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Adam Maloof - Research

Cambrian Explosion


The Cambrian explosion is the most spectacular evolutionary transition seen in the fossil record. Essentially all of the known, preservable animal body plans appear in the fossil record for the first time during this interval. In 1859, Charles Darwin famously considered the Cambrian explosion to be in potential conflict with the theory of evolution:

"I cannot doubt that all the [Cambrian] trilobites have descended from some one crustacean, which must have lived long before the [Cambrian] age... Consequently, if my theory be true, it is indisputable that before the lowest [Cambrian] strata was deposited, long periods elapsed, as long as, or probably longer than, the whole interval from [Cambrian] to the present day... The case must at present remain inexplicable; and may be truely urged as a valid argument against the views here entertained"


Why is there no fossil record of macroscopic animals during the first four billion years of Earth history? Are soft-bodied Ediacaran fauna (first appearance 575 million years ago (Ma)) and lightly calcified Cloudina and Namacalathus (first appearance 555 Ma) related to the animals that appear during the Cambrian? Why did the Ediacara and Cloudina/Namacalathus go extinct at the Precambrian-Cambrian boundary (542 Ma), and how is their extinction related to the coincident first appearance of complex animal burrows and a global -6 permil carbon isotope excursion? Classic explanations of the Cambrian explosion usually fall into one or more of the following three categories: (1) changes in the chemistry, temperature and geometry of the oceans; (2) changes in the genetic or developmental capacity of the organisms involved; and (3) changes in ecology. Our work in Morocco focusses on category (1), and attempts to place observations of developmental and ecological change in the context of environmental changes that are calibrated in absolute time.

In the Moroccan Anti-Atlas Mountains (photos), the Early Cambrian is represented by 2.5 km of nearly continuous carbonate sediment, punctuated with layers of volcanic ash. The very high sediment accumulation rates have allowed us to develop a high resolution record of carbon-isotopic variability [pdf]. In collaboration with Sam Bowring and Earth Time, we have performed precision ID TIMS U-Pb dating of single zircon crystals from interbedded ash flow units in order to calibrate carbon-isotope and paleontological events in absolute time. The results are a Geology paper that came out in July 2010, and a GSA Bulletin paper that became available online September 13.


Currently, we are in the process of determining the absolute duration of single carbon isotope excursions. To our surprise, some very large carbon isotope fluctuations were accomplished in just a few 100 kyr - too fast to fit into classical models of steady-state carbon cycling. The explanation for dynamic d13C variations will have profound implications for our interpretation of environmental and paleontological events of the Cambrian, as well as our basic understanding of carbon cycling on Earth's surface. In addition, we are collaborating with David Fike (Washington University) and Sam Bowring (MIT) to expand the chemostratigraphy to include d34S, d13Corganic87Sr/86Sr, and trace element geochemistry. By placing geochemical and paleontological events in the context of absolute time, this study will shed light on the age-old question: Do ecological and/or evolutionary changes drive changes in carbon cycling, or do environmental changes (reflected in perturbations to the carbon cycle), cause or allow for ecological and evolutionary developments?