This animal represents a more derived form, on the amniote stem lineage, but where there is not yet the complex lung and trachea seen in amniotes. The spiral valve in the conus arteriosus is shown here in more detail. The ventricle is shown as being undivided: although all extant amniotes have at least a partial ventricular septum, this is likely not homologous between the major amniote groups (mammals, lepidosaurs, and archosaurs), so would not have been present in their common ancestor. This animal is shown as retaining a truncus arteriosus as the system of separation of the major arteries leaving the heart independently is not strictly homologous among amniotes (the mammal condition cannot be derived from the sauropsid [reptiles and birds] condition of two separate systemic arches), and so the common amniote ancestor likely retained this more primitive condition. The posterior cardinal veins are shown as lost (reduced to azygous and hemiazygous veins), as is the condition in all amniotes.
© 2007-2010 Christine Janis, Professor of Ecology and Evolutionary Biology
Colleen Brogan '10, Student Technology Assistant
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