Major Patterns of Variation | 259 



Latimeria is a living representative of the coelacanth fishes, a group 

 once thought to have become extinct in the Cretaceous. Metasequoia, 

 the dawn redwood, was first described as a fossil from the Pliocene, 

 only to be found extant in China some years later. Another check of 

 our inferences drawn from comparing recent and fossil forms is the 

 discovery of previously postulated "missing links." The most famous 

 such find was perhaps Dubois's discovery of the remains of Homo 

 erectus (Pithecantliropus) , a fossil man with "ape-like" character- 

 istics (see Chap. 12). 



Fossils must be viewed as a very biased sample of the remains of 

 past life. Disturbances in the earth's crust have all but wiped out 

 critical parts of the fossil record, and the most interesting and vital 

 earlier portion is the most distorted. Parts of some organisms, such 

 as \'ertebrate bones, shells of mollusks and Foraminifera, woody 

 portions of plants, and cuticles of leaves, are more readily fossilized 

 than others. Organisms less well endowed with hard or resistant 

 parts (e.g., worms, larval insects, most algae and fungi) are less 

 often preserved. The habitat of the organism also has a large effect 

 on the probability of its becoming a fossil. A steppe-dwelling animal 

 or plant stood a much smaller chance of being preserved than one 

 from a swamp. Factors favorable for the preservation of plant re- 

 mains often are not suitable for animal preservation. (For example, 

 acid conditions that might preserve leaves would dissolve shells and 

 bones. ) Finally, certain specialized habitats, such as tar pits, mineral 

 water bogs, and volcanic slopes, may be preserved almost in toto at 

 times with whatever assortment of organisms might happen to be 

 present or attracted to them. 



Nevertheless, the fossil record is suflBciently ample and diverse 

 that paleontologists have recognized many patterns and described 

 processes thought to have been responsible for their development. 

 Many analytical problems are peculiar to paleontology. For example, 

 in addition to the taxonomic difficulties resulting from diversifica- 

 tion across a heterogeneous environment (horizontal speciation), 

 one must contend at the same time with differentiation through time 

 ( vertical speciation ) . 



Modes of Evolution 



Simpson and others have summarized what is known of the major 

 patterns of evolutionary change, particularly with respect to animals. 

 Simpson divides these patterns into three modes. Many instances of 

 the first mode-the splitting of phylogenetic lineages-may be traced 

 in the fossil record. These range from the division of the archosaurian 



