PALEOBIOGEOGRAPHY OF THE MARINE REALM 179 



dispersal through relatively brief larval stages more than it does 

 provincial environmental differences. 



In the photic zone of the shelf and bank seas adjacent to or 

 within these olenellid provinces, calcareous spongelike archaeo- 

 cyathids joined with a variety of lime-fixing algae to create reef- 

 like organic buildups from the mouth of the Khatanga River in 

 Siberia to central East Greenland and to the Beardmore Glacier 

 area of Antarctica. The presence of a variety of filamentous and 

 botryoidal algal structures associated with the archaeocyathids 

 confirms their preference for the shallower and probably warmer 

 parts of the Cambrian seas. 



Conversely, the detrital and weakly calcareous deposits of the 

 adjacent olenellid assemblages imply deeper, cooler waters for 

 them, which is consistent with the generally modest endemism and 

 smaller biotal variety of the olenellid provinces. Moreover, 

 although the archaeocyathids were locally able to establish 

 themselves in the shallower parts of the olenellid provinces, the 

 olenellids were seemingly not able to migrate across the broad 

 epicontinental reaches of the shallower and more extensive 

 archaeocyathid seas, as in Siberia. Around the central core of 

 North America the olenellids generally occur inside the belt of 

 archaeocyathids, which may mean either that the latter occupied 

 narrow shelf seas adjacent to offshore lands and defining the outer 

 edge of geosynclinal olenellid seas, or that deeper shelf seas with 

 olenellids were separated by shelf-edge archaeocyathid reefs from 

 the yet unknown geosyncline proper further offshore. More 

 evidence is needed to resolve this problem. 



Regardless of its more specific details, the described fourfold 

 biogeographic classification of the Early Cambrian biotas is fairly 

 interpreted as implying a regional sedimentological and bathy- 

 metric subdivision within a generally warm global climate of little, 

 or yet undeciphered, latitudinal variation. Information so far 

 synthesized neither demands nor opposes a different latitudinal 

 orientation from the present one. 



Gondwana 



Intermittently from middle Paleozoic (Devonian) through at 

 least early Mesozoic (Triassic) time, significant fractions of the 



