Page 449 - Introduction to Paleobiology and The Fossil Record

436 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Placoderms were fearsome predators, some of Other Devonian fishes were more modern
them, like Dunkleosteus from the Late Devo- in appearance. The fi rst shark-like chondrich-

nian of North America, reaching the impres- thyans, or cartilaginous fishes, came on the
sive length of 10 m. This was the largest scene during the Early Devonian. Acanthodi-

animal that had lived until then, and its size ans were small fishes, mostly in the range
and fearsome jaws may explain why so many 50–200 mm in length, and they bore numer-

Devonian fi shes were armored. ous spines at the front of each fin and in

Box 16.4 Genome duplications and vertebrate evolution

Vertebrates have larger genomes than other animal groups. The genome is the entire sequence of
genes contained on all the chromosomes within the nuclei of cells. Various worms and insects have

around 15,000 genes in their genomes, while the figure is 31,000 for humans, 30,000 for the mouse

and 38,000 for the pufferfish. However, vertebrates do not just have more genes than invertebrates,
they have two, four or even eight copies of many individual invertebrate genes. At one time, molecu-
lar biologists thought that humans had as many as 100,000 genes, but the reduced fi gure was
established in 2004 after the intense gene sequencing efforts of the Human Genome Project. What
does genome size mean?
Some have suggested that genome size maps on to the complexity of an organism. Surely, a single-
celled bacterium does not need many genes because it does not do much, and vertebrates, as much
more complex organisms, would need more genes. Humans ought to have the largest genomes since
we are somehow very complex and important. In fact, genome size is only loosely related to bodily
complexity: the largest genome reported so far comes from a lungfish! Much of the genome is so-

called junk DNA, or at least duplicate genes and non-coding sections, so the functional genome size
might be a better correlate of function or bodily complexity.
Whether functional or not, molecular biologists have proposed that there were at least
three genome duplication events (GDEs) in the history of vertebrates – times when evolutionary
change was dramatic and large sectors of the genome duplicated. GDEs are identified at the

origin of vertebrates, the origin of gnathostomes and the origin of teleosts, the hugely diverse modern

bony fishes (Furlong & Holland 2004). Could the evolutionary jump have caused the GDE, or

perhaps the GDE stimulated rapid and fundamental reorganization of the fishes at these three
points?
Donoghue and Purnell (2005) suggest that molecular biologists have been misled. By omitting
fossils, they see artificial morphological jumps in their cladograms, and then link this to the

postulated GDE. In fact, when fossils are inserted, the “jumps” seem less clear. For the origin of
gnathostomes, biologists have compared lampreys with sharks, and there is a wide gulf
between these two groups, so suggesting quite a leap in terms of anatomic change and in terms of
genome duplication. However, when fossils are inserted (Fig. 16.7), seven major ostracoderm and
placoderm clades fall between the living groups, and the evolutionary transition is stretched. Some
of the fossil groups (especially pteraspidimorphs, conodonts and placoderms) were diverse, and it is
not clear that the GDE drove, or permitted, a single dramatic burst of speciation, as had been
proposed. Further, it is not clear that there was a single reorganization of anatomy associated with
the origin of jaws and the GDE: the fossils show step-by-step character changes over a long
interval.
This is a developing field of study. The claim that genome duplication can drive major bursts

of evolution is dramatic, and perhaps overstated. Paleontologists can make profound contribu-
tions in new areas of science by working hand-in-hand with molecular and developmental
biologists.
Read more through http://www.blackwellpublishing.com/paleobiology/.