying substrate, locally and temporarily slowing down or stopping the apertural shell growth–could cause temporary deformations of growth lines. Such roughness, caused by stone, is described inside an egg capsule of Nautilus, which was fixed obliquely relative to the egg-laying substrate. This reduced the space between the inner and outer capsules, which locally fused together. The lateral-umbilical grooves, furrows, and deformations of growth lines were probably caused by the inner egg capsule during the prehatching stage. In fossil Nautilida, as in Nautilus, the size of this capsule was relatively small compared with the shell diameter at hatching. During the last stages of embryonic development, the shell extended backwards outside the egg capsule before hatching. This prehatching stage, during which the egg capsule continued to press against the shell, can be marked by a prehatching constriction. In fossil species, as in Nautilus, the inner capsule constituted a kind of “straitjacket” during the last stages of embryonic development. The expansion in whorl width at hatching, in normal as well as in abnormal shells, marks release of this straitjacket. Important deformations of the whorl section probably result from an abnormal form and size of the egg capsules mainly caused by the manipulations by the female during the egg laying on a hard and hollow substrate, increasing the straitjacket effect. An alternative explanation could be that the chorion did not expand adequately. From relatively early embryonic stages (approximately 180° adapical of the nepionic constriction) to hatching, both flaps of the hyponome could be turned backward under the shell, jammed between the inner wall of the egg capsule and the mantle margin, resulting in the formation of paired ventral parallel grooves. Many normal features of the embryonic development of nautiloids can be clarified through the study of the anomalies of embryonic shell growth.

The ammonitesKarakaschicerasandNeohoploceras(Valanginian Neocomitidae) from the Neuquen Basin, West-Central Argentina

Three species ofKarakaschiceras, K. attenuatus(Behrendsen, 1892),K. neumayri(Behrendsen, 1892), andK. lycoris(Leanza and Giovine, 1949), and one species ofNeohoploceras, N. arnoldi(Pictet and Campiche, 1860), are described and figured from the Valanginian strata of the Neuquén basin in west-central Argentina. The large collection of fossils available for study shows wide intraspecific variation regarding ornament and whorl section inKarakaschiceras neumayriandNeohoploceras arnoldi, casting some doubts on the current systematics applied to species of these genera. The biostratigraphic position of the four species in the Neuquén basin corresponds to theKarakaschiceras attenuatusSubzone of theOlcostephanus atherstoniZone. This subzone is correlated with the MediterraneanKarakaschiceras inostranzewiandSaynoceras verrucosumzones of Europe at the base of upper Valanginian sections.

Shell features, main colonized environments, and fractal analysis of sutures in Late Jurassic ammonites

A precise approach to the quantification of relationships between suture complexity, as measured by fractal analysis (step-line procedure), the architecture of shells, and the main colonized environments, has been made in a set of Late Jurassic ammonites (N=507). Statistically significant differences between fractal-dimension (Df) mean values of evolute and involute shells are interpreted as caused by differences in the surface:volume (S:V) ratio. Suture complexity is also related to the shape of whorl section. The lowest Df values correspond to subcircular whorl sections (low S:V ratio) and the highest ones to acute sections (high S:V ratio). The shape of flanks shows correlation with suture complexity. The highest values of Df are found in planulate shells and the lowest ones in whorl cross sections with convex flanks. Highly significant differences appear between Df mean values from unsculptured shells and those from ammonites with ribs and/or tubercles of medium to large size. Multivariate analysis shows a combined variation of shell features and suture complexity, resulting in a heterogeneous distribution of Df values within the ammonite morphospace, mainly according to structural (shell architecture) and ornamental (sculpture strength rather than density) factors. Finally, the data obtained on relationships between suture complexity and the colonized environments (epicontinental vs. epioceanic inhabitants) suggest that suture complexity is not primarily related to bathymetry, and/or that no major differences in habitat depths existed between epicontinental and epioceanic ammonites.

Covariation of morphological characters in the Triassic ammonoid Czekanowskites rieberi

The Triassic ammonoid Czekanowskites rieberi displays a covariation of morphological charac ters, which is rather common in ammonoids. Its morphological spectrum ranges from laterally compressed, involute, weakly ribbed forms to depressed, semiinvolute, strongly ribbed forms. In order to study this covariation, fifteen axially cut specimens have been analyzed by means of image analysis, which allows us to obtain the ontogenetic record of radii, area and perimeter of the individual whorl cross-sections. A logarithmic model of growth has been applied. Our data indicate that, owing to the covariation, the radii from the origin to the venter and to the umbil ical seam of a given whorl section vary inversely in order to maintain the relative position of the center of gravity of the whorl cross-section both throughout the ontogeny of single specimens and within the population. This influences hydrostatic parameters, such as the position of the center of mass and the orientation and stability of the shell. Since the ontogenetic record of the angular length of the body chamber is not known, we have calculated those hydrostatic varia bles using two mutually exclusive assumptions: (1) the angular length of the body chamber was constant throughout ontogeny and (2) the volume of the body chamber grew monotonically with the revolution angle. Fluctuations of the three hydrostatic variables were always less important in the first assumption. In any case, the spectrum of, for example, theoretical orien tations is comparable to those observed in the species of present-day Nautilus. The range of adult body-chamber length observed in C. rieberi is much narrower than the theoretical adult body-chamber length calculated under the second assumption which indicates that a certain control over this parameter existed in the natural population, probably in order to maintain a narrow range in orientation and stability. The excess or deficit in soft-body weight was probably compensated by inverse variations in shell-wall weight. The main conclusion is that, despite the extreme morphological variability, hydrostatic and, possibly, hydrodynamic properties of the population remained within narrow limits.

Sutural complexity and bathymetry in ammonites: fact or artifact?

The widespread assumption that sutural complexity in ammonites is mainly proportional to water depth is revisited. Fractal analysis has been used for the precise morphometric evaluation of sutural complexity in 131 Upper Jurassic ammonites. Suture lines belonging to twelve families have been analyzed, account being taken of shell structure (coiling, shape of whorl section), sculpture and paleoenvironments. Fractal dimensions obtained in epicontinental and epioceanic ammonites show the unlikelihood of precise relationships between suture complexity and depth, and/or the absence of major differences in habitat depth if bathymetry played any significant role in the configuration of intricate septa. Suture complexity appears to be better related to shell structural types. □Fractal analysis, ammonite sutures, Upper Jurassic.

LARDAROCERAS GEN. N., A NEW LATE ANISIAN AMMONOID GENUS FROM THE PREZZO LIMESTONE (SOUTHERN ALPS)

On the basis of ammonoids collected in the uppermost part of the Prezzo Limestone (Anisian), the new genus Lardaroceras and two new species L. krystyni (type species) and L. pseudohungaricum are described; a third one is left in open nomenclature. The genus Lardaroceras comprises involute and compressed ceratitids, whose subtrapezoidal whorl section is characterized by an evident periumbilical margin and a rounded ventral keel. The ornamentation is made of umbilical and lateroventral nodes, to which sometimes lateral nodes are added, and of lightly proverse and sinuous, primary, intercalatory and bifurcate ribs. The very distinctive suture line is subammonitic. The two species, L. krystyni and L. pseadohungaricam , differ mainly in the ornamentation and in the adult body chember: the former has umbilical and lareroventral nodes and its ornamentation fades on the adult body chamber, the latter has also lateral nodes and its ornamentation strengthens in the latest stage of growth. Because of the suture line the genus is attributed to the subfamily Beyrichitinae. The stratigraphic significance of the new genus is also discused.

NAIRITES-A NEW GENUS OF LATE TRIASSIC CERATITES FROM ARMENIA

A new genus of Triassic ammonite, Nairites Kiparisova and Zarian, and two new species, N. armenius and N. laevis, are described from collections made in coal seams in Lower Triassic beds exposed near Dzhermanis in the upper part of the Vedi river area, Armenia. The new genus Nairites differs from Noridiscites Spath by having a much narrower umbilicus, a higher whorl section, wider and shallower lobes and saddles, and a fairly long denticulated auxiliary lobe. N. armenius differs from N. laevis by having stronger ribbing, by the presence of tubercles on the flanks and by the presence of carinae (keels) on the margins of the truncated venter. Ralph W. Imlay

A New Species of Temnocheilus McCoy from Holy Island, Northumberland

SummaryAlthough British and American forms of this genus all bear points of resemblance to Temnocheilus ventro-concavus, only two small specimens from Eastern Europe, Nautilus dorso-armatus Abich, resemble it in their possession of a flattened concave venter and a whorl section of quasi-quadrilateral shape.