Nonparametric classification of fossil gymnosperm's foliar area
In Paleobotany, the features of leaf laminae linked to the climate have been used to infer past climatic conditions. However, in spite of the increasing number of paleoecological studies, studies worldwide have been focused on the angiosperms, leaving aside other important groups throughout geological time. In Mexico, for example, the most abundant and widely distributed fossil flora are found in Early and Middle Jurassic deposits and are constituted mainly of gymnosperms. So far, the contributions of investigations of the climate where this vegetal group dominated have only been carried out at high-latitude localities, counting genera not present in Mexico besides the use of an informal leaf-size proposal. It has been demonstrated that this character has a direct relationship with climate. Therefore, this research work was aimed at proposing for gymnosperms leaf-lamina size classification using foliar area of each of the 737 plant fossils collected from Jurassic outcrops at Mexico’s Oaxaca State Mixtec region in the Rosario, Zorrillo, Taberna- Simón, and Tecomazúchl formations and the Tecocoyunca group. To classify the area by its extension we used kernel density estimators (smoothed histograms) with the (empirical suggestion) half of optimal bandwidth (Härdle, 1991, Smoothing Techniques: With Implications in S [Springer]). The resulting multimodal distribution was the base to set up categories defined with a new algorithm denominated “antimodes”. All the estimations were performed with Mosqueda-Romo and Salgado-Ugarte’s programs amodes.ado (a new one), bandw.ado, warpdenm.ado, bhataplt.ado, bhatgauc.ado, and their updated versions for Stata 11. The results allow the demarcation of seven categories, the most abundant being foliar area number three (Microphile I, Zamites, Pterophyllum, and Ptilophyllum genera) with a 58.06% representation, while the group with the largest surface exposed to sunlight radiation (over 57.74%) but the least percentage representation at the study zone (0.53%) is the Pelourdea genus. The gathered data permit the inference that during the Jurassic age in the Mixtec terrain, xeric conditions prevailed, which conditioned the small size of the leaves.
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