Molar Size and Shape Variation in a Large Sample of Niptomomys (Microsyopidae, ) from the - Thermal Maximum: One Species or Two?

by

Rosa S. Felibert

Department of Anthropology

University of Florida

2017

ABSTRACT

The oldest euprimates first appear during a period of rapid, short-term, global

warming ~56 mya known as the Paleocene-Eocene Thermal Maximum (PETM).

Plesiadapiform primates of similar size and dental morphology to euprimates were present in North America before the PETM, and may have been affected by the arrival of euprimates as ecological competitors. Screenwashing PETM fossil localities in the

Bighorn Basin, Wyoming, has yielded many fossils (N≈600) of the microsyopid plesiadapiform Niptomomys. N. doreenae is known from before and after the PETM and may range through it. A second taxon, N. favorum, characterized by its small size and squarer M2 occlusal outline, was described from the large Castle Gardens locality

sample. To better characterize PETM diversity, we test the validity of N.

favorum against a sample of Niptomomys from Castle Gardens, other PETM localities,

and published measurements. M2 occlusal outlines (N=62) revealed a continuous range

from square to lingually compressed that encompasses the holotype of N. favorum.

Linear measurements of M1 (N=127) and M2 (N=163) indicated M1’s from Castle

Gardens are larger than those of later PETM localities (p=0.038), but produced no

outliers, suggesting the PETM fauna contains one species of Niptomomys. PETM lower

molars are smaller than all other measured Niptomomys teeth, paralleling the response

to warming effects recorded in larger-bodied lineages. These results are

consistent with either a single lineage of Niptomomys that became smaller during the

PETM, or a small immigrant taxon (N. favorum) that was transitionally present in the

Bighorn Basin during the PETM.

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INTRODUCTION

The Paleocene-Eocene Thermal Maximum (PETM) was a rapid warming event

that occurred about 56 million years ago (mya), during which global temperatures

increased ~5-8 degrees C in the span of 20,000 years or less. The entire event lasted

200,000 years and was caused by an increase in the amount of carbon dioxide (CO2) in

the atmosphere (McInerney & Wing, 2011). This warming event facilitated the dispersal

and dwarfing of in North America, as well as the migration of euprimates, or

primates of modern aspect (Bloch, Silcox, Boyer, & Sargis, 2007). The arrival of

euprimates to North America may have caused ecological competition with endemic

plesiadapiforms, a sister group of euprimates (Bloch, Silcox, Boyer, & Sargis, 2007;

Gingerich, 2003). Adaptive changes may have occurred during the PETM to fill new and

rapidly shifting ecological niches, and these provide the setting of this paper. One such

adaptive change is body size.

Many studies (Fleagle, 1978; Hutchinson & MacArthur, 1959; Newell, 1949;

Cope, 1887; Gould, 1966; Gingerich & Smith, 1985; Peters, 1986; Maurer, Brown, &

Rusler, 1992) have shown that body size is important when studying the ecology of past

organisms, as well as when attempting to reconstruct an extinct taxon’s biology and life

history. One such group of extinct organisms is the suborder of primitive primate-like

mammals, . Plesiadapiforms appear in the fossil record ~65 mya and

have been argued to have shared a recent common ancestor with euprimates (Bloch,

Silcox, Boyer, & Sargis, 2007). This group of mammals, which flourished during the

Paleocene and Early Eocene of North America, shares numerous traits with living

primates, even including a species with a nail instead of the more primitive claw (Bloch

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& Boyer, 2002). Understanding plesiadapiforms is a crucial component to understanding

primate evolution, and therefore, our own evolutionary history.

The suborder Plesiadapiformes contains 12 families (Bloch, Silcox, Boyer, &

Sargis, 2007; Silcox, 2001; Silcox & Gunnell, 2008). One of these is Microsyopidae, a

diverse family of over 20 species that ranged from the Late Paleocene through Middle

Eocene of North America (Fleagle, 2013; Gunnell, 1989; Silcox & Gunnell, 2008).

Fossils of the microsyopid plesiadapiform genus Niptomomys have been found in North

America ranging from before, during, and after the PETM. To date, there have been

three species of Niptomomys identified: N. doreenae, N. thelmae, and the more elusive

N. favorum. N. favorum from the Castle Gardens locality in the Bighorn Basin,

Wyoming, was diagnosed based on an isolated M1/2 (UCMP 212635) as being smaller

and squarer in occlusal outline than N. doreenae and N. thelmae, (Strait, 2001).

The extant primate literature has shown that there is a high correlation between

tooth size, specifically lower cheek teeth, and body size (Gingerich, 1974; Gingerich &

Schoeninger, 1979; Gingerich & Smith, 1985; Gingerich, Smith, & Rosenberg, 1982;

Pilbeam & Gould, 1974). This study assessed the occlusal outline of upper molars from

Castle Gardens and measured a large sample of M1 and M2 Niptomomys specimens to

determine whether N. favorum can be identified as a separate species existing

alongside N. doreenae during the PETM, or if specimens attributed to N. favorum from

Castle Gardens are merely a manifestation of dwarfing with a continuous N. doreenae

lineage through the PETM, as has been observed in other PETM lineages (Gingerich,

2003; Secord et al., 2012).

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LITERATURE REVIEW The Paleocene-Eocene Thermal Maximum

The PETM was a massive, rapid global warming event that caused temperatures to rise ~5-8 degrees Celsius at a rapid rate and persist for a span of ~200,000 years.

There have been numerous contributions made to the literature about the PETM, its onset, and its ecological consequences for mammals. McInerney and Wing (2011) thoroughly discussed changes in the carbon cycle, precipitation, ecosystems, and temperature during the PETM, and compared its effects to the effects that similar, even more rapid, anthropogenic global warming event could have in the future.

Chester and colleagues (2010) determined that rapid morphological change in

carnivorous species occurred during the PETM as a way of adapting to the changing

climate and the availability of food. This conclusion was based on Rosenzweig’s (1968)

study on Actual Evapotranspiration (AE), the amount of water that is evaporated from

soil and transpired from plants into the atmosphere, and its relationship with carnivore

body size. AE is correlated with the availability of water and solar energy and is

therefore a good predictor of mammalian carnivore productivity levels. Rosenzweig

found that environments lacking either or both of these resources are scarce in food,

which he found limited the body size of mammalian carnivores (Rosenzweig, 1968).

Chester and colleagues (2010) interpreted this research and concluded that selective

pressures, such as lack of water or increased temperature, could cause smaller body

sizes in mammals during the PETM.

Gingerich (2003) also related mammal body size to the PETM. His research

showed that mammalian body size dwarves in the same strata where the PETM

occurred, providing evidence that climate change and faunal size change are related.

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He posited that two different types of changes in body size occurred: transient and permanent. A transient change in body size is especially interesting to study because evidence shows body size decreasing during the PETM and returning to pre-PETM size after the PETM. Gingerich (2003) posited that these adaptive changes had to happen for immigrant taxa, which could only migrate due to the conditions the PETM caused, to coexist with endemic taxa.

Secord and colleagues (2012) related the PETM to mammal body size by discussing the change of body size in equids during and after the PETM. This was determined by examining the isotopic composition of mammal teeth and finding that body size in equids was negatively correlated with the temperatures inferred from the oxygen isotopes in those teeth. At the carbon isotope excursion (CIE) onset, equids had a 30% size decrease compared to a 76% size increase at the end of the CIE. This decrease in body size may have been caused by higher temperatures and increased

CO2 concentrations in the atmosphere. This paper also depicts the correlation between

the decrease of carbon-13 and the increase of oxygen-18 during the PETM, and

suggests that environmental and faunal changes during the PETM occurred extremely

rapidly.

Importance of Body Size in Ecology

Several studies have demonstrated the importance of body size in terms of

ecology and macro- and microevolution (Fleagle, 1978; Hutchinson & MacArthur, 1959;

Newell, 1949; Cope, 1887; Gould 1966; Speakman, 2005; Peters & Wassenberg, 1983;

Schmidt-Nielsen, 1984; Kelt & Van Vuren, 1999; Savage et al., 2007; Duncan, Forsyth,

& Hone, 2007; Capellini, Venditti, & Barton, 2010)

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Peters (1983) provided a comprehensive book explaining the ways in which organism body size affects ecology. On a fundamental level, body size affects a number of physiological functions, such as metabolism, heat production and heat loss, rate of energy use, and dynamics of locomotion. He discussed that these effects on physiological function are the reason for the wide range of diversity on the planet.

Schmidt-Nielsen (1984) also provided a comprehensive introduction to the topic of scaling and its importance in ecology. He described the concept of food extraction, the amount of food resources an animal consumes and how this directly affects population densities that determine the geographic distribution of different species.

Moreover, he explained the effects of body size on the metabolic rates and diets of . He discussed that small animals have a high metabolic rate and therefore are in constant need of food. This constant need for food prevents small animals from migrating over long distances, which places them in a vulnerable role. In the event of resource exhaustion, small animals are less likely to survive. This underscores the relevance of studying a small-bodied taxon like Niptomomys during a major ecological upheaval like the PETM. On the related subject of diet, he explains that body size directly affects sympatric competition in populations, as well as predator-prey relationships that constitute biological communities.

Fleagle (1978) studied size distribution of living and fossil primates, and how these size distributions varied across continents. He reached the conclusion that adaptive zones and body size ranges of primates are influenced by the presence or

6 absence of other primates due to possible food competition and the fitting of different environmental niches by different primate species.

Gingerich & Smith (1985) explained that body size is an important factor when studying evolution because it is indicative of diet and other life history patterns. They found that there are patterns of tooth size scaling which relate to diet and dentition, as well as evidence that primate tooth size scales geometrically with body weight. These findings challenge other results presented in the primate dentition literature (Pilbeam &

Gould, 1974; Gould, 1975). When tooth size in fossils is used to predict body size, one can more accurately predict the environment extinct primates lived in, how they interacted with it, and how these interactions altered life history patterns over time.

These factors provide insight that is useful in recognizing taxonomic relationships among fossil primate species.

Relationship between Tooth Size and Body Size

Previous research (Gingerich, 1974) has indicated that the best predictors of body size are the M1 and M2 positions of the tooth row. The best lower with which to estimate body size is the M1, as it is the least variable, probably because it is the first to erupt in the tooth row. Gingerich (1974) explains that sympatric species may not be accurately differentiated solely by form. It is important to estimate the size of the teeth to accurately diagnose whether there is another species present.

Gingerich & Schoeninger (1979) analyzed whether there was a pattern in dental variability in primates. The knowledge of the existence of a pattern is useful for understanding function in primate dentition, and understanding the relationships and adaptations of fossil primates. This research found that there is a definite pattern in

7 primate dental variability: canines are the most variable, cheek teeth least variable, and incisors show intermediate variability. Given that cheek teeth are the least variable, efforts have since focused on correlating these elements with body size.

Gingerich & Smith (1985) addressed the relationship between tooth size and body size in primates and insectivores by explaining the statistics behind allometric and isometric changes. They explained that shape remains the same only when length, area, and volume are all changed proportionally (isometry). If one is changed while others are not, the shape will change (allometry). This study also found a pattern of allometric scaling in primate dentition; increasing body size results in relatively larger anterior and posterior lower cheek teeth. Pilbeam and Gould (1974) conducted a tooth size survey and concluded that teeth in the cheek tooth series exhibit more positive allometry than teeth in other positions. As body mass increases, so does the size of these cheek teeth.

Gingerich, Smith, & Rosenberg (1982) also addressed the relationship between body size and tooth size using allometric scaling. They concluded that lower cheek teeth reach their definitive size before erupting, which makes them the best teeth to use when predicting the body size of an extinct primate species. This research is meant to be applied to generalized primates. Further research (Gingerich, Smith, & Rosenberg,

1982) has found that size of lower M1s or M2s are relatively accurate predictors of body size in living and fossil primates, including those that are specialized. They also made a good point that one should consider: any predicted body size result one calculates based on lower M1 and M2 measurements, even if based on a single specimen, is meant to be representative of the entire species. There are ways of making this body

8 size prediction more accurate (e.g., having a larger sample size, measuring cranial and postcranial elements if available), but researchers typically use body size results to characterize fossil species, not individuals.

Current Knowledge about Niptomomys

McKenna (1960) named the genus Niptomomys and the type species N. doreenae based on four jaw fragments from northwestern Colorado. McKenna allocated this new taxon to the family Anaptomorphidae. Since then, it has been reallocated to the family Microsyopidae (Szalay, 1969), where it is currently placed, on the basis of its lanceolate lower incisor morphology (Bown & Gingerich, 1972).

Szalay (1969) provided additional information on Niptomomys and its sister taxon

Uintasorex, including their similarities and differences, among them the fact that

Niptomomys is larger overall. He mentioned the possibility of N. doreenae being ancestral to Uintasorex parvulus, judging by the stratigraphy they are found in. The evidence for this, however, is circumstantial and has not been proven. Among the evidence evaluated are the minor qualitative differences that distinguish the molars of one genus from the other and the temporal distribution that separates them. This temporal distribution ranges the millions of years between the Wasatchian land mammal age of the Early Eocene (~55-50 mya) and the Bridgerian land mammal age of the

Middle Eocene (~50-46 mya). Both Niptomomys and Uintasorex are currently found in the tribe Uintasoricini (Bell & McKenna, 1997).

Gunnell & Gingerich (1981) introduced a new species of Niptomomys, N. thelmae. The diagnosis of this species is based on stratigraphic information and size difference between it and N. doreenae. This paper emphasized the importance of

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analyzing the stratigraphic position of specimens, as more than one species can be

found where before only one had been considered. This concept of stratophenetics, an

approach that combines stratigraphic information and phenetics to trace the evolution of lineages, was thoroughly defined in Gingerich (1979). This approach implements

Simpson’s (1943) chronocline and chronospecies concepts.

In this publication, Gingerich explained that the phylogeny of a group of

organisms cannot be fully understood without a continuous fossil record. He provided

an example in which specimens of Pelycodus, an adapid primate from the Early

Eocene, collected from the Bighorn Basin of Wyoming, were compared at different

stratigraphic levels. If specimens from a lower stratigraphic level were compared to

specimens from a newer level, their morphological distinction would place them in

separate species. However, with a complete fossil record available as is the case with

Pelycodus, one is able to compare multiple stratigraphic levels and notice that it is one

single evolutionary lineage that is gradually changing through time (Gingerich, 1979).

Gingerich (1989) described new Wa-0 mammals from 20 localities in the Bighorn

and Clarks Fork basins of Wyoming, providing the first descriptions of PETM mammals.

In this publication, he indicated the stratigraphic distribution of Niptomomys ranged from

the middle Clarkforkian land mammal age (Cf-2) of the late Paleocene through the Wa-2

stratigraphic biozone level of the early Eocene.

Strait (2001) briefly introduced another new species of Niptomomys found in the

Wa-0 Castle Gardens locality of the Bighorn Basin, Wyoming, N. favorum. This

classification is based on the smaller size of an isolated upper molar with a squarer

occlusal outline than is found in N. doreenae or N. thelmae.

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Rose et al. (2012) recovered numerous Niptomomys dental specimens from a

Wa-0 locality in the Sand Creek Divide section of the Bighorn Basin, Wyoming. These specimens were found to resemble the holotype for N. doreenae described by McKenna

(1960) save for some minor morphological and size differences which could place them in a distinct but closely related species. Despite these differences, the authors refrained from naming an additional species of Niptomomys or referring their specimens to N. favorum until further research demonstrates a more explicit contrast between the specimens collected and N. doreenae.

MATERIALS AND METHODS

Upper Molar Occlusal Outline

The diagnosis for Niptomomys favorum is based on a single specimen (UCMP

212635), a left M1/2, from the Castle Gardens locality in the Bighorn Basin, Wyoming.

This holotype is described as “substantially smaller and more squared in occlusal outline” than N. doreenae and N. thelmae (Strait, 2001: 134). To examine the validity of this diagnosis, the occlusal outlines of Niptomomys M1/2 specimens from Castle

Gardens were taken and compared to the occlusal outline of the holotype (Fig. 4).

Digital representations of Niptomomys teeth were generated via micro-CT

(computed tomography) scanning at the Duke University Shared Materials and

Instrumentation Facility on a Nikon XTH 225 ST and at the University of Florida

Nanoscale Research Facility on a GE v|tome|x 240. Scan resolutions varied between 4-

8 μm, though most specimens were scanned at the lower end of this range. Scans were reconstructed and exported in the form of TIF stacks. A TIF stack pertaining to a single

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upper molar specimen was selected and opened in the 3D visualization software, Avizo

Lite 9.2 (FEI Visualization Sciences Group, Berlin, Germany). Voxel size, which was

determined during CT scanning, was entered for the X,Y, and Z coordinates which

defined specific points in three-dimensional space. An isosurface was then created and

the three-dimensional image was manipulated until placed in occlusal view, with a

horizontal line passing through the paracone and metacone apices. A photo of the

digital specimen in occlusal view under orthographic projection was then taken using

Avizo’s snapshot button. Specimens of right M1/2s were reflected so that all snapshots corresponded to left upper molars. This procedure was then repeated for all 62 M1/2

specimens.

Snapshots were then opened in Adobe Photoshop, where I used the Magic

Wand tool to select the surface boundary. The entire surface was colored a solid black

color, creating a contrast of a solid black shape of the occlusal outline against a white

background. This image was then opened in Adobe Illustrator, and a vectorized shape

for each outline was created using the Image Trace tool. The fill was removed and the

outlines of all vectorized shapes were standardized to be the same width. These were

compared to the outline of the N. favorum holotype (UCMP 212635).

Collection of M1 and M2 Linear Dimensions Using Avizo

In Avizo, an isosurface of each scanned M1 and M2 specimen was rendered in

orthographic projection and placed in occlusal view. Maximum crown length and width

were measured using Avizo’s ruler tool. Measurements were recorded and input into a

12 master spreadsheet, which included basic information about each specimen, such as locality, biozone, tooth position, and species.

Collection of M1 and M2 Linear Dimensions Using Reticle Lens

In order to maximize sampling, I also measured those Niptomomys M1 and M2 specimens that had not been scanned. I handled each specimen under a microscope and placed it on modeling clay in occlusal view. Using a reticle lens, I measured length and width for the remaining specimens. The reticle lens was always set to a 0-mm starting position. The specimen was placed to the left of the vertical line of the crosshairs visible through the eyepiece.

Using the winding knob, the crosshairs were moved across the specimen until the vertical line reached the opposite side of the tooth. In the beginning, each measurement was taken three times, and if different from one another, an average of the measurements was taken. As I got more consistent measurements, I started measuring once for the length and once for the width. Measurements that were between two numbers or that were questionable due to potential accidental movement of the specimen were retaken to ensure accuracy. Measurements were recorded and input into the master spreadsheet.

Statistical Analyses

Previous research has demonstrated the effectiveness of using a moving window log-rate-interval analysis (mwLRI) to study time series dynamics of body size in mammal lineages (Secord, et al., 2012). While this analysis would have been useful to use in this research, specimens of Niptomomys are known from an insufficient number of independent vertical strata during the PETM and Early Eocene. Most specimens of

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Niptomomys were recovered from screenwashed localities, unlike the surface- prospected specimens of Secord et al. (2012), creating a large sample size, but with fewer distinct strata.

Specimens were separated first by tooth position, M1 or M2. Then they were

separated by three different categories representing different stages of the earliest

Wasatchian (Wa-0). Wa-0A represents a time before the arrival of euprimates to North

America, and includes the Castle Gardens locality. Wa-0B represents a time after the

arrival of euprimates but before the end of the PETM. Wa-0C, a smaller sample compared to the other two sample categories, represents the end of the PETM. In addition, linear measurements from the latest Clarkforkian (Cf-3) and younger biozones in the Wasatchian were gathered from the literature and used to contextualize the

PETM specimens. Length and width of each specimen were multiplied, then natural log-

transformed to correct for allometric scaling in Niptomomys crown area.

Analysis of variance (ANOVA) was used to determine whether the groups

represented by M1 and M2 specimens from different biozones were significantly different

from one another in size. Several assumptions must be met for ANOVA to be

appropriate: 1) the samples should be normally distributed, 2) sample sizes should be

comparable, and 3) sample variances should be approximately equal. Quantile-quantile

plots of M1 and M2 log-transformed area revealed that the underlying distribution of the

data was approximately normal (Figs. 2-3). However, neither sample sizes nor

variances were equal among the different biozones (much larger sample sizes and

variances in Wa-0A and Wa-0B versus all other samples). Therefore, a nonparametric

version of ANOVA, the Kruskal-Wallis rank-sum test, was used to assess differences in

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sample means. A Kruskal-Wallis test was performed using the base stats package in R

(R Core Team, 2016) on log-transformed M1 area from all biozones where Niptomomys

has been found and the three Wa-0 subzones. This procedure was repeated for the M2.

RESULTS Upper Molar Occlusal Outline

The comparisons of the N. favorum holotype M1/2 occlusal outline and those of

other Niptomomys M1 and M2 specimens from Castle Gardens are presented in Figure

4. The outline for the N. favorum holotype placed directly in the middle of the other

Niptomomys occlusal outlines, suggesting that the holotype was not more-squared or

less-squared in occlusal outline than other Niptomomys specimens (Fig. 4).

Differences in Size through Time

The differences in size among M1 and M2 of Niptomomys from different biozones

are summarized in Tables 1-2 and Figure 5. Results of the Kruskal-Wallis

nonparametric test of differences in means for both the M1 and M2 log-transformed

crown areas were highly significant (p-values < 0.0001 for each group). Figures 6 and 7

summarize these data for the M1 and M2 respectively. To further explore pairwise

differences in mean size among specific biozones and the subzones of Wa-0, a

nonparametric implementation of the Tukey’s Honestly Significant Differences test was

performed in R using the nparcomp package (Konietschke et al., 2012) on the Kruskal-

Wallis test output for both the M1 and M2. The results from these post-hoc comparisons

are shown in Tables 3 and 4. These tables demonstrate that time bins Wa-0A and Wa-

0B are very similar to each other but significantly different from all other time bins.

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DISCUSSION

This Niptomomys study is the first to study body size change trends during the

PETM using such a large sample size and investigating these trends in a small-bodied endemic mammal spanning the PETM. The large sample size and data set studied allows for more reliable statistical analyses and interpretations.

The holotype for N. favorum is not more squared in occlusal outline than other

Niptomomys specimens from Castle Gardens, as previously thought. When compared

with other Niptomomys M1/2, the holotype was well within the range of variation seen in

the occlusal outline. This suggests that the morphology of the M1/2 does not support an

additional species of Niptomomys. The only other diagnosis for N. favorum is the

“substantially smaller” molar size. I divided the Wa-0 biozone into three categories, Wa-

0A, Wa-0B, and Wa-0C, to study how the progression of the PETM and the arrival of

euprimates to North America affected Niptomomys body size.

Changes in body size and their correlation to the PETM and other hyperthermals

have been studied extensively in mammals (Clyde & Gingerich, 1998; D'Ambrosia,

Clyde, Fricke, Gingerich, & Abels, 2017; Gingerich, 1989; Gingerich, 2003; Gingerich,

2006; Rose, et al., 2012; Secord, et al., 2012). An important contribution in the research

has been studied in the context of the earliest known horses. This research found

evidence for a 30% decrease in equid body size at the beginning of the PETM, a

continuing decrease in body size throughout the event, and a 76% increase at the end

of the PETM. The smallest equid body size correlates to the warmest period in the

PETM (Secord, et al., 2012).

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This is not the case with the Niptomomys data I analyzed. Instead, evidence of the smallest Niptomomys place these specimens at the beginning of the PETM.

Evidence of an increasing Niptomomys body size pattern has been discovered from the warmest period of the PETM (Wa-0C). During this period, larger-bodied mammals, such as the earliest equids, were smallest in body size. This pattern is opposite that of the size pattern in Niptomomys.

Moreover, the size trend observed in Niptomomys is different from any body size pattern of small-bodied mammals currently known from the PETM. The only other research available on small mammal body size change during the PETM, that of small insectivores (Macrocranion junnei), has found that body size is consistent through Wa-

0A and Wa-0B levels, a finding consistent with that of our study (Vitek, unpublished).

There is no evidence for the branching off of Niptomomys into N. doreenae nor

N. favorum, as there is currently no evidence of two differently sized Niptomomys species during the PETM (Table 1). As there is also no evidence for the existence of an overall smaller species before or after the PETM, suggesting the existence of N. favorum would misattribute the size change caused by the evolutionary dwarfing mammals endured during the PETM interval and attribute it to the evolution of the genus. Our occlusal outline analysis suggests that N. favorum is not different from N. doreenae apart from its body size.

An interesting question about mammals during the PETM is whether endemic and immigrant species responded differently to the effects of the climate event. When the size trend observed in this study, that of an endemic primate Niptomomys, is compared to the size trend of immigrant primate Teilhardina brandti, one finds the two

17 trends to be notably different. T. brandti does not show a trend in which it starts off as being small in the beginning of the PETM and then increases in size significantly as the

PETM progresses. Instead, T. brandti is already relatively large at the beginning of the

PETM and increases slightly in size as the PETM progresses (Morse, in prep).

A different but related question is that of the ecology of endemic mammals and whether it was affected by the arrival of immigrant taxa with an ecology similar to theirs.

The results presented in this study suggest that in the case of primates, they were not.

These findings indicate that there is no significant change in size between specimens in the Wa-0A and the Wa-0B levels, the arrival of euprimates occurring during the culmination of Wa-0A and the beginning of Wa-0B (Figs. 6 and 7). If this study would have yielded a significant size change in Niptomomys between Wa-0A and Wa-0B periods, a reasonable conclusion would have been that the arrival of euprimates directly or indirectly caused this change in size. However, the findings of this study suggest the opposite.

Furthermore, the findings of this study indicate that the Niptomomys specimens found at Castle Gardens and throughout the PETM represent a single lineage. Evidence shows this lineage underwent an evolutionary dwarfing trend during the PETM, which correlates with other findings of temporary evolutionary dwarfing in mammals during that time. The size changes Niptomomys went through, although significant, cannot and should not be used to make an argument for the temporary evolution and branching off of two species. The evidence shows N. doreenae body size returning to the range in which it was before the PETM. A temporary change in this size is insufficient to diagnose a new species.

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Fig. 1. Size and morphological visual comparisons between N. favorum holotype specimen (UCMP 212635, left), and N. doreenae specimens, B (UCMP 231821) and C (UCMP 231729).

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Fig. 2 – Fig.3. Q-Q plots show an overall normal distribution of the data for both M1 and M2 data sets despite a few outliers on the low and high end of the distribution.

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Fig. 4. Occlusal outline of N. favorum

M1/2 holotype (UCMP 212635) in red

falls within the bounds of a

continuous range of variation, which

is most variable along the posterior

edge.

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Fig. 5. Occlusal view comparisons of M1 and M2 positions in Niptomomys through Wa-0 (A), Wa-1 (B), and Wa-2 (C) biozones, based on UF 328623 and UF 326323 (A), EPV-91343 (B), and UF 316901 (C). (Scale bar: 1 mm).

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Fig. 6 – Fig. 7. Box-plots denoting significant differences in sizes between groups found in different biozones. Cf-3 was excluded from these analyses to prevent erroneous skewing of data.

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Table 1. Summary of statistics for M1 measurements of Niptomomys. Numbers shown here based on natural log- transformed area of tooth measurements.

Standard Standard Coefficient of Biozone N Minimum Maximum Mean Deviation Error Variation Cf-3 1 0.95 0.95 2.59 N/A N/A N/A 0.61 Wa-0A 51 -0.26 0.53 1.06 0.04 0.07

0.62 Wa-0B 51 -0.39 0.65 1.04 0.04 0.07

0.61 Wa-0C 4 0.18 0.35 1.26 0.07 0.27

Wa-1 7 0.22 0.56 1.45 0.28 0.46 0.61 0.61 Wa-2 7 0.32 0.75 1.71 0.44 0.72

Wa-3 2 0.17 0.22 1.22 0.13 0.22 0.61

Wa-6 4 0.56 0.70 1.88 0.53 0.88 0.61

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Table 2. Summary of statistics for M2 measurements of Niptomomys. Numbers shown here based on logarithmic area of tooth measurements.

Standard Standard Coefficient of Biozone N Minimum Maximum Mean Deviation Error Variation Cf-3 5 0.26 0.42 1.44 0.27 0.44 0.61

Wa-0A 53 -0.22 0.40 0.99 0.001 0.002 0.61

Wa-0B 84 -0.58 0.42 1.03 0.17 0.04 0.62

Wa-0C 1 0.35 0.35 1.42 N/A N/A N/A

Wa-1 7 -0.06 0.48 1.21 0.17 0.23 0.62

Wa-2 7 0.22 0.52 1.46 0.10 0.46 0.61

Wa-3 3 0.27 0.52 1.51 0.10 0.52 0.61

Wa-6 3 0.56 0.86 1.95 0.13 0.98 0.61

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Table 3. Nonparametric Multiple Comparisons Table for M/1 Comparison Estimator Lower Upper Statistic p-value Wa-0A, Wa-0B 0.46 0.29 0.64 -0.63 0.9999891 Wa-0A, Wa0-C 0.92 0.70 0.98 4.62 <0.0001 Wa-0A, Wa-1 0.96 0.77 0.99 4.78 <0.0001 Wa-0A, Wa-2 0.99 0.84 0.99 4.70 <0.0001 Wa-0A, Wa-3 0.90 0.67 0.98 4.44 0.0002 Wa-0A, Wa-6 0.99 0.99 1.00 24.49 0.00 Wa-0B, Wa-0C 0.80 0.55 0.93 3.45 0.011 Wa-0B, Wa-1 0.90 0.69 0.98 4.54 0.0001 Wa-0B, Wa-2 0.97 0.74 0.99 4.38 0.0003 Wa-0B, Wa-3 0.75 0.52 0.89 3.22 0.02 Wa-0B, Wa-6 0.99 0.73 0.99 3.93 0.001 Wa-0C, Wa-1 0.86 0.22 0.99 1.75 0.74 Wa-0C, Wa-2 0.96 0.25 1.00 2.25 0.35 Wa-0C, Wa-3 0.25 0.00 0.97 -0.71 0.99 Wa-0C, Wa-6 0.99 0.99 1.00 24.49 0.00 Wa-1, Wa-2 0.84 0.27 0.99 1.86 0.65 Wa-1, Wa-3 0.04 0.00 0.75 -2.25 0.35 Wa-1, Wa-6 0.98 0.42 1.00 2.78 0.09 Wa-2, Wa-3 0.001 0.00 0.002 -24.49 0.00 Wa-2, Wa-6 0.82 0.19 0.99 1.54 0.88 Wa-3, Wa-6 0.99 0.99 1.00 24.49 0.00

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Table 4. Nonparametric Multiple Comparisons Table for M/2 Comparison Estimator Lower Upper Statistic p-value Wa-0A, Wa-0B 0.01 0.00 0.23 -4.09 0.001 Wa-0A, Wa0-C 0.01 0.002 0.17 -4.99 <0.0001 Wa-0A, Wa-1 0.17 0.01 0.81 -1.56 0.86 Wa-0A, Wa-2 0.40 0.06 0.88 -0.50 0.99 Wa-0A, Wa-3 0.73 0.04 0.99 0.72 0.99 Wa-0A, Wa-6 0.99 0.99 1.00 27.77 0.00 Wa-0B, Wa-0C 0.61 0.46 0.74 2.10 0.45 Wa-0B, Wa-1 0.81 0.39 0.97 2.28 0.32 Wa-0B, Wa-2 0.98 0.80 0.99 4.49 0.0001 Wa-0B, Wa-3 0.99 0.69 1.00 3.56 0.007 Wa-0B, Wa-6 0.99 0.99 1.00 27.77 0.00 Wa-0C, Wa-1 0.73 0.33 0.94 1.74 0.74 Wa-0C, Wa-2 0.98 0.84 0.99 5.39 <0.0001 Wa-0C, Wa-3 0.98 0.64 0.99 3.50 0.008 Wa-0C, Wa-6 0.99 0.99 1.00 27.77 0.00 Wa-1, Wa-2 0.80 0.22 0.98 1.56 0.86 Wa-1, Wa-3 0.86 0.21 0.99 1.70 0.77 Wa-1, Wa-6 0.99 0.99 1.00 27.77 0.00 Wa-2, Wa-3 0.62 0.05 0.98 0.43 1.00 Wa-2, Wa-6 0.99 0.99 1.00 27.77 0.00 Wa-3, Wa-6 0.99 0.99 1.00 27.77 0.00

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