In 1981 University of Nebraska paleontologists discovered a concentration of mammalian carnivore dens at Agate National Monument, Sioux County, Nebraska, that contained fossil remains of extinct Early Miocene beardogs, mustelids, and canids, dated at about 19.2 Ma. A decade of excavation (1981-1990) under National Park Service auspices produced six species of carnivores in association with bone fragments of probable prey (juvenile oreodonts, rhinoceros, and camel). Young, mature, and aged beardogs of a single species were found in the dens, including an adult female and her juvenile male offspring. This age distribution and analysis of sediments in the burrows suggest a relatively sudden death event, possibly a prolonged drought, in which animals died in their burrows within an ephemeral stream valley. After the death event their partially scavenged skeletons were buried by fine volcaniclastic silt and sand carried into the dens by wind and water over an unknown amount of time, presumably several months to years.
Another older den complex was recently found near Agate National Monument in 1991 and preserves the largest vertebrate burrows yet reported in the fossil record. The 22 million year-old site includes 5-6 large cylindrical burrow fills to 10m in length and 1-2m in diameter, distributed over 450m2. Bones of a wolf-sized beardog, a small fox-sized canid, and a rodent were found in association with the burrows. These den sites occur in extensive paleosol horizons indicating flat regional geomorphic surfaces, interpreted as semiarid Miocene grasslands of the upper Arikaree Group in western Nebraska and southeastern Wyoming.
Evidences in the fossil record of the ecology of extinct mammals are rare. Most often the remains of fossil vertebrates are preserved in stream, lake, or wind-deposited sediments with little or no information revealing the nature of their habitat, community associations, lifestyle, or other ecological relationships. Two Miocene sites discovered by the University of Nebraska in 1981 and 1991 proved an exception in that fossil remains of extinct carnivores were found within and in proximity to their dens. These den complexes were preserved intact, filled with fine-grained volcaniclastic sediments, and only revealed by recent erosion of Arikaree Group rocks in the Niobrara River valley, Sioux County, Nebraska. The carnivore dens occur in relationship to Early Miocene land surfaces (paleosols) within the Arikaree Group of western Nebraska. The two sites have been radiometrically dated at about 19 Ma and 22 Ma, respectively. They represent the oldest evidence of denning behavior by large mammalian carnivores known anywhere in the world.
QUARRY 3 DEN SITE (19.2 Ma)
The den complex was exposed by recent erosion of the west face of Beardog Hill, about 200m southeast of the Carnegie Hill waterhole bonebed at Agate National Monument. The site was discovered by O.A. Peterson of the Carnegie Museum (Pittsburgh) in 1904, and initially excavated by him and his co-workers in 1905. They found partially articulated skeletons of the beardog Daphoenodon superbus and less complete remains of other carnivores, but were unaware they had stumbled upon a den complex despite the large amount of carnivoran skeletal material at the site. Peterson named the locality Carnegie Quarry 3.
Quarry 3 remained undisturbed until 1981 when a University of Nebraska party relocated and opened the site in an attempt to explain the predominance of carnivoran fossils. Relative to ungulates which comprise almost all bones found in the waterhole bonebed at Carnegie and University Hills, carnivore remains are never abundant in the various waterhole bonebeds found at this stratigraphic level in the region. When we first excavated at Quarry 3 in September 1981 we were immediately rewarded by not only the discovery of large burrows but also by partial skeletons of previously undiscovered carnivores within the burrow fills. As work progressed from 1981 to 1990 the extent of the dens occupying an area of about 125m2 was defined and mapped (Fig. 1) and numerous carnivoran fossils were plotted and removed. Why Peterson and his men did not fully explore and excavate Quarry 3 remains a mystery: careful searches for field notes or diaries from the Carnegie excavations of 1904-1905 have proven fruitless, and we have only Peterson's brief comments in his publication of 1910 on the carnivores from Quarry 3.
Our census of the carnivores found in the dens, completed in 1994, includes our finds and Peterson's earlier material. There are two large beardogs (Family Amphicyonidae: Daphoenodon and a wolf-sized temnocyonine), two mustelids (Family Mustelidae: Megalictis and Promartes), and two small fox-sized canids (Family Canidae: Phlaocyon and Tomarctus). The carnivore present in greatest numbers in the burrows is Daphoenodon superbus, known from more than ten individuals ranging in age from juveniles to mature and aged adults. Among the most interesting discoveries during the excavation of Quarry 3 is evidence of marked sexual dimorphism in Daphoenodon. Males are large, robust, long-jawed with elongate premolars whereas females are much smaller, more gracile, and short-jawed with shorter premolars. Males display elaborate bony exostoses on the internal face of the distal radius which appear to be absent in the females.
A rare temnocyonine beardog is known from a skull and partial skeleton in Den 2 found less than 2m from a male Daphoenodon skeleton but segregated in its own burrow. The temnocyonine was a mature young adult, based on tooth wear: its death was surely premature as were the deaths of the juvenile Daphoenodon and its mother found nearby. In addition to these beardogs, several individuals of Megalictis, a large wolverinelike mustelid, and scarce remains of the small marten-sized Promartes were discovered during excavation of the northern part of the den complex (Fig. 1). Multiple individuals of the small canid Phlaocyon are represented by dental remains, and a few isolated teeth record the somewhat larger canid Tomarctus.
Fossils of the small canids are so rare and fragmented that one cannot be certain they were not prey of the larger animals in the dens. Arguing against this, however, is the fact that the only noncarnivoran bone found in the dens is a small, well-defined sample of limb, foot, and dental fragments of ungulates that seems to be the primary prey residue left in the dens. Such incomplete and comminuted fragments differ from the more complete, often partially articulated carnivoran skeletons in the dens, and are found scattered throughout the various burrows. These ungulate skeletal fragments belong to a limited range of animals, chiefly juveniles of the small oreodont Merychyus, the small rhinoceros Menoceras, and occasional rare camels. The small canids seem unlikely members of the prey suite recorded by the ungulate bone sample.
Carnivore bones in the dens bear the imprint of teeth and were scavenged by other mammals before sediment burial in the dens. Within the burrows the skeletons were covered by thinly-laminated, ash-rich, fine sand and silt carried into the dens by water and wind. Reexcavation of at least one burrow is evident after partial filling by such laminated sediments. Final burial of the den complex was accomplished by at least 10m of fine volcaniclastic sediment deposited over the site by a wide, shallow ephemeral stream that also buried the waterhole bonebed at Carnegie Hill. The stream valley was eventually filled by wind-deposited volcaniclastic loess that succeeded the local stream deposits, entombing the dens and their occupants until exposed by Quaternary erosion of the modern Niobrara River drainage.
HARRISON FORMATION DEN SITE (22 Ma)
Realizing that these dens were closely associated with fossil soils (paleosols) of the upper Arikaree Group in the vicinity of Agate National Monument, we began to search for additional evidence of burrows in the numerous paleosols of the region. In May 1991 we were fortunate to discover another den complex near Agate National Monument at a lower, hence older, stratigraphic level in the Harrison Formation (the Quarry 3 dens are in the base of the Upper Harrison beds). This new complex included the largest vertebrate burrows known to us, some measuring 10m in length and 1-2m in diameter (Fig. 2). These burrows differed from the Quarry 3 dens in their hardened outer rind of white siliceous rhizoliths (root casts) that formed a dense meshwork enclosing the burrow-fill. The siliceous encasing meshwork of roots creates a resistant capsule protecting the burrow-fill, hence these burrows are etched from the outcrop by erosion, creating a series of linear tubes standing in relief. In the vicinity of the burrows was a fully-articulated skeleton of a small canid and a partial skeleton of a rodent. Too small to have dug the dens, they probably explored and occupied them after they were developed by larger animals. Significantly, a few bones of a large beardog were found in place in one of the dens, suggesting the identity of the probable excavators of the tunnels. Our excavation of this site has been preliminary; future work may reveal more details of the site occupants and the areal extent of the den complex.
Our study of the upper Arikaree paleosols in the region east of the Hartville Uplift in western Nebraska and adjacent southeastern Wyoming indicates abundant paleosol horizons, dense rhizolith networks, and frequent preservation of vertebrate and invertebrate burrows. These discoveries have alerted us to the existence of a rich reservoir of paleoecological information providing insights into Early Miocene land surfaces, vegetation, infauna, and climate. Arikaree sediments of the central Great Plains retain the record of a 10 million year interval (19 to 29 Ma) when enormous volumes of fine volcanic sand and silt were introduced into the region by wind, much of it reworked by wind and local streams, eventually building a thick blanket of volcaniclastic sediments hundreds of meters thick in northwest Nebraska. The well-defined and regionally widespread sequence of ancient land surfaces marked by paleosols indicates periodic pauses in deposition of the upper Arikaree Group during the interval from about 19 to 22 Ma. These ancient soils appear to have been entisols and inceptisols with minimal development due to a seasonally semiarid climate in the North American midcontinent in the Early Miocene. Thus the dens are preserved as the result of a fortuitous combination of geologic and climatic events: the influx of fine volcanically derived sediment into the Great Plains; the episodic nature of Arikaree deposition leading to soil development on regional land surfaces east of the Rocky Mountain front; the alkaline chemistry of Miocene ground water and sediments conducive to preservation of phosphatic vertebrate bone mineral; and a semiarid climate that conserved the mineralogy of the fine volcaniclastics, and created chemical conditions favorable to development of dense rhizolith horizons. This dry climate also retarded the breakdown of vertebrate bone exposed to the Miocene environment until burial could take place. We can predict that fine volcaniclastics deposited under semiarid to arid conditions elsewhere in the world during the Cenozoic may also retain important diagenetic features (paleosols, plant traces, burrows, vertebrate fossils) useful to understanding the paleoecology of these regions.
Fig. 1. The Quarry 3 den complex at Beardog Hill, Agate Fossil Beds National Monument, Nebraska. Mammal bones occur in every den, and partial skeletons were found in Dens #1, 2, 3, and 5. Beardogs were common in the south part of the quarry (meters B to K, 1 to 7) and both mustelids and beardogs were found in the north part of the site (meters O to S, rows 0 to 4). The three distal terminations of Den 4 seen in row 4, H, J, and K, appear to be pupping chambers for young beardogs. Peterson's best Daphoenodon skeletons belonging to an adult female and her male offspring are believed to have come from Den 1.
Fig. 2. The Harrison Formation den complex contains enormous burrows, some with beardog bones in situ. Five major burrows have been identified: only preliminary excavation of Den 5 has taken place. The small carnivore skeleton in square B4 may occur within a burrow but its perimeter cannot be reliably defined at present. Den 3 is the largest vertebrate burrow presently known, at least 10m in length, and over 1m in diameter.
Fig. 3. Stratigraphic placement of the carnivore den complexes at and in the vicinity of Agate National Monument, Nebraska. The Quarry 3 den complex occurs as burrows excavated into calcareous tuff that contains the Carnegie Hill waterhole bonebed. This level at the base of the Upper Harrison beds is dated by the Eagle Crag Ash at about 19.2 Ma (fission track, zircon). A paleosol occurs above the dens that represents the Early Miocene land surface burrowed by these carnivores. The Harrison Formation den complex occurs in the Harrison Formation below the Agate Ash (21.9 Ma, potassium-argon, biotite) and is presumed to date to about 22 Ma. This is the oldest den complex of large carnivorans currently reported in the fossil record.
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