autapomorphy
noun
au·tap·o·mor·phy ȯ-ˈta-pə-ˌmȯr-fē 
plural autapomorphies
biological taxonomy
: a specialized character or trait that is unique to a monophyletic taxonomic group
… An eversible penis is a synapomorphy that unites lizards and snakes in a common monophyletic group, the Squamata, but is an autapomorphy of the Squamata within a larger monophyletic assemblage.
Robert L. Carroll, Patterns and Processes in Vertebrate Evolution, 1997
Robert L. Carroll, Patterns and Processes in Vertebrate Evolution, 1997These species share key traits as autapomorphies of their clade—and since these traits help to construct the relevant environment, sufficient similarity becomes, in part, an active construction of related organisms, not only a happenstance of common externalities. Stephen Jay Gould, The Structure of Evolutionary Theory, 2002
Stephen Jay Gould, The Structure of Evolutionary Theory, 2002 autapomorphic
ȯ-ˌta-pə-ˈmȯr-fik
adjective or less commonly autapomorph
ȯ-ˈta-pə-ˌmȯrf
These features are not known for any Jurassic and Cretaceous mammals and are autapomorphic for this new mammalian lineage. Zhe-Xi Luo and John R. Wible, Science, 1 Apr. 2005
Zhe-Xi Luo and John R. Wible, Science, 1 Apr. 2005 autapomorphous
ȯ-ˌta-pə-ˈmȯr-fəs
adjective The Chondrichthyes are a monophyletic clade and are principally distinguished on the basis of two unique autapomorphous character sets. Eileen D. Grogan and Richard Lund, in Biology of Sharks and their Relatives, 2004
Eileen D. Grogan and Richard Lund, in Biology of Sharks and their Relatives, 2004 
Word History
Etymology
autapomorph (borrowed from German, from aut- aut- + apomorph "apomorphic") + -y entry 2
First Known Use
1959, in the meaning defined above