SUTURAL PATTERN AND SHELL STRESS IN BACULITES WITH IMPLICATIONS FOR OTHER CEPHALOPOD SHELL MORPHOLOGIES
SUTURAL PATTERN AND SHELL STRESS IN BACULITES WITH IMPLICATIONS FOR OTHER CEPHALOPOD SHELL MORPHOLOGIES
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DOI：
10.1017/s0094837300010034
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发表时间：
1990-06-01
期刊：
影响因子：
2.7
通讯作者：
JACOBS, DK
中科院分区：
文献类型：
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作者：
JACOBS, DK
在直壳菊石Baculites中，由于珠层在张力下的有限强度而造成的壳形状的结构限制被层内的拱顶系统所绕过。拱形结构在复合氨石间隔缝合线的支撑下，在静水荷载引起弯矩时保持壳壁受压。为了确定这些拱顶如何在phragmocone中相互作用以抵抗流体静力载荷，对一套Baculites标本进行了测量。在杆状岩中，拱顶壳表面的周向曲率（曲率半径）与支撑隔层的缝合单元之间跨越的拱顶大小之间存在统计学上显著的反比关系。在这个拱顶系统中，曲率半径和跨度大小之间的反比关系导致在相邻拱顶相互作用的拱顶跨度两端产生可比的反作用力。这些反作用力的等效性防止了拱顶两端的侧向位移。因此，来自相邻拱顶的压应力叠加在弯曲的拉应力上，并降低了拉应力分量。限制拉伸应力对于由珍珠质等脆性材料组成的轻质外壳至关重要，珍珠质在压缩时很强，但在拉伸时很弱。杆状体特别适合于这项研究，因为它的直壳只在周向弯曲，从而简化了问题。然而，在盘绕的菊石中，缝线模式似乎同样受到限制，只能在片孔中产生弯曲度和跨度大小相反的拱顶。
In Baculites, a straight shelled ammonite, the constructional limits on shell shape resulting from the limited strength of nacre in tension are circumvented by a system of vaults in the phragmocone. Vaults bridge between regions of the phragmocone supported by the complex ammonite septal suture, and maintain the shell wall in compression when hydrostatic load induces bending moments. To determine how these vaults interact in the phragmocone to resist hydrostatic loading, measurements were made on a suite of Baculites specimens. In Baculites there is a statistically significant inverse relationship between circumferential curvature (radius of curvature) of the vaulted shell surface and the size of vaults spanning between sutural elements supporting the phragmocone. The inverse relationship between radius of curvature and the sizes of spans in this system of vaults results in the generation of comparable reactive forces at the ends of the vault spans where adjacent vaults interact. The equivalence of these reactive forces prevents the lateral displacement of the vault ends. Consequently, compressive stresses from adjacent vaults are superimposed on, and reduce, the tensional stress component of bending. Limiting tensile stress is of utmost importance in a lightweight shell composed of a brittle material such as nacre, which is strong in compression but weak in tension.Baculites is particularly appropriate for this study because its straight shell is curved only in the circumferential direction, thus simplifying the problem. However, sutural patterns in coiled ammonites appear to be similarly constrained to produce vaults in the phragmocone which vary inversely in curvature and span size.