A NEW DEFINITION AND A LECTOTYPIFICATION OF THE GENUS COOKSONIA LANG 1937

The genus Cooksonia Lang 1937 includes some of the earliest land plants. Specimens of Cooksonia pertoni Lang 1937 are considered the earliest Eutracheophytes. The definition of the genus is thus central to the delineation of the clade. However, the generic diagnosis is problematic. It is not restrictive enough, and most of the few diagnostic characters are plesiomorphic. Observations on new specimens of Cooksonia paranensis Gerrienne et al. 2001, a species very close to C. pertoni, considered along with a compilation of the Cooksonia literature, allow us to propose more precise diagnostic characters. An allometric study was performed on more than 100 specimens of C. paranensis. This study allows discrimination of true morphological variations from growth stages. The growth habit of Cooksonia is discussed. An emended diagnosis including apomorphic characters is given for the genus, as well as a lectotypification of the genus and the type-species.