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Innovative study clarifies evolutionary history of early complex single-celled organisms


BLACKSBURG, Va., March 16, 2004 – A billion years ago (the Neoproterozoic age), complex single-celled organisms, acritarchs, began to develop, grow, and thrive. Almost a billion years later, the study of the evolutionary history of acritarchs began to bog down amid inconsistencies in the reporting of the diversity of species.

Now, a Virginia Tech graduate student has devised a new way to study the ebb and flow of life in the Neoproterozoic and Early Cambrian ages, a period that includes two mass extinctions.

John Warren Huntley, of Asheville, N.C., a Ph.D. student in geosciences in the College of Science, will report on his strategy and results at the joint meeting of the Northeastern and Southeastern Sections of the Geological Society of America to be held March 25-27 in Tysons Corner, Va. Huntley received his bachelor's degree from Appalachian State University and his master's degree from the University of North Carolina at Wilmington.

"The evolutionary history of acritarchs reported in the literature has been based on the number of species," explains Huntley. "But there have been many workers collecting information and there is variation among these researchers on what is considered a species. This variation among workers could alter our understanding of what actually happened."

The strategy of a group of geoscientists at Virginia Tech is to use the quantitative data reported in the scientific literature to look at size and morphological complexity of specimens collected. So far, they have examined acritarch data spanning more than 700 million years – from 1270-million-year-old rocks deposited long before Neoproterozoic ice ages to Early Cambrian successions rocks deposited during the explosive evolution of early animals.

"Our preliminary results seem to confirm previous anecdotal evidence," says Huntley. "We're finding that complexity increases through time, which is to be expected."

However, complexity leveled off. "It appears that morphological complexity may have remained steady at high values, even when species diversity was fluctuating greatly," Huntley says.

As to size, there was a steady increase in size for at least 500 million years until the Ediacaran extinction, after which acritarchs remained very small compared to their pre-Ediacaran extinction size. "There had been anecdotal observations of the size change, which we have now quantified," Huntley says.

Huntley will present the paper, "Secular patterns in morphological disparity and body size of acritarchs through the Neoproterozoic and early Cambrian" (47-2) at 1:20 p.m. Friday, March 26, as part of the session on Pre-Cenozoic Paleontology in the Gunston A room at the Hilton McLean-Tysons Corner hotel. Co-authors are Virginia Tech geosciences professors Shuhai Xiao and Michal Kowalewski.

The trio began their study of acritarchs last October. "It is interesting to use novel techniques to study early life and this is a good opportunity to increase my knowledge in this important area," says Huntley, who has been studying mollusk evolution.

For more information, contact: John Warren Huntley, (540) 231-1913 or jhuntley@vt.edu; Michal Kowalewski (Huntley's major professor), (540) 231-5951 or michalk@vt.edu; or Shuhai Xiao, (540) 231-1366 or xiao@vt.edu.

Abstract:
Acritarchs are a group of phylogenetically heterogeneous microfossils that are interpreted as the resting cysts of eukaryotes. Their diversity through the Neoproterozoic and early Cambrian has been estimated by counting species. These estimates suggest that diversity increased steadily in the early Neoproterozoic and then decreased sharply between 700 and 600 Ma, a drop likely related to Neoproterozoic glaciations. An ephemeral diversification of process-bearing acritarchs (acanthomorphs) followed the glaciations, but many of these acanthomorphs disappeared in Ediacaran time. Acritarch species diversity increased again in the early Cambrian. Anecdotal evidence also suggests that acritarch size and complexity increased throughout the Neoproterozoic until the Ediacaran extinction, and that post-Ediacaran acritarchs reached the levels of morphological complexity, but not the large sizes, recorded for the Neoproterozoic.

This reconstruction of the evolutionary history of acritarchs has recently been brought into question, due to severe inconsistencies associated with acritarch taxonomy. We propose an independent strategy for investigating the evolutionary history of these early eukaryotes, by analyzing size and morphological complexity of acritarchs using quantitative data obtained from the literature. This method is not affected by the taxonomic problems of naming and counting species. Data are being assembled from monographs on Neoproterozoic to early Cambrian acritarchs. Size and morphologic data collected include: vesicle size and morphology, process size, morphology, and count, presence/absence of enveloping membranes, and excystment structures. Other data collected include: paleogeographic location, best estimated age, lithology, depositional environment, mode of preservation, and preparation technique. The resulting dataset is explored using multivariate and computer-intensive methods.

Preliminary data do not suggest any significant changes in acritarch vesicle size between720 and 580 Ma. Moreover, Multidimensional Scaling based on morphological characters does not suggest any major shifts within the occupied morphospace through this time interval. These pilot results should be interpreted with caution until more data are available.