The Rhynie chert is well known as a source of discoveries but the
one seen in this picture is one of the most unexpected. Click
on the image for enlargement. The
unique
scene, incidentally spotted in 2008 on a cut face of a small piece of chert,
shows a rotifer approaching a spherical alga colony superficially resembling
extant Volvox. Extant rotifers are known to snatch cells from
Volvox spheres to eat them. Apparently the picture indicates that
this snatching mode of grazing has been going on for at least 400 million
years.
Neither rotifers nor spherical algal colonies have ever been seen in
the Rhynie chert before. The oldest reported fossil rotifers were found in
amber dating to the Tertiary. The distinct features of the present fossil
specimen and its great age compared to any other known rotifer may justify
regarding it as a new species.
More problematic is the placement of the alga into an established clade.
By a lucky coincidence it has been cut right through its centre so that its
outer sheath is distinctly seen in cross-section. From the distribution of
the cells it can be concluded that there were about 500 in the whole sphere.
As it is not hollow it lacks the typical feature of volvocacean spheres.
There are only a few species with the habit of spherical colonies among the
vast numbers of Green Algae, Golden Algae, and even the basically different
Blue-green Algae (Cyanobacteria). (Thanks are due to Józef
Kazmierczak and Robert Lee for emphasizing a possible affiliation with the
latter.)
The dramatic scene featuring rotifer and alga is set among terrestrial
vegetation flooded by the Lower Devonian hot spring system at Rhynie. Apparently
the attack was spoilt by quick formation of silica gel making the rotifer
get stuck before it could harm the alga. One even can tell what came next:
The rotifer was bound to starve while the alga went on living within the
gel where it was provided with all necessities of life via diffusion. |