Cranial Remain from Tunisia Provides New Clues for the Origin and Evolution of Sirenia (Mammalia, Afrotheria) in Africa

Julien Benoit,Rodolphe Tabuce,Monique Vianey-Liaud,Hayet Khayati,Laurent Marivaux,Samuel Merigeaud,Sylvain Adnet,Essid El Mabrouk,Gilles Merzeraud,Mustapha Ben Haj Ali

doi:10.1371/journal.pone.0054307

Abstract

Sea cows (manatees, dugongs) are the only living marine mammals to feed solely on aquatic plants. Unlike whales or dolphins (Cetacea), the earliest evolutionary history of sirenians is poorly documented, and limited to a few fossils including skulls and skeletons of two genera composing the stem family of Prorastomidae (Prorastomus and Pezosiren). Surprisingly, these fossils come from the Eocene of Jamaica, while stem Hyracoidea and Proboscidea - the putative sister-groups to Sirenia - are recorded in Africa as early as the Late Paleocene. So far, the historical biogeography of early Sirenia has remained obscure given this paradox between phylogeny and fossil record. Here we use X-ray microtomography to investigate a newly discovered sirenian petrosal from the Eocene of Tunisia. This fossil represents the oldest occurrence of sirenians in Africa. The morphology of this petrosal is more primitive than the Jamaican prorastomids’ one, which emphasizes the basal position of this new African taxon within the Sirenia clade. This discovery testifies to the great antiquity of Sirenia in Africa, and therefore supports their African origin. While isotopic analyses previously suggested sirenians had adapted directly to the marine environment, new paleoenvironmental evidence suggests that basal-most sea cows were likely restricted to fresh waters.

Highlights

Sirenia include the living manatees and dugongs, but the widespread and rich fossil record of this order testify to its outstanding past successfulness [1]
Many of the earliest fossil records of these orders are found in the Paleogene of Africa [2,3,4,5], which implies that the Paenungulata, including Sirenia, probably shared a common ancestry on the African continent [6]
Molecular phylogenies support an African root for Paenungulata, considering that they belong to a clade of extant African mammals, the so-called supercohort Afrotheria [2,5]

Summary

Introduction

Sirenia include the living manatees and dugongs, but the widespread and rich fossil record of this order testify to its outstanding past successfulness [1]. The oldest and most primitive sirenians were exclusively found in the late Early to Early Middle Eocene of Jamaica [7,8] These primitive species belong to the family of Prorastomidae, a paraphyletic group consistently appearing as the basal-most offshoot within the Sirenia clade [4,9,10,11]. Among those ‘‘prorastomids’’, Prorastomus is primarily documented by an almost complete skull from the late Ypresian or early Lutetian (,48–50 Ma) of Jamaica [7] and, so far, this taxon represented the most ancient and primitive sirenian known. CBI-1-542 is a left petrosal, half the size of the one of Prorastomus (Fig. 1D, E, F, table 1), and represents the earliest cranial remain of a sirenian found in Africa

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