| That the Earth's climate has cooled over the last 100 million years has been recognized since the early nineteenth century. Today, the dynamics responsible for this change are being examined using computer models, especially General Circulation Models (GCM's). The success of modeling experiments can only be assessed by comparing results with observations, which, for paleoclimate, are drawn exclusively from the geological record. However, our knowledge of this record has been generally limited to temporal trends in climate.;This study examines the spatial distribution of paleoclimate during the transition from the 'hot-house' world of the Cretaceous (a time interval without significant polar ice-sheets) to the 'ice-house' world in which we now live. This has been done by mapping out the paleodistribution of climate proxies, specifically fossil crocodilians. Climatic interpretations based on these data are derived from analogy with the Recent. Potential changes in tolerances over time may be assessed by intercomparisons between the interpretations derived from diverse lines of evidence.;Fossil crocodilians have a relatively good fossil record and are climatically limited in the Recent. This makes them an ideal vehicle for examining other issues that impinge on the use of geologic data for reconstructing paleoclimate: taphonomy, sampling effects, time-averaging, tectonics, and, for fossils, paleobiogeography and evolution. Control groups provide one means of qualifying absences of data due to these effects.;The general conclusions reached are as follows: (1) during 'ice-house' intervals the latitudinal range of crocodilians is restricted to low latitudes and individuals are precluded from continental interiors, suggesting high latitude cooling, and increased seasonality and aridification in mid-latitudes; (2) the diversity of crocodilians also drops during such intervals, perhaps in response to range contraction; (3) crocodilians show no major extinction event at the K-T boundary implying that climate is not the dominant cause of extinctions at this boundary.;By mapping out the past distribution of climate, critical assessments of model results are possible. Through such work our understanding of the dynamics of the Earth system improves and our ability to predict future trends is enhanced. |
