The Early Cretaceous Crato Formation gymnosperms of north-east Brazil

  • Emily Alice Roberts

Student thesis: Doctoral Thesis

Abstract

The Early Cretaceous (?Aptian) Crato Formation of north-east Brazil is well-known for its diverse and well-preserved fossil assemblage, mostly pertaining to its insect and vertebrate fauna. However, the formation also yields a diverse flora including Isoetales, Polypodiopsida, Bennettitales, conifers, Gnetales, monocot and dicot angiosperms. In addition, sedimentary amber clasts and amber within the fossil plants have been reported. The flora is an allochthonous assemblage with plants of riparian origin deposited alongside vegetation from a semi-arid hinterland with lakes and includes several arid adapted species.
The flora has yielded many named and undescribed gnetaleans, a once diverse and abundant group that today is reduced to just three genera: Ephedra, Welwitschia and Gnetum. There are eight named Gnetales genera from the Crato flora and because of their rarity elsewhere in the fossil record the Crato gnetalean assemblage is of considerable significance.
Previously described Crato Formation gnetaleans are thought to have affinities to Ephedra and Welwitschia. A new gnetalean described here shares characteristics with Gnetum, including in situ echinate Gnetum-like pollen, previously unknown in the fossil record.
Of the described Crato Formation gnetaleans Welwitschiophyllum is of significance as it is only one of two foliar remains with affinities to the extant African Welwitschia. Detailed studies of both the fossil Welwitschiophyllum and extant Welwitschia reveal that despite some anatomical similarities there are also important differences. In addition a possible Welwitschiophyllum is described from the Cenomanian Gara Sbaa Member of Morocco.
Some Crato Formation macrophytes contain in situ amber, its location within the plant tissue varying between taxa. In the conifer Brachyphyllum obesum amber is located in either the main stem or possibly trapped between the stem and leaves. In Lindleycladus and Welwitschiophyllum it is situated within the leaves, and in three previously unidentified different cones it is located within their bracts.
Analysis, including ATR and micro-FTIR on ambers from the cones and from Brachyphyllum obseum demonstrates coniferalean affinities. Analysis of ‘amber’ in Welwitschiophyllum reveals a highly distinctive spectrum, unlike that of any fossil resin, yet remarkably similar to that of gum. Intriguingly, its proposed extant relative also produces a gum with a similar spectrum. Due to the soluble nature of gum its preservation in the fossil record is both unexpected and remarkable.
Date of AwardSept 2019
Original languageEnglish
Awarding Institution
  • University of Portsmouth
SupervisorDavid Martill (Supervisor) & Bob Loveridge (Supervisor)

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