Literature Review: Earliest Animal Life

by Carl Strang

Yesterday’s post featured reef communities of today and the distant past. Today I want to stay in those early times when multicellular animals first entered the fossil record. The first paper was published about a year ago in Science.

Erwin, Douglas H., et al. 2011. The Cambrian conundrum: early divergence and later ecological success in the early history of animals. Science 334:1091-1097.

They used a variety of improved molecular clock, fossil, developmental and ecological data to look at animal diversification which seemed to appear full blown in the Cambrian Period at the beginning of the Paleozoic Era. They concluded “that the major animal clades diverged many tens of millions of years before their first appearance in the fossil record,” with basic developmental toolkits appearing in the Cryogenian Period of the Proterozoic Eon (before that eon’s final, Ediacaran Period). The researchers place the split between sponges and other animals in the mid-Cryogenian about 750 million years ago (mya), with Cnidaria (the group that today includes corals, sea anemones and jellyfish) appearing around 700mya, Chordates around the beginning of the Ediacaran, arthropods around the beginning of the Cambrian, and vertebrates in the late Cambrian. Only some of the well-known fossil Ediacaran organisms can be tentatively tied to the animals of today: some possible sponges, mollusks and placozoa. Otherwise, there are only some suggestive trace fossils (e.g., burrows) from the Ediacaran Period that point to metazoan animals. Erwin’s group attributes the apparent “Cambrian explosion” to the evolution of predation, which applied selective pressure resulting in protective shells and other structures that were better preserved in the fossil record.

Barnacles, arthropods that protect themselves with shells.

Another paper, published earlier this year, added another dimension to the story.

Shanan E. Peters, Robert R. Gaines. Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion. Nature, 2012; 484 (7394): 363 DOI: 10.1038/nature10969

As reported in a ScienceDaily article. This study focused on the largest gap in the geologic record worldwide, dividing the Proterozoic Eon from the Cambrian Period which opened the Paleozoic Era, and tied that unconformity to a hypothesis about the sudden appearance of diverse life forms and skeletal features. They suggest that the erosion of preCambrian rock that produced the unconformity had the effect of adding concentrations of dissolved minerals to the sea. The resulting altered chemistry of their environment posed a challenge to living forms. The first production of biominerals thus was to remove those substances from organisms’ tissues. Having evolved that capability, animals then had the foundation for evolution of various uses of those minerals in shells and other skeletal formations, teeth, etc.

Thus the geological processes that grew the early continents, and lifted them above the sea, altered the chemistry of that sea. There were no land plants to resist the erosion. The marine animals, in addressing the challenge posed by the increased mineral content, found ways to create hard parts which in some were useful tools for predation, and in others were armor to resist that predation. The visible result of this biological arms race was the “Cambrian explosion,” in which multicellular life forms suddenly began to appear as fossils. But now evidence exists that points to those animals’ ancestors having diversified much much earlier.

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1 Comment

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