Our research focused on Upper-Ordovician brachiopods. We used species size, and relative abundance of species at different elevations to look at inter- and intra-species variation over time and space. Having identified trends in the aforementioned areas we compared these areas to the depositional environment. The collection site we chose was at a road-cut on Indiana State Route 1 south of Brookville. The road-cut crosscuts four formations of the Richmondian stage of the Cincinnatian series representing the upper-Ordovician period. These formations are Arnheim, Waynesville, Liberty and Whitewater. The location is situated on the Cincinnati Arch and the beds were deposited between 425 and 500 million years ago. The strata consists of alternating layers of shale and limestone. This site was chosen because of its terrific abundance of brachiopod fossils and the scope of time represented by the exposed formations.
This study focuses on changes in brachiopod populations in the upper-Ordovician as observed in the Richmodian stage. By quantifying and analyzing both changes in relative species abundance and changes in size within different species over time, trends in brachiopod variation over time became evident. Comparing trends in brachiopod variation with the changing depositional environments of the upper-Ordovician, a correlation (pronounced, in some cases) is observed between changing environments and some changes in brachiopod populations, both within and between species. Other changes do not necessarily correspond to changes in climate, but rather appear to relate to inter-species competition.
The wealth of brachiopod fossils in the Richmodian stage suggest that the brachiopods were quite successful in the climate and conditions offered by the upper-Ordovician period in the region that is today’s Cincinnati Arch. It follows that brachiopods found in the Richmodian prefer the environment that sees their populations thrive. As such, the dominant brachiopods of the different formations in the Richmodian stage (as evidenced by their abundance in our southeast Indiana site) are indicative of the environment associated with the region of study at the time of that formations deposition. It is in this sense that the brachiopod fossils included in this study can be used as proxy paleoclimatic data. Through the identification of fossil species within a stratum for which the depositional history is unknown the paleoenvironment can be determined based on the presence or absence of certain brachiopods.
With regards to paleoclimatic data, approximately 475 million years ago, North America lay on the equator. The area that is Ohio lay close to 26 degrees south of the equator. Ohio was engulfed in a tropical to subtropical environment and was covered in a relatively shallow marine sea. The warm water produced an abundance of life and eventually an abundance of fossils.
Method
Collection strategy was discussed before actually going into the field to collect our specimens. We had been to the site a several times prior to the collection outing and had drawings of the road-cut. So we knew generally how the site was excavated. We wanted to be as complete as possible, but, also not spend any more time than necessary in the field. The tools we used in the field were rock hammers, a tape measure, a hand lens, rope, and specimen bags. We determined our starting point by using maps, diagrams, landmarks and stratigraphy to find ground zero as represented by the base of the Arnheim formation. Having determined ground zero we went to the top of the first plateau and threw two ropes down to the bottom of the hill. The two ropes were set 30 feet apart. We then took a third rope and laid a level line across the formation at 5 feet above the base of the Arnheim as a guide for collection. This procedure was repeated every five feet up to an elevation of one hundred feet. When we got to the top of the different plateaus we took into consideration the excess at the top of the previous section and continued with vertical measurement into the next cut. We omitted the 25 foot level because of uncertainty about which formation was exposed at that elevation resulting from a disconformity. In the end we had collected 400+ specimens of brachiopods representing seven different species. These seven species were D. meeki, R. ponderosa, G. insculpta, P. subquadrata, H. occidentalis, S. planumbona, and L. capax.
When we had the specimens back in the laboratory we started the process of cleaning, identifying and measuring the specimens. After the cleaning and identification was completed we then measured the specimens using a set of calipers. We measured width and length and then determined if the specimen were pedicle or brachial valves or complete specimens. As each specimen was measured and classified, the information was entered on a spread sheet in the program EXCEL. The headings on the spread sheet by column were Genus species, formation, elevation, width(mm), length(mm) and valve. When all data was entered in EXCEL, it was then imported into the SuperANOVA and the Statview programs. Through the use of these two analytical programs the data was manipulated and evaluated in order to quantify patterns of inter- and intra- species variation.
Data
Having compiled a data set containing over 400 brachiopod specimens from the Arnheim, Waynesville and Liberty formations of the Richmodian stage, the data set was imported into two programs for analysis. These two programs were StatView, a integrated data analysis and presentation system and SuperANOVA, a general linear modeling package. Through the use of these analysis program the data set was organized and trends within the data set were quantified so as to be able to analyze the changes in brachiopod populations throughout space and time.
Using the SuperANOVA program to perform t-tests, it was determined that for all the species involved in this study there was no significant difference between measurements corresponding to different valves (i.e., brachial, pedicle, or both). This was determined by running a T-test using valve as the dependent variable and length as the independent variable. For all the species involved the P-value associated with each T-test was always greater than 0.05, thereby indicating that with regard to different valves there was no significant difference between different “populations” (in this case different valves). Therefore, it was determined that all the specimen measurements could be treated as if of the same population regardless of valve. So, for the purposes of this paper the column indicating which valve was measured to derive length and width measurements can be ignored.
Using the correlation function in the StatView program, a correlation matrix comparing length and width measurements was computed. The result was a correlation matrix showing a 97% correlation between length and width measurements. (see Figure 1)
For the sake of this study the implications of this correlation matrix were that length and width measurements were interchangeable for all intents and purposes and as such only one measurement was used in further analysis of the data. Length was arbitrarily chosen to be used in this study except when otherwise noted.
Having established the foregoing conclusions, attention could be directed at data analysis specifically addressing the focus of this study, namely brachiopod variation over time and space within the Richmodian stage. All of the data analysis that follows is concerned with differences between and within species through space and time. This study focuses on seven species of articulate brachiopods, henceforth referred to only by their species name; they are: meeki, ponderosa, occidentalis, insculpta, subquadrata, planumbona, and capax. The specimens were collected at five foot increments from 5 feet above ground zero (the base of the Arnheim formation as exposed at the Indiana Highway 1 site) to 100 feet above ground zero. The different heights above ground zero associated with the different specimens are labeled as elevation in this study. In summary, over 400 specimens of seven different species were collected at 19 different elevations (no specimens were collected at 25 feet because of uncertainty as to which formation was exposed there) and it is these specimens that constitute the data set involved in this study and the subsequent analysis.
In order to quantify the changes in relative abundance of species at different elevations, pie charts for each elevation were constructed to show the breakdown of relative species abundance. These pie charts quickly revealed two important variations that occurred with increasing elevation. As it happens, these changes in relative abundance of species at different elevations occur at the formation divisions. As can be seen in Figure 2 , the
Arnheim formation (5-20 feet) is dominated completely by meeki, whereas the Waynesville formation is also dominated by meeki but also contains about 21% ponderosa (and two stray occidentalis washed down from above and which represent nothing more than a source of error and are subsequently not mentioned again in this analysis), and finally the Liberty formation demonstrates a complete change from the two previous formations in that the Liberty holds absolutely no meeki. The ponderosa are still present but only account for about 5% of the Liberty specimens. The remaining 95% of the Liberty specimens are five species that are entirely absent from the Arnheim and Waynesville formations below. The specimens dominating the Liberty are insculpta, subquadrata, planumbona, capax, and to a lesser degree occidentalis. In summary, the breakdown of relative species abundance by formation is 100% meeki in the Arnheim, 79% meeki and 21% ponderosa in the Waynesville, and a combination of six different species in the Liberty with meeki completely absent.
While the relative abundance of species within the Arnheim and Waynesville formations are straight forward and well-nigh constant, the Liberty formation offers a different story. The initial Liberty collection at 75 feet contains a roughly equal distribution of insculpta, occidentalis, subquadrata, ponderosa and planumbona. (see Figure 3) But as elevation increases within the Liberty formation two interesting things occur. Firstly, insculpta dominates the Liberty beginning at 80 feet and continuing through 90 feet as can be seen in Figure 4.
Secondly, capax is entirely absent at elevations below 90 but suddenly appears and accounts for about 20% of the specimens at 90 feet. (see Figure 4) having appeared at 90 feet, capax quickly dominates the brachiopods in terms of abundance above the 90 foot mark, comprising just under 50% of the specimens at 95 feet and about 65% at 100 feet (see Figure 5). This development of capax’s domination above the 85 foot mark was the first indication that something occurred in association with the 85 foot mark in terms of depositional environment, other indications will be discussed shortly.
With regard to intra-species change over time, two methods were used to evaluate changes in size within species by elevation. Using the StatView program a size-frequency distribution was generated in the form of a histogram. For each species involved in the study histograms were generated for each elevation the species was found at, showing the percentage of specimens having lengths falling within different ranges (see Figure 6 for an example). Using the SuperANOVA program a linear model of the mean specimen size associated with each elevation was generated for each of the seven species of brachiopods used in this study (see Figure 7 for an example). Both of these methods of looking at size distribution by elevation demonstrated similar trends in the changing sizes of individual species over time and space. Therefore, attention was given to the linear models generated in the SuperANOVA program.
The linear models of mean size plotted against elevation showed different trends for different species. The most drastic change was demonstrated by meeki. For meeki, size and elevation were directly proportional (see Figure 7). Beginning with a mean length of 11mm at the 5 foot mark, the meeki steadily grew in size as time passed, reaching length of over 14mm before disappearing from the field site used in this study (at an elevation of 70 feet above the base of the Arnheim). Ponderosa, meeki’s only fellow brachiopod in the
Waynesville and the species represented over a greater vertical range than all other brachiopods involved in this study, demonstrated a remarkable consistently in size throughout the entire survey area. The width of ponderosa consistently remained around 40mm. The remaining species were found only in the Liberty formation above the 75 foot mark, as was discussed above. Of them, the species with the most interesting trend in mean size was insculpta. The beginning of the Liberty formation (75 feet) found the mean length of insculpta to be about 17.5mm, where it remained relatively steady up to the 95 foot mark where it begins a rise that sees the mean length shoot up to just under 19mm at 95 feet and about 20.5mm at the 100 foot mark (see Figure 8). Overall, this change represents a range of 3mm or an increase of nearly 20% in as many feet of rock! Subquadrata, which closely resembles insculpta in outward appearance, dropped in length significantly between 75 and 80 feet, after which, it climbed steadily back up, achieving near to its initial length by the 100 foot mark (see Figure 8). Both occidentalis and planumbona demonstrate a decrease in mean length with an increase in elevation (see Figure 9 and 10).
While different species demonstrated different trends in mean size change over time and space, the species found at every elevation below 90 feet within the Liberty formation, with the exception of planumbona (i.e., insculpta, subquadrata, and occidentalis) all shared one thing in common. All of these species demonstrated a depression in mean size around the 80 or 85 foot marks. For some species it is more pronounced than it is in others. For instance, occidentalis drops steadily from 75 feet to 85 feet, losing a total of about 14mm of mean length over that period (a 33% decrease in length) while insculpta and subquadrata reach there minimum low at the 80 foot mark with much of a less drastic percent change in length.
In summary, there were several notable trends in relative abundance (inter-species change) and mean size (intra-species change) observed in the data. With regard to relative abundance three trends are noteworthy. First, there exists a change at each formation break
with the Arnheim containing meeki only, the Waynesville also containing meeki as well as ponderosa, and the Liberty, representing a clean break with the preceding formations, contains no meeki but instead it contains five species not found in the Arnheim or Waynesville formations. Secondly, within the Liberty there is a steady growth in the relative abundance of insculpta between the 75 and 90 foot marks where capax is seen for the first time. Being introduced at the 90 foot mark, capax becomes the dominant species by the 95 foot mark and accounts for well over half the specimens at the 100 foot mark. (see Figure 5). With regard to intra-species changes as observed by mean size change with increasing elevation each species demonstrates its own unique trend, but three generalization can be made. One, elevation and mean size are directly proportional in meeki. Two, ponderosa’s size remains relatively constant throughout space and time. Three, several species demonstrate a depression in mean size associated with the 80 or 85 foot marks.
What we explored through research was the depositional environment of the upper Ordovician. The entire Ordovician period experienced many periods of extensive flooding. The water would invade the shallow epicontinental seas and then regress. Toward the end of the Ordovician (upper-Ordovician), the flooding reached it’s maximum extent and then some what receded. (Grolier 1992)
During the Ordovician period, the North American continent was located along the equator and Ohio was some 26 degrees below. From it’s ancient location we can derive that Ohio had a tropical to subtropical environment. The seas that engulfed the area were epicontinental and relatively shallow and were thought to receive sediments from bordering mountains. The tropical temperatures created warm beds which in turn were richly fossiliforuos. (Feldmann 42)
The two basic sediments, which make up the Upper-Ordovician layers, are shale and limestone. While both were deposited in shallow waters, shale was formed in a relatively deeper area. (Witzke 135) Shale is made up of fine silt deposits which are settled in areas with low wave energy. (Grolier 92) Any organism living in these areas must have adaptations to living on a muddy sea floor which was subject to frequent periods of sedimentation. (Frey 251). The shale units make up 60% of the exposed Ordovician in the Cincinnatian series and were dominated not by brachiopods, but by filter-feeders. (Frey 242)
The limestone units were also deposited in a tropical/subtropical environment, but in a relatively shallower area where wave activity was stronger and sediments couldn’t settle. (Grolier 1992). The limestone formations were more characteristic of intertidal lagoonal nearshore areas. (Harris 1296) It is in these areas where the variables that control life (temperature, light penetration, and nutrients supply) are most abundant. (Grolier 1992) Limestone is comprised of nearly half calcium deposits originating primarily as skeletal remains. (Kaufmann 119) Our interpretation of the lithology of the strata involved in this study is based primarily on visual observations of the site.
Analysis
Of the six different trends in brachiopod variation identified above, we explain two of them as direct results of changes in depositional environments. The other four identified trends in variation are interpreted as resulting from inter-species competition (or the lack there-of). The changes associated with changing depositional environments represent the broadest level of change observed in this study and are, therefore of prime interest.
The observed drastic change in relative species abundance associated with the transition from the Waynesville formation at 70 feet to the Liberty formation at 75 feet marks the most significant trend in brachiopod population variation observed in this study of the upper-Ordovician. Based on field observations and data evaluation the Liberty formation is distinguished by both rock type and fauna. Below the 71 foot mark (Waynesville and Arnheim formations) the rock is dominantly shale, whereas the Liberty formation is dominantly limestone. While the shale in the Arnheim and Waynesville contains only meeki and ponderosa, the Liberty limestone contains five species not found below (occidentalis, insculpta, subquadrata, planumbona, and capax) and is entirely lacking meeki. Because we know that limestone is indicative of a more shallow sea and that shale indicates deeper seas at the time of deposition, we can infer that the meeki of the Arnheim and Waynesville formations was a deeper water organism that cannot live too near shore, while the species found in the Liberty formation favor a shallow sea environment. The beginning of the upper-Ordovician period saw the inland sea well inland, beyond today’s southeastern Indiana, from which point it receded with dropping global sea level. When the sea was at its height, today’s modern Indiana was covered by a relatively deep sea that hosted a thriving meeki population. Once the water receded to the point of leaving our area of study under only a shallow sea, meeki presumably followed the deep water it was accustomed to, taking itself out of Indiana to deeper water as new species moved back down from further inland from what was the shallow coastal region associated with extensive advance of the inland sea to the newly located coastal zone associated with lower sea levels. In short, the five species absent from the Arnheim and Waynesville shale deposits but abundant in the Liberty limestone (capax, subquadrata, planumbona, insculpta, and occidentalis) likely followed the traveling shallow sea associated with the coast of the inland sea. Therefore, it follows that the presence of these five species in a stratum or formation indicates a shallower depositional environment, whereas the presence of meeki in a formation is indicative of a deeper environment. As such, the presence of these fossils can serve as proxy depositional environment data.
The findings concerning the migration of the coastline and its subsequent fauna change for regions raises a question that could not be addressed in this study. That is, whether or not the arrival of capax, insculpta, subquadrata, planumbona and occidentalis in the Liberty formation represents the first time those species occupied the region that is modern day Indiana. While the answer would appear to be yes based on the findings of this study, this may not be the whole story. Because this study only goes back in time to the Arnheim formation, the fossil record produced through this study begins in shale associated with deeper sea conditions. On its way to becoming the deep sea associated with Arnheim deposition the shoreline surely migrated through the region of study before the Arnheim, at which time it may or may not have brought the species in question through with it. The answer to this question would lie in the formations preceding the Arnheim formation.
Several of the species found in the Liberty formation show a relative drop in size around the 80 to 85 foot marks. Corresponding to this drop in size at those elevations is the presence of a shale member interrupting the otherwise dominantly limestone make-up of the Liberty formation. This shale member implies that the sea advanced further inland for a time, increasing the depth of water over the region of study, thereby pro
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