Terrestrial Palaeoecology and Global Change

330 Valentin A. Krassilov. Terrestrial Palaeoecology
feeding on taeniate pollen of the Lunatisporites, Protohaploxypinus and Vittatina
morphotypes produced by conifers and peltasperms. Mixed pollen loads contained two
or more such morphotypes. Hence the insects visited different sources of taeniate pollen
over their foraging territories. For them the taeniate structures might conceivably serve
as guides to the most rewarding pollen sources.
This hypothesis explains a high selective value of taeniate morphology for the insectpollinated
Permian gymnosperms. The pollinivorous insects might spread this morphology
over the diversity of gymnosperm lineages not only through selection, but also by
facilitating a microbial transduction of exogenous genetic material (VIII.1.4). The horizontal
gene transfer might have been most efficient in respect to palynomorphological
structures controlled by a few genes with insignificant pleiotropic effects.
Recurrence. A repeated occurrence of a character over times also falls in the
category of parallel, yet heterochronous, developments. There were several periods of
predominantly saccate pollen grains (in the Late Paleozoic and Cenozoic gymnosperms)
and of sculptured sporoderms (in the Late Cretaceous to Eocene angiosperms) alternating
with those marked by the prevalence of asaccate and unsculptured ones.
A recurrence of amphisporions (the dispersal units of heterosporous plants consisting
of megaspores bearing microspores in their sporoderm appendages) is a striking example.
Such structures first appeared in the Devonian Kryshtofovichia, a morphotype of
spiny megaspore with an apical “androcamera”, produced by some heterosporous lycopsids
(Nikitin, 1934). About 300 m.y. later, much similar structures reappeared in a
Cretaceous heterosporous fern Heroleandra (Krassilov & Golovneva, 1999, 2000;
Fig.50). The amphisporions provide for a joint long-distance dispersal of male and female
gametophytes, advantageous for colonizers. During the Cretaceous, fresh water
environments were invaded by floating heterosporous ferns filling a vacant ecological
niche as the heterosporous lycopsids did in the Devonian.
Certain characters of angiosperm pollen grains, considered as typical of the group,
are actually recurrent. These include the lamellate endexine, endoapertures and the tapetal
surface deposits, all found already in the Permian gymnosperm pollen grains (a lamellate
endexine in Cladatina, a porous endoaperture in Protohaploxypinus, a supratectal layer
in the in situ pollen grains of Permotheca: Krassilov et al., 1999c, 1999d). Further
studies will show how far the Permian gymnosperms anticipated the angiosperm ultrastructures.
Disparity of the sporophyte–gametophyte evolution rates. Still another aspect
of sporophyte–pollen grain correlation, or lack of such, is a temporal disparity of morphological
innovations, with the sporophytes taking the lead at one time, the pollen grains at
another. There are examples of an exceptionally conservative pollen morphology that
remained virtually unchanged over great intervals of geological time while the sporophytic
organs made a conspicuous progress, and vice versa.
Thus, the protosaccate Vesicaspora morphotype first appeared in the Pennsylvanian
callistophytes, a small group of seed ferns, the geological range of which did