Margaret Collinson

TROPICAL VEGETATION AND CLIMATE, EOCENE, UK.

The London Clay fruits and seeds from SE England are the most diverse Early Eocene flora with over 350 species curated in the collections of the Natural History Museum, London (NHMUK). Many of these fossils have nearest living relatives which are thermophilic flowering plants (e.g. Icacinaceae, Mastixiaceae and the mangrove palm Nypa) representing highly diverse paratropical forest vegetation during the Early Eocene Climatic Optimum (EECO). Courtesy of NHMUK, and my Honorary Research Fellowship there, these fossils are studied using CT at the Museum (Collinson et al., 2016). This enables non-destructive digital sectioning of the pyritised specimens without removing the protective silicone oil in which they are stored. CT study thus provides access to otherwise hidden detail of holotype specimens (those on which species are based) (Figure i). This ongoing research aims ultimately to re-examine all holotypes from this flora with CT, thus creating a permanent digital record and refining understanding of past vegetation and climate during the EECO.

Some of the thermophilic plants (e.g. Mastixiaceae) persist into the Late Eocene in the UK where they are represented by lignified fruits. The detailed cellular anatomy of type specimens of these fossils (Figure ii) has been revealed by SRXTM and used to confirm that these fossils represent extinct genera closely related to both modern genera (Diplopanax Hand.-Mazz and Mastixia Blume). Although limited to these two genera in Asia and Malesia at the present day the Mastixiaceae were widespread and more diverse in Europe and North America in the Cenozoic (Manchester & Collinson 2019).

(i) The holotype NHMUK V.22984 of the fossil fruit of Early Eocene Langtonia bisulcata Reid and Chandler (Mastixiaceae) from the London Clay flora, Sheppey, Kent, UK, preserved by pyrite permineralisation. A. Apical view by digital semi-transparent volume rendering of CT dataset. B. Digital transverse section by CT. This extinct genus has a distinctive E-shaped locule (seed containing cavity) and was geographically widespread in the Eocene occurring in both UK and North America. Specimen width 12mm. Specimen is curated by the Earth Department, NHMUK. Images copyright of the Trustees of the Natural History Museum.

(ii) Digital transverse section by SRXTM of cellular detail in the fossil fruit of Eomastixia bilocularis Chandler, NHMUK V20079a syntype from the Late Eocene Headon Hill Formation at Hordle, Hampshire. This genus has C-shaped seed-containing locules enclosed within variously oriented fibres of the fruit stone. Image width 3.5mm. Specimen is curated by the Earth Department, NHMUK. SRXTM dataset obtained at the Paul Scherrer Institut, Villigen, Switzerland, using the TOMCAT beamline X02DA of the Swiss Light Source.

References Tropical Eocene

• Collinson, M.E., Adams, N.F., Manchester, S.R., Stull, G.W., Hererra, F., Smith, S.Y., Andrew, M.J., Kenrick, P., Sykes, D. 2016. X-ray micro-computed tomography (micro-CT) of pyrite-permineralised fruits and seeds from the London Clay Formation (Ypresian) conserved in silicone oil: a critical evaluation. Botany 94, 697-711. https://doi.org/10.1139/cjb-2016-0078
• Manchester, S.R. & Collinson, M.E. 2019. Fruit morphology, anatomy and relationships of the type species of Mastixicarpum and Eomastixia (Cornales) from the late Eocene of Hordle, southern England. Acta Palaeobotanica 59, 51-67. https://doi.org/10.2478/acpa-2019-0006

CONTINENTAL TEMPERATURES AND ENVIRONMENTAL CHANGE ACROSS THE PALEOCENE EOCENE THERMAL MAXIMUM

As a palaeobiologist with special interests in the evolution of wetland floras I was excited to learn, in 1999, of a temporary exposure of the Cobham Lignite in a cutting for the channel tunnel rail link (Figure).  Thanks to permission and logistical support from Alfred McAlpine, AMEC and Channel Tunnel Rail Link in situ pillars of the lignite were excavated and collected. Research on these not only documented a wetland flora (Collinson et al., 2013) but also initiated wide-ranging collaborative studies (initially with University of Bristol and later including GNS Science New Zealand and others). Through those collaborations the well-preserved organic geochemistry in the thermally immature lignite has been exploited to understand the role of terrestrial methane emissions across the PETM (Paleocene Eocene Thermal Maximum) (Pancost et al 2007; Inglis et al., 2020). The availability of such lignites helped to stimulate work on a peat-specific temperature calibration (by David Naafs, Bristol). Utilising that calibration the Cobham Lignite became a key example for determining Eocene continental temperatures (Inglis et al., 2017; Naafs et al., 2018)

Cobham Lignite when exposed in a cutting for the channel tunnel rail link. With thanks to Alfred McAlpine, AMEC and Channel Tunnel Rail Link for access.

References PETM

• Collinson, M.E., Smith, S.Y., Van Konijnenburg-Van Cittert, J.H.A., Batten, D.J., Van Der Burgh, J., Barke, J., Marone, F. 2013. New observations and synthesis of Paleogene heterosporous water ferns. International Journal of Plant Sciences 174, 350-363. https://doi.org/10.1086/668249
• Inglis, G.N., Collinson, M.E., Riegel, W., Wilde, V., Farnsworth, A., Lunt, D.J., Valdes, P., Robson, B. E., Scott, A.C., Lenz, O. K., Naafs, B.D.A. And Pancost, R.D. 2017. Mid-latitude continental temperatures through the early Eocene in western Europe. Earth and Planetary Science Letters, 460, 86-96. http://dx.doi.org/10.1016/j.epsl.2016.12.009
• Inglis, G.N., Rohrssen, M., Kennedy, E.M., Crouch, E.M., Raine. J.I., Strogen, D.P., Naafs, D.A., Collinson, M.E., Pancost, R.D. Terrestrial methane cycle perturbations during the onset of the Paleocene-Eocene Thermal Maximum. Geology, 49,  https://doi.org/10.1130/G48110.1
• Naafs, B.D.A., Rohrssen, M., Inglis, G.N., Lähteenoja,  O., Feakins, S., Collinson, M.E., Kennedy,E., Singh, P.K., Singh,M.P., Lunt, D.J. And R.D. Pancost, R.D.  2018. High temperatures in the terrestrial mid-latitudes during the early Paleogene. Nature Geoscience, https://doi.org/10.1038/s41561-018-0199-0.
• Pancost, R.D. Steart, D.S., Handley, L., Collinson, M.E., Hooker, J.J., Scott, A.C., Grassineau, N.V. & Glasspool, I.J. 2007. Increased terrestrial methane cycling at the Palaeocene-Eocene thermal maximum. Nature, London 449, 332-335. https://www.nature.com/articles/nature06012

THE EOCENE AZOLLA EVENT

Eocene warm climate conditions led to a very unexpected consequence beginning in the latest Early Eocene and spanning more than 1.5 million years. During this time the freshwater free-floating water fern Azolla spread across and around the Arctic Ocean (from Canada to the Lomonosov Ridge), down the Norwegian Greenland Sea and into the North Sea (van der Burgh, Collinson et al., 2013).   Whole fertile fossil plants of Azolla from the Early Eocene of Wyoming, USA (Figure) are currently being studied. If this is the same species as any of the Arctic/Nordic species it would considerably extend the biogeographic range, and hence environmental impact, of the Eocene Azolla event.

A - Portion of fertile Azolla plant with tiny leafy shoots and a few scattered reproductive structures (dark brown) USNM P372411. Image width 25mm. B – Back scatter detector SEM image (JEOL JSM – IT 500 at NHMUK) from another area on the same rock specimen showing detail of the clusters of spore-bearing reproductive structures. Image width 3.5mm. Specimens are curated by the Dept. of Paleobiology, NMNH, Smithsonian, USA. Image B copyright of the Trustees of the Natural History Museum, UK.

Reference Azolla

• Van Der Burgh, J., Collinson, M.E., Van Konijnenburg-Van Cittert, J.H.A., Barke, J., Brinkhuis, H. 2013.  The freshwater fern Azolla (Azollaceae) from Eocene Arctic and Nordic Sea sediments : new species and their stratigraphic distribution. Review of Palaeobotany and Palynology, 194, 50-68. https://doi.org/10.1016/j.revpalbo.2013.02.002