Manatees and the dugong, known collectively as sirenians, are herbivorous, aquatic mammals that are generally restricted to shallow, tropical waters. Like whales, sirenians have reduced the amount of hair on their bodies, have forelimbs shaped into flippers, lack external hindlimbs and have a paddle-like tail.
Despite their overall similarities, the DNA of sirenians is much more similar to that of elephants and the DNA of whales is much more similar to hoofed mammals like cows and giraffes. This suggests that both sirenians and whales independently adapted to living in an aquatic medium. In fact, the fossil record of whales suggests a transition from a land-based habitat to a completely aquatic way of life. Do sirenians show the same pattern?
Indeed they do! One of the earliest sirenian species appearing in the fossil record is Pezosiren, a mammal with an unmistakably sirenian-like skull, but the rest of its skeleton clearly suggests that it had the ability to walk on land. Unlike modern sirenians, this animal had forelimbs that are not shaped like flippers but instead were used to support their weight on land, as well as hindlimbs and a distinct hip bone (pelvis). Pezosiren has been found in rocks estimated to be ~47.8 million years in age, similar to the age that walking ‘whales’ were also found roaming the earth.
More recent sirenian fossils show evidence of fully committing to an aquatic lifestyle, including Halitherium, which is found in rocks estimated to be ~38 million years old. This species and similarly dated fossils had modified their forelimbs into flippers, possessed thick and dense ribs to provide ballast, and reduced their hindlimbs, presumably to minimize drag. Though it looked very much like modern manatees (see below) and dugongs, it had a better-developed, albeit very reduced, pelvis+hindlimb complex.
In addition to their better-developed limbs, early species of sirenians had a number of other traits that manatees and the dugong have lost. Pezosiren and others had multiple vertebrae in the hip region (sacral), much like their land-dwelling elephant relatives, whereas most fossil and all modern sirenians have fused these into a single bone. Modern sirenians also have reduced their teeth to a great extent compared to their fossil forebears. Earlier species had permanent premolars, canines and incisors, whereas modern species lack these teeth (dugongs have only one incisor). See below for a diagram of a fossil sirenian (Protosiren, ~47.8 million years old) showing the positions of the premolars (P), canine (C) and incisors (I).
Questions for Creationists
Where did the dozens of other sirenian species go? Given their aquatic adaptations, wouldn’t they have survived Noah’s flood? Is it just a coincidence that molecular phylogenetics shows sirenians as being genetically similar to land mammals like elephants and there are sirenian fossils that appear to document a transition from land to water? Is it coincidence that we see parallel evidence of a land to water transition in whales?
1. Springer, M. S., Signore, A. V., Paijmans, J. L., Vélez-Juarbe, J., Domning, D. P., Bauer, C. E., … & Meredith, R. W. (2015). Interordinal gene capture, the phylogenetic position of Steller’s sea cow based on molecular and morphological data, and the macroevolutionary history of Sirenia. Molecular phylogenetics and evolution, 91, 178-193.