Oral Paper

         Paleobotany

Reexamination of Arthmiocarpus hesperus from the Late Cretaceous of South Dakota: expanding the fossil record of bisexual climbers in Araceae

Presenting Author
Jeronimo Morales Toledo
Description
The study and re-evaluation of Cretaceous plant fossils is an opportunity to understand the early evolution of angiosperms, prior to the establishment of modern ecosystems. The most diverse family within Alismatales, Araceae, are the sister group of all other extant monocots (other than Acorales) and have a fossil record that can be traced back to the Early Cretaceous (110 to 120 Ma). However, the only confidently assigned aroid fossils prior to the Cretaceous-Paleogene (K-Pg) mass extinction come from the early divergent lineages (i.e., Orontioideae and Lemnoideae), obscuring our complete understanding of the historical evolution of the aroid subfamilies in deep time. In this study, we investigated the morphology, anatomy, and taxonomy of Arthmiocarpus hesperus (Wieland) Delevoryas, a permineralized fossil from the Late Cretaceous Fox Hills Sandstone, Cottonwood Creek, South Dakota. This specimen was previously interpreted as a drupe from the fig family (Moraceae), but shows features consistent with Araceae. Thus, reinvestigating the morphology and classification of this fossil could provide greater understanding of the aroid evolutionary history. Thin sectioning and X-ray micro-computed tomography (μCT) were used to re-examine this species’ type material, resulting in 3D reconstructions of the seed and characterization of structures in multiple planes of section. This allowed us to resolve conflicting interpretations of fruit-seed morphology and anatomy, and to identify additional characters useful in refining potential taxonomic affinities. The helically arranged sessile berries on a central spadix confirm affinities to Araceae, and features of the fruits and seeds such as berries that are hexagonal in cross section, fibrous pericarp, a smooth seed coat, and a monocotyledonous L-shaped embryo suggest it is most closely related to members within the subfamily Monsteroideae. However, Arthmiocarpus presents a unique set of characters that differentiate it from extant and extinct members of Araceae. This study is an example of how the subfamilies of extant angiosperms were constituents of Cretaceous ecosystems prior to the K-Pg mass extinction and the establishment of modern ecosystems.