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The mode of life in ammonoids

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Abstract

The following structural features clearly indicate that ammonoid shells were adapted to withstand considerably higher hydrostatic pressures thanNautilus shells: (1) the corrugated and marginally fluted septa gave the shell wall efficient support against implosion; (2) the secondary connecting rings could grow a great deal in thickness; and (3) the last formed chambers remained full of liquid which supported the last septum. On the basis of the following characters it is concluded that ammonoids were incapable of swimming efficiently by jet-propulsion: (1) the retractor muscles were weakly developed; (2) the life position was unstable and highly variable; and (3) in animals with a ventral apertural rostrum the hyponome was probably absent.

Ammonoids are considered here as having been pelagic cephalopods which lived in the upper 1000 m of the oceans, and which probably undertook considerable diurnal vertical migrations, similar to those inSpirula. Only some groups may have adopted a life in shallow epicontinental seas. In the late Mesozoic, ammonoids have been replaced by modern oceanic squids which are extremely numerous in the corresponding pelagic environment.

Zusammenfassung

Folgende strukturelle Eigenschaften weisen darauf hin, daß die Ammonitengehäuse angepaßt waren, einen viel höheren Wasserdruck auszuhalten als dieNautilus- Gehäuse. 1. die gefalteten Septen verstärkten die Gehäusewand wirkungsvoll gegen den Wasserdruck; 2. die sekundären Siphonalhüllen hatten ein stärkeres Dickenwachstum; und 3. die zuletzt gebildeten Kammern blieben flüssigkeitsgefullt und schützten daher das letzte Septum. Auf Grund folgender Eigenschaften ist anzunehmen, daß die Ammoniten zu aktivem Schwimmen weitgehend unfähig waren: 1. die Retraktormuskeln waren nur schwach ausgebildet; 2. die Schwimmstellung der Tiere war instabil und variabel; und 3. den Tieren mit einem ventralen aperturalen Rostrum fehlte wahrscheinlich ein Trichter.

Die Ammoniten werden daher als Cephalopoden angesehen, die pelagisch in den oberen 1000 m der Ozeane lebten und die wahrscheinlich bedeutende tägliche vertikale Migrationen unternahmen, etwa analog denen der rezentenSpirula. Nur einzelne Gruppen mögen an das Leben in flachen epikontinentalen Meeren angepaßt gewesen sein. Am Ende des Mesozoikums wurden die Ammoniten durch moderne Tintenfische ersetzt, die heute äußerst zahlreich in einer entsprechenden pelagischen Umgebung leben.

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Correspondence to Harry Mutvei.

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Mutvei, H. The mode of life in ammonoids. Paläontol. Z. 49, 196 (1975). https://doi.org/10.1007/BF02987658

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Keywords

  • Neutral Buoyancy
  • Body Chamber
  • Retractor Muscle
  • Nacreous Layer
  • Life Position