What do I mean by the title of this post?
Here's the back story. As some of you may, or may not know, sirenians are closely related to elephants, and more distantly to other African mammals, such as rock hyraxes and aardvarks, among others (Asher et al., 2003); altogether these organisms are classified as Afrotherians (= African beasts). As the name implies, these organisms are either endemic of the African continent, or evolved there originally. The earliest proboscidean (elephants and their kin) appeared in what is now Morocco in the middle Paleocene (~60 million years ago) (Gheerbrant, 2009). This implies that the split between sirenians and proboscideans occurred in the early Paleocene (65.5-61.7 mya) somewhere in the northern half of the African continent. However, as we know, the fossil record is far from perfect, and for many years, that of sirenian implied that they originated outside of the African continent. The oldest-most-primitive sirenians, Prorastomus sirenoides Owen, 1855, and Pezosiren portelli Domning, 2001, (collectively known as prorastomids), were known only from early-middle Eocene (~48-40 million years ago) deposits in the island of Jamaica in the Caribbean region, making the origins of sirenians a paleobiogeographical conundrum. This, however, has changed in the last several months thanks to new discoveries.
|Composite skeletal reconstruction of Pezosiren portelli, (modified from Domning, 2001).|
Prorastomids in Africa
Last fall, in the August issue of Journal of Vertebrate Paleontology, a paper came out describing the first prorastomid remains from Africa (Hautier et al., 2012). The fossil was found in phosphatic deposits in the country of Senegal in the western part of the continent. Only one fossil was recovered, a single posterior thoracic vertebra. Fortunately, prorastomid vertebrae are quite different from those of other sirenians. The Senegalese fossil shows those telltale features of prorastomid vertebrae, such as the triangular outline of the neural canal, and the T-shaped spinous process (Domning, 2001; Hautier et al., 2012). This fossil, although from slightly younger deposits than the Jamaican prorastomids, confirmed the presence of these ancient sirenians in Africa.
A new fossil confirms the African origin of Sirenia
Just last week a paper by Julien Benoit and colleagues described a sirenian petrosal* found in early Eocene (~50 mya) freshwater deposits in Tunisia (northern Africa). The pretrosals of sirenians, unlike those of cetaceans (whales & dolphins) are not regarded as diagnostic. However the petrosal from Tunisia is distinctively primitive, more so than that of Prorastomus sirenoides (Savage et al., 1994; Benoit et al., 2013). That, combined with the older age of the Tunisian petrosal, confirms the presence of early sirenians in northern Africa. This new find, also adds additional information to the origins of sirenians. Sirenians were though to have gone aquatic earlier and more directly than cetaceans, which went through a freshwater phase (Clementz et al., 2006). The Tunisian sirenian was collected from freshwater deposits, showing that they did went through a freshwater phase first, then became fully marine (and some have eventually gone back to freshwater [i.e. Amazonian manatee]).
*This is the part of the skull that houses the hearing and balance organs; known as the petrous part of the temporal bone in humans. For more information on sirenian petrosals go here.
|Comparison of the Tunisian petrosal (A-C) with that of Prorastomus sirenoides (D-F) (from Benoit et al., 2013).|
Other questions still remain. Early sirenians, like Pezosiren and most likely Prorastomus and the Tunisian sirenians were amphibious; still having fully developed hindlimbs (see first figure). So, the question is, how did they made it from northern Africa to the Caribbean; did they crossed directly across the Atlantic, or did they followed the nearly continuous coastline present in the northern Atlantic in the early Eocene?
Personally, I favor the first. Cetaceans do seem to have taken the northern Atlantic route, whereas evidence for sirenians doing so, is still lacking (Beatty & Geisler, 2010). In addition, other tetrapods have crossed from Africa to the Caribbean and South America during the Cenozoic. Gavialoids crossed from northwestern Africa to the Caribbean sometime during the late Eocene, giving rise to gryposuchines, an endemic group of Caribbean-South American gavialoids (Vélez-Juarbe et al., 2007). Platyrrhine primates, caviomorph rodents and hoatzins did as well crossed from African to South America during the Eocene and/or Oligocene (Poux et al., 2006; Mayr et al., 2011; Antoine et al., 2012). The distance from Africa to South America and the Caribbean was shorter then than it is now, which likely made these multiple trans-Atlantic crossings more likely.
|Map showing dispersal routes of early sirenians and cetaceans (from Benoit et al., 2013).|
For a nice post about the Tunisian sirenian, go read Brian Switek's post over at NatGeo.
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