This represents a hypothetical “proto-mammal” condition. All living mammals have lost the right systemic arch (= right aorta), at least as a complete structure, but parts of the original arch have been incorporated into the right brachiocephalic artery (leading to the right arm and right carotid artery going to the head). Thus the ancestral form likely retained a truncus, as shown here with two complete systemic arches (as seen in mammalian embryology). This animal is also shown as retaining a conus arteriosus with a spiral valve: in mammals, this has been incorporated into the base of the aorta. Exactly when these features were changed to the more derived mammalian condition is unknowable, but it is unlikely to have been the situation in the pelycosaur shown here. Most likely this accompanied the transition to endothermy as evidenced in derived therapsids (cynodonts).
© 2007-2010 Christine Janis, Professor of Ecology and Evolutionary Biology
Colleen Brogan '10, Student Technology Assistant
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