Julie A. Meachen, Ph.D.
My research involves examining the morphology of extant and extinct vertebrates, with a focus on the mammalian order Carnivora, to investigate how these animals co-exist, evolve and adapt to biotic and environmental change. I am interested in functional adaptations as tools for examining species ecological interactions in the past and present to make predictions for future generations of species. To answer these functional questions I use a morphometric approach which involves a combination of techniques including geometric morphometrics, radiographs, CT scans, and linear measurements. I am also interested in paleontological fieldwork and have participated in field work from sites across the U.S.
My current research program as a postdoctoral associate at the National Evolutionary Synthesis Center (NESCent) involves exploring guild structure and resource partitioning within an ecosystem, and functional diversity over space and time in the mammalian order Carnivora. Competition among the carnivorans is compelling; character displacement may be more dependent on the presence of other carnivore species than on prey specialization. One phenomenon that results from resource partitioning is non-random size structure within a guild. Carnivorans may also partition their niches through diet or habitat specialization which is usually reflected in dental and/or post-cranial morphology.
In collaboration with Trina Roberts of the Iowa Museum of Natural History, I am deriving a carnivore morphospace constructed from body mass, dental and post-cranial characters to assess niche partitioning and functional diversity in mammalian carnivores. Together, Trina and I have come up with a way to identify patterns of clustering or repulsion among measurements (mass, cranial, and postcranial) at multiple scales. This is a critical element of understanding community assembly, as it is possible that different factors structure diversity at different scales. For example, shared ancestry (phylogenetic constraints) might act to keep all members of a family close in size, appearing as clustering at a large scale, but competition might structure them within that size range, appearing as evenness at a smaller scale. Our combination of modern and paleontological data, multiple sites, and multiple types of functional measurements makes it possible to examine community assembly in three dimensions, across time, across space (localities) and across variables, thereby allowing us to extract novel combinations of factors that affect community composition. For example, dietary partitioning may be important under some circumstances, whereas mass may be more important in another time or locality. Looking at multiple communities in a hierarchical way gives a clearer picture at what processes are shaping carnivore communities through time and space.
Following from my original NESCent project involving carnivore guild structure, I have started a project in collaboration with Lars Werdelin of the Swedish Museum of Natural History which examines the functional diversity of extant carnivorans to investigate the influences of temperature, precipitation and habitat type on postcranial morphology. Our questions for this project include: How does variation in habitat and climate induce morphological variation? How is the relationship between habitat/climate and morphological variation constrained by phylogeny? And are habitat and/or climate related to functional richness in carnivore communities?
To answer these questions we have combined linear measurements with multivariate analyses to examine morphological disparity and how it relates to climate and habitat.
Recently, I have submitted a grant to NSF with Co-PI Alan Cooper of the University of Adelaide to re-open Natural Trap Cave in Wyoming. We propose to re-open this exceptional site with the goal of investigating causes of the megafaunal extinction and paleoecological responses to climate change at the end of the Pleistocene. NTC gives us the opportunity to track genetic changes through a long period of time that spans the pre- to post-extinction events. A key strength of our approach is to bring a broad range of scientific methods and researchers together so that the same materials are analyzed using morphological, biogeochemical, and genetic techniques. This synergistic approach will establish a new standard for the analysis of recent paleontological material and promises to produce a far more resolved picture of paleoecological change than has previously been possible. Using ancient DNA, accelerated mass spectrometry (AMS) radiocarbon dates, ash-layer dating, stable isotope analyses, and results from previous studies of NTC we will correlate the timing of the extinction with climate records and genetic variation in species found at this locality. Here, we propose to investigate the questions: Was the Pleistocene extinction event preceded by, or concurrent with a large loss of genetic variation? Do mammal species that did not go extinct exhibit similar genetic changes? Can population metastructure be detected through time, and how is this influenced by climate?
If funded, we would begin excavations as early as summer 2012. Please contact me if you would like more information on this project or if you are interested in participating in the excavations.
Along with collaborator Robin O'Keefe from Marshall University, I am involved in a project at Rancho La Brea that will examine the timing and causes of the mass extinction event as it happened at this locality. Based on previous work by Robin and others, we are examining developmental stress in carnivores from Rancho La Brea. Developmental stress can give an indication of the competitive environment and food availability during the time of pit deposition. Surprisingly, in pit 61/67 (mean age 11.5 Ka), the pit that was most likely concurrent with the megafaunal extinctions in North America, there is relatively little developmental stress in carnivores, suggesting that during the mass extinction time period, carnivores were doing relatively well at Rancho La Brea. We hope to answer the questions: What is the timing of the extinction event at Rancho La Brea? Why were the carnivores thriving right before/during the extinction event? Was human overhunting initially contributing to the success of carnivores before their ultimate demise?
Many pits at Rancho La Brea have been radiocarbon dated, but not with the accuracy that is required to answer these questions about the megafaunal extinction event. We plan to submit an NSF proposal in January 2012 to obtain numerous, accurate radiocarbon dates for Rancho La Brea to test our hypotheses.
Publications
Meachen-Samuels, J. In Press. Morphological convergence of the prey-killing arsenal of sabertooth predators. Paleobiology.
Van Valkenburgh, B., A. Curtis, J. X. Samuels, D. Bird, B. Fulkerson, J. Meachen-Samuels, and G. Slater. 2011. Aquatic adaptations in the nose of carnivorans: evidence from the turbinates. Journal of Anatomy 218: 298-310.PDF
Meachen-Samuels, J. and B. Van Valkenburgh. 2010. Radiographs Reveal Exceptional Forelimb Strength in the Sabertooth Cat, Smilodon fatalis. PLoS ONE 5: e11412. PLoS ONE link
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Meachen-Samuels, J. 2010. Radiographic analysis of the humerus in the Felidae and the Canidae. Journal of Mammalian Evolution 17: 193-209.PDF
Meachen-Samuels, J. and W. Binder. 2010. Age determination and sexual dimorphism in Panthera atrox and Smilodon fatalis (Felidae) from Rancho La Brea. Journal of Zoology, London. PDF
Press Coverage:
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Samuels, J. X., J. Meachen-Samuels and P. Gensler. 2009. The first mid-Blancan occurence of Agriotherium (Ursidae) in North America: a record form Hagerman Fossil Beds National Monument, Idaho. Journal of Paleontology 83:597-603. PDF
Meachen-Samuels, J. and B. Van Valkenburgh. 2009. Forelimb indicators of prey-size preference in the Felidae. Journal of Morphology 270:729-744. PDF
Meachen-Samuels, J. and B. Van Valkenburgh. 2009. Craniodental indicators of prey-size preference in the Felidae. Biological Journal of the Linnean Society 96:784-799. PDF
Meachen, J. 2005. A new species of Lamine from the Plio-Pleistocene of Florida. Bulletin of the Florida Museum of Natural History 45:435-447. PDF
Webb, S.D. and J. Meachen. 2004. On the origin of the Lamine Camelidae with a new genus from the late Miocene of the high plains. Bulletin of the Carnegie Museum 36:349-362. PDF