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Sexual dimorphism in Nautilus from Palau

Published online by Cambridge University Press:  08 April 2016

W. Bruce Saunders  and
Claude Spinosa
W. Bruce Saunders
Affiliation:
Department of Geology, Bryn Mawr College; Bryn Mawr, Pennsylvania 19010
Claude Spinosa
Affiliation:
Department of Geology and Geophysics, Boise State University; Boise, Idaho 83725
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Abstract

Shells of 375 sexed specimens of Nautilus cf. N. pompilius trapped at 300–700 ft (90–215 m) depth off Mutremdiu Point and Ngemelis Island, Palau, exhibit distinctive dimorphism in shell proportion and overall size. Of 221 mature animals, males have larger shells (mean diameter = 209 mm), a broader aperture (mean maximum width = 99 mm; mean width at apertural salient = 92 mm) and greater weight (mean shell + body weight = 1426 gm). Female shells are smaller (mean diameter = 198 mm), narrower (mean maximum width = 91 mm; mean apertural salient width = 81 mm) and the overall weight is less (mean = 1157 gm). Dimorphic differences are not apparent in young shells (less than 180 mm diameter), but develop during the final ½ to ¼ whorl. This is accompanied by development of the spadix in males, indicating that shell dimorphism reflects sexual maturity. Of 375 animals trapped, 28% were female; no depth segregation was apparent. Traditional identification of larger fossil dimorphic cephalopods as females and smaller forms as males is the reverse of observed dimorphism in Nautilus and should be discontinued in favor of macroconch and microconch designations.

Type
Research Article
Information
Paleobiology , Volume 4 , Issue 3 , Summer 1978 , pp. 349 - 358
Copyright
Copyright © The Paleontological Society 

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