Flowering after disaster: Early Danian buckthorn (Rhamnaceae) flowers and leaves from Patagonia

Abstract

Southern-Hemisphere terrestrial communities from the early Paleocene are poorly known, but recent work on Danian plant fossils from the Salamanca Formation in Chubut Province, Argentina are providing critical data on earliest Paleocene floras. The fossils described here come from a site in the Salamanca Formation dating to ca. 1 million years or less after the end-Cretaceous extinction event; they are the first fossil flowers reported from the Danian of South America, and possible the entire Southern Hemisphere. They are compressions and impressions in flat-laminated light gray shale, and they belong to the family Rhamnaceae (buckthorns). Flowers of Notiantha grandensis gen. et sp. nov. are pentamerous, with distinctly keeled calyx lobes projecting from the hypanthium, clawed and cucullate emarginate petals, antepetalous stamens, and a pentagonal floral disk that fills the hypanthium. Their phylogenetic position was evaluated using a molecular scaffold approach combined with morphological data. Results indicate that the flowers are most like those of extant ziziphoid Rhamnaceae. The associated leaves, assigned to Suessenia grandensis gen. et sp. nov. are simple and ovate, with serrate margins and three acrodromous basal veins. They conform to the distinctive leaves of some extant Rhamnaceae in the ziziphoid and ampelozizyphoid clades. These fossils provide the first unequivocal megafossil evidence of Rhamnaceae in the Southern Hemisphere, demonstrating that Rhamnaceae expanded beyond the tropics by the earliest Paleocene. Given previous reports of rhamnaceous pollen in the late Paleogene and Neogene of Antarctica and southern Australia, this new occurrence increases the possibility of high-latitude dispersal of this family between South America and Australia via Antarctica during the Cenozoic.

Fossil flowers from the early Palaeocene of Patagonia, Argentina, with affinity to Schizomerieae (Cunoniaceae)

“…The fossils described here are the second taxon of Danian flowers known from the Southern Hemisphere, after Notiantha Jud, Gandolfo, Iglesias & Wilf (Rhamnaceae), from a different locality in the Salamanca Formation ( Jud et al , 2017 ). Lacinipetalum spectabilum is the oldest reliable occurrence of crown-group Cunoniaceae, and it is most likely sister to the extant Schizomerieae because it shares some synapomorphies of that tribe.…”

“…The Las Flores locality, where two specimens of Lacinipetalum were found, is similarly angiosperm-dominated with rare conifers and ferns, but so far it has received little attention ( Donovan et al , 2016 ). Other lineages identified from various Salamanca Formation and other Palaeocene localities in Patagonia include Cheirolepidiaceae ( Barreda et al , 2012 ), Podocarpaceae, Cupressaceae ( Ruiz et al , 2017 ), cycads ( Petriella, 1972 ), ferns and lycophytes ( Archangelsky, 1973 ), Arecaceae ( Romero, 1968 ; Petriella, 1972 ; Archangelsky, 1973 ; Futey et al , 2012 ), Rhamnaceae ( Jud et al , 2017 ), Myrtaceae ( Ragonese, 1980 ), Boraginaceae ( Brea and Zucol, 2006 ), Elaeocarpaceae, Euphorbiaceae, Rutaceae and Cunoniaceae ( Weinmannioxylon species) ( Petriella, 1972 ). Together, these occurrences demonstrate that the flora included tropical and southern-wet forest elements; the biome probably resembled the temperate forests of Eastern Australia, where Schizomeria and Ceratopetalum are native today.…”

“…Among these fossils are delicate flowers with well-preserved features that permit high confidence in their systematic identifications (e.g. Jud et al , 2017 ).…”

Estimating divergence times and ancestral breeding systems inFicusand Moraceae

“…Our set of fossil age constraints includes four in Moraceae and eight distributed among the remaining families of Rosales (Supplementary Data Table S2). To revise fossil calibrations in Rosales, we proceeded as follows (Parham et al, 2012;Sauquet et al, 2012): (1) we started from lists of calibrations used in previous molecular dating studies (Zerega et al, 2005;Cruaud et al, 2012;Magallón et al, 2015) and completed this list with specific reviews (Collinson, 1989;Friis et al, 2011) and recently published fossil taxa (Manchester, 1999;Calvillo-Canadell and Cevallos-Ferriz, 2007;Manos et al, 2007;Benedict et al, 2011); (2) for each fossil, we critically assessed the phylogenetic assignment based on original descriptions and subsequent reviews, and using the latest reference phylogeny for each family (Manchester, 1999;Calvillo-Canadell and Cevallos-Ferriz, 2007;Manos et al, 2007;Benedict et al, 2011;Friis et al, 2011;Jud et al, 2017); (3) for each fossil, we also critically revised the absolute age or age range of the fossil using the latest stratigraphy and geological time scale from the International Commission on Stratigraphy (Cohen et al, 2017). Because none of these fossil taxa has been included in total evidence phylogenetic analyses, our assignments here are at best 'apomorphy based' and therefore we have been particularly conservative in both selecting our final calibrations and assigning them to clades.…”

52 million years old Eucalyptus flower sheds more than pollen grains

“…During the last 15 years, persistent fieldwork in Argentinean Patagonia has yielded several superbly preserved fossil flowers and fruits (Zamaloa et al, 2006;Gandolfo et al, 2009Gandolfo et al, , 2011Futey et al, 2012;Hermsen et al, 2012;Hermsen and Gandolfo, 2016;Gandolfo and Hermsen, 2017;Jud et al, 2017Jud et al, , 2018aWilf et al, 2018Wilf et al, , 2019, which in many cases represent the oldest records globally for the taxa and the clades they belong. Among these, Gandolfo et al (2011) and Hermsen et al (2012) reported a diverse and beautifully preserved suite of fossils belonging to the iconic Australian genus Eucalyptus.…”