Trilobites Found in Ash Layers

Trilobites are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (521 million years ago).

Trilobites featured three lobes on their exoskeletons and had half-moon-shaped skulls through which they breathed using their legs. These creatures have been found in layers of petrified ash within sandstone long the coast of Ko Tarutao island in Thailand. The layers were formed by ancient volcanic eruptions that settled on the sea floor, resulting in a green layer known as tuff.

"The tuffs will allow us to not only determine the age of the fossils we found in Thailand, but to better understand parts of the world like China, Australia, and even North America where similar fossils have been found in rocks that cannot be dated," said Shelly Wernette, the first author of the monograph, which details the new fossil findings.

The researchers from the University of California identified 12 types of trilobites which had previously been documented in other parts of the world, but never in Thailand.

Nigel Hughes, monograph co-author and UC Riverside geology professor, said “Because continents shift over time, part of our job has been to work out where this region of Thailand was in relation to the rest of Gondwanaland. It’s a moving, shape-shifting, 3D jigsaw puzzle we’re trying to put together. This discovery will help us do that."

The findings have been described in a monograph published in the journal Papers in Palaeontology.

Abstract:

Tuff-bearing upper Cambrian to lowermost Ordovician strata on Ko Tarutao island, Satun province, southernmost peninsular Thailand, contain a rich trilobite fauna relevant to global biostratigraphy, peri-Gondwanan palaeogeography and shifting evolutionary mode. This area of Sibumasu, a lower Palaeozoic marginal Gondwanan terrane, is shown to have been closely associated with Australia, North China (Sino-Korea) and other continental fragments from the supercontinent's northern equatorial sector, including South China at that time. Shared faunas also suggest a Kazakhstani and Laurentian association. Collections from eight sections yielded 10 newly discovered species and one new genus from ancient shoreface and inner shelf siliciclastic deposits. With the new taxa and revision of taxa known previously, we refine the age of the upper two formations of the Tarutao Group to the middle of Cambrian Stage 10, and lower–middle Tremadocian. Two biozones are erected for Sibumasu: the Eosaukia buravasi Zone, encompassing all Cambrian sections from Ko Tarutao, and the Asaphellus charoenmiti Zone, encompassing the Tremadocian fauna discussed herein. The new genus is Tarutaoia and new species are Tsinania sirindhornae, Pseudokoldinioidia maneekuti, Pagodia? uhleini, Asaphellus charoenmiti, Tarutaoia techawani, Jiia talowaois, Caznaia imsamuti, Anderssonella undulata, Lophosaukia nuchanongi and Corbinia perforata. Other taxa reported for the first time from Tarutao are Mansuyia? sp., Parakoldinioidia callosa Qian, Pseudagnostus sp., Homagnostus sp., Haniwa mucronata Shergold, Haniwa sosanensis? Kobayashi, Lichengia simplex Shergold, Pacootasaukia sp., Wuhuia? sp., Plethopeltella sp., Apatokephalus sp., Akoldinioidia sp. 1 and Koldinioidia sp.

Theories About the Cambrian Explosion

Cambrian-Ordovician rock succession at Shingle Pass in the Egan Range of Nevada. The SPICE carbon isotope event is in Late Cambrian strata at the far left, and the succession becomes younger into the Middle Ordovician toward the right. Plankton and animal biodiversity increases from the Cambrian to the Ordovician (left to right). Image Credit: Dr. Matthew R. Saltzman, Ohio State University

The Cambrian period, from 542 to 488 million years ago, is the subject of great interest to earth scientists, botanists, and biologists. The Cambrian explosion saw a relatively sudden increase in organic diversity. Different theories exist as to what triggered the change. 

One explanation involves the increase in atmospheric oxygen about 500 million years. According to this theory, photosynthetic plankton pulled carbon dioxide out of the atmosphere and released large amounts of oxygen, creating an atmosphere that we would consider breathable.

The evidence is found in rock fossils called acritarchs. Rock samples containing these fossils preserve an indirect record of oxygen amounts in the form of carbon isotopes. Isotopes of an element have the same number of protons but different numbers of neutrons. There are two stable carbon isotopes in nature: the most common, C-12, has 6 protons and 6 neutrons; but about 1.1 percent is C-13, with 6 protons and 7 neutrons. In an atmosphere with high levels of oxygen, photosynthesis within plankton favored carbon-12 over the heavier carbon-13, a signature that was preserved in the fossil record.

The Cambrian period also was marked by extinction events. One explanation for the extinction involves a condition called anoxia where the ocean's oxygen levels drop dramatically, causing many organisms to die. This event, known as the Steptoean Positive Carbon Isotope Excursion (SPICE), was characterized by the burial of large amounts of organic matter on the ocean floor.

Recently, a team from the University of Exeter's Global Systems Institute proposed a theory involving plate tectonics. According to this theory during the formation of the supercontinent there was an increase in continental arc volcanism. Long chains of volcanoes formed where continental and oceanic tectonic plates collided. This led to the increased release of CO2 from ancient, subducted sedimentary rocks. The Exeter team calculated an increase in atmospheric CO2 and warming of the planet, which in turn amplified the weathering of continental rocks. This process supplied phosphorus to the ocean to drive photosynthesis and oxygen production.

Clearly, the factors that led to a habitable Earth and organic diversity are complex. Many questions remain to be answered. One wonders how an increase in oxygen levels explains the emergence of bilateral symmetry in those early living organisms? Why that characteristic and not some other?

In the Precambrian organisms we find neither bilateral symmetry nor any bivalves. The potential for greater diversity and complexity seems to coincide with the appearance of binary features. Too little attention is given to such signals or triggers in the fossil record.

The unity of organic life can be explained by several paradigms. Evolutionary processes, divine supervision and intervention, Earth's physical forces, etc. In science we must go with the paradigm or hypothesis that best aligns with the physical evidence. The appearance of bilaterally symmetric invertebrates in the latest Precambrian Ediacaran suggest that bilateralism was a precursor to greater complexity and hierarchies. Consider how binary language used in computer science has led to great complexity. Random sequences degrade. Binary features and binary sets increase complexity. 

Related reading: Does the Cambrian Explosion Pose a Challenge to Evolution?; Does the Binary Feature Signal Greater Complexity?; Cause of Cambrian Explosion —Terrestrial or Cosmic?