12.5: Early Paleozoic Life - The Cambrian and Ordovician

The Cambrian Explosion: Hard Parts and Burrowing

The beginning of the Paleozoic Era is marked by the first appearance of hard body parts like shells, spikes, teeth, and scales; and the appearance in the rock record of most animal phyla known today. That is, most basic animal body plans appeared in the rock record during the Cambrian Period. This sudden appearance of biological diversity is called the Cambrian Explosion. Scientists debate whether this sudden appearance is more from a rapid evolutionary diversification as a result of a warmer climate following the late Proterozoic glacial environments, better preservation and fossilization of hard parts, or artifacts of a more complete and recent rock record. For example, fauna may have been diverse during the Ediacaran Period, setting the state for the Cambrian Explosion, but they lacked hard body parts and would have left few fossils behind. Regardless, during the Cambrian Period 541–485 million years ago marked the appearance of most animal phyla.

Figure \(\PageIndex{1}\): Anomalocaris reconstruction by the MUSE science museum in Italy.

Why did animals develop hard, mineralized exoskeletons in abundance during the Cambrian? It was likely that shells were the evolutionary answer to a biological need. Increased animal diversity at this time would lead to greater competition for resources, and an external shell offered several evolutionary advantages: 1) it offered protection from physical and chemical variations in the environment, such as changes in water temperature and salinity; 2) it offered protection from biological predators; and, 3) it allowed muscles additional mechanical leverage during locomotion, making it easier to burrow.

Burrowing by a variety of new invertebrates churned up the sediment, disturbing the continuity of the bedding. This bioturbation is a “trace fossil” signal of the Cambrian Explosion. Indeed, the boundary between the Ediacaran and Cambrian Periods is defined by the appearance of a trace fossil, Treptichnus pedum, which indicates an expansion of animal mobility and more complex interactions in the ecosystem. The development of burrowing also increased organisms' ability to withstand seasonal shifts in currents and exploit food sources below the seafloor, which would further increase their chances of survival.

Figure \(\PageIndex{2}\): Treptichnus pedum defines the Ediacaran / Cambrian transition at the GSSP at Fortune Head, Newfoundland. (C. Bentley annotation of Martin Smith photo via Wikimedia.)

The Cambrian Fauna

Fossil diversity throughout the Archean and Proterozoic is challenging to track because of the lack of hard parts for most of the organisms alive during these times. Once paleontologists recognized the shift to skeletonized animals during the Cambrian at the beginning of the Phanerozoic, many big questions bubbled to the top of paleobiological research. Has diversity constantly been increasing through time? If periods of mass extinction (intervals when many genera or even families die off simultaneously) exist, are there similar periods of diversification, or origination, when many species or higher taxa appear at once? How long is the average species around on Earth? To tackle some of these questions, Jack Sepkoski, a paleontologist at the University of Chicago, developed what is now one of the most famous graphs in the world of paleobiology, his Phanerozoic diversity curve.

Figure \(\PageIndex{3}\): Sepkoski’s (2002) Phanerozoic marine diversity curve. Total height of graph is total number of genera. Blue shading represents the Cambrian fauna, red shading is the Palozoic fauna, green shading is the Modern fauna, and black represents microfossils.

This graph shows the number of marine genera through the Phanerozoic. It is important to note this graph does have a subtle taphonomic bias to it, as it focuses on marine animals after the development of a shelly exoskeleton. When looking at the curve, several major features can be identified: 1) the Cambrian Explosion, a time of significant diversity increase through the Cambrian; 2) the Paleozoic Diversification and Plateau, a time of extensive radiation of invertebrate groups followed by relatively stable diversity from the Ordovician through Permian; and 3) the Cenozoic Rise, a time of exponential diversity growth from the Triassic to today. The five major mass extinctions are also quite noticeable as rapid, sharp declines in diversity at the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous.

When Sepkoski was gathering his data, he recognized another fascinating set of patterns in his Phanerozoic diversity curve. Doing some basic analyses, he was able to identify groups of marine taxa with hard skeletal material that tend to originate at the same time, co-exist for a few geologic periods, and then go extinct at relatively the same time. He called these groups evolutionary faunas. The Phanerozoic diversity curve is divided into three of these evolutionary faunas: the Cambrian fauna, the Paleozoic fauna, and the Modern fauna. It is important to recognize that these groups of organisms are not mutually exclusive, and indeed, throughout much of the Phanerozoic, they coexist. The idea is that, typically, one of the groups is most dominant in ecosystems at particular points of time. Therefore, if you have a general sense of which fossil groups “hang out together” you will be able to make quick deductions about the age of the rocks containing the specimens.

The Cambrian fauna is responsible for the incredible increase in diversity known as the Cambrian Explosion. Some major taxa included in the Cambrian fauna are trilobites, inarticulate brachiopods, and early representatives of echinoderms. Trilobites are a superphylum (Trilobitomorpha) of arthropods that are a hallmark fossil of not only the Cambrian, but also much of the Paleozoic. Many of the taxa of the Cambrian fauna, particularly the trilobites, were hit hard by the end-Ordovician mass extinction, and trilobites are one of many taxa that go extinct during the end-Permian mass extinction.

Figure \(\PageIndex{4}\): Asaphiscus wheeleri, Cambrian trilobite from Utah. (Public domain image.)

Inarticulate brachiopods are rather distinctive and typically fossilize with flattened, dark-colored shell with distinctive curved lines on the shell surface. Some genera, like Lingula sp. possessed a long, fleshy stalk to burrow into shallow marine sediments. While still around today, inarticulate brachiopods are much less common than they were in the early Paleozoic.

Figure \(\PageIndex{5}\): Inarticulate brachiopod Lingula sp. from the Cambrian Conasauga Formation of Alabama. (Photo courtesy of Encyclopedia of Alabama)

Exceptional Preservation: Burgess Shale Fauna

One of the best fossil sites for the Cambrian Explosion was discovered in 1909 by Charles Walcott (1850–1927) in the Burgess Shale in western Canada. The Burgess Shale is a Lagerstätte, a site of exceptional fossil preservation that includes impressions of soft body parts. This discovery allowed scientists to study Cambrian animals in immense detail because soft body parts are not normally preserved and fossilized. Other Lagerstätte sites of similar age in China and Utah have allowed scientist to form a detailed picture of Cambrian biodiversity. The biggest mystery surrounds animals that do not fit existing lineages and are unique to that time. This includes many famous fossilized creatures: the first compound-eyed trilobites; Wiwaxia, a creature covered in spiny plates; Hallucigenia, a walking worm with spikes; Opabinia, a five-eyed arthropod with a grappling claw; and Anomalocaris, the alpha predator of its time, complete with grasping appendages and circular mouth with sharp plates. Most notably appearing during the Cambrian is an important ancestor to humans. A segmented worm called Pikaia is thought to be the earliest ancestor of the Chordata phylum that includes vertebrates, animals with backbones.

Figure \(\PageIndex{6}\): Original plate from Walcott’s 1912 description of Opabinia, with labels: \(fp\) = frontal appendage, \(e\) = eye, \(ths\) = thoracic somites, \(i\) = intestine, \(ab\) = abdominal segment.

The Great Ordovician Biodiversification Event: Reefs Expand

By the end of the Cambrian, mollusks, brachiopods, nautiloids, gastropods, graptolites, echinoderms, and trilobites covered the sea floor. Although most animal phyla appeared by the Cambrian, the biodiversity at the family, genus, and species level was low until the Ordovician Period. During the Great Ordovician Biodiversification Event (GOBE), vertebrates and invertebrates (animals without backbone) became more diverse and complex at family, genus, and species level. The cause of the rapid speciation event is still debated but some likely causes are a combination of warm temperatures, expansive continental shelves near the equator, and more volcanism along the mid-ocean ridges. Some have shown evidence that an asteroid breakup event and consequent heavy meteorite impacts correlate with this diversification event. The additional volcanism added nutrients to ocean water helping support a robust ecosystem. Many life forms and ecosystems that would be recognizable in current times appeared at this time. Mollusks, corals, and arthropods in particular multiplied to dominate the oceans.

Figure \(\PageIndex{7}\): A modern coral reef.

One important evolutionary advancement during the Ordovician Period was reef-building organisms, mostly colonial coral. Corals took advantage of the ocean chemistry, using calcite to build large structures that resembled modern reefs like the Great Barrier Reef off the coast of Australia. These reefs housed thriving ecosystems of organisms that swam around, hid in, and crawled over them. Reefs are important to paleontologists because of their preservation potential, massive size, and in-place ecosystems. Few other fossils offer more diversity and complexity than reef assemblages.

Figure \(\PageIndex{8}\): Guadalupe Mountains National Park in Texas is made of a giant fossil reef complex.

The Paleozoic Fauna

The Paleozoic fauna includes many of the taxa that are responsible for creating and maintaining the Paleozoic plateau of diversity from the Ordovician to the end of the Permian. These groups developed during the Great Ordovician Biodiversification Event (GOBE) as noted from the steep incline on Sepkoski's diversity curve at the onset of the Ordovician. The GOBE is a summation of many increases in diversity among planktonic, benthic, and reef communities during the Ordovician. The Paleozoic benthic seas were dominated by a wide array of relatively abundant invertebrates, including trilobites, articulate brachiopods, bryozoans, reef building tabulate and rugose corals, crinoids, and blastoids.

Figure \(\PageIndex{9}\): Diorama of a Late Ordovician seafloor with many representatives of the Paleozoic fauna. (Creative Commons Attribution 2.0 Generic license)

Interestingly, after initial bursts of species diversity of benthic organisms, there are also increases in the use of ecospace through the Paleozoic. Many of the taxa that evolved during the GOBE were filter feeding organisms, and despite diversification and abundance of phytoplankton and other prokaryotes and unicellular eukaryotes at the base of the food web, there was an increase in competition for resources. Therefore, as the Paleozoic continues, there is an increase in the number of organisms burrowing deeper under or growing taller (e.g., stalked crinoids and blastoids) above the seafloor. These adaptations also aid in maintaining diversity as sediments eroding into epicontinental seas from tectonic activity increase turbidity.

End-Ordovician Mass Extinction

At the end of the Ordovician period, the only animals on Earth were invertebrates living in the ocean. Certain varieties of mollusks, trilobites, graptolites, eurypterids, brachiopods, conodonts, corals, echinoderms, and other groups all went extinct about 443 Ma. These were all key members of the Paleozoic Fauna. Of the species paleontologists have documented, about 52% of them disappeared at the Ordovician/Silurian boundary, never to return. It is worth emphasizing that — uniquely among the Big Five — the end-Ordovician mass extinction was entirely limited to marine organisms, since land had yet to be colonized at that time. After the mass extinction was over, it took 50 million years for Earth’s oceans to recover their former levels of diversity.

The cause of the late Ordovician extinction is inferred to likely be global cooling. There is evidence of glaciation during the late Ordovician in the southern supercontinent Gondwana: this is known from sedimentological documentation of tillites bearing faceted, striated clasts, and dropstones in what is today South America, Arabia and Africa. There are also vast areas where the glaciers appear to have bulldozed pre-existing unlithified sediments, deforming them as the ice flowed along. Using isotopic evidence, Finnegan et al. (2011) have estimated that global seawater temperatures dropped 5 \(^{\circ}\)C during the latest Ordovician. Five degrees may not sound like much, but because of water’s exceptional heat capacity and the huge volume of Earth’s seawater, it represents a truly enormous amount of heat energy lost from the world ocean. Glass sponges, a deep-water siliceous sponge-like animal, are well-adapted to cold conditions, and they did well in the aftermath of the end-Ordovician extinction.

Figure \(\PageIndex{10}\): Faceted and striated clast extracted from Ordovician strata in Arabia. (Modified from Figure 3 of Masri (2017).)

How could a cooling climate trigger a mass extinction? One key variable is a change in the ocean temperature, with negative impacts for those organisms who are specialized for warmer waters. This is particularly acute in the tropical/equatorial regions of the planet’s ocean. Another key impact is in the change in sea level that accompanies the build up of glacial ice mass. Glaciers are made of water, and that water has to come from somewhere. As we are currently seeing in modern times, melting glaciers add water to the ocean, which raises sea level. The reverse is also true: at the height of the most recent ice age, sea level was ~120 meters lower, and the shoreline was at the edge of the continental shelf. This is bad news for marine invertebrates, most of which live in the shallow marine realm. Dropping sea level by more than 330 feet reduces the amount of habitat they have in which to dwell, and increases competition between individuals.

Figure \(\PageIndex{11}\): Map showing the inferred extent of end-Ordovician glaciation in Gondwana. (Map by Callan Bentley, redrawn after an original by Hoffmann and Linnemann, 2013.)

What caused the cooling climate? It appears to be a coincidence of several events: (1) the position of Gondwana over the south polar region, drifting there due to plate tectonics, and (2) a lowering of greenhouse gas levels through some perturbation to the climate. We can break condition “2” down further into a couple of hypotheses: (2a) enhanced silicate weathering by rainwater carbonic acid due to an episode of mountain-building in ancestral North America, the Taconian Orogeny, and (2b) reduced sunlight influx due to stratospheric dust kicked up by a series of meteorite impacts. So, the earliest of five mass extinction events documented in the fossil record was initiated by cooling climate.

4-minute video describing mass extinctions.

Key Terms

• bioturbation - fossil-based evidence of burrowing found as the continuity of the bedding is disrupted
• Burgess Shale - a Cambrian lagerstätte that allowed scientists to study Cambrian animals in immense detail
• Cambrian Explosion - the sudden appearance of biological diversity at the start of the Paleozoic Era
• chordata - a major animal phylum containing all vertebrates and some specialized invertebrates all sharing specific characteristics such as having a hollow dorsal nerve cord
• evolutionary fauna - a group of marine animal taxa with hard skeletons that appear, evolve, and go extinct together as a distinct, dominant community over a particular geological time
• Great Ordovician Biodiversification Event (GOBE) - an event during the late Ordovician in which vertebrates and invertebrates became more diverse and complex at family, genus, and species level
• Lagerstätte - a site of exceptional fossil preservation that includes impressions of soft body parts
• mass extinction - a widespread, rapid decrease in the Earth's biodiversity, where at least 75% of all species perish within a geologically short interval