Biomedical subjects

P Steemans

Publications and source records attributed to P Steemans.

2 recordsLinked to original sources

PubMed2004

Runcaria, a middle devonian seed plant precursor.

The emergence of the seed habit in the Middle Paleozoic was a decisive evolutionary breakthrough. Today, seed plants are the most successful plant lineage, with more than 250,000 living species. We have identified a middle Givetian (385 million years ago) seed precursor from Belgium predating the earliest seeds by about 20 million years. Runcaria is a small, radially symmetrical, integumented megasporangium surrounded by a cupule. The megasporangium bears an unopened distal extension protruding above the multilobed integument. This extension is assumed to be involved in anemophilous pollination. Runcaria sheds new light on the sequence of character acquisition leading to the seed.

Belgium↗

PubMed2000

Miospore evolution from the Ordovician to the Silurian.

The definition of the term cryptospore is amended to include only spores thought to be produced by embryophytes and to exclude all enigmatic palynomorphs. Cryptospores are included here in the miospore group.The oldest published assemblage of typical cryptospores is Llanvirn in age. The evolution of cryptospore assemblages is very slow from the Llanvirn to the early Llandovery. The Telychian and Sheinwoodian stages are periods of drastic impoverishment in cryptospore biodiversity, followed by an important modification in miospore assemblages. This step in the evolution of the vegetation is correlated with the rapid transgression of seas on the continent during the early Llandovery. On the other hand, the Hirnantian glaciation does not affect diversity of the miospore assemblages.

Journal Article↗