Revisiting the Cenozoic History and the Origin of the Eastern Pacific Coral Fauna

Abstract

Distribution, composition and functioning of eastern Pacific (EP) coral communities and reefs have resulted from modern and ancient processes. While much has been learned from living systems, still many questions remain regarding how pre-Holocene events shaped modern EP coral communities and reefs. From the late Paleocene to late Miocene, fossil outcrops were spatially restricted to the Washington-Seattle and California regions, whereas from the late Miocene to late Pleistocene corals have been mainly recovered from the Gulf of California, but scarcely (just two outcrops) from western Mexico and Central America. From the Paleocene to Recent, 191 reef-building species, including living taxa without fossil representatives, have inhabited the eastern Pacific region. Of the 53 identified genera, 11 are living and 42 are regional (i.e., currently restricted to the Indo-Pacific or Caribbean and western Atlantic) or globally extinct; 49 species are living and 142 are extinct. Fourteen of the 48 living EP species have fossil records that extend back to the early (Porites panamensis) and middle (Pocillopora capitata) Pliocene, while the remainder appeared during the middle to late Pleistocene in the Gulf of California. Considering the age and number of taxa per assemblage, genera and species ranged from 1 to 21 (mean = 2.98 ± 3.75 SD), and 1–39 (mean = 3.87 ± 6.12 SD) respectively. The highest numbers of genera (>11) and species (>12) correspond with the middle Eocene to early Miocene of Central Chiapas, and the late Eocene of Panama. As such, EP coral communities and reefs probably never attained high species richness; in addition, depositional evidence and paleoenvironmental reconstructions of coral-bearing deposits from Washington-Seattle, Central California and the Gulf of California suggest that from late Paleocene to late Pleistocene, reefs were small and paucispecific as are today’s formations. Based on coral occurrences, species richness increased during the Late Eocene/Early Oligocene (55 species), decreased during the early and middle Pleistocene (5 species), rose again during the late Pleistocene (13 species) and finally peaked again during Recent times (46 species). Bootstrap tests of the temporal changes in species richness indicate that except for the numbers recorded in Late Eocene-Early Oligocene, Late Pleistocene and Recent, changes elsewhere were non-significant. From the spatio-temporal distribution of the fossil outcrops, our knowledge regarding events directly related to the origin and evolution of the current coral fauna is biased since it represents the evolutionary history of subtropical coral communities and reefs from the Gulf of California. Quantitative analysis of presence/absence coral data suggests that during the last six million years, Gulf coral communities and reefs experienced dramatic turnover, in particular (a) the extinction of Caribbean-related regional endemics, and (b) since the middle Pleistocene, the steady arrival of Indo-Pacific taxa likely via the North Equatorial Counter Current. Lastly, morphologic, electrophoretic and molecular analyses clearly support a strong affinity of the eastern Pacific endemics, Porites panamensis and Porites sverdrupi, with the Caribbean and western Atlantic Porites clades, suggesting that the eastern Pacific poritid fauna resulted from local speciation of Caribbean/Atlantic populations and long-distance dispersal of Indo-Pacific taxa (e.g., Porites australiensis, Porites lutea). Hence, the vicariance and dispersal hypotheses regarding the origin of the eastern Pacific coral fauna, at least for some taxa, are complementary.