Hostname: page-component-586b7cd67f-rcrh6 Total loading time: 0 Render date: 2024-11-27T13:41:25.607Z Has data issue: false hasContentIssue false
  • English
  • Français

Form and function of orthoconic cephalopod shells with concave septa

Published online by Cambridge University Press:  08 April 2016

G. E. G. Westermann
G. E. G. Westermann*
Affiliation:
Department of Geology, McMaster University, Hamilton, Ontario L8S 4M1, Canada
Get access Rights & Permissions [Opens in a new window]

Abstract

Models of “ideal” orthoconic shells having simple concave septa with minimal weight and maximal strength and analysis of 72 species of fossil orthocones and cyrtocones yield important insights into the physical principles underlying cephalopod shell design. The ideal septum is a spherical cap weighing only 77% of a hemispherical septum of equal strength. The septa of most longicones approximate this ideal shape while those of brevicones are less curved, probably owing to buoyancy problems. Increase in septal strength leads to weight increase unless the shell becomes more logiconic or septal spacing increases or both. However, increased spacing requires more cameral liquid for septum formation, thus reducing buoyancy. In ideal longicones, septal spacing resembles the cone radius for thick, strong septa but declines to half of the cone radius for thin, weak septa. In ideal intermediates and brevicones, spacings are respectively reduced by factors of about 2 and 4, with similar additional dependence on septal thickness. Most real septa resemble these ideal models.

The relative length of the body chamber to the phragmocone varies greatly between about 0.2 and 1.5, depending mainly on the wall thickness and to a lesser degree on the septal thickness, apical angle and body density. Removal of cameral liquid in the adult must be compensated for by additional growth to retain neutral buoyancy. The conditions for neutral equilibrium calculated for longicones with different “counterweights” indicate that: (1) cameral liquid only is least feasible; (2) half-and-half calcium carbonate and liquid results in one-third length and one-quarter volume reduction of the body chamber; (3) with calcium carbonate only, body chamber reduction is minimal. Real ‘counterweights’ appear to be intermediate between (2) and (3), providing the animal with horizontal stability, which is missing in (3). Most uncalcified siphuncles reduce the body chamber only slightly although they improve horizontal stability. If the wall attains full thickness only at the apical end of body chamber, the liquid-only ‘counterweight’ becomes feasible.

Type
Research Article
Information
Paleobiology , Volume 3 , Issue 3 , Summer 1977 , pp. 300 - 321
Copyright
Copyright © The Paleontological Society 

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Literature Cited

Alexander, R. M. 1968. Animal Mechanics. 346 pp. Univ. Washington Press; Seattle, Wash.Google Scholar
Blind, W. 1975. Über die Entstehung und Funktion der Lobenlinie bei Ammonoideen. Paläntol. Z. 49:254–267.Google Scholar
Currey, J. D. 1976. Further studies on the mechanical properties of mollusc shell material. J. Zool. 180:445–453.Google Scholar
Currey, J. D. and Taylor, J. D. 1974. The mechanical behaviour of some molluscan hard tissues. J. Zool. 173:395–406.Google Scholar
Denton, E. J. and Gilpin-Brown, J. B. 1973. Floatation mechanisms in modern and fossil cephalopods. Adv. Mar. Biol. 11:197–268.Google Scholar
Flower, R. H. 1964a. The nautiloid Order Ellesmeroceratida (Cephalopoda). State Bur. Mines Min. Res. New Mex. Inst. Mining Tech., Mem. 12:1–234.Google Scholar
Flower, R. H. 1964b. Nautiloid shell morphology. State Bur. Mines Min. Res. New Mex. Inst. Mining Tech., Mem. 13:1–79.Google Scholar
Flower, R. H. 1968. The first great expansion of the actinoceroids. State Bur. Mines Min. Res. New Mex. Inst. Mining Tech., Mem. 19:1–16.Google Scholar
Freudenthal, A. M. 1966. Introduction to the Mechanics of Solids. John Wiley & Sons, Inc., New York.Google Scholar
Heptonstall, W. 1970. Buoyancy control in ammonoids. Lethaia. 3:317–328.Google Scholar
Jeletzky, J. A. 1966. Comparative morphology, phylogeny, and classification of fossil Coleoidea. Univ. Kansas Paleontol. Contrib., Mollusca. 7:1–162.Google Scholar
Merkt, J. 1966. Über Austern und Serpeln als Epöken auf Ammoniten-Gehäusen. N. Jb. Geol. Paläontol., Abh. 125:467–479.Google Scholar
Moore, R. C., ed. 1964. Treatise on Invertebrate Paleontology. Part K, Mollusca 3, Cephalopoda-general features-Endoceratoidea-Actinoceratoidea-Nautiloidea-Bactritoidea. 519 pp. Geol. Soc. Am. and Univ. Kansas Press; Lawrence, Kansas.Google Scholar
Mutvei, H. and Reyment, R. E. 1973. Buoyancy control and siphuncle functions in ammonoids. Palaeontology. 16:623–636.Google Scholar
Raup, D. and Chamberlain, J. 1967. Equations of volume and center of gravity in ammonoid shells. J. Paleontol. 431:566–574.Google Scholar
Reyment, R. 1958. Some factors in the distribution of fossil cephalopods. Stockholm Contrib. Geol. 1:97–184.Google Scholar
Reyment, R. 1973. Factors in the distribution of fossil cephalopods. Part 3: experiments with exact models of certain shell types. Bull. Geol. Inst. Univ. Uppsala, N.S. 4:7–41.Google Scholar
Riccardi, A. C. and Sabattini, N. 1975. Cephalopoda from the Carboniferous of Argentina. Palaeontology. 18:117–136.Google Scholar
Seilacher, A. 1975. Mechanische Simulation und funktionelle Evolution des Ammoniten-Septums. Paläontol. Z. 49:268–286.CrossRefGoogle Scholar
Spaeth, C. 1975. Zur Frage der Schwimmverhältnisse bei Belemniten in Abhängigkeit vom Primärgefüge der Hartteite. Paläontol. Z. 49:321–331.Google Scholar
Tanabe, K. 1975. Functional morphology of Otoscaphites puerculus (Jimbo), an Upper Cretaceous ammonite. Trans. Proc. Palaeontol. Soc. Japan, N.S. 99:109–132.Google Scholar
Taylor, J. D. and Layman, M. 1972. The mechanical properties of bivalve (Mollusca) shell structures. Palaeontology. 15:73–87.Google Scholar
Teichert, C. 1964. Morphology of hardparts. Pp. K13–K53. In: Moore, R. C., ed. Treatise on Invertebrate Paleontology, Part K, Mollusca 3. Geol. Soc. Am. and Univ. Kansas Press; Lawrence, Kansas.Google Scholar
Thompson, D'Arcy. 1942. On Growth and Form. 1116 pp. Cambridge Univ. Press; London.Google Scholar
Ward, P. and Westermann, G. E. G. 1977. First occurrence, systematics and functional morphology of Nipponites (Cretaceous Lytoceratina) from the Americas. J. Paleontol. 51:367–372.Google Scholar
Westermann, G. E. G. 1971. Form structure and function of shell and siphuncle in coiled Mesozoic ammonoids. Life Sci. Contrib. R. Ont. Mus. 78:1–39.Google Scholar
Westermann, G. E. G. 1973. Strength of concave septa and depth limits of fossil cephalopods. Lethaia. 6:383–403.CrossRefGoogle Scholar
Westermann, G. E. G. 1975a. Architecture and buoyancy of simple cephalopod phragmocones and remarks on ammonites. Paläontol. Z. 49:221–234.Google Scholar
Westermann, G. E. G. 1975b. Model for origin, function and fabrication of fluted cephalopod septa. Paläontol. Z. 49:235–253.CrossRefGoogle Scholar