The Cambrian in North America

The Cambrian Period, the first period of the Paleozoic Era (“era of ancient life”), was named in 1835 by the British geologist Adam Sedgwick. He derived the moniker from Cambria (or Cumbria) in North Wales–where the type section was located–and which was the Latin name for the region (having its etymology in the Welsh word “cymry,” meaning “countryman” or “compatriot” against Anglo-Saxon invaders).

underwater fossil

Evolution had been busy for billions of years before the first fossils of multicellular creatures are recorded in the sediments. Although in 1991, the International Subcommission on Cambrian Stratigraphy formalized the Precambrian-Cambrian boundary at the first appearance of the horizontal burrow Trichophycus pedum in the reference section at Fortune Head, Newfoundland (age 543 million years ago, or mya), this corresponds in practicality to the appearance of a widespread, diverse fauna possessing easily fossilizable hard parts (the traditional beginning of the Cambrian).

Scientists as far back as the nineteenth century, including Charles Darwin, recognized that something special marked the onset of the Cambrian; life, in all its fabulous diversity, seemed to spring almost magically and instantaneously from the vast void of the Precambrian.

Until the advent of radiometric dating, there was no way of knowing how long ago that was, or how much time the Cambrian Period represented. The earliest radiometric dates indicated that the Cambrian began shortly after 600 mya and spanned almost 100 million years.

More accurate dates obtained in the last two decades have resulted in a revision of the chronology and a compression of the length of time the Cambrian Period represents. The overall timeframe has been shifted towards the recent in the process. In 1998 the Cambrian-Ordovician Boundary Working Group adopted a biostratigraphical definition of that boundary with an age of 490 mya. Thus the current definition of the Cambrian encompasses an interval of 53 million years.

Before we consider what evolution was doing during the “Cambrian Explosion”–a term popularized in the 1970s for this seemingly sudden proliferation of lifeforms–lets orient ourselves with an overview of the paleogeography and climate. You may recall the term “Grenville Orogeny” from our discussion of the tectonic activity leading to the creation of the core of the North American continent. This was a really big tectonic episode occurring around a billion years ago that had ramifications for all the continents, not just North America. It resulted in the assembly of the first well-known “supercontinent,” variously known as Pannotia, Rodinia, Proto-Pangea, or Ur-Pangea.

Dont confuse this supercontinent with the familiar Pangea of Triassic times–this was a much earlier precursor. During the late Precambrian, portions of this supercontinent were located at southern polar latitudes and experienced extensive glaciations (sometimes referred to as “snowball earth”). By around 700 mya, this supercontinent was breaking up and some of the fragments, including Laurentia (which consisted of most of North America and Greenland, along with portions of what are now Great Britain, France, and Scandinavia) drifted northward into more equable climes.

CNAmBy the onset of the Cambrian, Laurentia occupied the midlatitudes of the southern hemisphere and was oriented about 90° clockwise from North Americas position today; thus what is now the Pacific Coast of North America lay approximately parallel to the Tropic of Capricorn, and Greenland and Québec extended to around 50° south. Sea levels were high and much of North America, especially the lands which comprise the southern and western United States today, lay underwater.

Parts of modern Newfoundland, Nova Scotia, and New England had not yet docked with the remainder of North America, instead forming a submarine portion of the continent of Avalonia, which was located near the South Pole. Calm, shallow seas occupied shelf environments marginal to Laurentia, and were ideal for the formation of extensive limestones and reef deposits. Slumping and mudslides along the scarp of one of these reefs in what is now British Columbia formed the spectacular fossil deposits of the Burgess Shale about 505 mya.

The climate during the Cambrian is not well known, although there are no indications of any glaciations occurring during this time. Without large continental landmasses located in the high latitudes of either hemisphere, oceanic currents would have been able to circulate freely and distribute heat relatively well between the equator and the poles. Thus, it is likely that worldwide climates were fairly equable, being neither overly hot nor terribly cold.

Early in the Cambrian there is evidence of aridity over much of the North American continent–consistent with its positioning underneath the southern subtropical dry belt created by the persistent high pressure of the convergence zone between the equatorial and midlatitude Hadley cells (of atmospheric circulation). Later on in the Cambrian, Laurentia had drifted further northwards such that Alaska straddled the equator. Tropical climates probably existed along the modern western (ancient northern) margin of the continent, and warm-temperate ones along the modern eastern (ancient southern) shore.

What was happening on land? Basically, not much. Microbes may have colonized continental hot springs, but other than that, the land surfaces were pretty much barren. Although plants had diverged from animals possibly as much as 2 billion years ago or so, there is little, if any, evidence to indicate that they had yet begun to colonize the land. A single report, in Russian, of a supposedly Cambrian land plant (Aldanophyton) has been published, but this occurrence is questionable and lacks substantiation from any other fossil evidence uncovered thus far.

In the seas, however, it was quite another story. We have seen previously that during the Vendian (latest Precambrian), there is ample fossil evidence of a diverse and abundant array of soft-bodied lifeforms, the so-called Ediacaran biota. These, however, were preserved only under the most unusual of circumstances because of their lack of hard parts. The traditional definition of the onset of the Cambrian cites the sudden appearance of almost all the modern phyla of animals, complete with easily-fossilizable shells.

The $64,000 question surrounding this “Cambrian Explosion” is: did these animals spring de novo from the abyss at the beginning of the Cambrian, or did something change, merely allowing their fossils to first be preserved at this time? In other words, was there really an explosion of evolution, or is the sudden plethora of fossils a preservational artifact? And if the former, what could have driven it?

Let us begin with the assumption that the explosion is real–that all the modern phyla with readily fossilizable hard parts, i.e. all those with significant preservation potential (with the exception of sponges, which are known from the latest Precambrian, and bryozoans, which do not appear until the Ordovician)–did indeed evolve during the 53 million years of the Cambrian (and, the best evidence suggests, within a 10-million-year interval between 530 and 520 mya).

Contained within this assumption is the further postulate that those modern phyla without any hard parts, and thus possessing low preservation potential (e.g. flatworms), also arrived on the scene at this time. Arthropods (especially trilobites, but also crustaceans and other weird forms), molluscs, echinoderms, corals, annelids (segmented worms), brachiopods, graptolites, tunicates, and chordates are all represented in Cambrian sediments.

seleno

Additionally, lagerstätten such as the Burgess Shale of western Canada, Chengjjiang in China, and Sirius Passet in Greenland document a wide array of creatures not identifiable with any known living phylum–all of which also thrived in Cambrian seas. What might have driven such a burst of evolution, and why has the pace slowed since?

Many reasons have been proposed over the years, some more and some less robust in the face of intense scrutiny. For example, the breakup of the late Precambrian supercontinent would have resulted in an increase in continental shelf area and a corresponding increase in habitat for benthic and shallow-water marine creatures. Although Pannotia/Rodinia is the earliest well-known supercontinent, others likely existed and fragmented in the 3+ billion years before.

Why wasnt a burst of evolution associated with those events? Possibly it was because, at those times, only unicellular life existed, so evolution lacked the “raw material” from which to fashion the elaborate body plans that it could when working with the Ediacaran biota. Another suggestion has been that evolution was spurred on to refill econiches that were vacated by extinctions related to the late Precambrian glaciations.

Both of these suggestions have merit, but similar circumstances occurred later on in the geologic record (e.g., subsequent to the end-Triassic extinctions), yet they were accompanied by no evolution of new phylum-level body plans. I suspect that they were, at most, contributing factors…and that we must look elsewhere for the primary cause(s).

Another intriguing suggestion is that the Cambrian Explosion corresponds with the invention of sexual reproduction. Evolution suddenly could create variety far more quickly and easily than it could through mutation alone. This certainly sounds plausible, but is difficult, if not impossible, to test. Another reasonable-sounding suggestion is that the rate of evolution was accelerated by an “arms race” between predators and prey that had not existed during the peaceful days of the Precambrian, when presumably most, if not all, animals were sessile filter-feeders.

It is true that trace fossils–which indicate vagile animals–are rare prior to the Cambrian, which lends some credence to this possibility. Also, Ediacaran fossils showing clear evidence of defensive adaptations are unknown, whereas the Burgess Shale is full of critters sporting spines, horns, and other structures which could have been used for protection. Obvious predators, notably anomalocarids and conodonts, abound.

Arthropods in the Burgess Shale are particularly diverse, and show that trilobites, while the dominant fossils of the Cambrian, were merely a minor branch of the arthropod family tree at the time. In fact, the finest lesson of the Burgess Shale may be to show us how incomplete and skewed the Cambrian fossil record really is. Perhaps the hard parts which exponentially increased the preservation potential of Cambrian organisms were a necessary defensive response to the evolution of active predators.

What about the proposition that the Cambrian Explosion is an illusion–that complex metazoan life existed far back into the Precambrian, and is only revealed to us by fossils when hard parts, and greatly increased preservation potential, evolved? Reason to suspect this comes from recent studies of genetic divergence and the idea of a “molecular clock.”

brooksella

This is the concept that mutations in an organisms DNA occur at a statistically constant rate throughout time (although this is merely a reasonable assumption). If true, the number of mutations separating the DNA of two organisms–or genetic difference between them–provides an estimate of the time since they diverged from a common ancestor. Such a “clock” must be calibrated against fossil data (which provides an absolute timeframe for the divergence of, say, humans and chimpanzees) and is only accurate if the mutation rate has remained constant throughout time; therefore, it can only give ballpark estimates, but these are useful nevertheless.

It has been pointed out that such studies reveal that evolution at the molecular (genetic) level frequently proceeds much more quickly than at the level of morphology (which is all the fossil record can reveal). Two organisms that differ greatly in their genetic sequence may not appear that far apart morphologically. Prior to genetic sequencing, Bacteria and Archea were classified together because of their strong physical resemblance.

DNA sequencing reveals that they differ from each other as much as do ostriches and liverworts–and therefore obviously have been on separate evolutionary paths for a very long time. Fossil evidence cannot reveal this disparate history. Proponents of this point of view point out that molecular genetic studies place the common ancestors of the modern phyla deep within the Precambrian. That supports the “illusion” theory of the explosion.

However, I have my doubts about this analysis. The reverse is often true: a small difference in DNA can result in a paleontologically significant difference in morphology. We share 99+% of our DNA with chimpanzees, yet no good paleontologist would mistake a chimp for a human in the fossil record. This is especially true when the mutations occur in homeobox (Hox) genes–those which control the timing and trajectory of developmental processes (rather than the synthesis of proteins).

A minor mutation in a developmental gene can result in as significant a change in morphology as a bird growing teeth or not. (It has even been argued that mutations in Hox genes fuel the “jumps” in punctuated equilibrium.) Additionally, I find the assumption that genetic mutations accumulate at a constant rate, especially over the expanses of geologic time and across diverse lineages, rather tenuous.

A small difference in the rate of mutation between two lineages can have a tremendous effect on the divergence dates calculated. This is not to say that I do not believe that various phyla diverged from one another far back in the Precambrian. They well may have–but I am skeptical of putting too much faith in molecular clock studies as evidence of this.

One fairly robust piece of data indicating that there was nowhere near the diversity of metazoans during the Late Precambrian as later is the paucity of trace fossils found in the older sediments. Had there been a diversity of vagile (movement-capable) animals, even soft-bodied ones, one would expect to find an abundant and varied trace fossil record during the Late Precambrian. This is not the case. Horizontal burrows do not occur in sediments older than about 575 mya, and vertical burrows before 543 mya. This is clear evidence that many econiches were not filled during the latest Precambrian, and circumstantial evidence that the basic types of organisms –possibly the phyla themselves–capable of exploiting those niches had not yet evolved.

cambgraphSo what can we conclude from all of this? Taken together, there is not a clear-cut case for either a wide radiation of phylum-level body plans (archetypes) deep within the Precambrian, nor for the entirety of the Cambrian Explosion being explained by “evolution in overdrive.” My take on the whole situation is that it was probably some of both. Likely many lineages had embarked on widely disparate trajectories long before the Cambrian began, yet evolution itself also spun into high gear during the interval from 530 to 520 mya.

One explanation for this which I find to be quite convincing is that the timespan encompassing the Cambrian Explosion represents the maximum slope of a sigmoid growth curve–the portion subsequent to the slow-increase, “get off the ground” beginning and the asymptotic amelioration that sets in when econiches become filled or limits on food supply make themselves felt.

Say what?–puzzle you non-mathematical types! Ok, a little explanation is called for. Envision a graph shaped more or less like a forward-slanting S, flat on the top and the bottom (at the beginning and ending of the stroke) and almost vertical, rather than backwardly diagonal, in the middle.

Height is population or diversity (could be number of individuals, number of species, or number of archetypes) and increasing distance to the right represents time. Say you have bacteria dividing in a petri dish (or organisms giving rise to new species). Say every minute (or million years) they double–give rise to two daughter cells (or species). Each of those progeny doubles during the next unit of time, etc. etc. Try it on a calculator, starting with 1 and multiplying by 2 each time.

Youll have a million in less time than you think. But this cant go on forever–those organisms need resources (food, living space) and those resources are finite. So at some point, some of them begin to starve (or die off for whatever reason), and you dont have the entire lot of them doubling their population with each iteration. The longer you go, the more of them cant compete, and the slower the growth rate gets until its effectively zero and the population is limited by the carrying capacity of the environment; you are at the top of the S–time goes on, but your population (or number of new species) fails to increase with it.

This may have been what happened during the Cambrian. New taxa evolved at an ever-increasing rate until the limits of ecospace began to impinge on their partying. Why have no new archetypes or phylum-level body plans appeared subsequent to this interval (with the exception of the Bryozoa, which didnt lag by much)? Perhaps ecomorphospace was filled and no one new could squeeze in. Perhaps the necessary early-stage mutations were all lethal. Who knows. Suffice it to say that by the end of the Cambrian, the blueprint for the next half billion years of life on earth had been laid.

Ok, enough about the explosion. What did a Cambrian seafloor look like? If all the modern phyla of animals were present, would it have appeared very different from a tropical reef today?

Well, yes, there were the first reefs, but they werent formed by corals–they were created by skeletonized sponges called archaeocyathids, which had conical, cup-like forms and grew in dense colonies. The Cambrian is known to many as the “Age of Trilobites” because trilobites are the most common fossils found in Cambrian rocks. But the Burgess Shale and other lagerst otten teach us that whereas yes, arthropods were the dominant phylum, trilobites themselves were relegated to a minor role compared with non-trilobite arthropods. It would have been amazing enough to be transported back to a trilobite reef, but one populated by the bizarre variety of arthropods revealed in the Burgess Shale would have been even more awesome.

Although the other modern phyla were present, their numbers and ecological importance paled in comparison with that of the arthropods. Anomalocarids–considered by some to be arthropods, by others to be a separate phylum–preyed on the trilobites and their kin. Brachiopods consisted mostly of inarticulate forms. Molluscs were rare; hyolithids (an extinct group), bivalves, and gastropods were present, but the cephalopods were the first group to gain significant ecological importance. Echinoderms consisted of mostly now-extinct forms such as edrioasteroids and helicoplacoids; the presence of Cambrian crinoids is questionable, and the familiar starfish and sea urchins had not yet evolved. Jawless chordates, including forms such as the enigmatic conodonts and the familiar Pikaia, swam through shallow waters, but fish would not appear until the Ordovician.

The Cambrian came to a close with a mass extinction that decimated 75% of the trilobite families, half the sponge families, and a significant portion of the brachiopods and snails. A possible reason for this was changing climates associated with the onset of the Ordovician glaciations.

North America has extensive sediments in which the denizens of Cambrian seas can be seen and/or collected. The most famous is undoubtedly British Columbias Burgess Shale, and although the quarries are closed to all but researchers with special permits, the Smithsonian has an extensive collection of these fossils. Charles D. Wolcotts original material is housed there. Cambrian sediments are also widely exposed in the Great Basin, which was continental shelf half a billion years ago, and much of whose land is owned by the BLM today. My favorite collecting area is the Wheeler Amphitheater/Antelope Spring trilobite beds in the House Range of west-central Utah.

Many fine specimens of Elrathia kingi, Asaphiscus wheeleri, Peronopsis interstricta, and small inarticulate brachiopods come from the black shales which formed in anoxic bottom waters there 500+ million years ago. Other types of trilobites come from various-aged Cambrian sediments nearby and in Nevada. Spending a few days there, under a crystal blue sky amongst desert piñon and juniper, listening only to the hum of insects and looking out over the valleys paved with salt crusts, is not only refreshing to the soul but one of the best ways I know to really connect with those vanished denizens of long ago.

Leave a Reply