Fossil crocodilians as indicators of Late Cretaceous and Cenozoic climates: implications for using palaeontological data in reconstructing palaeoclimate

Abstract That the Earth's climate has cooled over the last 100 million years has been recognized since the early nineteenth century—the transition from the ‘hot-house’ world of the Cretaceous (a time interval without significant polar icesheets) to the ‘ice-house’ world in which we now live. Today, the dynamics (process) responsible for this change are being examined using computer models, especially General Circulation Models (GCM's). Through such work our understanding of the dynamics of the Earth system improves and our ability to predict future trends is enhanced. But, the success of modelling experiments can only be assessed by comparing results with observations (the pattern), which, for palaeoclimate, are drawn exclusively from the geological record. Palaeoclimate interpreted from geological data is invariably based on analogy with the Recent, the validity of which depends on corroboration from multiple lines of evidence. This requires the compilation and investigation of large, global datasets of well-constrained geological climate proxies. This study uses the palaeodistribution of fossil crocodilians to examine the spatial distribution of palaeoclimate during this transition. Fossil crocodilians have a relatively good fossil record and are climatically limited in the Recent. They also provide a manageable global dataset with which to investigate other issues that impinge on the use of geologic data for reconstructing palaeoclimate: taphonomy, sampling effects, time-averaging, tectonics, and, for fossils, palaeobiogeography and evolution. As such this study provides a possible template for examining other climate proxies in order to validate and hone palaeoclimatic interpretations. An analysis of modern crocodilians indicates that temperature is the principal influence on their global distribution, with the Coldest Month Mean temperature (CMM) of ≈ 5.5°C marking the minimum thermal limit for the group, corresponding today to a minimum Mean Annual Temperature (MAT) of ≈ 14.2°C. The duration of warmth during the year (partly indicated by the Mean Annual Range of Temperature, MART) is also important, as this affects early juvenile development and survival. The presence of standing water provides an essential thermal buffer against temperature extremes. By analogy, the palaeodistribution of fossil crocodilians suggests the following: during the Late Cretaceous and early Palaeogene MAT's in excess of 14.2°C (CMM's > 5.5°C) permeated throughout mid-latitudes and coastal regions in high latitudes; during the Oligocene the restriction of fossil crocodilians to low latitudes suggests high-latitude cooling, as indicated by numerous other lines of evidence—coeval restriction of crocodilians to coastal regions in mid-latitude North America and much of Asia, consistent with increased seasonality and aridification in continental interiors, may reflect local orographic changes as well as global climate change. During the Miocene fossil crocodilians return to the Great Plains of North America, but do not reoccupy the continental interior of Asia; by the Pliocene, crocodilians are again restricted to low-latitudes and coastal regions in mid-latitudes, coincident with further high-latitude cooling, and aridification and cooling of mid-latitude continental interiors. Minor distributional changes during the Holocene may reflect human effects.