Montsechia vidalii from the Barremian of Spain, the earliest known submerged aquatic angiosperm, and its systematic relationship to Ceratophyllum

Montsechia vidalii is an abundant plant fossil from the Barremian of northeastern Spain. Systematic affinities have been suggested with several living plant clades (liverworts, horsetails, conifers, Gnetales), particularly with angiosperms. We provide detailed descriptions, documentation, and discussion of morphology and histology based on hand specimens of isolated leafy stems with fruits examined under the stereomicroscope, and with light and scanning electron microscopy. Montsechia vidalii shows shoots with flexible axes of two phyllotaxies and leaf morphologies. One morphology has opposite-decussate leaves and stems, with small linear leaves. The other has spirally arranged leaves and stems, with inconspicuous leaves. The cuticle is thin and shows anomocytic stomata. Some stems bear terminal pairs of indehiscent fruits, probably developing from bicarpellate reproductive complexes, probably corresponding to two simple flowers each with a single carpel and arranged in indeterminate inflorescences. Every fruit contains a single seed and shows both similarities to the uniovulate ascidiate carpel that has been reconstructed as ancestral in angiosperms and derived features such as orthotropous, pendent and unitegmic ovules that are shared with the aquatic living genus Ceratophyllum. Phylogenetic analyses confirm that these similarities reflect a systematic relationship to Ceratophyllum and relatives. This and other recently described fossils, in particular Pseudoasterophyllites, suggest that this clade was diverse and ecologically significant in the Early Cretaceous. Flexible axes, thin cuticles, low number of stomata, and indehiscent fruits, as well as sedimentological and taphonomic evidence, strongly suggest a freshwater habit for M. vidalii.