Comparison of archaeocyathans at the Harkless reef and Stewart’s Mill reef, and an explanation of the variations in central cavity diameter and the abundance of specimens.
Introduction
Archaeocyaths were reef-building organisms with a calcareous skeleton composed of two porous walls separated by an interavallum. They appeared after the first calcareous cyanobacteria and they eventually evolved to be part of the cyanobacteria mounds that were already being built by these early “reef” builders (Rowland & Gangloff, 1988). They were the dominant reef builders in the early Cambrian and were extinct by the upper Cambrian (Savarese 1992). They fed themselves through the passive flow of water through their central cavities. Currents passing over the central cavity caused water to be drawn up through the central cavity of the organisms, through the pores that passed through to the intervallum, and the sponge could then filter feed off the water flowing through it (Savarese 1992). We examined the archaeocyath fossils present at the Harkless reef and the upper and lower sections of the Stewart’s Mill reef (both of which will hence force be referred to as the Upper and Lower reefs) with the goal of comparing the abundance of specimens and the size of their central cavity diameters in the three reefs with the hope of explaining the differences and similarities of the three.
Methods.
At the Stewarts mill location data was collected from the lower section of the reef and the upper section. At the Harkless mill reef data was collected only once as there was only one layer present. We used a point counting grid to assess the proportional volume of the rock occupied by archcaeocyathans, and measured the central cavity diameters of the archaeocyathans present in the grid. We used a 6x6 grid and recorded whether there were microbial thrombolites, cement, or archaeocyathans present at each of the 36 points of intersection. Twenty data sets were collected at all three sites. The data was then compiled and analyzed through RStudio and two box plots were created. One that gave the percentage that the central cavity diameter was of the overall diameter of the specimens, and one that gave the percentage of the rock face that was made up of archaeocyathans.
Results
Looking at the box plot from figure 1 for the Harkless reef we can see that the median value for the central cavity diameter versus total diameter was roughly 40%, 50% of the data points were within the range of 32%-50%, and the overall range was from 12%-60%. From the box plot for the lower part of the Stewart’s Mill reef we see that the median value was roughly 38%, half the data set was in the rage of 32%-50%, and the overall range was 20%-75%. The box plot from the upper section of the Stewart’s Mill reef shows us that the median was about 30%, half of the data was in the rage of 25%-40% and the overall range was from 10%-45%
Figure 1.1 shows the percent of rock surface area made up of archaeocyaths. Of the 20 data sets taken from the Harkless reef, half of them were in the rage of 25%-35%. Meaning the rock face was 25%-35% made up of archaeocyaths. The overall rage for the Harkless reef was from 15%-55% with a median of roughly 52%. For the Lower reef the median was found to be 15%, the data sets ranged from 5%-30%, and half of the data sets were with in the 10%-30% range. The Upper reef had a median value of 30% and an overall range of 10%-55%. Half the data points were in the 25%-35% range.
Figure 1.0 Box plot of central cavity diameters compared to the total diameter of the Archaeocyathans at the three locations.
Figure 1.1 Proportion of archaeocyaths present at each site.
Discussion
From looking at figure 1.0 we can see that the Harkless reef central cavity diameters were very similar to those found on the Lower
