Fruit morphology, anatomy and inferred brassicalean affinities of Scalarifructus coloradensis (Knowlton) gen. et comb. nov. from the Eocene of North America and Europe

Background and Aims Distinctive bicarpellate, elongate, capsular fruits from the early to middle Eocene Green River flora of Colorado, Utah and Wyoming, USA, previously named Danaea coloradensis Knowlton 1923, match those of Carpolithus scalariformis Reid and Chandler 1933 from the early Eocene London Clay flora of southern England. We call attention to this biogeographical disjunction, revise the taxonomic nomenclature, and investigate the morphology and potential affinities of the newly recognized genus, Scalarifructus. Methods Fossils were investigated by reflected light and X-ray micro-computed tomography scanning, with external morphology provided by the impression fossils and internal anatomy by the permineralized fruits. Key Results Although specimens from the Green River Formation are preserved as impressions in shale, they match well with the morphology preserved in the pyrite-permineralized specimens from the London Clay Formation, and are considered to be conspecific. Anatomically preserved fruits from the London Clay provide details of internal tissues of the pericarp and seeds, while impression specimens from the Green River give additional features of the exocarp. Scalarifructus coloradensis (Knowlton) gen. et sp. comb. nov. fruits are septicidal capsules bearing two rows of seeds per carpel in parietal placentation, separated longitudinally by a false septum. Conclusions Although these fruits from floras ca. 47–53 Ma do not match any extant genus, and even the familial assignment remains elusive, their morphology is consistent with features found today in the order Brassicales. The occurrence of this taxon in North America and Europe parallels previously recognized disjunctions between the Palaeogene floras of Europe and Cordilleran North America.