Evaluating the Findings
The change in internal
volume and shell thickness over time among Anadara and
Astarte shells are evidence of
evolution,
due to the high correlation between directional
change in morphology versus time. How do we know that
the changes we see in shell thickness and internal volume are
evolution and not short-term response to an environmental stressor,
such as sediment accumulation rate? By examining the sedimentary
environments in the area, Dr. Kelley determined that there was no
correlation to any major
environmental change, in other words, the types of rock that these
fossils were found in do not record any significant changes that
correspond to the incremental changes in shell features.
Dr. Kelley was also able to
negate the factors of ontogeny by determining the age of the shells
based on growth lines. While the shell features
of Astarte and Anadara were observed to change over time, other
genera of bivalves studied by Kelley do not share these same results.
These genera often demonstrated non-consistent patterns of temporal
change, and widely fluctuating non-directional trends.
(see below).
