Literature Review: Paleozoic

by Carl Strang

This week’s assembly of papers from 2011 is a smattering of studies I encountered pertaining to the Paleozoic Era. In the Paleozoic we see the elaboration of multicellular life, first in the sea and then on land as both plants and animals solved the challenges of terrestrial life.

Peter Van Roy, Derek E. G. Briggs. A giant Ordovician anomalocaridid. Nature, 2011; 473 (7348): 510 DOI: 10.1038/nature09920     Anomalocaris is the most impressive predatory animal of the early Paleozoic, but this invertebrate previously was known only from the Cambrian Period which opened the era. This newly described species was larger than the Cambrian ones, at 3 feet long, and extends the group 30 million years beyond the Cambrian into the Ordovician Period.

G. D. Cody, et al. Molecular signature of chitin-protein complex in Paleozoic arthropods. Geology, 2011; DOI: 10.1130/G31648.1     They found surviving remnants of chitin and protein in a scorpion exoskeleton from the northern Illinois Pennsylvanian Period (310 million years ago) and in a eurypterid from Ontario’s Silurian Period (417 million years ago). The previous oldest preserved organic material was protein from dinosaur fossils. Ten years ago none of this would have been thought possible.

The next paper is about fish jaws such as this example of the extinct group known as placoderms. Field Museum of Natural History exhibit, Chicago.

Philip S. L. Anderson, Matt Friedman, Martin D. Brazeau, Emily J. Rayfield. Initial radiation of jaws demonstrated stability despite faunal and environmental change. Nature, 2011; DOI: 10.1038/nature10207     They did a comparative functional morphology study of early fish jaws across tens of millions of years, looking also at changes in fish communities. They found that jawless fish diversity was unchanged over 30 million years of overlap, calling into question the assumption that jawed fishes outcompeted them. When they did decline, there is no sign that jawed fishes expanded into abandoned ecological space. The initial diversification of jaw structure stabilized well before 400 million years ago, and subsequently remained stable in the face of significant environmental change.

Gerienne, Philippe, et al. 2011. A simple type of wood in two Early Devonian plants. Science 333:837. They describe slender fossil stems from France (407 million years ago) and from Canada (397 million years ago) that become the earliest examples of woody plants. Prior to this discovery, only herbaceous plants were known from this time, and the previous earliest woody plants were from the Middle Devonian, 397-385 million years ago. The size of the stems and details of their cellular structure support the idea that wood first evolved for fluid transport within the plant rather than for support. The timing of this development coincides with a drop in atmospheric carbon dioxide, which would have driven improvements in transport within the plant. The fossils resemble Psilophyton, a precursor of ferns and other vascular plants.

Club moss

Banks, Jo Ann, et al. 2011. The Selaginella genome identifies genetic changes associated with the evolution of vascular plants. Science 332:960-963. They sequenced the genome of a club moss, Selaginella moellendorffii, and compared the result to the genomes of angiosperm plants. Both lineages are vascular, but they diverged shortly after vascular plants appeared 410 million years ago. Club mosses are much less diverse than angiosperms, and much less important ecologically. This may be related to their lack of whole-genome duplications which happened several times in angiosperm history. Most of the genes that direct angiosperm development are present in Selaginella. Comparisons with other groups indicate that the number of genes nearly doubled, from 3800 to 6800, between aquatic green algae and early land plants represented by mosses. A smaller increase of more than 400 genes characterized the common ancestor of Selaginella and Angiosperms, then there was a further increase of 1000 genes in the Angiosperms. After that the whole-genome duplications began.

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