232 L. HILLIS-COLINVAUX 



low numbers of Halimedae in these places may not be so much the 

 result of a MacArthur and Wilson equilibrium as an almost infinitely 

 prolonged early colonization stage. 



B. Palaeobiogeography and prehistory 



Halimeda lived and contributed its segments to ancient reefs much 

 as it does today. The known fossil Halimedae have been found in 

 limestone facies of the Tethyan (Mediterranean and Persian Gulf) 

 region (Elliott, 1960), Mexico and Texas (Johnson, 1969). The genus 

 itself is first recorded from the Cretaceous (Elliott, 1960; Johnson, 

 1969), but would date from the Middle Jurassic if the two genera, 

 Boueina and Arabicodium, which are similar to it, are included in the 

 genus as recommended by Elliott (1965) and Johnson (1969). The 

 epithet Halimeda has priority. These early Halimedae are small- 

 segmented, and are considered to have developed from hybridizations 

 between the species groups Arabicodium and Bouenia (Elliott, 1965). 

 The existence of such a long historical record, uncommon among 

 macroalgae, has the potential of providing considerable information 

 about the evolutionary history of the taxon as well as contributing to 

 the understanding of present-day distribution. However, the data for 

 Halimeda are few at present. 



The known distribution of fossil material as indicated above is 

 along part of the borders of the Tethys Sea and the western North 

 Atlantic. It also seems likely that halimediform algae grew in shallow- 

 water sites over a wider range of these particular shores as well 

 as extending westward into the Indo-Pacific, along the eastern 

 "Pacific" shores of the existing land masses, and by Miocene times 

 possibly along the northern shores of Australia. Examination of new 

 sites may prove that this is so. The faunas for the Caribbean and 

 Mediterranean regions of the time appear to have much in common 

 (Valentine, 1973; Berggren and Hollister, 1974a; Hallam, 1975), which 

 is possibly the outcome of a current system common to much of the 

 palaeoequatorial region, and to the low variability of climate. Perhaps 

 the Halimeda fiora also was fairly similar for much of the region. 



Within the Tethyan area itself, five fossil species of Halimeda, 

 H. nana Pia, H. praemonilis Morellet and Morellet, H. eocaenica 

 Morellet and Morellet, H. praeopuntia Morellet and Morellet and an 

 unnamed Halimeda species, have been reported for the Tertiary 

 preceding the Messinian crisis (Morellet and Morellet, 1922, 1941; 

 Elliott, 1960). One species of Boueina also occurred. There was, 

 therefore, considerably more species diversity of Halimeda in this 



