A reassessment of the Oligocene hyracoid mammals from Malembo,Cabinda, Angola

The Oligocene Malembo locality, Cabinda exclave, Angola, has yielded a rich vertebrate fauna represented by fragmentary remains. This fossiliferous locality is the only definite occurrence of Oligocene terrestrial mammals in sub-Saharan West Africa. The hyracoids from Malembo have only been very succinctly described and compared thus far, so that their systematic attribution is not consensual among specialists. A revision now allows the identification of three (or four) medium to large-sized species represented by Geniohyus dartevellei, Pachyhyrax cf. crassidentatus, and two undetermined taxa. The species G. dartevellei is revived for the holotype of Palaeochoerus dartevellei Hooijer, 1963; this species is unique to Malembo but appears close to Geniohyus mirus, a species only known from the early Oligocene of the Fayum, Egypt. Other species of Geniohyus and Pachyhyrax crassidentatus are also only known from the early Oligocene of the Fayum. The presence of Geniohyus and Pachyhyrax cf. crassidentatus at Malembo thus supports an early Oligocene age for the fauna.     

山西天镇后裂爪兽属(哺乳动物纲,蹄兔目)化石初步报道

山西天镇的蹄兔化石被定为Ｐｏｓｔｓｃｈｉｚｏｔｈｅｒｉｕｍｉｎｔｅｒｍｅｄｉｕｍ。这是该属目前所知最好的一批材料,使我们对该属头骨、下颌和牙齿的特征有了更多、更确切的了解。该属头骨吻部拉长变宽,门齿间有长的齿隙;头骨和下颌骨具多个凹陷;上臼齿单面高冠,白垩质发育。这些特征足以把它和所有其他相近的属区别开来。后裂爪兽属似有明显的雌、雄性差异,主要表现在个体大小和门齿的形态及粗壮程度上。天镇产后裂爪兽化石地层的地质时代,根据蹄兔化石的进化水平和整个动物群性质判断,比泥河湾典型层位要早,可能应在１。８Ｍａ~２．６Ｍａ之间。     

Revision of the Early Miocene Hyracoidea (Mammalia) of East Africa

Three genera of hyracoids were recorded from the Early Miocene of East Africa by Whitworth [18], but there has been considerable divergence of opinion about their status. Despite differences in cranial and dental morphology from Megalohyrax and Bunohyrax, Whitworth [18] classified two species in these genera that are recorded from much earlier deposits (Early Oligocene) in the Fayum, Egypt. One of his genera (Meroehyrax) was new. His classification has been the subject of debate, with some researchers [6,13] doubting the hyracoid status of one of his species (Bunohyrax sp), and changing the generic status of another (Megalohyrax championi). Meyer [6] recorded a fourth genus (Prohyrax) from Kenya, linking it to material from Namibia described by Stromer [16,17]. New samples of two hyracoid species collected by the Uganda Palaeontology Expedition throw light on their systematic position and taxonomy. It is concluded that there are three hyracoid genera (Afrohyrax, Brachyhyrax and Meroehyrax) in the Early Miocene deposits of East Africa, the first two of which are new. A fourth genus (Prohyrax) occurs in southern Africa, but is not reliably known from East Africa. To cite this article: M. Pickford et al., C. R. Palevol 3 (2004).

Résumé

Révision des Hyracoidea (Mammalia) du Miocène inférieur de l'Afrique de l’Est. Trois espèces d’Hyracoidea ont été signalées dans le Miocène inférieur d’Afrique orientale par Whitworth [18]. Malgré des différences importantes de la morphologie cranio-dentaire, deux des espèces kenyanes étaient classées dans Megalohyrax et Bunohyrax, genres connus dans les dépôts beaucoup plus anciens du Fayoum en Égypte. Le troisième genre décrit par Whitworth (Meroehyrax) était nouveau. La classification proposée [18] a été débattue ; certains auteurs [6,13] ont remis en cause le statut d’Hyracoïde d’une de ses espèces et ont modifié le statut générique d’une autre. Meyer [6] a signalé un quatrième genre (Prohyrax) au Kenya, sur la base des ressemblances avec le genre namibien décrit par Stromer [16,17]. De nouveaux spécimens récoltés par l’Uganda Palaeontology Expedition permettent d’éclaircir la position systématique et la taxonomie de deux des espèces. Nous concluons qu’il n’existe que trois genres d’Hyracoïdes dans les dépôts du Miocène inférieur d’Afrique orientale (Afrohyrax, Brachyhyrax et Meroehyrax), dont les deux premiers sont nouveaux. Par ailleurs, un quatrième genre (Prohyrax) est connu d’Afrique australe. Pour citer cet article : M. Pickford et al., C. R. Palevol 3 (2004).     

Molecular evolution of the mitochondrial 12S rRNA in Ungulata (mammalia)

The complete 12S rRNA gene has been sequenced in 4 Ungulata (hoofed eutherians) and 1 marsupial and compared to 38 available mammalian sequences in order to investigate the molecular evolution of the mitochondrial small-subunit ribosomal RNA molecule. Ungulata were represented by one artiodactyl (the collared peccary, Tayassu tajacu, suborder Suiformes), two perissodactyls (the Grevy's zebra, Equus grevyi, suborder Hippomorpha; the white rhinoceros, Ceratotherium simum, suborder Ceratomorpha), and one hyracoid (the tree hyrax, Dendrohyrax dorsalis). The fifth species was a marsupial, the eastern gray kangaroo (Macropus giganteus). Several transition/transversion biases characterized the pattern of changes between mammalian 12S rRNA molecules. A bias toward transitions was found among 12S rRNA sequences of Ungulata, illustrating the general bias exhibited by ribosomal and protein-encoding genes of the mitochondrial genome. The derivation of a mammalian 12S rRNA secondary structure model from the comparison of 43 eutherian and marsupial sequences evidenced a pronounced bias against transversions in stems. Moreover, transversional compensatory changes were rare events within double-stranded regions of the ribosomal RNA. Evolutionary characteristics of the 12S rRNA were compared with those of the nuclear 18S and 28S rRNAs. From a phylogenetic point of view, transitions, transversions and indels in stems as well as transversional and indels events in loops gave congruent results for comparisons within orders. Some compensatory changes in double-stranded regions and some indels in single-stranded regions also constituted diagnostic events. The 12S rRNA molecule confirmed the monophyly of infraorder Pecora and order Cetacea and demonstrated the monophyly of suborder Suiformes. However, the monophyly of the suborder Ruminantia was not supported, and the branching pattern between Cetacea and the artiodactyl suborders Ruminantia and Suiformes was not established. The monophyly of the order Perissodactyla was evidenced, but the relationships between Artiodactyla, Cetacea, and Perissodactyla remained unresolved. Nevertheless, we found no support for a Perissodactyla + Hyracoidea clade, neither with distance approach, nor with parsimony reconstruction. The 12S rRNA was useful to solve intraordinal relationships among Ungulata, but it seemed to harbor too few informative positions to decipher the bushlike radiation of some Ungulata orders, an event which has most probably occurred in a short span of time between 55 and 70 MYA. Correspondence to: E. Douzery     

Space partitioning between two small mammals in a rocky desert

An analysis was made in Augrabies Falls National Park, South Africa of the factors that allow coexistence of two small herbivorous mammals: the rock hyrax Procavia capensis and the dassie rat Petromus typicus. The rocky habitat provides adequate food and living shelters of varying sizes. There appears to be competition for shelters: Petromus occupies only those shelters that are too small for Procavia with the result that the population of Petromus is small and restricted to certain favourable areas whereas Procavia is abundant and widespread.     

Maximum-Likelihood Analysis of the Tethythere Hypothesis Based on a Multigene Data Set and a Comparison of Different Models of Sequence Evolution

Hyracoids have been allied with either perissodactyls or tethytheres (i.e., Proboscidea + Sirenia) based on morphological data. The latter hypothesis, termed Paenungulata, is corroborated by numerous molecular studies. However, molecular studies have failed to support Tethytheria, a group that is supported by morphological data. We examined relationships among living paenungulate orders using a multigene data set that included sequences from four mitochondrial genes (12S rRNA, tRNA valine, 16S rRNA, cytochrome b) and four nuclear genes (aquaporin, A2AB, IRBP, vWF). Nineteen maximum-likelihood models were employed, including models with process partitions for base composition and substitution parameterizations. With the inclusion of partitions with a heterogeneous base composition, 18 of 19 models favored Hyracoidea + Sirenia. All 19 models favored Hyracoidea + Sirenia after excluding heterogeneous base composition partitions. Most of the support for Hyracoidea + Sirenia derived from the mitochondrial genes (bootstrap support ranged from 51 to 99%); Tethytheria, in turn, received 0 to 19% support in different analyses. Bootstrap support deriving from the nuclear genes was more evenly split among the competing hypotheses (3 to 45% for Tethytheria; 17.5 to 62% for Hyracoidea + Sirenia). Lineage-specific rate variation among both mitochondrial and nuclear genes may contribute to the different results that were obtained with mitochondrial versus nuclear data. Whether Tethytheria or a competing hypothesis is correct, short internodes on the molecular phylogenies suggest that paenungulate orders diverged from each other over a 5- to 8-million-year time window extending from the late Paleocene into the early Eocene. We also used likelihood-ratio tests to compare different models of sequence evolution. A gamma distribution of rates results in a greater improvement in likelihood scores than does an allowance for invariant sites. Twenty-one rate partitions corresponding to stems, loops, and codon positions of different genes result in higher likelihood scores than a gamma distribution of rates and/or an allowance for invariant sites. Process partitions of the data that incorporate base composition and substitution parameterizations result in significant improvements in likelihood scores in comparison to models that allow only for relative rate differences among partitions.