Middle: Castorocauda lutrasimilis (Docodonta), Shenshou lui (Haramiyida); Bottom: Tachyglossus aculeatus (Monotremata), Ursus arctos (Theria).

In particular, trait-based taxonomy generally includes Adelobasileus and Sinoconodon in Mammalia, though they fall outside the Mammaliaformes definition.

To compensate, mammals developed prismatic enamel, characterized by crystallite discontinuities that helped spread out the force of the bite.

[6] Lactation, along with other characteristically mammalian features, is also thought to characterize the Mammaliaformes, but these traits are difficult to study in the fossil record.

[7] Combined with the more basal tritylodontids that also display evidence of lactation,[8] this seems to imply that milk is an ancestral characteristic in this group.

However, the fairly derived Sinoconodon appears to have uniquely discarded milk altogether[citation needed].

Prior to hatching, the milk glands would provide moisture to the leathery eggs, a situation still found in monotremes.

[22] Tritylodontidae Pachygenelus Adelobasileus Sinoconodon Megazostrodon Morganucodon Haldanodon Castorocauda Thomasia Haramiyavia Megaconus Shenshou Eleutherodon Sineleutherus Arboroharamiya Xianshou Shuotheriidae Monotremata Fruitafossor Eutriconodonta Tinodon Multituberculata expanded below Expanded from above Spalacotheriida Henkelotherium Dryolestes Amphitherium Peramus Vincelestes Nanolestes Kielantherium Aegialodon Marsupialia Placentalia Cladogram based on Rougier et al. (1996)[23] with Tikitherium included following Luo and Martin (2007).

[24] Tritylodontidae Adelobasileus Sinoconodon Morganucodon Tikitherium Haldanodon Castorocauda Ornithorhychus (platypus) Tachyglossidae (echidna) Triconodonts, multituberculates,marsupials, and placentals