Premise of research. Small angiosperm fossils are diverse in Early Cretaceous mesofossil floras from Portugal and eastern North America. Investigations of these fossils have revealed an unexpectedly high diversity of extinct angiosperms related to lineages that are now species poor, such as Austrobaileyales, Nymphaeales, and Chloranthaceae. Here we analyze Early Cretaceous fruits and seeds from Portugal that are related to eumagnoliid angiosperms and that are also important for understanding extinct diversity in early angiosperms.

Methodology. The fossils were prepared by sieving in water; cleaned with HF, HCl, and water; and studied using scanning electron microscopy and synchrotron radiation X-ray tomographic microscopy. The systematic conclusion based on comparative studies was tested in a phylogenetic analysis.

Pivotal results. We recognize a new group of angiosperms based on fruits and seeds united by features that are otherwise unusual among angiosperms. Two genera, Serialis and Riaselis, are established and 10 species described. Both have unilocular fruiting units formed from a single carpel. Serialis has fruits with two or more seeds, while fruits of Riaselis are always one seeded. In Serialis, seeds are permanently attached to each other and dispersed as a unit. Both genera have anatropous and mesotestal-endotestal seeds with a tiny embryo and a distinctive vasculature in the testa extending from the hilum to the chalaza and then also on the antiraphal side to the micropyle. The fossils are most similar to seeds of Magnoliales but also share some features with seeds of Austrobaileya.

Conclusions. Serialis and Riaselis are the earliest fossils that can be assigned to the Magnoliales but are sufficiently different from those of all Magnoliales that they cannot be assigned to any extant family. Serialis and Riaselis provide further documentation of extensive extinct diversity among early angiosperms, and their abundance in the mesofossil floras suggests that they were common and widespread in early angiosperm communities.