Environments of Mid-Cretaceous Saharan dinosaurs

Recent studies of the oceanic record suggest that the Earth was a global greenhouse during middle Cretaceous time. A review of topographic, sedimentary and biologic data pertaining to terrestrial mid-Cretaceous equatorial environments broadly supports the climatic inferences of marine studies. In particular, analyses of widely-occurring low latitude Saharan sediments support the Cretaceous greenhouse hypothesis. In comparison to marine ecosystems, terrestrial ecosystems respond more sensitively to atmospherically transported heat and nutrients because of the more tenuous presence of the hydrosphere on land. Indeed, the morphologies of terrestrial biota suggest that: (1) equatorial mid-Cretaceous climates were episodic rather than seasonal; (2) convective storms although infrequent were violent; and (3) a moist intertropical convergence zone was absent. Cretaceous atmospheric dynamics apparently differed importantly from those of the present. Circumstantial evidence suggests that (1) higher atmospheric carbon dioxide levels probably stimulated the emission of other greenhouse gases; and (2) higher humidity levels reduced diurnal temperature variations but impaired evaporative thermoregulatory mechanisms. That terrestrial ecosystems withstood greenhouse conditions in low latitudes as well as they did underscores the adaptability of terrestrial life. Analogies for future environmental stresses and responses, whether anthropogenic or resulting from other causes, might be found in the terrestrial record of the Cretaceous equatorial zone.