Evolution of the Amniotic Egg Presented by: Teaire Carmichael & Zamyra C. Lambert

Embryo Retention: What is it? Embryo Retention is the mechanism by which the embryo sustains viability. Most recent studies suggest that early amniotes lacked extended embryo retention. The evolution of the amniotic egg is somewhat a mystery.

The 411 on the Amniotic Egg The earliest known records of amniotes dates back to 310 million years ago. Oldest known fossils date back to the Upper Triassic, but the nest suggest possible existence in the Lower Triassic. Due to poor preservation of eggs that lack mineralized external shell. The origin of the amniotic egg must be studied indirectly.

Approach A Adopted by Laurin Used optimization by squared change parsimony, independent contrast, and permuted multiple linear regressions.

Approach B Used by Laurin & Reisz, Wilkinson & Nussbaum, Laurin & Girondot, Laurin et al. and Wilkinson et al. Optimized the presence or absence of extended embryo retention to test Lombardi’s hypothesis. Lombardi’s hypothesis- some structures of amnion and serosa evolved in response to selective pressure for hypertrophied respiratory exchange surfaces in an embryo-retaining ancestor.

Conclusion of Approach A & B Hypothetical ancestors of amniotes probably didn’t exhibit extended embryo retention, thus rejecting Lombardi’s hypothesis. Based on the widely accepted phylogeny of sarcopterygians. The presence or absence of extended embryo retention in the ancestors of amniotes cannot be known with the available data and analytical techniques.

Character 1 & Character 2 Character 1 (extended embryo retention): Requires three steps over the reference phylogeny, and parsimony suggests that the ancestral amniote lacked extended embryo retention. Character 2 (embryonic stage at oviposition): requires 9 steps over the referred phylogeny. The optimization suggest ancestral amniotes laid eggs at the gastrula development stage, which is equivalent to the absence of extended embryo retention.

Embryonic stage at Oviposition

Extended Embryo Retention

Extended Embryo Retention

Charts

Stages of Developement

Conclusion Character optimization cannot be expected to yield reliable info about the ancestral condition of the various nodes of the tree for Character 1 or 2.

Origin of the Amniotic Egg Long considered the key adaptation of the amniotes. Enables amniotes to serve their reproductive ties with water. Referred to as a terrestrial egg Laurin and Reisz proposed that extraembryonic membranes evolved in a embryo-retaining form, as a pathway for fetal-maternal interaction rather than for protection.

The parts of the Egg The amnion surrounds the embryo and creates a fluid-filled cavity in which the embryo develops. The chorion forms a protective membrane around the egg. The allantois is closely applied against the chorion, where it performs gas exchange and stores metabolic wastes.

Work Cited Laurin, Michael. Embryo retention, character optimization, and the orgin of the extra-embryonic membranes of the amniotic egg. 10 August 2005. Wilkinson, Mark, Oommen, V. , Richardson, M. , Wilkinson, M. Extended embryo retention, caecilian oviparity and amniotic orgins. 5 June 2001. Laurin, Michel and Jacques A. Gauthier. 1996. Amniota. Mammals, reptiles (turtles, lizards, Sphenodon, crocodiles, birds) and their extinct relatives. Version 01 January 1996. http://tolweb.org/Amniota/14990/1996.01.01 in The Tree of Life Web Project, http://tolweb.org/

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