Ghost of seagrasses past: Using sirenians as a proxy for historical distribution of seagrasses

Abstract

Abstract Seagrasses are a notable component of shallow marine habitats, especially in tropical and subtropical regions. Their fossil record extends back to the Mesozoic, but it is relatively poor and fragmentary, with large temporal and geographical gaps. As a result, very little is known about the paleobiogeography of these plants and how physical drivers, such as climatic or oceanic events, have affected their distribution. One approach is to infer the past distribution of seagrasses using fossils of organisms dependent of seagrasses with more complete records as proxies. Seagrass consumers, such as sirenians (seacows, manatees and dugongs), are much better represented in the fossil record than seagrasses are. The characteristically dense bones of sirenians together with the fact that they are usually found in marginal marine environments increases the potential for preservation and recognition of their fossils. The long evolutionary history of sirenians, extending throughout most of the last 50 million years, together with their diet permits the use of their fossils as a proxy for inferring the paleobiogeography of seagrasses. Here I looked at the fossil record of sirenians and seagrasses from the Eocene, through the Miocene epochs. This comparison produced several inferences about seagrass paleobiogeography and how physical drivers, such as climate change, ocean currents and tectonic events, have been influential in their distribution: 1) seagrasses were well-established in the Western Atlantic-Caribbean prior to the middle Eocene, making possible at least two instances of trans-Atlantic sirenian dispersal events, either with the aid of Tethyan currents or along the nearly continuous Northern Atlantic coastline that was present in the Eocene; 2) climatic cooling during the early Oligocene seemed to have limited the extent of seagrasses and sirenians, although these groups recovered and further diversified and expanded their distributions by the late Oligocene in tandem with a climatic warming event; 3) by the Miocene, seagrasses and sirenians reached the southern Western Atlantic and the Eastern Pacific aided by the presence of the Central American Seaway, achieving a distribution similar to, and sometimes, surpassing that of today. The fossil record of sirenians can provide a broad overview of seagrass paleobiogeography through time. However, several aspects, such as when sirenians and seagrasses arrived to Australia and the seemingly late arrival of seagrasses to South America and the Eastern Pacific, still need further investigation.