Geometric Morphometrics of Dentaries in Myotis: Species
Identification and Its Implications for Conservation and the
Fossil Record
Kyle Jansky, Blaine W. Schubert, and Steven C. Wallace
Northeastern Naturalist, Volume 23, Issue 1 (2016): 184–194
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K. Jansky, B.W. Schubert, and S.C. Wallace
22001166 NORTHEASTERN NATURALIST 2V3(o1l). :2138,4 N–1o9. 41
Geometric Morphometrics of Dentaries in Myotis: Species
Identification and Its Implications for Conservation and the
Fossil Record
Kyle Jansky1,*, Blaine W. Schubert1,2, and Steven C. Wallace1,2
Abstract - Dentaries of 6 species of Myotis from eastern North America were analyzed,
using landmark-based geometric morphometrics, and were distinguished with 83.3% accuracy,
although sexes were poorly discriminated using this technique. Fossils of Myotis
from Bat Cave, KY, were studied in an attempt to identify these specimens to species level.
Southeastern Bats and endangered Indiana Bats dominated the fossil sample, with some
Eastern Small-footed Bats and endangered Gray Bats. Such results demonstrate the ability
to differentiate Myotis from historic and prehistoric sites and provide a tool for researchers
to understand and potentially to conserve these species.
Introduction
Over 100 extant species of Myotis occur worldwide (Simmons 2005), and identification
of fossils of Myotis to the level of species historically has been difficult. In
many areas, several morphologically similar species occur, and complete skeletons
rarely are preserved intact. Many caves, though, contain remains of Myotis that
could help illuminate the natural history of individual species and have broader
implications for cave paleoecology because bats differ interspecifically in terms of
roost preferences (Schubert and Mead 2012). Identifying fossils of Myotis might
also contribute to conservation by highlighting caves that were formerly roosting
sites for endangered or at-risk species (Toomey et al. 2002).
Six species of Myotis live in eastern North America: M. austroriparius (Rhoads)
(Southeastern Bat), M. grisescens Howell (Gray Bat), M. leibii (Audubon and
Bachman) (Eastern Small-footed Bat), M. lucifugus (Le Conte) (Little Brown Bat),
M. septentrionalis (Trouessart) (Northern Long-eared Bat), and M. sodalis Miller
and Allen (Indiana Bat). Indiana and Gray Bats are endangered, whereas Northern
Long-eared Bats are threatened in the United States (USFWS 1982, 2007, 2015).
All are currently being impacted by white-nose syndrome, an exotic fungal disease
that has greatly reduced many populations (Frick et al. 2010).
Our study is primarily an attempt to develop a procedure for identifying
dentaries of Myotis in eastern North America using landmark-based geometric
morphometrics, a technique previously employed to distinguish species of Pipistrellus
(Sztencel-Jabłonka et al. 2009) and western Myotis (Gannon and Rácz
2006). Any attempt to identify dentaries of Myotis to species presupposes that
they can be accurately identified to genus. In North America, only Corynorhinus
1Don Sundquist Center of Excellence in Paleontology, East Tennessee State University,
Johnson City, TN 37614. 2Department of Geosciences, East Tennessee State University,
Johnson City, TN 37614. *Corresponding author - kjjansky@gmail.com.
Manuscript Editor: Allen Kurta
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K. Jansky, B.W. Schubert, and S.C. Wallace
2016
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and Lasionycteris have a lower alveolar formula similar to Myotis (dental formula
3–1–3–3, with a single-rooted p2 and p3 and a double-rooted p4; Czaplewski et al.
2002). Fortunately these genera can easily be distinguished by dental and dentary
morphology (Czaplewski et al. 2002, Fauteaux et al. 2014, Gaudin et al. 2011). For
instance, Corynorhinus has a more laterally directed angular process, no lingual
cingulum on the molar trigonids, a broad talonid, and mandibular foramen typically
exposed. Lasionycteris has a lingual cingulum on molar trigonids, a lateral mental
foramen that opens between the roots of p2 and p3, wide p2 and p3, narrow talonid,
subequal incisors, and a distinct lingual bulge on the hypoconid of p3. In Myotis,
the lingual cingulum of molar trigonids is present, and widths of p2 and p3 are subequal;
the talonid is narrow, i3 is larger than i1 and i2, and p4 is longer than wide.
Attempts to identify the 6 species of Myotis of eastern North America using
osteological and dental material have met with partial success. Fauteaux et al.
(2014) were able to distinguish among Myotis in eastern Canada, in part because
only 3 species occur there. However, identification of fossils based on geographic
criteria must be done with caution, because geographic ranges may have changed
over time (Bell et al. 2009). Several authors have noted characteristics that distinguish
Northern Long-eared Bats from other members of the genus, including strong
lingual cingula around the protocones of the upper molars, a rectangular p3 with
anteroposterior length greater than labiolingual width, and middle labial cusp of i3
small (Czaplewski et al. 2002, Fauteaux et al. 2014, Gaudin et al. 2011). Gray Bats
and Eastern Small-footed Bats can sometimes be distinguished on the basis of size
(Fauteaux et al. 2014, Gaudin et al. 2011). Menzel et al. (2005) were able to distinguish
among these 6 species using cranial characters, though they did not analyze
morphology of the dentary in detail.
Our study had several objectives. First, we sought to assess the viability of using
landmark-based geometric morphometrics to identify dentaries of the 6 species of
Myotis present in eastern North America, and especially the 3 species most difficult
to identify (Southeastern Bat, Little Brown Bat, and Indiana Bat). Second, we evaluated
whether sexual dimorphism was likely to confound such analyses. Finally, we
applied these methods to fossil specimens from Bat Cave, KY, to determine which
species were present at the site.
Field Site
Bat Cave is located in Mammoth Cave National Park, KY, and contains thousands
of remains of bats, likely deposited by 1 or more floods (Colburn et al. 2015).
Radiocarbon dating of radii of Myotis suggests that the deposits span much of the
Holocene epoch, from ~9510 to 2250 years before present (Colburn et al. 2015).
Most identifiable material consists of crania, dentaries, and humeri (Colburn et al.
2015, Jegla 1961, Macgregor 1993).
Previous studies of fossils from Bat Cave identified Eptesicus fuscus (Palisot de
Beauvois) (Big Brown Bat), Perimyotis subflavus (Cuvier) (Tricolored Bat), Corynorhinus
sp. (big-eared bat), Eastern Small-footed Bat, Little Brown Bat, Indiana Bat,
and a large amount of material, mostly dentaries, classified as Myotis but not assigned
to any particular species (Colburn et al. 2015, Jegla 1961). Two crania were identified
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2016 Vol. 23, No. 1
as probable Little Brown Bats, 4 crania as probable Indiana Bats, and 6 dentaries
were assigned to the Eastern Small-footed Bat based on small size (Colburn et al.
2015). A number of species also were collected from or observed to live in Bat Cave,
including Southeastern Bats, Gray Bats, Little Brown Bats, Northern Long-eared
Bats, Indiana Bats, Tricolored Bats, Big Brown Bats, and Corynorhinus rafinesquii
(Lesson) (Rafinesque’s Big-Eared Bat) (Colburn et al. 2015; Smithsonian Institution
National Museum of Natural History collections records,Washington, DC).
Materials and Methods
Adult bats (n = 224) from 6 species in the genus Myotis were included in various
statistical analyses. Southeastern Bats (n = 29; 10 male, 18 female, 1 unknown),
Gray Bats (n = 40; 23 male, 16 female, 1 unknown), Eastern Small-footed Bats
(n = 37; 24 male, 10 female, 3 unknown), Little Brown Bats (n = 38; 21 male, 15
female, 2 unknown), Northern Long-eared Bats (n = 39; 28 male, 8 female, 3 unknown),
and Indiana Bats (n = 39; 15 male, 18 female, 6 unknown) were studied
because these are the only species of Myotis that occur in eastern North America.
These modern specimens were from the Smithsonian Institution National Museum
of Natural History, whereas fossils from Bat Cave that were analyzed (n = 48) were
curated at the Illinois State Museum.
We selected the dentary for this project because it is among the most common
elements in the fossil record of North American Myotis (e.g., Czaplewski and
Peachey 2003), and previous studies using geometric morphometrics to distinguish
among dentaries of other taxa of bats have met with success (Gannon and Rácz
2006, Sztencel-Jabłłonka et al. 2009). We used 17 landmarks, most of which were
tips of processes and the posterior margins of alveoli (Fig. 1, Table 1). We omitted
dental characteristics because teeth often disarticulate from dentaries post-mortem.
We photographed dentaries in labial view, with the orientation standardized by
ensuring that the tip of the coronoid process, tip of the angular process, and distal
tip of the mandible were on a single plane, perpendicular to the photographic angle.
We then used TPSUtil to convert the photographs into thin-plate-spline (TPS) files
Figure 1. Dentary of Myotis, showing the landmarks used in this project.
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Table 1. Definitions of landmarks used.
Landmark Definition
1 Tip of coronoid process: point of maximum curvature
2 Angle of mandibular condyle and coronoid process: point of maximum curvature
3 Tip of mandibular condyle: point of maximum curvature
4 Angle of mandibular condyle and angular process: point of maximum curvature
5 Tip of angular process: point of maximum curvature
6 Most ventral point of mandibular symphysis: point of maximum curvature
7 Ventral angle of masseteric fossa: point of maximum curvature
8 Posterior of the base of m3 (at the alveolus)
9 Posterior of the base of m2 (at the alveolus)
10 Posterior of the base of m1 (at the alveolus)
11 Posterior of the base of p3 (at the alveolus)
12 Posterior of the base of p2 (at the alveolus)
13 Posterior of the base of p1 (at the alveolus)
14 Posterior of the base of c (at the alveolus)
15 Posterior of the base of i3 (at the alveolus)
16 Posterior of the base of i2 (at the alveolus)
17 Posterior of the base of i1 (at the alveolus)
(Rohlf 2010a). Landmarks were digitized with tpsDIG2 (Rohlf 2010b) and transformed
into shape variables by a Procrustes superimposition, using TPSSUPER
(Rohlf 2004). We used SPSS (IBM Corporation 2012) to carry out principal component
analyses (PCA) and discriminant function analyses (DFA). Discriminant
analyses analyzed the shape variables, not principle components, and results were
cross-validated
We analyzed dentaries from the 6 species of Myotis from eastern North America
to determine whether landmark-based geometric morphometric analysis is a viable
method for identifying these species. We also analyzed these dentaries for sexual
dimorphism using DFA. Specimens of unknown sex were included in analyses as
unknowns, so superimposition would be based on a more robust sample size. Restricting
analyses to certain species of Myotis introduced several assumptions. We
assumed the fossil dentaries belonged to 1 of the 6 species of Myotis that currently
occur in eastern North America (i.e., the fossils did not belong to an extinct species
or a species today restricted to another geographic region) and further assumed that
mandibular morphology had not substantially changed since deposition of the fossils
and that modern specimens have been correctly identified to species.
We included specimens from Bat Cave as unknowns to identify species present
in the fossil record. To allow for the possibility that fossils might have violated the
above assumptions, we classified unknowns as none of the species if their typicality
probabilities fell outside the 95% confidence interval around the mean for the
most likely species, a procedure suggested by Kovarovic et al. (2011). Variants of
this analysis were also performed excluding fossils, but these results did not differ
substantially from those reported here. Early results indicated neither Little Brown
Bats nor Northern Long-eared Bats were major components of the fauna of Bat
Cave (i.e., no fossils were classified as either species), so we repeated the analyses
excluding these 2 species to determine more accurately which remaining species
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Table 2. Discriminant analysis of the 6 Myotis of eastern North America, with fossils from Bat Cave
treated as unknowns. For each species, the number of dentaries assigned is followed by the percent in
parentheses. Values reported for specimens of known species are cross-validated. Fossils were classified
as “none” if the typicality value for the most likely species was below 0.05. Specimens of known
species were correctly classified in 83.3% of cases (93.2% when not cross-validated).
Predicted Species
Eastern Northern
Small- Little Long-
Bat species Southeastern Gray footed Brown eared Indiana None Total
Southeastern 21 (72.4) 4 (13.8) 0 (0) 4 (13.8) 0 (0) 0 (0) 29
Gray 5 (12.5) 32 (80) 0 (0) 2 (5) 0 (0) 1 (2.5) 40
Eastern Small-footed 0 (0) 0 (0) 34 (91.9) 1 (2.7) 0 (0) 2 (5.4) 37
Little Brown 1 (2.6) 3 (7.9) 1 (2.6) 29 (76.3) 0 (0) 4 (10.5) 38
Northern Long-eared 0 (0) 0 (0) 0 (0) 0 (0) 38 (97.4) 1 (2.6) 39
Indiana 1 (2.6) 1 (2.6) 2 (5.1) 3 (7.7) 1 (2.6) 31 (79.5) 39
Fossils (species 16 (33.3) 2 (4.2) 1 (2.1) 0 (0) 0 (0) 12 (25) 17 (35.4) 48
unknown)
were present. Analysis excluding these 2 species was intended to supplement, not
supersede, the primary analysis of the fossils.
Southeastern Bats, Little Brown Bats, and Indiana Bats are difficult to distinguish
based on mandibular characteristics (Gaudin et al. 2011) and are often
grouped together as simply “medium-sized Myotis” (Colburn et al. 2015, Toomey
et al. 2002). For this reason, and because DFA is more successful if fewer groups
are analyzed, we also performed a DFA of these 3 species alone to assess the viability
of distinguishing them with landmark-based geometric morphometrics.
Results
A PCA of the 6 species of Myotis of eastern North America and the fossils exhibited
a gradient along the first principal component, with Northern Long-eared
Bat on 1 side, Eastern Small-footed Bat, Little Brown Bat, and Indiana Bat in the
middle, and Southeastern Bat and Gray Bat on the other side (Fig. 2). Most fossils
clustered together and scored along the middle-to-high end of the first principal
component and positively along the second component. Some fossils extended
beyond the morphospace occupied by known species.
Overall, 83.3% of the 224 modern specimens were correctly identified by
DFA (Table 2). Correct identification for individual species ranged from 72.4%
(Southeastern Bat) to 97.4% (Northern Long-eared Bat), and the number of false
positives ranged from 1 (2.5%) for Northern Long-eared Bat to 10 (25.6%) for the
Little Brown Bat. Of the 48 fossils, 16 (33.3%) were classified as Southeastern
Bats, 12 (25%) as Indiana Bats, 2 (4.2%) as Gray Bats, and 1 (2.1%) as an Eastern
Small-footed Bat. No fossils were assigned to the Little Brown Bat or Northern
Long-eared Bat. Seventeen fossils (35.4%) were classified as none of the 6 species
because the typicality probabilities for their most likely species were below 0.05.
When we reran the analysis excluding Little Brown Bat and Northern Longeared
Bat, the remaining 4 species were correctly classified 86.2% of the time
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Figure 2. First 2 axes of a principal component analysis of the dentaries of the 6 species of
Myotis from eastern North America and fossils from Bat Cave, KY. PC 1 explained 22.8%
of the variance, and PC 2 explained 14.4%.
Figure 3. First 2 axes of a discriminant function analysis of the dentaries of the Southeastern
Bat, Gray Bat, Eastern Small-footed Bat, and Indiana Bat, treating fossils from Bat Cave,
KY, as unknowns. DF 1 explained 64.8% of the variance, and DF 2 explained 23.4%.
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Table 3. Results of discriminant analysis of the Southeastern Bat, Little Brown Bat, and Indiana Bat.
For each species the number of dentaries assigned is followed by the percent in parentheses. Values
reported are cross-validated. Specimens were correctly classified in 82.1% of cases (97.2% when not
cross-validated).
Predicted Species
Species Southeastern Bat Little Brown Bat Indiana Bat Total
Southeastern Bat 24 (82.8) 4 (13.8) 1 (3.4) 29
Little Brown Bat 4 (10.5) 29 (76.3) 5 (13.2) 38
Indiana Bat 1 (2.6) 4 (10.3) 34 (87.2) 39
(Fig. 3). Nineteen fossils (39.6%) were assigned as Southeastern Bat, 11 (22.9%)
as Indiana Bat, 4 (8.3%) as Gray Bat, 2 (4.2%) as Eastern Small-footed Bat, and 12
(25%) as none of these 6 species. The first PCA axis separated Eastern Small-footed
Bats from Southeastern Bats and Gray Bats, whereas Indiana Bats were intermediate.
Indiana Bats largely separated from the other 3 species along the second axis.
Fossils mostly clustered between Indiana Bats and the Southeastern Bat/Gray Bat
group, with only a few specimens plotting close to Eastern Small-footed Bats.
A DFA of 3 medium-sized species of Myotis, a group previously difficult to differentiate
in the fossil record, correctly classified 82.1% of individuals (Table 3).
Five Southeastern Bats were misidentified, and there were 5 false positives. The
number of misidentifications and false positive were 9 and 8, respectively, for Little
Brown Bats, and 5 and 6, respectively, for Indiana Bats.
Analyses of sexual dimorphism typically did not distinguish sexes substantially
better than chance. The only exception was the Northern Long-eared Bat, which
had the least-balanced sex ratio, with 28 males and 8 females. The sexes in this
species were discriminated with 69.4% accuracy.
Discussion
Although Northern Long-eared Bats exhibited some sexual dimorphism, the
analysis was based on an unbalanced ratio of males to females; thus, these results
must be interpreted with caution. Because the other 5 species were not sexually
dimorphic, landmark-based geometric morphometrics is unlikely to be useful for
determining sex ratios of fossil populations of Myotis. Nevertheless, our results
suggest that sex was unlikely to be a major confounding factor in other analyses.
The main goal of our research was to devise a method of identifying dentaries
of Myotis to species. DFA correctly identified 83.3% of specimens, a success rate
high enough that general conclusions may be drawn regarding species composition
of a sample of fossils. That is, although individual identifications must be treated
with caution, if many fossils are attributed to the same species, then there is a high
probability that the species in question was present in the fauna.
Menzel et al. (2005) discriminated among these 6 species with a 99.4% success
rate by incorporating both cranial measurements and external body measurements
recorded on specimen tags. Using only cranial dimensions, they had a success
rate of 96.9% (Menzel et al. 2005). Our analysis relied on the dentary alone and
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provides a viable alternative for situations in which an adequate sample of cranial
material is not available. Furthermore, our method identified certain species (i.e.,
Eastern Small-footed Bat and Northern Long-eared Bat) in over 90% of cases, and
if some species can be ruled out in advance, analysis of remaining species is more
effective. In particular, Gray Bats often can be distinguished based on size alone
(Gaudin et al. 2011), and if size were combined with our methods, Gray Bats probably
could be recognized reliably. We initially planned to incorporate size into our
study, but were unable to do so because the program used to append scale bars to
our digital photographs provided erroneous measurements, a fact not noticed until
after data had been collected.
Once other species have been removed (either a priori or with a preliminary
DFA), Southeastern Bats, Little Brown Bats, and Indiana Bats—species previously
difficult to distinguish—were recognized with 82.1% accuracy. The endangered
Indiana Bat was identified with 87.2% accuracy, with only 5 false positives, and
this success rate could have important applications for conservation. If a fossil
population is analyzed and a significant proportion of specimens are assigned to this
species, Indiana Bats likely were indeed present. If the bats have disappeared due to
recent alteration or disturbance to their roost, steps might be taken to restore the site
to earlier conditions, in hopes that a colony would be reestablished (Toomey et al.
2002). Such restoration could prove important, because only 23 hibernacula serve
approximately 82% of all Indiana Bats (USFWS 2007). Restoring altered caves has
proven a viable conservation method for Gray Bats (Decher and Choate 1995).
Discriminant analysis of material from Bat Cave strongly indicated presence of
Southeastern Bats and Indiana Bats, with some support for presence of Gray Bats
and Eastern Small-footed Bats. Many fossils had extremely low typicality probabilities
for their most likely species, suggesting that they did not belong to any
species of Myotis found in the region today. No evidence of Little Brown Bats or
Northern Long-eared Bats was found. Though this did not prove absence of these
species, our results implied that neither species contributed substantially to the assemblage.
It is notable that our analysis did not identify any fossils as Little Brown
Bats since previous studies attributed some cranial material to this taxon (Colburn
et al. 2015), and it is the most common species at the cave today; typically 200–300
Little Brown Bats hibernated there each year before the appearance of white-nose
syndrome, though only 97–161 animals have overwintered there annually since
2009 (R. Toomey, Director, Mammoth Cave International Center for Science and
Learning, Mammoth Cave National Park, Mammoth Cave, KY, 2015 pers. comm.).
Our results suggested that the predominance of Little Brown Bats at Bat Cave today
was a recent development. Absence of Northern Long-eared Bats was less surprising,
because few have been observed at the cave in modern times (R. Toomey, 2015
pers. comm.).
Once the Little Brown Bat and Northern Long-eared Bat were removed from
the analysis, relative proportions of the other 4 species (Southeastern Bat, Gray
Bat, Eastern Small-footed Bat, and Indiana Bat) remained largely unchanged. Most
fossils appeared to be Southeastern Bats and Indiana Bats. The large proportion of
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Southeastern Bats (39.6%) was intriguing. This species had not previously been
identified among the fossils and had rarely been encountered in the hibernaculum
(Colburn et al. 2015, Hall 1961). Nevertheless, this result was consistent with previous
attribution of many fossils from Bat Cave to “medium-sized Myotis” (Colburn
et al. 2015:94).
Presence of Indiana Bats was expected. Indiana Bats have consistently been present
in the cave, with a population comparable to that of Little Brown Bats, though
their numbers have decreased to 30–40 animals in recent years (R. Toomey, 2015
pers. comm.). Additionally, several crania from Bat Cave have been identified as Indiana
Bats (Colburn et al. 2015). Consequently, our results provide additional support
for long-term use of Bat Cave as a hibernaculum by this endangered species.
Apparent presence of Gray Bats and Eastern Small-footed Bats should be
interpreted cautiously because our analyses were less than 100% successful at distinguishing
among species. However, a bachelor colony of Gray Bats roosts in the
cave during summer (Colburn et al. 2015), and several dentaries from the deposit
have previously been identified as Eastern Small-footed Bats due to their small size
(Colburn et al. 2015). Thus, our detection of these species among the fossils at Bat
Cave is consistent with previous research.
When all 6 Myotis from eastern North America were analyzed, 35.4% of specimens
fell below the 0.05 cutoff for typicality probabilities; this value dropped to
25% when Little Brown Bats and Northern Long-eared Bats were excluded. That
so many specimens fell outside the 95% confidence interval for their assigned species
strongly suggested that 1 or more groups not represented among the modern
specimens that we included in our analyses were present at Bat Cave in the past.
Possible explanations included presence of an extinct species, presence of an extant
species no longer found in eastern North America, and change in the shape of the
dentary since the fossils were deposited, though we could not assess the relative
probability of these hypotheses. Most specimens with typicality probabilities below
0.05 were assigned to either Southeastern Bat or Indiana Bat. Thus, future research
may revise relative proportions of these species within the fossil deposit because
other specimens may fall within the 95% confidence interval for these species but
be closer to the group centroid of a species not represented in our study. Similarity
to these 2 species could also be used as a starting point for design of future studies
to assess fossils from Bat Cave (i.e., by suggesting inclusion of species likely to be
morphologically similar to these species but not currently found in the region).
From our results, it seems that Bat Cave has long been home to several species of
bats, though at proportions differing from today. Specifically, the Southeastern Bat
and Indiana Bat appear to have had substantial colonies at Bat Cave in the past. One
or more species of Myotis no longer present in eastern North America (whether due
to extinction or extirpation) may have made substantial contributions to the fossil
deposit. A small number of Gray Bats and Eastern Small-footed Bats may also have
been present, along with a few Big Brown Bats, Tricolored Bats, and Corynorhinus
sp. (Colburn et al. 2015, Macgregor 1993). Although Indiana Bats continue to use
the cave today, Southeastern Bats have largely abandoned this site. At some point,
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possibly after deposition of the fossils, a small colony of Little Brown Bats became
established. Explaining these inferred changes would be an interesting subject for
future research.
Acknowledgments
We thank the Smithsonian Institution National Museum of Natural History, especially S.
Peurach, for access to specimens and general assistance. Thanks are also due to the Illinois
State Museum for permitting study of fossils from Bat Cave, and particularly to C. Widga
for photographing those specimens. The research of R. Toomey and M. Colburn provided
much background information. J. Mead, C. Jansky, and A. Cardini provided helpful feedback
on an earlier version of this manuscript, and D. Jansky helped prepare the figures. This
research was supported in part by the National Science Foundation (EAR-0958985) and
East Tennessee State University through the Office of Research and Sponsored Programs
and the Don Sundquist Center of Excellence in Paleontology.
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