Reinvestigating the fossil leaf Welwitschiophyllum brasiliense Dilcher et al. (2005), from the Lower Cretaceous Crato Formation of Brazil
Introduction
Early Cretaceous floras typically have a higher abundance of gymnosperms than seen in many floras today, where angiosperms (flowering plants) have become dominant following the Cretaceous Terrestrial Revolution (Lloyd et al., 2008). One gymnosperm group in particular that flourished during the Mesozoic was the Gnetales (Crane, 1996), today represented only by three genera that vary significantly in morphological appearance and habitat. Ephedra Linnaeus, 1753 is a plant with reduced leaves, usually shrubby, although a few are small trees and climbers, and it has 50 species distributed in subtropical, arid and temperate environments (Kubitzki, 1990). Gnetum Linnaeus, 1767 has 39 species, mostly lianas, but includes trees and shrubs, which are all broad leaved with pinnate-reticulate venation, and inhabiting sub-tropical to tropical environments (Ickert-Bond and Renner, 2016). Welwitschia mirabilis Hooker (1863) is a plant with only two elongated, strap-like leaves on the woody caudex and a large taproot, and is restricted to the Namib Desert with only one species. W. mirabilis is sometimes divided into subspecies W. mirabilis ssp. mirabilis and W. mirabilis ssp. namibiana (Leuenberger, 2001).
The Gnetales of the Mesozoic display a rich diversity not reflected in the extant genera (Crane, 1996, Rydin et al., 2006). This diversity is well documented in the Lower Cretaceous Crato Formation of north-east Brazil, where eight genera have been erected to date, and many more plants identified as gnetalean are yet to be described (Rydin et al., 2003, Dilcher et al., 2005, Martill et al., 2007, Kunzmann et al., 2009, Kunzmann et al., 2011, Ricardi-Branco et al., 2013, Löwe et al., 2013). Four of these Crato Formation plant fossils are considered to be relatives of Welwitschia; the seedling Cratonia cotyledon Rydin et al. (2003), young paired cotyledon Priscowelwitschia austroamericana Dilcher et al. (2005), male reproductive strobili Welwitschiostrobus murili Dilcher et al. (2005), and the leaf taxon Welwitschiophyllum brasiliense Dilcher et al. (2005). The diversity of these Welwitschia-like macro-remains are of interest, as its history is chiefly represented by pollen in the fossil record.
Other Lower Cretaceous macro-remains with possible Welwitschiaceae affinities are known from the United States, China, Mongolia, Russia, and Europe. These fossils include the reproductive structures Gurvanella dictyoptera Krassilov (1982) (=Chaoyangia liangii Duan, 1998, Rydin et al., 2006), Heerala antiqua (Heer) Krassilov and Bugdaeva (1988), Angarolepis odorata Krassilov and Bugdaeva (1988), Eoantha zherikhinii Krassilov (1986), and the dispersed seeds, Bicatia Friis et al., 2014. Drewria potomacensis Crane and Upchurch (1987) and Conospermites hakeaefolius Ettinghausen (1867) are the only confirmed foliar remains of Welwitschiaceae found outside of the Crato Formation.
Welwitschiophyllum is a rare occurrence of a non-cotyledonous foliar fossil assigned to Welwitschiaceae. They occur as distinctive detached leaves reaching up to ~850 mm in length. In order to better understand the relationship of this fossil taxon to Welwitschia, which previously was only based on morphological comparisons, we provide and review the Welwitschiophyllum anatomy based on thin sections and compare its histology to that of extant Welwitschia.
Section snippets
Geological setting
The Lower Cretaceous (~120 million years old) Crato Formation of north-east Brazil outcrops on the flanks of the Chapada do Araripe, in the Araripe Basin, mainly in the states of Ceará and Pernambuco (Batten, 2007, Martill, 2007) (Fig. 1). The best exposures are of anthropic origins (stone quarries) in the neighbourhood of Nova Olinda and Santana do Cariri in southern Ceará. All the fossils described here come from the quarries in this region. The formation comprises a heterolithic sequence of
Materials and methods
Fossil repositories: The fossil material from the Crato Formation examined here comprises three isolated leaves of W. brasiliense Dilcher et al., (2005) (Specimens UERJ 13-P1, UERJ 14-P1, and UOP-PAL-MC0003). Specimens UERJ 13-P1, UERJ 14-P1 are accessioned at Rio Janeiro State University and UOP-PAL-MC0003 is accessioned at the University of Portsmouth.
CITES Permit: Analysis of extant W. mirabilis was performed on samples obtained on CITES permit No. 152606.
Petrographic thin sections: To
4.1. Morphology and anatomy of fossil Welwitschiophyllum leaves
Welwitschiophyllum brasiliense Dilcher et al. (2005) is a relatively common fossil leaf found in the Crato Formation. They are long (up to ~850 mm in length), lanceolate, detached leaves that taper to an acute apex and have a curved base (Fig. 2A). Where preserved, the leaves have a dense parallel venation (9–15 veins per cm). A single specimen comprising two apparently basally attached leaves that may be inferred as forming part of a rosette or other structure, was not available for study (
Discussion
Welwitschiophyllum was originally placed in the Welwitschiaceae based on the isobilateral form of the leaves, possible thickening of the epidermis, triangular elongated leaf shape with a wide base, parallel equidistant first-order veins that are convergent near the apex, with some veins disappearing into the margin, plus longitudinal splitting from a frayed leaf apex, and somewhat thickened or creased mid-leaf area (Dilcher et al., 2005). We observed some of these features but not any possible
Conclusion
The original description (Dilcher et al., 2005) of Welwitschiophyllum related it to Welwitschia using the following characteristics: the isobilateral form of the leaves, possible thickening of the epidermis, triangular elongated leaf shape with a wide base, longitudinal splitting from a frayed leaf apex, and numerous parallel veins. This study has shown that Welwitschiophyllum shares some key features with extant Welwitschia, i.e., having long, coriaceous, parallel veined leaves that produce
Acknowledgements
We warmly thank the two anonymous reviewers whose constructive comments strengthened the paper and the support from our editor Dr Koutsoukos. We would like to thank Dr Anthony Hitchcock, Karin Behr and Benjamin Festus (SANBI, Pretoria, SA) and Michael Plewka (Gevelsberg) for assistance obtaining CITES permits and supplying Welwitschia samples. Norbert Lehrl (Vienna Palm House) is thanked for his expertise on Welwitschia. At the University of Portsmouth Geoff Long kindly produced thin sections,
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