Burrowers of the Oligocene

Burrowers of the Oligocene:
Taphonomic Studies and Interpretation
the White River Formation Underground.

By: Dr. Kent A. Sundell,
Casper College, Wyoming

The following is an update of Oligocene vertebrate fossil material collected from private lands in Converse County, Wyoming. This data is a direct result of a commercial fossil business that supports continuous collection, preparation, display, and scientific study of vertebrate fossils from a rapidly eroding badlands environment. All specimens have individual specimen numbers (i.e. DF0101), locality data, and accurate stratigraphic data. Many of the specimens have been sold and donated to preparators, collectors and museums prior to preparation so many identifications (id's) are field id's and are broad groups rather than genus and species level id's. A total of 3,200 specimens have id's and are included in these statistics. All specimens are at least upper skulls or more than ½ of a skeleton. No severely weathered skulls, isolated jaws, teeth or bones are included in these data. Because of the intense annual detailed surface collecting methods (searching for Reptiles comprise 14.5% of the total population, including tortoises (9.9%), lizards (3.5%), amphisbaenids (0.7%), and snakes (1.1%). Carnivores make up 5.1% of the total population, including sabertooth cats (1.2%), large dogs (0.2%), small dogs (2.3%), Hyaenodon (0.8%), mustelids (0.2%), and entelodonts (0.5%). Insectivores comprise 3.20% of the entire population, including leptictids (2.0%) and small insectivores (1.20%). Marsupials (mainly Peratherium) are 2.8% of the fauna. Ungulates make up 40.08% of the population, including oreodonts (25.25%), camels (7.5%), Hypisodus (2.1%), Leptomeryx (1.9%), horses (1.2%), large rhinoceros (0.6%), small rhinoceros (0.4%), and titanothere (0.2%). Rabbits make up 23.5% of the population and rodents comprise 9.6%, including Ischyromys (1.8%) and small rodents (7.8%). Birds are very rare and are primarily represented by eggs (0.4%) and a few skeletal remains.

The most striking ecological aspect of this fauna is that more than 81% of the individual specimens have some type of denning or burrowing lifestyle. Tortoises, oreodonts and rabbits are certainly the most common burrowers. These burrows of many sizes undoubtedly provided shelter and cover for other non-digging animals in the ecosystem. Due to the large sample size and consistent collecting techniques, the presence of many well articulated skeletons of any taxon is an indication that it may be using the burrows even if it did not dig it. The many complete skeletons of Hypisodus found at Douglas suggest this small deer-like artiodactyl was using burrows. Conversely, the absence of many complete skeletons are an indication that organism does not use burrows (horses, rhinos, entelodonts, and Leptomeryx). The lack of articulated skeletons of the large rodent Ischyromys suggests that it is primarily a tree dwelling squirrel.

The presence of many dens and burrows helps explain the relatively common occurrence of well articulated vertebrate specimens from this locality. Several other peculiarities include the proportionately few number of horse specimens, small running rhinoceros, and titanotheres relative to faunas in Nebraska and South Dakota and the abundance of well preserved small reptiles such as snakes and lizards. The Douglas, Wyoming White River Oligocene fauna was dominated by a denning and burrowing community. The presence of so much burrowing activity suggests easy digging stratum (ash rich claystones), low water tables, and drier climate.

Oreodonts (Merycoidodontoidea) are the most commonly collected fossil from the White River Formation of central North America. They locally comprise 25% of the fauna. They have previously been thought of as sheep-like ruminants, migrating in large herds across the early developing grasslands of North America, presumably protected by sheer numbers and musk ox-like defensive techniques from sabertooth cats, Hyaenodons, Archaeotheriums, and other large carnivore predators. The White River Formation was composed primarily of soft yet firm silty and ash-rich mudstones a perfect digging and denning medium. Recent extensive collections of multiple (several closely associated individuals) oreodont specimens (36 multiples) indicate they were burrowing denning mammals using their distinctive large canines for defensive purposes, fending off predators at the mouth of their burrows. Of eight observed specimens with unborn fetuses, three had 4 fetuses, two had 3+ fetuses, two had 2 fetuses, and one had 1 fetus. At least nine specimens were composed of subadults (siblings?) with 3-5 individuals in calcareous enriched nodules (dens) that varied from 24"x18"x6" up to 60"x24"x10". Three of the subadult specimens are fully articulated (not scavenged). Two thirds of all multiple specimens are disarticulated (predated) with mostly skulls, feet and miscellaneous broken skeletal elements in den-size (24"x36"x 8") nodules. The occurrence of these multiple oreodont specimens in close proximity, both predated and not predated is the primary physical proof of the burrowing, denning, and social behavior of oreodonts.

Multiple fetuses (2-5), digging feet with four claws, large heads relative to body size, short stocky bodies, large genetic diversity, and even the diagnostically large canines are additional supportive evidence of the oreodonts burrowing lifestyle. Two spectacular multiple specimens with 6 and 10 individuals of different ontogenetic ages (adults, subadults, and juveniles) prove the social nature of oreodonts. The largest specimen contained 1 adult, 4 subadults, and 5 juveniles in a single large burrow.
In summary, oreodonts dug many burrows and dens into the soft firmly consolidated sediments, they raised small litters of offspring , occupied the dens most of the year, and defended themselves from predators by backing into the den and exhibiting their characteristic large canines. When carnivores were successful in penetrating the dens they commonly feasted on the juveniles and subadults, leaving the leftover skulls, feet, and broken bones on the floor of the den, which later collapsed and form the core of concretionary nodules that we find the specimens in today. Oreodont dens can be distinguished from carnivore dens because they contain only oreodont material (carnivore dens have fragments of many (5+) species) and commonly of similar size subadults and juveniles (siblings). Oreodonts successfully developed and occupied an ecological niche most similar to modern prairie dogs. They used the burrows for shelter and protection from predators, but commonly ventured out to browse on nearby vegetation. When danger threatened they probably alerted the group by sounds and sights, ran back to the burrow, and easily defended the entrance with their large sharp canines. Abandoned burrows provided habitat and shelter for many other smaller animals during the middle Tertiary of North America. Because oreodonts were one of the most common mammals during the Oligocene and dug many large burrows they were the keystone species of a dominantly burrowing community of organisms during deposition of the White River Formation.