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Ground sloth Megalonyx from Pleistocene deposits of the Old Crow Basin, Yukon, Canada.

(Received 17 August 1999; accepted in revised form 26 January 2000)

ABSTRACT. The bear-sized ground sloth Megalonyx, endemic to North America, was widespread during the Pleistocene, reaching as far north as Alaska, Yukon, and Northwest Territories. Twenty-two specimens collected from 10 localities in the Old Crow Basin, northern Yukon, include several bones that can be referred to Jefferson's ground sloth (Megalonyx jeffersonii) on the basis of the distinctive morphology of the upper and lower caniniform teeth. All of the Yukon specimens are relatively small, suggesting a geological age earlier than Wisconsinan--probably Sangamonian. The Alaskan, Yukon, and Northwest Territories records imply that the species occupied a broad east-west range in northwestern North America during a warm phase of the late Pleistocene.

Key words: Jefferson's ground sloth, Megalonyx jeffersonii, late Pleistocene, vertebrate fossils, Yukon

RESUME. Le paresseux marcheur Megalonyx, de la taille d'un ours, qui etait endemique a l'Amerique du Nord, occupait au pleistocene une vaste aire s'etendant au Nordjusqu'en Alaska, au Yukon et aux Territoires du Nord-Ouest. Vingt-deux specimens recueillis a 10 emplacements dans le bassin de Old Crow (partie nord du Yukon), comprennent plusieurs os que l'on peut attribuer au paresseux marcheur de Jefferson (Megalonyx jeffersonii) si l'on se fie a la morphologie particuliere des dents superieures et inferieures en forme de canines. Tous les specimens du Yukon ont une taille relativement petite, ce qui suggere qu'ils datent d'un age geologique anterieur au wisconsinien -- probablement le sangamonien. Les releves faits en Alaska, au Yukon et dans les Territoires du Nord-Ouest laissent entendre que l'espece occupait une vaste aire s'etendant d'est en ouest dans la partie nordoccidentale de l'Amerique du Nord durant une phase de rechauffement du pleistocene tardif.

Mots cles: paresseux marcheur de Jefferson, Megalonyx jeffersonii, pleistocene tardif, fossiles vertebres, Yukon

Traduit pour la revue Arctic par Nesida Loyer.


Megalonyx, the most widespread North American sloth genus, is known from the late Miocene (Hemphillian ca. 6 Ma) until the end of the Pleistocene (Rancholabrean ca. 10000 B.P.). The megalonychids represent the earliest members of the South American mammalian fauna to disperse into North America during the late Cenozoic. The earliest representative of the group, Pliometanastes, is known from several localities in the southern United States (Hirschfeld and Webb, 1968) and evolved into Megalonyx (Hirschfeld, 1981). The latter genus includes a lineage of several species that culminated in M. jeffersonii, its largest and most recent representative member (McDonald, 1977).

During this time Megalonyx became widely distributed across the North American continent and extended its range as far north as Alaska (Stock, 1942) and the Northwest Territories in Canada (Stock and Richards, 1949). While these two records have at least indicated the presence of Megalonyx in the northern parts of the continent, the specimens, each a single tooth, have provided only limited information on the taxon. The first detailed account of Yukon ground sloth remains (Harington, 1977:177-185) comprised descriptions of 12 specimens assigned to Megalonyx cf. M. jeffersonii. In the Old Crow Basin, C.R. Harington (Canadian Museum of Nature project 1966 to 1987) and W.N. Irving (Northern Yukon Research Programme based at the University of Toronto 1975 to 1983) and their field parties recovered a diverse fauna that includes additional remains of Megalonyx. These remains improve our understanding of the northernmost populations of this genus and are the subject of this paper.


Megalonyx fossils have been recovered from 10 localities in the Old Crow Basin, Yukon (Fig. 1). Although none of the specimens were found in stratigraphic position except the second phalanx from Locality 44, their possible geological, geochronological and paleoenvironmental context is considered below (see Discussion). Not all recovered material is diagnostic, but those specimens that are diagnostic can be identified as the species M. jeffersonii (Fig. 2), and all material is referred to that species. The 22 specimens are listed in Table 1. All are housed in the vertebrate paleontology collections of the Canadian Museum of Nature (CMN).

TABLE 1. Specimens of Megalonyx jeffersonii from the Old Crow
Basin, Yukon, Canada.

Locality        Catalog Number       Skeletal Element

  11A           CMN 48628            right third metacarpal
  11A           CMN 48646            second phalanx digit 4 pes
  11A           CMN 24192            molariform
  11A           CMN 48661            left upper caniniform
  11A           CMN 22567            proximal phalanx
  11A           CMN 26193            proximal half ungual digit 3 manus
  11A           CMN 48449            second phalanx manus
  11A           CMN 24194            proximal right calcaneum
  11A           CMN 48656            proximal right calcaneum
  22            CMN 14528            molariform
  22            CMN 19203            upper fifth molariform
  24            CMN 14883            second phalanx
  29            CMN 14882            proximal half ungual
  42            CMN 33247            molariform
  44            CMN 43457            second phalanx
  66            CMN 25148            left astragalus
  66            CMN 31146            proximal phalanx
  66            CMN 24215            left lower caniniform
  66            CMN 23042            proximal phalanx digit 3 manus
  84            CMN 31778            left upper caniniform
  144           CMN 28550            second phalanx digit 3 manus

D CMN 43284 proximal phalanx


While all of the specimens listed can be identified as megalonychid sloth and can be referred to the genus Megalonyx, only a few specimens can be referred to the species M. jeffersonii with any certainty. These specimens will be discussed further below. Measurements of the more diagnostic specimens described below are provided in Table 2.

TABLE 2. Measurements of selected specimens of Megalonyx
jeffersonii from the Old Crow Basin, Yukon. All measurements are
in millimetres (mm).

Right Third Metacarpal (CMN 48628) Locality 11A
  Length                                                      95.9
  Proximal end, mediolateral width                            50.2
  Proximal end, dorsoventral width                            47.7
  Distal end, mediolateral width                              39.6
  Distal end, length of carina                                52.0

Left upper caniniform (CMN 48661) Locality 11A
  Length                                                      31.5
  Width                                                       15.4

Left upper caniniform (CMN 31778) Locality 84
  Length                                                      34.4
  Width                                                       20.0

Left lower caniniform (CMN 24215) Locality 66
  Length                                                      34.2
  Width                                                       14.6

Left astragalus (CMN 25148) Locality 66
  Anteroposterior length                                     102.2
  Width anterior edge trochlea                                83.2
  Width posterior edge trochlea                               71.9
  Anteroposterior length of lateral edge of trochlea          80.8
  Dorsoventral height navicular process                       48.4

Mediolateral width navicular process 69.7

One of the distinctive dental features of Megalonyx is the shape of the enlarged anteriormost tooth, or caniniform. Changes in morphology of the tooth, with the enlargement and increasing prominence of the lingual column in both upper and lower caniniform teeth, permit the chronospecies of Megalonyx to be distinguished. In the Blancan to earliest Irvingtonian species, M. leptostomus, the lingual bulge is the least developed, but is positioned along the midline of the tooth. Longitudinal grooves mesial and distal to the lingual column are generally absent or only incipiently present in the latest members of this species. In M. wheatleyi, from the middle Irvingtonian, the lingual column is more prominent, and there are weak grooves along its mesial and distal edge. The mesial and distal grooves along the edge of the lingual column are well developed in M. jeffersonii. These characters are seen in the three caniniforms from the Old Crow Basin: a left upper caniniform (CMN 48661) from Locality 11A (Fig. 3c); a left upper caniniform (CMN 31778) from Locality 84 (Fig. 3e); and a left lower caniniform (CMN 24215) from Locality 66 (Fig. 3d). Although the morphology of the upper and lower caniniform teeth conforms to that of M. jeffersonii, these teeth fall at the small end of the size range for the species (Figs. 4 and 5).

The sample of Megalonyx includes four molariform teeth. The general similarity between the upper and lower molariform teeth makes it difficult to provide more precise identification of isolated teeth. The teeth are triangular in cross section with the mesial and distal lophs of the occlusal surface converging towards the apex of the triangle, a feature characteristic of megalonychid sloth teeth. Like all sloth teeth, they lack roots and have an open pulp cavity.

Heaton and McDonald (1993) described a third metacarpal from the Kuchta Sand Pit locality in South Dakota and documented size differences between Blancan, Irvingtonian, and Rancholabrean species of Megalonyx. The graph provided by Heaton and McDonald (1993) has been modified to include the right third metacarpal (CMN 48628) from Locality 11A (Fig. 3a). Morphologically the specimen agrees with other specimens of Megalonyx, and its size places it at the small end of the size range of M. jeffersonii (Fig. 6).

The astragali of megalonychid sloths are distinguished from those of other ground sloths by the absence of the modification of the medial trochlea into an odontoid process. While the lack of this modification gives the astragalus an appearance generally similar to those of carnivores and rodents, the sloth astragalus can readily be distinguished by the concave articular surface on the navicular process. The left astragalus (CMN 25148) from Locality 66 (Fig. 3b) possesses all of the features typical of Megalonyx astragali. Like the other specimens, it is from an individual at the small end of the size range for M. jeffersonii (Fig. 7).

Many of the Megalonyx specimens from the Old Crow Basin are phalanges: four proximal, five second, and two ungual. All members of the order Xenarthra are distinguished by the proximodistal shortening of the proximal phalanges in both manus and pcs. This distinctive feature is seen in the Megalonyx proximal phalanges from the Old Crow Basin. The second phalanges are elongated with a distal articular surface that forms almost three-quarters of a circle, reflecting the extreme flexion of the unguals while the sloth is walking. The ungual process of the two unguals has the typical tall, narrow triangular cross section present in megalonychid sloths. Measurements of phalanges from the Old Crow Basin are provided in Table 3.

TABLE 3. Measurements of phalanges of Megalonyx jeffersonii from
the Old Crow Basin, Yukon. All measurements are in millimetres


CMN No.   Locality            Mediolateral    Proximodistal   Antero-
                              Width (max.)    Depth (max.)    posterior

14882     Old Crow Loc. 29
          (distal half
           lacking)               29.3            50.5          61.7+
48646     Old Crow Loc. 11A       34.5            42.7          67.2
28550     Old Crow Loc. 144       39.7            52.6          76.2
48449     Old Crow Loc. 11A       27.7            34.0          58.0
31778     Old Crow Loc. 84        36.7            43.8          31.0+
22567     Old Crow Loc. 11A       37.7            53.9          40.0
43284     Old Crow Loc. D         49.3            56.0          34.3
43457     Old Crow Loc. 44        35.6            49.0          63.5
26193     Old Crow Loc. 11A
23042     (distal half
           lacking)               33.3            62.3          73.0+
14883     Old Crow Loc. 66        43.2            55.4          35.9
          Old Crow Loc. 29
          (proximal end           30.8            28.2+         53.2

damaged) (approx.)


All of the Yukon specimens fall at the small end of the size range for Megalonyx jeffersonii and are only slightly larger than the largest individuals of the earlier ancestral species, Megalonyx wheatleyi (Figs. 4-7). There are two possible explanations for the small size of the Yukon individuals. One is related to the geological age of the specimens. During its phylogeny, the Megalonyx lineage increased in body size through time, so each succeeding species was on the average larger than the preceding species; the terminal species Megalonyx jeffersonii is the largest.

The earliest record of M. jeffersonii is from the type locality of the Irvingtonian which Repenning (1987) placed in the earlier part of his Irvingtonian II (900 000 to 400 000 [+ or -] 25 000 years ago). The small size of the Old Crow Basin specimens may merely be indicative of their Sangamonian age. For example, a substantial vertebrate fauna of possible Sangamonian age has been excavated from Locality 44 (Harington, 1977, 1990), the source of a Megalonyx second phalanx (CMN 43457).

Alternatively, the small size of the individuals may indicate a negative Bergmann's response, reflecting an overall trend of a decrease in body size with increasing latitudes. It is not easy to document size trends against latitude for Megalonyx in the Sangamonian, since few faunas include the genus; also, the same skeletal element is not always preserved, making direct comparisons difficult. As a preliminary examination of this trend, the sample from Old Crow can be compared with samples from American Falls Reservoir in Idaho and Ingleside Texas. All three faunas are considered Sangamonian in age, reducing the chance that any size trends reflect a change with time as opposed to latitude. Upper and lower caniniforms of Megalonyx are known from all three localities and permit some comparison. While the overall samples are small (upper caniniform: Old Crow N = 2, American Falls N = 2, and Ingleside N = 1; lower caniniform: Old Crow N = 1, American Falls N = 4, and Ingleside N = 3), a plot of average tooth length against latitude does show a trend. As shown in Figure 8, Megalonyx was smaller in the northern part of its range than in the south. This suggests that the overall smaller size of Megalonyx jeffersonii from Old Crow compared to other individuals of the species may reflect both its geological age and a pattern of smaller size in the lineage at higher latitudes.

A similar pattern has been observed in the very small Peary caribou Rangifer tarandus pearyi of the Canadian Arctic Islands (Banfield, 1961). One explanation for this smaller size in higher latitudes might be that it represents a response to a decrease in the availability of suitable vegetation. Given the low basal metabolism of sloths (McNab, 1985), we might expect that overall body size would increase at higher latitudes, to improve thermal inertia and help maintain a constant body temperature in a cooler environment. Akersten and McDonald (1991) discussed the problems of maintaining body temperature in the sloth Nothrotheriops, which lived in desert habitats, and the possible effect on its distribution, so it seems likely that thermal stress would have been a critical factor for Megalonyx living at high latitudes as well. Of course, we cannot tell whether the insulation of the pelt in Megalonyx jeffersonii varied greatly from south to north. This departure from the expected pattern in size change over the range of the animal points out how much more needs to be learned about the paleophysiology of this extinct group.

Average temperatures during the Sangamonian may have been as much as 3 [degrees] C warmer than at present (Karrow, 1990), and this may have been one important factor that permitted Megalonyx to expand its range as far north as the Old Crow Basin. Analysis of plant macrofossils and invertebrates of the possible Sangamonian unit at Locality 44 indicates that climate was at least as warm as at present (Harington, 1990). A significant change in climate from cool (dwarf birch) to warmer (spruce-birch) is reflected in pollen assemblages extending upward from the reworked Lower Lake clay to the vertebrate fossil-bearing unit (Lichti-Federovich, 1973). Large spruce (Picea sp.) logs up to 1.5 m in circumference, along with a nearly equal number of tamarack (Larix sp.) logs (identified by J.S. Gonzalez, Forintek Canada Corp., pers. comm. 1986) indicate a period warmer than at present. The tamarack fossils are clearly north of the modern range of the species (Little, 1971; Viereck and Little, 1975). Furthermore, the presence of short-faced skunks Brachyprotoma obtusata more than 4300 km north of their known paleo-range (Youngman, 1986)--particularly if they had habitat requirements similar to those of their closest living relatives, the spotted skunks Spilogale putorius--may also indicate a warmer phase. In addition, the fossiliferous unit that is the probable source of the ground sloth bones lies between the widespread Lower Lake and Upper Lake beds, considered to be of Illinoian and Wisconsinan ages, respectively (Harington, 1977; Jopling et al., 1981). This fact suggests that the unit is of the last interglacial age.

One taxon found with Megalonyx at two localities, 11A and 66, is wolverine, Gulo gulo (Bryant, 1987). The modern distribution of this species is circumpolar and corresponds to the boreal zone of the Northern Hemisphere (Kvam et al., 1988). Its possible association with Megalonyx might at first suggest that the sloth was capable of surviving in a cooler environment than might be expected. Bryant (1987) did note that the ancestral form Gulo schlosseri is associated with decidedly warm faunas in Europe, and early North American Gulo may have been adapted to a similar environment. Bryant feels that Gulo gulo is not inherently a purely "boreal" or "tundra" species, given that its range extended as far south as Pennsylvania until the mid-1800s.

The southernmost sloth in South America is found at Ultima Esperanza Cave in Chile, at 51.5 [degrees] S. This species, Mylodon darwinii, has a larger body size than Megalonyx from the Old Crow Basin. At 68 [degrees] N, within the Arctic Circle, Megalonyx from the Old Crow Basin may represent the greatest tolerance to extreme temperatures of any of the extinct ground sloths. Both the southernmost and the northernmost records indicate that some of the extinct sloths may not have been restricted to as narrow a range of environmental temperatures as their modern relatives. Whether this greater tolerance reflected a higher basal metabolism or merely greater body size that provided thermal inertia is unknown at this time.


The Old Crow Basin has produced the best samples of Megalonyx jeffersonii from Canada. Previous records included only isolated occurrences of single specimens (Table 4). While M. jeffersonii is common in Rancholabrean faunas in the eastern United States (McDonald, 1977), it has not yet been found in any faunas in eastern Canada. Nor is it known from Pleistocene faunas north of the Great Lakes and St. Lawrence River, although many taxa with which it is commonly associated, such as the American mastodon Mammut americanum, have been found there. (For example, three mastodon teeth and "elephant" bones--perhaps representing that species too--are known from the Moose River Basin on northern Ontario, as observed by C.R. Harington in 1998; and another mastodon tooth found near Chambord, Lac Saint Jean, Quebec, was reported by Pierard and Tremblay, 1980). The presence of Megalonyx jeffersonii at higher latitudes in the western part of its range than in the east parallels the distribution seen in the stag-moose Cervalces scotti, the giant beaver Castoroides ohioensis, and the helmeted muskox Bootherium bombifrons. All of these taxa are also closely associated with Megalonyx in Pleistocene faunas from the eastern United States. It is worth noting that American mastodon, giant beaver, and helmeted muskox remains are recorded from Pleistocene deposits in the Old Crow Basin, and that the first two species have been found in place in the possible Sangamonian interglacial unit at Locality 44. The FAUNMAP Working Group (1996) found that Pleistocene mammalian faunas followed a Gleasonian model in responding to environmental change: that is, each species responded in accordance with its individual tolerance limits, resulting in range shifts with varying rates, at different times, and in divergent directions. Yet the similarity of these three species in expanding their northernmost ranges into the same general area suggests either a close association with a specific environment or strong similarities in the ecological requirements of these herbivores.

Pinsof (1996) considered 62 faunas in North America as Sangamonian in age. He listed 20 from Canada, 4 from Alaska, 35 from the rest of the United States, and 3 from Mexico. Five of the Canadian faunas include Megalonyx, although it should be noted that while Pinsof listed Old Crow as one of the faunas, he did not include Megalonyx in the faunal list. Seven of the Sangamonian faunas in Canada are east of Manitoba, and none of these seven include Megalonyx. Eleven of the 35 localities in the United States (excluding Alaska) include Megalonyx, but except for American Falls Reservoir in Idaho, all are restricted to the southern United States.

The Yukon fossils are important, not only because they constitute the largest samples of ground sloth remains from Canada, but also because they can be identified as Jefferson's ground sloth (Megalonyx jeffersonii). Although no specimen has been found in its original stratigraphic context except the specimen from Locality 44, we suggest that the species lived in the proto-Old Crow Basin during the last (Sangamonian) interglacial. Considered together, the Alaskan (Fairbanks area), Yukon (Old Crow Basin), and Northwest Territories (Lower Carp Lake) records suggest that the species occupied a rather broad east-west range in northwestern North America during a warm phase of the late Pleistocene (Harington, 1978, 1993).

TABLE 4. Non-Yukon records of Megalonyx from Canada.

Specimen                  Locality          Reference and Remarks


   Right upper cheek      Medicine Hat      Churcher, 1969:154; Stalker
    tooth fragment        (Mitchell         and Churcher, 1982.
    (field number          Bluff)           Identified as ?Megalonyx sp.
    MB-68-310)                              Considered to be of
                                            Sangamonian age.

   Left femur shaft and   Medicine Hat      Confirmed as Megalonyx by
    fragment of shaft     (Island Bluff)    H.G. McDonald. Both specimens
    (P960.17.40 and 39)                     are probably from the same
                                            individual (J.A. Burns, pers.
                                            comm. 1999).

   Right femur midshaft   Edmonton area     Referred to M. jeffersonii
   (P90.6.21)                               (J.A. Burns, pers. comm.
                                            1999). See general reference
                                            to Megalonyx in Burns and
                                            Young, 1994.

   Right femur midshaft   Edmonton area     Referred to M. jeffersonii
   (P98.6.28)                               (J.A. Burns, pers. comm.

   First thoracic         Fort              Referred to Megalonyx sp.
    vertebra               Saskatchewan     spinours process (J.A. Burns,
   (P89.13.68)                              pers. comm.  1999).

   Left first rib         Fort              Referred to M. jeffersonii
    proximal third         Saskatchewan     (J.A. Burns, pers. comm.
   (P89.13.549)                             comm. 1999).

British Columbia
   Ungual (ROM 3339),     Quesnel Forks     Cowan, 1941; Harington, 1977,
    cast of missing                         1996. From river gravel.

Northwest Territories
   Lower molariform       Lower Carp        Stock and Richards, 1949.
   (ANSP 15208)            Lake             Found with fragments of an
                                            American mastodon tooth.


   Tooth fragment         Saskatoon area    Churcher, 1969. Identified as
    (ROM 5538)            (Pike Lake)       Megalonyx.
                                            Considered to be late
                                            Pleistocene in age.

   Condylar region of     Stewart Valley    Churcher, 1969:55; Stalker,
    left mandible         (Wellsch Valley   1971. Identified as
    (WV-76a)               fauna)           ?Megalonyx sp. Considered to

be of Kansan age.


We thank Dr. James A. Burns for information on Megalonyx specimens in the Provincial Museum of Alberta Quaternary paleontology collection; Dr. Kevin L. Seymour for similar information on Royal Ontario Museum collections; David Brynaert and Canadian Geographic for permission to use Fig. 2; Donna Naughton (Canadian Museum of Nature) for Fig. 1; and Gail Harington for assistance with word processing. C.R. Harington is particularly grateful to the late Peter Lord, Jerry Fitzgerald, Charlie Thomas, and Freddy Frost for their help in collecting the specimens. The authors would also like to express their appreciation to the two anonymous reviewers for their comments.


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