Article: Morphology and phylogenetic significance of the angiosperm Platanites hebridicus from the Palaeocene of Scotland
Fossil platanoid leaves from the Palaeocene of Mull, north-west Scotland, are assigned to Platanites hebridicus Forbes. The leaves closely resemble those of extant Platanaceae, and differ only in being pinnately compound. Each leaf consists of a shallowly three-lobed terminal leaflet and two smaller asymmetric lateral leaflets. Reproductive structures associated with the fossil foliage are also similar to those of extant Platanaceae, and the only unequivocal differences are the ellipsoidal achene shape and the smaller number of achenes per fruiting head in the Palaeocene material. The combined information from leaves and reproductive structures establishes the 'P. hebridicus plant' as one of the most completely understood fossil Platanaceae. The recognition of pinnately compound leaves in this critical angiosperm group has important implications for understanding the early divergence of major clades within the dicotyledons. During the mid-Cretaceous, simple, palmate platanoid leaves and pinnately compound Sapindopsis leaves exhibit partially intergrading patterns of venation and cuticular structure, and this has been used to suggest a close phylogenetic relationship between these early primitive representatives of the extant dicotyledonous subclasses Hamamelidae and Rosidae. The occurrence of extinct Platanaceae with compound leaves adds to the similarities between platanoid and Sapindopsis foliage, strengthens the proposed close relationship between the Platanaceae and 'Rosidae, and highlights the need to clarify relationships within the mid-Cretaceous platanoid-Sapindopsis complex.