ECOLOGY AND TAXONOMY OF Holimeda 237 



species such as lacunalis, macrophysa, macroloha or borneensis may be 

 successfully replacing them. An alternative explanation is that their 

 low numbers may be an artifact of the collections available. 



The time-scale for the first events in the history of Halimeda has 

 been ascertained by the location of fossils in the appropriate strata, 

 and by inference from our understanding of subsequent historical 

 events. Some modern species, however, may have existed earlier than 

 hitherto implied. The reason for questioning the time-scale lies in the 

 present disjunct distribution of H. cuneata. One explanation for its 

 modern occurrence in the southern subtropical regions, delimited by 

 the south-eastern tip of South Africa, southern tip of Madagascar, 

 south-western and south-eastern coasts of Au&tralia, and in the northern 

 subtropical region of the Gulf of Cutch in north-west India (Fig. 75) 

 is that this species had evolved at least by the time the regions 

 represented were located in the same latitude, that is, before India had 

 joined with continental Asia. The latest that they were in close proxi- 

 mity was in the Late Cretaceous (Smith and Briden, 1977). li cuneata 

 existed then, it seems likely that other modern pantropical species had 

 also evolved, and therefore would have coexisted with the fossil species 

 which, as already indicated, have been described for a later epoch. An 

 alternative explanation is that cuneata formerly occurred along much 

 of the east coast of Africa and the western shores of India, that it 

 eventually extended into subtropical waters and was displaced, or 

 essentially so, from tropical regions. This line of reasoning does 

 not require such an early differentiation of the taxon. Informa- 

 tion from fossils may provide the decisive support for one of these 

 hypotheses. 



In conclusion it may be said that there is much direct evidence in 

 the fossil record for Halimeda species being both conservative and 

 ancient. The data allow the working hypothesis that many of the 

 present panoceanic stocks can be identified as early as the beginning 

 of the Tertiary. Local speciation has occurred only where isolation has 

 been very prolonged. The possibility that diversity is a function of very 

 poor and spasmodic opportunities for recruitment, rather than due to 

 the establishment of species equilibria, is encouraged. 



D. A biogeographical approach to the phylogeny of the Caulerpales 



Morphological characteristics have provided most of the information 

 for placing Halimeda in phylogenetic schemes (Gepp and Gepp, 1911; 

 Gilmartin, 1966; Parker, 1970), but again biogeographical data can 

 contribute useful support or indicate weaknesses. 



