Stomata in fossil monocots

Leaf and inflorescence evidence for near-basal Araceae and an unexpected diversity of other monocots from the late Early Cretaceous of Spain

Sender L. M., Doyle J. D., Upchurch G. R. Jr., Villanueva-Amadoz U., Diez J. B. (2019)
Luis Miguel Sender, James A. Doyle, Garland R. Upchurch Jr, Uxue Villanueva-Amadoz, José B. Diez,

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Journal of Systematic Palaeontology 17(15): 133-1346 – ISSN: 1477-2019 (Print) – 1478-0941 (Online 2018) – https://doi.org/10.1080/14772019.2018.1528999 –

https://www.tandfonline.com/doi/full/10.1080/14772019.2018.1528999

Abstract

Phylogenetic analyses imply that monocots were a key group in the early radiation of angiosperms, yet they are much rarer than other major clades in the Early Cretaceous macrofossil record.

Here we describe a well-preserved leaf and several inflorescences related to the near-basal monocot family Araceae and abundant monocot leaves of uncertain affinities from two latest Albian localities in north-eastern Spain. Orontiophyllum ferreri sp. nov. has a multistranded midrib, several orders of parallel-pinnate veins, two orders of transverse veins, and paracytic-oblique stomata. This suite of characters (but with both anomocytic and paracytic-oblique stomata) is characteristic today of Orontium in the near-basal araceous subfamily Orontioideae, and later Cretaceous and early Cenozoic leaves assigned to Orontiophyllum have similar architecture.

Sedimentology and anatomy suggest a (semi)aquatic ecology. Other monocot leaves at the same locality are linear and parallel-veined but have similar stomata. Although anomocytic stomata have been proposed as ancestral in monocots, O. ferreri, the associated linear leaves, Albian–Cenomanian cuticles from Australia and Portugal, and extant data are consistent with the hypothesis that variable paracytic-oblique stomata are ancestral. 

Turolospadix bogneri gen. et sp. nov., from the other locality, includes spadices of ebracteate flowers with four tepals, a central gynoecium, and a long stipe (vs a spathe attached just below the fertile zone as in most Araceae). Phylogenetic analyses indicate that the character combinations seen in O. ferreri and T. bogneri are ancestral for Araceae, and they could be either sister to Araceae or nested within a basal grade of the family. Together with fossils from the Aptian–Albian of Brazil and Portugal, the Spanish fossils indicate that Araceae are among the oldest extant monocot families, but they were associated with diverse linear-leaved monocots of uncertain affinities.