MORPHOLOGIC VARIABILITY OF SWEETOGNATHIDS: PHENOTYPIC OR GENOTYPIC?

The role of phenotypic plasticity in population divergence is an emerging field in ecologic and evolutionary biology. There are examples of phenotypic plasticity in cichlid fish pharyngeal jaws developing under changing ecologic conditions and food resource partitioning. Is it possible that the high degree of morphologic plasticity of teeth in the conodont genus Sweetognathus is related to shallow-marine instability sustained by repeated sea-level fluctuations associated with long eccentricity (405 Kyr) cyclothems during the late Paleozoic Ice Age (LPIA), as well as other detrimental environmental conditions? Sweetognathus originates during the early Asselian in the mid-west USA and/or Bolivia with the evolution of Sw. expansus, followed by a lineage including Sw. sulcatus, Sw. wardlawi, Sw. merrilli, Sw. whitei and Sakmarian Sw. obliquidentatus. Populations of these taxa exhibit very high morphologic plasticity, but only rare specimens of some taxa have been found as far away as the Canadian Arctic and the Pamirs and only the latter species in the Urals. During the late Asselian a new morphotype originates; this species is first recovered in Nevada and migrated to the Urals. This form is interpreted as the ancestor of a second Sweetognathid lineage including Sw. binodosus, Sw. anceps, Sw. asymmetrica (species name replaces Artinskian homeomorph of Sw. whitei), Sw. behnkeni, Sw. clarki as well as Neostreptognathodus pequopensis and N. pnevi. Morphologic plasticity is still present, but to a much lower degree compared to the earlier lineage. Survival in the more stable post-LPIA shallow marine environment may not have required access to as many morphotypes. The expansion of this lineage is far more successful than the first and descendant forms range up to the Changhsingian. This biostratigraphically useful group of species exhibit near global distribution and will contribute to the definition of Artinskian and Kungurian Stages. A genetic basis for this morphologic plasticity is interpreted, but success is variable depending on ecologic setting. Overall morphologic trends in both lineages start with a pustulose ridge-like carina, later separated into nodes, then the nodes expand transversely, and finally they split apart into two rows. Is this an adaptive response to comparable adaptive peaks?