Western North American Naturalist Western North American Naturalist

Volume 61 Number 1 Article 9

1-29-2001

Taphonomy and significance of Jefferson's ground sloth Taphonomy and significance of Jefferson's ground sloth

(Xenarthra: Megalonychidae) from Utah (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 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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Records of the sloths Megalonyx jeffersonii,Paramylodon harlani, and Nothrotheriops shas-tensis in the Great Basin are rare. All currentlyknown records for sloths are located aroundthe region’s periphery. This scant record is alsoreflected in Utah, which has a very limitedrepresentation of Pleistocene ground sloths.The first reporting of a sloth, the mylodontParamylodon, from Utah was based on 2 teethand a partial vertebra (Miller 1976), presumablyfrom the same individual, in the Silver Creeklocal fauna. The site yielding this fauna is about48 km northeast and 568 m higher in elevationthan the sloth from Point-of-the-Mountain(hereafter referred to as PM). Nothrotheriops,the smallest of the 3 ground sloths, has beenidentified from cave sites in the southwesternpart of Utah based solely on dung samples(Mead et al. 1984, Mead and Agenbroad 1992).The first Utah discovery of Megalonyx wasonly recently made (Gillette et al. 1999) andwas based on several bones of a single individ-ual found near the city of Orem, about 26 kmsoutheast of PM. The material described hererepresents a substantial amount of a single,very large individual and is only the 2nd knownrecord of Megalonyx from the entire GreatBasin. It seems appropriate to provide as muchinformation as possible about this specimenand the depositional setting from which it wasrecovered and the inferences that can be maderegarding the ecology of this extinct species.

The partial Megalonyx skeleton describedhere was recovered from Lake Bonnevilleshoreline sediments in the southernmost partof Salt Lake County. Equipment operator JoeMiller, who works for Geneva Rock ProductsCompany, made its discovery in the summerof 1996. With the kind permission of that com-pany’s management, he presented the fossilmaterial to the Earth Science Museum atBrigham Young University, Provo, Utah, whereit has been accessioned.

The ground sloth bone-bearing unit lieswithin Pleistocene-age Lake Bonneville shore-line deposits in the Jordan River cut of theTraverse Range (Fig. 1). A Trimble TDC1 GPSunit was used to calculate the elevation of thebone-bearing unit at 1381.7 m (Fig. 2). Thebone-bearing unit lies approximately 10 mbelow the present land surface. Machette(1992) mapped the surface sediments of thestudy area as upper Pleistocene lacustrinegravels that were deposited during the Provoregressive phase of the Bonneville lake cycle.These data broadly bracket the time of bonedeposition from the maximum transgressivephase of the Stansbury Level at approximately22 ka (thousands of years before present) tothe Provo regression at approximately 13 ka(Currey et al. 1984, Currey 1990).

Research has shown that pre-Bonnevillelakes occupied the lower parts of the north-eastern portion of the Great Basin back to a

Western North American Naturalist 61(1), © 2001, pp. 64–77

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 LakeBonneville, Utah, sloths have been notably rare. Three species of ground sloth, Megalonyx jeffersonii, Paramylodon har-lani, and Nothrotheriops shastensis, are known from the western United States during the Pleistocene. Yet all 3 are rarein the Great Basin, and the few existing records are from localities on the basin margin. The recent discovery of a partialskeleton of Megalonyx jeffersonii at Point-of-the-Mountain, Salt Lake County, Utah, fits this pattern and adds to ourunderstanding of the distribution and ecology of this extinct species. Its occurrence in Lake Bonneville shorelinedeposits permits a reasonable age determination of between 22 and 13 ka.

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

1Hagerman Fossil Beds National Monument, Box 570, Hagerman, ID 83332. Present address: National Park Service, Geologic Resources Division, Scienceand 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

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 StansburyLevel and of all ages are from Currey et al. (1984).

Point-of-the-MountainMegalonyx site

date of possibly 200 ka (Scott et al. 1983, McCoy1987). Lake Bonneville, as generally under-stood, appears to have had its beginning some-time between 26.5 and 30.0 ka (Oviatt et al.1992, Oviatt et al. 1994, Lemons et al. 1996).Maximum lake depth, as viewed elsewhere inthe Great Basin, reportedly was 320 m about14.5–15.0 ka (Jarrett and Malde 1987, Oviattet al. 1992) when the Bonneville Level devel-oped at 1554 m above mean sea level. A well-documented flood of gigantic proportions thenoccurred, quickly lowering the lake to theProvo Level. Interstate Highway 15, whichruns immediately east of the fossil site, is situ-ated on the resulting wave-built bench in thisarea (Fig. 2).

The Megalonyx skeleton reported here wasrecovered at 1381.7 m elevation, approximately10 m above the Stansbury Level. According toCurrey et al. (1984), this places the unit be-tween the Stansbury (approximately 1371.6 m)and Provo Level (1463 m) at PM. The Stans-bury Level probably formed before any otherlevels between 20 and 23 ka (Currey et al.1984). The lowest Bonneville shoreline is theGilbert Level at 1310 m, and it is significantlylower than the fossil occurrence. This shore-line developed during the latest part of thePleistocene, probably 10–11 ka.

LOCALITY DESCRIPTION

The sand and gravel quarry owned byGeneva Rock Products Company in which thesloth was found is in SW1/4, NE1/4, Sec. 23,T4S, R1W, Jordan Narrows 7.5′ Quadrangle,Salt Lake County, Utah, at 111°55′W longitude,40°27′30″N latitude. This is BYU locality num-ber 802. Sediments from which the sloth wasrecovered were deposited on the north flankof the Traverse Mountains of the WasatchRange that forms PM. This range marks theeasternmost boundary of late Pleistocene LakeBonneville.

SEDIMENTOLOGY AND

DEPOSITIONAL HISTORY

Sedimentology

Bone was found on and within an oxidized,poorly sorted, sandy siltstone bed. Thicknessand lateral extent of this ferruginous bed areunknown. Detailed study of the bone-bearingbed is incomplete because shortly after recov-

ery of the sloth, excavation by the gravel pitoperator resulted in the bed being completelycovered. The currently exposed (1999) stratalunit above the bone-bearing bed is composedof sand and gravel. The stratal unit below thebed is primarily composed of sand as viewedelsewhere in the pit.

The ferruginous bone-bearing bed is over-lain by high-angled (31°), NW dipping fore-sets of large-scale, sigmoidal, cross-stratifiedbeds (Fig. 3). The unit containing the bone-bearing beds is approximately 10 m thick andis composed of sand and gravel. Foreset bedscan be traced laterally and upward into gravel-rich topset beds, which are largely low-angle(12°) to planar beds. Measurements takenfrom bedforms laterally to the south of topsetbeds indicate an azimuth direction of approxi-mately 260°. Grain-size distribution of 5 sam-ples from the base to top of the unit indicatesa coarsening upward trend (Fig. 4). Gravel-and pebble-sized lithoclasts are subangular tosubrounded and are composed of limestone,quartzite, and granite. Mica is a common com-ponent of the sand. The unit is extremely fri-able to nearly unlithified, as grains can be dis-aggregrated by rubbing. A thin (0.6 m) soilhorizon is presently developing on the top ofthe unit in the study area.

The bone-bearing unit is underlain by an11.6-m-thick, sand-dominated unit that displayslow-angle planar to undulating beds as seen inmore deeply excavated pit areas. This unit isalso extremely friable. The lower portion of theunit displays soft sediment deformation in theform of convolute bedding. The scale of con-volute bedding is on the order of 2 m.

There is a sharp contact at the base of thislower sand with a relatively thick (1 m) gray-green clay. Within the excavated pits, this claylayer acts as a permeability barrier and locallyponds water (Fig. 5). Elevation of this clay layer(1370 m) is within 1.5 m of the Stansbury Levelelevation (Currey et al. 1984) and is approxi-mately 12.2 m above the elevation of the Jor-dan River.

The stratal unit overlying the bone-bearingbed is interpreted to represent progradation ofa large spit developing at the PM on the north-ern flank of the Traverse Range. Perennial windsfrom the west and northwest working on theextensive fetch of Lake Bonneville would haveproduced strong southwest-oriented longshorecurrents. Similar spits at Rocky Ridge and Little

66 WESTERN NORTH AMERICAN NATURALIST [Volume 61

Mountain near the town of Payson to the southand Stockton Bar to the west have been iden-tified elsewhere in Lake Bonneville deposits(Bissell 1968). Longshore currents were ableto entrain large gravel-sized clasts and push alarge quantity of sediment toward and aroundPM, which projected into Lake Bonneville at

that time. Sediments here were shed from thenearby Wasatch Front as indicated by graniteand limestone lithoclasts that match the lithol-ogy there. This spit built a relatively shallowterrace where the strong currents were con-centrated. Sediments were pushed along thetop of this terrace until they avalanched into

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. Thesebeds are interpreted to have been deposited by a prograding spit during the Provo regressive phase of the Bonnevillelake cycle.

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

deeper water. Once deposited as foreset bedswithin water as deep as 10 m, longshore cur-rents no longer entrained the sediments.

The ferruginous bone-bearing siltstone isinterpreted as either a deeper water hemi-pelagic deposit that developed slightly basin-ward of the prograding spit, or possibly as apaleosol. The underlying unit is interpreted torepresent shoreline deposits developed basin-ward of strong longshore currents. Convolutebedding indicates fast sedimentation rates and/or quick burial or possibly fluctuations in thegroundwater table after subaerial exposure.

Depositional History

The above interpretations lead to 2 possiblescenarios concerning the depositional historyof the ground sloth bones. The 1st scenarioinvolves deposition and burial by spit progra-dation during the Provo regressive phase ofthe Bonneville lake cycle (Machette 1992). It

represents the simplest explanation and is pre-ferred by the authors. Sedimentologic andtaphonomic data suggest that the sloth carcassmay have washed into Lake Bonneville rela-tively intact. The carcass was transported somedistance along the Wasatch Front by longshoredrift. As the carcass continued to be trans-ported toward PM, it made its way to the edgeof the wave-built terrace of a large spit. Thespit would be migrating to the west, but thelongshore current and direction of the spitwould have produced foreset beds oriented tothe northwest. The carcass eventually fell offthe wave-built terrace of the spit and settledinto deeper water (approximately 10 m) thathad been receiving only suspended-load, hemi-pelagic sediments. Eventually, foreset lamina-tion from spit progradation buried the carcass,thus preventing scavenging and disassociationof the skeleton. With continued regression ofLake Bonneville, the bone and associated silt-stone entered the vadose zone. The combina-tion of atmospheric oxygen penetrating thelarge pore spaces of the coarse-grained sedi-ment and occasional meteoric water thatperched on the impermeable siltstone uponwhich the skeleton lay, altered the siltstoneinto a groundwater laterite.

A 2nd possible scenario involves depositionof the sloth during Stansbury time and burialeither by spit migration during the Bonnevilletransgressive phase or much later burial byspit migration during the Provo regressivephase. This more complicated scenario sug-gests that the ferruginous siltstone developedunder subaerial conditions resulting in a soilhorizon. The paleosol developed during theshort regression that followed the Stansburyhighstand. The animal died on dry land andbecame iron-stained during soil-forming pro-cesses. Sometime later it was buried, eitherduring the Bonneville transgressive phase orthe Provo regressive phase of the Bonnevillelake cycle. Taphonomic observations, includ-ing the lack of bite or gnaw marks on bones,the absence of any indication of weathering ofthe bone such as described by Behrensmeyer(1978), the lack of scattering of bones, andenough bones to produce both an articulatedleft and right manus (Figs. 6A, B), suggest thatscavenging must not have been a significantfactor. These observations weaken this 2ndscenario.

68 WESTERN NORTH AMERICAN NATURALIST [Volume 61

Fig. 5. Clay layer at 1370 m elevation. This unit is veryclose to the Stansbury Level (Currey et al. 1984). Note thewater associated with this relatively impermeable clay. Theferruginous siltstone bed within which the Megalonyx wasdiscovered may have been oxidized post-depositionally bya perched water table such as this.

2001] JEFFERSON’S GROUND SLOTH TAPHONOMY 69

Fig. 6. Selected bones of the especially large Megalonyx jeffersonii (BYU 13610) from Point-of-the-Mountain, SaltLake 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.

A B

C

E

D

SPECIMEN AGE

Since no collagen was present in the Mega-lonyx bones (Austin Long, personal communi-cation, 1998) a carbon-14 age could not beobtained. Therefore, its age needs to be esti-mated based on Bonneville lake levels thathave been dated (Oviatt et al. 1992, 1994). Theage of the bone can be bracketed betweenapproximately 22 ka and 13 ka based on lakelevel ages previously derived (Currey et al.1984, Currey 1990). Assuming the bone wasdeposited subaqueously and quickly buried, asin the 1st scenario discussed above, the agewould be approximately 13 ka. This interpreta-tion concurs with Machette (1992), who mappedsurface exposures in the study area as beingdeposited during the Provo regressive phaseof the Bonneville lake cycle. If the sloth wasdeposited subaerially, as in the 2nd scenariodiscussed above, the age would be approxi-mately 22 ka.

The large size of the specimen would sug-gest the younger of the 2 alternative interpre-tations for the age of the sloth. As discussedbelow, there was a trend of size increase inMegalonyx from its first appearance in the lateHemphillian ca 5.2 ma (Hirschfeld and Webb1968) through the Tertiary and Quaternary,with the largest specimens being latest Pleis-tocene in age.

SPECIMEN DESCRIPTION

Preserved portions of the partial skeleton ofMegalonyx jeffersonii (BYU 802/13610) fromPM include the following elements: leftscapula, portions of both humeri, left radius(Fig. 6E), left manus: scaphoid, lunar, mag-num, unciform, metacarpal II, metacarpal III,metacarpal IV, metacarpal V (Fig. 6A), partialright radius, right manus: scaphoid, lunar,ulnare, trapezoid, magnum, unciform, meta-carpal I, metacarpal II, metacarpal III, proxi-mal and second phalanges of digit III (Fig.6B), parts of left tibia, left astragalus (Fig. 6D),left navicular (Fig. 6C), left partial cuboid,fragment of calcaneum, right fibula minus dis-tal end.

Bone preservation is excellent despite thecomplete depletion of collagen. None of thebones show any signs of weathering as describedby Behrensmeyer (1978), suggesting immedi-ate burial following death of the animal. Whilethe mode of discovery by heavy equipment

prevented any observation of the specimen insitu, recovery of most bones of the manus onboth sides, as well as numerous other smallbones, makes it reasonable to infer that theanimal was buried as a fully intact or reason-ably complete carcass. Fresh fracture surfaceson several bones indicate significant breakageand probable loss of some skeleton during bull-dozing operations. Examination of recoveredparts of the skeleton did not reveal any evi-dence of tooth marks caused by scavenging bypredators or gnawing by rodents, also suggest-ing rapid burial.

While we postulate that burial of the car-cass may have occurred following some trans-port in the lake by longshore currents, transittime must have been minimal. Schaefer (1972)noted that both whales and seals tend to sinkat death unless the animal has a high fat con-tent, in which case it may float longer beforesinking. Submerged carcasses may later becomebuoyant and float following gas buildup in thebody cavity due to decomposition. This occursonly if the animal does not sink into anaerobicwaters where conditions are unfavorable forbacterial activity so that insufficient gases areproduced to cause the carcass to surface. Ourpostulated water depth of 14 m for the finalresting place of the carcass was probably tooshallow to produce anaerobic conditions. Ifthe original location where the animal diedwas on a shallower wave-built terrace of thespit, then the carcass may have become par-tially bloated and been able to float a shortdistance before sinking into deeper water onthe face of the prograding spit. It is possiblethat some parts of the skeleton had alreadyseparated from the carcass prior to this sec-ondary transport. As noted by Schaefer (1972),the jaw and then skull are among the firstparts to separate from the carcass in a floatinganimal. While the skull and jaw of the PMspecimen were not recovered, their absencemay reflect either their separation from thecarcass prior to its transport to its final restingplace or destruction by heavy equipment beforerecognition of the existence of the specimen.However, no traces of teeth or skull boneswere found after careful picking of all thebone material.

While all preserved bones of the PM speci-men can be readily referred to the genusMegalonyx, none of the available materialactually includes those portions of the skeleton

70 WESTERN NORTH AMERICAN NATURALIST [Volume 61

that provide diagnostic features used in identi-fying the species as M. jeffersonii (McDonald1977). The Orem specimen described byGillette et al. (1999) included the co-ossifiedproximal and second phalanges of the thirddigit of the pes, one of the features used byMcDonald (1977) to distinguish M. jeffersoniifrom all earlier species of Megalonyx. In ear-lier species of the genus the 2 bones are sepa-rate. Other features such as dentition andskull features, parts absent in the PM speci-men, can also be used to distinguish M. jeffer-sonii from other species. Currently, only a sin-gle late Pleistocene species of Megalonyx, M.jeffersonii, is recognized. The extremely largesize of the individual (with caveats discussedbelow), the age of the deposits from which itwas recovered, and the geographic proximityof the new specimen to other Megalonyx thatcan be securely referred to the species make itreasonable to refer the PM sloth to Megalonyxjeffersonii.

One of the evolutionary trends of Mega-lonyx has been an increase in overall body sizethrough time. The PM specimen is among thelargest individuals known of M. jeffersonii(Fig. 6, Table 1). The only other known speci-men of M. jeffersonii similar in size to the PMindividual is from Darke County, Ohio (Mills1975). The Ohio specimen is dated at 12,190 ±215 BP, and so it is close to the younger of the2 possible ages for the PM specimen. The 2skeletons have 4 bones in common, the radius(Fig. 7), second metacarpal (Fig. 8), third meta-carpal (Fig. 9), and navicular (Fig. 10), whichpermit direct comparison of their size anddimensions. These bones and their measure-ments are given in Table 1. The Megalonyxfrom Orem is also a large individual, butbecause it does not share any bones with thePM specimen, a direct comparison cannot bemade.

The closest sample of Megalonyx outsidethe Bonneville Basin is on the Snake Riverplain, the possible source area for dispersal ofMegalonyx into the basin (Gillette et al. 1999).The PM specimen is larger than Megalonyxfrom American Falls Reservoir, Idaho, consid-ered to be Sangamon in age, ca 100,000 years(Pinsof 1998), and thus considerably olderthan the PM individual.

Since so few articulated specimens of M.jeffersonii are available, this new specimenpermits an evaluation of the range of variabil-

ity that exists within the species. A compari-son of relative lengths of metacarpals of theUtah specimen with an articulated hand fromAmerican Falls Reservoir (IMNH 23034) andthe 3 associated metacarpals of the type of M.jeffersonii from West Virginia (ANSP 12508;Fig. 11) indicates that despite differences insize, relative proportions of this segment ofthe hand remain constant. Figure 11 also pro-vides an indication of the relative size rangethat existed in M. jeffersonii with regard to therange of metacarpal lengths. It must be re-membered that samples for each metacarpalrepresent a span of time and are not from con-temporaneous individuals. Since there was asize increase in M. jeffersonii from its initialappearance in the late Irvingtonian land mam-mal age (McDonald 1977) until its extinctionat the end of the Rancholabrean, this producesa considerable range of size for the species.This is best illustrated by comparing thelargest and smallest adult fifth metacarpals inFigure 11. The largest specimen, from theDarke County, Ohio, individual, has a length of135.8 mm, which is 154% larger than the small-est individual at 88.1 mm from San JosecitoCave, Nuevo Leon, Mexico. The age of the SanJosecito fauna has been recently dated between27 and 11 ka (Arroyo-Cabrales et al. 1995),which makes it roughly contemporaneous withboth the Point-of-the-Mountain and DarkeCounty Megalonyx.

Most known specimens of M. jeffersonii donot have associated absolute dates. Thereforeit is not possible to quantify precisely the rateof size increase through time for the species.Size may be useful in a biostratigraphic con-text but only in a generalized way. An addedproblem is that the overall sample size for eachbone is so small that it is not possible to takeinto account size differences that may reflectdifferences in latitude or other environmentalgradients that might affect the size of a localpopulation. Size differences between the largerspecimens from the north, PM (40.5°N) andDarke County (40°N), and the individuals fromSan Josecito Cave (24°N) suggest that a posi-tive Bergmann’s response may exist in Mega-lonyx. Confirmation of this pattern will requirea larger sample than is presently available.

While precise quantification of size versustime is not presently possible, there is noquestion that within the Megalonyx lineagethere was a trend toward an overall size in-

2001] JEFFERSON’S GROUND SLOTH TAPHONOMY 71

crease through time. This is clearly indicatedfor a variety of bones: radius (Fig. 7), secondmetacarpal (Fig. 8), third metacarpal (Fig. 9),navicular (Fig. 10), and astragalus (Fig. 12). Ascan be seen in most cases, specimens identi-fied as M. jeffersonii are larger than the ances-tral species, M. wheatleyi, based on samplesfrom Port Kennedy Cave, Pennsylvania, andthe McLeod Limerock Mine in Florida. Whilethe large sample size of the second metacarpal(Fig. 8) does result in slight overlap in sizebetween the 2 species, this would be expectedin an evolving population. Largest individualsof M. wheatleyi are from younger populationsof the species, while smallest M. jeffersonii arefrom older populations. Identification to species

in the different samples is based on discretemorphological criteria and not size. In allcases the PM specimen is the largest individ-ual within the M. jeffersonii sample, whichmost likely reflects its relatively young age.

DISCUSSION

While Megalonyx was widely distributedacross North America and is present in numer-ous Rancholabrean faunas in the western UnitedStates (Gillette et al. 1999), until recently itwas not known from Utah. This is surprisinggiven the number of localities in the state thathave produced remains of Pleistocene verte-brates (Jefferson et al. 1994). McDonald (1996)

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 NaturalHistory G-25748 from Darke County, Ohio.

Bone BYU DMNH

Left radiusLength (center of proximal articular surface to centerof distal articular surface) (435) (450)

Anteroposterior width proximal end 66.8 76.4Mediolateral width proximal end 66.7 64.0Anteroposterior 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 lunarMediolateral width 36.9 —Dorsoventral depth 50.1 —Anteroposterior length 63.3 —

Right ulnareMediolateral width 80.8 —Dorsoventral depth 52.4 —Anteroposterior length 62.1 —

Left unciformMediolateral width 64.5 —Dorsoventral depth 61.9 —Anteroposterior length 54.1 —

Right first metacarpalLength 63.8 —Mediolateral width proximal end 41.8 —Dorsoventral height distal end 30.5 —Mediolateral width distal end 26.9 —

Left second metacarpalLength 98.2 98.4Width lateral side proximal end 60.1 58.2Width dorsal side proximal end 48.2 44.9Width ventral side proximal end 58.7 48.7Width of distal end 38.8 43.2Length of carina 58.5 59.4

noted that the 3 species of ground sloths foundin the western United States tended to utilizedifferent habitats, thus reducing the probabil-ity of association with each other in faunas.While this is also true of the Utah record, theextremely small sample size is not a valid testof this hypothesis. However, the rarity of all 3sloths in Utah, compared to other large herbi-vores, suggests either that suitable habitats thatcould support these animals were marginal andmay have supported only small populations orthat we have not yet sampled their preferredhabitats within the state.

Paramylodon harlani is known from only onelocality, Silver Creek (Miller 1976), at higherelevations away from the Bonneville Basin.Nothrotheriops shastensis is reported fromCowboy Cave and Bechan Cave on the Col-orado Plateau based on dung (Mead et al.1984, Mead and Agenbroad 1992) and is alsoabsent from the Bonneville Basin. Since theonly 2 records of Megalonyx found in Utah areassociated with deposits that accumulated

along the eastern margin of Lake Bonneville,it may be that Megalonyx was restricted toriparian habitat closely associated with thelake margin. These nearshore deposits of sandsand gravels (see above discussion on sedimen-tology) form a narrow band along the easternshore of the lake at the Wasatch Front, whichacted as a sediment source.

Most vertebrate remains recovered fromsands and gravels of Lake Bonneville consistof isolated bones, many of which are tumbledand abraded (Nelson and Madsen 1980). Thissuggests that most vertebrate remains pre-served in sand and gravels associated withLake Bonneville were transported as bones andnot as carcasses. While the entire skeleton ofthe sloth was not saved, the presence of mostbones of each manus suggests that, prior todisturbance by heavy equipment, the PM wasprobably still articulated and at least repre-sented a partial carcass. This was also the caseof the Megalonyx found at Orem in which manyof the recovered bones were still articulated

2001] JEFFERSON’S GROUND SLOTH TAPHONOMY 73

TABLE 1. Continued.

Bone BYU DMNH

Left third metacarpalLength 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 metacarpalLength 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 metacarpalLength 122.6 135.8Dorsoventral length proximal end 47.3 49.9Mediolateral width proximal end 53.0 49.2Dorsoventral length distal end 54.3 57.6Mediolateral width distal end 29.1 31.7

Left astragalusWidth 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 navicularDorsoventral length 91.4 90.2Mediolateral width 85.8 81.3Anteroposterior thickness 49.6 45.2Length of ectocuneiform facet 72.3 85.5Length of meso and entocuneiform facets 76.1 76.3

(Gillette et al. 1999). Burial and preservationof at least a reasonably complete carcass sug-gest minimal transport and also indicate thatthe animal may have been living close to theshoreline and deposition site. None of the re-mains of either of the 2 most common speciesfound in these gravel deposits, Ovis canaden-sis (mountain sheep) or Bootherium bom-bifrons (musk ox), have been found as partialskeletons.

Due to the alertness of workers at GenevaRock Products sand and gravel pits at PM,especially Richard Trotter, a number of Pleis-tocene vertebrates have been discovered anddonated to the BYU Earth Science Museumover the past 15 years. These include speci-mens of Mammuthus (mammoth), Equus (horse),

Odocoileus (deer), Camelops (camel), Boother-ium (musk ox), and Bison (buffalo), with themusk ox being best represented. Similar taxaalong with several others have been found inLake Bonneville deposits along the WasatchFront, both north and south of the presentlocality (Nelson and Madsen 1978, McDonaldand Ray 1989, Jefferson et al. 1994, Gilletteand Miller, 1999, Miller in press).

A majority of the species recovered aregrazers. Besides Megalonyx the only otherbrowsers recovered are Odocoileus (deer) andBootherium (musk ox). Bootherium appears tohave been an eclectic feeder capable of feed-ing in woodlands but also able to use morexeric grassland vegetation, including Agropyon-like, Bromus-like, and Poa-like grasses (Guthrie

74 WESTERN NORTH AMERICAN NATURALIST [Volume 61

Fig. 7. Scatter diagram of the length of the radius againstthe anteroposterior length of its distal end in Megalonyxjeffersonii and Megalonyx wheatleyi. � = Point-of-the-Mountain Megalonyx jeffersonii, � = Megalonyx jeffer-sonii, � = Megalonyx wheatleyi.

Fig. 8. Scatter diagram of the length of the second meta-carpal against the mediolateral width of the proximal endin Megalonyx jeffersonii and Megalonyx wheatleyi. � =Point-of-the-Mountain Megalonyx jeffersonii, � = Mega-lonyx jeffersonii, � = Megalonyx wheatleyi.

Fig. 9. Scatter diagram of the length of the third meta-carpal against mediolateral width of the proximal end inMegalonyx jeffersonii and Megalonyx wheatleyi. � = Point-of-the-Mountain Megalonyx jeffersonii, � = Megalonyxjeffersonii, � = Megalonyx wheatleyi.

Fig. 10. Scatter diagram of the dorsoplantar length ofthe navicular against its mediolateral width in Megalonyxjeffersonii, Megalonyx wheatleyi, and Megalonyx leptosto-mus. � = Point-of-the-Mountain Megalonyx jeffersonii, �

= Megalonyx jeffersonii, � = Megalonyx wheatleyi, ✳ =Megalonyx leptostomus.

1992). Any attempt to reconstruct an integratedecology of the Bonneville shoreline 13,000years ago is hampered by a distinct preserva-tional bias. While the majority of vertebrateremains are from the eastern side of LakeBonneville (Nelson and Madsen 1980), thedocumented paleobotanical record is primar-ily from the western margin of the lake (Rhodeand Madsen 1995).

Rhode and Madsen (1995) recognized 3sequential vegetation formations in the regionduring the Pleistocene–Holocene transition:cold montane steppe from 14,000 to 13,000years ago, limber pine woodlands from 13,000to 10,800 years ago, and woodland/steppemosaic and xeric desert scrub after 11,000years. The paleobotanical record indicates thatmontane shrub vegetation covered altitudesup to at least 2000 m in the western Bon-neville Basin. The remains of wood recoveredfrom lacustrine sediments in the northeasternBonneville Basin (Scott et al. 1983) indicatethat the Wasatch Front east of the basin mayhave supported coniferous forests. Rhode andMadsen (1995) suggest that differences in veg-etation on the eastern and western sides of theBonneville Basin may have resulted from moremesic effects and enhanced precipitation fromlake effects. This in turn possibly would havelimited the distribution of Megalonyx withinthe Bonneville Basin to the eastern margin ofthe lake where some type of woodland habitatexisted.

As previously noted, while Pleistocene ver-tebrates (almost exclusively mammals), espe-cially large ones, are not rare in Bonnevilledeposits along the Wasatch Front, neither arethey abundant. Extensive excavations havetaken place along the Wasatch Front, especiallyin the past decade. It might therefore beassumed that larger quantities of fossils shouldbe uncovered than is the case. However, con-sidering that the eastern shore of Lake Bon-neville at all its levels lapped up against thesteep Wasatch Front, only limited habitatwould have been available to animals living inthis restricted zone. With this consideration,fossils would not be expected to be abundanteven though preservational conditions werefavorable. The one exception is Ovis canadensis(Stokes and Condie 1961, Stock and Stokes1969, Nelson and Madsen 1980), one of themost common species preserved in Lake Bon-neville–related sediments in this area. The rel-ative abundance of mountain sheep comparedto other species probably reflects their abilityto utilize the steep habitat of the Wasatch Front,certainly not a habitat that would be conduciveto the success of a ground sloth.

While admittedly the 2 Utah records pro-vide only a limited sample upon which to baseany broad conclusions concerning the ecologyof Megalonyx, the strong similarity in theoccurrence of both does suggest a pattern.Despite the recovery of numerous mammaliantaxa represented by a reasonable number ofsamples from sediments associated with LakeBonneville (Nelson and Madsen 1980, Miller

2001] JEFFERSON’S GROUND SLOTH TAPHONOMY 75

Fig. 11. Graph showing relative lengths of metacarpalsin Megalonyx jeffersonii. Solid symbols are from articu-lated hands. Open symbols indicate isolated specimensand provide an indication of the size range of each bone.� = Point-of-the-Mountain, Utah (BYU 13610), � =American Falls Reservoir, Idaho (IMNH 23034), � =Cromer Cave, West Virginia (ANSP 12508, holotype ofMegalonyx jeffersonii).

Fig. 12. Scatter diagram of anteroposterior length ofastragalus against mediolateral width of the posterior edgeof the trochlea in Megalonyx jeffersonii and Megalonyxwheatleyi. � = Point-of-the-Mountain Megalonyx jeffer-sonii, � = Megalonyx jeffersonii, � = Megalonyx wheat-leyi.

1982, Jefferson et al. 1994), only 2 records of sloth, both Megalonyx, have been found. Bothwere recovered from deposits associated withthe Provo Level of Lake Bonneville. In markedcontrast to other mammalian records, whichconsist of isolated bones, both sloth specimensare partial, articulated skeletons, suggestingrapid burial. It can be inferred that Megalonyxwas restricted to a habitat closely associatedwith the margin of Lake Bonneville and thatits absence in other Pleistocene faunas in thearea reflects this habitat preference.

ACKNOWLEDGMENTS

Special appreciation is given to Joe Millerof Geneva Rock Products Company for dis-covering the partial Megalonyx skeleton dis-cussed in this paper and his wife, Kristina, forkindly bringing this very important specimento the Earth Science Museum at Brigham YoungUniversity (BYU). Deep gratitude is also ex-pressed to the management of the above com-pany for their part in the acquisition of thisfossil and other specimens that were earlierrecovered from their PM gravel pit. RichardTrotter, especially, and other co-workers atGeneva have also provided important fossilsfrom this site over the years. Their thoughtful-ness and kindness in bringing them to theabove museum for scientific study is greatlyappreciated. John Chiles of Geneva Rock Prod-ucts is thanked for permitting the authors ofthis paper access to the gravel pit to studydeposits there and photograph relevant areas.Ken Stadtman of the BYU Earth ScienceMuseum did the necessary preparation of thesloth bones. Appreciation is extended to himfor this. We also thank James Miller of theGeology Department at Brigham Young Uni-versity for helping produce Figure 1. DonaldCurrey of the University of Utah provided use-ful information, for which he is kindly acknowl-edged. Austin Long at the University of Ari-zona is thanked for his attempts to obtain acarbon-14 age based on bone samples of thepresent specimen. We also thank Ginger Cooleyof BYU for the sedimentological processing ofmaterial used for analysis. We would like toextend our appreciation and thanks to Drs.Jim Mead and Dave Gillette for their thought-ful reviews of the manuscript and suggestionsfor its improvement.

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Received 8 September 1999Accepted 13 March 2000

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