Comparable Ages for the Independent Origins of Electrogenesis in African and South American Weakly Electric Fishes

One of the most remarkable examples of convergent evolution among vertebrates is illustrated by the independent origins of an active electric sense in South American and African weakly electric fishes, the Gymnotiformes and Mormyroidea, respectively. These groups independently evolved similar complex systems for object localization and communication via the generation and reception of weak electric fields. While good estimates of divergence times are critical to understanding the temporal context for the evolution and diversification of these two groups, their respective ages have been difficult to estimate due to the absence of an informative fossil record, use of strict molecular clock models in previous studies, and/or incomplete taxonomic sampling. Here, we examine the timing of the origins of the Gymnotiformes and the Mormyroidea using complete mitogenome sequences and a parametric Bayesian method for divergence time reconstruction. Under two different fossil-based calibration methods, we estimated similar ages for the independent origins of the Mormyroidea and Gymnotiformes. Our absolute estimates for the origins of these groups either slightly postdate, or just predate, the final separation of Africa and South America by continental drift. The most recent common ancestor of the Mormyroidea and Gymnotiformes was found to be a non-electrogenic basal teleost living more than 85 millions years earlier. For both electric fish lineages, we also estimated similar intervals (16–19 or 22–26 million years, depending on calibration method) between the appearance of electroreception and the origin of myogenic electric organs, providing rough upper estimates for the time periods during which these complex electric organs evolved de novo from skeletal muscle precursors. The fact that the Gymnotiformes and Mormyroidea are of similar age enhances the comparative value of the weakly electric fish system for investigating pathways to evolutionary novelty, as well as the influences of key innovations in communication on the process of species radiation

The Global Standard Stratotype-section and Point (GSSP) for the base of the Eocene Series in the Dababiya section (Egypt)

The GSSP for the base of the Eocene Series is located at 1.58 m above the base of Section DRH in the Dababiya Quarry, on the east bank of the Nile River, about 35 km south of Luxor, Egypt. It is the base of Bed 1 of the Dababyia Quarry Beds of the El Mahmiya Member of the Esna Formation, interpreted as having recorded the basal inflection of the carbon isotope excursion (CIE), a prominent (3 to 5%) geochemical signature which is recorded in marine (deep and shallow) and terrestrial settings around the world. The Paleocene/Eocene boundary is thus truly a globally correlatable chronostratigraphic level. It may be correlated also on the basis of 1) the mass extinction of abyssal and bathyal benthic foraminifera (Stensioina beccariiformis microfauna), and reflected at shallower depths by a minor event; 2) the transient occurrence of the excursion taxa among the planktonic foraminifera (Acarinina africana, A. sibaiyaensis, Morozovella allisonensis); 3) the transient occurrence of the Rhomboaster spp. - Discoaster araneus (RD) assemblage; 4) an acme of the dinoflagellate Apectodinium complex. The GSSP-defined Paleocene/Eocene boundary is approximately 0.8 my older than the base of the standard Eocene Series as defined by the Ypresian Stage in epicontinental northwestern Europe.link_to_subscribed_fulltex

Chronostratigraphic terminology at the Paleocene/Eocene Boundary

link_to_subscribed_fulltex

RESEARCH ARTICLE Body-size structure of Central Iberian mammal fauna reveals semidesertic conditions during the middle Miocene Global Cooling Event

We developed new quantitative palaeoclimatic inference models based on the body-size structure of mammal faunas from the Old World tropics and applied them to the Somosaguas fossil site (middle Miocene, central Iberian Peninsula). Twenty-six mammal species have been described at this site, including proboscideans, ungulates, carnivores, insectivores, lagomorphs and rodents. Our analyses were based on multivariate and bivariate regression models correlating climatic data and body-size structure of 63 modern mammal assemblages from Sub-Saharan Africa and the Indian subcontinent. The results showed an average temperature of the coldest month higher than 26ÊC for the Somosaguas fossil site, a mean annual thermal amplitude around 10ÊC, a drought length of 10 months, and an annual total precipitation greater than 200 mm per year, which are climate conditions typical of an ecotonal zone between the savanna and desert biomes. These results are congruent with the aridity peaks described over the middle Aragonian of Spain and particularly in the local biozone E, which includes Somosaguas. The aridity increase detected in this biozone is associated with the Middle Miocene Global Cooling Event. The environment of Somosaguas around 14 Ma was similar to the current environment in the Sahel region of North Africa, the Horn of Africa, the boundary area between the Kalahari and the Namib in Southern Africa, south-central Arabia, or eastern Pakistan and northwestern India. The distribution of modern vegetation in these regions follows a complex mosaic of plant communities, dominated by scattered xerophilous shrublands, semidesert grasslands, and vegetation linked to seasonal watercourses and ponds.Peer reviewe