Abstract Detail

Paleobotany

Elgorriaga, Andres [1], Atkinson, Brian [1].

Unaccounted diversity of Cycad reproductive structures: A permineralized pollen cone from the Late Cretaceous of the Ladd Formation, California, USA.

Cycadales are the sister group to all other living groups of gymnosperms and have a fossil record that extends at least back into the Permian. Except for Cycadaceae, molecular divergence time analyses estimate a Cretaceous origin for most living cycad clades. However, extant generic diversity is estimated to be not much older than Neogene. Thus, the cycad phylogeny has extremely long branches/ghost lineages and their evolution in deep time is poorly understood. Although their fossilized reproductive structures, especially structurally preserved ones, have much potential to help elucidate the early evolution of cycads, such fossils are exceptionally underreported, with most of the cycad fossil record consisting of leaf impressions/compressions. Here we report an anatomically preserved mature pollen cone from the Campanian Holz Shale member of the Ladd Formation located in Silverado Canyon, Southern California, USA. The cone was studied via sectioning, scanning electron microscopy, physical tomography, and phylogenetic analyses. The cone is 2.0 mm wide and six endarch bundles, mainly composed of narrow tracheids with scalariform to reticulate wall thickening patterns, form its vascular cylinder. Helically arranged and imbricate microsporophylls with a rhomboid scutellum are attached to the cone’s axis by a short yet massive stalk, having two pollen sacs partially embedded in the abaxial surface. Although the pollen sacs are mostly dehisced, in-situ pollen was found, being boat-shaped, monosulcate, up to 14–19 μm long, and with foveolate ornamentation. A single vascular bundle enters each microsporophyll and immediately bifurcates, with each daughter bundle providing a small trace for each pollen sac. Daughter bundles extend distally and are eventually replaced by an extensive network of wide transfusion tracheids, with reticulate to pitted wall thickening patterns, extending acropetally throughout the scutellum. Secretory cavities are present in both microsporophylls and the cone’s axis. Microsporophyll and pollen morphology of this cone indicate it is assignable to Skyttegaardia, a recently described genus based on dispersed charcoalified microsporophylls from the Early Cretaceous of Denmark. Possible cycadalean affinities for Skyttegaardia were suggested by previous workers based on microsporophyll and pollen morphology, however, the small number and embedded attachment of pollen sacs, precluded a straightforward cycadalean assignment. However, additional data provided by this new cone, including general cone organization and anatomical features, further indicate cycadalean affinities for Skyttegaardia. Moreover, preliminary phylogenetic analyses recover this genus as a member of crown-group Cycadales, nested within the Zamiaceae. These findings support a Cretaceous diversification of Zamiaceae and indicate that cycads underwent considerable morphological diversification of their reproductive structures. In this sense, Skyttegaardia is evidencing an exceptional case of allometric change that resulted on a diminutive size and low number of pollen sacs per microsporophyll. This previously unaccounted grade of morphological change in a cycadalean suggest that the fossil record possibly contains more unique morphologies, at least as early as the Late Cretaceous, that are yet still to be identified as members of this unique and ancient group. 

1 - University Of Kansas, Ecology And Evolutionary Biology, 1200 Sunnyside Avenue, Lawrence, KS, 66045, United States

Keywords:
Mesozoic
Cretaceous
North America
Cycadales
pollen cone
Anatomy
Evolution
phylogeny.

Presentation Type: Oral Paper
Number:
Abstract ID:860
Candidate for Awards:None