| Method |
The minimum constraint on the mammal-bird split, equivalent to the minimum age of the origin of clade Amniota corresponds to the age of the Joggins Formation. This is 314.5 – 313.4 Ma ± 1.1 Myr, a date based on biostratigraphy (palynology) and exact dating from elsewhere, conferring a minimum constraint of 312.3 Ma. The soft maximum constraint on the bird-mammal split is based on the richly fossiliferous units such as the East Kirkton locality, source of a diverse fauna of batrachomorphs and reptiliomorphs (see human-toad split BNID 105157), but that has hitherto not yielded anything that could be called either a diapsid (group containing extant reptiles and birds) or a synapsid (group containing extant mammals). Further fossiliferous sites of similar facies lie below the East Kirkton level, and they have not yielded reptile remains. Researchers take the age of the fossiliferous Little Cliff Shale of the East Kirkton locality (Brigantian 328.8-326.4 Ma ± 1.6 Myr) as the basis for the soft maximum age constraint of 330.4 Ma. |