Taphonomy and Significance of Jefferson's Ground Sloth (Xenarthra: Megalonychidae) from Utah

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Western North American Naturalist

Volume 61 Number 1 Article 9

1-29-2001

Taphonomy and significance of Jefferson's ground sloth (Xenarthra: Megalonychidae) from Utah

H. Gregory McDonald Hagerman Fossil Beds National Monument, Hagerman, Idaho

Wade E. Miller

Thomas H. Morris

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Recommended Citation McDonald, H. Gregory; Miller, Wade E.; and Morris, Thomas H. (2001) "Taphonomy and significance of Jefferson's ground sloth (Xenarthra: Megalonychidae) from Utah," Western North American Naturalist: Vol. 61 : No. 1 , Article 9. Available at: https://scholarsarchive.byu.edu/wnan/vol61/iss1/9

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TAPHONOMY AND SIGNIFICANCE OF JEFFERSON’S GROUND SLOTH (XENARTHRA: MEGALONYCHIDAE) FROM UTAH

H. Gregory McDonald1, Wade E. Miller2, and Thomas H. Morris2

ABSTRACT.—While a variety of mammalian megafauna have been recovered from sediments associated with Lake Bonneville, Utah, sloths have been notably rare. Three species of ground sloth, Megalonyx jeffersonii, Paramylodon harlani, and Nothrotheriops shastensis, are known from the western United States during the Pleistocene. Yet all 3 are rare in the Great Basin, and the few existing records are from localities on the basin margin. The recent discovery of a partial skeleton of Megalonyx jeffersonii at Point-of-the-Mountain, Salt Lake County, Utah, fits this pattern and adds to our understanding of the distribution and ecology of this extinct species. Its occurrence in Lake Bonneville shoreline deposits permits a reasonable age determination of between 22 and 13 ka.

Key words: Megalonyx, Paramylodon, ground sloth, Great Basin, Lake Bonneville, Sangamon, Pleistocene, spit.

Records of the sloths Megalonyx jeffersonii, The partial Megalonyx skeleton described Paramylodon harlani, and Nothrotheriops shas- here was recovered from Lake Bonneville tensis in the Great Basin are rare. All currently shoreline sediments in the southernmost part known records for sloths are located around of Salt Lake County. Equipment operator Joe the region’s periphery. This scant record is also Miller, who works for Geneva Rock Products reflected in Utah, which has a very limited Company, made its discovery in the summer representation of Pleistocene ground sloths. of 1996. With the kind permission of that com- The first reporting of a sloth, the mylodont pany’s management, he presented the fossil Paramylodon, from Utah was based on 2 teeth material to the Earth Science Museum at and a partial vertebra (Miller 1976), presumably Brigham Young University, Provo, Utah, where from the same individual, in the Silver Creek it has been accessioned. local fauna. The site yielding this fauna is about The ground sloth bone-bearing unit lies 48 km northeast and 568 m higher in elevation within Pleistocene-age Lake Bonneville shore- than the sloth from Point-of-the-Mountain line deposits in the Jordan River cut of the (hereafter referred to as PM). Nothrotheriops, Traverse Range (Fig. 1). A Trimble TDC1 GPS the smallest of the 3 ground sloths, has been unit was used to calculate the elevation of the identified from cave sites in the southwestern bone-bearing unit at 1381.7 m (Fig. 2). The part of Utah based solely on dung samples bone-bearing unit lies approximately 10 m (Mead et al. 1984, Mead and Agenbroad 1992). below the present land surface. Machette The first Utah discovery of Megalonyx was (1992) mapped the surface sediments of the only recently made (Gillette et al. 1999) and study area as upper Pleistocene lacustrine was based on several bones of a single individ- gravels that were deposited during the Provo ual found near the city of Orem, about 26 km regressive phase of the Bonneville lake cycle. southeast of PM. The material described here These data broadly bracket the time of bone represents a substantial amount of a single, deposition from the maximum transgressive very large individual and is only the 2nd known phase of the Stansbury Level at approximately record of Megalonyx from the entire Great 22 ka (thousands of years before present) to Basin. It seems appropriate to provide as much the Provo regression at approximately 13 ka information as possible about this specimen (Currey et al. 1984, Currey 1990). and the depositional setting from which it was Research has shown that pre-Bonneville recovered and the inferences that can be made lakes occupied the lower parts of the north- regarding the ecology of this extinct species. eastern portion of the Great Basin back to a

1Hagerman Fossil Beds National Monument, Box 570, Hagerman, ID 83332. Present address: National Park Service, Geologic Resources Division, Science and Technical Services Branch, Box 25287, Denver, CO 80225-0287. 2Department of Geology, Brigham Young University, Provo, UT 84602.

64 2001] JEFFERSON’S GROUND SLOTH TAPHONOMY 65

Point-of-the-Mountain Megalonyx site

Fig. 1. Index map of the north central portion of Utah where Megalonyx jeffersonii fossils were collected.

Fig. 2. Topographic profile and location of key beds and lake levels at Point-of-the-Mountain. Elevations of the Bonne- ville and Provo Benches are taken from the Jordan Narrows 7.5-minute topographic map. Elevations of the Stansbury Level and of all ages are from Currey et al. (1984). 66 WESTERN NORTH AMERICAN NATURALIST [Volume 61 date of possibly 200 ka (Scott et al. 1983, McCoy ery of the sloth, excavation by the gravel pit 1987). Lake Bonneville, as generally under- operator resulted in the bed being completely stood, appears to have had its beginning some- covered. The currently exposed (1999) stratal time between 26.5 and 30.0 ka (Oviatt et al. unit above the bone-bearing bed is composed 1992, Oviatt et al. 1994, Lemons et al. 1996). of sand and gravel. The stratal unit below the Maximum lake depth, as viewed elsewhere in bed is primarily composed of sand as viewed the Great Basin, reportedly was 320 m about elsewhere in the pit. 14.5–15.0 ka (Jarrett and Malde 1987, Oviatt The ferruginous bone-bearing bed is over- et al. 1992) when the Bonneville Level devel- lain by high-angled (31°), NW dipping fore- oped at 1554 m above mean sea level. A well- sets of large-scale, sigmoidal, cross-stratified documented flood of gigantic proportions then beds (Fig. 3). The unit containing the bone- occurred, quickly lowering the lake to the bearing beds is approximately 10 m thick and Provo Level. Interstate Highway 15, which is composed of sand and gravel. Foreset beds runs immediately east of the fossil site, is situ- can be traced laterally and upward into gravel- ated on the resulting wave-built bench in this rich topset beds, which are largely low-angle area (Fig. 2). (12°) to planar beds. Measurements taken The Megalonyx skeleton reported here was from bedforms laterally to the south of topset recovered at 1381.7 m elevation, approximately beds indicate an azimuth direction of approxi- 10 m above the Stansbury Level. According to mately 260°. Grain-size distribution of 5 sam- Currey et al. (1984), this places the unit be- ples from the base to top of the unit indicates tween the Stansbury (approximately 1371.6 m) a coarsening upward trend (Fig. 4). Gravel- and Provo Level (1463 m) at PM. The Stans- and pebble-sized lithoclasts are subangular to bury Level probably formed before any other subrounded and are composed of limestone, levels between 20 and 23 ka (Currey et al. quartzite, and granite. Mica is a common com- 1984). The lowest Bonneville shoreline is the ponent of the sand. The unit is extremely fri- Gilbert Level at 1310 m, and it is significantly able to nearly unlithified, as grains can be dis- lower than the fossil occurrence. This shore- aggregrated by rubbing. A thin (0.6 m) soil line developed during the latest part of the horizon is presently developing on the top of Pleistocene, probably 10–11 ka. the unit in the study area. The bone-bearing unit is underlain by an LOCALITY DESCRIPTION 11.6-m-thick, sand-dominated unit that displays low-angle planar to undulating beds as seen in The sand and gravel quarry owned by more deeply excavated pit areas. This unit is Geneva Rock Products Company in which the also extremely friable. The lower portion of the sloth was found is in SW1/4, NE1/4, Sec. 23, unit displays soft sediment deformation in the T4S, R1W, Jordan Narrows 7.5′ Quadrangle, form of convolute bedding. The scale of con- Salt Lake County, Utah, at 111°55′W longitude, volute bedding is on the order of 2 m. 40°27′30″N latitude. This is BYU locality num- There is a sharp contact at the base of this ber 802. Sediments from which the sloth was lower sand with a relatively thick (1 m) gray- recovered were deposited on the north flank green clay. Within the excavated pits, this clay of the Traverse Mountains of the Wasatch layer acts as a permeability barrier and locally Range that forms PM. This range marks the ponds water (Fig. 5). Elevation of this clay layer easternmost boundary of late Pleistocene Lake (1370 m) is within 1.5 m of the Stansbury Level Bonneville. elevation (Currey et al. 1984) and is approximately 12.2 m above the elevation of the Jor- SEDIMENTOLOGY AND dan River. DEPOSITIONAL HISTORY The stratal unit overlying the bone-bearing bed is interpreted to represent progradation of Sedimentology a large spit developing at the PM on the north- Bone was found on and within an oxidized, ern flank of the Traverse Range. Perennial winds poorly sorted, sandy siltstone bed. Thickness from the west and northwest working on the and lateral extent of this ferruginous bed are extensive fetch of Lake Bonneville would have unknown. Detailed study of the bone-bearing produced strong southwest-oriented longshore bed is incomplete because shortly after recov- currents. Similar spits at Rocky Ridge and Little 2001] JEFFERSON’S GROUND SLOTH TAPHONOMY 67

Fig. 3. Topset and foreset sand and gravel beds in the stratal unit directly above the sloth bone-bearing unit. These beds are interpreted to have been deposited by a prograding spit during the Provo regressive phase of the Bonneville lake cycle.

Fig. 4. 3-D histogram displaying grain-size distributions of 5 beds within the sigmoidal cross-stratified unit above the ground sloth bone-bearing unit. Bed 1 is at the base of the unit and represents foreset to bottomset beds. Sample 5 represents topset beds. The diagram illustrates the coarsening upward trend typical of a prograding spit.

Mountain near the town of Payson to the south that time. Sediments here were shed from the and Stockton Bar to the west have been iden- nearby Wasatch Front as indicated by granite tified elsewhere in Lake Bonneville deposits and limestone lithoclasts that match the lithol- (Bissell 1968). Longshore currents were able ogy there. This spit built a relatively shallow to entrain large gravel-sized clasts and push a terrace where the strong currents were con- large quantity of sediment toward and around centrated. Sediments were pushed along the PM, which projected into Lake Bonneville at top of this terrace until they avalanched into 68 WESTERN NORTH AMERICAN NATURALIST [Volume 61

represents the simplest explanation and is preferred by the authors. Sedimentologic and taphonomic data suggest that the sloth carcass may have washed into Lake Bonneville relatively intact. The carcass was transported some distance along the Wasatch Front by longshore drift. As the carcass continued to be transported toward PM, it made its way to the edge of the wave-built terrace of a large spit. The spit would be migrating to the west, but the longshore current and direction of the spit would have produced foreset beds oriented to the northwest. The carcass eventually fell off the wave-built terrace of the spit and settled into deeper water (approximately 10 m) that had been receiving only suspended-load, hemi- pelagic sediments. Eventually, foreset lamina- tion from spit progradation buried the carcass, thus preventing scavenging and disassociation of the skeleton. With continued regression of Lake Bonneville, the bone and associated siltstone entered the vadose zone. The combina- tion of atmospheric oxygen penetrating the large pore spaces of the coarse-grained sediment and occasional meteoric water that Fig. 5. Clay layer at 1370 m elevation. This unit is very close to the Stansbury Level (Currey et al. 1984). Note the perched on the impermeable siltstone upon water associated with this relatively impermeable clay. The which the skeleton lay, altered the siltstone ferruginous siltstone bed within which the Megalonyx was into a groundwater laterite. discovered may have been oxidized post-depositionally by A 2nd possible scenario involves deposition a perched water table such as this. of the sloth during Stansbury time and burial either by spit migration during the Bonneville transgressive phase or much later burial by deeper water. Once deposited as foreset beds spit migration during the Provo regressive within water as deep as 10 m, longshore cur- phase. This more complicated scenario sug- rents no longer entrained the sediments. gests that the ferruginous siltstone developed The ferruginous bone-bearing siltstone is under subaerial conditions resulting in a soil interpreted as either a deeper water hemi- horizon. The paleosol developed during the pelagic deposit that developed slightly basin- short regression that followed the Stansbury ward of the prograding spit, or possibly as a highstand. The animal died on dry land and paleosol. The underlying unit is interpreted to became iron-stained during soil-forming pro- represent shoreline deposits developed basin- cesses. Sometime later it was buried, either ward of strong longshore currents. Convolute during the Bonneville transgressive phase or bedding indicates fast sedimentation rates and/ the Provo regressive phase of the Bonneville or quick burial or possibly fluctuations in the lake cycle. Taphonomic observations, includ- groundwater table after subaerial exposure. ing the lack of bite or gnaw marks on bones, the absence of any indication of weathering of Depositional History the bone such as described by Behrensmeyer The above interpretations lead to 2 possible (1978), the lack of scattering of bones, and scenarios concerning the depositional history enough bones to produce both an articulated of the ground sloth bones. The 1st scenario left and right manus (Figs. 6A, B), suggest that involves deposition and burial by spit progra- scavenging must not have been a significant dation during the Provo regressive phase of factor. These observations weaken this 2nd the Bonneville lake cycle (Machette 1992). It scenario. 2001] JEFFERSON’S GROUND SLOTH TAPHONOMY 69

C D

Fig. 6. Selected bones of the especially large Megalonyx jeffersonii (BYU 13610) from Point-of-the-Mountain, Salt Lake County, Utah: A, dorsal view of articulated partial left manus; B, dorsal view of articulated partial right manus; C, proximal view of left navicular; D, proximal view of left astragalus; E, lateral view of left radius. 70 WESTERN NORTH AMERICAN NATURALIST [Volume 61

SPECIMEN AGE prevented any observation of the specimen in situ, recovery of most bones of the manus on Since no collagen was present in the Mega- both sides, as well as numerous other small lonyx bones (Austin Long, personal communi- bones, makes it reasonable to infer that the cation, 1998) a carbon-14 age could not be animal was buried as a fully intact or reason- obtained. Therefore, its age needs to be esti- ably complete carcass. Fresh fracture surfaces mated based on Bonneville lake levels that on several bones indicate significant breakage have been dated (Oviatt et al. 1992, 1994). The and probable loss of some skeleton during bull- age of the bone can be bracketed between dozing operations. Examination of recovered approximately 22 ka and 13 ka based on lake parts of the skeleton did not reveal any evi- level ages previously derived (Currey et al. dence of tooth marks caused by scavenging by 1984, Currey 1990). Assuming the bone was predators or gnawing by rodents, also suggest- deposited subaqueously and quickly buried, as ing rapid burial. in the 1st scenario discussed above, the age While we postulate that burial of the car- would be approximately 13 ka. This interpreta- cass may have occurred following some trans- tion concurs with Machette (1992), who mapped port in the lake by longshore currents, transit surface exposures in the study area as being time must have been minimal. Schaefer (1972) deposited during the Provo regressive phase noted that both whales and seals tend to sink of the Bonneville lake cycle. If the sloth was at death unless the animal has a high fat con- deposited subaerially, as in the 2nd scenario tent, in which case it may float longer before discussed above, the age would be approxi- sinking. Submerged carcasses may later become mately 22 ka. buoyant and float following gas buildup in the The large size of the specimen would sug- body cavity due to decomposition. This occurs gest the younger of the 2 alternative interpre- only if the animal does not sink into anaerobic tations for the age of the sloth. As discussed waters where conditions are unfavorable for below, there was a trend of size increase in bacterial activity so that insufficient gases are Megalonyx from its first appearance in the late produced to cause the carcass to surface. Our Hemphillian ca 5.2 ma (Hirschfeld and Webb postulated water depth of 14 m for the final 1968) through the Tertiary and Quaternary, resting place of the carcass was probably too with the largest specimens being latest Pleis- shallow to produce anaerobic conditions. If tocene in age. the original location where the animal died was on a shallower wave-built terrace of the SPECIMEN DESCRIPTION spit, then the carcass may have become par- Preserved portions of the partial skeleton of tially bloated and been able to float a short Megalonyx jeffersonii (BYU 802/13610) from distance before sinking into deeper water on PM include the following elements: left the face of the prograding spit. It is possible scapula, portions of both humeri, left radius that some parts of the skeleton had already (Fig. 6E), left manus: scaphoid, lunar, mag- separated from the carcass prior to this sec- num, unciform, metacarpal II, metacarpal III, ondary transport. As noted by Schaefer (1972), metacarpal IV, metacarpal V (Fig. 6A), partial the jaw and then skull are among the first right radius, right manus: scaphoid, lunar, parts to separate from the carcass in a floating ulnare, trapezoid, magnum, unciform, meta- animal. While the skull and jaw of the PM carpal I, metacarpal II, metacarpal III, proxi- specimen were not recovered, their absence mal and second phalanges of digit III (Fig. may reflect either their separation from the 6B), parts of left tibia, left astragalus (Fig. 6D), carcass prior to its transport to its final resting left navicular (Fig. 6C), left partial cuboid, place or destruction by heavy equipment before fragment of calcaneum, right fibula minus dis- recognition of the existence of the specimen. tal end. However, no traces of teeth or skull bones Bone preservation is excellent despite the were found after careful picking of all the complete depletion of collagen. None of the bone material. bones show any signs of weathering as described While all preserved bones of the PM speci- by Behrensmeyer (1978), suggesting immedi- men can be readily referred to the genus ate burial following death of the animal. While Megalonyx, none of the available material the mode of discovery by heavy equipment actually includes those portions of the skeleton 2001] JEFFERSON’S GROUND SLOTH TAPHONOMY 71 that provide diagnostic features used in identi- ity that exists within the species. A compari- fying the species as M. jeffersonii (McDonald son of relative lengths of metacarpals of the 1977). The Orem specimen described by Utah specimen with an articulated hand from Gillette et al. (1999) included the co-ossified American Falls Reservoir (IMNH 23034) and proximal and second phalanges of the third the 3 associated metacarpals of the type of M. digit of the pes, one of the features used by jeffersonii from West Virginia (ANSP 12508; McDonald (1977) to distinguish M. jeffersonii Fig. 11) indicates that despite differences in from all earlier species of Megalonyx. In ear- size, relative proportions of this segment of lier species of the genus the 2 bones are sepa- the hand remain constant. Figure 11 also pro- rate. Other features such as dentition and vides an indication of the relative size range skull features, parts absent in the PM speci- that existed in M. jeffersonii with regard to the men, can also be used to distinguish M. jeffer- range of metacarpal lengths. It must be re- sonii from other species. Currently, only a sin- membered that samples for each metacarpal gle late Pleistocene species of Megalonyx, M. represent a span of time and are not from con- jeffersonii, is recognized. The extremely large temporaneous individuals. Since there was a size of the individual (with caveats discussed size increase in M. jeffersonii from its initial below), the age of the deposits from which it appearance in the late Irvingtonian land mam- was recovered, and the geographic proximity mal age (McDonald 1977) until its extinction of the new specimen to other Megalonyx that at the end of the Rancholabrean, this produces can be securely referred to the species make it a considerable range of size for the species. reasonable to refer the PM sloth to Megalonyx This is best illustrated by comparing the jeffersonii. largest and smallest adult fifth metacarpals in One of the evolutionary trends of Mega- Figure 11. The largest specimen, from the lonyx has been an increase in overall body size Darke County, Ohio, individual, has a length of through time. The PM specimen is among the 135.8 mm, which is 154% larger than the small- largest individuals known of M. jeffersonii est individual at 88.1 mm from San Josecito (Fig. 6, Table 1). The only other known speci- Cave, Nuevo Leon, Mexico. The age of the San men of M. jeffersonii similar in size to the PM Josecito fauna has been recently dated between individual is from Darke County, Ohio (Mills 27 and 11 ka (Arroyo-Cabrales et al. 1995), 1975). The Ohio specimen is dated at 12,190 ± which makes it roughly contemporaneous with 215 BP, and so it is close to the younger of the both the Point-of-the-Mountain and Darke 2 possible ages for the PM specimen. The 2 County Megalonyx. skeletons have 4 bones in common, the radius Most known specimens of M. jeffersonii do (Fig. 7), second metacarpal (Fig. 8), third meta- not have associated absolute dates. Therefore carpal (Fig. 9), and navicular (Fig. 10), which it is not possible to quantify precisely the rate permit direct comparison of their size and of size increase through time for the species. dimensions. These bones and their measure- Size may be useful in a biostratigraphic con- ments are given in Table 1. The Megalonyx text but only in a generalized way. An added from Orem is also a large individual, but problem is that the overall sample size for each because it does not share any bones with the bone is so small that it is not possible to take PM specimen, a direct comparison cannot be into account size differences that may reflect made. differences in latitude or other environmental The closest sample of Megalonyx outside gradients that might affect the size of a local the Bonneville Basin is on the Snake River population. Size differences between the larger plain, the possible source area for dispersal of specimens from the north, PM (40.5°N) and Megalonyx into the basin (Gillette et al. 1999). Darke County (40°N), and the individuals from The PM specimen is larger than Megalonyx San Josecito Cave (24°N) suggest that a posi- from American Falls Reservoir, Idaho, consid- tive Bergmann’s response may exist in Mega- ered to be Sangamon in age, ca 100,000 years lonyx. Confirmation of this pattern will require (Pinsof 1998), and thus considerably older a larger sample than is presently available. than the PM individual. While precise quantification of size versus Since so few articulated specimens of M. time is not presently possible, there is no jeffersonii are available, this new specimen question that within the Megalonyx lineage permits an evaluation of the range of variabil- there was a trend toward an overall size in- 72 WESTERN NORTH AMERICAN NATURALIST [Volume 61

TABLE 1. Measurements of specimens of Megalonyx jeffersonii from Point-of-the-Mountain, Salt Lake County, Utah. All measurements in mm. Numbers in parentheses ( ) are approximate. BYU = Brigham Young University Earth Sci- ence Museum 802/13610 from Point-of-the-Mountain, Salt Lake County, Utah; DMNH = Dayton Museum of Natural History G-25748 from Darke County, Ohio. Bone BYU DMNH Left radius Length (center of proximal articular surface to center of distal articular surface) (435) (450) Anteroposterior width proximal end 66.8 76.4 Mediolateral width proximal end 66.7 64.0 Anteroposterior width distal shaft 102.7 — Mediolateral width distal shaft 35.3 — Mediolateral width distal end 86.2 (79) Anteroposterior width distal end 122.5 (107) Right scaphoid Mediolateral width 77.5 — Dorsoventral depth (medial side) 71.5 — Anteroposterior length 52.5 — Right lunar Mediolateral width 36.9 — Dorsoventral depth 50.1 — Anteroposterior length 63.3 — Right ulnare Mediolateral width 80.8 — Dorsoventral depth 52.4 — Anteroposterior length 62.1 — Left unciform Mediolateral width 64.5 — Dorsoventral depth 61.9 — Anteroposterior length 54.1 — Right first metacarpal Length 63.8 — Mediolateral width proximal end 41.8 — Dorsoventral height distal end 30.5 — Mediolateral width distal end 26.9 — Left second metacarpal Length 98.2 98.4 Width lateral side proximal end 60.1 58.2 Width dorsal side proximal end 48.2 44.9 Width ventral side proximal end 58.7 48.7 Width of distal end 38.8 43.2 Length of carina 58.5 59.4

crease through time. This is clearly indicated in the different samples is based on discrete for a variety of bones: radius (Fig. 7), second morphological criteria and not size. In all metacarpal (Fig. 8), third metacarpal (Fig. 9), cases the PM specimen is the largest individ- navicular (Fig. 10), and astragalus (Fig. 12). As ual within the M. jeffersonii sample, which can be seen in most cases, specimens identi- most likely reflects its relatively young age. fied as M. jeffersonii are larger than the ances- tral species, M. wheatleyi, based on samples DISCUSSION from Port Kennedy Cave, Pennsylvania, and the McLeod Limerock Mine in Florida. While While Megalonyx was widely distributed the large sample size of the second metacarpal across North America and is present in numer- (Fig. 8) does result in slight overlap in size ous Rancholabrean faunas in the western United between the 2 species, this would be expected States (Gillette et al. 1999), until recently it in an evolving population. Largest individuals was not known from Utah. This is surprising of M. wheatleyi are from younger populations given the number of localities in the state that of the species, while smallest M. jeffersonii are have produced remains of Pleistocene verte- from older populations. Identification to species brates (Jefferson et al. 1994). McDonald (1996) 2001] JEFFERSON’S GROUND SLOTH TAPHONOMY 73

TABLE 1. Continued. Bone BYU DMNH Left third metacarpal Length 114.6 — Dorsoventral length proximal end 62.4 — Mediolateral width proximal end 77.7 — Length of distal carina 71.0 — Mediolateral width distal end 45.1 — Left fourth metacarpal Length 128.4 — Dorsoventral length proximal end 59.6 — Mediolateral width proximal end 55.3 — Length of distal carina 69.2 — Mediolateral width distal end 48.2 — Left fifth metacarpal Length 122.6 135.8 Dorsoventral length proximal end 47.3 49.9 Mediolateral width proximal end 53.0 49.2 Dorsoventral length distal end 54.3 57.6 Mediolateral width distal end 29.1 31.7 Left astragalus Width posterior end of trochlea 120.5 — Width anterior end of trochlea (80) — Dorsoventral height navicular process 102.8 — Anteroposterior length 92.9 — Dorsoventral height of trochlea on lateral side 105.9 — Mediolateral width navicular process 70.7 — Left navicular Dorsoventral length 91.4 90.2 Mediolateral width 85.8 81.3 Anteroposterior thickness 49.6 45.2 Length of ectocuneiform facet 72.3 85.5 Length of meso and entocuneiform facets 76.1 76.3

noted that the 3 species of ground sloths found along the eastern margin of Lake Bonneville, in the western United States tended to utilize it may be that Megalonyx was restricted to different habitats, thus reducing the probabil- riparian habitat closely associated with the ity of association with each other in faunas. lake margin. These nearshore deposits of sands While this is also true of the Utah record, the and gravels (see above discussion on sedimen- extremely small sample size is not a valid test tology) form a narrow band along the eastern of this hypothesis. However, the rarity of all 3 shore of the lake at the Wasatch Front, which sloths in Utah, compared to other large herbi- acted as a sediment source. vores, suggests either that suitable habitats that Most vertebrate remains recovered from could support these animals were marginal and sands and gravels of Lake Bonneville consist may have supported only small populations or of isolated bones, many of which are tumbled that we have not yet sampled their preferred and abraded (Nelson and Madsen 1980). This habitats within the state. suggests that most vertebrate remains pre- Paramylodon harlani is known from only one served in sand and gravels associated with locality, Silver Creek (Miller 1976), at higher Lake Bonneville were transported as bones and elevations away from the Bonneville Basin. not as carcasses. While the entire skeleton of Nothrotheriops shastensis is reported from the sloth was not saved, the presence of most Cowboy Cave and Bechan Cave on the Col- bones of each manus suggests that, prior to orado Plateau based on dung (Mead et al. disturbance by heavy equipment, the PM was 1984, Mead and Agenbroad 1992) and is also probably still articulated and at least repre- absent from the Bonneville Basin. Since the sented a partial carcass. This was also the case only 2 records of Megalonyx found in Utah are of the Megalonyx found at Orem in which many associated with deposits that accumulated of the recovered bones were still articulated 74 WESTERN NORTH AMERICAN NATURALIST [Volume 61

Fig. 7. Scatter diagram of the length of the radius against Fig. 8. Scatter diagram of the length of the second meta- the anteroposterior length of its distal end in Megalonyx carpal against the mediolateral width of the proximal end jeffersonii and Megalonyx wheatleyi. G = Point-of-the- in Megalonyx jeffersonii and Megalonyx wheatleyi. G = Mountain Megalonyx jeffersonii, I = Megalonyx jeffer- Point-of-the-Mountain Megalonyx jeffersonii, I = Mega- sonii, L = Megalonyx wheatleyi. lonyx jeffersonii, L = Megalonyx wheatleyi.

Fig. 9. Scatter diagram of the length of the third meta- Fig. 10. Scatter diagram of the dorsoplantar length of carpal against mediolateral width of the proximal end in the navicular against its mediolateral width in Megalonyx Megalonyx jeffersonii and Megalonyx wheatleyi. G = Point- jeffersonii, Megalonyx wheatleyi, and Megalonyx leptosto- of-the-Mountain Megalonyx jeffersonii, I = Megalonyx mus. G = Point-of-the-Mountain Megalonyx jeffersonii, I jeffersonii, L = Megalonyx wheatleyi. = Megalonyx jeffersonii, L = Megalonyx wheatleyi, ✳ = Megalonyx leptostomus.

(Gillette et al. 1999). Burial and preservation Odocoileus (deer), Camelops (camel), Boother- of at least a reasonably complete carcass sug- ium (musk ox), and Bison (buffalo), with the gest minimal transport and also indicate that musk ox being best represented. Similar taxa the animal may have been living close to the along with several others have been found in shoreline and deposition site. None of the re- Lake Bonneville deposits along the Wasatch mains of either of the 2 most common species Front, both north and south of the present found in these gravel deposits, Ovis canaden- locality (Nelson and Madsen 1978, McDonald sis (mountain sheep) or Bootherium bom- and Ray 1989, Jefferson et al. 1994, Gillette bifrons (musk ox), have been found as partial and Miller, 1999, Miller in press). skeletons. A majority of the species recovered are Due to the alertness of workers at Geneva grazers. Besides Megalonyx the only other Rock Products sand and gravel pits at PM, browsers recovered are Odocoileus (deer) and especially Richard Trotter, a number of Pleis- Bootherium (musk ox). Bootherium appears to tocene vertebrates have been discovered and have been an eclectic feeder capable of feed- donated to the BYU Earth Science Museum ing in woodlands but also able to use more over the past 15 years. These include speci- xeric grassland vegetation, including Agropyon- mens of Mammuthus (mammoth), Equus (horse), like, Bromus-like, and Poa-like grasses (Guthrie 2001] JEFFERSON’S GROUND SLOTH TAPHONOMY 75

Fig. 11. Graph showing relative lengths of metacarpals Fig. 12. Scatter diagram of anteroposterior length of in Megalonyx jeffersonii. Solid symbols are from articu- astragalus against mediolateral width of the posterior edge lated hands. Open symbols indicate isolated specimens of the trochlea in Megalonyx jeffersonii and Megalonyx and provide an indication of the size range of each bone. wheatleyi. G = Point-of-the-Mountain Megalonyx jeffer- G I = Point-of-the-Mountain, Utah (BYU 13610), = sonii, I = Megalonyx jeffersonii, L = Megalonyx wheat- L American Falls Reservoir, Idaho (IMNH 23034), = leyi. Cromer Cave, West Virginia (ANSP 12508, holotype of Megalonyx jeffersonii). As previously noted, while Pleistocene vertebrates (almost exclusively mammals), espe- 1992). Any attempt to reconstruct an integrated cially large ones, are not rare in Bonneville ecology of the Bonneville shoreline 13,000 deposits along the Wasatch Front, neither are years ago is hampered by a distinct preserva- they abundant. Extensive excavations have tional bias. While the majority of vertebrate taken place along the Wasatch Front, especially remains are from the eastern side of Lake in the past decade. It might therefore be Bonneville (Nelson and Madsen 1980), the assumed that larger quantities of fossils should documented paleobotanical record is primar- be uncovered than is the case. However, con- ily from the western margin of the lake (Rhode sidering that the eastern shore of Lake Bon- and Madsen 1995). neville at all its levels lapped up against the Rhode and Madsen (1995) recognized 3 steep Wasatch Front, only limited habitat sequential vegetation formations in the region would have been available to animals living in during the Pleistocene–Holocene transition: this restricted zone. With this consideration, cold montane steppe from 14,000 to 13,000 fossils would not be expected to be abundant years ago, limber pine woodlands from 13,000 even though preservational conditions were to 10,800 years ago, and woodland/steppe favorable. The one exception is Ovis canadensis mosaic and xeric desert scrub after 11,000 (Stokes and Condie 1961, Stock and Stokes years. The paleobotanical record indicates that 1969, Nelson and Madsen 1980), one of the montane shrub vegetation covered altitudes most common species preserved in Lake Bon- up to at least 2000 m in the western Bon- neville–related sediments in this area. The rel- neville Basin. The remains of wood recovered ative abundance of mountain sheep compared from lacustrine sediments in the northeastern to other species probably reflects their ability Bonneville Basin (Scott et al. 1983) indicate to utilize the steep habitat of the Wasatch Front, that the Wasatch Front east of the basin may certainly not a habitat that would be conducive have supported coniferous forests. Rhode and to the success of a ground sloth. Madsen (1995) suggest that differences in veg- While admittedly the 2 Utah records pro- etation on the eastern and western sides of the vide only a limited sample upon which to base Bonneville Basin may have resulted from more any broad conclusions concerning the ecology mesic effects and enhanced precipitation from of Megalonyx, the strong similarity in the lake effects. This in turn possibly would have occurrence of both does suggest a pattern. limited the distribution of Megalonyx within Despite the recovery of numerous mammalian the Bonneville Basin to the eastern margin of taxa represented by a reasonable number of the lake where some type of woodland habitat samples from sediments associated with Lake existed. Bonneville (Nelson and Madsen 1980, Miller 76 WESTERN NORTH AMERICAN NATURALIST [Volume 61

1982, Jefferson et al. 1994), only 2 records of LITERATURE CITED sloth, both Megalonyx, have been found. Both were recovered from deposits associated with ARROYO-CABRALES, J., E. JOHNSON, H. HAAS, M. DE LOS RIOS PAREDES, R.W. RALPH, AND W. T. H ARTWELL. the Provo Level of Lake Bonneville. In marked 1995. First radiocarbon dates for San Josecito Cave, contrast to other mammalian records, which Nuevo Leon, Mexico. Quaternary Research 43: consist of isolated bones, both sloth specimens 255–258. are partial, articulated skeletons, suggesting BEHRENSMEYER, A.K. 1978. Taphonomic and ecological information from bone weathering. Paleobiology 4: rapid burial. It can be inferred that Megalonyx 150–162. was restricted to a habitat closely associated BISSELL, H.J. 1968. Bonneville—an Ice-Age lake. Brigham with the margin of Lake Bonneville and that Young University Geology Studies 15. 66 pp. its absence in other Pleistocene faunas in the CURREY, D.A. 1990. Quaternary paleolakes in the evolu- area reflects this habitat preference. tion of semidesert basins, with special emphasis on Lake Bonneville and the Great Basin, U.S.A. Palaeo- geography, Palaeoclimatology, and Palaeoecology 76: ACKNOWLEDGMENTS 189–214. CURREY, D.A., G. ATWOOD, AND D.R. MAYBE. 1984. Major Special appreciation is given to Joe Miller levels of Great Salt Lake and Lake Bonneville. Utah Geological and Mineral Survey, Map 73. of Geneva Rock Products Company for dis- GILLETTE, D.D., H.G. MCDONALD, AND M.C. HAYDEN. covering the partial Megalonyx skeleton dis- 1999. The first record of Jefferson’s ground sloth, cussed in this paper and his wife, Kristina, for Megalonyx jeffersonii, in Utah (Pleistocene, Rancho- kindly bringing this very important specimen labrean land mammal age). Pages 509–521 in D.D. Gillette, editor, Vertebrate paleontology in Utah. to the Earth Science Museum at Brigham Young Utah Geological and Mineral Survey Miscellaneous University (BYU). Deep gratitude is also ex- Publication 99-1. pressed to the management of the above com- GILLETTE, D.D., AND W.E. MILLER. 1999. Catalogue of pany for their part in the acquisition of this new Pleistocene mammalian sites and recovered fos- fossil and other specimens that were earlier sils from Utah. Pages 523–530 in D.D. Gillette, editor, Vertebrate paleontology in Utah. Utah Geologi- recovered from their PM gravel pit. Richard cal and Mineral Survey Miscellaneous Publication Trotter, especially, and other co-workers at 99-1. Geneva have also provided important fossils GUTHRIE, R.D. 1992. New paleoecological and paleoetho- from this site over the years. Their thoughtful- logical information on the extinct helmeted muskoxen from Alaska. Annales Zoologici Fennici 28: ness and kindness in bringing them to the 175–186. above museum for scientific study is greatly HIRSCHFELD, S.E., AND S.D. WEBB. 1968. Plio-Pleistocene appreciated. John Chiles of Geneva Rock Prod- megalonychid sloths of North America. Bulletin of ucts is thanked for permitting the authors of the Florida State Museum, Biological Sciences 12: this paper access to the gravel pit to study 213–296. JARRETT, R.D., AND H.E. MALDE. 1987. Paleodischarge of deposits there and photograph relevant areas. the late Pleistocene Bonneville flood, Snake River, Ken Stadtman of the BYU Earth Science Idaho, computed from new evidence. Geological Museum did the necessary preparation of the Society of America Bulletin 99:127–134. sloth bones. Appreciation is extended to him JEFFERSON, G.T., W.E. MILLER, M.E. NELSON, AND J.H. MADSEN, JR. 1994. Catalogue of late Quaternary ver- for this. We also thank James Miller of the tebrates from Utah. Natural History Museum of Los Geology Department at Brigham Young Uni- Angeles County Technical Report 9. 34 pp. versity for helping produce Figure 1. Donald LEMONS, D.R., M.R. MILLIGAN, AND M.A. CHAN. 1996. Currey of the University of Utah provided use- Paleoclimate implications of late Pleistocene sediment yield rates for the Bonneville Basin, northern ful information, for which he is kindly acknowl- Utah. Palaeogeography, Palaeoclimatology, and Palaeo- edged. Austin Long at the University of Ari- ecology 123:147–159. zona is thanked for his attempts to obtain a MACHETTE, M.N. 1992. Surficial geologic map of the carbon-14 age based on bone samples of the Wasatch Fault Zone, eastern part of Utah Valley, Utah County and parts of Salt Lake and Juab coun- present specimen. We also thank Ginger Cooley ties, Utah. U.S. Geological Survey, Miscellaneous of BYU for the sedimentological processing of Investigations Series, Map I-2095. material used for analysis. We would like to MCCOY, W.D. 1987. Quaternary aminostratigraphy of the extend our appreciation and thanks to Drs. Bonneville Basin, western United States. Geological Jim Mead and Dave Gillette for their thought- Society of America Bulletin 98:99–112. MCDONALD, H.G. 1977. Description of the osteology of ful reviews of the manuscript and suggestions the extinct gravigrade edentate, Megalonyx, with for its improvement. observations on its ontogeny, phylogeny and func- 2001] JEFFERSON’S GROUND SLOTH TAPHONOMY 77

tional anatomy. Master’s thesis, Department of Zool- OVIATT, C.G., D.R. CURREY, AND D. SACK. 1992. Radiocar- ogy, University of Florida, Gainesville. 328 pp. bon chronology of Lake Bonneville, eastern Great ______. 1996. Biogeography and paleoecology of ground Basin, U.S.A. Palaeogeography, Palaeoclimatology, sloths in California, Arizona and Nevada. San Ber- and Palaeoecology 99:225–241. nardino County Museum Association Quarterly 43: OVIATT, C.G., G.D. HABIGER, AND J.E. HAY. 1994. Varia- 61–65. tion in the composition of Lake Bonneville marl: a MCDONALD, J.N., AND C.E. RAY. 1989. The autochthonous potential key to lake-level fluctuations and paleocli- North American muskoxen Bootherium, Symbos, and mate. Journal of Paleolimnology 11:19–30. Gidleya (Mammalia: Artiodactyla: Bovidae). Smith- PINSOF, J.D. 1998. The American Falls local fauna: late sonian Contributions to Paleobiology 66:1–77. Pleistocene (Sangamonian) vertebrates from south- MEAD, J.I., AND L.D. AGENBROAD. 1992. Isotope dating of eastern Idaho. Pages 121–145 in W.A. Akersten, Pleistocene dung deposits from the Colorado Plateau, H.G. McDonald, D.J. Meldrum, and M.E.T. Flint, Arizona and Utah. Radiocarbon 34:1–19. editors, And whereas . . . papers on the vertebrate MEAD, J.I., L.D. AGENBROAD, P.S. MARTIN, AND O.K. DAVIS. paleontology of Idaho honoring John A. White. Vol- 1984. The mammoth and sloth dung from Bechan ume 1. Idaho Museum of Natural History Occa- Cave in southern Utah. Current Research in the sional Paper 36. Pleistocene 1:79–80. RHODE, D., AND D.B. MADSEN. 1995. Late Wisconsin/early MILLER, W.E. 1976. Late Pleistocene vertebrates of the Holocene vegetation in the Bonneville Basin. Qua- Silver Creek local fauna from north central Utah. ternary Research 44:246–256. Great Basin Naturalist 36:387–424. SCHAEFER, W. 1972. Ecology and paleoecology of marine ______. 1982. Pleistocene vertebrates from deposits of environments. University of Chicago Press, Chicago, Lake Bonneville, Utah. National Geographic Society IL. 568 pp. Research Reports 14:473–478. SCOTT, W.E., W.D. MCCOY, R.R. SHROBA, AND M. RUBIN. ______. In press. Quaternary vertebrates of the northeast- 1983. Reinterpretation of the exposed record of the ern Bonneville Basin and vicinity of Utah. In: J.W. last two cycles of Lake Bonneville, western United Gwynn, editor, Great Salt Lake 2000: an overview of States. Quaternary Research 20:261–285. change. Utah Geological and Mineral Survey Mis- STOCK, A.D., AND W.L. STOKES. 1969. A re-evaluation of cellaneous Publication. Pleistocene bighorn sheep from the Great Basin and MILLS, R. 1975. A ground sloth, Megalonyx, from a Pleis- their relationship to living members of the genus tocene site in Darke County, Ohio. Ohio Journal of Ovis. Journal of Mammalogy 50:805–807. Science 75:147–155. STOKES, W.L., AND K.C. CONDIE. 1961. Pleistocene bighorn NELSON, M.E., AND J.H. MADSEN, JR. 1978. Late Pleis- sheep from the Great Basin. Journal of Paleontology tocene musk oxen from Utah. Transactions of the 35:598–609. Kansas Academy of Science 81:277–295. ______. 1980. A summary of Pleistocene fossil vertebrates Received 8 September 1999 in the northern Bonneville Basin of Utah. Pages Accepted 13 March 2000 97–113 in J.W. Gwynn, editor, Great Salt Lake, a scientific, historical, and economic overview. Utah Geological and Mineral Survey Bulletin 116.