Sid Mosdell/Flickr (CC BY 2.0)

A rare New Zealand reptile doesn't have a penis -- but it may help explain phallic evolution.

Researchers at the University of Florida in Gainesville, led by Thomas Sanger, found that the tuatara develops tiny nubbins as an embryo.


But the development of these nubbins stalls and they never forms into a proper penis. This initial growth, researchers have now claimed, suggests the phallus developed only once throughout the evolution of mammals and reptiles.

Sanger's team used 82 Victorian microscope slides of tuatara embryos to examine the phalluses. The slides were digitally cleaned and compiled into a 3D image -- which revealed the paired nubbins characteristic of tuataras. In reptiles, these nubbins grow into pairs of insertable organs, and in mammals the two buds fuse to form a penis.

The nubbins represent an early trace of the phallic developmental process that has long been up for debate -- whether or not tuataras 'lost' an organ or whether other reptiles 'gained' a pair.

This research indicates that the tuataras lost a phallus -- indicating that the basic penis evolved only once.