246 



Origin of Biomes and Succession 



Fig. 106. The region (shaded area) where the fi\e reptiles CrotapJnjius 

 collaris, Hollhrookia texana, Phnjnosoma cornatiim, Phnjnosomo modc.stum, 

 and Eiimeces obsoletus now occur together. The black dot indicates where the 

 five occurred together during the Sangamon interglacial period of the Pleis- 

 tocene, the location of the Cragin Quarry beds. (After Etheridge.) 



the Caribbean. Chace (1958) described a squillid crustacean Ly- 

 siosquiUa graiji from the western Atlantic, which he thought most 

 closely related to L. decemspinosa, a species occurring in similar 

 habitats only in the Pacific waters of Peru and Costa Rica. Accord- 

 ing to Morrison ( 1959) at least five genera of tropical or subtropical 

 salt marsh snails include pairs of very closely related species, one 

 on the west Atlantic and one on the east Pacific shores of the Ameri- 

 cas, indicating that the rise of Central America di\ided not only 

 the aqueous marine commimities existing between North and South 

 America, but the beach and salt marsh communities also. Hopkins 

 (1959) pointed out evidence in Mollusca indicating past divisions 

 of marine communities in the Bering Sea region. 



Geomorphic action leads to reunion as well as division of com- 

 munities, as is shown in Fig. 104. Also biotas can mo\'e in more 

 than one direction. In contrast to the lizard example in Fig. 106, 

 collections of fossil fish found in an Oklahoma deposit of probably 

 Illinoian (mid-Pleistocene) age contain an aggregation of species 

 which now occur together only considerabh' north and east of the 

 fossil locality (Fig. 107) (Smith, 1954). Alternating community 

 movements are reflected in the mixtures of closely related species 

 now living within the same area. The forest-floor inhabiting milliped 

 genus Brachoria (Fig. 108) is especially suggestive of such a history, 



