title: Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix.
abstract: A recently improved understanding of gut immunity has merged with current thinking in biological and medical science, pointing to an apparent function of the mammalian cecal appendix as a safe-house for symbiotic gut microbes, preserving the flora during times of gastrointestinal infection in societies without modern medicine. This function is potentially a selective force for the evolution and maintenance of the appendix, and provides an impetus for reassessment of the evolution of the appendix. A comparative anatomical approach reveals three apparent morphotypes of the cecal appendix, as well as appendix-like structures in some species that lack a true cecal appendix. Cladistic analyses indicate that the appendix has evolved independently at least twice (at least once in diprotodont marsupials and at least once in Euarchontoglires), shows a highly significant (P < 0.0001) phylogenetic signal in its distribution, and has been maintained in mammalian evolution for 80 million years or longer.
authors.affiliation: Department of Basic Medical Sciences, University of Arizona College of Medicine-Phoenix, in Partnership with Arizona State University, Phoenix, AZ, USA.
authors.forename: H F
authors.initials: HF
authors.lastname: Smith
authors.forename: R E
authors.initials: RE
authors.lastname: Fisher
authors.forename: M L
authors.initials: ML
authors.lastname: Everett
authors.forename: A D
authors.initials: AD
authors.lastname: Thomas
authors.forename: R Randal
authors.initials: RR
authors.lastname: Bollinger
authors.forename: W
authors.initials: W
authors.lastname: Parker
mesh_terms: D000818:Animals; D002432:Cecum / Q000033:anatomy & histology* / Q000382:microbiology; D008322:Mammals / Q000033:anatomy & histology* / Q000145:classification*; D010802:Phylogeny