Periodic growth lines can be used to construct a record of variations in the growth rate of a fossil or recent shell. Because growth rate is largely dependent upon environmental conditions, such a record serves also as an indication of variations in the environment during the lifetime of the shell. Although the very presence of such variations is of some paleoecologic interest, the main potential of growth lines in paleoecology lies in an extension of this argument. This is the concept that the contemporaneity of fossil organisms could be tested by examining their growth rate records for similarities; these would be expected because contemporaneous organisms could be responding to the same variations in environmental conditions.
To test this concept, a contemporaneous population of the scallop Pecten maximus was obtained from the Firth of Clyde, Scotland. The specimens included several year classes, but all were collected and killed on the same day. Measurements were made between the fine growth lines to construct growth-rate records for periods as long as four years. Cross-correlation of these records revealed the following: 1) growth records must consist of at least ninety lines to insure a significant correlation; 2) correlations concerning growth in the very early part of the growing season should be given less significance when compared to those concerning mid- and late season growth; 3) growth records of contemporaneous years generally displayed a high positive correlation; 4) growth records of non-contemporaneous years generally displayed poor correlations; 5) successive seasons, spanning as many as four years were found to correlate well with individuals of similar age; 6) in most cases where high correlations of non-contemporaneous years existed, the high values were found to be related to broad seasonal trends and the similar age class between individuals.
Investigations involving cluster analysis of the data established the following: 1) high frequency (short term) growth variation yielded more consistent and more reliable correlations than did the growth variations involving the original unfiltered data, but neither could be used alone for completely reliable results; 2) various approaches concerning weighting and averaging of the two sets of data improved the resolution of the cluster analysis and in some instances yielded groupings of contemporaneous seasons; 3) at least to some degree individuals of a particular age class respond to environmental variance in a manner similar to that of others within the same age class; 4) younger individuals seem to be more responsive to high frequency environmental variation than older specimens.
Other applications of this study concern growth and life habits: 1) the amount of growth within each fine lamella becomes progressively less as an individual ages from season to season; 2) the number of lamellae deposited within a season generally decreases for each season beyond the third year of growth; 3) younger specimens generally begin growth slightly earlier in the season than do older specimens and continue to grow considerably longer in the fall; 4) growth continues throughout the summer season with no marked decrease at any given time; 5) good correlations between individuals as far back as their 1969 seasons of growth seem to indicate a non-motile existence for Pecten maximus, at least after the first year of growth.
This study demonstrates that the growth increments of contemporary individuals of Pecten maximus generally do correlate to a high degree, and that the contemporaneous nature of their growth can be established through these correlations. These observations confirm the potential of growth line analysis in the reconstruction of paleocommunities.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
George R. Clark
Second Committee Member
George Roger Jiracek
Third Committee Member
Jonathan Ferris Callender
Oceanography Section of the National Science Foundation
Dillon, John F.. "Growth Line Analysis Of A Recent Scallop Population And It'S [Sic] Potential For Paleoecology." (1974). https://digitalrepository.unm.edu/eps_etds/318