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PeerJ. 2014 Oct 23;2:e624. doi: 10.7717/peerj.624. eCollection 2014.

X-ray Synchrotron Microtomography of a silicified Jurassic Cheirolepidiaceae (Conifer) cone: histology and morphology of Pararaucaria collinsonae sp. nov.

Author information

1
Department of Earth Sciences, Invertebrates and Plants Division, Natural History Museum , London , United Kingdom ; Department of Agricultural Sciences, LaTrobe University , Melbourne, Victoria , Australia.
2
Department of Earth Sciences and Engineering, Imperial College London , London , United Kingdom.
3
School of Earth, Atmospheric and Environmental Sciences and The Manchester X-ray Imaging Facility, School of Materials, The University of Manchester , Manchester , United Kingdom.
4
School of Geography, Earth and Environmental Sciences, University of Birmingham , Edgbaston, Birmingham , United Kingdom.
5
Department of Earth Sciences, Invertebrates and Plants Division, Natural History Museum , London , United Kingdom.
6
Tisbury, Salisbury, Wiltshire , United Kingdom.

Abstract

We document a new species of ovulate cone (Pararaucaria collinsonae) on the basis of silicified fossils from the Late Jurassic Purbeck Limestone Group of southern England (Tithonian Stage: ca. 145 million years). Our description principally relies on the anatomy of the ovuliferous scales, revealed through X-ray synchrotron microtomography (SRXMT) performed at the Diamond Light Source (UK). This study represents the first application of SRXMT to macro-scale silicified plant fossils, and demonstrates the significant advantages of this approach, which can resolve cellular structure over lab-based X-ray computed microtomography (XMT). The method enabled us to characterize tissues and precisely demarcate their boundaries, elucidating organ shape, and thus allowing an accurate assessment of affinities. The cones are broadly spherical (ca. 1.3 cm diameter), and are structured around a central axis with helically arranged bract/scale complexes, each of which bares a single ovule. A three-lobed ovuliferous scale and ovules enclosed within pocket-forming tissue, demonstrate an affinity with Cheirolepidiaceae. Details of vascular sclerenchyma bundles, integument structure, and the number and attachment of the ovules indicate greatest similarity to P. patagonica and P. carrii. This fossil develops our understanding of the dominant tree element of the Purbeck Fossil Forest, providing the first evidence for ovulate cheirolepidiaceous cones in Europe. Alongside recent discoveries in North America, this significantly extends the known palaeogeographic range of Pararaucaria, supporting a mid-palaeolatitudinal distribution in both Gondwana and Laurasia during the Late Jurassic. Palaeoclimatic interpretations derived from contemporaneous floras, climate sensitive sediments, and general circulation climate models indicate that Pararaucaria was a constituent of low diversity floras in semi-arid Mediterranean-type environments.

KEYWORDS:

Diamond Light Source; Fossil forest; Pararaucaria; Purbeck; Tithonian

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