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Language：English   Publisher：The Palaeontological Society of Japan (PSJ)  

Polyptychoceras, a Cretaceous heteromorph ammonite genus, is characterized by a trombone-like shell called a "hamitoid" shell. In order to clarify the shell-forming mechanism, a large sample, which consists of more than 320 specimens of P. pseudogaultinum (Yokoyama) obtained from the Upper Santonian of Hokkaido, was biometrically analyzed. Besides the shell coiling, cyclic changes of growth pattern are recognized by the analyses of the shell ornamentation, the relative growth rate of shell height, and the distance between septa. Intermittent shell growth, which was also deduced from the ontogenetic stage distribution in the population samples, is probably the cause of such peculiar shell coiling. Also, we carried out some computer simulations to reconstruct hydrostatically the ontogenetic change of the living attitude of P. pseudogaultinum. It is suggested that the rate of absolute shell growth possibly depends on the living attitude of this ammonite in the water column; the shell grows slowly when the shell aperture faces upward, and grows rapidly when the aperture faces in other directions. It is likely that every individual of this ammonite spent most of its life time with an upward-facing aperture.

DOI： 10.2517/prpsj.1.29

CiNii Books

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Language：English   Publisher：The Palaeontological Society of Japan (PSJ)  

Polyptychoceras, a Cretaceous heteromorph ammonite genus, is characterized by a trombone-like shell called a "hamitoid" shell. In order to clarify the shell-forming mechanism, a large sample, which consists of more than 320 specimens of P. pseudogaultinum (Yokoyama) obtained from the Upper Santonian of Hokkaido, was biometrically analyzed. Besides the shell coiling, cyclic changes of growth pattern are recognized by the analyses of the shell ornamentation, the relative growth rate of shell height, and the distance between septa. Intermittent shell growth, which was also deduced from the ontogenetic stage distribution in the population samples, is probably the cause of such peculiar shell coiling. Also, we carried out some computer simulations to reconstruct hydrostatically the ontogenetic change of the living attitude of P. pseudogaultinum. It is suggested that the rate of absolute shell growth possibly depends on the living attitude of this ammonite in the water column; the shell grows slowly when the shell aperture faces upward, and grows rapidly when the aperture faces in other directions. It is likely that every individual of this ammonite spent most of its life time with an upward-facing aperture.

DOI： 10.2517/prpsj.1.29

CiNii Books

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Language：English  

Arm autotomy was induced in a living specimen of Metacrinus rotundus (Echinodermata: Crinoidea). An arm was autotomized at a ligamentary articulation known as a cryptosyzygy, following incision by scissors distal to the break point. Although sessile stalked crinoids cannot entirely escape from a predatory attack by arm autotomy and they do not have an active defense, arm autotomy at cryptosyzygies reduces damage and arm loss by effective distribution, and by minimizing trauma and facilitating subsequent regeneration. The paradigmatic distribution of cryptosyzygies in which arm loss is set at a minimum, compared with the actual distribution, shows that these two patterns are similar and that actual specimens successfully reduce arm loss by the effective distribution of cryptosyzygies. The crinoid branching pattern also affects arm loss, and two different paradigms are discussed: anti-predatory and harvesting. Arm branching patterns of various isocrinids have tended toward the anti-predatory configuration from the Jurassic to the Recent, suggesting that the isocrinids have coped with increased predation. Shallow-water comatulids generally adopt the anti-predatory paradigm in their branching pattern, whereas many deep-water, stalked crinoids adopt a harvesting paradigm, reflecting that shallow-water comatulids receive more predatory attacks than do deep-water crinoids. © 1994, Paleontological Society. All rights reserved.

DOI： 10.1017/S0094837300011118

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Country：Japan

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Country：Japan

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