researchers can better understand past environments where extinct species thrived. When

Transcription

1

2 researchers can better understand past environments where extinct species thrived. When animals die their carcasses are often scavenged and re-deposited, if not destroyed by erosional processes. A majority of the animals that once persisted on Earth literally have left no record, and are only known from the rare fossils of their relatives. Even if an animal is preserved in pristine conditions, the stratigraphy it is contained in could be reworked, destroying the stratigraphic succession. This shows the importance of preserving and excavating sites that contain well preserved faunal remains that are within a stratigraphic context. In this study, I investigate changes in tooth morphology in the genus Peromyscus. The specimens examined were taken from Parker s Pit, a pit cave in the Black Hills of South Dakota. Background Geology of the Black Hills The Black Hills, South Dakota has a unique geology, and is an isolated range of mountains, surrounded by the Northern Great Plains (Turner, 1974) (see Figure 1). It is located in the South- Western portion of the state and is home to the national monument Mount Rushmore. Geologically, the hills consist of Early Mesozoic red-beds and sandstone, Paleozoic limestone, and Precambrian metamorphic rock. The eastern part of the state is composed of glacial till on top of Cretaceous clay, sandstone, shale and chalk deposits (South Dakota Geology, 2011). In essence, the Black Hills are dome with the top eroded away, with abundant cave formations in the Mississippian limestone (Schwarcz et al., 2004). 2

3 Figure 1 The geology of South Dakota. The location of the site in this study can be located in the left corner of South Dakota, the Black Hills. The Black Hills are an isolated mountain range that has weathered to expose its inner core of metamorphic rocks, in red. The white star indicates Parker s Pit, the site of research for this report. (Source: South Dakota Geology, Environments and Ecology of the Black Hills 3

4 From east to west, the Black Hills stretch 100 km wide and 200 km in length from north to south. The Black Hills are a forested uplift of rock, surrounded by short and mid-grass prairies. Because they are an isolated forest environment surrounded by prairie, many species found there are unique to the area and not geographically connected to their relatives. The predominant vegetation in the forest of the Black Hills is the Ponderosa pine, and houses 19 known families of mammals (Turner, 1974). Extreme fluctuations in climate over the past 2.5 million years have controlled the size of the environments in the Black Hills. Unstable, changing environments promote the movement of organisms in search of habitat with sustainable resources. The presence of some plants and animals in the Black Hills can be explained by migration pressures due to glacial and interglacial periods (Schultz and Bertrand, 2012). One of the most unique features about the Hills is their cave systems. The Black Hills are home to Wind Cave National Park, a cave system that ranks one of the most complex in the world. It is also one of the longest cave systems in the world, winding 139 miles, and ranking 5 th in the world (2011). Glacial and Interglacial periods Over the course of the Quaternary, the last 1.6 million years ago to present day, global climate has varied rhythmically (Lowe, 2007). The Quaternary is divided into two epochs; the Pleistocene and the Holocene. The Pleistocene is characterized by the last ice age, in which the Laurentide Ice Sheet covered nearly all of Canada and north parts of the United States, where it is called the Wisconsinan Glacial episode. These massive glaciations span from approximately 125,000 to 14,000 years ago. The Holocene, starting 12,000 years ago, marks the end of the late 4

5 Pleistocene glaciations (Montanari, 2002). The Black Hills has an extensive record of glacial and interglacial periods, making it an interesting area to study paleoecology, see Figure 1 in the Appendix. Over this time period there is no evidence that glacial ice sheets reached the Black Hills; however, at the height of the Wisconsin glaciations around 18,000 years ago, the ice sheet was 150 miles east from the hills. The presence of these glaciers promoted southern migration by northern communities. These northern communities include organisms such as shrews, voles, lemmings, squirrels, caribou, muskoxen, and moose. In the interglacial periods, southern species migrated north, where the environments could reach sub-tropical climates. This variation of glacial and interglacial created temporary isolation within the Black Hills. Genetic flow and species diversity would then increase over the next period of glacial advance and retreat. (Turner, 1974). Forest fires are also historical occurrences within the Black Hills, and it is thought that they were due in part to increased early human activity during the last glacial maximum. Today, the prairies are inhabited by species that are also adapted to the forest biota. The Site Parker s Pit is a unique cave; it is a pit cave and acts as a trap, once animals have fallen in most die within the cave. The cave is located on a side of a hill on the (direction) range, see Map 1, appendix. Erosional processes create a stratigraphic sequence, with oldest sediment and remains on the bottom and youngest sediments on top. In this report, I compare different loci and stratigraphic units within the cave by comparing the consistency and morphology of the genus Peromyscus, which are commonly known as deer mice. They are abundant within the cave 5

6 sediment, making them a more statistically viable candidate in the analysis conducted in this paper. I will be seeing if there is a correlation between different stratigraphic units and locations in size of teeth; length and width. Parker s Pit contains two separate cones, representing two different openings to the cave, one that has since closed off. The first, Main cone, is the entrance that is still accessible, and is used to access the cave. The Red cone is an old opening to the cave that has accumulated enough sediment to reach the surface. It was once a pit cave but as it filled in it became a cave that was accessible, and could be used as a den. There is evidence that supports the idea that it was inhabited by weasels.. It is thought that the weasels brought Peromyscus into the den to feed their young, as weasel teeth are found in abundance in the red cone Methods To examine the question of body size change in regards to climate, it was necessary to narrow the study to one species and then collect the molars of that species. The species that is being focused on is Peromyscus, or the common deer mouse. This species is short-lived and abundant in the Black Hills region, making it a good candidate for research. Dentary specimens with molars intact are preferred for this type of study, as they are most often the only elements of a skeleton preserved. Excavation Procedures In order to get to Parker s Pit, myself and my advisor, Russell Graham had to travel three days by van. We arrived in Custer, South Dakota, on July 3 rd, 2011, to meet with members of the 6

7 Western Interior Paleontological Society, who were volunteering from the Denver Museum of Nature and Science. We stayed in a group camp site with these volunteers, approximately 45 minutes from Parker s Pit. The following day we went to the forestry office to retrieve caving equipment that was stored in their facilities. We then transported the equipment to the site and assembled an A frame that would be placed over the entrance to the cave (see below, Picture 1). Picture 1 Parker s Pit is located on the side of a hill, facing towards the south. As shown, the A frame is in place, the generator would be located to the left of this picture. (Source: The other equipment we brought with us included a generator, mechanical winch, tarps, sediment bags, lights, harnesses, extension cords, helmets and excavation tools. After the cave preparation was complete, we were able to start excavation. Over the course of the next two weeks we traveled to the pit every day at approximately 8 am and returned to the camp site around 5 pm. Various tasks needed to be assigned while at the cave. Since the areas of 7

8 excavation were relatively small, no more than 3 x 3 meters, members of the excavation team took turns excavating, bagging sediments, and writing identification cards. It was also essential that at least two people stay outside of the cave at all times to operate the mechanical winch. See the Appendix for a map of the interior of the cave. Sediment was brought up in bags which included a card that identified the location, unit, square and level the sediment was excavated from. These cards were kept in plastic zip-lock bags to protect them from water and dirt. When we filled enough bags of sediment, about 30, half of the volunteers left the pit to go to the screen-washing pond (see Picture 2 below) to start the long process of recovering bones. In this process, wooden frames with screens on the inside were placed on wooden horses. Water was pumped out of the pond and fed through hoses so that the pressure was sufficient enough to sift through the sediment. While sifting through sediments, fragile bones including large bones and jaws were taken out of the sediment and placed in pill bottles for protection during transport. After the majority of sediment was washed away, the remaining bones and gravel were left to dry in the sun on large white tarps. 8

9 Picture 2 - Screen washing site, with volunteers from the Western Interior Paleontological Society. (Source: Eric Grimm, 2011) After the sediment was dry, the remaining gravel and bones were transferred to zip-lock bags, along with their location identification cards. The final step in the excavation process was to pick the bones out of the gravel. The bags were slowly sorted through using forceps to pick out any bone material. The bones were placed in pill bottles and small containers, with a label identifying their original location in the cave. The small containers and bags of unpicked sediment were transported back to State College for further preparation and research purposes. Analysis of Specimens 9

10 To investigate the question of body changes with regard to climate change, I utilized dentary elements of the species, Peromyscus. I used the molars (M1, M2 and M3) as these parts of the skeleton are most often preserved. Some of the jaws were already identified as Peromyscus specimens, while others I had to identify myself. The mandibles that were already identified as Peromyscus were identified by Joel Christine, for his thesis (Christine, 2008). I measured the length and width of the M1, M2 and M3, as were available in the sample, see Picture 3 below. I also measured the total length of the three teeth. See Table 1, 2, and 3 in the Appendix for the complete data set. From these values I could graph the length of the M1 versus width of the M1 in millimeters, in order to show distribution of size of the M1. Picture 3 Red line indicates length measurements, and black line indicates width measurements. (Source: (Christine, 2008)) I looked at three locations within the cave, the Red Cone, Main Cone 1, and Main Cone 2. By studying three different excavation sites it is possible to study a greater interval of time, as each location was deposited at different times. It is thought that the oldest part of the cave is the Red 10

11 Cone, which gets its name from the iron oxides, which give the soil a red-rusty color. This part is thought to be the oldest because it was formed from an entrance to the cave that was open at one time but modernly is not. Fossil remains from the top of the cone reveal that it was closed off some 40,000 years ago. The opening once leading to the Red Cone is much different from the opening to Main Cone 1 and 2. Red Cone gets its name from the iron-oxide rich soils which stain everything with rust. Red Cone was formed by an entrance to the cave that has been filled in modern times. Red Cone was an inclined entrance that animals used as a den and could come and go as they pleased. Main Cone 1 and 2 deposits are composed of dark sediments, from two sides of the same cone, the entrance cone (see Map 1, Appendix). In this modern opening, animals fall into the pit through a 40 foot vertical drop. Some small animals can withstand this high of a fall; however, all of the animals that fall in do not come out and die within the pit. Results Table 1 Summary of results Location in Cave Number of individuals, N Length M1 Range Width M1 Range Average M1 length (in mm) Average M1 width (in mm) Red Cone Main Cone Main Cone

12 Graph 1 Box and whisper plot of results. The second and third quartiles shown by the boxes, with the first and 4th quartiles as the whiskers. Graph 2 Plot of length versus width of all M1 specimens. Main Cone 1 specimens are shown in red, Main Cone 2 specimens are shown in blue and Red Cone specimens are shown in green. Discussion and Conclusion The results of this study indicate that there is no apparent change in size in Peromyscus at Parker s Pit Cave. Disproving the original null hypothesis of this thesis, the next question to ask is why Peromyscus does not change morphologically with changes in climate. Long-term environmental changes create a selective pressure for genes that are favorable within a given environment. This process is called microevolution, and it is thought to keep populations alive and continue to evolve (Reale et al., 2003). 12

13 Different locations within the cave pertain to certain intervals of time. Specimens from Red Cone have been radiocarbon dated to greater than 20,000 years ago, around the height of the Wisconsin glaciation. Specimens from Main Cone 1 date to 4,000 14,000 years ago, spanning late Pleistocene to the warm stage in the mid-holocene. The fact that there is no change in size in Peromyscus over this type of climate interval proves that they are a stable genus morphologically, and are resistant to extreme changes in climate and environment. Peromyscus maniculatus, the common deer mouse, is wide spread across North America. The small rodents make caches, storing food which includes seeds and berries; however, they also eat insects. Deer mice have been described as not having a preference when it comes to habitat. On the Oregon coast they range from beach to forest ecosystems. In Colorado they prefer alpine and conifer forests; however, they have also been reported from islands off of the coast of British Columbia, Canada (Sullivan, 1995). Subspecies of Peromyscus maniculatus are characterized by their habitation preferences, and currently there are 67 known subspecies. An interesting aspect of Peromyscus is that they are the only rodent found in association with Ponderosa Pine (Pinus ponderosa). The significance of this is that Ponderosa is a fire-dependent plant and can only disperse its seeds at high temperature. Research has even shown that in numerous deer mouse communities, the number of individuals in burned areas is higher than those areas with no burn. This is mostly likely due to a flux in seeds, annual plants and insects to a recently burned area. Deer mice are often the first species to invade a post-burn habitat (Sullivan, 1995). Another interesting aspect of this study is that there is only one species of Peromyscus that we find in the cave. If there were more than one, we would expect to see isolated clusters on the length versus width graph. This is significant in that it shows that this species of Peromyscus was not ecologically replaced by another species as a result of changing climates and 13

14 environments. An interesting concept to study would be to look at the Peromyscus population in the Black Hills and determine whether the modern population is related to the fossil population. This would bolster the idea that Peromyscus is resilient and unresisting to changes in climate. Future studies should focus on why certain mammals follow Bergmann s Rule and why others do not. Human activity is accelerating climate change, as we add more and more greenhouse gases to the atmosphere. This will change ecosystem dynamics and services drastically, but to understand the full implications we need to know how the organisms on Earth will physically change. As a population, humans are incredibly exploitative. We consume much more than we can produce, and we consume exhaustible resources that will not be regenerated for the next generation. If human induced climate change continues, global temperatures will rise. If most organisms do follow Bergmann s rule, humans could be faced with smaller organisms, which would result in a smaller food supply. As a population, we must understand and take responsibility for our actions on this planet, and prepare for future changes. 14

15 References Christine, J.A., 2008, Examining the Age Distribution of Peromyscus Remains in Parker's Pit Cave, Custer County, SD [Senior Thesis thesis]: State College, The Pennsylvania State University. Lowe, J.J., 2007, Understanding Quaternary Climatic Change, Elsevier, p Montanari, A.S.G.a.R., G., 2002, Drops of Time, Strolling among stones, Geological Observatory and Pollyphonic Studios of Coldigioco. Natural Features and Ecosystems, 2011, Natural Features and Ecosystems, National Park Service, U.S. Department of the Interior. 15

16 Reale, D., McAdam, A.G., Boutin, S., and Berteaux, D., 2003, Genetic and plastic responses of a northern mammal to climate change: Proceedings of the Royal Society of London Series B-Biological Sciences, v. 270, p Schultz, L.D., and Bertrand, K.N., 2012, Long Term Trends and Outlook for Mountain Sucker in the Black Hills of South Dakota: American Midland Naturalist, v. 167, p Schwarcz, H.P., Ford, D.C., and Baldwin, S., 2004, Late Pleistocene paleoclimate in the Black Hills of South Dakota from isotope records in speleothems: Palaeogeography, palaeoclimatology, palaeoecology, v. 203, p Sheridan, J.A., and Bickford, D., 2011, Shrinking body size as an ecological response to climate change: Nature Climate Change, v. 1, p Sullivan, J., 1995, Peromyscus maniculatus. In: Fire Effects Information System, in U.S. Department of Agriculture, F.S., ed., Rocky Mountain Research Station, Fire Sciences Laboratory Turner, R., 1974, Mammals of the Black Hills of South Dakota and Wyoming, University of Kansas, p Watt, S.M., Volker Salewski, 2010, Bergmann's rule; a concept cluster?: Oikos, v. 119, p

17 Appendix Map 1 An interior map of Parker s Pit (modified from Ohms, Walz, and Shafe, 2005) with stars denoting the location of the excavation areas included in this study. 17

18 Figure 1 - Graph constructed using Greenland Summit Ice cores. It demonstrates the rapid periods of warming with gradual cooling following. (Source: Slingerland, 2012). Cat # Tooth Length total L Width (mm) (mm) (mm) unit level sq date 1 m ? TBD 8/1/ m m m /1/ m m m1 1.6 N/A no date 18

19 9 m m3 N/A N/A 10 m N/A TBD 8/1/ m m3 N/A N/A 11 m1 1.4 N/A ?? 11 m m3 N/A N/A 46 m N/A /17/ m m3 N/A N/A 47 m N/A /17/ m m3 N/A N/A 48 m N/A /17/ m

20 48 m3 N/A N/A 49 m /17/ m m m /17/ m m m /17/ m m m1 1.5 N/A /17/ m m3 N/A N/A 53 m1 1.6 N/A /18/ m m3 N/A N/A 20

21 54 m /18/ m m m /18/ m m m /18/ m m m /18/ m m m N/A /18/ m m3 N/A N/A Table 1 Main Cone 1 location specimens, 18 in total. Cat # Tooth Length (mm) total L (mm) Width (mm) Unit Level Square date 21

22 6 m m m m m MC2 7/29/ m m N/A m m3 N/A 0 N/A 13 m m m m m m m m m m1 N/A N/A N/A 16 m m m m m m m

23 18 m m m m m m m m m m m m m m m m m N/A m m3 N/A 0 N/A 25 m m m m N/A m m3 N/A 0 N/A 27 m1 1.4 N/A m

24 27 m3 N/A 0 N/A Table 2 Main Cone 2 specimens, 18 in total. The average length in M1 in millimeters is Cat Tooth Length Total Width # (mm) Length (mm) (mm) unit level sq date 3 m TBD 8/1/ m m m /1/ m m m Surface TBD 8/1/ m m m Surface TBD 8/1/ m m

25 28 m Surface TBD 7/1/ m m m Surface TBD 7/1/ m m m Surface 7/1/ m m m Surface TBD 7/1/ m m m1 N/A N/A N/A Surface TBD 7/1/ m m m Surface 7/1/

26 33 m m m Surface TBD 7/1/ m m m Surface 7/1/ m m m N/A 0.92 Surface TBD 7/1/ m m3 N/A 0 N/A 37 m N/A 0.98 Surface TBD 7/1/ m m3 N/A 0 N/A 38 m N/A 0.94 Surface TBD 7/1/ m2 N/A 0 N/A 26

27 38 m m N/A 0.84 Surface TBD 7/1/ m m3 N/A 0 N/A 40 m N/A 1.04 Surface TBD 7/1/ m m3 N/A 0 N/A 41 m1 N/A N/A N/A Surface 7/1/ m m m1 1.6 N/A 1 Surface 7/1/ m m3 N/A N/A 43 m1 N/A N/A N/A Surface TBD 7/1/ m m3 N/A 0 N/A 27

28 44 m1 1.5 N/A 0.92 Surface 7/1/ m m3 N/A N/A 45 m N/A 0.92 Surface TBD 7/1/ m m3 N/A 0 N/A Table 3 Red Cone specimens, 22 in total. The average length of M1 in millimeters is