Page 213 - Introduction to Paleobiology and The Fossil Record

200 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

could specialize within an organism, some (sperms and eggs) between organisms. Typi-
being adapted for feeding, others for repro- cally, the male provides sperm that fertilize
duction, defense or communication. the egg from the female. Gametes have half
But it seems that multicellularity required the normal DNA complement, and the two

sex as well. The first organisms almost cer- half DNA sets zip together to produce a dif-
tainly reproduced asexually, that is, their cells ferent genome in the offspring, but clearly
divided and split. Asexual reproduction, or sharing features of father and mother. In
budding as it is sometimes called, is really just eukaryotes, the DNA exists as two copies,
a form of growth: cells feed and grow in size, each strand forming one half of the double-
and when they are big enough they split by helix structure. Cell divisions in sexual repro-
mitosis to form two organisms. The DNA duction are called meiosis, where the DNA
splits at the same time and is shared by the unzips to form two single copies, one going
two new cells. Sexual reproduction, on the into each gamete, prior to fusion after
other hand, involves the exchange of gametes fertilization.

Box 8.3 Bangiomorpha: origin of multicellularity and sex

Red algae (rhodophytes) today range from single cells to large ornate plants, and they may be toler-
ant of a wide variety of conditions. The modern red alga Bangia, for example, can survive in a full
range of salinities, from the sea to freshwater lakes. The oldest fossil red alga was announced in

1990, and described in detail by Nick Butterfield from the University of Cambridge in 2000. The

specimens are preserved in silicified shallow marine carbonates of the Hunting Formation, eastern
Canada, dated at 1.2 Ga, together with a variety of other fossils, both prokaryote and eukaryote.
In his 2000 paper, Butterfield quippishly named the new form Bangiomorpha pubescens, the

species name pubescens chosen “with reference to its pubescent or hairlike form, as well as the con-
notations of having achieved sexual maturity”. The name Bangiomorpha pubescens has even made
it into the dictionaries of bizarre and cheeky names; one web site notes “The fossil shows the fi rst
recorded sex act, 1.2 billion years ago. The ‘bang’ in the name was intended as a euphemism for
sex.” The fossils do not show sex acts, and the commentators surely exaggerate: Nick Butterfi eld
may be based at the University of Cambridge in England, home of smutty humor since medieval
times (if not before), but he is Canadian by birth!
Bangiomorpha grew in tufts of whiskery strands attached to shoreline rocks by holdfast structures

made from several cells (Fig. 8.13a). The individual filaments are up to 2 mm long, and the cells are
less than 50 μm wide. The cell walls are dark and enclose circular to disk-like cells, and the whole

plant is enclosed in a further thick external layer. The individual filaments may be composed of a
single series of cells, or of several series running side by side, or a combination of the two (Fig.
8.13b). Multiple-series fi laments are composed of sets of wedge-shaped cells that radiate from the
midline of the strand, a diagnostic feature of the modern Bangia and of all so-called bangiacean red
algae.
Many dozens of specimens of Bangiomorpha have been found, and these show how the fi laments
developed. Starting with a single cell, the filament grew by division of cells (mitosis) along the fi la-

ment axis. One cell divided into two, then two into four, and so on. Along the fi laments (Fig. 8.13b),
disk-shaped cells occur in clusters of two, four or eight, and these refl ect further cell divisions within

the filament. Some broader filaments show clusters of spherical, spore-like structures at the top end;

if correctly identified, these prove that sexual reproduction and meiosis were taking place. Close

study of the filaments, and of series of developmental stages, shows that Bangiomorpha was not
only multicellular but that it showed differentiation of cells (holdfast cells versus fi lament cells),
multiple cycles of cell division, differentiated spores and sexually differentiated whole plants.

Read more about Bangiomorpha in Butterfield’s (2000) paper and at http://www.
blackwellpublishing.com/paleobiology/.