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Free Content Zoogeography and Evolution in the Octocorallian Family Gorgoniidae

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Abstract:

The predominance of Gorgoniidae and Plexauridae in the shallow-water alcyonarian fauna of the West Indian and warm-water east Pacific regions is noted. The family Gorgoniidae is briefly characterized and its subdivisions down to genus defined. The subfamily designations Gorgoniinae and Lophogorginae are established. The nature of spiculation and external morphology is described and the significance of certain features in establishing an evolutionary sequence is explained. At the present time, two distinct lines of evolution can be detected within the family: one, well established and differentiated, characterized by simple spindle sclerites, centered as a whole in the American tropics but the most generalized species of world-wide warm-water distribution; and a second, less clearly differentiated but closely paralleling the development of the first, characterized by modified spicules, and limited to the Antillean region. The modern distribution is interpreted with the help of palaeogeography, which reveals Tertiary marine seaways through Central America. During the period of interoceanic communication, the homogeneous Atlantic-Pacific gorgonian population probably consisted of spindle-bearing species which had begun to undergo differentiation of branching pattern and were at the “anastomosed fan” level; spicular differentiation toward disk spindles had probably commenced also, since some Atlantic species have disk spindles nearly as well developed as those of typical Pacific Eugorgias. As the Pliocene wore on and interoceanic communication came to an end, the Gulf-Caribbean area apparently suffered changes in conditions that caused all but the hardiest species, or those in areas still reasonably favorable for gorgonian growth, to die out. In the subsequent ages, there has arisen from the small nucleus of undifferentiated species that survived, an entire new lineage which has flourished and now occupies as dominant a position in the shallow Caribbean waters as the relatively unchanged group from whose ancestors it arose still does in the eastern Pacific. It has followed much the same road of morphological development, but the physiological adjustment that the isolated eastern segment of the fauna achieved in the struggle for survival was accompanied by modification of spicule type from which developed the scaphoid sclerites that characterize all members of the lineage. That it is a newer group than the spindle lineage is suggested by the low level of differentiation between species. In the West Indian region the old spindle lineage now occupies a subordinate position and has not continued to develop as it has on the west coast of the Americas, where an extensive array of species has evolved.

Document Type: Research Article

Publication date: 1953-01-01

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