Fig. 5.

Interior surface structures. (A) Digital PPC-SR-μCT section slicing vertically through the main funnel opening shown in Fig. 1A. (B) Magnification of the marked area in A, showing interior surface pit structures. (C) Scanning electron micrograph of the big opening, showing the same interior surface pit structures as in B. (D and E) Close-up views of these interior surface structures of C, showing details of pits and surrounding features. A reticular structure surrounds the regularly spaced pits, which often display clearly preserved circular collars (red arrowheads).