Quaternary International xxx (2015) 1e14
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Quaternary International
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Dietary reconstruction of pygmy mammoths from Santa Rosa Island of
California
Gina M. Semprebon a, *, Florent Rivals b, c, d, Julia M. Fahlke e, William J. Sanders f,
€ hlich h
Adrian M. Lister g, Ursula B. Go
a
Bay Path University, Longmeadow, MA, USA
Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain
Institucio
de Paleoecologia Humana i Evolucio
Social (IPHES), Tarragona, Spain
Institut Catala
d
Area de Prehistoria, Universitat Rovira i Virgili (URV), Tarragona, Spain
e
€tsforschung, Berlin, Germany
Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversita
f
Museum of Paleontology and Department of Anthropology, University of Michigan, MI, USA
g
Natural History Museum, London, UK
h
Natural History Museum of Vienna, Vienna, Austria
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online xxx
Microwear analyses have proven to be reliable for elucidating dietary differences in taxa with similar
gross tooth morphologies. We analyzed enamel microwear of a large sample of Channel Island pygmy
mammoth (Mammuthus exilis) molars from Santa Rosa Island, California and compared our results to
those of extant proboscideans, extant ungulates, and mainland fossil mammoths and mastodons from
North America and Europe. Our results show a distinct narrowing in mammoth dietary niche space after
mainland mammoths colonized Santa Rosa as M. exilis became more specialized on browsing on leaves
and twigs than the Columbian mammoth and modern elephant pattern of switching more between
browse and grass. Scratch numbers and scratch width scores support this interpretation as does the
Pleistocene vegetation history of Santa Rosa Island whereby extensive conifer forests were available
during the last glacial when M. exilis flourished. The ecological disturbances and alteration of this
vegetation (i.e., diminishing conifer forests) as the climate warmed suggests that climatic factors may
have been a contributing factor to the extinction of M. exilis on Santa Rosa Island in the Late Pleistocene.
© 2015 Elsevier Ltd and INQUA. All rights reserved.
Keywords:
Island Rule
Niche occupation
Proboscidea
Tooth microwear
Paleodiet
1. Introduction
1.1. Background
Endemic to the California Channel Islands, the pygmy mammoth
(Mammuthus exilis, Maglio, 1970) was initially discovered on the
Island of Santa Rosa and later on Santa Cruz and San Miguel in the
Channel Island archipelago of California. M. exilis is a small
mammoth considered to be a dwarfed form of its likely ancestor,
the Columbian mammoth (M. columbi), which occupied the mainland of North America (Madden, 1977, 1981; Johnson, 1978). Today,
the Channel Islands are comprised of eight islands (Fig. 1). In the
Late Pleistocene, the four Northern Channel Islands formed a single
* Corresponding author.
E-mail
addresses:
(G.M. Semprebon).
[email protected],
[email protected]
super-island, dubbed Santarosae by Orr (1968) and lay closer to the
mainland than today's Channel Islands. Even during periods of
glaciation in the Pleistocene when sea levels were much lower than
they are today, the islands were separated from the California coast
by a relatively small water gap of around 6.5e8 km (Roth, 1996;
Muhs et al., 2015). As sea levels rose due to the melting of continental ice, 76% of Santarosae disappeared (Johnson, 1972) leaving
only the highest elevations exposed e now known as the islands of
San Miguel, Santa Cruz, Santa Rosa, and Anacapa. Of these modern
islands, all but Anacapa have produced mammoth remains
(Agenbroad, 2001). The breakup of Santarosae is believed to have
taken place about 11,000 cal. BP (Kennett et al., 2008).
Researchers have long been interested in the Channel Islands for
several reasons. First, the islands are part of one of the richest
marine ecosystems in the world and are home to over 150 endemic
species such as the island fox (Urocyon litteralis) e a small fox with
six subspecies each unique to the island it lives on. Hence, the
Channel Islands are often referred to as the North American
http://dx.doi.org/10.1016/j.quaint.2015.10.120
1040-6182/© 2015 Elsevier Ltd and INQUA. All rights reserved.
Please cite this article in press as: Semprebon, G.M., et al., Dietary reconstruction of pygmy mammoths from Santa Rosa Island of California,
Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.10.120
2
G.M. Semprebon et al. / Quaternary International xxx (2015) 1e14
Fig. 1. The Channel lslands and Southern California Coast (Modified from Rick et al. (2012) (Fig. 1)). Dotted line indicates Late Pleistocene shoreline of the “super island” of
Santarosae.
Galapagos. Second, because island species are generally regarded as
more susceptible to human-induced extinctions than those on
continents, and the Channel Islands were initially occupied by
humans during a period of extensive extinction in North America
(and elsewhere) around 13,000 cal. BP (Erlandson et al., 2011), data
from such islands has proven useful for providing context for late
Quaternary extinctions on continents such as the Americas and
Australia (Steadman and Martin, 2003; Wroe et al., 2006). Third,
islands are invaluable for studying evolution and diversification,
including the effects of insularity on fauna (Palombo, 2008) such as
the so-called “Island Rule”. The Island Rule was first stated by Foster
(1964) after comparing numerous island species to their mainland
varieties. He proposed that the body size of a species becomes
smaller or larger depending on the resources available to it in its
environment. The Island Rule posits that certain island species
evolve larger size when predation pressure is relaxed (due to the
absence of some mainland predators), while others evolve smaller
size due to resource constraints regarding availability of food and
land area (Whittaker, 1998; McNab, 2010).
The history of the discovery and excavation of pygmy mammoths from the islands is chronicled in Agenbroad (2001).
Mammoth remains have been known from the Northern Channel
Islands of Santa Rosa, San Miguel, and Santa Cruz since 1856
(Stearns, 1873). A spectacular find was made in 1994 by Park Service
researchers led by Larry Agenbroad (a Santa Barbara Museum
research associate at the time) of a nearly complete adult male M.
exilis skeleton e a mature male of about 50 years of age (Agenbroad,
1998). After this discovery, a thorough pedestrian survey of the
islands using GPS was begun, to document and pinpoint each discovery. More than 160 new localities were recorded with the majority of localities found on Santa Rosa Island (Agenbroad et al.,
2007). Several mainland-size mammoth (Mammuthus columbi) elements in addition to remains of M. exilis were recovered as a result
of this survey. The survey of Santa Rosa revealed a ratio of
approximately 3:10 large mammoth:small mammoth remains
(Agenbroad, 2012).
As many as three species ostensibly of different sizes have been
proposed by researchers over the years (Orr, 1956a,b, 1968; Roth,
1982, 1996). However, Agenbroad (2009) showed convincingly,
using less fragmentary material, that only two species are likely
present e M. columbi and M. exilis (Fig. 2).
There has been much speculation regarding the origin of
mammoths on the Northern Channel Islands, and the finding of
Columbian mammoth remains is intriguing given that these remains may represent remnants of an ancestral population, unless
they represent later migrants to the island. The oldest remains are
found in the basal conglomerate of the Garanon Member of the
Santa Rosa Island Formation. U/Th results formerly suggested an
age of at least 200 ka (Orr, 1968), but these are not now considered
reliable and new U/Th data indicate an age of ca. 80 ka (Muhs et al.,
2015). Mammoth remains attributed to both M. columbi and M.
exilis have been found throughout the entire Formation (the latest
calibrated direct radiocarbon date on M. exilis being 10,700 ± 90 BP
(B-14660), equivalent to c. 12,600 cal BP (Agenbroad, 2012).
Also intriguing is the speculation as to how mainland mammoths arrived on the islands. Initially, it was assumed that the
ancestral form of M. exilis was either Mammuthus (formerly Archidiskodon) imperator or Mammuthus columbi. It has now been shown
that M. imperator and M. columbi are conspecific (Slaughter et al.,
1962; Miller, 1971, 1976; Agenbroad, 2003) and further research
has suggested that M. columbi represents the likely ancestor of M.
exilis (Johnson, 1978; Madden, 1981; Roth, 1982, 1996). This suggests that M. exilis evolved according to the Island Rule (Foster,
1964). That is, a large continental species (M. columbi) adapted to
an island environment becoming in time a new smaller species e
M. exilis (Fig. 2).
The question remains e how did Columbian mammoths reach
the islands? Historically, it was assumed that ancestral mammoths
could not have swum to the islands. Consequently, various land
bridges linking the Northern Channel Islands to the mainland have
long been hypothesized (e.g., Clements, 1955; Van Gelder, 1965;
Valentine and Lipps, 1967; von Bloeker, 1967; Remington, 1971).
This early idea, that insular mammoth remains proved the existence of a land bridge, persisted for many decades (see synopsis by
Johnson, 1978). The idea was deeply entrenched but began to give
way in light of accumulating geological and biological evidence
(Savage, 1967; Johnson, 1978). For example, Johnson (1972, 1978)
pointed out that a land bridge was not a sine qua non for explaining mammoths on the Northern Channel Islands, citing research
that elephants are excellent distance swimmers, among the best of
all land mammals and highly skilled at crossing water gaps. Hence,
currently it is now understood that sea-level fall brought the
Please cite this article in press as: Semprebon, G.M., et al., Dietary reconstruction of pygmy mammoths from Santa Rosa Island of California,
Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.10.120
G.M. Semprebon et al. / Quaternary International xxx (2015) 1e14
3
Fig. 2. Comparative sizes of the mainland mammoth (Mammuthus columbi) and the pygmy mammoth of the Channel Islands (Mammuthus exilis). Modified from Agenbroad (2012)
(Fig. 2). Estimated height data from Agenbroad (2009).
islands only a few kilometers from the shore, enabling mainland
mammoths an opportunity to gain access even if a land bridge was
not present.
Today the local vegetation of the Northern Channel Islands is
dominated by grasslands and shrublands with some patchy
woodlands, and mild temperatures with little annual temperature
fluctuation (Junak et al., 2007). To date, studies of the vegetation
history of the island have included pollen analyses on Santa Rosa
Island (Cole and Liu 1994; Kennett et al., 2008), San Miguel Island
(West and Erlandson, 1994), continuous offshore pollen cores from
the Santa Rosa Basin (Heusser, 1995, 2000) and paleobotanical
studies on Santa Cruz Island (Cheney and Mason, 1930). During the
Late Pleistocene, when these islands existed as the single superisland of Santarosae, forest vegetation was present including cypress, pine, and Douglas fir (Cheney and Mason, 1930; Anderson
et al., 2008). Climatic changes at the end of the Pleistocene have
been invoked as the cause for the loss of coastal forest (Anderson
et al., 2008), and major fires coincident with the last known
occurrence of pygmy mammoths (around 13,000 cal BP) have been
demonstrated (Kennett et al., 2008). Anderson et al. (2010) combined pollen analysis with an analysis of the fire disturbance
regime of Santa Rosa Island and reported that a coastal conifer
forest covered the highlands of Santa Rosa during the last glacial
but that by ca. 11,800 cal. BP Pinus stands, coastal sage scrub and
grassland replaced the forest as the climate warmed.
1.2. Aims of the study
The purpose of this study is twofold: 1) to explore the paleodietary ecology of Mammuthus exilis from Santa Rosa Island, California using stereomicrowear analysis, and 2) to compare
microwear patterns of M. exilis to those of extant elephants and to
other mammoths and mastodons from North America and Europe,
especially its ancestor M. columbi. To test the hypothesis that M.
exilis was exploiting the forest vegetation during the Late Pleistocene, we employed light microscopy dental microwear on a large
sample of pygmy mammoth teeth from Santa Rosa Island. If forest
vegetation including woody browse was exploited on Santa Rosa
Island during the Late Pleistocene, we predict that: 1. M. exilis
would exhibit more scratches in the low scratch range (i.e., between 0 and 17) typical of extant mammals that feed on leaves; 2.
Some individuals of M. exilis would display the unusually deep and
wide scratches (i.e., hypercoarse scratches) found in extant
proboscideans and other mammals known to ingest large quantities of twigs and bark.
1.3. Microwear
Microwear has been employed for over three decades as a
technique to visualize scars in dental enamel caused by food items
such as plant phytoliths or by exogenous grit or dust coating the
surface of vegetation (e.g., Rensberger, 1978; Walker et al., 1978;
Solounias and Semprebon, 2002; Semprebon et al., 2004;
Merceron et al., 2004, 2005; Scott et al., 2005, 2006; Ungar et al.,
2008, 2010) including studies of mammoths and other proboscideans (Rivals et al., 2012, 2015). Microwear turns over relatively rapidly, thus giving insight into the dietary behavior of the
last days, or weeks, before an animal's death e the so-called “Last
Supper Effect” (Grine, 1986). As such, it provides a window into the
short-term dietary behavior of a taxon as opposed to the deep-time
adaptation of gross tooth morphology, giving insight into what a
taxon was actually eating despite what it might originally have
been adapted to eat (Rivals and Semprebon, 2011). Importantly, this
snapshot into the habitat and dietary behavior of a taxon is largely
taxon-independent.
While microwear analyses have proven to be very reliable in
diagnosing dietary behavior of mammals, two considerations are
relevant to all microwear approaches: 1) potential error involved in
the analyses, and 2) the causal factors producing microwear scars.
Different microwear methodologies have different strengths and
weaknesses, but all have value depending on questions being
studied. The three methods currently used involve scanning electron microscopy (SEM), light microscopy for dental microwear
(LDM), and dental microwear texture analysis (DMTA). Because
SEM and LDM rely on observer measurements, while DMTA employs an automated approach, studies have been done to identify
and/or quantify both inter- and intra-observer error involved in
each (e.g., Grine et al., 2002; Galbany et al., 2005; Fraser et al., 2009;
Mihlbachler et al., 2012 DeSantis et al., 2013; Williams and Geissler,
2014; Hoffman et al., 2015).
While these studies have provided valuable insight regarding
elucidating potential error in microwear studies, it is important to
not generalize results utilizing one methodology to another. Thus,
microwear error studies aimed at testing LDM (other than
Semprebon et al., 2004), while adding to the cannon of knowledge,
have not duplicated the original LDM methodological regime (i.e.,
Please cite this article in press as: Semprebon, G.M., et al., Dietary reconstruction of pygmy mammoths from Santa Rosa Island of California,
Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.10.120
4
G.M. Semprebon et al. / Quaternary International xxx (2015) 1e14
have employed different magnifications, used different counting
areas, have not counted all features visible, and/or assessed microwear from photographs e sometimes using untrained observers).
The original methodology developed by Semprebon, 2002 and
Solounias and Semprebon, 2002 (and utilized in this study) involves counting pits and scratches directly using a stereoscope at 35
times magnification following the same standard microscope calibration and systematic counting methods employed every day in
diagnostic medicine and life science research to accurately count
substances such as blood cells and platelets using light microscopy
(where differential light refraction as used in this study is used to
identify the objects being counted).
For example, the procedure used to construct the comparative
extant ungulate and proboscidean databases (Semprebon, 2002;
Rivals et al., 2012) and to analyze the fossil proboscideans used in
this study relies on employing a calibrated ocular reticle subdivided
into 9 subsquares which greatly enhances the ability to not lose
track of features counted rather than attempting to count numerous
features scattered within a larger, undivided field of view. Additionally, features are counted according to a strict protocol that
ensures that features present are not skipped over or counted more
than once (Estridge and Reynolds, 2012). Also, a relatively large
counting area is employed and features are counted twice (to
reduce error due to potential variable scar distribution). When this
protocol is followed, both interobserver and intraobserver error has
been demonstrated to be low (Semprebon et al., 2004).
In some studies, however, feature counts have been made from
images using much higher magnifications than used in Semprebon
et al. (2004) and counting error would naturally be expected to be
higher due to the greater number of features visible at higher
magnifications. For example, Mihlbachler et al. (2012) reported that
interobserver error was higher than that reported by Semprebon
et al., 2004, although evidence was found (2012) that error diminished significantly with increased experience of the observer and
data collected by all observers was highly correlated (Mihlbachler
et al., 2012). Also, while DMTA has been employed to extract 2D
microwear data from photosimulations of scanned areas of DMTA,
(e,g., DeSantis et al., 2013), stereomicroscopy was invented to provide depth of field and a three dimensional visualization of an object
being examined and thus is widely used in microsurgery, dissection,
forensics, medicine, industry, and quality control. In microwear
analysis as employed here and in Solounias and Semprebon (2002)
and Semprebon et al. (2004), stereomicroscopy allows relative
depth and 3D shape of different features to be assessed.
While fully automated methodologies such as DMTA have been
developed and have proven useful to reduce subjectivity and
interobserver error (Ungar et al., 2003; Galbany et al., 2005) it is
important to note, that DMTA reduces interobserver error only
when the same exact location on the same tooth surface is scanned
twice, as results obtained on the same tooth and same facet may be
quite different when different regions of that tooth and facet are
scanned (see Fig. 8 in Scott et al., 2006). This difference is due to the
natural heterogeneity of microwear on tooth surfaces, a propensity
which affects all microwear methodologies but is more problematic
at higher versus lower magnifications and when only small areas of
a tooth are analyzed. If relatively low magnification and a large
counting area is assessed such as in the Solounias and Semprebon
(2002) methodology, both inter- and intra-observer error may be
minimized (see Williams and Geissler, 2014).
Regardless, the effects of potential error should be evaluated
within the context of whether accurate dietary categorization of
living mammals with known diets can be obtained by single observers. A variety of different variations on the original LDM
methodology have produced great fidelity in terms of partitioning
living mammals (e.g., carnivores, cetaceans, proboscideans,
primates, marsupials, ungulates, and rodents) into their known
trophic groups (Semprebon et al., 2004; Merceron et al., 2004,
2005; Rivals et al., 2007, 2012; Townsend and Croft, 2008; Fraser
et al., 2009; Williams and Patterson, 2010; Fraser and Theodor,
2011, 2013; Williams and Holmes, 2011; Bastl et al., 2012; Fahlke
et al., 2013; Christensen, 2014; Williams and Geissler, 2014).
The causal factors involved in producing microwear scars are
only beginning to be understood. Microwear patterns must be
influenced by the differing physical properties of enamel and
ingested abrasives such as biogenic silica (i.e., phytoliths) from
plants, or abiotic silica from dust or grit. While it is also possible
that differing tooth morphologies and masticatory biomechanics
may impact the nature of microwear scars, a similar pattern of
microwear scars have been observed across various orders of
mammals consuming similar dietary items which seems to argue
that such effects are relatively minor (e.g., Semprebon et al., 2004)
although further study is needed. A number of studies have tested
the effects of abrasives in producing microwear scars and have
expanded our thinking away from considering phytoliths alone as
contributing to microwear scar topography (Covert and Kay, 1981;
Kay and Covert, 1983; Maas, 1991, 1994; Gügel et al., 2001;
Mainland, 2003; Sanson et al., 2007; Lucas et al., 2013; Schulz
et al., 2013; Müller et al., 2014; Hoffman et al., 2015). Studies
examining the relative hardness of enamel versus phytoliths have,
however, produced contradictory results (Baker et al., 1959; Sanson
et al., 2007; Rabenold and Pearson, 2014; Xia et al., 2015). The most
promising studies regarding teasing apart causal factors involve
controlled feeding experiments. These studies have elucidated
patterns in such widely diverse taxa as American opossums (Covert
and Kay, 1981; Kay and Covert, 1983), rabbits (Schulz et al., 2013),
and ungulates (Hoffman et al., 2015). Such studies have documented an increase in scratch numbers in rabbits when their diet
contained grass with more biogenic abrasives (Schulz et al., 2013)
and in opossums that were fed fine-grained pumice (Covert and
Kay, 1981; Kay and Covert, 1983). Hoffman et al. (2015) provided
the controlled feeding experiment with domesticated sheep, and
tested the effects of abiotic abrasives of different sizes. This study
reported a significant increase in pit features that was correlated
with an increase in grain size. This observed grit effect supports the
interpretation of increased grit consumption by extant ungulates
from semi-arid and arid environments due to increased pitting
observed in these forms (Solounias and Semprebon, 2002).
LDM (after Semprebon, 2002; Solounias and Semprebon, 2002)
was used in this study as it allows for the attainment of relatively
large sample sizes (numbers of specimens), and a very large
comparative stereomicrowear database exists which is comprised
of extant artiodactyls, perissodactyls, and proboscideans of known
diets compiled by a single observer (GMS). Relatively few studies
have concerned themselves with proboscidean microwear patterns
(examples are Palombo and Curiel (2003), Palombo (2005), Green
et al. (2005), Todd et al. (2007) and Calandra et al. (2008, 2010)).
Rivals et al. (2012, 2015) studied dietary diversity patterns in
Pleistocene representatives of Mammut, Anancus, Palaeoloxodon
and Mammuthus from a variety of localities in Europe and North
America. In this study, we test the effect of insular dwarfism on
dietary niche occupation by comparing Pleistocene mammoths
from the North American mainland with island mammoths from
the Channel Islands off the coast of California.
2. Materials and methods
2.1. Sample
Molar teeth of Mammuthus exilis from the Channel Islands of
California were sampled from the collections of the Santa
Please cite this article in press as: Semprebon, G.M., et al., Dietary reconstruction of pygmy mammoths from Santa Rosa Island of California,
Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.10.120
G.M. Semprebon et al. / Quaternary International xxx (2015) 1e14
5
intensity fiber-optic light source (Dolan-Jenner) was directed across
the surface of casts at a shallow angle to the occlusal surface.
The mean number of pits (rounded features) versus mean
number of scratches (elongated features) per taxon were counted
within a 0.4 mm square area (0.16 mm2) by GMS. It was also noted if
more than four large pits (i.e., deep pits with low refractivity that
are at least twice the diameter of highly refractive small pits) were
present or absent per microscope field (within the 0.4 mm square
area) and whether gouges (i.e., depressions with ragged, irregular
edges that are 2e3 times larger than large pits) were present or
absent (after Semprebon, 2002; Solounias and Semprebon, 2002).
Results were compared with extant ungulate and proboscidean
microwear databases to determine the dietary categories of
browser versus grazer (Semprebon, 2002; Solounias and
Semprebon, 2002). Scratch textures were assessed as being either
fine, coarse, hypercoarse, a mixture of fine and coarse, or a mixture
of coarse and hypercoarse scratch types per tooth surface using the
differential light refractivity of each type of scratch to categorize
them (after Semprebon et al., 2004). In addition, a scratch width
score (SWS) was obtained by assigning a score of 0 to teeth with
predominantly fine scratches per tooth surface, 1 to those with a
mixture of fine and coarse types of textures, 2 to those with predominantly coarse scratches, 3 to those with a mixture of coarse
and hypercoarse textures, and 4 to those with predominantly
hypercoarse scratches. Univariate statistics, ANOVA, and Tukey's
post-hoc test for honest significant differences were calculated
using PAST software (Hammer et al., 2001).
Because extant seasonal mixed feeders and those that alternate
their diet when migrating to different regions alternately feed on
browse and grass may overlap the browsing or grazing mean
scratch/pit morphospaces, the percentage of individuals within a
taxon that possess raw scratch values falling into the low scratch
range (i.e., between 0 and 17) were calculated, since extant leaf
browsers, grazers, and mixed feeders show distinctive and
discriminating low scratch range patterns (Semprebon, 2002;
Semprebon and Rivals, 2007).
Fig. 3. Mandible of Mammuthus exilis from Santa Rosa Island, California (Specimen
from Santa Barbara Museum of Natural History e catalogue number SBMNH-VP 1078).
Arrow depicts area sampled for microwear analysis (i.e., the central portion of the
central enamel bands of the grinding facet of the occlusal surface). Scale bar ¼ 5 cm.
Barbara Museum of Natural History, California. The specimens
were screened under a stereomicroscope for potential taphonomic alteration of dental surfaces. Those with badly preserved
enamel or taphonomic defects (i.e., features displaying unusual
morphologies or fresh features made during the collecting process or during storage) were removed from the analysis
following King et al. (1999). We also excluded slightly or heavily
worn teeth.
Fifty-one M. exilis adult molar casts from Santa Barbara Island,
two from San Miguel Island, and one from Santa Cruz Island, as well
as one tooth of M. columbi from Santa Rosa Island, were determined
to be suitable for analysis and subjected to microwear analysis by a
single observer (GMS) who also analyzed the extant ungulate and
proboscidean comparative samples.
3. Results
3.1. Comparative extant proboscidean microwear
Results of the microwear analysis are shown in Table 1. It is clear
from Fig. 4A that extant Loxodonta africana and Elephas maximus
exhibit a varied dietary behavior e taking both grass and browse,
whereas the forest elephant, Loxodonta cyclotis, exhibits less variability and is more focused on the browse end of the spectrum
(note, however, that fewer specimens were available for L. cyclotis
than for the others). This corroborates what is known from field
studies (Sukumar, 2003).
2.2. Microwear technique
Tooth surfaces were cleaned, molded, cast and examined at 35
times magnification with a Zeiss Stemi-2000C stereomicroscope
after the methodology of Solounias and Semprebon (2002). The
analysis was made from the central portion of the central enamel
bands of the occlusal surface (Fig. 3). Microscopic scars were
visualized using external oblique illumination. An M1-150 high-
Table 1
Microwear summary data for the second molars of extant and fossil proboscidean samples.
Species
N
Pit mean Pit SD Scratch mean Scratch SD SWS % LP
Loxodonta africana
33 22.9
Loxodonta cyclotis
6 29.8
Elephas maximus
10 20.9
Mammuthus exilis e Santa Rosa Island
51 26.9
Mammuthus exilis e San Miguel Island
2 24.0
Mammuthus exilis e Santa Cruz Island
1 22.0
Mammuthus columbi e Santa Rosa Island
1 31.5
3.9
4.0
2.6
5.99
e
e
e
17.4
12.9
18.3
10.78
7.8
8.5
19.5
5.3
5.1
4.3
4.59
e
e
e
3.1
2.8
3.1
2.0
4.0
1.0
2
54.6
50.0
70.0
37.3
50.0
0.0
100
%G
%F
36.4
33.3
50.0
13.7
0.0
0.0
0
0.0
0.0
0.0
0.0
0.0
16.7
0.0
0.0
0.0
7.84
11.77
33.33
0.0
0.0
0.0
0.0
0.0
100
0
100
0
%C
%M
% CH
%H
% 0e17 S Diet
87.9
12.1
39.4
66.7
16.7
83.3
90.0
10.0
40.0
47.06
0.0
94.1
0.0
100
100
0.0
0.0 100
0
0
0
MF
B
MF
B
B
B
G
Abbreviations: N ¼ Number of specimens; Pit Mean ¼ Mean number of pits; Pit SD ¼ standard deviation of Pits; Scratch ¼ Mean number of scratches; Scratch SD ¼ standard
deviation of scratches; SWS ¼ Mean Scratch width score from 0 to 4 (0 ¼ fine scratches only, to 4 ¼ hypercoarse scratches only; %LP ¼ Percentage of individuals per taxon with
large pits; %G ¼ Percentage of individuals per taxon with gouges; %F ¼ Percentage of individuals per taxon with fine scratches; %C ¼ Percentage of individuals per taxon with
coarse scratches; %M ¼ Percentage of individuals per taxon with a mixture of fine and coarse scratches; %CH ¼ Percentage of individuals per taxon with a mixture of coarse and
hypercoarse scratches; %H ¼ Percentage of individuals per taxon with hypercoarse scratches; %0-17S ¼ low scratch percentage (i.e., Percentage of individuals per taxon with
scratch counts between 0 and 17; B ¼ leaf browser; MF ¼ mixed feeder; G ¼ grazer. All extant and extinct specimens were scored by GMS.
Please cite this article in press as: Semprebon, G.M., et al., Dietary reconstruction of pygmy mammoths from Santa Rosa Island of California,
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3.2. Pygmy mammoth molar microwear compared with extant
proboscideans
The raw scratch distribution of M. exilis is skewed more toward
the low scratch browsing range, much like that of Loxodonta cyclotis
(Fig. 4B), indicating a less heterogeneous dietary regime than that
found in L. africana or E. maximus. Two teeth of M. exilis from San
Miguel Island and one from Santa Cruz Island were available and
these specimens also plot within the extant ungulate browsing
ecospace.
An ANOVA (Table 2) indicates that M. exilis scratch numbers are
significantly different (i.e., lower) from those of L. africana (Tukey's
HSD Test, p ¼ 0.0047) and E. maximus (Tukey's HSD Test,
p ¼ 0.0011) but are not significantly different from scratch numbers
of L. cyclotis (Tukey's HSD Test, p ¼ 0.6844). Also, M. exilis pit
numbers are significantly different (i.e., higher) from those of E.
maximus (Tukey's HSD Test, p ¼ 0.0189) but not significantly
different from those of L. africana (Tukey's HSD Test, p ¼ 0.193) or
L. cyclotis (Tukey's HSD Test, p ¼ 0.4508).
Table 2
ANOVA and Tukey's HSD test results for differences in numbers of pits, numbers scratches, and scratch width score. Significant differences are indicated in bold.
Number of scratches
ANOVA results e Test for Equal Means:
Sum of Squares
df
Mean square
Between groups:
1107.22
3
369.075
Within groups:
2259.03
96
23.5315
Total:
3366.25
99
Tukey's honest significance test: Pair-wise comparisons e q values below the diagonal, p values above the diagonal.
L. cyclotis
L. africana
E. maximus
Loxodonta cyclotis
e
0.0972
0.0319
Loxodonta africana
3.303
e
0.9688
Elephas maximus
3.945
0.6417
e
Mammuthus exilis
1.57
4.873
5.514
F
15.68
p
<0.001
M. exilis
0.6844
0.0047
0.0011
e
Number of pits
ANOVA results e Test for Equal Means:
Sum of Squares
Between groups:
636.545
Within groups:
2428.51
Total:
3065.06
Tukey's honest significance test:
L. cyclotis
Loxodonta cyclotis
e
Loxodonta africana
4.936
Elephas maximus
6.313
Mammuthus exilis
2.099
df
3
96
99
Mean square
212.182
25.297
F
8.388
L. africana
0.0041
e
1.377
2.837
E. maximus
0.0002
0.7648
e
4.214
M. exilis
0.4508
0.193
0.01893
e
df
3
96
99
Mean square
10.2437
0.689884
F
14.85
L. africana
0.8198
e
0.0908
4.882
E. maximus
0.8511
0.9999
e
4.791
M. exilis
0.0544
0.0046
0.0056
e
df
3
96
99
Mean square
0.577029
0.182385
F
3.164
L. africana
0.998
e
1.135
1.884
E. maximus
0.7607
0.8531
e
3.019
M. exilis
0.6572
0.5448
0.1496
e
df
3
96
99
Mean square
0.402204
0.24691
F
1.629
L. africana
0.9957
e
1.106
1.237
E. maximus
0.7431
0.8626
e
2.342
M. exilis
0.9172
0.818
0.3526
e
p
<0.001
Scratch width score
ANOVA results e Test for Equal Means:
Sum of Squares
Between groups:
30.7311
Within groups:
66.2289
Total:
96.96
Tukey's honest significance test:
L. cyclotis
Loxodonta cyclotis
e
Loxodonta africana
1.232
Elephas maximus
1.141
Mammuthus exilis
3.65
p
<0.001
Gouges
ANOVA results e Test for Equal Means:
Sum of Squares
Between groups:
1.73109
Within groups:
17.5089
Total:
19.24
Tukey's honest significance test:
L. cyclotis
Loxodonta cyclotis
e
Loxodonta africana
0.2522
Elephas maximus
1.387
Mammuthus exilis
1.632
p
0.0280
Large pits
ANOVA results e Test for Equal Means:
Sum of Squares
Between groups:
1.20661
Within groups:
23.7034
Total:
24.91
Tukey's honest significance test:
L. cyclotis
Loxodonta cyclotis
e
Loxodonta africana
0.3252
Elephas maximus
1.431
Mammuthus exilis
0.9117
p
0.1877
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7
Among Mammuthus exilis from Santa Rosa Island (Table 1,
Fig. 5A), 94.1% of individuals have scratches that fall between 0 and
17, a pattern typical of browsing ungulates and those of the extant
Loxodonta cyclotis individuals analyzed here, but distinctive from
extant species with a greater breadth of scratch values such as
ungulate mixed feeders, Loxodonta africana and Elephas maximus.
When additional microwear variables are considered, Mammuthus exilis (Fig. 6), exhibits some distinct patterns from those of
extant elephants. Differences in large pitting and gouging (Fig 6A)
are not statistically significant when M. exilis is compared to living
elephants (Table 2) but there are statistically significant scratch
textural differences between M. exilis and Loxodonta africana and
Elephas maximus (Fig. 6B), while scratch textural differences between M. exilis and Loxodonta cyclotis are not statistically significant
(Table 2). Scratch texture is a reflection of differences in widths and
depths of scratches e i.e., fine scratches are the narrowest and
shallowest while hypercoarse scratches are the widest and deepest.
Thus, the enamel surface of M. exilis typically displays finely
textured and mixtures of finely textured and coarse scratches, and
fewer mixtures of coarse and hypercoarse scratches or purely
hypercoarse scratches than seen in the African savannah and Asian
elephants (Table 1, Fig. 6B).
Fig. 7 shows that typical extant leaf-dominated ungulate
browsers as a group have scratch width scores that range between
0 and 0.46 (i.e., they possess mostly finely textured scratches);
whereas typical grazers range from 0.70 to 1.67 (i.e., they have
fewer fine and more coarse scratches) and do not overlap leaf
browsers. Mixed feeders that consume browse and grass regionally
or seasonally predictably overlap both leaf browsers and grazers
and range from 0 to 1.14 (data from Semprebon, 2002). What is
striking about Fig. 7, is that both modern proboscideans and M.
exilis are truly unique in terms of their larger mean scratch width
scores when compared to other herbivorous mammals studied
with stereomicrowear data thus far (but note that M. exilis has
narrower scratches than modern proboscideans).
3.3. Pygmy mammoth microwear compared to mainland
mammoths and mastodons
Fig. 4. A. Bivariate plot of the number of pits versus the number of scratches for the
extant proboscideans analyzed graphed in relation to Gaussian confidence ellipses
(p ¼ 0.95) on the centroid for extant ungulate browsers (B) and grazers (G) (convex
hulls) adjusted by sample size (ungulate data from Semprebon (2002) and Solounias
and Semprebon (2002); extant proboscidean data compiled by GMS). B. Bivariate
plot showing the raw scratch and pit distribution of M. exilis compared to that of extant
ungulates from Semprebon (2002) and Solounias and Semprebon (2002). C. Mean
scratch versus mean pit results for M. exilis from Santa Rosa Island compared to extant
ungulates (convex hulls), and living and fossil proboscidean mean scratch/pit values.
Key: 1 ¼ Mammut americanum (Phosphate Beds, SC USA); 2 ¼ M. americanum
(Ingleside, San Patricio Co., TX, USA); 3 ¼ M. americanum (Various Localities, FL,USA);
4 ¼ M. exilis, Santa Rosa Island, CA, USA; 5 ¼ M. columbi (Phosphate Beds, SC, USA);
6 ¼ M. columbi (Santa Rosa Island, CA USA); 7 ¼ M. columbi (Ingleside, San Patricio Col,
Mammuthus exilis clearly has mean scratch and pit results nearly
identical to the forest elephant (Loxodonta cyclotis) and distinct
from L. africana and Elephas maximus. Mammuthus exilis has mean
scratch and pit results that are more similar to those found in prior
studies for the mastodon Mammut americanum than those found in
mainland mammoths (Figs. 4C and 5B), the latter taxa typically
displaying values either intermediate between the browsing and
grazing extant morphospaces, or skewed towards grazing,
depending on the species (Green et al., 2005; Rivals et al., 2012).
In addition, an examination of enamel surfaces (Fig. 8) suggests
a more attritive diet for M. exilis compared to most other mammoths studied. Note the relatively polished, flat, and shiny enamel
surface with relatively few food scars in M. exilis compared to the
relatively dull and more irregular enamel surface with many food
scars in M. primigenius. Fortelius and Solounias (2000) provide a
comprehensive study and discussion of attritive versus abrasive
molar wear due to mastication of various types of food items. Taxa
that consume relatively soft food items such as browse experience
much tooth-on-tooth (attritive) polishing of opposing wear facets
as molars cut completely through relatively soft food and produce
TX, USA); 8 ¼ M. columbi (Quarry G., Sheridan Co., NE, USA); 9 ¼ M. columbi (Grayson,
Sheridan Co., NE, USA); 10 ¼ M. primigenius (Fairbanks Area, AK, USA);
11 ¼ M. primigenius (Brown Bank, North Sea); 12 ¼ M. trogontherii (Crayford - Slade
Green, UK); 13 ¼ M. trogontherii (Ilford, UK). Data: 1e2, 5, 7e14 from Rivals et al.
(2012); 3 from Green et al. (2005); 4, 6 from this study).
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G.M. Semprebon et al. / Quaternary International xxx (2015) 1e14
Fig. 5. Box plot of the percentage of scratches per taxon that fall within the low scratch range (i.e., between 0 and 17). Boxes represent results from extant ungulates. A. Low scratch
range scores for Mammuthus exilis (from Santa Rosa Island) compared to results for extant ungulates and extant proboscideans. B. Low scratch range scores for mainland mammoths
(key as in Fig. 7 legend). The box represents the central 50% of the scratch values and the bars represent the range (minimal and maximal values).
relatively polished and flat enamel facets. Taxa that consume
relatively abrasive foods such as grass experience more abrasive
food-on-tooth wear which leads to a less polished and more
irregular enamel surface.
4. Discussion
4.1. Pygmy mammoth microwear compared to extant elephants
In this study, M. exilis exhibited a microwear scratch pattern
similar to (and not significantly different from) that of Loxodonta
cyclotis (i.e., scratch results concentrated toward the browsing
morphospace), in contrast to that of L. africana or Elephas maximus (scratch results that extend more into the mixed feeding and
grazing zones). These results are consistent with known differences in habitat and dietary habits of L. cyclotis, L. africana and E.
maximus.
Loxodonta cyclotis (African forest elephant), found today in
western and central Africa, mainly occupies lowland tropical rainforests, swamps, and semi-evergreen and semi-deciduous tropical
rainforests. Forest elephants are known to change habitats
seasonally, occupying lowland rainforest during the wet season and
moving to swampy areas during the dry season (Merz, 1986a,b; Fay
and Agnagna, 1991; Dudley et al., 1992; White et al., 1993; Tangley,
1997). Loxodonta cyclotis subsists mainly on leaves, bark, fruit, and
twigs of rainforest trees and varies its diet regionally according to
the availability of trees and fruits (White et al., 1993; Tangley, 1997;
Eggert et al., 2003). An extensive seven-year observational study of
Reserve in Gabon (White et al., 1993)
L. cyclotis from the Lope
determined that 70% of the diet of L. cyclotis in terms of the number
of species and amounts eaten came from leaves and bark.
In contrast, Loxodonta africana (African savannah elephant) and
Elephas maximus (Asian elephant) are more generalist feeders,
relying on more widely distributed resources including grasses,
shrubs, herbs, palms and vines and broadleaved trees (Nowak,
1999; Osborn, 2005; Archie et al., 2006; Rode et al., 2006;
Wittemyer et al., 2007). Loxodonta africana extended historically
from north of the Sahara Desert to the southern tip of Africa but is
now found in increasingly fragmented habitats (deserts, savannas,
forests, river valleys and marshes (Estes, 1999). E. maximus is found
today in Southeast Asia from India to Borneo but was formerly
more widely distributed from the Levant, throughout Southeast
Asia, and in China as far north as the Yangtze River. Like L. africana,
E. maximus used a greater variety of habitats in the past than it does
today, as currently ecotones with an interdigitation of grass, forests,
and low woody plants are favored as continuously forested areas
without regular clearings do not support high densities (Shoshani
and Eisenberg, 1982). Both L. africana and E. maximus alternate
between a predominantly browsing strategy with that of grazing
because of seasonal variation of food plants, seasonally changing
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Fig. 6. Bar charts showing percentages of individuals per taxon in M. exilis and extant
proboscideans with large pits and gouges (Fig. 6A) and the percentage of individuals
per taxon of M. exilis and extant proboscideans displaying fine, mixed fine and coarse,
coarse, mixed coarse and hypercoarse and hypercoarse scratch textures (Fig. 6B).
mineral content, and migration habits (Joshi and Singh, 2008;
Mohaptra et al., 2013).
Our microwear results suggest that pygmy mammoths from
Santa Rosa engaged in a fairly restricted dietary regime more like
9
that of the extant L. cyclotis than the more highly migratory L.
africana and E. maximus. Thus, some microwear features inscribed
into enamel of M. exilis, such as scratch numbers and textures and
the appearance of the enamel surfaces themselves, are more
congruent with a less abrasive diet than African savannah and
Asian elephants. The enamel surface of M. exilis is flatter (enamel
band edges sharp and less rounded), more polished and less scarred
than African savannah and Asian elephants and more similar to L.
cyclotis and other predominantly browsing forms (Solounias and
Semprebon, 2002).
Interestingly, no individual of M. exilis exhibits mostly hypercoarse scratches as is seen in some individuals of all species of living
elephants (Solounias and Semprebon, 2002). Hypercoarse
scratches are relatively unusual features that have thus far mainly
been reported in those taxa known to consume appreciable
amounts of bark (Solounias and Semprebon, 2002; Green et al.,
2005) or are known hard-object processors such as primate hardfruit and seed consumers (Semprebon et al., 2004). It is possible
that these scratch textural results are due to bark consumption, as
puncture-like large pits typical of fruit and seed consumers were
not observed. The fact that fewer individuals of M. exilis possess
mixed coarse and hypercoarse or purely hypercoarse scratches than
the African savannah and Asian elephants raises interesting questions regarding the relative importance of bark consumption in
these forms.
It is conceivable, however, that because proboscideans are large
animals with large jaw muscles, they might concentrate more
pressure when biting on a hard object compared to smaller more
selective-feeding mammals and that this, rather than bark consumption, might produce hypercoarse scratches. While this should
not be discounted as a possibility or as a contributing factor,
hypercoarse scratches are not simply deeper than coarse or fine
scratches, they are also wider. This augmented scratch width is
concordant with the tendency of bark to include clustered silica
whereby soil minerals are concentrated and cemented into silica
aggregates (Sangster and Hodson, 1992; Albert and Weiner, 2001).
Also, if bite force alone were responsible for producing hypercoarse
scratches, other microwear scars would be expected to be deeper
and we have not found this to be the case. In addition, an important
finding that bears on this question is the presence of hypercoarse
scratches in the enamel of Florida mastodons with preserved
Fig. 7. Box plot of scratch width scores for extant ungulates and proboscideans and M. exilis. A score of 0 ¼ teeth with mostly fine scratches, 1 ¼ those with a mixture of fine and
coarse scratches, 2 ¼ those with mostly coarse scratches, 3 ¼ those with a mixture of coarse and hypercoarse scratches, and 4 ¼ those with mostly hypercoarse scratches. In each
plot, the center vertical line marks the median of the sample, the central 50% of the scratch values fall within the range of the box, and the top and bottom bars represent the range
of scratch values.
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Fig. 8. Photomicrographs of molar enamel surfaces (@35X magnification) of M. exilis e SBMNH e VPN 838 (left) compared to M. primigenius e AMNH 651012-1952 (right). Note the
relatively flat and polished enamel surface of M. exilis, typical of attritive feeders versus the uneven topography and duller enamel surface of M. primigenius which is typical of
abrasion-dominated feeders. Both specimens were analyzed using the same lighting regime. Scale bar ¼ 0.4 mm.
digesta consisting of a heavy concentration of cypress twigs (Webb
et al., 1992; Mihlbachler, 1998; Green et al., 2005). While the above
are consistent with hypercoarse scratches being caused by bark
consumption, further study is needed to tease apart the contributing factors that produce such distinctive scratches in
proboscideans.
Rivals et al., 2012) which indicate more seasonal or regional mixed
feeding (i.e., alternate consumption of browse and grass) in the
majority of forms, although exceptions do exist. For example,
Mammuthus columbi from the Phosphate Beds of South Carolina in
North America apparently browsed (Fig. 4C, point labeled 5; Rivals
et al., 2012).
4.2. Pygmy mammoth microwear compared to mainland
mammoths and mastodons
4.3. Channel Island Paleoecology
Our results indicate that M. exilis most likely engaged in a diet of
relatively low abrasion (polished enamel) and less dietary breadth
(few scratches) than the majority of mainland mammoths studied
thus far, including its likely ancestor e M. columbi.
Our data show (Table 1 and Fig. 4B) that 94% of individuals in our
sample of M. exilis have scratch numbers that fall within a relatively
narrow low scratch (browsing type) range while only the forest
elephant (L. cyclotis) and one of the mammoth samples we studied
(Mammuthus columbi from the Phosphate Beds of South Carolina e
Fig. 4C) also fall within this range. Two questions to consider when
interpreting these data are: (1) do the relatively narrow browsedominated scratch results of M. exilis reflects its actual dietary
range or were the specimens analyzed collected from a particular
horizon or facies that may underrepresent the diversity of habitats
(and diets), and (2) since microwear represents a snapshot in time
of dietary behavior (i.e., the diet at the time of an animal's death), is
it possible that a concentration of deaths in a particular season
might have led to a “last supper effect” (Grine, 1986)?
Firstly, the sample available to us was drawn from a variety of
facies (Agenbroad, 2001) so our results most likely show fidelity to
the actual dietary range of M. exilis. However, if food supply or
climate were highly seasonal on Santa Rosa Island during the
tenure of M. exilis on the island, it is quite plausible that deaths
would not have been spread evenly throughout the year, especially
on an island where seasonal migration is not possible.
Interestingly, the enamel surfaces and mean scratch/pit patterns
of M. exilis are more similar to those found in prior studies for the
American Mastodon e Mammut americanum (Green et al., 2005)
than to the results reported for Middle to Late Pleistocene Mammuthus from North America and Europe (Perez-Crespo et al., 2012;
Today the local vegetation of the Northern Channel Islands is
dominated by grasslands and shrublands with some patchy
woodlands, and mild temperatures with little annual temperature
fluctuation (Junak et al., 2007). To date, studies of the vegetation
history of the island have included pollen analyses on Santa Rosa
Island (Cole and Liu 1994; Kennett et al., 2008) and San Miguel
Island (West and Erlandson, 1994), and paleobotanical studies on
Santa Cruz Island (Cheney and Mason, 1930). During the Late
Pleistocene, when these islands existed as the single super-island of
Santarosae, forest vegetation was present including cypress, pine,
and Douglas fir (Cheney and Mason, 1930; Anderson et al., 2008).
Climatic changes at the end of the Pleistocene have been invoked as
the cause for the loss of coastal forest (Anderson et al., 2008), and
major fires coincident with the last known occurrence of pygmy
mammoths (around 13,000 cal. BP) have been demonstrated
(Kennett et al., 2008). Anderson et al. (2010) combined pollen
analysis with an analysis of the fire disturbance regime of Santa
Rosa Island and reported that a coastal conifer forest covered the
highlands of Santa Rosa during the last glacial but that by ca.
11,800 cal. BP Pinus stands, coastal sage scrub and grassland
replaced the forest as the climate warmed.
Our molar microwear results support our hypothesis that M.
exilis was exploiting forest vegetation on Santa Rosa during the Late
Pleistocene since M. exilis individuals possessed scratches mainly in
the low scratch range typical of leaf browsing extant ungulates and
other mammals as well as the unusually deep and wide scratches
typical of bark and twig consumers such as extant proboscideans
and black rhinos (see Solounias and Semprebon, 2002). These results are consistent with what is known of the Pleistocene vegetation history of Santa Rosa Island whereby extensive coastal
conifer forests were available during the last glacial as well as Pinus
Please cite this article in press as: Semprebon, G.M., et al., Dietary reconstruction of pygmy mammoths from Santa Rosa Island of California,
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stands and sage scrub as the climate warmed (Anderson et al.,
2010). Therefore, abundant browse was apparently available to M.
exilis, and consumption of leaves and twigs, as hypothesized here is
compatible with available Pleistocene vegetation on the island.
4.4. Extinction of M. exilis
There is much debate surrounding the extinction of Late Quaternary megafauna. The leading hypotheses are human overkill,
climatic fluctuations, or vegetational change, with widespread
disease, or extraterrestrial impact events sometimes invoked
(Martin and Klein, 1984; Grayson, 2001, 2007; Grayson and Meltzer,
2002; Koch and Barnosky, 2006; Firestone et al., 2007; Haynes,
2007; Faith and Surovell, 2009). It has long been thought that island species are more susceptible to human-induced extinctions
than those on continents and, consequently, the Channel Islands
have generated much interest to proponents of overkill hypotheses.
While early researchers reported that the Northern Channel Islands
were colonized by humans at least 40,000 years ago and claimed an
association between human artifacts and mammoth remains as
well as what were interpreted as “fire features” (Berger and Orr,
1966; Orr and Berger, 1966; Orr, 1968), closer scrutiny puts the
date of the first occupation of the islands by humans at around
13,000 cal. BP), and what were interpreted as “mammoth roasting
pits” are now thought to be natural burn features or due to other
natural processes (Wendorf, 1982; Cushing et al., 1986; Rick et al.,
2012; Pigati et al., 2014).
While a possible minimum temporal overlap between
mammoth and Homo sapiens of 250 years has been suggested for
Santa Rosa Island using radiocarbon dating (Agenbroad et al.,
2005), faunal remains associated with archeological sites dated
between 12,200 and 11,500 cal. BP on the Channel Islands are
devoid of large mammals (Erlandson et al., 2007; Rick et al., 2012)
and there is no direct evidence of human hunting of mammoths on
the Channel Islands (Rick et al., 2012).
Much is known about Channel Island mammoths. It is likely that
Columbian mammoths gained access to the Northern Channel
Islands during a low stand of MIS 6, ca. 130 ka (Muhs et al., 2015)
and gave rise to island pygmy mammoths via an approximately 50
percent linear size reduction (Agenbroad, 2009). This decrease in
size would provide the advantage of a lesser need for food intake
which might have proven to be an advantage in times of environmental stress (McNab, 2010; Lister and Hall, 2014).
However, the survival ability of the pygmy mammoths would
have been compromised by the vast reduction of island land area as
sea levels rose at the end of the Pleistocene. The super-island of
Santarosae shrank to about 75% of its original landmass as sea levels
rose (Kennett et al., 2008) causing inevitable ecological disturbances and alteration of available vegetation and freshwater sources. In addition, the results of this study clearly show that M. exilis
mainly browsed on leaves and twigs, a clear shift in niche from its
mainland ancestor. As warming ensued, conifer forests were
replaced by sage scrub, Pinus stands and grasslands by about
11,800 cal. BP (Heusser, 1995, 2000; Anderson et al., 2010). Our
microwear results are consistent with the last glacial (MIS4-2)
vegetative history of Santa Rosa Island and suggest that climatic
factors contributed to the extinction of Channel Island Pygmy
Mammoths on Santa Rosa Island as the shrinking of the forest
habitat would have impacted a species that relied heavily on leaves,
and twigs.
Muhs et al. (2015) point out that since the earliest dated remains
of Mammuthus from the California Channel islands are in late MIS 5
(ca. 80ka), it is likely that Mammuthus arrived in a low-stand of MIS
6 (or conceivably MIS 8). In this case, they survived the warm
phases of MIS 5 where vegetation changes similar to those of the
11
MIS2/1 transition occurred (reduction in pine and other trees;
expansion of open vegetation). Muhs et al. (2015) conclude that
climate and vegetation change may not have been a major
contributor to M. exilis extinction. It remains plausible, however,
that habitat change, especially in the situation of a small island
area, would have reduced population size, making the species more
likely to go extinct from these causes either stochastically or via
another agent (Lister and Stuart, 2008). While our results suggest
that climatic factors may have played a role in the extinction of
Channel Island pygmy mammoths on Santa Rosa Island, the
determination of the relative contribution of climatic factors versus
human overkill would benefit from futher study. These suggestions
require further work including on the diet of the early (M. columbi)
immigrants in comparison with M. exilis.
5. Conclusions
This analysis has provided for the first time microwear evidence
of feeding behavior (i.e., mainly browsing on leaves and twigs) in M.
exilis from Santa Rosa Island of California. This strongly suggests
that a constrained niche was imposed upon M. exilis, compared to
the broader dietary breadth apparent in its mainland ancestor (M.
columbi). Lastly, our analysis offers a glimpse into a possible
contributing factor for the extinction of Channel Island pygmy
mammoths, as the woodland dietary preferences of this taxon were
beginning to diminish and be replaced by grasslands as the climate
warmed.
Acknowledgements
We thank Paul Collins from the Santa Barbara Museum of Natural History for access to the specimens and Jelena Hasjanova,
Michaela Tarnowicz, Kristina Rogers, and Mia Russo from Bay Path
University for assistance in making casts.
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