Proposal (626) to South American Classification Committee
Recognize rheas as an order, Rheiformes
The higher classification of the ratites and tinamous has been turbulent through the decades, with just about every permutation possible in terms of ranking schemes. Our current classification treats the Rheidae as a family in the order Struthioniformes, implicitly following many classifications that treat all the ratites in a single order. Our footnote reads as follows:
Recent genetic data (Harshman et al. 2008, Phillips et al. 2010, Smith et al. 2012) indicate that the Struthioniformes is paraphyletic with respect to the Tinamiformes. Cracraft (2013) elevated the rheas to ordinal rank, Rheiformes, as in some older classifications. Proposal badly needed to change SACC classification to reflect this.
Rather than transfer Rheidae to the Tinamiformes, or Tinamidae to Struthioniformes, Cracraft (2013) elevated rheas to the rank of order, along with ostriches, tinamous, and kiwis, and placed cassowaries and emus in the same order. He placed Rheiformes and Tinamiformes adjacent in the linear sequence.
Recommendation: I personally favor recognizing all these ancient lineages at the rank of order regardless of their inter-relationships given their age relative to other taxa ranked as orders, so would vote for this regardless of tinamou-rhea sister relationship. Regardless, the genetic data require treating these two lineages at the same rank, and so I recommend elevating the rheas to the rank of order. Treatment of the rheas in their own order actually follows many classifications, including even the online Merriam-Webster dictionary, so this is not a radical proposal.
CRACRAFT, J. 2013. Avian higher-level relationships and classification: nonpasseriforms. Pp. xxi-xliii in The Howard and Moore Complete Checklist of the Birds of the World, 4th Edition, Vol. 1. Non-passerines (E. C. Dickinson & J. V. Remsen, Jr., eds.). Aves Press, Eastbourne, U.K.
(see SACC Biblio for the others)
Van Remsen, March 2014
Comments from Stiles: “YES. Given the age of all these lineages as well as the distinctive features of the birds themselves, a separate order for the rheas seems eminently reasonable.”
Comments from Nores: “YES. If Ratites do not form a monophyletic group, a separate order for the rheas seems quite reasonable.”
Comments from Zimmer: “YES, for reasons stated in the proposal.”
Comments from Pacheco: “YES. The suggested treatment seems most appropriate instead to gather Tinamous and Rheas in expanded Struthioniformes.”