(descriptions from Nicol 2015a, and as noted)
(Morrow and Nicol 2012; Nicol 2015a, and as noted)
(Augee et al. 2006; Rismiller and McKelvey 2009; Nicol 2015a, and as noted)
"When we look at the details of echidna biology, we are not looking at a 'living fossil' that has failed to join modern mammals such as ourselves in the 'fast lane', but [rather, the echidna has] found a niche in the 'slow lane' that is so successful that it has remained there for millions of years."
— Augee et al. 2006, Echidna: Extraordinary Egg-laying Mammal