Four new lepidosaurs from the Early Cretaceous deposits of the Tlayua Quarry, Central Mexico are described, establishing their phylogenetic relationships using cladistic methodology. These lepidosaurs have unique characters never present in related forms suggesting that they have evolved in isolated environments far from immediate ancestors, indicating the insular nature of the quarry. The sphenodontid Pamizinsaurus tlayuaensis is covered with unique rows of small rounded osteoderms that could have protected it against predation in open environments. Ankylosphenodon pachyostoseus has unusual teeth ankylosed deep into the dentary with probable continuous growth, which combined with propalinal action of a deep lower jaw suggest herbivory. Unique pachyostotic ribs and vertebrae, delay in the ossification of the epiphyses, and zygapophyses horizontally oriented to stiffen the vertebral column could be related to aquatic behavior. The unique morphology of Pamizinsaurus and Ankylosphenodon argue against the concept of low morphological diversification of sphenodontians. / The lizard Huehuecuetzpalli mixtecus shows most iguanian features, but still retains a divided premaxilla, amphicoelous vertebrae, thoracolumbar intercentra, and the second distal tarsal, supporting a phylogenetic position as sister-group of squamates. Although late in the fossil record, Huehuecuetzpalli provides important information on early transformation of characters in lizard evolution. Tepexisaurus tepexii is an early scincomorph relatively more primitive to all known scincoids. The lack of osteoderms indicates that Tepexisaurus and some paramacellodids are not scincoids, suggesting that the Paramacellodidae is not monophyletic. The relative primitive morphology of Tepexisaurus in Albian deposits can be correlated with the late presence of sphenodontians and the relictual nature of Huehuecuetzpalli in Tlayua. This suggests that TIayua was a refuge for archaic terrestrial forms. / Character transformation at the origin of the Squamata was explored through a phylogenetic analysis including basal lepidosauromorphs, the best known early squamates, and extant squamate "families". Results using a rigorously reviewed data set, show that many characters suggested to be squamate autapomorphies are certainly along a lineage basal to the Squamata, which includes Marmoretta, Tamaulipasaurus, the Ardeosauridae (redefined to include Bavarisaurus), and Huehuecuetzpalli. The name Squamatoidea to group all taxa basal to Squamata + squamates is suggested. The Total Branch Support index obtained falls between values of other published phylogenies. The low values seem to be affected by the inclusion of several fossil taxa with incomplete information and the redistribution of a limited number of characters in a greater number of branches.

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This thesis presents the results of a study to examine long-term forest dynamics in the high-altitude mountains of West-Central Mexico. Vegetation dynamics on temporal scales ranging from 10² to 10³ years were reconstructed in order to provide essential information on the temporal variability of ecological patterns and processes in these forests; information that is of direct relevance for their current and future conservation and management strategies. Vegetation and palaeoecological methods undertaken included fossil and modern pollen analysis, vegetation surveys, microfossil charcoal analysis, magnetic susceptibility, inorganic and organic geochemistry, radiocarbon and ²¹⁰Pb dating. These were used to evaluate the long-term dynamics of three forest types; Pine Forest, Cloud Forest and Transitional Forest on timescales spanning the past 4260, 1340 and 1230 years respectively. The main drivers of change were climate and disturbance events induced by climate fluctuations, for example increased fire frequency. The reconstructed records indicate that the sequences from the Cloud Forest and the Transitional Forest spanned two wet and one dry climatic interval while the Pine Forest sequence spanned two dry and two wet periods. The impact of these climatic fluctuations was significant on all three forest types and resulted in variations in forest diversity, taxonomic turnover and successional change. The climate change episodes observed in these records seem to be the local manifestation of climatic events that were occurring throughout Mexico at these intervals in time. Human influences were evident in the three forests through the appearance of cultural taxa, particularly during the driest period (~ 1200 yr BP). There is little evidence from these records, however, to suggest a widespread clearance of the landscape for agriculture. Results from this study support the current conservation and management recommendations for Cloud Forest to exclude timber extraction, grazing and agricultural activities from this forest type. In the Pine Forest, human interventions such small-scale agriculture, prescribed burning and silvicultural actions are in agreement with the longterm pine ecology and as such, total exclusion of human activities is not necessary. For the Transitional Forest, results from this study suggest that there needs to be the establishment of adequate plans to reduce frequent fires to arrest the development of prone-to-fire taxa.

No description available.