POSSIBLE ANAGENETIC EVOLUTION WITHIN THE AMMONITE GENUS GUNNARITES FROM THE LATE CRETACEOUS OF ANTARCTICA

The planispiral of kossmaticeratid ammonite genus Gunnarites is a common member of Campanian-Maastrichtian ammonite assemblages from Gondwanan land masses, including Antarctica, New Zealand, Australia, South Africa, India, and South America. In Antarctica, specifically the James Ross Basin (JRB), Gunnarites is an important biostratigraphic indicator (Assemblage Zones 9 and 10 of Olivero, 2012) that spans the Campanian-Maastrichtian boundary, a period which marks the transition from a cosmopolitan to a more endemic ammonite fauna in the JRB. This interval is also notable as being represented only by a variety of small (usually < 100 stratigraphic meters) discontinuous exposures, preserved along coastlines or nunataks, for which correlations are hard to develop. In contrast, the Santonian-Campanian and Maastrichtian-Danian intervals are represented by long (> 1000 stratigraphic meters) exposures.

While the genus Gunnarites has been used as a reliable biostratigraphic indicator, the geographic and stratigraphic distribution of its species has been poorly explored. Spath (1953) defined eight species (and several subspecies morphotypes) in the JRB, but these have been poorly defined quantitatively and largely ignored in research on the area. We describe detailed morphometric analysis of Gunnarites specimens (n > 50) from different locations across the JRB. Current results do not support the partitioning of Antarctic Gunnarites into eight species, but perhaps as few as two or three with moderate intra-specific variability. Furthermore, analysis of whorl expansion rate and umbilical percentage suggest a gradual evolution of some characters between different locations within the JRB. These characters suggest that species level distinctions within Gunnarites may provide further biostratigraphic refinement of the Campanian-Maastrichtian interval. This transition may represent an example of anagenetic evolution within ammonites, similar to patterns observed in Baculites in the Western Interior of North America, though this observation is currently tentative.