Paleoecological Archives in Lake Deposits 2: Records from Important Groups

The lacustrine fossil record comprises a mixture of endogenic fossils, such as cladocerans, derived from lakes, and exogenic fossils, such as insects or pollen, which are carried into lakes, by wind and water from surrounding areas. Our primary emphasis here will be on the endogenic fossil record of lakes; we will only briefly consider general aspects of the taphonomy and paleoecological significance of exogenic fossils for terrestrial plant and insect fossils. Information about lake fossils varies greatly between groups. Some taxa, such as diatoms, are virtual workhorses of the field, with numerous investigators, and established methods of sampling, analysis, and interpretation. At the other extreme are organisms such as copepods, which, despite their importance in lacustrine ecosystems, are so poorly fossilized that they are unlikely to ever play a major role in paleolimnology. In between these extremes lie the majority of lacustrine organisms. Many relatively common groups have great potential for paleoecological interpretation, but, for reasons of inadequate study, a lack of researchers, or difficulties in taxonomy, have thus far been little used by paleolimnologists. Major opportunities await new students in the field who are willing to take up the challenges of studying these clades. Despite their importance in lacustrine communities, cyanobacteria remain a relatively unexploited source of information for paleolimnology. Isolated cells have poor preservation potential, and fossil cyanobacterial cells are preserved in Late Quaternary lake muds primarily by their more resistant reproductive spores (akinetes), or occasionally by filaments. Planktonic cyanobacteria are only rarely recorded in older sediments. In contrast, benthic cyanobacterial communities are well represented in ancient lake beds by their constructional deposits, lithified algal mats, stromatolites, and thrombolites. Although their body fossils have been used only rarely to solve paleolimnological problems, planktonic cyanobacteria have great potential for this purpose, given their obvious importance in many lacustrine communities. Relatively resistant akinetes might be very useful for understanding changes in plankton communities, especially in cases where better- studied siliceous microfossils (diatoms and chrysophytes) are not well preserved, for example, in very alkaline lakes. However, almost nothing is known of the taphonomic biases that control the planktonic cyanobacterial fossil record.

The First Shipping Constructed in New Spain

Amid the mixed emotions with which Cortés prepared A% to enter the Mexican capital in early November, 1519, was the gnawing fear that his military position therein would be an untenable one. Even the most courageous and least professional military man among the invaders must have realized that a Spanish force of fewer than five hundred men, far within the interior of a hostile and strange land, literally had entered a lion’s den as it marched into the populous island-city of Tenochtitlán via the narrow and ever so vulnerable causeway that stretched some seven miles between it and the mainland. So it was that even as his courage and curiosity took him into Tenochtitlán, his military mind and his sense of responsibility for the welfare of his men led Cortés to seek a means whereby the Spaniards could dominate the waters of Lake Texcoco and the lacustrine communities, including Tenochtitlán itself. Only after the Spaniards had become masters of the adjacent countryside—in this instance the waters and shores of a lake—could they know that inner satisfaction and quiet confidence that comes from a sincere sense of physical security. Only when the Spaniards had established their mastery over the lake area could it be said that the initiative so important to continuance of the conquest rested in their hands. With his infantry, his cavalry and his artillery momentarily stalled in the insular setting in which they found themselves, Cortés needed to create a new military force which, when thrown into the balance, would tip the scales once again in favor of the Spaniards.