Complete ammonoid jaw apparatuses from the Solnhofen plattenkalks: implications for aptychi function and microphagous feeding of ammonoids
Abstract:Completely preserved jaw apparatuses of the ammonite genera Physodoceras and ? Fontannesiella containing laevaptychi and lamellaptychi, respectively, are described from the Lower Tithonian Solnhofen plattenkalks. In both cases, the unmineralized upper jaw elements are reduced and only constructed by the "inner lamella", while the strongly calcified lower jaw elements (aptychi) reduced the "inner lamella" and enlarged the "outer lamella". A functional analysis of the jaw apparatus of modern cephalopods, in conjunction with additional observations of fossils, particularly from Upper Jurassic plattenkalks (colour pattern, healed injuries), confirms the transformation from an original biting apparatus to an operculum. The prerequisite for this functional change was that the aptychi-possessing ammonites became microphagous. The position of the shell aperture, demonstrated by the pattern of settlement of cirripedian epizoans, and often preserved crop and stomach contents – particularly of oppeliids of the Solnhofen plattenkalks – confirm the planktotrophic feeding of these taxa. Ten arms (tentacles) are presumed to be the plesiomorphic character of all neocephalopods. A possible reduction of the arm-number (? six) is discussed for the Aptychophora as a morphological consequence of the transformation of the jaw apparatus into an operculum.
Document Type: Research Article
Publication date: 2007-07-01
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