Literature Review: The Paleozoic Era

by Carl Strang

Last week I shared notes on some published papers from last year which cast light on the early Earth, when life first appeared. The Paleozoic Era was marked by the sudden appearance of abundant, diverse new fossils. What has become increasingly clear is that readily fossilized shells were the major new development, making the “Cambrian explosion” possible (the Cambrian Period is the opening chapter of that era).

Smith, M. Paul, and David A.T. Harper. 2013. Causes of the Cambrian explosion. Science 341:1355-1356. They reviewed major themes that have emerged from research in this area. Some important components are the rising sea/sinking continents which released high mineral concentrations into the oceans (providing material for constructing shells), along with the huge increase in shallow sea habitat (providing much area in which ecological interactions could take place, including predator-prey interactions which would drive the evolution of shells for prey protection and improved predator effectiveness), and growing evidence that earlier, Proterozoic organisms in fact were ancestors of Cambrian groups, which includes molecular clock estimates as well as fossil connections.

Diverse marine invertebrates appeared in the early Paleozoic.

Diverse marine invertebrates appeared in the early Paleozoic.

Jie Yang, Javier Ortega-Hernández, Nicholas J. Butterfield, Xi-guang Zhang. Specialized appendages in fuxianhuiids and the head organization of early euarthropods. Nature, 2013; 494 (7438): 468 DOI: 10.1038/nature11874  From a ScienceDaily article. They described fossils of an early-Cambrian-explosion arthropod, Chenjiangocaris kunmingensis, in which the anterior-most limbs are modified for feeding, apparently for shoveling sediment into the mouth. It was generally soft-bodied, but had a carapace. This is also the earliest known species with a central nervous system extending back from the head. It came from a new site in south China called Xiaoshiba which promises to provide many additional insights into the earliest evolution of animals.

The appearance of shells in the Cambrian, probably resulting from predator-prey interactions, made abundant fossils from that time a possibility.

The appearance of shells in the Cambrian, probably resulting from predator-prey interactions, made abundant fossils from that time a possibility.

Tanaka,Gengo, et al. 2013. Chelicerate neural ground pattern in a Cambrian great appendage arthropod. Nature 502 (7471): 364 DOI: 10.1038/nature12520  From a ScienceDaily article. They described a new species whose central nervous system is preserved well enough to show that it was a chelicerate arthropod, and thus that chelicerates split from mandibulates more than 520 million years ago. Alalcomenaeus belonged to an extinct group of large-clawed arthropods, the megacheirans, that previously had been difficult to place. It had two pairs of large eyes at the front of the head. Earlier, a similar analysis of another Chinese fossil from this deposit, Fuxianhuia, revealed a nervous system that connected it to the crustaceans, which are mandibulates (chelicerate and mandibulate arthropods are the two major groups, the former today including spiders and scorpions, the latter insects and crabs, among others).

Jean-Bernard Caron, Simon Conway Morris, Christopher B. Cameron. Tubicolous enteropneusts from the Cambrian period. Nature, 2013; DOI: 10.1038/nature12017  As reported in ScienceDaily. They described an acorn worm or hemichordate, Spartobranchus tenuis from the Burgess Shale, which is the oldest of its group by 200 million years. Its group is one of the two main groups of hemichordates, and this fossil ties the two together, as well as having connections to echinoderms and chordates. They were extremely abundant in places, and may have been important marine sediment dwellers, their function similar to terrestrial earthworms today.

Pennisi, Elizabeth. 2013. Eating was tough for early tetrapods. Science 339:390. This news article covered recent research on early terrestrial vertebrates (from much later in the Paleozoic than the species mentioned above) that looked at the challenge posed by swallowing prey on land rather than in the water. Developing that capability may have taken 80 million years, with early ones feeding in the water or carrying captured prey into the water where they could have swallowed it along with gulps of water. At some point, tongues evolved that could assist.

Linda A. Tsuji, Christian A. Sidor, J.- Sébastien Steyer, Roger M. H. Smith, Neil J. Tabor, Oumarou Ide. The vertebrate fauna of the Upper Permian of Niger—VII. Cranial anatomy and relationships of Bunostegos akokanensis (Pareiasauria). Journal of Vertebrate Paleontology, 2013; 33 (4): 747 DOI: 10.1080/02724634.2013.739537  As described in a ScienceDaily article. This pareiasaur was endemic to the center of Pangaea, which has been established geologically as a desert region (the continents fused together in the late Paleozoic, and the resulting supercontinent of Pangaea had an interior far from any sea). Apparently that region was so distinct from surrounding more moderate biomes that the boundary served as an isolating barricade for species on either side. This one was a cow-sized herbivore.

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