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Final Report Summary - MDKPAD (The radiation of modern mammals: release from dinosaur incumbency or response to environmental change?)

The extinction of the dinosaurs is one of the best known events in the history of life, and the dinosaurs’ demise has long been perceived as a major reason why humans are on this planet today. Mammals did not radiate into their present-day diversity of sizes and specializations until the Cenozoic era (starting 66 million years ago), and the mammals who lived alongside the dinosaurs during the Mesozoic era (252 – 66 million years ago) have long been perceived as vermin-like critters, suppressed in their evolutionary potential by the dominance of the ruling dinosaurs.
But what if this popular mythology was just that – a myth? Could other factors have been at play in the suppression of mammalian radiation besides the presence of dinosaurs? Palaeobotanical research has shown that the rise of the angiosperms, the flowering plants that dominated Cenozoic terrestrial ecosystems, reached a critical point during the final 10 million years of the Cretaceous (the final period of the Mesozoic). By this time the angiosperms were forming their own microhabitats, leading to the type of vegetational habitat complexity typical of today’s world, where mammals dominate the ecosystems. Could this vegetational change have been more important for mammal evolution than the demise of the dinosaurs?
To answer this question I sought to investigate whether mammal behaviour was changing along with these changing habitats: specifically, whether the floral change was associated, or even drove, the pattern of substrate use in mammalian communities: i.e., were the mammals at various time intervals primarily terrestrial (ground living) or primarily arboreal (tree-living). I proposed to conduct the research by investigating the skeletal anatomy of Cretaceous fossil mammals.
I chose the University of Bristol as the place to conduct this research, even though the critical data were all in North American institutions, because of their long-standing programme n the study of Mesozoic ecosystems. A prime aim in obtaining a Marie Curie incoming fellowship was to effectively retire back to my country of origin, having spent most of my professional life in the US. I had spent a couple of sabbaticals at the University of Bristol, an institution where I could not only continue to do scientific research, but could also usefully contribute to the teaching and training of young scientists interested in palaeontology. Additionally, I have numerous research links with colleagues in the European Union, and living in the UK would aid those associations.
The first research task was to establish a data set of photographs of the anatomy of living mammals of known behaviour to serve as a reference for interpreting the fossil bones. Previous opinion held that the anatomy small mammals (rabbit size and under, the size of the Mesozoic mammals) would not reflect their behaviour well, as mammals this small all tend to be highly generalized. As most of the Cretaceous mammal skeletal remains consist of fragments, I focused on the anatomy of the joints, those bits of the skeleton that are highly ossified and so are more likely to be preserved. To this end I established a dataset of 84 species of living mammals, from a wide diversity of taxonomic groupings. Statistical analysis shows that many of the joint surfaces can be used as correlates of behaviour, distinguishing the animals into arboreal, semi-arboreal, and terrestrial modes of locomotion. The overlap between categories is greater, and the statistical power less, than for larger mammals, but the predictive ability is still good.
I also established a similar dataset of photographs of the bones of Cretaceous mammals (and some from the earliest Cenozoic, right after the dinosaur extinctions, found in the same generalized fossil localities as the latest Cretaceous ones). I took over 2000 photographs, representing around 500 individual bones. Unfortunately, only around 100 individual specimens were complete and undamaged enough to yield useful information. However, this sample was sufficient to reveal some ecological trends over time. Analysis of the data show that most of the mammals of the latest Cretaceous and earliest Cenozoic were generalized, semi-arboreal ones (as are most living small mammals are today, such as squirrels). However, in the period around 10 million years before the end of the Cretaceous at least a couple of specialized, highly arboreal mammals could be identified, as arboreal as any mammal known today (an unexpected discovery for the Mesozoic). Unfortunately the sample size for this time period is small, and it is not yet clear if this difference can be established statistically. Nevertheless, a potential difference in locomotor behaviour over this time period matches results from other researchers that show a difference in dietary preferences, and the skeletal and dental data support each other in showing an ecological change that correlates with patterns in floral change preceding any change in the dinosaur faunas.
The attachment provides an example of the analysis of one of the joints (the distal humerus), and shows how the anatomy of this bone correlates with substrate use among living mammals, and how it can be used to interpret the behaviour of fossil ones.

Another important aspect of my Marie Curie grant has been in the interaction with European colleagues, and the transfer of knowledge. This is a summary of my activities during the funded period.
1. Advising fourth year undergraduate students in the programme in GeoBiology (MSci) and Masters students in the program in Palaeobiology (MSc). Primary advisor for 2 MSci students (one graduated, one ongoing), and secondary advisor for 2 more (both graduated, one of whom has published his thesis where I am a coauthor). Primary advisor for 2 MSc students (co-authored paper on thesis for the 1st in revision for the Journal of Morphology, co-authored paper on thesis for the 2nd in preparation), secondary advisor for 4 MSc students (1 graduated, co-authored paper in preparation, 3 ongoing).
2. Formal teaching in undergraduate/Master’s level courses. Three lectures and one lab in course at the University of Bristol (2015 and 2016), four lectures and two labs as a visiting lecturer at the University of Cambridge (2016). These will continue to be ongoing obligations.
3. Informal teaching at the University of Bristol. In the spring term I run a reading group for interested students, comprised of undergraduates, Masters students, and graduate students.
4. Interaction with European researchers. I have continued, and will be continuing, collaboration with Borja Figueirido Juan Pérez-Claros, and Paul Palmqvist (University of Málaga); Mikael Fortelius, Jussi Eronen, Laura Säila and other members of the NOW database (University of Helsinki); Marcus Clauss (University of Zurich); and the German Centre for Integrative Biodiversity Research (the first three in an active capacity, the last in a more advisory role on manuscripts submitted using my database on North American fossil mammal occurrences). Although none of these papers directly involved the Marie Curie project, these are the publications that have come out during the funded period. Figuerido et al. Nature Communications, 2015 (CMJ last out of 4 authors); Eronen et al., Proceedings of the Royal Society B, 2015 (CMJ 2nd out of 4 Figueirido et al. Paleobiology, 2016 (CMJ last out of 3 authors); Fritz et al., Proceedings of the National Academy of Sciences USA 2016 (CMJ 5th out of 8 authors).

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UNIVERSITY OF BRISTOL
United Kingdom

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Life Sciences
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