THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 29 



The possibility of error is clearly illustrated in the occurrence in the mid- 

 Tertiary of forms (the Ancyclopoda) undoubtedly ungulate in most char- 

 acters, but with strong clawed feet. Such exceptions to the general rules 

 are rare, but the exceptions serve to compel caution. The inferences drawn 

 from one portion of the skeleton of these animals and fragmentary skeletons 

 are by far the most common remains of vertebrates would be at total 

 variance with those drawn from another part. 1 



Similar examples could be drawn in large numbers from among the 

 invertebrates, although the specializations and adaptations are not always 



so striking. 



(A) FAUNAL ELEMENTS AS TIME-MARKERS. 



Obvious enough to the trained stratigrapher, it is still important to warn 

 other workers of the varying value of fossils as indicators of passing time 

 and changing conditions. Such forms as the brachiopod Lingula and the 

 star-fishes are classical examples of genera and groups that have remained 

 unchanged through long periods of time and are valueless as time-markers 

 unless the most careful specific and varietal determinations are made. 

 Other more plastic forms, as the ammonites of the Mesozoic, record in their 

 rapid changes short intervals of time and rapid and slight changes of en- 

 vironment. Caution is also necessary in accepting as archaic, types which 

 recall ancient forms of life. The Tuatara lizard Sphenodon, of New Zealand, 

 long regarded as an example of a survival from Mesozoic or even late Paleo- 

 zoic times, is now under suspicion as possibly being very specialized, with 

 but few very archaic characters. 2 



(*) THE FLORA OF THE UNIT. 



Much that has been said concerning the fauna of any unit is equally 

 true of the flora, and similar checks must be used in interpreting the fossil 

 remains. 



One difference between the animal and plant worlds has been given 

 great weight in all considerations of paleogeography, that is, the compara- 

 tive immobility of plants. For this reason they have been considered as 

 especially good measures of climate and climatic fluctuations. This idea 

 must not, however, remain unchallenged. Attention has often been brought 

 to the fact that invertebrates, the most common resource of stratigraphers, 

 are dispersed by forces entirely independent of their own motile powers. 

 Eggs and free-swimming embryos of fixed forms are dispersed by currents 

 of water or air; adult individuals of non-sessile kind are equally readily 

 swept into new regions; their ultimate extinction or preservation in the new 

 region is entirely independent of the mode of migration. Even vertebrates, 



1 Scott, W. B., Land Mammals of the Western Hemisphere, pp. 353 and 383, 1913. 

 * Ruedemann, R., The Paleontology of Arrested Evolution. N. Y. State Museum Bull. 

 No. 196, 1918. 



