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The Rise of Placental Mammals: Dissecting an Evolutionary Radiation

Periodic Reporting for period 2 - PalM (The Rise of Placental Mammals: Dissecting an Evolutionary Radiation)

Reporting period: 2019-07-01 to 2020-12-31

Mammals are ubiquitous in today’s world. Over 5,000 species, from hamsters to humans, are distributed across the globe. But how did mammals become so successful? This question gets to the heart of a much wider mystery: how major groups of plants and animals rise up over evolutionary time. How do their evolutionary radiations unfold: when do they happen, how quickly, and what drives them? Answering these questions will help us better understand how major groups become successful over evolutionary time and how biodiversity is affected by large, infrequent events like mass extinctions. It will also give unique insight into the early stages of our own evolutionary story.

Two main handicaps have held back generations of researchers. First, tackling the mystery of the mammal radiation requires multiple lines of evidence that transcend traditional research boundaries. Second, we still know very little about those mammals that flourished during the ~10 million years after the end-Cretaceous extinction (the early Paleogene). They are often ignored, because they mostly belong to ‘archaic’ groups, with uncertain relationships to both Cretaceous and modern mammals. How to place these ‘archaic’ species—some 200+ species of incredible anatomical, dietary, and body size diversity— on the family tree of mammals is one of the great unsolved problems in palaeontology.

A wealth of new fossils (including spectacular specimens collected over the past six years by our team) and new multidisciplinary analytical techniques together provide an unprecedented opportunity to untangle the biology and phylogeny of these critical early Paleogene species, and then to use that information to better understand how mammals ascended to dominance and what role the end-Cretaceous extinction played in this story. In doing so, we will integrate, for the first time, a wealth of data on the anatomy, genetics, ages, genealogy, body sizes, diets, and behaviours of early mammals and their modern relatives, providing the most detailed look yet at a major evolutionary radiation in the fossil record.

The main objective of this proposal is to use state-of-the-art imaging technology to study fossils and novel numerical techniques to build and analyse the largest and most comprehensive dataset of early Paleogene mammal phylogeny and evolution. We will achieve this by establishing a large, interdisciplinary team straddling the geological and biological sciences, centred at the University of Edinburgh, called the Paleogene Mammal Working Group (PalM).

Detailed Objectives: By establishing a Paleogene Mammal Working Group and using state-of-the-art analytical tools (some developed by the PI, and other novel, ground-breaking methods that will be developed as part of this project) to study an ever-growing roster of Paleogene mammal specimens (including a wealth of new material collected by the our team over the past decade), we will address the following questions:
1) What are the genealogical relationships of Paleogene mammals: how are they related to each other and to their Cretaceous forebears and living mammals? I will test whether some/all ‘archaic’ Paleogene species are early members of major living mammal groups, failed experiments in mammalian evolution that did not produce any descendants, and/or linked to Cretaceous species that lived alongside the dinosaurs.
2) When did placental mammals and major subgroups originate? I will test the time component of the three main hypotheses for placental diversification—explosive, long-fuse, and short-fuse—and determine which best fits the available data, thus establishing whether placentals and major subgroups originated alongside, or after the extinction of, dinosaurs.
3) What effect did the end-Cretaceous extinction have on mammalian evolution? I will test the rate component of the three main hypotheses for placental diversification, and determine whether there were changes in biodiversity across the Cretaceous-Paleogene boundary, whether the extinction preferentially wiped out certain types of mammals, and how mammals emerged from the extinction and radiated to become the diverse animals we know today.
The first 30 months of the PalM project have seen considerable progress towards our objectives. PI Brusatte has built the core of his research team, which includes three PhD students funded by this grant. Our team (University of Edinburgh core group and international project partners) held two meetings: one in Pittsburgh (USA) in March 2019 and one in Albuquerque (USA) in May 2019. Associated with both meetings were visits to key museum collections (including the American Museum of Natural History in New York, Peabody Museum at Yale, New Mexico Museum of Natural History and Science), which house important fossils of early mammals, along with fieldwork in New Mexico that recovered ca. 150 new specimens of Paleocene-aged mammal fossils. During this time, we also obtained computed tomography (CT) scans of ca. 25 fossil mammal skulls, purchased computing equipment necessary for the project (HP workstation with Wacom touch screen, Lenovo workstation), purchased software necessary for the project (CT processing software Mimics), purchased photographic equipment necessary for the project (two Nikon cameras with macrolenses to photograph fossils). Some other planned meetings, data collections trips, and joint work with our colleagues was cancelled because of the Covid-19 pandemic, however (see details below).

Our main achievements include the following: six peer-reviewed papers (including one in Proceedings of the National Academy of Sciences), 18 papers (talks or posters) presented at major international conferences, ca. 50 talks (academic and popular) by PI Brusatte, and a major popular science article by PI Brusatte (for BBC Science Focus Magazine). We have constructed ca. half of our ‘framework dataset’ of anatomical features of extinct and extant mammals to be used for phylogenetic analysis.
The PalM project will provide two major novel insights. First, the ‘archaic’ mammals provide an unprecedented glimpse at an evolutionary radiation. Fossils are our only way of addressing how certain groups become successful over evolutionary time scales. Early Paleogene mammals, on the other hand, offer the potential of a well-dated phylogeny and data on their body sizes, diets, and behaviours, gleaned from many well preserved fossils (particularly new specimens my team and I have found). By combining all of this information into a single project, the ‘archaic’ mammals are poised to become the prime exemplar of an evolutionary radiation in the fossil record. In this project, we will dissect this radiation in detail, providing an unparalleled case study of how evolutionary radiations occur.

Second, the ‘archaic’ mammals lived in the aftermath of one of the most severe mass extinctions in Earth history, in which more than 75% of species died out after a burst of environmental change. There is growing consensus that human activities are now causing something analogous to a mass extinction. The end-Cretaceous extinction is the most recent of the ‘big five’ mass extinctions over the past 500+ million years and therefore struck a fauna that is most similar to today’s, including mammals that were either early members of the modern groups and/or close relatives. However, the poor understanding of where the ‘archaic’ mammals fit on the mammal family tree hampers a better understanding of how their diversity, body size, and evolution were affected by the extinction. This will be rectified with the PalM project, making these mammals a prime exemplar of how a real group of animals responded to a real interval of sudden and devastating environmental change in the past.

In sum, my team and I will be using a wealth of new fossil material and ground-breaking analytical techniques to solve some of the biggest mysteries in vertebrate evolution: the relationships of ‘archaic’ mammals, how evolutionary radiations occur, and how environmental changes affect organisms on long-term scales. These questions have frustrated palaeontologists for generations, but the convergence of new fossils, new methodology, and my strong research team make us uniquely placed to break the impasse.

Our key expected results include:

1) Phylogenetic Analysis and Genealogy of Early Mammals: We will address the key questions: How are Paleocene mammals related to each other and to their Cretaceous forebears and living mammals? We will build the largest and most comprehensive phylogenetic dataset, combining anatomical and molecular (DNA) data, and including Cretaceous, Paleogene, and modern mammals. This will produce a phylogeny (family tree) of these mammals.
2) Timing of the Placental Radiation: We will address the key question: When did placental mammals and major subgroups originate? We will apply various dating and calibration algorithms, using Bayesian statistics, to our phylogenetic tree, to estimate the origination date of Placentalia and major subdivisions.
3) Effects of the end-Cretaceous extinction: We will address the key question: What effect did the end-Cretaceous extinction have on mammalian evolution? We will use various statistical macroevolutionary techniques to determine whether there were changes in placental mammal biodiversity across the Cretaceous-Paleogene boundary, whether the extinction preferentially wiped out certain types of mammals, and how mammals emerged from the extinction and radiated to become the diverse animals we know today.
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