Many have wanted to see how the facial anatomy of these animals looks under the surface. Trunkos (mammoth trunko shown here) are the most advanced/derived softbilled birds. They have one very hyptertophied tentacle on the upper jaw and one smaller prehensile one on the lower jaw, a reduction from the six-armed radial faces of their earlier ancestors. Trunkos have evolved large flanges of bone on either side of their heads to which a large number of complex muscles attach, supporting the powerful upper trunk; it is these flanges that gives them their characteristic "cheeks", and not their jawlines which are set more forward and behind the flanges. An additional attachment point for the appendage is the front of the skull which has become flattened and slightly concave, with a smaller band-like muscle pair wrapping entirely around the skull just over the eye sockets. The lower tentacle is controlled by muscles that wrap around the lower back of the skull.

Trunkos, and all other tentacled softbilled birds, evolved their complex soft tissue originally from lip-like gape flanges on either side of the mouth which over tens of millions of years expanded to form a fleshy sheath over most of the beak, allowing a heightened sense of touch. Small papillae evolved over the surface of this bill sheath and it is from these which movable tendrils originally evolved. The muscles that moved them originally were repurposed from the minute muscles present in the skin of all birds which served to move the feather follicles on their faces so as to extend or retract the plumage. Before they connected to the earliest small fleshy protrusions on the snout they controlled the whisker-like quills used by the trunko's ancestors to feel around in the dark. As the facial appendages grew in size and complexity, their muscular attachments became equally exaggerated to support them, anchoring all around the head and requiring some changes to the underlying skull.

This is the first I have illustrated the skull of any softbilled bird, and hopefully it clears up a misconception I have heard a lot lately that the trunk IS the beak. The beak, normally hidden, is still relatively normal, though it has been internalized as the tentacles have gotten larger and the face obscured by heavy soft tissue, only visible when the mouth is opened.

The mammoth trunko also has a large nasal cavity formed entirely by soft tissues which is used to warm cold air and filter dust before reaching the lungs.

The structure of the neck pouch is also illustrated. It is basically a muscular bag, a sphincter operates the opening to seal the egg safely inside and smaller muscles coming off the neck serve to stiffen and support the pouch.