In several recently published phylogenetic analyses, two Lower Devonian taxa, Doliodus and Pucapampella, both fall on the chondrichthyan stem, very close to the base of ‘conventionally defined chondrichthyans’ (i.e., forms possessing tessellated mineralization of the cartilaginous endoskeleton). These two taxa nevertheless exhibit strongly discordant morphologies from each other. A summary of the anatomical data concerning these taxa is presented here, including new, as well as previously published, findings. A new family Pucapampellidae is erected, containing Pucapampella and a newly recognized genus from South Africa. Morphological evidence is summarized for the monophyly of crown elasmobranchs (sharks and rays), holocephalans (chimaeras) and other chondrichthyans. Based on these data, Doliodus and pucapampellids both fall outside the chondrichthyan crown, but their relative phylogenetic positions on the chondrichthyan stem are unclear. Pucapampellid interrelationships are particularly hard to assess because little is known beyond their cranial and visceral arch morphology and also because pucapampellids possess a suite of ontogenetically primitive (and thus potentially neotenic) features. By contrast, the phylogenetic position of Doliodus seems less elusive; it possessed an ‘acanthodian-like’ complex of dermal spines, including pectoral fin spines, prepectoral, admedian, and prepelvic spines, and possibly dorsal and pelvic fin spines, in conjunction with numerous ‘chondrichthyan-like’ endoskeletal features and a heterodont ‘sharklike’ dentition. Doliodus can be viewed as a quintessential component of the evolutionary transition between ‘acanthodians’ and ‘conventionally defined chondrichthyans’, leaving little doubt that the chondrichthyan total group includes ‘acanthodians’ (now widely perceived to be a paraphyletic group, populating the basal part of the chondrichthyan stem). Although Doliodus has been resolved as a basal member of the ‘conventionally defined chondrichthyans’, it could occupy a more basal position on the chondrichthyan stem.
The three most basic drivers of energy demand are economic activity, population, and technology. Longer-term trends in economic growth for a particular economy depend on underlying demographic and productivity trends, which in turn reflect population growth, labor force participation rate, productivity growth, national savings rate, and capital accumulation (USEIA, 2011).
Several historic shifts are likely to fundamentally alter global demographics over the coming decades. First, as developing nations move from poverty to relative affluence, there is a fundamental shift from agriculture to more energy-intensive but much more productive commercial enterprises. Second, labor forces in the developed countries are aging considerably, which has implications on many fronts, including energy use and employment structures. Third, for the first time the majority of the world's population has become urbanized, with the largest urban centers emerging in developing regions where energy access is a serious constraint. All of these will have immense impacts on the level and quality of energy demand and on concerns about energy security.
Global energy security and sustainability in the twenty-first century will depend less on the total global population than on incomes and their distribution. This in turn will depend to a large extent on how effectively the lack of energy services, which now limit economic opportunities in the less developed regions, is addressed. In addition, energy security will depend on the ability of countries to maintain reliable sources of energy to meet their needs.
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