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For more than 60 years, evolutionary biologists have debated the issue of whether the processes of genetic change observable within populations (microevolution) can provide an adequate explanation for the large-scale patterns in the history of life (macroevolution). In general, population geneticists have argued in favor of microevolutionary extrapolation, whereas paleontologists have sought to establish an autonomous and hierarchical macroevolutionary theory based on the operation of selection at several levels of biological organization (especially species). The massive variation in eukaryotic genome sizes (haploid nuclear DNA contents, or “C-values”) has similarly been a subject of debate for more than half a century, and it has become clear that no one-dimensional explanation can account for it. In this article, the basic concepts of macroevolutionary theory are reviewed and then applied to the long-standing puzzle of genome size variation (the “C-value enigma”). Genome size evolution provides a clear example of hierarchy in action and therefore lends support to the theoretical approach of macroevolutionists. Perhaps more importantly, it is apparent that genome evolution cannot be understood without such a hierarchical approach, thereby providing an intriguing conceptual link between the most reductionistic and expansive subjects of evolutionary study.
The siliciclastic succession of the late Neoproterozoic Vendian Group in the White Sea area demonstrates a wide range of lithofacies, some recurring in a vertical succession. Significantly, each lithofacies contains a distinct assemblage of Ediacaran fossils that represents in situ benthic paleocommunities smothered in life position. These lithofacies define (1) a monospecific Inaria assemblage, restricted to the lower-shoreface muds; (2) a Charnia assemblage, within the middle-shoreface graded siltstone-shale couplets; (3) a Dickinsonia-Kimberella assemblage, confined to the interstratified sandstone and shale of prodelta; and (4) a Onegia-Rangea assemblage, preserved within channelized sandstone beds of the distributary-mouth bar.
In the White Sea area a strong correlation exists between taxonomic composition, biostratinomic features, and paleoecological context of the Ediacaran fossil assemblages. Facies-controlled distribution is also evident in other Ediacaran localities, demonstrating the recurrence of similar facies relationships on a global scale. This pattern is interpreted as representing Ediacaran biofacies with Avalon-type biotas distributed in deep marine habitats, Ediacara-type biotas inhabiting microbial biofilms in shallow marine prodeltaic settings, and infaunal Nama-type biotas found in distributary-mouth bar shoals. This in turn reveals a marked degree of environmental sensitivity and ecological specialization. Correspondence between depositional environment and taxonomic composition speaks against any obvious biogeographic provinciality of the Ediacaran biotas, and also casts doubt on claims of substantial evolutionary change.
Historical changes in the coastline of Kikai Island of the Ryukyu Islands in the southeast part of Japan were estimated by using a numerical simulation based on a glacio-hydro-isostasy model. Temporal changes in the area of the island during the last 40 Kyr were compared with temporal changes in species diversity in fossil land snails of the island. The species number in the past was theoretically estimated by the area of Kikai Island in the past and a species-area relationship among the modern land snail fauna of the Ryukyu Islands. The theoretical species numbers are very close to the actual ones. This suggests that the change in island area is the main cause of the change in species diversity in Kikai Island. In addition, we discuss causes other than the area, such as island elevation, distance to the nearest large island, climate change, human activity, and imperfection of fossil data. We also discuss the change in Fisher's alpha and body size against the change in the area.
A morphologically diverse assemblage of organic-walled fossils from the middle Neoproterozoic Svanbergfjellet Formation, Spitsbergen, is identified as a monospecific assemblage representing the Gongrosira-phase of a vaucheriacean xanthophyte alga. As such, it provides a range of additional criteria with which to identify fossil vaucheriaceans and confirms the identification of Palaeovaucheria in the Mesoproterozoic Lakhanda Formation. Pronounced taxonomic inflation, through the practice of form-taxonomy, suggests that overall estimates of eukaryotic diversity in the Proterozoic need to be adjusted downward. Combined with positive evidence for low levels of speciation and extended stasis, pre-Cambrian eukaryotes are seen to evolve at a fundamentally lower rate than their Phanerozoic counterparts. This slower turnover accounts for the “delayed” appearance of animals without appeal to external triggers or constraints. The Cambrian acceleration of evolutionary rates was a direct consequence of newly introduced animals, whereas the much slower overall rates of the Proterozoic imply an absence of earlier metazoans.
Modern histological techniques allow paleontologists to investigate the internal microstructure of bone tissue. We apply high resolution images of histological thin sections from an ontogenetic series (not conspecific) of pachycephalosaurid frontoparietal domes to test the hypothesis that these Late Cretaceous dinosaurs used their heads as battering rams, analogous to the behavior of the bighorn sheep, Ovis canadensis, or as a thermoregulatory device. Our analysis reveals that the internal structure of the pachycephalosaur dome is a dynamic tissue that reflects the changeable expansion and vascularity of the dome throughout ontogeny. The radiating structures within the frontoparietal dome, used previously to support “head-butting” hypotheses, are unexpectedly transitory, diminishing in mature individuals and nearly absent in adult skulls where head-butting behavior is presumed to occur. The unique architecture of the pachycephalosaurid dome is dividable into three distinct Zones. We demonstrate that the relative vascularity, associated tissue structures, and orientation and density of Sharpey's fibers within these Zones are modified during growth. Evidence for an external dome covering in vivo precludes the determination of the final shape of the pachycephalosaur skull. On the basis of these new observations, we propose that cranial display in support of species recognition and communication is a more parsimonious interpretation of the function of the pachycephalosaurid dome. Sexual display behaviors were probably secondary.
Anomodont therapsids were the most diverse and abundant group of terrestrial vertebrate herbivores during the Late Permian and much of the Triassic. Their success has been widely attributed to a complex mastication system that was based on a propalinal sliding of the lower jaw. Traditionally, anomodont phylogeny has been viewed as an essentially linear sequence of taxa, each with more specializations for propaliny, culminating in the radiation of the dicynodont anomodonts. However, recent phylogenetic work has shown that similar specializations for propaliny can be found in members of two distinct anomodont clades, the endemic Russian Venyukovioidea and the unnamed clade consisting of Galeops Dicynodontia. These specializations have been inferred to be present in the common ancestor of the venyukovioids and dicynodonts, implying a single origin of propaliny in anomodonts. Here a reevaluation of the anomodont feeding system is presented, and the possibility that propaliny evolved twice within the group is specifically addressed. First, I examine current hypotheses regarding anomodont phylogeny, using a modified version of data sets presented by previous authors. Then I consider the evolution of four characters related to propaliny in the context of the resulting phylogeny. The results of this investigation suggest that a dual origin of propaliny within anomodonts is not unlikely, but it depends largely on the interpretation of equivocal or poorly preserved features in some critical taxa, especially Otsheria. Given that the power stroke of mastication in basal synapsids involved the application of a posterodorsally directed force on the lower jaw, the evolution of propaliny in basal synapsids may not have been difficult. Furthermore, the independent evolution of propaliny in several other nonmammalian synapsid clades (e.g., Edaphosauria, Gorgonopsia) suggests that homoplasy may be the rule rather than the exception for this feature. Finally, the possibility of a dual origin of propaliny in anomodonts has important implications for the more general problem of the evolution of herbivory in terrestrial vertebrates. In particular, it suggests that propaliny alone was not enough to guarantee success as a herbivore during the late Paleozoic and early Mesozoic.
Cenozoic sediments of Florida contain one of the most highly fossiliferous sequences of extinct sirenians in the world. Sirenians first occur in Florida during the Eocene (ca. 40 Ma), have their peak diversity during the late Oligocene–Miocene (including the widespread dugongid Metaxytherium), and become virtually extinct by the late Miocene (ca. 8 Ma). Thereafter during the Pliocene and Pleistocene, sirenians are represented in Florida by abundant remains of fossil manatees (Trichechus sp.). Stable isotopic analyses were performed on 100 teeth of fossil sirenians and extant Trichechus manatus from Florida in order to reconstruct diets (as determined from δ13C values) and habitat preferences (as determined from δ18O values) and test previous hypotheses based on morphological characters and associated floral and faunal remains. A small sample (n = 6) of extant Dugong dugon from Australia was also analyzed as an extant model to interpret the ecology of fossil dugongs.
A pilot study of captive manatees and their known diet revealed an isotopic enrichment (ϵ*) in δ13C of 14.0‰, indistinguishable from previously reported ϵ* for extant medium to large terrestrial mammalian herbivores with known diets. The variation in δ18OV-SMOW reported here is interpreted to indicate habitat preferences, with depleted tooth enamel values (≈25‰) representing freshwater rivers and springs, whereas enriched values (≈30‰) indicate coastal marine environments. Taken together, the Eocene to late Miocene sirenians (Protosirenidae and Dugongidae) differ significantly in both δ13C and δ18O from Pleistocene and Recent manatees (Trichechidae). In general, Protosiren and the fossil dugongs from Florida have carbon isotopic values that are relatively positive (mean δ13C = −0.9‰) ranging from −4.8‰ to 5.6‰, interpreted to represent a specialized diet of predominantly seagrasses. The oxygen isotopic values (mean δ18O = 29.2‰) are likewise relatively positive, indicating a principally marine habitat preference. These interpretations correlate well with previous hypotheses based on morphology (e.g., degree of rostral deflection) and the known ecology of modern Dugong dugon from the Pacific Ocean. In contrast, the fossil and extant Trichechus teeth from Florida have relatively lower carbon isotopic values (mean δ13C = −7.2‰) that range from −18.2‰ to 1.7‰, interpreted as a more generalized diet ranging from C3 plants to seagrasses. The relatively lower oxygen isotopic values (mean δ18O = 28.1‰) are interpreted as a more diverse array of freshwater and marine habitat preferences than that of Protosiren and fossil dugongs. This study of Cenozoic sirenians from Florida further demonstrates that stable isotopes can test hypotheses previously based on morphology and associated floral and faunal remains. All these data sets taken together result in a more insightful approach to reconstructing the paleobiology of this interesting group of ancient aquatic mammalian herbivores.