Literature Review: Paleozoic Era

by Carl Strang

The first animals which unambiguously connect to present day forms appear in the fossil record early in the Paleozoic Era, which began 542 million years ago, billions of years after the planet first formed. Here are some notes from studies of this era published in 2014.

American alligator. One of the following studies places the split between the reptilian crocodile-dinosaur-bird group and the lizard-snake group at the very end of the Paleozoic Era.

American alligator. One of the following studies places the split between the reptilian crocodile-dinosaur-bird group and the lizard-snake group at the very end of the Paleozoic Era.

Cong, Peiyun, et al. 2014. Brain structure resolves the segmental affinity of anomalocaridid appendages. Nature, DOI: 10.1038/nature13486 They studied the brain structure of Lyrarapax unguispinus, a fossilized relative of Anomalocaris, and found it was both simpler than those of its contemporary prey, and very similar to those of today’s onychophorans, or velvet worms, terrestrial southern hemisphere forest floor predators with unusual antennae that connect to the brain in the same way that the pair of grasping appendages connected to the brain of Lyrarapax. The similarities suggest a common ancestry.

Jourdan, F., et al. 2014. High-precision dating of the Kalkarindji large igneous province, Australia, and synchrony with the Early-Middle Cambrian (Stage 4-5) extinction. Geology 42 (6): 543. DOI: 10.1130/G35434.1 From a ScienceDaily article. The first major extinction event, which took out 50% of species in the Middle Cambrian, was caused by a mass volcanic eruption in Australia according to this study.

Morris, Simon Conway, and Jean-Bernard Caron. 2014. A primitive fish from the Cambrian of North America. Nature, DOI: 10.1038/nature13414 New Burgess shale fossils from the Cambrian of 505mya (million years ago) show detail in one of the earliest fishes, Metaspriggina, in which branchial arches are revealed as paired, with the first pair slightly thicker than the others (a step toward the first jaw). They had large eyes, and probably were good swimmers.

Shubin, Neil H., Edward B. Daeschler, and Farish A. Jenkins, Jr. 2014. Pelvic girdle and fin of Tiktaalik roseae. PNAS, DOI: 10.1073/pnas.1322559111 From a ScienceDaily article. They describe the anatomy of the rear part of this fish, previously known only from anterior portions. This animal was transitional toward terrestrial life, living in a delta environment where the ability to cross over land from stream to stream was advantageous. It was large, as much as 9 feet long, with large teeth making it somewhat reminiscent of a crocodile. It was lobe-finned, had a flexible neck, and rudimentary lungs. Its well-developed shoulder girdle previously was known, but it had been assumed that it crawled with only its front fins. The surprise was that the pelvic girdle also is developed, with a ball and socket joint and strong hind fins, so these fish had rudiments of four, rather than just two legs.

Ezcurra, M.D., T.M. Scheyer, and R.J. Butler. 2014. The origin and early evolution of Sauria: reassessing the Permian saurian fossil record and the timing of the crocodile-lizard divergence. PLoS ONE 9(2): e89165. doi:10.1371/journal.pone.0089165 They took a close look at Permian fossils in an attempt to resolve debate on when the split happened between the reptilian line leading to crocodiles, dinosaurs and birds on the one hand (archosauromorphs) and lizards and snakes on the other (lepidosauromorphs). They concluded that only the former have been found in the Permian, and place the earliest possible time for the split at 254.7 million years ago (very late Permian).

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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