Two New Oligocene Desmostylians and a Discussion of Tethytherian Systematics

Domning, Daryl P.; Ray, Clayton E.; McKenna, Malcolm C.

doi:10.5479/si.00810266.59.1

Cited by 107 publications

(92 citation statements)

References 16 publications

(21 reference statements)

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“…Desmostylians represent an extinct order of semiaquatic herbivorous mammals, known only from the Oligocene and Miocene marginal marine deposits of the north Pacific (Domning et al, 1986). For such a small group of mammals (six currently recognized genera), there has been a great deal of debate about their lifestyle and their relationship to other mammals.…”

Section: Sirenian and Desmostylian Phylogenymentioning

confidence: 99%

Evolution of marine mammals: Back to the sea after 300 million years

Uhen

2007

The Anatomical Record

162

113

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The fossil record demonstrates that mammals re-entered the marine realm on at least seven separate occasions. Five of these clades are still extant, whereas two are extinct. This review presents a brief introduction to the phylogeny of each group of marine mammals, based on the latest studies using both morphological and molecular data. Evolutionary highlights are presented, focusing on changes affecting the sensory systems, locomotion, breathing, feeding, and reproduction in Cetacea, Sirenia, Desmostylia, and Pinnipedia. Aquatic adaptations are specifically cited, supported by data from morphological and geochemical studies. For example, analysis of oxygen isotopes incorporated into fossil tooth enamel indicates whether these mammals foraged in (and, therefore, ingested) fresh water or sea water. Comparisons between groups are made to see if there are any common patterns, particularly relating to adaptations to aquatic life. Results show that aquatic characteristics evolved in mosaic patterns and that different morphological solutions to aquatic conditions were achieved separately in each of these groups. Changes in the axial and appendicular skeleton assist with locomotion for aquatic foraging. Nostril and eye placement modifications accommodate wading versus underwater foraging needs. All groups exhibit aquatic adaptations directly related to feeding, particularly changes in the dentition and rostrum. The earliest representatives of these clades all show morphological features that indicate they were feeding while in the water, suggesting that feeding ecology is a key factor in the evolution of marine mammals.Anat Rec 290:514-522, 2007.2007 Wiley-Liss, Inc.Key words: cetacea; sirenia; pinnipedia; carnirora; desmostylia; feeding ecology; aquatic adaptation A recurrent theme in tetrapod evolution is a return to an aquatic habitat. Whereas amphibians never quite parted with the water, all other tetrapod groups (reptiles, birds, and mammals) have several representatives that have returned to the water for at least an amphibious lifestyle of some degree, if not a fully aquatic existence. Mammals in particular have returned to the water in at least seven separate lineages (Cetacea, Sirenia, Desmostylia, Pinnipedia, Ursus maritimus [polar bear], Enhydra lutris [sea otter], and Thalassocnus spp. [aquatic sloths]). Some of these lineages have retained most of their terrestrial form while spending a great deal of time in the water, whereas others have changed their morphology dramatically and spend almost all, if not all, of their time in the water.Before exploring how the anatomical changes for life in water came about, one must know how marine mammals are related to one another, and particularly to their terrestrial relatives. A brief introduction to the phylogeny of each group of marine mammals is presented here, based on the latest studies using both morphological and molecular data. CETACEAN PHYLOGENYThe relationships of cetaceans to terrestrial mammals and other marine mammals was a great mystery for many

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Section: Sirenian and Desmostylian Phylogenymentioning

confidence: 99%

Evolution of marine mammals: Back to the sea after 300 million years

Uhen

2007

The Anatomical Record

162

113

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“…Moeritherium was tapir-sized and possibly had a prehensile upper lip rather than a trunk (8). Moeritherium's lifestyle has been debated for over a century, with different lines of evidence supporting an aquatic, semiaquatic, or terrestrial mode of life (7,(9)(10)(11)(12)(13). Cranial features of Moeritherium that are seen in some aquatic or semiaquatic mammals, such as a long, tubular cranium and anteriorly positioned orbits, have previously been cited as evidence for an amphibious lifestyle, but they are not restricted to aquatic mammals (10).…”

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“…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). Moeritherium was tapir-sized and possibly had a prehensile upper lip rather than a trunk (8).…”

mentioning

confidence: 99%

“…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) …”

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confidence: 99%

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Stable isotope evidence for an amphibious phase in early proboscidean evolution

Liu

Seiffert

Simons

2008

Proc. Natl. Acad. Sci. U.S.A.

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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)

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Continental Paleobiogeography as Phylogenetic Evidence

Rossie

Seiffert

Primate Biogeography

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Two New Oligocene Desmostylians and a Discussion of Tethytherian Systematics

Cited by 107 publications

References 16 publications

Evolution of marine mammals: Back to the sea after 300 million years

Evolution of marine mammals: Back to the sea after 300 million years

Stable isotope evidence for an amphibious phase in early proboscidean evolution

Continental Paleobiogeography as Phylogenetic Evidence

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