Risella Carter and Laxtorum Blome, two genera from the diverse Rhaetian
fauna of the Sandilands Formation, Queen Charlotte Islands, are used to
illustrate phyletic trends in latest Triassic Radiolaria. Several
distinct morphotypes constituting a lineage are recognized for each
genus. These lineages are homogenous, evolved in situ, and show a
continuum of variation through time. The evolution of Risella takes
place entirely in the Rhaetian and all species disappear at the end of
the Triassic. Earliest species of Laxtorum appear in the upper Norian
and evolve rapidly in the Rhaetian. All Rhaetian species go extinct at
the end of the Triassic but the genus survives marginally into the Lower
Jurassic.
Morphological transformations in Risella (a paronaellid) are manifest in
the external/cortical shell as the shape changes from triangular to
three-rayed. In Laxtorum, distal post abdominal chambers become
constricted and eventually develop a terminal tube while, at the same
time, an increase in size and sphericity is coupled with a reduction in
the number of post abdominal chambers.
Evolutionary transitions in the Risella lineage probably represent a
reversion of the normal hypothesized trend for paronaellid radiolarians.
In the Laxtorum lineage, comparisons with other groups and species
displaying similar homeomorphies suggest the evolutionary trends are
fundamental and occur repeatedly in faunas of all ages.