Rytiodus

Abstract: Corystosiren varguezi, gen. et sp. nov. (Dugongidae: Rytiodontinae), is described on the basis of a skull from Yucatan, Mexico, and skull and tusk fragments from the Bone Valley district, Florida, USA. The age of this form is most probably early Pliocene, but it appears to be the sister taxon of the early Miocene Rytiodus Lartet, 1866, from France and Libya. Both had strongly deflected rostra and very large, flattened, self-sharpening tusks. Corystosiren is additionally distinguished by a broad and extremely thick skull roof. An unnamed Miocene taxon from South Carolina may be phylogenetically intermediate between C. varguezi and other rytiodontines.

Abstract: A species of the dugongine dugongid genus Rytiodus from the Miocene (Burdigalian—Langhian; 19–14 Ma) deposits of Jabal Zaltan (= Gebel Zelten), Libya, is re-described, considered a valid species differing from Rytiodus capgrandi from the lower Miocene (Aquitanian) of the Aquitaine Basin (SW France), and named Rytiodus heali. R. heali, together with R. capgrandi and Corystosiren varguezi appear to constitute a taxon of dugongines characterized by broad, mediolaterally compressed, blade-like tusks with enamel mainly on the medial side, and a modified nasal process of the premaxilla. They are regarded as specialized seagrass feeders, using their tusks in excavation of seagrass rhizomes. Rytiodus lived sympatrically with the generalist small-tusked halitheriine dugongid genus Metaxytherium.

Abstract: The evolution of the Neogene sirenians along the Euro-North African coasts (southern North Sea, northeastern Atlantic, Mediterranean and Paratethys) has been investigated in correlation with the palaeogeographic and palaeoclimatic evolution of the region. A better chronostratigraphic setting and a systematic review of the main specimens (listed in a database) have been provided and nine valid Neogene Euro – North African sirenian species have been recognized: Miosiren kocki, Prohalicore dubaleni, Rytiodus capgrandi, Rytiodus sp. from Libya, Metaxytherium krahuletzi, M. medium, M. petersi, M. serresii and M. subapenninum; while “Halitherium” bellunense, previously referred to the Early Miocene, has been determined to be Late Oligocene in age and confirmed to be a dugongine close to Rytiodus spp.. These species, except Prohalicore dubaleni known only from an incomplete mandible and M. petersi under study by Domning and Pervesler, have been included in a cladistic analysis. This analysis has been carried out with the PAUP version 40b10 program; developed for 31 species of sirenians and the primitive Proboscidea Phosphatherium escuilliei as outgroup, and based on 57 characters of cranium, mandible and dentition. The results confirm the anatomical observations and show a close correspondence between palaeobiogeographic and chronostratigraphic distribution of the taxa and their positions on the tree. A reconstruction of Euro-North African sirenian palaeoecology and palaeobiogeography during the Neogene has been proposed in relation to the long-term trend of climatic cooling and to the decreasing of marine influence, which characterized the evolution of the region. The sirenian palaeoecology has been reconstructed by combining data from seagrass evolution and ecology, sirenian distribution and morphology, stable isotope (C and O) analyses of sirenian tooth enamel. On the whole, the Neogene Euro-North African sirenians appear to be fully aquatic herbivorous mammals strictly linked to coastal environments. During the Early Miocene, which was a period of global warming, at least three different sirenian lines lived along the Euro-North African coasts: the euryhaline mixed-feeder Miosiren kocki (Trichechidae, Miosireninae), living in isolation on the southernmost margin of the North Sea; the generalist small-tusked Metaxytherium krahuletzi (Dugongidae, Halitheriinae), which appears to have been widespread along the Euro-North African coasts; and the specialized large-tusked genus Rytiodus (Dugongidae, Dugonginae), recorded with confidence just in the Aquitaine Basin (SW France) and in the Sirte Basin (Libya), where it most probably lived sympatrically with Metaxytherium krahuletzi. From the Middle Miocene onward, however, the only sirenians surviving in the Euro-North African region belonged to the generalist cosmopolitan halitheriine genus Metaxytherium. The only exception is the miosirenine trichechid Prohalicore dubaleni, suggesting that Trichechidae survived until the Serravallian, probably living together with the…

Abstract: The near lack of vertebrate fossils from the Cenozoic of Madagascar has left many of the details regarding the origin and evolution of the island's extant faunas unknown. However, recent fossil discoveries from Madagascar's nearshore marine deposits have begun to elucidate this mystery. These finds include sharks, bony fish, turtles, crocodylians, a middle Eocene sirenian (Eotheroides lambondrano), and the island's first fossil dolphin. We report here at least three (possibly four) different early (or possibly later) Miocene dugongid sirenians recovered from the island of Nosy Makamby, Mahajanga Basin, northwestern Madagascar. These include (1) a fragmentary braincase originally attributed to the genus Halitherium but here reidentified as a previously named species known only from Libya (Rytiodus heali; Dugonginae); (2) a newly named genus and species (Norosiren zazavavindrano) interpreted as a primitive relative of Xenosiren (Dugonginae); (3) a probable dugongine not yet identified with any known species; and (4) a taxon reported here as Metaxytherium cf. krahuletzi (Halitheriinae), the first Neogene halitheriine credibly reported from the Indian Ocean basin. This pattern of shallow marine environments harboring multispecies sirenian paleofaunas is seen elsewhere in the world, and these three or four contemporaneous sirenians represent the first glimpse into Madagascar's sea cow diversity during the Miocene. This specific time period is a poorly known and critical interval for interpreting Madagascar's past, and these specimens are potentially highly significant for reconstructing sirenian evolutionary and biogeographic history. Surprisingly, this sirenian fauna, so far, shares no genera with the roughly contemporaneous and relatively nearby one from Kutch, western India.