Tachyglossus aculeatus (Shaw 1792) is the echidna and the only other genus of egg-laying mammal. It protects itself with sharp spines and has a long, ant-catching tongue. The hands and feet are adapted to digging with short, almost immobile proximal elements and long claws. Prepubic bones precede the pubis. A proximal process sits atop the fibula. The leathery snout without whiskers is sensitive to vibrations.

Distinct for its sister, Ornithorhynchus, and many other mammals, the acetabulum is perforated. The lateral temporal fenestra is absent. So are the teeth. Like the hedgehog, the echidna can roll itself into a ball for protection.

Cifelliodon wahkarmoosuch (Huttenlocker et al. 2018; Early Cretaceous, 126 mya) was originally considered a sister to Vintana nesting within the Multituberculata, but here nests with Tachyglossus as an ancestor echidna. It has fewer teeth that do not erupt above the alveoli, leading to toothlessness in echidnas. The pterygoid is deeper than the mandible in both.

Fruitafossor windscheffeli (Luo and Wible 2005; Late Jurassic) was originally considered a digging basal mammal based on the shape of the scapula and sprawling forelimbs. Here Fruitafossor nests as a basal echidna from Colorado, prior to tooth loss. The teeth are blunt, as in xenarthrans, and the four fingers (perhaps five originally) have broad, digging claws with short phalanges. The torso was wider than deep with a long lumbar area that included xenarthran (interlocked) vertebrae. The skeleton was µCT scanned (see above).