Page:Popular Science Monthly Volume 77.djvu/75

Rh faunas in which few of the genera and none of the species are identical with those now living the problem becomes more difficult and the conclusions are much less definite, as the comparisons must be more general. Proofs of actual temperatures as measured in degrees should not be expected unless the botanists can furnish data. There is, however, great local differentiation of faunas and it is fair to ask the question to what extent this is due to differences in climate. One of the earliest discussions of this question was by Ferdinand Roemer, who more than fifty years ago in "Die Kreidebildungen von Texas" noted the fact that the Cretaceous of the highlands in Texas is lithologically and faunally much like the Cretaceous of southern Europe and the Mediterranean region, that it differs from the Cretaceous of New Jersey in about the same way that the southern European Cretaceous differs from that of England and northwestern Germany, and that in each case the European deposit is approximately 10° farther north than its American analogue. He concluded that the differences between the northern and southern facies were due to climate and that the climatic relations between the two sides of the Atlantic were about the same in Cretaceous time as they are now. Roemer's conclusion that there were climatic zones in the Cretaceous may be true, but his reasoning was based on false premises so far as the American deposits are concerned, for the New Jersey type of marine Cretaceous extends with little change all the way from New Jersey to the Rio Grande, and the "Cretaceous of the highlands" with which he contrasted it, now known as the Comanche series, is not represented by marine beds on the Atlantic coast. This shows the necessity for careful stratigraphic and areal work as well as for good paleontology before such broad conclusions can be safely made.

The more general work of Neumayr recognized in the Jurassic and Cretaceous of Europe three faunal provinces designated as boreal, central European, and alpine or equatorial, which on account of their zonal distribution he regarded as indicating climatic differences. He believed that these zones are recognizable throughout the northern hemisphere and cited evidence to show that similar zones exist south of the equator. In recent years Neumayr's conclusions have been questioned by many because in so many instances genera supposed to be characteristic of one zone have been found mingled with those of another. For example, the alpine ammonite genera Lytoceras and Phylloceras occur in Alaska (lat. 60°) associated with the boreal Aucella, and Aucella itself ranges from the Arctic Ocean to the torrid zone. Still, in spite of such exceptions and anomalies in distribution, there is much evidence for a real distinction between boreal and southern faunas in the Jurassic and in the Cretaceous which may indicate a zonal distribution of temperature in Mesozoic time. It should be