Biomedical subjects

Else Marie Friis

Publications and source records attributed to Else Marie Friis.

3 recordsLinked to original sources

PubMed2003

Cratonia cotyledon gen. et sp. nov: a unique Cretaceous seedling related to Welwitschia.

The fossil history of most extant seed plant groups is relatively well documented. Cycads, conifers and Ginkgo all have an extensive fossil record, and the understanding of early angiosperm diversity is increasing. The Gnetales are an exception. Few macrofossils have been described, and character evolution within the group is poorly known. Cratonia cotyledon is a new gnetalean fossil from the Early Cretaceous Crato Formation of Brazil. This well-preserved seedling consists of two cotyledons, a feeder and a root. The leaf surface shows polygonal epidermal cells and apparently paracytic or actinocytic stomata. The cotyledons have a very specific venation pattern, shared only by Cratonia and Welwitschia, with parallel primary veins and secondary veins fusing to form inverted 'Y's between the main veins. Based on the 'Y'-venation and the presence of a feeder, we assign Cratonia to the Gnetum-Welwitschia clade. Fossil seedlings are unusual and this complete specimen with unambiguously welwitschioid characters is spectacular. Cratonia indicates that the evolutionary split between Gnetum and Welwitschia had occurred in the Early Cretaceous. Further, the close relationship between a West African plant and an east South American Early Cretaceous fossil is consistent with a major geological event: the rifting of the Gondwana continent.

Cotyledon↗

PubMed2003

Archaefructus--angiosperm precursor or specialized early angiosperm?

With molecular analyses indicating that angiosperms are not closely related to any other extant seed plant group, information from fossils might provide the only basis for reconstructing their origin. Therefore the description of a new Early Cretaceous angiosperm, Archaefructus, placed as the sister of all extant angiosperms, has created much debate and optimism. However, we question both the interpretation and the analysis of Archaefructus, concluding that it might be a crown-group angiosperm specialized for aquatic habit rather than a more primitive relative.

Biological Evolution↗

PubMed2000

Seed Size, Fruit Size, and Dispersal Systems in Angiosperms from the Early Cretaceous to the Late Tertiary.

Fossil data from 25 angiosperm floras from the Early Cretaceous (∼124 million years ago) to the Pliocene (∼2 million years ago) were compiled to estimate sizes of seeds and fruits and the relative proportion of two different seed-dispersal systems by animals and by wind. The results suggest that, first, seed and fruit sizes were generally small during most of the Cretaceous, in agreement with previous suggestions, but the trend of increasing sizes started before the Cretaceous-Tertiary boundary; second, there was a decrease in both seed and fruit sizes during late Eocene and Oligocene, reaching a level that has continued to the Late Tertiary; third, the fraction of animal dispersal was, in contrast to previous suggestions, rather high also during the Cretaceous but increased drastically in the Early Tertiary and declined congruently with the declining seed and fruit sizes from the late Eocene; and fourth, the fraction of wind dispersal showed a bimodal pattern, being high in the Late Cretaceous and in the Oligocene-Miocene but with a drop in between. We find that the observed trends are only weakly related to the availability of animal fruit dispersers. Instead, the trends are congruent with a climate-driven change in environmental conditions for recruitment, where larger seeds are favored by closed forest vegetation. The prevalence of semiopen, dry, and probably herbivore-disturbed vegetation during the Cretaceous, the development of closed multistratal forests in the Eocene, and the later development of a more open vegetation and grasslands starting in the Oligocene-Miocene, are reflected in the distribution of angiosperm seed and fruit sizes and in the dispersal systems.

Cretaceous‐Tertiary boundary↗