Size-structure of the herbivore guild in the Mesozoic and Cenozoic
Variation in skewness and kurtosis in local mammalian herbivore size distributions through time.
Understanding how communities are structured and how this structure changes through time are fundamental objectives of ecology and paleobiology. Variation in the shapes of guild-level body-size distributions can help explain how niche partitioning and competition affect patterns of species co-existence and energy flows in an ecosystem. Terrestrial herbivores occupy a wide variety of sizes, making them an ideal system to study how local environmental and ecological processes shape within guild body mass distributions. Working with Dr. Matthew Carrano at the Smithsonian’s National Museum of Natural History, I am analyzing the body-mass distributions of mammals and dinosaurs from well-sampled quarries and bonebeds to determine the generalities and variability in the shape of these distributions through time. This study has three goals: 1) to understand whether the replacement of species through time has an impact on the size- structure of the local herbivore guild and therefore niche space occupation, 2) to compare the assembly of the Mesozoic and Cenozoic herbivore guild to determine whether dinosaur-dominated ecosystems are fundamentally different from mammal-dominated ones and 3) whether there is a common structure to the mammalian herbivore guild irrespective of taxonomy, time interval, or geography.
Presentations Jukar, A. M., M. T. Carrano. The structure of the mammalian and dinosaurian herbivore guild. Society of Vertebrate Paleontology 79th Annual Meeting, Brisbane, Australia, 9-12 October.
Extinction Dynamics at the Beginning of the Anthropocene
Late Quaternary extinctions in South Asia
Extinction chronology for six species found in late Quaternary sites in the Indian peninsula. Black dots are the median calibrated 14C dates with 95% confidence intervals (CI) ranked <12. Red dots are median calibrated 14C dates ranked >12. Grey dots are the non-radiocarbon dates with 95% CI. Each vertical bar represents a site where a particular species was found; multiple dots on a site represent the youngest and oldest dates and error bars. Blue dots are the median expected extinction dates estimated using GRIWM with 95% CI. Brown shaded regions represent the two phases of human dispersal into the Indian Subcontinent, and early phase from ~96ka to 60ka, and a later phase from 60 ka to 30 ka (Bae et al. 2017b). The grey shaded bar represents archaeological phases. Shading indicated the transition from one phase to another. We present the δ18O record from the GICC05 Greenland ice core (Andersen et al. 2006, Rasmussen et al. 2006, Seierstad et al. 2014, Steffensen et al. 2008) as a proxy for global temperature and a composite record of δ18O from Bittoo (Kathayat et al. 2016) and Mawmluh cave (Berkelhammer et al. 2012, Dutt et al. 2015) speleothems as a proxy for monsoonal intensity in the Subcontinent. Horizontal blue bars represent the last glacial maximum (LGM) and Younger Dryas (YD) cold periods.
The causes of the late Quaternary extinction event have been debated for decades. This extinction event, in which several large species were wiped out, has been studied extensively in North America, Northwestern Eurasia, and more recently in some detail in Australia, South America, and Africa. However, we know little about the extinction event, let alone the causes in South and Southeast Asia. It has been suggested that a combination of climate change and human hunting resulted in the extinction.
I have complied the most extensive dataset of dated South Asian mammal occurrences over the last 100,000 years, and am using this dataset to understand patterns of distribution change and eventual extinction. This is the first study of its kind to explore the pattern of extinctions in the Late Quaternary in this understudied part of the world.
In a separate project, I collaborated with researchers from India, Dr. Rajeev Patnaik and Dr. Parth Chauhan, and Taiwan, Dr. H-C Li and Dr. J-P Lin to report on the first and only directly dated specimen of Hexaprotodon from the Indian Subcontinent, it's environment context, and reasons for its eventual extinction.
Publications Jukar, A. M., S. K. Lyons, P. J. Wagner, M. D. Uhen. Late Quaternary extinctions in South Asia. Science. In Review.
Presentations Jukar, A. M., S. K. Lyons, P. J. Wagner, M. D. Uhen. Late Quaternary Extinctions in India. North American Paleontological Convention, Riverside, CA, June 23-27. Jukar, A. M. Late Quaternary Extinctions in South Asia. Society for Vertebrate Paleontology 77th Annual Meeting, Calgary, Canada, August 23-26, 2017.
Jukar, A. M. Megafaunal extinction in South Asia: challenges and prospects. Ecological Society of America Annual Meeting, Portland, OR, August 6-12, 2017. Jukar, A. M. India’s missing megafauna: the megafaunal extinction in South Asia. University of Pennsylvannia-Smithsonian Geobiology Symposium 25. Washintgon D.C., February 24, 2017.
Evolution of Terrestrial Ecosystem (ETE) Research
I am a member of the Evolution of Terrestrial Ecosystems program at the Smithsonian's National Museum of Natural History. The research that we do is aimed at understanding the assembly of Earth's biotas, and the changing dynamics of communities and ecosystems. I am involved with a research project that is using functional traits to explain patterns of statistically significant pair-wise associations of species in North America in three time periods, the late Pleistocene, Holocene and modern era, and understand assembly rules and how they change with increasing anthropogenic activity. We also recently published a paper in Science about the reorganization of mammal communities after the end Pleistocene megafaunal extinction. Assembly rules change after the extinction of large species, and the role of biotic interactions in structuring communities on the landscape lessens.
Publications Pineda-Munoz, S., A. M. Jukar, K. Amantagelo, M. Balk, W. A. Barr, A. K. Behrensmeyer, J. Blois, M. B. Davis, A. Du, J. Eronen, D. L. Fraser, N. J. Gotelli, C. Looy, J. Miller, A. Shupinski, L. Soul, A. Toth, A. Villaseñor, S. Wing, S. K. Lyons. Body mass-related changes in mammal community assembly patterns during the late Quaternary of North America. Ecography. Submitted.
Presentations Tóth, A. M., S. K. Lyons, A. M. Jukar, A. Du, W. Barr, A. Villaseñor, A. K. Behrensmeyer, D. Fraser, J. Miller, L. Soul, N. Gotelli, S. Pineda-Munoz, J. Blois, M. Davis, J. Eronen, J. T. Faith, J. B. Alroy. The end-Pleistocene megafaunal extinction caused a fundamental shift in survivor mammal community structure. International Biogeography Society 2019 Meeting, Malaga, Spain, January 8-12. Pineda-Munoz, S., Jukar, A. M., ETE Working Group, Lyons, S. K. Human Impact on North American Mammal Faunas from the Pleistocene. Ecological Society of America Annual Meeting, Portland, OR, August 6-12, 2017. Pineda-Munoz, S., Jukar, A. M., ETE Working Group, Lyons, S. K. Human Impact on North American Mammal Faunas. Society for Vertebrate Paleontology 76th Annual Meeting, Salt Lake City, October 26-29, 2016
Large Mammal Paleobiogeography
Taxonomy and biogeography of equids from South Asia
Comparison of the new specimen of Plesiohipparion huangheense with Chinese and Turkish specimens. a) Line drawing of NHMUK PV OR 15790 with morphological features labeled. b) NHMUK PV OR 15790 occlusal view. c) THP10097 occlusal view. d) HU9102 occlusal view modified from Bernor & Lipscomb (1991). e) NHMUK PV OR 15790 labial view.
Horses have inhabited the Indian subcontinent from the Miocene to the present. I am part of a collaborative project with Dr. Ray Bernor of Howard University to understand the systematics and biogeography of Pliocene hipparionine horses, and the early evolution of the genus Equus in the Old World.
Using museum collections, we have shown that a major turnover occurred in hipparionines in South Asia during the Latest Miocene and Earliest Pliocene whereby the Sivalhippus lineage was replaced by Cremohipparion, Plesiohipparion and Eurygnathohippus in the Late Pliocene. We also show that there was a radiation of Equus in the Old World starting ~2.6 million years ago after the dispersal of Equus simplicidens from North America, and Grevy's Zebra likely descended from this radiation.
Turnover in South Asian Plio-Pleistocene herbivorous mammals
Locator map of the Siwalik Group and Karewas of Kashmir in South Asia with the stratigraphic range of the Upper Siwalik subgroup.
The four circled regions represent the four fossiliferous Upper Siwalik regions in South Asia. The Upper Siwalik subgroup and Karewas of Kashmir span the Upper Pliocene and Lower-Middle Pleistocene and are divided into two faunal zones: Tatrot and Pinjor. The Boulder Conglomerate formation (BC) caps the Pinjor faunal zone in a time transgressive manner from 1.77 Ma to 0.6 Ma. Magnetostratigraphically, the Tatrot faunal zone spans the Gauss Chron while the Pinjor faunal zone spans the Matuyama Chron and the lower 0.18 Ma of the Brunhes Chron. The ages shown on the magnetostratigraphic column show the age boundaries of the five time bins used in this study. Figure modified from (Patnaik and Nanda 2010).
The Siwalik Group in India and Pakistan consists of sediments laid down from the middle Miocene to the middle Pleistocene and stretches from the Potwar Plateau in Pakistan to Northeastern India. The fauna from these rocks is collectively called the Siwalik fauna, and elements can be found in the Karewas of Kashmir as well. While the Miocene mammal faunas have been studied extensively, comparatively little work has been done on the Plio-Pleistocene (upper Siwalik) mammals from this group. In this study, using pair-wise dissimilarity indices, I explored patterns of faunal turnover in the Plio-Pleistocene large herbivore guild from the Upper Siwaliks of the Indian Subcontinent. I then used patterns of species appearances and disappearances to understand the processes of immigration and extirpation in driving turnover. I further studied changes in the proportional representation of different herbivore families and changes in the overall body-mass distribution through time to determine whether taxonomic turnover was accompanied by changes in community structure. Finally, I used hypsodonty to determine whether the turnover is consistent with what is expected from environmentally forced faunal change in the context of increasing aridity.
Publications Jukar, A. M., S. K. Lyons, M. D. Uhen. Large herbivore turnover in the Indian Subcontinent during the Plio-Pleistocene. IN PREP.
Presentations Jukar, A. M. Faunal change in the Plio-Pleistocene of the Indian Subcontinent. International Biogeography Society 2017 Meeting, Bangalore, India, September 26-28, 2017. A 5 minute lightning talk accompanies this poster. Awarded Best Student Poster. Jukar, A. M., Turnover of Large Herbivorous Mammals in the Indian Subcontinent during the Plio-Pleistocene. Society for Vertebrate Paleontology 76th Annual Meeting, Salt Lake City, October 26-29, 2016.
The effect of topographic heterogeneity on temporal turnover at a major climatic interval
Sample of the FAUNMAP II dataset binned into two temporal bins and spatial bins based on physiographic provinces. Blue dots represent the full-glacial time bin; yellow dots represent the deglacial time bin.
Species respond to climate change by shifting their distributions geographically. Along elevation gradients, species tend to move up or down in elevation in response to climate change. Topographically heterogeneous landscapes tend to include climatic refugia, and are also known to buffer the effects of climate change. I am interested in understanding how regional assemblages of mammals change across intervals of climate change, and how this turnover is affected by the topography of the region they inhabit. While modern datasets of mammals are inadequate to study long term community change, the FAUNMAP II dataset of late Pleistocene and Holocene mammals in North America offers both the geographic and temporal scales necessary to track regional communities through periods of intense climate change in the late Pleistocene.
In this study, I am estimating temporal turnover using pair-wise dissimilarity indices that measure overall community change, change in species composition, and changes in species richness. Communities are binned temporally into two climatic periods–Full Glacial and deglacial, and spatially into physiographic provinces. The values for turnover are then regressed with the range of elevation (a proxy for topographic heterogeneity) to determine if topography has an effect on community turnover across climatic intervals. All analyses were done using R, JMP, and ArcGIS. FAUNMAP II ages were calibrated using OxCal.
Publications Jukar, A. M., S. K. Lyons, M. D. Uhen. Temporal turnover of mammal communities in response to climate change in different topographical landscapes. Global Ecology and Biogeography. IN PREP.
Presentations Jukar, A. M. Temporal turnover of mammal communities in response to climate change in different topographical landscapes. Oral presentation at the 100th Ecological Society of America Annual Meeting, Baltimore, MD, August 11, 2015.
Ecology and Evolution of the Proboscidea
Body-mass estimation in the Proboscidea
Skeletal measurements Used in this study. OCB, Occipital Condyle Breadth; HL, Total Humerus Length; HMC, Humerus Minimum Circumference; FL, Total Femur Length; FMC, Femur Minimum Circumference
Body mass is an important ecological trait that can be easily measured in the fossil record, and is informative in studies of ecology, physiology, and evolution. I developed a novel method to estimate the body mass of extinct proboscideans.
Two allometric scaling relationships are typically used to estimate body mass in proboscideans, one, which relates body mass to shoulder height, and another, which relates body mass with various limb bone dimensions. The challenge with using these relationships for some extinct species, and many taxa that I study, are known or are identifiable from cranial and dental remains alone.
Therefore, I developed allometric equations that predict the total lengths and minimum circumferences of the humeri and femora from the breadth of the occipital condyles. I sampled both extant and extinct species from four families: Gomphotheriidae, Stegodontidae, Mammutidae and Elephantidae, and developed equations for the entire proboscidean sample, and for individual families when possible.
Using a new deinothere dental material from Kutch in western India, colleagues from India and I reassessed dental apomorphies associated with Indian deinotheres, and report on the first occurrence of Deinotherium indicum from the region. This study has implications for the evolutionary replacement, taxonomy, and biogeography of South Asian deinotheres.
Publications Singh, N. P., A. M. Jukar*, R. Patnaik, M. Sharma, N. A. Singh, Y. P. Singh. The first occurrence of Deinotherium indicum (Mammalia, Proboscidea, Deinotheriidae) from the late Miocene of Kutch, India. Journal of Paleontology. In Review.*corresponding author
Variation in dental morphology of Plio-Pleistocene Elephantidae from the Indian Subcontinent
Elephas hysudicus third upper molar from the Pinjor zone of the Upper Siwaliks of India
Elephantids first appear in the Indian Subcontinent in the Pliocene, with Elephas planifrons. This is a long ranging species which extends from roughly 3.6Ma to 0.6Ma. Subsequently, around 2.7Ma, another species appears, Elephas hysudricus, which is suggested to be the ancestor of the modern Asian elephant Elephas maximus. The exact stratigraphic range of E. hysudricus is unclear, but definitive remains have been found from 2.7Ma to 0.6Ma. Another species, Elephas platycephalus has been documented in the Siwalik hills, with a suggested age range of 2.0-0.6Ma. From the middle Pleistocene to the late Pleistocene, the enormous Palaeoloxodon namadicus is found in the Indian peninsular region, along with a species of Elephas that could be a late stage E. hysudricus or E. maximus.
The goal of this study is to quantify the variation in the dental morphology of the these species through time in an effort to understand dental evolution in conjunction to climate and habitat change. I am collaborating with Dr. Adrian Lister, from the Natural History Museum, London.
Coral Reef Ecology
Associations between herbivore functional group diversity and density and ecosystem functioning in the Caribbean
Fish functional groups used in this study
For my masters thesis, I wanted to measure the relative contribution of within functional group diversity and density to ecosystem functioning. I used the coral reef model because of the vast literature on the function of herbivory on reefs and it's contribution to suppressing the overgrowth of macroalgae.
It's been suggested that the diversity of species within a functional group contributes to 1) the portfolio effect, i.e., the ability for a function to persist even when some species are lost, and 2) complementarity of function, i.e. different species contribute slightly differently to the overall function. However, studies have also shown that the density of functional groups also contributes towards normal ecosystem functioning. I used a large database of Caribbean coral reefs, which included information on the density of individual fish species and the percent cover of macroalgae and coral.
Coral reef fish can be split into functional groups based on the mode of feeding, size, and on how mobile they are. Using these functional groups, I used the Shannon-Weiner Diversity Index to measure within functional group diversity, calculated the average density of functional groups, and then used a generalized mixed effects model to understand the associations between diversity and density and the cover of macroalgae. The major finding of this study was that the density of large parrotfish, which function as scrapers and excavators was the most significantly associated with low cover of macroalgae. Another interesting finding was that the density of territorial damselfish was negatively associated with the cover of macroalgae, and a detailed look at the species that make up the functional group revealed that one species was dominant, and that species of damselfish prefers habitats with high coral cover.
Phenotypic Plasticity and Thermal Physiology
Variation in developmental patterns in two geographically separated populations of Bombina orientalisin the Republic of Korea
This thesis used a common garden experiment, conducted in the field in the Republic of Korea, to investigate the effect of the interaction between variable levels of maternal investment and different levels of diel thermal fluctuations on the early development of two populations of Bombina orientalis, one mountain, and one lowland, in order to determine whether the two populations are locally adapted to their native habitats. In addition, in order to understand the patterns of morphological variation induced by diel thermal fluctuations, a laboratory study was modeled to predict field responses from responses in constant laboratory environments. The common garden experiment revealed that maternal investment was positively correlated with total length, tail length, snout-vent length (SVL) and the ratio of tail length to SVL while thermal variance was negatively correlated with total length, tail length and the ratio of tail length to SVL. These relationships were consistent at the hatching stage and 24 hours after hatching. Significant population level differences were also observed for total length, tail length and the ratio but these effects were modified by egg size and thermal variance. Using total length and the ratio of tail length to SVL as surrogates of fitness, the experiment suggested that large mountain eggs produced the largest and fastest hatchlings in the low thermal variance environment while large lowland eggs produced the largest and fastest hatchlings in the high thermal variance environment. Since the low thermal variance and high thermal variance environments were modeled after the mountain and lowland environments respectively, the results of the experiment suggest that the two populations may be locally adapted. The laboratory model for development at different temperature ranges did not match with the field results suggesting that certain morphological traits might be buffered against thermal variance or there might be an interaction between mean temperatures and thermal variance. This study helps answer questions about rapid adaptive evolution in new environments and expands on the role of environmental factors as inducers of phenotypic variation and agents of natural selection.