Ash Hollow Formation

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The Ash Hollow Formation is a lithostratigraphic formation in central North America and extends mainly over the US states of Nebraska , South Dakota and Wyoming . It is known for its excellently preserved and sometimes abundant vertebrate fossils. These are sediments deposited by slowly flowing waters in which a thick package of volcanic ash is stored. The entire formation dates to the Upper Miocene around seven to twelve million years ago.

Geological framework

The Ash Hollow Formation covers large parts of the Great Plains in central North America and is one of the geological deposits of the Ogallala Group , of which it forms the uppermost layer. The entire group overlays the Oligocene and Early Miocene Arikaree groups . The Ogallala group can be divided into several formations. The sands and silts of the Valentine Formation are found in the lying wall, while various sediment bodies follow a hiatus in the hanging wall . In the eastern distribution area these are loose sands and gravels of the Long Pine Formation , while in the western area it is the Broadwater Formation . Both are to be placed in the Pliocene early Pleistocene .

Structure and lithology

Outcrop of the Ash Hollow Formation (light band below the top of the terrain) in the Ashfall Fossil Beds State Historical Park in Nebraska

The formation consists mainly of solid sands and sandstones cemented by lime and thereby banked , in which lenses of conglomerates are incorporated. The entire deposit unit reaches a thickness of up to 60 m and is divided into the lower Cap Rock Member and the upper Merritt Dam Member . It was created when sediment material was washed ashore during individual flooding phases, which was then baked together in dry phases through chemical weathering . In the lower part of these solid deposits, in the Cap Rock Member , there is a layer of light, white volcanic ash, which is between 30 cm and 3 m thick and which is partially underlain by loose gravelly sand. This layer of ash is known as Ashfall Fossil Beds , but it is not continuous, but forms more or less large lenses. The ash is composed of almost 100% glassy rhyolites , which have a diameter of 70 to 200 µm, with an average of 90 µm. Another layer of ash follows with the Grove Lake ash in the Merritt Dam Member . Measurement data obtained from the volcanic ashes with the help of fission track dating in the 1970s give the lower ash layer an age of around 8 and the upper layer of around 6.8 million years. More recent studies with the uranium-lead method from 2018 date the Ashfall Fossil Beds to around 11.8 million years and the Grove Lake ash to around 6.4 million years. The origin of both ash layers lies in several catastrophic volcanic eruptions. The Ashfall Fossil Beds are assigned to the Bruneau-Jarbridge volcano , which is located about 1400 km west of today's Idaho and entered a very active phase between 12.7 and 10.5 million years ago. For the younger Grove Lake ash , the Heise volcanic field , which is also in Idaho, comes into question. This showed higher activity 6.6 to 4.3 million years ago. According to the age data, the Ash Hollow Formation belongs to the Upper Miocene (locally stratigraphically Clarendonian to Early Hemphillian ).

Fossil finds

Fossil conservation is excellent and gives the Ash Hollow Formation the properties of a preserve and concentrate deposit at the same time. The Ashfall Fossil Beds in the lower Cap Rock Member should be emphasized here . The State Historical Park of the same name in eastern Nebraska is named after the fossil site. Mass piles of partially complete skeletons have been excavated here, but the Ash Hollow State Historical Park is also an important fossil site, as is the Lemoyne Quarry. both located in western Nebraska. The majority of the finds come from the volcanic ash layer, the living beings there died in the volcanic eruption. Individual finds also come from the lying and hanging sand layers, but have no direct relation to the fossils of the ash layer. There are now over two hundred fossil-bearing outcrops within the formation, which enable a very good reconstruction of the landscape at that time.

flora

More than 30 plant species have been documented so far, mostly in the form of pollen or seeds, rarely as whole leaves or as wood residues. Almost half of the plants belong to grasses . These include representatives of genera such as Piptochaetium , Stipa and Nassella , which are now common in open landscapes in South America . Berriochloa grasses, which alone comprise at least five species, also occur very frequently, and rice grasses have also been identified. Are still dominant Boraginaceae such as Anchusa , Cryptantha and Biorbia . Those of the hackberry tree are found among the seeds . Among the algae continue to diatoms and charophytes known.

fauna

Finds of a Teleoceras (1) and a Cormohipparion skeleton (2) in the Ashfall Fossil Beds State Historical Park in Nebraska
Finds of a female Teleoceras skeleton (3) and a rhinoceros calf directly above it in Ashfall Fossil Beds State Historical Park in Nebraska

The abundant wildlife includes vertebrates and invertebrates. The invertebrates often include mollusks , ostracods and sponges , which are usually passed on through their hard calcareous shells or the calcareous skeleton. All of these finds are associated with fresh water sources.

The most extensive and important are the vertebrate finds, the mammal fauna is very rich. The largest occurring animal is Serridentinus , a trunk animal with four tusks and a weight of up to 5 tons that belongs to the gomphotheria . Unpaired ungulates are very common , with rhinos with Teleoceras making up the largest proportion. So far, more than one hundred skeletons, some of them complete, are known and give a good insight into the life of this animal with only a small horn and a cylindrical body, which weighed up to 2 tons and was the dominant herbivore of that time. Horses are occupied with five genera alone: ​​the three- toed Pseudhipparion , Cormohipparion , Neohipparion and the single-hoofed Pliohippus and Protohippus . A complete skeleton of Tapirus is one of the earliest records of this genus in North America. The cloven-hoofed species mainly include three types of camels, such as the characteristic, huge, over 1 t heavy and with a giraffe-like long neck, and the two present-day ones representatives resembling Procamelus and Protolabis . In addition, Longirostromeryx is a hardy animal with saber-toothed canines. Among the predators, Cynarctus and Leptocyon, two dog-like animals are to be named, a third form being proven by bite marks. The occurrence of the beaver Dipoides and smaller mammals such as mice, including silk pocket mice and Mylagaulus as well as several species of red- toothed shrews , bats and moles are also important.

In addition to the mammals, remains of the bird world have been handed down, including an ancestor of today's crowned crane Balearica and the bird of prey Apatosagittarus . Among the reptiles, in addition to snakes and lizards, two representatives of turtles are particularly worth mentioning: the giant tortoise Hesperotestudo , which can be proven by partially complete, up to 120 cm long carapaces, and the musk tortoise Sternotherus . There are also frogs, salamanders and fish.

Landscape reconstruction

Based on the fossil remains and the sedimentological conditions, an open, largely flat savannah landscape can be concluded, which was only sporadically interspersed with trees. This was inhabited by large herbivores that specialized in grass forage. Individual joint finds of rhinoceros cows with calves lying close together, but also horse mares with foals, suggest a close social association among these animals. The Berriochloa grasses should also be emphasized, some of which were found as clumped remains in some parts of the mouth of rhinoceros skulls and indicate that these were a preferred food of these animals. Another interesting finding are small, filled caves with accumulations of semen and remains of harvest mice, which are obviously remnants of the food stores of these small mammals. In these landscapes, as the fine-grained sediments show, there were periodic floods. The climate was mild and frost-free.

Cause of death

The reconstructed savanna-like landscape was abruptly disrupted by the volcanic eruption of the Bruneau-Jarbridge volcano and covered by a layer of ash up to several meters thick. The countless fossil remains can be traced back to a mass extinction that was directly caused by the ash deposits. The larger ungulates , however, more likely succumbed to long-term exposure to the volcanic ash and did not die directly in the course of the volcanic eruption. Several factors indicated this. A vertical sequence of fossils can be established, the individual layers of which are separated by 10 to 15 cm of fossil-free ashes. At the bottom there are mostly fragmented remains of small animals such as turtles and water birds. This is followed by medium-sized animals such as camels and horses. Only above this, a good 25 cm above the base of the ash, do the large skeletons of the rhinos appear. This vertical structure suggests a death sequence that was directly related to body size. In addition, abnormal osteoarthritis similar to hypertrophic osteoarthropathy could be found on the bones of the larger ungulates . The swellings, in turn, are due to damage to the lungs , such as those that can occur when the volcanic ash is inhaled for a long time.

Research history

Hubbard Rhino Barn at Ashfall Fossil Beds State Historical Park in Nebraska

The first finds were known as early as the 1920s when young people discovered some bones on a slope outcrop. In 1953, Donald Peterson found a near-complete rhinoceros skull, which can now be seen in the museum at Ashfall Fossil Beds State Historical Park. In 1971, heavy spring rains flushed the skull of a young rhinoceros clear on the grounds of Colson Farm, which was later documented by paleontologist Mike Voorhies. Voorhies has been conducting regular geological surveys in this area since 1969. This further rhinoceros discovery finally led to specific scientific investigations, which were carried out between 1977 and 1979 and which included the discovery of innumerable fossil remains on an excavation area of ​​more than 600 m². From these excavations, the establishment of the Ashfall Fossil Beds State Historical Park resulted in 1991, whereby a more than 180 m² large, covered excavation area enabled the now annual examinations. After these excavations were completed and new ones continued, it was replaced in 2007 by the more than 1,600 m² Hubbard Rhino Barn .

Individual evidence

  1. ^ A b c d e f Robert Diffendal, Jr., Roger K. Pabian, and JR Thomasson: Geologic History of Ash Hollow State Historical Park, Nebraska. University of Nebraska, Lincoln, 1996 ( [1] )
  2. ^ WI Rose, CM Riley, and S. Dartevelle: Sizes and Shapes of 10-Ma Distal Fall Pyroclasts in the Ogallala Group, Nebraska. The Journal of Geology 111, 2003, pp. 115-124
  3. ↑ a b Jon J.Smith, Elijah Turner, Andreas Moeller, RM Joeckel and Rick E. Otto: First U-Pb zircon ages for late Miocene Ashfall Konservat deposit and Grove Lake ashes from eastern Great Plains, USA. PLoS ONE 13 (11), 2018, p. E0207103 doi: 10.1371 / journal.pone.0207103
  4. ^ Ada Swineford and Paul C. Franks: Opal in the Ogallala formation in Kansas. The Society of Economic Palaeontologist and Mineralogists (SEPM): Silica in Sediments (SP7). , 1959, pp. 111-120
  5. ↑ a b c d e Ashfall Fossil Beds State Historical Park. Designated a national natural landmark. ( PDF )
  6. ↑ a b c d Michael R. Voorhies and Joseph R. Thomasson: Fossil grass anthoecia within miocene rhinoceros skeletons: diet in an extinct species. Science 206 (Oct 19), 1979, pp. 331-333
  7. ↑ Joseph R. Thomasson: Berriochloa gabeli and Berriochloa huletti (Graminae: Stipaea), two new grass species from the Late Miocene Ash Hollow Formation of Nebraska and Kansas. Journal of Paleontology 79 (1), 2005, pp. 185-199
  8. ↑ Donald R. Prothero: The evolution of North American rhinoceroses. Cambridge University Press, 2005 (pp. 94-124)
  9. ↑ Bruce J. MacFadden: Fossil Horses. Systematic, Paleobiology, and Evolution of the family Equidae. Cambridge University Press, 1992 (pp. 71-72)
  10. ^ Richard C. Hulbert Jr .: Late Miocene Tapirus (Mammalia, Perissodactyla) from Florida, with description of new species Tapirus webbi. Bulletin of the Florida. Museum of Natural History 45 (4), 2005, pp. 465-494
  11. ↑ Thomas M. Bown: The fossil Insectivora of Lemoyne Quarry (Ash Hollow Formation, Hemphillian), Keith County, Nebraska. Transactions of the Nebraska Academy of Sciences 8, 1980, pp. 99-122
  12. ↑ a b c d Ashfall Fossil Beds State Historical Park ( [2] )
  13. ↑ MR Voorhies: Fossil Pocket Mouse Burrows in Nebraska. American Midland Naturalist 91 (2), 1974, pp. 492-498