Abstract

Certain ammonoids changed the mode of coiling or the growth angle of their last body chamber, constricted the terminal aperture, or developed apertural processes, which restricted all life functions. The modified terminal body chamber of macroconchs apparently functioned as a floating egg case for a single breeding phase. The young that hatched from tiny eggs fed on the enclosed female corpse. This same breeding strategy is executed by the extant octopod Argonauta. As a nude cephalopod, the sexually mature female secretes an egg case, which resembles Cretaceous ammonites, for the tiny eggs. The remarkable similarity in mode of breeding between Argonauta and ammonoids with modified terminal body chambers suggests that the ancestral argonautid was a nude ammonoid. Other octopods, which lay large, yolk-rich eggs attached onto substrates, likewise originate from ancestral nude ammonoids, which, however, did not breed in a floating egg case. Nude ammonoids crossed the Cretaceous-Tertiary boundary, as did the genuine coleoids comprising rudimentary endoskeletons.

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