Graduate Thesis Or Dissertation
 

Unearthing the Cretaceous Diversification of Cornales

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  • The order Cornales (dogwoods) is the earliest diverging lineage within the most diverse group of flowering plants, the asterids (>80,000 species). Although molecular phylogenetics have significantly improved our understanding of cornalean systematics, early phylogenetic relationships remain uncertain due to an initial rapid radiation. The fossil record of Cornales is extensive and indicates that the Late Cretaceous was an important time for the initial diversification of the order. Moreover, molecular divergence time estimates suggest that most of the major cornalean lineages diverged between 100-84 Ma. Many cornalean fossils are represented by fruits, which contain numerous systematically informative characters. Therefore, cornalean fossil fruits are crucial for reconstructing the early evolution of Cornales. The over-arching goal of this dissertation is to reconstruct the early evolution of Cornales by utilizing the fossil record. There are three main objectives: 1) increase the taxon sampling density of Late Cretaceous cornalean fruits, 2) develop new hypotheses on the early evolution of Cornales, and 3) elucidate the initial diversification of the order by conducting phylogenetic and quantitative morphological analyses. Herein, seven taxa, six of which are new, are characterized based on permineralized fruits from several Upper Cretaceous deposits. All fossils described in this study are anatomically preserved within shallow-marine calcareous concretions. Specimens were prepared using the cellulose acetate peel technique and studied using light microscopy. In Chapter 2 two permineralized cornalean fruits from the Campanian of Washington Stae are described. Fruits are ellipsoidal, tetra-locular, and consist of valvate endocarps. Each locule is crescent-shaped in cross section with one apically attached seed and have valves that are restricted to the apical third of the endocarp. Endocarp tissue consists of isodiametric to elongate sclereids and secretory cavities. Vascular bundles occur in rows in each septum, there are rows of vascular bundles. These fossil fruits display unique character mosaic indicative of several cornalean families, but not conforming to any family or fruit from known extinct taxa. Therefore, the fossil fruits are assigned to a new cornalean genus and species, Suciacarpa starrii gen. et sp. nov. In Chapter 3, a permineralized fossil fruit assignable to Cornus subg. Cornus is described from the Campanian of Vancouver Island, British Columbia, Canada. The fossil fruit consists of a tri-locular woody endocarp with dorsal germination valves. Endocarp contains elongate and isodiametric sclereids and secretory cavities. Central vasculature is lacking and the endocarp is interpreted to have been vascularized by bundles located along the outer periphery of the septa. There is one seed per locule. The interpreted vascular pattern and presence of secretory cavities strongly suggest that the fossil fruit is assignable to Cornus subg. Cornus, with a morphology and anatomy o is most similar to Cornus piggae, which was originally described from the Paleocene of North Dakota. This Cretaceous fossil fruit is the earliest evidence for crown-group Cornaceae and its discovery in western Canada contradicts the hypothesis that Cornaceae originated in Europe. In Chapter 4, two new taxa are described based permineralized fruits from the Campanian Shelter Point locality of Vancouver Island. One fruit represents a new species, Suciacarpa xiangae sp. nov., that is represented by a large ovoid tetra-locular sclerenchymatous endocarp. Locules are crescent shaped in cross section and accompanied by short dorsal germination valves. Valves consist of small isodiametric sclereids and numerous large secretory cavities. The septa and central axis are composed of transverse fibers and elongate sclereids. Central vascular bundles are lacking, but there are several small bundles within the septa. There is one apically attached seed in each fertile locule. The second fruit type represents a new cornalean genus and species, Sheltercarpa vancouverensis gen. et sp. nov., that is characterized by a single smooth tetra-locular woody endocarp. The endocarp is made up of isodiametric and, to a lesser extent, elongate sclereids. Germination valves are short. The fruit lacks a central vascular bundle. Fruits of Suciacarpa xiangae and Sheltercarpa vancouverensis display character mosaics that are reminiscent of at least two different major cornalean clades. The cornalean fruit diversity from the Late Cretaceous suggests that the Campanian was a critical time for the initial diversification of Cornales. Chapter 5, three new taxa from the early Coniacian (~89 Ma) of North America, are characterized: Eydeia vancouverensis sp. nov., Obamacarpa edenensis gen. et sp. nov., and Edencarpa grandis gen. et sp. nov. The fruits of each species have thick-walled woody endocarps with dorsal germination valves, one apically attached seed per locule, several rows of vascular bundles in each septum, but no central vascular bundle. However, each species differs from the others, and from previously described cornaleans in endocarp size, sculpturing, and anatomy. The taxa characterized in this chapter are the same age as the previously recognized oldest cornalean, Hironoia fusiformis, from the early Coniacian of Japan. The diversity, geographic distribution, and stratigraphic age of these early cornalean fruits strongly suggest that the initial diversification of Cornales and asterids was well underway by the Coniacian. In Chapter 6, the Cretaceous diversification of Cornales is reconstructed using phylogenetic, morphospace, and disparity analyses. A cladistic matrix consisting of 77 fruit morphological characters and 58 taxa (35 extant, 23 extinct) was constructed. This matrix was used for two main phylogenetic parsimony analyses: 1) fossil inclusive and 2) fossil exclusive (living taxa only). The fossil inclusive and exclusive parsimony analyses resulted in trees with a basal grade consisting of Loasaceae, Hydrangeaceae, Hydrostachyaceae, Grubbiaceae, and Curtisiaceae, respectively, leading to a core cornalean clade comprising Cornaceae, Alangiaceae, Nyssaceae, Mastixiaceae, and Davidiaceae. In the analysis excluding extinct taxa, relationships among the core cornalean clades are largely unresolved. However, inclusion of fossils in the analysis results in more highly resolved relationships among Cornaceae, Alangiaceae and a clade consisting of Nyssaceae, Mastixiaceae, and Davidiaceae. These results clearly demonstrate that the novel character mosaics found within Cretaceous cornalean fruits play a vital role in resolving deep-node relationships within Cornales. In addition, a modified fruit morphological matrix (drupaceous fruits only) was used for morphospace and disparity analyses. The results of these analyses indicate that the morphological diversity of cornalean fruits has significantly changed and increased since the Late Cretaceous. A combination of ecological and developmental factors most likely explain the morphological diversification of Cornales that occurred during the Paleogene (66-56 Ma).
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  • 2017-10-21 to 2018-06-12

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