Page 38 - Science

INSIGHTS | PERSPECTIVES

EARTH SCIENCE
Early plants and the rise of mud

Mudrock deposition in rivers increased by an order of magnitude after plants first evolved

By Woodward W. Fischer deposits on Mars, created where ancient riv- the fabric of the geological record, and taking
ers drained into a lake at the bottom of Gale account of this is particularly important for
he geological record of our planet pro- crater. The martian deposits—like those gen- terrestrial deposits, which are inherently rare
vides evidence for a handful of ways erated on Earth before plants—contain little (8). Using a couple of different approaches,
in which life has fundamentally al- in the way of mudrock (7). Although the sedi- McMahon and Davies demonstrate that the
tered processes and environments at mentary trend has been well-documented fractional abundance of mudrock rose by an
Earth’s surface. It was the evolution of on Earth, it has remained poorly quantified. order of magnitude or more following the
T photosynthesis nearly 2.5 billion years McMahon and Davies set out to measure the evolution of plants. This reflects the tremen-
ago that oxygenated the atmosphere and mudrock trend and narrow down its timing. dous impact that plants have had on the dis-
oceans (1), greatly increasing the spectrum Geologists describe and quantify sedi- tribution of sediment in river corridors.
of minerals found in rocks (2). Over the past mentary rocks using measured stratigraphic Particularly fascinating is the timing of
250 million years, the production of mineral sections, which are one-dimensional repre- this transition. From the estimates of McMa-
skeletons by algae in the oceans transformed sentations that capture the sequence and hon and Davies, increase in mudrock began
the way in which sediments accumulate in thickness of layers of sediment in the order in the Late Ordovician to Silurian (450 to
marine basins (3). On page 1022 of this issue, in which they were deposited at a given lo- 420 million years ago). This implicates land
McMahon and Davies (4) illustrate how plants, cation. McMahon and Davies collated strati- plants, but is earlier than expected. The old- Downloaded from
too, have left an indelible mark in the geo- graphic sections from river deposits before est fossil plants are Ordovician in age; earlier,
logical record, their signature written in mud. and after plant evolution. They combined more equivocal Cambrian-age microfossils
Mudrocks (fine-grained sedimentary rocks these observations with a survey of literature hint at the earliest stages in plant evolution
composed of silt- and clay-sized particles) data to arrive at a data set covering hundreds (see the figure). But it was not really until the
are rare in the sedimentary deposits left by of different sedimentary units, deposited on Late Devonian (about 370 million years ago)
Precambrian and early Paleozoic (500 mil- a range of continents over the past 3 billion that plant ecosystems were sufficiently well
lion years and older) rivers, in which sand years. From each of these sections, they ex- developed to be regarded as forests.
and gravel are common (5); and the rise of tracted the percentage of mudrock present in Early plant evolution occurred over a http://science.sciencemag.org/
mudrocks in river deposits in the geological the river deposits as a function of time. 100-million-year interval of increasing eco-
record somehow reflects changes in the rout- The raw data readily recapitulated the physiological complexity and landscape oc-
ing of sediment by rivers associated with the expected pattern: Mudrocks were rare be- cupation (9, 10). Primitive plants were mostly
evolution of plants and their colonization of fore the appearance of plants and common bryophytes (related to mosses and liver-
the landscape (6). This pattern formed part thereafter. But to obtain more quantitative worts), and early vascular plants in the later
of the basis for interpretation of sedimentary information, statistical treatment of the data Silurian and early Devonian (about 425 mil-
was required, because the number of strati- lion years ago) had stem lengths measured in
graphic units preserved in any given interval centimeters, little water-conducting tissue,
Division of Geological and Planetary Sciences, California on March 1, 2018
Institute of Technology, Pasadena, CA 91125, USA. Email: in the record is highly uneven. This type of no woody tissue, and were limited to wet en-
wfischer@caltech.edu bias is an omnipresent feature woven into vironments. It is interesting that such primi-

Plants reshaped the sedimentary deposits left by a river
Muddy floodplains were rare on prevegetated landscapes, compared with those developed after the evolution of plants and their colonization
of the landscape. The rise of mudrock coincided with some of the earliest events in plant evolution.

Precambrian Cambrian Ordovician Silurian Devonian Carboniferous
Vascular plants
woody trees and shrubs
Mudrock Sand
Vascular plants perennial herbs
Bryophytes mosses, hornworts, liverworts

Microbes mats, crusts, bio,lms
Vegetation Vegetation
Mudrock Lesser Mudrock
Amount of mudrock abundance GRAPHIC: V. ALTOUNIAN/SCIENCE

4500 500 400 300 Today
Time (millions of years before present)

994 2 MARCH 2018 • VOL 359 ISSUE 6379 sciencemag.org SCIENCE
Published by AAAS

DA_0302Perspectives.indd 994 2/28/18 11:03 AM