CLUMPED OR DISPERSED? USING NEAREST NEIGHBOR ANALYSIS TO ASSESS VARIATIONS IN SETTLEMENT PATTERNS OF FOUR ORDOVICIAN ISOROPHID EDRIOASTEROIDS

Obrution occurs almost instantaneously, preserving environments in situ, providing paleontologists with a snapshot of the living community and some of the most complete ecological information in the fossil record. Since population studies are most reliable when applied to census assemblages, catastrophically buried communities of edrioasteroids, an extinct group of sessile echinoderms, are ideal for such analysis.

Nearest Neighbor Analysis (NNA), which analyzes the distribution of distances between nearest neighbors of each organism, was applied to a population of 233 isorophid edrioasteroids (Carneyella ulrichi (n=112), Carneyella pliea (n=2), Curvitriordo n. sp. (n=110), and Streptaster vorticellatus (n=9)) found on an Upper Ordovician hardground collected near Maysville, Kentucky. The observed distance between nearest neighbors was compared to the expected mean distance between randomly distributed edrioasteroids to assess the spatial distribution. The ratio of these values (R) measures the departure from a random distribution. An R < 1 represents a clumped distribution, an R = 1 indicates a random distribution and an R > 1 indicates a dispersed distribution. The Z statistic was calculated to test if the observed pattern varies significantly from a random distribution. If Z is < -1.96 or > +1.96, we are 95% confident that the edrioasteroids are not randomly distributed.

NNA of the 233 edrioasteroids revealed a score of R = 0.45 and a Z statistic of -30, indicating a clumped distribution. When looked at separately, specimens of Carneyella ulrichi received a score of R = 0.87 and a Z statistic of -20, also indicative of a clumped distribution, whereas Curvitriordo n. sp. received a score of R = 1.16 with a Z statistic of +28, indicating a random pattern, and Streptaster vorticellatus received a score of R = 1.98 with a Z statistic of +7216, indicating a dispersed pattern.

To test sensitivity of scale, the hardground was arbitrarily divided into three sections each bearing approximately the same number of edrioasteroids. NNA was performed on each. Area 1 had a NNA score of R = 0.52 with a Z statistic of -16, area 2 had a NNA score of R = 0.57 and a Z statistic of -71, and area 3 had a NNA score of R = 0.48 with a Z statistic of -65. Each signified a clumped distribution. Possible reasons for the clumping of individuals are discussed.