The Emergence of Animals: The Cambrian Breakthrough (I)

Here’s what I wrote about the book on goodreads:

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“This book is almost 25 years old, and this is one of the main reasons why I did not give it five stars. Parts of this book is just amazing, but the fact that I felt that it was necessary to continually look up terms and ideas covered in the book made it slightly less fun to read than it could have been. Some parts of the scientific vocabulary applied throughout the book are frankly outdated, and this aspect reflects not only a change in which words are used but also, more importantly, a change in how people think about these things. That progress has been made since the book was written is a good thing, but it did subtract a little from the overall reading experience that I very often felt that I had to be quite careful about which specific conclusions to accept and which to question. It does not help that some of the main conclusions towards the end of the book seem to have been proven, for lack of a better word, wrong.

But all in all it’s really a very nice book – there’s a lot of fascinating stuff in there.”

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A few sample quotes from the book:

“a distinction needs to be made between the two major types of animal fossils — body fossils and trace fossils. Body fossils are either actual parts of the organism’s body (such as a shell or a bone), or impressions of body parts (even if the parts themselves have been dissolved away or otherwise destroyed). The imprint of a feather or leaf or the external surface of a shell are examples of body fossils. […] Trace fossils are markings in the sediment (usually made while the sediment was still soft) left by the feeding, traveling, or burrowing activities of animals. Familiar examples of trace fossils include tracks and trails made by worms as they plow through sediment looking for food and ingesting sediment. […] Completely unrelated organisms can make trace fossils which are indistinguishable to paleontologists. Trace fossils are part of the fabric of the sediment, and therefore can be very resistant to destruction by metamorphism of the surrounding rock. Body fossils, on the other hand, are often destroyed by chemical reactions with the surrounding sediment. But body fossils are the only fossil type that can consistently give reliable information about the identity of the organism which left the remains. […] The worst problem in the search for the oldest animal fossils is mistaken identity. Sedimentary rocks are replete with irregular structures and small scale disturbances or interruptions of the horizontal bedding or layering. Some of these disturbances are caused by organisms, but many are not. […] Usually a well-preserved and well-formed trace fossil is unquestionably biologic in origin, and all paleontologists would agree that the trace was formed by an animal. Yet it can be difficult to define precisely what it is about a trace fossil that makes it convincingly biogenic (formed by life). […] A sedimentary structure that resembles, but is in fact not, a trace fossil (or a body fossil, for that matter) is called a pseudofossil. Pseudofossils have plagued the study of Precambrian paleontology because many inorganic sediment disturbances look deceptively like fossils.”

“Convincing trace fossils are known from the late Precambrian, sometimes in association with the soft bodied Ediacaran fossils (Glaessner 1969). These trace fossils are generally simpler, less common, and less diverse than Cambrian trace fossils. There is a significant difference in the complexity and depth of burrowing between Cambrian and Precambrian trace fossils, and it has been argued that the changeover from simple trace fossils to more complex types of traces occurred at more or less the same time as the Cambrian explosion, the first appearance of abundant Cambrian shelly fossils. […] Even shallow, sediment surface burrows in the Cambrian show a marked change in character over their Precambrian predecessors. […] something outstanding happened to the abilities of trace-fossil makers across the Precambrian-Cambrian boundary. Animals discovered a large number of ways to effectively use the sediment as a food resource, and also began to move deeper into the substrate for deposit feeding and homebuilding.”

“Seilacher (1984, 1985) recognizes that flattened body shapes maximize surface area for the takeup of oxygen and food dissolved in seawater, and perhaps also for the absorption of light. “Normal” metazoan animals generally have plump, more or less cylindrical, bodies. For very small, thin skinned animals, cells near the body surface can get oxygen and expel waste by simple diffusion across the cell surface membranes. Waste products such as carbon dioxide will be supersaturated inside of the animal’s body, and will tend to migrate out of its cells and into the open environment. The reverse is true for oxygen; it will tend to migrate into the cells because its concentration is greater on the outside than on the inside of an oxygen-respiring animal. Animals such as frogs and salamanders are able to respire (at least in part) in this way. But for most large, cylindrical animals, diffusion respiration will not work because diffusion is ineffective for cells buried deep within the animal’s body. This is a consequence of the fact that as an animal increases its size, its total volume outstrips its surface area by a large margin. […] metazoans have developed intricate systems of pipework and tubing to deliver nutrient and waste removal services to interior cells. Circulatory systems, digestive tracts, gills, and lungs are all solutions to the problems associated with volume increase.”

“Monoplacophorans […] are cap-shaped shells distinguished by two rows of muscle scars on the interior of the shell. They were thought extinct until living specimens were dredged from the deep sea and described in the late 1950s. Monoplacophorans have had an unusual history of discovery. They are the only group of animals that has been: (a) described hypothetically before being discovered; (b) found as fossils before being found alive,- and (c) dredged from the depths of the oceans before being collected from shallower marine waters (Pojeta et al. 1987). […] Rostroconchs are a major, extinct, order of mollusks that first appeared in the earliest Cambrian. Rostroconchs have a shell that is shaped like a clam shell, except that instead of having an organic ligament connecting the two valves, the two halves of a rostroconch shell are fused together to form a single valve. Despite this fusion, larger rostroconchs look very much like clam fossils with valves still articulated, which partly explains why rostroconchs were not recognized as a major, distinct, group until the 1970s. […] Slightly after the first appearance of rostroconchs, the first true clams or bivalves appear. Clams probably had the same ancestor as the rostroconchs […]. Instead of keeping the two valves fused as in rostroconchs, clams hinged the valves with articulating teeth and a tough, organic ligament. This evidently proved to be the more successful approach, since bivalve shells now litter the beaches all over the earth, whereas rostroconchs dwindled to extinction in the Permian.”

“Of the earliest Cambrian shelly fossils, many groups are truly problematic in the sense that not only do we have no idea what kind of animal made them, but also we have no clear conception of the function or functions of the skeletal remains. […] there is an anomalously high proportion of small shelly fossils that do not belong to later phyla. “Living fossils” are creatures alive today that have undergone very little morphologic change for long stretches (sometimes 100 million years or more) of geologic time. Few living fossils remain from the earliest Paleozoic fauna. […] Many of the groups that were most important in the Cambrian are unimportant or extinct today, for example, the trilobites, the inarticulate brachiopods, hyoliths, monoplacophorans, eocrinoids, the sclerite-bearers, and phosphatic tube-formers. True metazoans were undoubtedly present before the Cambrian, but they were all, with [few] exception[s] […], soft-bodied. New types of soft-bodied animals appear in the Cambrian as well, but our understanding of these forms is restricted to rare finds of Cambrian soft-bodied fossils, which are even rarer than finds of the Ediacaran fauna.”

I’ll just quote that last part again: “our understanding of these forms is restricted to rare finds of Cambrian soft-bodied fossils”.

They’re talking about the findings of soft-bodied organisms who did not make shells or anything like that which lived more than 500 million years ago. To get a sense of perspective in terms of how long ago this is, have a look at this picture – that’s one guess at what we think the Earth might have looked like back then. In my mind, the fact that we know anything at all about soft-bodied animals living back then is pretty amazing to think about.

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I could easily write perhaps four posts about this book, but I’m not going to do that. Instead I have decided for now to limit my coverage here to the stuff above and some links to relevant stuff I looked up while reading the book, which I have posted below – I was surprised how much relevant stuff wikipedia has on related matters, and if you’re curious you should really go have a look at some of those links. I should note that I will probably add another post about the book later on with some more observations from the book – it seems wrong to me to limit coverage of this great book to one post, but there’s no way I can cover all the good stuff in there anyway.

Here are as mentioned some relevant wiki links to the kinds of stuff they talk about in this book – most of the links are in my opinion links to articles of what I’d consider to be a ‘reasonable’ length/quality, and although I have not read all of them I’d note that some of them are quite good:

Cambrian explosion.
Ediacara biota (featured).
Kleptoplasty.
Trace fossil.
Cloudinid (‘good article’).
Sclerite.
Archaeocyatha.
Trilobite.
Echinoderm.
Brachiopod (‘good article’).
Bivalvia (featured).
Chiton.
Bryozoa (‘good article’).
Adam Sedgwick.
Roderick Murchison.
Global Boundary Stratotype Section and Point (noteworthy in this context is that the Precambrian/Cambrian boundary GSSP at Fortune Head had not been decided upon when this book was written – they have a whole chapter about these and related things).
Manorian glaciation (this is not what it’s called in the book, but that is what they’re talking about anyway).
Snowball Earth.
Timeline of glaciation.
Cryogenian.
Rodinia.
Mirovia.
Curie temperature.
Autotroph.
Heterotroph.
Microbial mat.
Anomalocaris.
Stromatolite.
Acritarch.
Great Oxygenation Event.
Methane clathrate.