Sirenians, seagrasses, and Cenozoic ecological change in the Caribbean

Abstract The Western Atlantic–Caribbean region preserves the longest (some 50 million years) and most diverse fossil record of the mammalian order Sirenia in the world. Sirenians are aquatic herbivores; in marine waters, they eat mainly seagrasses (Hydrocharitaceae and Potamogetonaceae). Though the fossil record of seagrasses and other marine macrophytes is meager, we can use it together with that of sirenians to frame interesting hypotheses about marine plant evolution, and speculate about the nature and dynamics of the Caribbean seagrass ecosystem during the ages when (unlike today) it was dominated by diverse large mammalian herbivores. In this synthesis, I define the aquatic megaherbivore adaptive zone, and identify those of its ecological dimensions that are likely to have been most critical in feeding-niche partitioning among fossil sirenians. Possible morphological indicators of such partitioning are specified for each of these dimensions, using data on functional anatomy and ecology of living sirenians. Faunal lists of Caribbean sirenians (including unpublished taxonomic and morphological data) show what suites of species have occurred sympatrically at different times during the Cenozoic. Based on cranial morphology, I suggest how these herbivores most probably partitioned their resources. This makes it possible to follow the history of individual feeding niches within the sirenian guild through geological time, and, in turn, to speculatively reconstruct gross aspects of Caribbean seagrass communities and their changes through time. Eocene Caribbean sirenian faunas included prorastomids, protosirenids, and early dugongids. These were diverse in both locomotor and feeding adaptations, ranging from amphibious to fully aquatic forms and from selective browsers to incipiently indiscriminate grazers. Eocene Caribbean seagrass communities were already essentially modern in aspect, but included a greater diversity of species with mid-sized to large rhizomes. Oligocene faunas comprised as many as six apparently sympatric species. These included only dugongids, but were differentiated into halitheriines (generalized feeders on seagrass leaves and smaller rhizomes, or specialists on leaves) and dugongines (specialists on larger rhizomes). Miocene faunas of marine sirenians continued the Oligocene pattern. In freshwater habitats of South America, trichechids appeared in the record and adapted to an increase in abundance of fibrous, abrasive aquatic grasses (Gramineae). Pliocene marine sirenian faunas in the Caribbean consist only of dugongine dugongids, plus a trichechid species which evidently extended its range to North America. The dugongines most specialized for eating large rhizomes apparently died out during this epoch. Pleistocene and Recent faunas comprise only trichechids (manatees). Large-tusked dugongines probably acted as “keystone species”, disrupting climax seagrass communities and thereby increasing their productivity and diversity (including sirenian diversity). Under these conditions (which prevailed up to about 2–3 Ma), most primary productivity was presumably consumed by herbivores, in contrast to the detritus-based seagrass ecosystems of today, which include few if any large herbivores. Pliocene oceanographic changes following closure of the Central American Seaway probably led to the collapse of this diverse system, causing extinction of several large, “climax” seagrass species and producing the florally and faunally depauperate seagrass communities of the present-day Caribbean. The resulting “vacuum” in marine sirenian niches may have enabled manatees to spread from South America into the Caribbean and North America, as well as West Africa.