Does reduction of the eggshell occur concurrently with or subsequent to the evolution of viviparity in phrynosomatid lizards?

• Published in: Biological journal of the Linnean Society Vol. 71; no. 4; pp. 719 - 736
• Main Authors: MATHIES, TOM, ANDREWS, ROBIN M
• Format: Journal Article
• Language: English
• Published: Elsevier Science Ltd 01.12.2000
• Subjects: egg retention; embryonic development; placentation; reptiles; squamates; Urosaurus; placentation; squamates; egg retention; embryonic development; reptiles
• ISSN: 0024-4066; 1095-8312
• DOI: 10.1006/bijl.2000.0471

Viviparity and placentation have evolved many times within squamate reptiles, but the sequence in which the attendant morphological modifications occur remains unclear. In particular, it is unknown whether a reduction of the egg shell occurs concurrently with longer periods of egg retention (i.e. increasingly advanced stages of embryogenesis at oviposition) or whether such thinning occurs after viviparity has evolved. To investigate this question, we evaluated the prediction that shell morphology and permeability vary systematically with the capacity to support embryonic development in utero (as judged by the maximum embryonic stage attainable in utero) in five species of oviparous sceloporine lizards and one lizard species in the sister genus Urosuarus. Despite major differences among species in the capacity to support embryogenesis, shell morphology (structure, thickness) and physiology (permeability to water vapour) did not vary as predicted. These results raise the intriguing possibility that other features associated with simple placentation (e.g. increased oviductal and chorioallantoic vascular density) evolve concurrently with longer periods of egg retention and viviparity and that shell thinning may occur subsequent to the evolution of viviparity, at least in sceloporine lizards.