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New insights into Mesozoic cycad evolution: an exploration of anatomically preserved Cycadaceae seeds from the Jurassic Oxford Clay biota

DOI: 10.7717/peerj.3723 DOI Help

Authors: Alan R. T. Spencer (Imperial College London) , Russell J. Garwood (University of Manchester; The Natural History Museum London) , Andrew R. Rees (University of Birmingham) , Robert J. Raine (Geological Survey of Northern Ireland) , Gar W. Rothwell (Oregon State University; Ohio University) , Neville T. J. Hollingworth (University of Birmingham; Science and Technology Facilities Council) , Jason Hilton (University of Birmingham)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Peerj , VOL 5

State: Published (Approved)
Published: August 2017
Diamond Proposal Number(s): 9244

Open Access Open Access

Abstract: Most knowledge concerning Mesozoic Era floras has come from compression fossils. This has been augmented in the last 20 years by rarer permineralized material showing cellular preservation. Here, we describe a new genus of anatomically preserved gymnosperm seed from the Callovian–Oxfordian (Jurassic) Oxford Clay Formation (UK), using a combination of traditional sectioning and synchrotron radiation X-ray micro-tomography (SRXMT). Oxfordiana motturii gen. et sp. nov. is large and bilaterally symmetrical. It has prominent external ribs, and has a three-layered integument comprising: a narrow outer layer of thick walled cells; a thick middle parenchymatous layer; and innermost a thin fleshy layer. The integument has a longitudinal interior groove and micropyle, enveloping a nucellus with a small pollen chamber. The large size, bilateral symmetry and integumentary groove demonstrate an affinity for the new species within the cycads. Moreover, the internal groove in extant taxa is an autapomorphy of the genus Cycas, where it facilitates seed germination. Based upon the unique seed germination mechanism shared with living species of the Cycadaceae, we conclude that O. motturii is a member of the stem-group lineage leading to Cycas after the Jurassic divergence of the Cycadaceae from other extant cycads. SRXMT—for the first time successfully applied to fossils already prepared as slides—reveals the distribution of different mineral phases within the fossil, and allows us to evaluate the taphonomy of Oxfordiana. An early pyrite phase replicates the external surfaces of individual cells, a later carbonate component infilling void spaces. The resulting taphonomic model suggests that the relatively small size of the fossils was key to their exceptional preservation, concentrating sulfate-reducing bacteria in a locally closed microenvironment and thus facilitating soft-tissue permineralization.

Journal Keywords: Palaeobotany; Fossil; Cycad; Cycadeospermum; Cycas; Oxford Clay Formation; Taphonomy; Gymnosperm; Ovule; 3D Reconstruction

Subject Areas: Earth Science

Instruments: I12-JEEP: Joint Engineering, Environmental and Processing

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