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Species and Speciation in the Fossil Record$
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Warren D. Allmon and Margaret M. Yacobucci

Print publication date: 2016

Print ISBN-13: 9780226377445

Published to Chicago Scholarship Online: May 2017

DOI: 10.7208/chicago/9780226377582.001.0001

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Towards a Model for Speciation in Ammonoids

Towards a Model for Speciation in Ammonoids

(p.238) Chapter Eight Towards a Model for Speciation in Ammonoids
Species and Speciation in the Fossil Record

Margaret M. Yacobucci

University of Chicago Press

Ammonoid cephalopods show a remarkably high rate of speciation when compared to other metazoans although the drivers of this evolutionary volatility are not clear. Ammonoid evolution is characterized by high degrees of homeomorphy, heterochrony, and other patterns that reveal a flexible developmental growth program. Ammonoid biodiversity through time appears to be linked to major environmental changes, particularly sea level change. The model of speciation presented synthesizes these observations with contemporary views on ecological speciation, emphasizing the role of developmental flexibility in permitting the rapid production of new anatomical variants that then sort into ecological niches and diverge. In this model, a newly formed habitat space plays host to the rapid endemic radiation of ammonoids from one or a few ancestral species. Anatomical variants are produced via changes in developmental timing and then sort into different niches based on microhabitats within the environment. Assortative mating and disruptive selection lead to reproductive isolation and speciation among these morphs. The same processes will occur each time sea level rises; given developmental constraints on shell form, homeomorphic species will result. More data on the phylogeny, biogeography, ecology, and developmental flexibility of ammonoids, will allow us to test this speciation model in other ammonoid clades.

Keywords:   cephalopods, homeomorphy, heterochrony, sea level, sympatry

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