I seek to understand the tempo and mode of eukaryotic and animal evolution in Precambrian seas. Changing biological and environmental factors during this period of Earth history set the stage for the radiation of animals and the emergence of Phanerozoic ecosystems and diversity. However, biotic events in the Precambrian are often obscured due to gaps in the fossil record, poor age constraints, and uncertainty surrounding the taxonomic affinity of fossils. Through my research program, I seek to overcome these problems by integrating data from the fossil record, geochemical proxies, biomarkers, and modern phylogenetics and physiology. I am also interested in interpreting the Phanerozoic organic-walled microfossil record in light of modern work on eukaryotic resting stages.
Organic biomarkers from the Chuar Group
We are investigating organic biomarkers from the Neoproterozoic Chuar Group of the Grand Canyon. The Chuar is an ideal setting in which to look at potentially coupled environmental and biological signals because there is a wealth of robust geochemical and paleontological data throughout the Group. I am working with Roger Summons from MIT and Carol Dehler from Utah State, as well as with MIT undergraduate Kim Barker and lab technician Emily Matys.
Enigmatic Mineralized Scale microfossils from the Tindir Formation, Yukon Territory:
I am actively involved in a reinvestigation of enigmatic protistan mineralized scales found in Neoproterozoic rocks of the Tindir Formation of the Yukon Territories and Alaska (translation - strange microscopic fossils made of hard stuff). These fossils, first described systematically by Alison and Hilgert (1981), are unique to the Tindir formation. In June 2007, I and colleague Francis Macdonald returned to the original site and re-collected many of the chert beds originally sho
wn to be fossiliferous. Our results indicate that the unit from which the fossils originate is 150-250 million years older than previously suggested (See Macdonald et al. 2010, Geology). Importantly, we have found that the fossils are not only being preserved in chert, but also in the limestone in which the chert is hosted. This means we can extract the microfossils using weak acid and view them under a scanning electron microscope. This allows us to see the fossils in previously unrecognized and unaccessed detail. Using both chert hosted and limestone hosted specimens, we have shown that the fossils are made of calcium apatite and organic carbon, and are likely the oldest evidence of eukaryotic biomineralization. This work was published in the June 2011 issue of Geology; the pdf may be downloaded here. I collaborated with Nicholas Butterfield at Cambridge University and with Bill Schopf's lab at UCLA on the morphology, composition, and taxonomic affinity of the Tindir scale fossils.
Press relating to the June Geology article Phosphate biomineralization in mid-Neoproterozoic protists
Subsequent re-sampling of the Tindir Formation in June of 2008 resulted in hundreds of additional chert samples from both the original locality and importantly, from new sections laterally distributed within a few kilometers of the original site. We expect that these new samples will shed light on the temporal and spatial distribution of these unique fossils as well as provide us with a greater sense for their biodiversity.
A Composite Stratigraphy of the Neoproterozoic:
One of the great challenges in Neoproterzoic Earth history is improving our ability to tell time. In the Phanerozoic, biostratigraphy (using fossils to correlate strata) allows us an easy way to place events along the geologic timeline when absolute radiometric dates are not available. Along with my collaborators Pete Sadler and Mary Droser of UC Riverside, we are working towards creating a composite stratigraphy of the Ediacaran time period in order to gain insight into the dynamics of Ediacaran biotic evolution and Earth systems. We are approaching this by creating a composite biostratigraphy of the Ediacaran using Pete Sadler’s correlation program CONOP9. Acritarchs offer the obvious and perhaps only option for a biostratigraphic zonation comparable with that applied to early Paleozoic rocks. Acritarchs are decay-resistant, closed, organic-walled microfossils that represent metazoan cysts and/or phytoplankton. In Ediacaran strata they are abundant, diverse, and widely distributed forms with distinctive complex shapes. We are assembling a sequence of first and last appearances for all acritarch taxa from late Cryogenian to early Cambrian, to add other potentially time-significant Ediacaran events, and to supply an uncertainty interval for the position of each event in the sequence; i.e. rather than a suite of global biozones we will start to build a reproducible time line of Ediacaran events, anchored on the first and last appearances of acritarch taxa.
Taxonomic Affinity of Ediacaran Acritarchs:
Acritarchs, closed organic walled microfossils, have historically been considered phytoplankton because of their small size, their environmental placement, and their similarity to Phanerozoic forms such as prasinophyte algae and dinoflagellates. These affinities, however, do not necessarily form the most parsimonious categorization of acritarchs, especially the diverse taxa found in Ediacaran rocks. In terms of pure morphological similarity, many Ediacaran acritarchs bear closer resemblance to the diapause egg cases of modern invertebrate groups, 
especially those of certain arthropods. I use Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) to analyze the ultrastructure of fossil and modern forms in order to resolve the issue of Ediacaran acritarch taxonomic affinity. Comparison of both morphology and ultrastructure between modern analogs and a diversity of acritarch taxa reveals further similarity between some groups of Ediacaran acritarchs and modern metazoan cysts. Placement of some Ediacaran acritarchs within the Metazoa requires reinterpretation of the acritarch record and invites ecological and environmental hypotheses regarding the presence of recalcitrant cysts in Ediacaran seas.
Read more about this project here and here.
Macroscopic tubular fossils from the Ediacaran Nama group, Namibia:
In June 2006, I participated in the Agouron Advanced Field Course investigating the terminal Ediacaran and early Cambrian of Namibia. As part of this collaborative project, I have headed up work on abundant macroscopic tubular fossils found in terminal Neorproterozoic shales of the Nama Formation.
Hear more about my thoughts on my research (and my outreach work) in this interview with Robyn Dahl