Trunko Internal Anatomy

Serinan tentacle birds have evolved highly novel facial musculature, unique among birds, that controls their highly distinctive appendages. Though in modern species the structures are extremely large and prominent, they had very slight beginnings. Softbills evolved their complex soft tissue originally from lip-like gape flanges on either side of the canary's mouth, which over tens of millions of years expanded to form a fleshy sheath over the nostrils, providing a surface for the formation of a mucous membrane to increase their sense of smell, and then over most of the beak, allowing a heightened sense of touch, since the earliest forms were nocturnal and nearly blind. 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 not just in the skin but down to the skull, and requiring some changes to the underlying bones to accommodate them.

Trunkos (the mammoth trunko is shown here) are now among the most advanced 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.

Softbilled birds do not actually have soft beaks, rather their term originates from the ancestral forms having all but the tip of the beak covered in a fleshy sheathe. Trunkos have taken this to an extreme and fully internalized the beak, but it is still present and well-developed, used for tearing food very effectively and even capable of a degree of chewing made possible by cranial kinesis, or movements of the upper jaw bones to grind against the lower jaw. The fleshy facial structure has given them lips, keeping chewed food from falling out of their mouths. 

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. This structure is not muscular, and primarily formed of thin cartilage and skin. 

The structure of the characteristic trunko neck pouch is also illustrated. It is basically a muscular bag; a sphincter muscle has evolved, now able to seal the egg safely inside like the pull string of a change purse, and smaller muscles coming off the neck serve to stiffen and support the pouch.