Takao Ubukata

Functional constraints on coiling geometry and aperture inclination in gastropods

Abstract We studied the morphological diversity of gastropod shell forms from the viewpoint of theoretical morphology, emphasizing the relationships of shell form to postural stability and the available space for soft body, which we assessed in terms of the moment of force and soft-tissue ratio calculations, respectively. The results of computer simulations suggest a functional trade-off between postural stability and available space for soft body: a compact shell possessing a low spire and small umbilicus exhibits high postural stability, whereas a less overlapped shell form with a high spire and large umbilicus makes available space for soft body. A functional morphospace analysis using theoretical models reveals that outward and downward inclination of the aperture moderates the functional trade-off between these parameter values and permits compatibility between stable posture and efficient shell construction. The hypothetical optimum that realizes this compatibility is consistent with the observed range of forms estimated from 359 extant gastropod species. The biometric results also suggest that land snails are more highly constrained than marine species in achieving a balance between postural stability and available space for soft body.

The sequence of local recolonization of warm-water marine molluscan species during a deglacial warming climate phase: a case study from the early Pleistocene of the Sea of Japan

Abstract Changes in the fossil assemblage of offshore molluscs within each sixth-order (41-kyr) depositional sequence of the early Pleistocene Omma Formation document repeated recolonizations of warm-water species into the Sea of Japan. Each recolonization started as soon as the warm Tsushima Current began to flow in the Sea of Japan during a deglaciation and subsequent interglacial period. Here we examined in detail the stratigraphic distribution of six selected significant warm-water species and calculated confidence intervals for their first (local) appearances in a total of 10 deglaciations using continuously sampled data. They were subdivided into two groups, based on the chronological order of their first appearance during warming phases. Comparison of geological with biogeographic data reveals that these migration patterns are not always consistent with the modern geographic distributions of the species. These results show that fossil records may anticipate how the assemblage of local species would change in response to climate warming.

Theoretical morphology of bivalve shell sculptures

A theoretical morphologic model defining patterns of shell sculptures in Bivalvia is introduced. It is based on the displacement of sculptural elements along the growing shell margin and introduction of new sculptural elements. The kinematics of the sculptural elements are defined in terms of the following parameters: maximum speed of displacement of sculptural elements, position along the growing shell margin where a migrating element attains maximum speed, and position of the divergence axis of the riblets. Computer models successfully mimicked most of the diverse patterns of bivalve shell sculptures. Morphometric analysis revealed that the displacement speed of a sculptural element is not constant but depends on the relative position of the element on the shell margin. It was revealed that the primary component of variation in bivalve shell sculptures could be explained by variation in the displacement speed of sculptural elements around the divergence axis of riblets.

Pattern of growth rate around aperture and shell form in Bivalvia: a theoretical morphological study

Abstract Shell growth and morphogenesis were studied in nine species of Bivalvia from the viewpoint of theoretical morphology. The aperture map, or pattern of relative rate of shell accretion for each point around the aperture, received particular attention. Morphometric analyses indicate that the basic pattern of the aperture map is generally maintained throughout ontogeny, whereas both shell convexity and aperture shape commonly change with growth. Computer simulations show that posterior elongation of the aperture with growth cancels the effect of ontogenetic shell inflation to move the maximal growth point anteriorly. In the species examined, the coiling axis is generally inclined to the hinge axis toward the anterodorsal direction and is plunging to the dextral side of the valve. This condition allows ontogenetic shell inflation without modification of the basic pattern of the aperture map. The result indicates that ontogenetic change of shell form is architecturally constrained by a basic pattern of the aperture map, which is kept throughout ontogeny.

A theoretical morphologic analysis of bivalve ligaments

Abstract A theoretical morphologic model defining ligament formation in the Bivalvia is introduced. It is based on the spacing of a lamellar layer, the spacing of a fibrous layer, and the relative growth rate of the expanding ligament with respect to enlargement of the ligamental area. Most of the diverse patterns of bivalve ligaments are successfully modeled by computer simulations. Wide intraspecific variation of the ligamental pattern is observed in an arcid species, Tegillarca granosa. This appears to be a consequence of allometric change of morphogenetic parameters during growth, adjusted to maintain the relationship between ligament strength and shell weight. The distribution of actual ligaments, which does not fill the theoretical morphospace, shows potential evolutionary pathways of bivalve ligaments. Thus, it implies phylogenetic relationships between ligament types from the viewpoint of pattern formation.

Late Cretaceous Record of Large Soft-Bodied Coleoids Based on Lower Jaw Remains from Hokkaido, Japan

The origin and phylogenetic relationships of most modern coleoid groups have not yet been explained by reliable fossil evidence, in large part because of the reduction or disappearance of a calcified chambered shell during their evolutionary history. Herein we describe two exceptionally large coleoid lower jaws from the Upper Cretaceous strata in Hokkaido, Japan. On the basis of the comparison of gross morphology and morphometric data of the lower jaws of modern and fossil coleoids, we assigned the two lower jaws to the following new taxa: Nanaimoteuthis hikidai sp. nov. of the order Vampyromorpha (superorder Octobrachia) and Haboroteuthis poseidon gen. et sp. nov. of the order Teuthida (superorder Decabrachia). The lower jaw of N. hikidai is distinguished from other species of the same genus from the Upper Cretaceous of Vancouver Island (Canada) and Hokkaido by having a broader, more anteriorly curved hood of the outer lamella. The lower jaw of H. poseidon seemingly exhibits mosaic features like those of modern teuthids and sepiids but is assigned to Teuthida on the basis of the overall shape of the outer lamella and the development of a distinct fold on the lateral wall. Because of the unusually large lower jaws, these new taxa appear to be comparable in body size to modern giant squids (Architeuthis spp.) and the Humboldt squid (Dosidicus gigas). This and other discoveries of large jaws referable to octobrachian and decabrachian coleoids from the Upper Cretaceous strata of the North Pacific fill the gap in the relatively poor fossil record of mainly soft-bodied coleoids.

Piggyback whorls: A new theoretical morphologic model reveals constructional linkages among morphological characters in ammonoids

A new theoretical morphological model is proposed for the analysis of growth, form and morphospace of ammonoid shells. In this model, the shape of a radial cross section through the shell is simulated by “piggybacking” of successive whorls. The “piggyback whorls model” is defined in terms of the enlarging rate of the perimeter and the proportion of the dorsal wall to the whorl periphery, if an isometric relationship is assumed between perimeter and area of the cross-sectioned whorl. Allometric coefficients on these growth parameters determine how compressed and evolute shells are formed. The present model successfully reproduced some correlations among purely geometric variables that have been reported in previous works and were also observed in our biometric analyses. This model yields a hypothesis of “constructional linkages” between aperture shape and coiling geometry that might provide a functional coupling between hydrostatic and hydrodynamic characters. The model may partly explain Buckmans Law of Covariation between rib features and shell shapes.

Enantiomorphs differ in shape in opposite directions between populations

Abstract Development is left–right reversed between dextral and sinistral morphs of snails. In sympatry, they share the same gene pool, including polygenes for shell shape. Nevertheless, their shell shapes are not the mirror images of each other. This triggered a debate between hypotheses that argue either for a developmental constraint or for zygotic pleiotropic effects of the polarity gene. We found that dextrals can be wider or narrower than sinistrals depending on the population, contrary to the prediction of invariable deviation under a developmental constraint. If the pleiotropy is solely responsible instead, the mean shape of each morph should change, depending on the frequency of polarity genotype. Our simulations of this mean shape change under zygotic pleiotropy, however, show that the direction of interchiral difference remains the same regardless of genotype frequency. Our results suggest the presence of genetic variation among populations that changes the maternal or zygotic pleiotropic effect of the polarity gene.

Theoretical morphology of hinge and shell form in Bivalvia: geometric constraints derived from space conflict between umbones

Abstract Geometric properties of the shells of 123 species of extant Bivalvia were analyzed from the viewpoint of theoretical morphology. The effects of shell form and the structure of ligament on the interumbonal space and the maximum shell opening received particular attention. The results of computer simulation and morphospace analysis indicate that possessing both prosogyrous shell form and an extended hinge without the parivincular ligament tends to cause space conflict between umbones or dorsal shell margins of right and left valves. To a large degree, a prosogyrous shell form with a long parivincular ligament helps shell opening without umbonal conflict, if the shell is flat enough to avoid the mutual interference between dorsal shell margins of closed valves. Extension of the ligament and plunging of the anterior part of the coiling axis into the ventral side provide enough space along the dorsal shell margins in which a parivincular ligament and its substrata are developed.

Modelling various sculptures in the Cretaceous bivalve Inoceramus hobetsensis

The geometry of the external shell sculpture in the Late Cretaceous inoceramid bivalve Inoceramus hobetsensis Nagao & Matsumoto, 1939 was studied both empirically and theoretically. A large sample, collected from the Upper Cretaceous of Hokkaido, Japan, shows remarkably high intraspecific variation in the shell sculptural pattern. Quasi-commarginal ribs, slightly oblique to the external growth increments, occur in some specimens. These sculptures are commonly irregular in strength and spacing, and their features are successfully modelled by computer simulations when the commarginal ribs are superposed with nearly concentric divaricate rib. Computer models indicate that the divergent sculpture element, often found in other inoceramids, was present throughout the evolution of I. hobetsensis and was developing in the evolutionary lineage from I. hobetsensis nonsulcatus to I. hobetsensis hobetsensis, although it was only weakly expressed. The results also suggest that some apparently distinct sculptural patterns of I. hobetsensis are the result of minor changes in the morphogenetic program.