An Aptian angiosperm leaf mistaken for a cycad reveals an unexpected extinct line in the Early Cretaceous radiation of angiosperms

COIRO, MARIO; DOYLE, JAMES; UPCHURCH, GARLAND; MARTINEZ, LEANDRO CARLOS ALCIDES

Tucson

Conferencia; Botany Conference 2019; 2019

A conspicuous element in the Early Cretaceous macrofossil record of angiosperms (particularly in the Aptian and Albian) consists of lobed leaves, often ternately organized, with generally low-complexity fine venation (e.g., Vitiphyllum, Vernifolium, Crataegites, Jixia, Sujfunophyllum). Such leaves have been traditionally compared with eudicots, particularly the early-divergent clade Ranunculales. They appear to be distinct from Albian Sapindopsis and ?platanoid? leaves (Proteales), which show different patterns of lobation and higher-order venation. In some cases tricolpate pollen is not known in palynofloras from the same beds, resulting in an apparent conflict with the pollen record. During an investigation of leaf anatomy and the fossil record of cycads, we examined the late Aptian foliage fossil Mesodescolea S.Archang., from the Anfiteatro de Ticó and Punta del Barco Formations of the Baqueró Group in Patagonia, Argentina. Mesodescolea was originally interpreted as a pinnate leaf in which the pinnules have a midrib, anastomosing venation, and a submarginal vein. It was assigned to the cycads and linked particularly with the extant monotypic genus Stangeria (Zamiaceae), a South African endemic with highly divergent cuticular micromorphology. New observations on the cuticles of Mesodescolea and Stangeria lead us to reject a close relationship between the two genera. Instead, the cuticle of Mesodescolea has typical angiosperm characters, in particular guard cells with poles at the same level as the stomatal aperture and a conspicuous outer vestibulum. The stomatal complexes are laterocytic to variable, with a pattern of variation reminiscent of many extant early-divergent angiosperms and fossils from Zone I (Aptian-early Albian) of the Potomac Group. Another feature shared by Mesodescolea and these taxa is structures previously interpreted as trichome bases, now identified as radiostriate idioblasts (probable oil cells). Closer examination of micromorphology and venation architecture shows that Mesodescolea is a pinnately lobed leaf with low-complexity tertiary venation, more or less as in the Aptian-Albian lobed leaves considered early eudicots. A phylogenetic analysis suggests that Mesodescolea is most closely related to either Austrobaileyales or Chloranthaceae, and not to eudicots. These results suggest that Mesodescolea and possibly other Early Cretaceous lobed leaves belong to a previously unrecognized extinct line and should not be assigned to the eudicots without supplemental evidence. They also imply that the evolution of leaf morphology in early angiosperms was more complex than previously thought.