Figure S1. Photographs of specimens AUM 5017 (A; a hatchling platypus) and AUM 5014 (B; an echidna soon after hatching).

Figure S2. Additional CT scan reconstruction of monotreme specimens, including postcranial skeleton of platypus SAM 646 (A), the spine of platypus specimen AUM5017/2 (B), and a low-resolution medical CT scan of Echidna SAM 9591 (C).

Figure S3. Three-dimensional reconstructions of specimens of Vombatus ursinus (A) and Macropus eugenii (B).

Figure S4. Camera lucida drawing of a clear-stained specimen of Monodelphis domestica.

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