THE SPECIES PROBLEM WITH FOSSIL ANIMALS 



ters. In addition, M. senonensis tends to be relatively taller than 

 M. coranguinum. 



Studies of living spatangoid echinoids make possible a reason- 

 able interpretation of the data presented above in terms of en- 

 vironmental adaptations. According to Kermack's view, the 

 evolution of the main M. leskei-corangninum stock is to be inter- 

 preted as progressively better adaptation to a burrowing habit. 

 This view is based primarily on known functions of the labrum 

 and subanal fasciole. In burrowing urchins, the labrum increases 

 the efficiency of the mouth as an organ of ingestion; and dense 

 cilia associated with the subanal fasciole produce strong posteri- 

 orly directed water currents which aid in discharging water from 

 the burrow. 



The M. senonensis stock, on the other hand, seems to have 

 evolved adaptations for living on rather than in the bottom. For 

 this mode of life a subanal fasciole is unnecessary. The domelike 

 form of the test, moreover, might facilitate cleaning of the sur- 

 face; at least it would not hinder movement on the bottom. 



We now come to the problem of expressing the facts and 

 inferences about Micraster taxonomically. At least three possible 

 criteria can be used for subdivision of such a continuum. First, 

 boundaries between species (or subspecies) might be placed at 

 those points in the phylogenetic tree where branching takes 

 place. Assuming that the phylogeny has been adequately docu- 

 mented, this criterion has at least the merit of objectivity. But 

 units so delineated may differ very greatly in scope. In the case 

 of Micraster — assuming a relatively constant morphological evo- 

 lutionary rate — the oldest lineage segment would be much 

 smaller in morphological scope than the youngest. 



Stratigraphic boundaries offer another possible basis for the 

 delimitation of taxonomic units. Micraster, for example, might 

 be split into eleven species with taxonomic boundaries placed 

 at the limits of the six stratigraphic zones. Although to a biologist 

 such a course might appear rather arbitrary, to a stratigrapher 

 units so defined might be quite useful. 



From a strictly biological point of view probably the most 

 satisfactory basis for subdividing an unbranched lineage is mor- 



