Historical biogeography of the Icacinaceae Miers during the PETM.
Résumé
The Icacinaceae Miers are a basal lamiid family of trees, shrubs,
and lianas with a pantropical distribution. The group has been the
subject of intensive phylogenetic studies over the past 20 years,
leading to a highly reduced circumscription of the family including around 23 genera and 160 species. This family is well known
in the Paleocene and Eocene fossil record, particularly from the
Northern Hemisphere, although fossil evidence from the Southern Hemisphere has recently been increasing. In the Paris Basin,
Icacinaceae are represented by numerous fossil endocarps from
five main localities. Among these, Rivecourt (Thanetian) and Le
Quesnoy (Ypresian) are geographically and temporally very close.
The Paleocene-Eocene thermal maximum (PETM) event occurs
(temporally) between these two fossil outcrops. Thus, the study of
the Icacinaceae fossil in those localities provides a good opportunity to elucidate the impact of the PETM on this family in the Paris
Basin. Furthermore, available data from recent molecular phylogenetic studies allows us to reconstruct a dated, global biogeographic
history for the Icacinaceae family, which may compliment the information given by fossils.
Based on 266 fossil specimens attributed to Icacinaceae, from the
five Paris Basin localities, we conducted detailed anatomical (using
SEM) and morphological studies. In addition, using the same process, we analyzed fruits representing as 64 ingroup species and 6
outgroup species. Next, we conducted phylogenetic analyses (molecular and combined molecular-morphological) to reconstruct
phylogenetic relationships among extant and extinct species of
Icacinaceae. Finally, we used this combined phylogeny as the basis
for divergence dating and biogeographic analyses.
The study of fossils from the Paris Basin led to the recognition of
nine new species from two extant genera. No clear extinction is
shown through the PETM, but a floral change across the PETM
is demonstrated, possibly due to immigration of new plant taxa
(in concert with faunal immigrants). The phylogenic framework
allows us to date major diversification events for Icacinaceae and
to reconstruct biogeographic processes during the Paleocene-Eocene transition. We discuss the implication of global patterns for
understanding the changes occurring in the Paris Basin during the
PETM and most generally for the Northern Hemisphere during the
Cenozoic.