Mammoths and mylodonts: Exotic species from two different continents in North American Pleistocene faunas

ARTICLE IN PRESS

Quaternary International 142–143 (2006) 229–241

Mammoths and mylodonts: Exotic species from two different continents in North American Pleistocene faunas H. Gregory McDonalda,, Steve Pelikanb a

Geologic Resources Division, National Park Service, P.O. Box 25287, Denver, CO 80225, USA Department of Mathematical Sciences, University of Cincinnati, Cincinnati, OH 45221-0025, USA

b

Available online 19 May 2005

Abstract Throughout the Cenozoic, the North American mammalian fauna has been enriched by the appearance of new taxa originating on different continents. During most of the Tertiary, the primary source area of these new taxa was Eurasia with dispersal across some version of the Bering Land Bridge. In the late Pliocene (Blancan) ca. 2.5 mya, the creation of the Panamanian Land Bridge permitted the northward dispersal of species of South American origin including ground sloths. One of these sloths was ‘‘Glossotherium’’ chapadmalense, which in turn gave rise to the Pleistocene species Paramylodon harlani. Mammoths first appear in North America at the beginning of the Irvingtonian ca. 1.9 mya. Despite originating on two different continents, the two species are often found together in North American Pleistocene faunas and shared a common habitat. Both of these lineages are commonly interpreted as grazers, indicative of open grassland habitat, and both of these exotic species shared this habitat with North American endemic species such as horses, also interpreted as grazers. Despite their association in North American faunas, mammoths did not disperse into South America and mylodont sloths were unable to disperse into Eurasia. This suggests there were some aspects of their ecology they did not have in common and there existed a limited zone of conditions that permitted them to share common habitat. There is no evidence that the appearance of either species in North America resulted in the extinction of any native species. The question is how these different species, immigrants and endemics, were able to avoid competition, coexist, and become integrated into a single fauna, thus enriching the overall North American Pleistocene fauna. Published by Elsevier Ltd.

1. Introduction As noted by Guthrie (1984), it is axiomatic that two sympatric species never use all the same resources in exactly the same way since such competition would result in the demise of both. Resource partitioning among species sharing a common habitat decreases competition and is critical for the survival of each species. A question an ecologist and a paleoecologist must then answer is in what way do two species sharing a common habitat partition food or other resources? It would seem reasonable that since mammoths and Corresponding author. Current address: Museum Management Program, National Park Service, 1201 Oakridge Drive, Suite 150, Fort Collins, Colorado 80525, USA. E-mail addresses: [email protected] (H.G. McDonald), [email protected] (S. Pelikan).

1040-6182/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.quaint.2005.03.020

mylodont sloths evolved on different continents under very different ecological and evolutionary conditions that their respective ecologies would be very different and they would have little in common with regard to their ecological requirements, hence there would be little if any potential competition. Given these expected differences, it might not be expected that they would share a common habitat. Yet, despite these very different evolutionary and presumably adaptive differences, mylodont sloths and mammoths seem to be closely associated in North American faunas. The challenge is to determine not only what aspects of their ecology are common to the two species and allowed them to share a common habitat, but equally important is to identify the ecological differences that allowed them to share this habitat. Both genera are widely distributed in North America and their ranges overlap. Does their association in a fauna reflect a chance taphonomic

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occurrence resulting from their widespread distribution or does this association indicate they had similar habitat preferences and thus reflects common traits regarding their respective ecologies. The goal of this paper is to look at the complimentary aspects of the ecology of the North American mylodont sloth, Paramylodon harlani, and its association with mammoth, primarily Mammuthus columbi, with regard to their distribution and utilization of a common habitat. A secondary question is what impact the appearance of these exotic species had on the grassland ecosystems and the endemic North American grazers, and how they adapted to this ecosystem following their appearance. With successive dispersal of grazers originating on other continents (mylodont sloths, mammoths and eventually bison) into North America it would appear that the grazing niche became increasingly ‘‘saturated’’, with each succeeding wave of immigrants there would be greater degree of competition for a limited resource. Yet all of these different groups managed to survive until the collapse of the ecosystem at the end of the Pleistocene.

2. Discussion 2.1. Size Size affects many ecological parameters and as such is an important component to any understanding of the ecology of a species (Peters, 1987). Calculation of body size has been one approach utilized to infer the ecology and evolution of extinct species (Damuth and MacFadden, 1990). Mylodont sloths and mammoths were among the largest grazers present in the North American grassland habitat during the Pleistocene. As such there is the question as to whether their body size resulted in any potential competition between them. There is a general size increase in the North American mylodont lineage from the Blancan ‘‘Glossotherium’’ chapadmalense, with an estimated body weight of 236 kg, to the Irvingtonian P. harlani at 660 kg to the Rancholabrean form with an estimated body weight of 985 kg. (McDonald, in press). Shipman (1992) calculated a mean body-size estimate of 7368 kg for M. columbi based on 28 late Pleistocene specimens. So while Paramylodon was among the larger members of the North American late Pleistocene fauna, the Columbian mammoth was 7.5 times larger. 2.2. Evidence of a common diet between Mammoths and Mylodonts While there is direct evidence of diet for the Columbian mammoth from dung preserved in Bechan Cave (Davis et al., 1984; Mead et al., 1986) and Cowboy Cave, Utah (Hansen, 1980) no such direct evidence is

available for Paramylodon. Hair recovered from Bechan Cave has been referred to Paramylodon but none of the dung samples were assigned to the genus. As a basis for comparison of the diets of mammoths and mylodont sloths, the dung of the close relative, Mylodon darwinii, preserved from Mylodon Cave in Chile (Moore, 1978), has been used to provide a reasonable extrapolation as to the types of vegetation consumed by Paramylodon. Mammoth dung from Bechan Cave was composed of 95% crushed culms and leaves of grasses and sedges, the remaining 5% included browse such as saltbrush (Atriplex sp.), sagebrush (Artemisia tridentata), water birch (Betula occidentalis), and blue spruce (Picea pungens) (Mead et al., 1986). Dung of mammoth from Cowboy Cave consisted of 99% grass (Hansen, 1980). Isotopic study of the tooth enamel of Mammuthus from the Rancholabrean of Florida also supports the interpretation that Mammuthus was a C4 grazer/mixed feeder (Feranec and MacFadden, 2000) and there was no difference in its isotopic signature from that of two other sympatric grazers, Equus and Bison. In the latter study, the three genera had similar seasonal carbon isotope variations, which were interpreted as either a lack of forage specificity between the three taxa or the presence of abundant resources, making forage partitioning unnecessary. Feranec and MacFadden (2000) concluded that, since the three taxa co-existed in the same faunas, niche-partitioning must have occurred but in a manner not discernable through the analysis of either carbon or oxygen isotopes. Mylodont sloths first appear in the Blancan of Florida and Paramylodon is a common member of the late Pleistocene fauna and along with the other three taxa, would have been yet another taxon competing for these food resources. Plants in the dung of M. darwinii consisted primarily of sedges such as Carex (Cyperaceae), grasses (Gramineae) and herbs associated with an open, moist, cool, boggy sedge-grassland (Moore, 1978). Moore (1978) interpreted the dung as indicative of an animal that selectively grazed a diverse grassland-forest zone. Based on the diet of Mylodon, it is not unreasonable to infer that its North American relative, Paramylodon, also included a large percentage of grasses and sedges in its diet, a diet it had in common with Mammuthus. 2.3. Digestive physiology All extant xenarthrans lack a caecum and presumably all of the extinct forms did also, including the mylodont sloths. Living sloths have a large complex stomach partially divided into four chambers (Goffart, 1971). The stomach functions in a similar fashion to ruminants except that it lacks the equivalent of the reticulum to filter digesta and prevent large particles from entering the intestines or the omasum to reabsorb water. The absence of this filter is supported by the preserved dung

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in two extinct ground sloths, Nothrotheriops shastense and M. darwinii, which is characterized by large plant fragments. Therefore, the functional differences between ruminants and living sloths are more significant than the anatomical similarities and living sloths are considered to be foregut fermenters. The low rate of basal metabolism in modern sloths allows them to benefit from a long-retention, slow fermentation system of digestion (Cork and Foley, 1991). As a result, defecation in modern sloths is infrequent, and only occurs every 8–10 days in Bradypus and at about 6 day intervals in Choloepus (Goffart, 1971). The slow transit time through the digestive system permits more complete extraction of nutrients. In contrast the digestive system of modern elephants, and presumably the extinct mammoths, is based on postgastric (hindgut) fermentation. While the stomach is large, its primary function is to store ingested food and enzymes in the stomach only partly break down vegetation. Most nutrients are extracted in the enlarged caecum and large intestine (Van Hoven et al., 1981). In modern elephants, the transit of food through the digestive system is rapid, 24–54 h, and an adult African elephant produces on the average 11 kg of dung every 2 h, or 100–150 kg of dung each day (Guy, 1975). Guthrie (1984) included mammoths as monogastric caecalids along with equids. Based on our knowledge of the anatomy and digestive physiology of modern sloths, they cannot be considered equivalent to mammoths in this regard, but may be more comparable to nonruminant artiodactyls such as peccaries and camelids, neither of which is a pure grazer (Dompierre and Churcher, 1996). In ruminants, the rate of passage of vegetation is controlled by the filtering function of the reticulum, and the size of food particles and their ability to pass through it. This limits the ability of ruminants to increase the volume of vegetation consumed in order to compensate for low nutritional value of forage. Sloths, like mammoths, would not have been limited in this way and both groups could feed on vegetation with a high fiber and low nutrient content by increasing the volume of vegetation consumed. Therefore mylodonts would have filled the niche of a foregut grazer. As described by Haynes (1991), the elephant’s feeding strategy is to eat great quantities of vegetation, ferment it and extract the nutrients quickly, followed by excretion of the relatively large indigestible fraction while continuing to ingest more food. Therefore, even though both mammoths and mylodonts were both grazers, utilization of grasses and sedges was quite different, judging from differences in methods of digestion utilized by their modern relatives. The combination of lower basal metabolism, slower and more complete absorption of nutrients, and smaller body size meant that sloths probably did not need to consume the same quantities of grass or other vegetation, as did mammoths. Since sloths could have

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extracted a larger percentage of nutrients per unit volume of vegetation than mammoths, they would not have needed to feed as often. One of the consequences of this difference in feeding strategy is that sloths probably would not have been as active as mammoths and would not have needed to be constantly on the move to locate new forage. Consequently, mylodont sloths probably had a smaller home range, stayed within a fixed area, and did not migrate on a seasonal basis. Olivier (1982) felt that mammoths, like living elephants, must have exhibited resource-oriented movements and that mammoths had separate summer and winter ranges analogous to the wet and dry seasonal ranges of extant elephants. The idea that mammoths in North America undertook seasonal migrations was discussed by Churcher (1980). Mammoths probably migrated during times of year when the nutritional value of forage was the poorest (either the dry or winter season), while sloths may have been able to still feed in the area year-round and utilize this poorer forage without competition from mammoths. Therefore, unlike mammoths, mylodont sloths probably did not need to continually move into new areas to obtain sufficient graze to maintain a population. In addition, given their limb morphology, sloths were not likely to have undertaken active seasonal migrations, although given sufficient time they were certainly capable of dispersing as evidenced by their movement out of South America. Consequently, sloths probably were year-round residents of the local habitat and only shared it on a seasonal basis when mammoths migrated through the area. 2.4. Social organization and reproduction There is no evidence that mylodont sloths formed family groups like mammoths (Fox et al., 1992) and they were probably solitary animals with a smaller number of individuals per unit area than mammoths. Many records of Paramylodon represent remains of single individuals. The few samples of multiple individuals from a single fauna, such as Rancho La Brea in California or American Falls Reservoir in Idaho, are time-averaged samples that accumulated over thousands of years. There are no known sites of catastrophic origin preserving multiple individuals that would indicate Paramylodon had any type of extended family structure or herd behavior. The primary social interaction of this species was probably limited to mating and to females with young. This may explain the smaller representation of Paramylodon in the North American fossil record compared to mammoth. Living sloths produce one young and the same was probably true for Paramylodon, although no articulated skeletons of a pregnant female Paramylodon have been recovered to support this supposition. In the extant tree

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sloths, the gestation period ranges from 6 to 8 months. It is probable that the gestation period in much larger sloths like Paramylodon was longer but there is no way to determine if it was as long as in proboscideans. Modern elephants may serve as a model to extrapolate the timing of sexual maturity in mammoths. However, given the very different lifestyles between living arboreal sloths and extinct ground sloths, one cannot confidently apply reproductive information from living to extinct sloths, thus limiting any possible comparisons of reproductive strategies. Mylodonts and mammoths can both be interpreted as being K selected, their populations were maintained near the carrying capacity of their range, reproduction was low with a small number of offspring, and the female produced a single young after a long gestation period. Individuals of both groups were probably longlived with a long reproductive life. 2.5. Dispersal Mammoths and mylodonts dispersed in opposite directions, with mammoths from north to south and mylodonts from south to north. As they dispersed, each population needed to adjust in opposite ways to changes in the average annual seasonal temperature and to seasonal extremes. Mammoths needed to acclimatize to warmer average temperatures as they dispersed south and the mylodont sloths to cooler temperatures as they moved north. This suggests that the warmest winter temperatures limited the northern dispersal of sloths and the coldest summer temperatures limited the southern

dispersal of the mammoths, even though mammoths dispersed as far south as Central America, as discussed below. If this is the case, their presence in North America together is an area of overlap that provided the appropriate range of temperatures required by both populations. While not explicitly discussed before, the presence of both mammoths as a ‘‘northern’’ species and mylodonts as a ‘‘southern’’ species in a fauna can be considered disharmonious. As such their association is equivalent to that described for many smaller species as indicative of a disharmonious or non-analog fauna (Stafford et al., 1999). The only difference is that once it was established in the Irvingtonian, this association seems to have been maintained until the extinction of both lineages at the end of the Pleistocene. Mylodont sloths first appeared in temperate North America during the Blancan, ca. 2.5 mya, and at that time they were already as far north as Idaho. The first appearance of mammoths in North America has been used to define the Blancan-Irvingtonian boundary at 1.9 mya. The earliest Irvingtonian faunas that record both Paramylodon and Mammuthus are Vallecito Creek, California, at ca. 1.5 mya and the Gilliland fauna in Texas, ca. 1.1 mya (ages follow Repenning, 1987). The northernmost Irvingtonian faunas that include both genera are Gordon and Hay Springs in Nebraska. 2.6. Distribution Based on the distribution maps in the FAUNMAP database, there is a close overlap in the distribution of M. columbi and P. harlani (Fig. 1). This association

Fig. 1. Distribution map of P. harlani and M. columbi showing their distribution in the Rancholabrean. Data from FAUNMAP database.

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Table 1 Distribution of Irvingtonian P. harlani and its association with mammoths and mastodons. EE ¼ latitude  107+elevation in meters Locality

Latitude

Longitude

California Anza-Borrego

33.27

116.33

37.1 33.49 38.7

Fairmead Pauba Mesa Colorado Porcupine Cave Florida Haile 16A Inglis 1A Leisey Shell Pits Tucker Borrow Pit Kansas Courtland Canal Kanopolis Nebraska Bartek Brothers Fossil Quarry Gordon Fossil Quarries Hay Springs New Mexico Mesilla Mesa Fauna C Oklahoma Holloman Quinlan Pennsylvania Port Kennedy Cave Texas Gilliland Martin Ranch Rock Creek White Knob Mexico El Golfo

Elevation in meters

EE

Mammoth present

Mastodon present

305

3864

M. meridionalis

120.2 117.1

64 366

4034 3949

M. columbi M. cf. meridionalis

Yes Stegomastodon No Yes

105.9

2900

7041

No

No

29.7 29 27.7 27.83

82.6 82.7 82.5 80.81

24 0 0 7

3202 3103 2964 2985

No No M. hayii M. cf. hayii

No Yes Yes Yes Cuvieronius tropicus

41 38.7

98.1 98

497 479

4884 4620

No M. cf. columbi

gomphothere No

41.2 42.8 42.7

96.8 102.2 102.7

403 1083 1165

4811 5663 5734

M. imperator M. imperator M. imperator

Yes No No

31.97

106.68

1226

4647

sp.

No Stegomastodon cf. mirificus No

34.4

99

421

4102

36.45

99

528

4428

M. imperator M. columbi No

40.1

75.4

31

4322

No

Yes

33.8 34.4 34.5 35.9

99.5 101.4 101.4 100.1

440

4057

991

4683

M. imperator M. imperator M. imperator No

No No No Cuvierionius bensonensis

34.3

114.5

0

3496

M. cf. meridionalis

Cuvieronius sp.

does not seem to exist between Paramylodon and other species of late Pleistocene mammoth, Mammuthus jeffersonii and M. primigenius. While we know that the two genera are found in a number of Irvingtonian faunas (see Table 1), the current state of the taxonomy of Irvingtonian mammoths precludes any interpretation of an association of Paramylodon with a particular species of mammoth, if there is more than one species. The northernmost record of P. harlani is from Sequim, Washington at 48.11 north latitude. While there are mammoth species in the Rancholabrean (i.e. the wooly mammoth, M. primigenius), with a more northernly distribution, M. columbi, (the species with which Paramylodon is most often associated), is found in Alberta at 55.11 north (Jim Burns, pers. com.) and in the Yukon ca. 651 north latitude (FAUNMAP Working Group, 1994) (Table 2). The southernmost records of M. columbi are from the Orillas del Humuya fauna of Honduras at about 14.51 north latitude (Webb and Perrigo, 1984) and Hacienda

del Silencio, Costa Rica, at 9.91 north latitude (Lucas et al., 1997). As with mammoths, Central American records of mylodonts are rare and have not been adequately described. Webb and Perrigo (1984) did not list any mylodont sloths from faunas in El Salvador while specimens referred to both Mylodon and Paramylodon by Barnum Brown were recovered in a mixed assemblage from Rı´ o de la Pasio´n, Guatemala (Woodburne, 1969) and a single mylodont ungual from Tiba´s 1, Costa Rica (Lucas et al., 1997). Presently mylodont material from Central America can only be confidently identified to family and a confirmed identification of Paramylodon has not been made from any locality south of Mexico. The southernmost localities in Mexico that include Paramylodon are Santa Cruz Aguiahua at 19.21 north latitude and Valesquillo at 191 north latitude (McDonald, 2002). Given the paucity of records of mammoths and mylodonts in Central America, and these few records consist mostly of single specimens and not extensive

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Table 2 Distribution of Rancholabrean P. harlani and its association with Mammuthus and Mammut. * indicates single specimen and not part of an associated fauna. EE ¼ latitude  107+elevation in meters. Data from FAUNMAP Working Group (1994) and supplemented with data from Jefferson et al. (2002), Kost (1987), Morgan et al. (2001), Pajak et al. (1996), Silva-Barcenas (1969), Smith and Cifelli (2000) Locality

Alabama Bogue Chitto Creek Arizona Shonto Springerville California ARCO Arena Costeau Pit Dudley Ridge Harbor Freeway Hawver Cave La Brea and San Vincente Lake Manix Los Angeles Brickyard #3 Maricopa McKittrick Mountain View Dump Naval Housing Unit Point Concepcion Rancho La Brea San Pedro Stevenson Bridge Teichert Gravel Pit Tulare Lake Colorado Bennett Sand and Gravel Bijou Creek Crook Dutton Lamb Springs La Salle Magna Site Selby Walsenberg Widefield Florida Aucilla River 1A Cutler Hammock Hornsby Springs Ichetucknee River 3B Mefford Cave 1 Melbourne North Havana Road Page/Ladson Reddick 1A Rock Springs Seminole Field Silver Springs Vero 2 Ward Island 1 West Palm Beach Georgia Brunswick Isle of Hope Idaho Acequia Gravel Pits American Falls Fauna Chicken Ramsey Gravel Pit

Latitude

Longitude

Elevation in meters

EE

Mammoth present

Mastodon present

34

3501

M. imperator

Yes

32.4

87.3

36.57 34.27

110.65 109.33

1860 1906

5773 5573

sp. sp.

No No

38.7 33.62 35.95 33.9 38.87 34 34.87 34.1 35 35.32 37.43 33.7 34.5 34.07 33.7 38.53 38.53 36

121.4 117.62 119.78 118.3 120.87 118.3 116.5 118.2 119.37 119.62 122.08 118.12 120.5 118.35 118.3 121.85 121.37 119.8

4 90 58 44 393 38 520 92 260 320 0 0 37 15 34 16 8 58

4145 3687 3910 3671 4566 3676 4265 3741 4005 4119 4005 3606 3729 3653 3640 4136 4128 3910

sp. M. columbi M. columbi No sp. No M. columbi M. cf. columbi No M. columbi M. columbi No No M. columbi No M. columbi sp. No

No No No No No No No Yes Yes Yes No No No Yes No No No No

39.8 39.6 40.9 39.62 39.5 40.3 38.1 39.87 37.5 38.7

104.4 104 102.8 102.25 105.1 104.7 106.1 102.25 104.75 104.7

1602 1586 979 1255 1696 1418 2330 1173 1886 1790

5861 5819 5355 5482 5923 5730 6407 5453 5899 5937

sp.

No

*

*

*

*

M. columbi M. columbi No sp. M. columbi

No No No No

*

*

*

*

30.1 25.62 29.85 29.98 29.25 28.07 27.09 30.12 29.37 28.75 27.8 29.2 27.65 30.12 26.7

83.0 80.32 82.6 82.77 81.87 82.68 82.43 84 82.18 81.5 82.75 82.2 80.4 84 80.2

0 5 3 9 55 3 5 2 24 6 3 12 3 6 5

3221 2746 3197 3217 3185 3006 2904 3225 3166 3082 2978 3136 2962 3141 2862

No M. columbi No M. columbi No M. columbi sp. No sp. sp. M. columbi M. columbi sp. M. columbi M. columbi

Yes Yes Yes Yes No Yes Yes Yes Yes Yes Yes No Yes No Yes

31.1 33.2

81.5 81.1

0 1

3328 3425

M. columbi No

Yes Yes

42.6 43 42.88

113.5 112.67 112.65

1266 1340 1351

5830 5941 5939

sp. M. columbi

No Yes

*

*

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Table 2 (continued ) Locality

Latitude

Longitude

Elevation in meters

EE

Mammoth present

Mastodon present

Dam Local Fauna Duck Point Fauna Massacre Rocks Fauna McTucker Creek/Island Michaud Gravel Pit Rainbow Beach Kansas Bluff Creek Butler Spring Cragin Quarry Delphos Hamman Gravel Pit Jinglebob Fauna Oatville Sand Pit Turner Gravel Pit Wilson Co. #2 Kentucky Big Bone Lick Louisiana Avery Island Little Bayou Sara Missouri Autolite Cave Boney Spring Jones Spring Kimmswick Montana Doedon Nebraska Mullen II Nevada Nevada State Prison New Mexico Albuquerque Gravel Products Badlands Ranch Blackwater Draw Jal Oklahoma Chickasha Oregon Fossil Lake La Grande Legion Park South Carolina Edisto Beach Texas Aubrey Avenue Local Fauna Big Wichita River Ingleside Kickapoo Kincaid Shelter Utah Bechan Cave Silver Creek Washington Sequim Mexico Actun Spukil Aguascalientes Bustamente El Cedral

42.77 42.62 42.68 43.04 42.89 42.88

112.83 112.87 112.93 112.65 112.63 112.72

1430 1342 1324 1333 1351 1335

6004 5911 5891 5938 5940 5936

sp. sp. cf. sp. sp. sp.

Yes No Yes No No No

37.4 37.0 37.3 39.3 38.4 37.1 37.62 37.24 37.5

99.9 100.5 100.4 97.7 97.8 100.5 97.37 97.38 95.4

744 756 763 419 442 763 393 357 259

4746 4715 4754 4625 4551 4733 4818 4342 4272

38.87

84.62

145

29.87 30.87

91.75 91.37

37.75 38.1 38.07 38.38

89.75 93.38 93.35 90.38

*

*

No M. columbi

No No

*

*

M. columbi M. columbi sp.

No No No

*

*

sp.

Yes

4291

sp.

Yes

0 0

3196 3303

sp. No

Yes Yes

519 215 221 127

4558 4292 4298 4234

No No M. jeffersonii No

No Yes Yes Yes

793

5758

sp.

Yes

46.5

106

42.15

101.22

1019

5529

M. imperator

Yes

39.25

119.75

1400

5600

M. columbi

No

35.12 35.1 34.28 32.12

106.5 103.5 103.32 103.18

1617 1235 1280 930

5375 4991 4948 4598

M. columbi sp. M. columbi sp.

Yes No No Yes

30.1

97.89

1150

4371

M. columbi

Yes

43.25 45.3 45.1

120.37 118.07 122.9

1310 848 54

5938 5695 4880

M. columbi No sp.

No No Yes

32.52

80.28

0

3480

M. columbi

Yes

33.25 30.25 34 28 33.6 29.37

96.87 97.75 98.9 97.1 98.8 99.47

211 138

3774 3375

1.5 104 92

2998 3705 3235

sp. sp. M. columbi M. columbi M. cf. columbi sp.

No Yes No Yes Yes No

37.37 40.72

110.87 111.5

1280 1952

5279 6307

M. columbi M. columbi

No No

48.1

123.03

0

5147

sp.

No

20.38 21.28 26.5 23.82

89.75 102.08 101 101.72

60 1978 1001 1700

2241 4255 3837 4249

No M. cf. meridionalis No M. columbi

No Yes No Yes

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236 Table 2 (continued ) Locality

Latitude

Longitude

Elevation in meters

EE

Mammoth present

Mastodon present

Santa Cruz Aguiahua Tajo de Tequixquiac Tequesquinahua Valsequillo Venustianao

19.2 20 19.6 19 20.1

98.3 99.8 99.2 98 104.7

2220 2225 2280 2050 560

4274 4365 4377 4083 2711

? M. cf. imperator sp. M. columbi No

? Yes No Yes No

Table 3 State by state comparison of association between Mammuthus sp. and Paramylodon in the Rancholabrean State

Number of localities with Mammuthus only

Number of localities with Paramylodon only

Number of localities with both genera

Simpson coefficient C/N1

Dice coefficient 2C/(N1+N2)

Jaccard coefficient C/(N1+N2C)

Arizona California Colorado Florida Idaho Kansas New Mexico Texas Utah Colorado Plateau Total IRV Total RLB

34 172 11 14 44 139 36 74 34 41 19 371

0 28 5 8 3 5 0 0 0 0 6 40

2 22 5 5 10 5 5 6 2 3 12 57

1.00 0.44 0.50 0.63 0.77 0.5 1.00 1.00 1.00 1.00 0.67 0.59

0.11 0.18 0.38 0.31 0.30 0.06 0.24 0.14 0.11 0.12 0.49 0.24

0.06 0.10 0.24 0.19 0.18 0.03 0.14 0.08 0.06 0.07 0.32 0.14

C ¼ number of localities with the two genera in common, N1 is the number of localities for the less common genus, N2 is the number of localities for the more common genus.

faunas, only North American records have been used in the following analysis. The few Central American records are important, however, in documenting the most southerly overlap of range between mylodonts and mammoths. In Table 3 there is a major discrepancy in the number of records of Mammuthus and Paramylodon for the North American Rancholabrean where their distributions overlap. The ratio of Mammuthus to Paramylodon records is 4.4–1 in the Rancholabrean, but is only 1.7–1 in the Irvingtonian, possibly reflecting the overall smaller sample size. Given the inverse relationship between the body size and population density in mammals, an a priori assumption is that Paramylodon should be more common than Mammuthus since its body size in the Rancholabrean is only 13% that of the mammoth. As mentioned above, the differences in their relative abundance may reflect differences in the social structure of the two species. Organizing the data by states provides a random sampling, since state boundaries are arbitrary and independent of the original distribution of the two species. As such, each state can be considered an independent subsample of the entire data set which allows one to an examine whether there is a consistent pattern across the area of distributional overlap of the two genera, or whether the degree of

association varies geographically or merely reflects a sampling bias for a particular area. On the other hand, their association may be defined by physiographic features. Agenbroad and Mead (1989) documented the presence of mammoths on the Colorado Plateau. Their compilation included 41 records of Mammuthus and, within this sample there are only three records of Paramylodon, all from faunas that included Mammuthus. As can be deduced from the data presented in Table 3, the different coefficients of association are roughly similar across the range of distributional overlap for the two species. Harris (1985) noted that the distribution of Mammuthus in the west ranged between 4550 and 6200 Equatorial Equivalent (EE) [EE ¼ site’s elevation in meters+(107  latitude of the site)] within the Interior Division (the area west of the Sierra Nevada and Cascades Ranges and east of the Great Plains), although he did note that mammoth on the west coast is found throughout the entire range of elevations. At the lower range of its EE distribution in the Interior Division, mammoth drops out at the base of the Sagebrush Steppe-woodland biome. Fig. 2 shows the distribution of M. columbi and P. harlani by EE and shows there is a similar pattern of distribution for both species by this parameter.

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30 Mammuthus

Number of Localities

25

Paramylodon Both Genera

20 15 10 5 0 20012500

25013000

30013500

35014000

40014500

45015000

50015500

55016000

60016500

65017000

Equatorial Equivalent Fig. 2. Frequency distribution of P. harlani and M. columbi by Equatorial Equivalent.

Mammoths have been traditionally considered grazers and indicative of open habitat. In contrast, mastodons have been traditionally considered browsers with a preference for forest habitat. Both the American Mastodon, Mammut americanum, and Columbian Mammoth, M. columbi, are widely distributed across the North American continent, but there is an inverse relationship in their relative abundance to one another. There is an east-west gradient across North America, with the number of localities containing mastodon highest in the forested, eastern part of the continent and decreasing westward, while the highest concentration of Columbian mammoths is in the west and decreases eastward. This gradient for mammoths was first documented by Agenbroad (1984) and is illustrated on the distribution maps produced by the FAUNMAP Working Group (1994) for the two species. The only records of the Columbian mammoth in eastern North America are from Florida and South Carolina (Faunmap Working Group, 1994), which generally reflects the strong western influence on the Pleistocene fauna of Florida along the Gulf Coast Corridor (Webb and Wilkins, 1984; McDonald, 1985). The presence of mastodon in the west is determined by the distribution of forest habitat; consequently, records of mastodon in the west tend to be at higher elevations (Miller, 1987; Lucas and Morgan, 1997). While there are a few records of Paramylodon at higher elevations, none are associated with mastodons. Limited associations of mammoth with mastodon exist in the southern part of their ranges. Polaco et al. (2001) listed 15 localities containing mastodon in Mexico and at five of these there was co-occurrence with mammoth. Paramylodon was present in three of these faunas with mammoth and mastodon (Table 2)

and one locality has mastodon and Paramylodon but no mammoth. If Paramylodon and M. columbi had similar ecological requirements and consequently shared similar habitats, one would expect a low association between Paramylodon and Mammut. This appears to be true. While both genera were widely distributed in North America during the Rancholabrean and the range of Paramylodon overlaps with the much wider range of mastodon, their association in faunas is less common than that of Paramylodon and M. columbi (Tables 2 and 3). This low association pattern of Paramylodon with Mammut, or the other group of browsing proboscideans (gomphotheres), also existed in the Irvingtonian (Tables 1 and 4). To determine whether the association of Paramylodon with Mammuthus in a fauna is random and reflects a simply chance occurrence resulting from broadly overlapping ranges, Poisson regression models were fit to the data. The extent to which the presence of both genera in a fauna cannot be explained by chance could be attributed to similarity of habitat requirements. As a check on the association as related to feeding strategy (grazer versus browser), two other widespread taxa were included in the analysis; a second grazer, Bison, and a browser, Mammut. A total of 475 records of these four taxa were analyzed based on the FAUNMAP database and supplemented with additional data, including single records of each species not listed in FAUNMAP. There are 16 possible patterns for the co-occurrence of the four taxa; the frequency with which these were found in the data is presented in Table 5. The Poisson regression models were surrogates for multinomial models (Venables and Ripley, 1994). The

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Table 4 State by state comparison of association between Mammut and Paramylodon in the Rancholabrean State

Number of localities with Mammut only

Number of localities with Paramylodon only

Number of localities with both genera

Simpson coefficient C/N1

Dice coefficient 2C/(N1+N2)

Jaccard coefficient C/(N1+N2C)

Arizona California Colorado Florida Idaho New Mexico Oklahoma Utah Total IRV Total RLB

1 88 5 13 4 5 1 2 5 119

2 46 5 3 10 4 1 2 20 72

0 13 0 10 3 1 0 0 5 27

— 0.22 — 0.77 0.43 0.20 — — 0.50 0.38

— 0.16 — 0.56 0.30 0.18 — — 0.29 0.28

— 0.09 — 0.38 0.18 0.10 — — 0.17 0.16

C ¼ number of localities with the two genera in common, N1 is the number of localities for the less common genus, N2 is the number of localities for the more common genus. Table 5 Tabulation of the frequencies of the 16 possible co-occurrences between Paramylodon, Mammuthus, Mammut, and Bison in the late Rancholabrean

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Bison

Paramylodon

Mammut

Mammuthus

Frequency

Model 1

Model 2

0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1

0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1

0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1

0.00 87.00 2.00 9.00 205.00 23.00 3.00 10.00 60.00 27.00 5.00 9.00 9.00 12.00 2.00 12.00

1.78 85.49 0.22 10.51 185.23 29.39 22.77 3.61 57.88 32.06 7.12 3.94 9.80 21.37 1.20 2.63

1.97 85.85 0.03 10.15 200.99 26.19 7.01 6.81 61.32 24.87 3.68 11.13 9.72 12.10 1.28 11.90

The data is based on 475 sites from FAUNAMP database with supplementary data from additional faunas. This data includes sites with a single find and may not be part of a fauna. The last two columns present the predictions of two Poisson Regression models of the association of Paramylodon with Mammuthus, Mammut and Bison. See text for explanation of procedure.

Table 6 Results of Poisson regression analysis for the two models of the association of Paramylodon with Bison, Mammut, and Mammuthus Model

Deviance residual

Degrees of freedom

AIC

Null Model Model 1 Model 2

864.050 77.086 22.971

15 7 4

161.3 113.19

Model 1: Frequency ¼ BisonMammutMammuthus+Paramylodon. Model 2: Frequency ¼ BisonMammutMammuthus+Paramylodon: (Bison+Mammut+Mammuthus).

minimal model (model 1, allowing for all interactions between the non-Paramylodon genera) is compared with model 2, which accounts for interactions between Paramylodon and the other genera. Table 6 summarizes the fit of the models and shows that model 2 is a

substantially better explanation of the data than the minimal model. The first model says that the chance of finding Paramylodon in a fauna does not change with the presence or absence of the other genera while the second

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model says that the presence of Paramylodon in a fauna does depend on the other genera. The predictions of the co-occurrence frequencies based on these models are shown in the last two columns of Table 5. The second model does a much better job accounting for the observed frequencies; it has significantly smaller residual deviance and smaller AIC (Akaike, 1974). AIC is a measure of the quality of models that takes into account both the goodness of fit (measured as likelihood) and the number of parameters used in the model. The smaller the value of AIC, the better is the model’s goodness of fit. Having shown that the presence of Paramylodon is not random with respect to the presence of at least some of the other genera, the nature of the relationships among the genera was assessed by predicting the presence of Paramylodon in terms of the presence and absence of the other genera. This was accomplished using logistic regressions, shown in Table 7, and the analysis shows that the presence of Mammuthus and of Bison, the grazers, make a significant positive contribution to predicting the presence of Paramylodon in a fauna while Mammut, the browser, does not. The presence of all three taxa in a fauna probably reflects a random taphonomic bias such as the accumulation of the fauna near an ecotone at the margin of the preferred habitat of the respective taxa. 2.7. Mammoths and mylodonts and grassland habitat Coevolution between grasses and herbivorous mammals has been mediated by the physical defenses of grasses, since they have relatively low concentrations of absorbable secondary metabolites (Freeland, 1991). These physical defenses include cellulose content and structure, lignin, and silica content. Freeland (1991) suggested that any investigation of reciprocity between the evolution of grasses and herbivores would be most

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profitably conducted on mammals introduced into alien environments. The appearance of large-bodied grazers such as mylodont sloths in the late Pliocene and mammoths in the middle Pleistocene, provides a natural experiment to examine the impact resulting from the appearance of two exotic species with different adaptations to grazing on North American grasslands. Based on a study of equids introduced into Australia, Freeland (1991) noted that these large gramnivores existed at population densities several times higher than would be predicted from either their body size or knowledge of population densities in their native habitat. Higher than expected population density was explained as possibly the result of the absence of significant predation combined with a limited pathogen load. As a result of these high population densities of grazers, there may be a greater selection pressure on grasses to evolve greater resistance to grazing and possibly reciprocal adaptation by the herbivores. McDonald (1995) noted that there was an increase in the depth of the mandible in P. harlani from the Blancan through the Irvingtonian and Rancholabrean reflecting an increase in hypsodonty, presumably an adaptation to more efficient grazing and mastication of grasses. In Mammuthus in North America, there is an increase in the number of enamel plates in the teeth from Irvingtonian to Rancholabrean species, reflecting a structural adaptation to more effective processing of grasses. While the North American grasslands evolved along with native grazers (equids, antilocaprids and some camelids), the appearance of mylodont sloths in North America in the Blancan, mammoths in the Irvingtonian, and bison in the Rancholabrean may have resulted in an ecological overload of the habitat. However, until the Pleistocene extinction event, the different adaptations of these groups permitted accommodation and integration of endemics and exotics into a single fauna.

Table 7 Results of fitting three separate logistic regression models for predicting the presence of Paramylodon in a fauna based solely on the presence/absence of Bison, Mammut or Mammuthus Estimate Bison model Intercept Bison Mammut model Intercept Mammut Mammuthus model Intercept Mammuthus

Standard error

t Value

Pr(4|t|)

0.04196 0.16968

0.01784 0.02827

2.353 6.001

0.0191* 3.8e09***

0.12563 0.02780

0.02216 0.02907

5.669 0.956

2.5e08*** 0.339

0.07080 0.13509

0/01667 0.03115

4.248 4.337

2.60e05*** 1.77e05***

The t value (coefficient estimate divided by standard error) has an approximate t-distribution and P(4|t|) is the significance level at which we may conclude that the estimated coefficient is nonzero. The number of *s indicates the degree of significance with which the estimates are non-zero. Only the effect of Mammut is not significant.

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3. Summary While mylodont sloths first appeared in the Blancan and mammoths did not enter North America until the Irvingtonian, P. harlani and Mammuthus spp. are associated in the earliest Irvingtonian faunas in which mammoths appear. During the Rancholabrean the range of P. harlani closely coincides with M. columbi, but not with that of any of the other species of mammoth. The overlap in the ranges of these two species included not only areas of the United States and Mexico in North America but also Central America. Both groups share many ecological traits such as being gramnivores with a diet dominated by grasses and sedges. While both groups were K-selected, they had different social structures, with mammoths having a family structure similar to that of extant elephants while the sloth tended to be more solitary. Both genera were widespread in North America but the presence of both taxa in a fauna is not random. Poisson analysis of their co-occurrence indicates that the presence of either Mammuthus or Bison in a fauna significantly increases the chances of Paramylodon being present. The mastodon, Mammut does not have a significant role in predicting the presence of Paramylodon in a fauna. This suggests that the Columbian mammoth and Harlan’s ground sloth shared a common habitat. Although both Harlan’s ground sloth and the Columbian mammoth are among the larger members of the Pleistocene fauna in North America and shared a common habitat there does appear to have been niche partitioning. This was probably the result of differences in their digestive physiology, with sloths having a longer transition time of food in the digestive tract allowing for more complete extraction of nutrients while mammoths had a rapid turn-around in consumed vegetation. Reduction in competition for forage between the two species may have also been facilitated by the mammoth’s greater ability to migrate seasonally while sloths were probably year around residents in an area.

Acknowledgements We thank Russ Graham and Bart Weiss of the Denver Museum of Nature and Science for help with data from the FAUNMAP database and museum collections. Pam Owen, University of Texas, Richard Hulbert, Florida Museum of Natural History, Mary Thompson, Idaho Museum of Natural History, Pat Holroyd Museum of Paleontology, University of California, Berkeley, and Gary Morgan, New Mexico Museum of Natural History kindly provided locality information on faunas in their collections. Joaquin Arroyo-Cabrales kindly helped with data from Mexico. Chris Shaw reviewed the paper and

provided critical comments that greatly helped clarify some of the ideas presented.

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