Mammal madness: Is the mammal tree of life not resolved yet?
Professor Emma Teeling, University College Dublin, Ireland
Living mammals (~5,400 species), inhabit every biome on earth, and are arguably one of the most phenotypically diverse group of vertebrates. From the largest, 170 ton blue whale, to the smallest, ~2 g flying, echolocating bumblebee bat, the huge diversity and extraordinary adaptive radiations in mammalian form and function have fascinated evolutionary biologists for centuries. Most molecular phylogenetic studies place all placental mammals into four superordinal groups: Laurasiatheria (e.g. dog, bat, cow), Euarchontoglires (e.g. human, rodent, flying lemur), Xenarthra (e.g. armadillo, ant-eater) and Afrotheria (e.g. elephant, sea-cow, tenrec) and estimate that they last shared a common ancestor 110 million years ago. This phylogeny has been the basis for many functional and comparative studies. However, a recent total evidence study examined the phylogenetic relationships amongst representative living and fossil mammals using a large supermatrix that combined ~4500 phenomic (morphological) characters, the largest morphological data set to date, with DNA sequences for 27 nuclear genes. Their results dramatically contrasted with prevailing molecular clock studies and suggested that approximated 10 interordinal mammalian divergences occurred in as little as 200,000 years, suggesting ‘viral rates’ of DNA mutations for mammals. Only 22/44 nodes that are supported by molecular studies were supported and many highly convergent ‘morphological’ clades received high support in their analyses. In addition despite the high level of congruence amongst most molecular studies, questions still remain regarding the position and divergence time of the root of placental mammals, and certain ‘hard clades’ such as Laurasiatheria and Paenungulata seem impossible to resolve. Here, we explore recent consensus and conflicting mammal phylogenetic studies and explore the reason for this conflict. The question of whether the mammal is or tree can be ever resolved will be addressed.
Molecular Palaeobiology of arthropod terrestrialisation: is the gap closing?
Dr Davide Pisani, University of Bristol, UK
Members of the phylum Arthropoda (e.g. insects, centipedes, spiders, crabs and their allies) are the first animals to appear in the terrestrial fossil record. Accordingly, dating the arthropod radiation is fundamental to understand animal terrestrialisation more broadly. However, there is still substantial disagreement between the fossil record of the terrestrial arthropods, and the molecular divergence times for the major arthropod lineages. Furthermore, to clarify how terrestrial arthropods colonized lands it is paramount to clarify the phylogenetic relationships between marine and terrestrial arthropod lineages. For example, it has recently been suggested that the remipedes, a small group of crustaceans exclusively known from marine caves, might represent the sister group of the insects. If this result was confirmed it would suggest a complex route to insect terrestrialisation. However, the evidence proposed to support a close relationship between remipedes and insects is weak at best, and an alternative scenario where the branchipod crustaceans (e.g. the fairy shrimps) that are mostly known from freshwater represent the sister group of the insects should still be considered a valid alternative. Here we shall revise current knowledge of arthropod phylogeny and molecular divergence times within this lineage and interpret these results to better elucidate what processes might have led animals to first colonise the land.
Inferring the influence of past climate change on megafaunal population dynamics
Professor Beth Shapiro, University of California Santa Cruz, USA
In combination with a molecular clock, genomic data isolated from the remains of long-dead plants and animals can reveal the timing and nature of demographic processes. When coupled with environmental data, genetic reconstructions can begin to address not only when populations grew or shrank, but also why. Broad sampling of faunal remains across the northern hemisphere has shown, for example, that most megafaunal population sizes fluctuate considerably over the last glacial cycle (ca. 125,000 years). However the sparseness of sampling and wide range of environmental variation across the northern hemisphere makes it difficult to identify which environmental processes are driving these demographic changes. In this session, I will present the results of a new analysis of genetic data from caballine horses (Equus caballus) and a variety of environmental proxies, including isotopic, insect, and plant macro- and microfossil data, that were collected at a single location – the Klondike region of Canada’s Yukon Territory. Our data span the last 50,000 years, a period that includes both the transition into and out of the coldest part of the last ice age, making it possible to trace the influence of specific environmental changes on North American horse survival and extinction.
Fossilized birth/death dating applied to beeches (Fagus) and ferns (Osmundaceae), clades with excellent fossil records
Professor Susan Renner, University of Munich, Germany
The fossilized birth-death (FBD) method of calibrating molecular clocks makes use of both young and old fossils, and is a tip-dating method in that it treats the fossils as tips in MCMC runs. Simulations have shown that method is able to infer speciation, extinction, and fossil recovery rates. With colleagues, I have applied FBD dating to the northern hemisphere genus Fagus, using 45 fossils and nuclear sequences for all nine species, and the worldwide fern clade Osmundaceae, using 35 fossils and sequences for all 12 living species. For both clades, the inferred divergence times were older than obtained with standard node dating. Visualization of the results was easy for the Osmundaceae consensus chronogram, but not for Fagus for which network visualization helped identify and place ‘rogue’ fossils. Inferred speciation and extinction rates imply a ca. 5x higher evolutionary turnover in Fagus than in Osmundaceae, fitting the hypothesized low turnover in plants, such as these ferns, adapted to low nutrient conditions. Only three empirical studies have used the FBD approach so far, which seems surprising given its conceptual superiority to node dating.