PhD: Early Angiosperms and the Origin of the Latitudinal Diversity Gradient

OVERVIEW
Life is extraordinarily diverse, with an estimated total of ~8.7 million species living on Earth today
(Mora et al. 2011). Studies of species distributions show that the tropics are considerably richer in
species than higher latitudes, a pattern known as the latitudinal diversity gradient (LDG) (Wiens &
Donoghue 2004). This global-scale biogeographic pattern is remarkably consistent across geographic
areas, scales and habitats, and it has been quantified hundreds of times in marine and terrestrial
taxa (Hillebrand 2004).

Angiosperms (flowering plants) are the most taxonomically diverse group of land plants and they
largely conform to this classical pattern: they are most diverse in the tropical rainforests of Central
and South America. Fossil pollen from North America has been used to show that the angiosperm
LDG is geologically ancient, dating from at least 55 million years ago, and that low plant diversity at
high latitudes is not dependent on freezing temperatures (Harrington 2004). However, the first
angiosperms evolved during the Cretaceous, a period relative global warmth and equable climate,
and the nature of the angiosperm LDG at this critical period in plant evolution is largely unknown.

It is currently unclear whether a latitudinal gradient in plant diversity could have existed in the
relatively warm climate of the mid-Cretaceous, with little or no ice at the poles. While previous work
has shown that the mid-Cretaceous angiosperm LDG was either flat or inverted (in strong contrast to
the present day) (Mejia-Velasquez et al. 2012), there is a pressing need for more data from both
tropical and temperate latitudes to understand the biogeographical evolution of angiosperm
diversity. To do this the project will:

(1) Generate new data on the diversity of mid-Cretaceous angiosperms using fossil pollen extracted
from sediments in southern England.
(2) Use these data to generate comparisons of angiosperm diversity between southern England,
South America and West Africa during the mid-Cretaceous.
(3) Undertake quantitative morphometric analyses to establish which morphological traits are
associated with higher diversity, and to generate measures of morphological diversity to compare
with taxonomic diversity.

METHODOLOGY
Fieldwork will be undertaken to collect sediment samples from classic mid-Cretaceous exposures on
the southern Coast of England. These sediments were studied by Kemp (1968) and Laing (1975) and
preserve a diverse assemblage of fossil pollen that represents some of the first angiosperms to
evolve. Fossil pollen will be released from these sediment samples by acid digestion. These pollen
grains will be imaged using a semi-automated microscope at the Royal Botanic Gardens, Kew, and
will be analysed in a digital microscopy environment (OMERO) (https://www.openmicroscopy.org/).

Using this environment, counts of different taxa will be produced, and statistical analyses of these
count data will be used to quantify the diversity of mid-Cretaceous vegetation in southern England.
The diversity of this vegetation will be compared to the diversity of mid-Cretaceous vegetation in
South America and West Africa in order to reconstruct the origin of the LDG among flowering plants.
Analyses of morphological traits of these pollen assemblages will allow for comparison of taxonomic
diversity with different measures of biodiversity, and will establish a link to the evolution of
angiosperm functional morphology

COLLABORATION
This project involves collaboration with Dr Sarah Phillips (Royal Botanic Gardens, Kew) and
Dr Carlos Jaramillo (Smithsonian Tropical Research Institute, Panama).

REFERENCES
Harrington G.J. 2004. Structure of the North American vegetation gradient during the late
Paleocene/early Eocene warm climate. Evolutionary Ecology Research, 6, 33–48.
Hillebrand H. 2004. On the generality of the latitudinal diversity gradient. The American Naturalist,
163, 192–211.
Kemp E.M. 1968. Probable angiosperm pollen from British Barremian to Albian strata.
Palaeontology, 11, 421–434.
Laing J.F. 1975. Mid-Cretaceous angiosperm pollen from Southern England and Northern France.
Palaeontology, 18, 775–808.
Mejia-Velasquez P.J, Dilcher D.L., Jaramillo, C.A., Fortini L.B. and Manchester S.R. 2012. Palynological
composition of a Lower Cretaceous South American tropical sequence: climatic implications and
diversity comparisons with other latitudes. American Journal of Botany, 99, 1819–1827.
Mora C., Tittensor D.P., Adl A., Simpson A.B. and Worm B. 2011. How many species are there on
Earth and in the ocean? Plos Biology, 9, e1001127.
Wiens J.J. and Donoghue M.J. 2004. Historical biogeography, ecology and species richness. Trend in
Ecology and Evolution, 19, 639–644.