Stable isotope evidence for an amphibious phase in early proboscidean evolution

Abstract

The order Proboscidea includes extant elephants and their extinct relatives and is closely related to the aquatic sirenians (manatees and dugongs) and terrestrial hyracoids (hyraxes). Some analyses of embryological, morphological, and paleontological data suggest that proboscideans and sirenians shared an aquatic or semiaquatic common ancestor, but independent tests of this hypothesis have proven elusive. Here we test the hypothesis of an aquatic ancestry for advanced proboscideans by measuring ␦ 18 O in tooth enamel of two late Eocene proboscidean genera, Barytherium and Moeritherium, which are sister taxa of Oligocene-to-Recent proboscideans. The combination of low ␦ 18 O values and low ␦ 18 O standard deviations in Barytherium and Moeritherium matches the isotopic pattern seen in aquatic and semiaquatic mammals, and differs from that of terrestrial mammals. ␦ 13 C values of these early proboscideans suggest that both genera are likely to have consumed freshwater plants, although a component of C 3 terrestrial vegetation cannot be ruled out. The simplest explanation for the combined evidence from isotopes, dental functional morphology, and depositional environments is that Barytherium and Moeritherium were at least semiaquatic and lived in freshwater swamp or riverine environments, where they grazed on freshwater vegetation. These results lend new support to the hypothesis that Oligocene-to-Recent proboscideans are derived from amphibious ancestors.Barytherium ͉ Eocene ͉ Fayum ͉ Moeritherium ͉ Proboscidea T he elephants Elephas and Loxodonta (order Proboscidea) are the only living remnants of a major adaptive radiation whose origin can now be traced back to the earliest Eocene (Ϸ55 million years ago) in Africa (1). Genomic data place proboscideans within the placental mammalian superorder Afrotheria (2) and the more restricted supraordinal clade Paenungulata, which also contains the aquatic manatees and dugongs (order Sirenia) and terrestrial hyraxes (order Hyracoidea). Genetic evidence has thus far failed to resolve relationships among paenungulate orders (2, 3), but a recent analysis of genomic and morphological evidence provided weak support for a Proboscidea-Sirenia clade (Tethytheria) to the exclusion of Hyracoidea (4). A monophyletic Tethytheria has long been seen as the best explanation for available morphological evidence (5) and is key to the hypothesis [also based on developmental (6) and paleontological (5, 7) evidence] that the common ancestor of elephants and sea cows might have been at least semiaquatic.Eocene proboscideans were radically different from living elephants in their size, skeletal and dental morphology, and presumably many aspects of their ecology and behavior as well (5,(7)(8)(9)

Reassessment of the Generic Attribution ofNumidotherium savageiand the Homologies of Lower Incisors in Proboscideans

“…The recent isotope analysis of Barytherium's enamel sug− gested that it grazed freshwater vegetation in a swamp or riverine environment (Liu et al 2008), supporting this sugges− tion. Similar results were obtained from the enamel of Moeri− therium.…”

“…The recovery of a few weathered remains of an old individual at the begin− ning of the twentieth century in the Qasr El Sagha Formation of the Fayûm led to the description of its type species, B. grave Andrews, 1901. Very few discoveries (consisting of isolated remains only) referable to the genus have been re− ported in the region since (Harris 1978;Holroyd et al 1996), though many new specimens have been recovered from the Birket Qarun Formation in recent years (Seiffert 2007;Liu et al 2008; Seiffert et al 2008). In contrast, hundreds of cra− nial, dental and postcranial elements referable to Barythe− rium, including a partial skeleton, were discovered at Dor El Talha (Savage 1969).…”

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

“…In Proboscidea, isotopic analysis suggest that stem taxa ( Moeritherium and Barytherium ) may have feed on freshwater plants and lived in freshwater swamps [40]. This assertion is supported by the study of developmental growth in modern elephants which has shown that they display some characters involving a past adaptation to life in water (e.g.…”

The new framework for understanding placental mammal evolution

“…(33,35,50) However, many other sequence-based studies do not support this clade, some favoring Sirenia-Hyracoidea (7,32,38,40) and others favoring Proboscidea-Hyracoidea. (37) Recent support (56,57) for the relatively old idea that some early proboscideans were semiaquatic (54) underscores the morphological basis for the proboscidean-sirenian clade. Instability within Afroinsectiphilia (insectivoran-grade afrotherians plus the aardvark) concerns primarily the position of Tubulidentata.…”