Fossils

Alan V. Morgan

My first fossil find was, literally, a life-changing experience. I was eight years old, playing with my younger brother and friends when I crawled under a large blackberry bush to hide. In the semi-dark a large snail rested on the ground beside me. Gingerly I gently poked it until it rolled over. It was strangely heavy and I picked it up. My game was over. I rushed home to my mother who told me that what I had found was a fossil snail. She had been brought up in the South Wales coalfield and was used to fossil ferns and plants that were occasionally brought back by members of our mining family.

One week later we went to the National Museum of Wales in Cardiff. Emlyn Evans, the School's Service Officer, to whom I owe a tremendous debt, had the specimen (Figure 1, below) identified as Pleurotomaria anglica (J. Sowerby). It was from the Lower Lias of the Jurassic and about 175 million years old. I was hooked! What was this "thing"? Why did it have a funny name? How had it been turned to stone? How did the geologists at the National Museum know how old it was? What on earth was the "Lower Lias" and the Jurassic?

Figure 1. Pleurotomaria anglica

Emlyn invited me to the Saturday morning "classes" conducted by the museum. For two years I traveled back and fore to Cardiff on the local train, being met and returned to the station in Cardiff by Emlyn (the sort of thing that probably would not happen today). I was told that if I wanted to become a geologist I would need a university degree in geology, perhaps even a Ph.D. When, two years later, I moved up to the grammar school system I could see, somewhere on that distant educational horizon, some sort of career in geology. My two geology teachers in Barry Grammar school, A.J. James and Jack Edney are also owed a great debt of gratitude.

Of course, now at the "other end" of my life I simply did not realise what excitement and pleasure that inadvertent chance discovery would provide me. It has taken me to most corners of the world, provided me with challenges and adventures and a constant wonder of this intriguing place that we are permitted to be part of, albeit for a short time. So this brief introduction brings me back to the topic of fossils, a word derived from things that are "dug up" from the Earth.

Fossils and the geological past

Fossils are part of a tangible record of organisms that have inhabited our planet in the distant past. They are defined as "the remains or traces of formerly living organisms." When I was starting my geological "career" about half of a century ago, conventional wisdom was that true fossils were confined to the last part of the geological record, from the Cambrian to the present. The vast expanse of geological time (see What On Earth V.2; No. 2, pp. 12, 13 and related text) known generally as the "Precambrian", was regarded as unfossiliferous and those fossils that had been reported from Precambrian rocks were generally thought to be of non-biological origin (gas bubbles, concretions and other inorganic features). In fact, we now know that life - albeit in microscopic form - was present back to the earliest times of our planet's history. Our world was subject to intense meteorite and asteroid bombardment from its origins, about 4.56 billion years ago, to about 3.8 billion years ago. When this massive bombardment stopped we see geochemical carbon isotope "signatures" in rocks from Greenland that indicate organic life forms were present. Where these early life forms originated has long been a subject of scientific speculation. Early ideas were that they were produced by electrical discharges in an "organic soup" that prevailed in a primitive reducing atmosphere on Earth. These ideas have more recently been cast into doubt by the realisation that a reducing atmosphere might have been short-lived, and a second explanation, that life might have arrived from space, could be a more attractive alternative hypothesis. We know that complex organic compounds exist in space and might have been transported to our world by comets that plunged into our world's early atmosphere and oceans.

Organisms in our world today belong to three main groups. The first are the eukaryotes (organisms with genetic material contained within a cell nucleus and protected by a cellular membrane), bacteria (single-celled organisms without an internal cell membrane) and "archeobacteria". The archeobacteria and very primitive bacteria are autotrophs; that is they make their own food by a process of "capture and synthesise" by utilising chemical energy outside the organism. We also believe that this took place frequently in the past, perhaps at sites where hot springs and sulphide vents associated early ocean spreading took place in many parts of the early crust. "Real" fossils appear as microscopic filamentous bacteria dated to approximately 3.5 billion years in rock in western Australia. These are part of a group of organisms that include the cyanobacteria, photosynthesizing algae known as stromatolites. Their modern counterparts survive today in hypersaline ocean water in places like Hamelin Pool in Shark Bay, western Australia. They are rather uninteresting blackened mounds consisting of sheets of algal filaments that have trapped layers of sediment. Hypersalinity helps to protect these colonies from the predation of grazing molluscs, a problem that did not exist back in the Precambrian. Stromatolites are important since their photosynthetic activity helped to create the oxygen-rich atmosphere that is essential to most advanced life forms.

These advanced life forms (eukaryotic cells and metazoans) date back to at least 2.1 billion years, and there are many localities throughout the world that contain different examples. In the last part of the Precambrian animal life started to develop the diversity of forms that we see today. Encysting stages of phytoplankton are recorded at 1.6 to 1.4 billion years. The earliest description of truly diversified metazoans (Aspidella) were reported by Elkanah Billings in 1872, from the Avalon Peninsula, south of St. Johns in Newfoundland. Other advanced Precambrian fossils were found in Namibia in 1933, an exceptionally diverse fauna at Ediacara in South Australia in 1946, in Charnwood Forest, Leicestershire, England in 1957 and subsequently in the White Sea Region, in the United States, China, and elsewhere. The oldest dated advanced form is a "giant" metazoan, Charnia wardi that is about 575 million years old from the south coast of Newfoundland. Some of these organisms are illustrated in the centrefold of this issue.

Figure 2 (Above): Precambrian stromatolite colonies, Pilbara, Western Australia.

It was not until the advent of Cambrian time that the full diversity of fossil life is well seen in the geological record from many different parts of the world. With what appears to be of amazing geological rapidity these 542 million year old rocks are crowded with conventional and bizarre life forms. Some appear as "dead-ended" forms that literally flashed on, and off, the world's stage. Others plodded along until their modern "descendents" appear to be virtually unchanged from their earliest ancestors. I will go on to explore some of the various important fossil groups that we see in the fossil record, but before that we should talk a little about how fossils are preserved.

Figure 3 (below): Modern stromatolites at Hamelin Pool, Western Australia.

Fossil preservation

Fossils can be preserved in a multitude of different ways. These depend on a variety of factors that might include;
rock lithology (a reflection of the original sediment in which good preservation shows a preference for fine rather than coarse grained rocks). Consequently limestones, mudstones and shales are better lithologies for preservation than conglomerates, breccias, and coarse sandstones. This, in turn might be a reflection of the original depositional environments since finer sediments are generally present in less energetic environments. Marine and lacustrine sediments are better repositories than most terrestrial environments since erosion and transport generally breaks potential fossil material.

The nature of the original organism is important. Large, thick-shelled or heavily armoured organisms will likely preserve better through time than thin and fragile organisms. Post-mortem and post-depositional events are important. Shortly after death predation by large and small carnivores, scavengers and micro-organisms can complete destroy potential fossil remains. In a longer time frame fossils can be destroyed by percolating fluids (for example, dolomitisation). These changes are known as diagenetic changes. In still longer time frames metamorphism can stretch and destroy fossils by stress or by heat and pressure as new minerals form.

Figure 4 (Above). Small fly trapped in amber, Baltic region.

Assuming that organisms survive these inherent risks there are a variety of modes of preservation. Amazingly, soft parts of fossils can be preserved although this usually requires a very rapid removal of the organism from the destruction of both predators and bacteria. One of the classic methods is by entombing the organism in resin which eventually changes to amber (Figure 4). Fossil insects - shades of "Jurassic Park" - are moderately frequently found in this way and date back at least to late Cretaceous time. On a different scale much larger animals, such as mammoth (Figure 5), bison and horses have been recovered from relatively recent permafrost.

Figure 5. Baby Mammoth in melting permafrost, Siberia, Russia.

In the youngest parts of the geological column many shells contain the original aragonitic structures that they possessed in life. Further back in time there is a slow alteration of the aragonite to calcite (a more stable configuration). Shell or plant materials can be replaced on a cell-by-cell basis by other minerals carried through percolating fluids that migrate through sediments and rock sequences. Classic examples of replacement may be by silica, by pyrite, or by iron carbonates such as siderite. In the case of plant materials (and less so in the animal kingdom) the change is by carbonation. Here the volatiles in the organism are gradually driven off finally leaving a film of carbon that often preserves quite delicate structures from the original plant or animal. Examples of each of these preservational modes are shown in the centrefold pages.

Classification of fossils

The modern system of classification dates back to 1758 when Carolus Linnaeus devised a binomial classification for animals and plants. Today we still used a modified form of this system. I tell my students to remember the gnomonic, Kind People Care Only For Giant Slugs, where the initial letter stands for the major elements of the classification, i.e. Kingdom, Phylum, Class, Order, Family, Genus, Species. For example the animal illustrated below (Figure 6) is an Australian dingo. The full classification of the dingo would be: KINGDOM Animalia; (Sub kingdom) Metazoa; PHYLUM Chordata; (Sub phylum) Vertebrata; (Super class) Tetrapoda, CLASS Mammalia; (Sub class) Theria; (Infra class) Eutheria; (Cohort) Ferungulata; (Super order) Ferae; ORDER Carnivora; (Sub order) Fissipeda; (Super family) Canoidea; FAMILY Canidae; (Sub family) Caninae; (Tribe) None; GENUS Canis; (Sub genus) None; SPECIES Canis familiaris; (Sub species) None. (whew)! The common name is "Dog", and you might further differentiate it as a "Dingo", rather than a "Poodle" or a "Pit Bull", or a "Chihuahua". In fact there are over 400 varieties of dogs, but they all have two things in common - they all can interbreed, and produce viable offspring (the definition of a species) - and all are Canis familiaris. It is important to use the Latin classification because confusion can arise by using "common" names. For example the term "Robin" means completely different birds to a North American versus a European. The "robin" of North America is a moderately large orange-breasted bird whose Latin name is Turdus migratorius. The European "robin" is also an orange-breasted bird, but is much smaller, and its Latin name is Erithacus rubecula.

Figure 6. Australian Dingo, Canis familiaris.

When we look at fossils we have some problems. We have to work with limited information and we have unknown variables, especially if the species has been long extinct. However, we can work with some basic scientific assumptions and we are constantly refining identifications, which is why there is a major scientific furor when new fossils are found, or "living fossils" - like the Coelocanth - turn up after being "absent" from the geological record for millions of years. We do try to follow the Linnaean classification which is why "my" snail is Pleurotomaria anglica (J. Sowerby). Pleurotomaria is its generic name, anglica is its specific name and J. Sowerby was the person who named it. The rest of the classification precedes it, right back to the "Kingdom" level. I might mention that there are five currently accepted "Kingdoms" that all things fall into. These are MONERA (All Prokaryotes, and bacteria); PROTOCTISTA (Protozoa, slime molds, n.algae);FUNGI (also Lichens); PLANTAE (all Eukaryotic plants) and ANIMALIA (all Eukaryotic animals).

The reality of fossil preservation

Most of the fossils that have been illustrated above and in the centre pages are examples of "better than average" preservation. There are different localities in the world where excellent fossil preservation is not unusual; however, the majority of fossils are of "average" preservation, and usually result from being "internal" or "external" molds. These are formed in the following way.

Figure 7. Three diagrams illustrating (left to right); 
Figure 8. The two rows above illustrate how an internal mould (mold) - (middle line) and external mould - (bottom line) is formed as a fossil weathers from rock. 
The photograph shows two clams that illustrate both forms.

1. A clam (pelecypod) in growth position. Sediment is falling to the sea floor.
2. The clam has died. Sediment has completely entombed it and the sediment has lithified to rock. The interior of the specimen has filled with sediment but the shell is still intact and the sediment (internal and external) has preserved the morphology of the exterior and interior of the shell.
3. The rock that entombed the (now) fossil shell has been involved in tectonic activity. Stress has distorted the shell morphology in this mildly metamorphic rock type.

Figure 9 (below). Internal and External Casts

A moderate amount of explanation is required for the centrefold. First let me say that all specimens (with one exception) are "real" fossils. The images are all copyright of the author, (except *), and a number are from my personal collection. The specimen sizes (not illustrated) vary considerably and range from microfossils to organisms that are up to about 10 m in length.

The central part of the image represents the geological time scale. This has been explained in moderate detail - including time boundaries - in the last issue of What On Earth (Spring Issue, 2004). Fossils have been present in the geological record almost back to the start of Earth's history, but the vast bulk of geological time is not well represented in this diagram. Instead I have concentrated on illustrating three images from the "earlier" part of the Precambrian, and another three from the "youngest" part of the Precambrian record. Most of the images (7 to 28) are fossils from the Phanerozoic Eon, the youngest part of the column when life forms proliferated in numbers and diversity. The Phanerozoic is split into the Paleozoic ("Ancient Life" - images 7 to 15); the Mesozoic (" Middle Life" - images 16 to 25) and the Cenozoic (" Recent Life" images 26 to 28). The "life positions in time" of each of the fossils is illustrated by a corresponding number on the Geological Time scale. These are relatively accurate but generalised.

Deciding on what images to introduce was also difficult. These are not all Canadian examples, although many representatives would have close relatives present in the various rock sequences across Canada. The bulk of the illustrations are all Eukaryotes. These include invertebrates (Animalia), but a number are vertebrates (also Kingdom Animalia) a few are plants (Kingdom Plantae) and several Prokaryotes represent the Kingdom Monera. The Kingdom Protoctista is illustrated by one image, and the only Kingdom not represented is the Fungi.

I have tried to illustrate forms that are relatively common and many of these are used as zonal fossils. I have also tried to illustrate very general lineages through a number of images (i.e. amphibians to reptiles to mammals). However, given the space allocation I apologise in advance for not being more comprehensive. The following is a brief description of each of the fossils. E. is Early, M. is Middle and L. is Late (early being the oldest); BY = Billion Years, MY = Million Years.

1: Stromatolite Cyanobacteria (Pilbara, W. Australia) E. Precambrian, about 3.5 BY. Top view 10cm.

2: Unicellular prokaryotes (Bungle Bungles, W. Australia) M. Precambrian, 1.6 BY. Microns across.*

3: Stromatolite Cyanobacteria (Belt Series, Montana) L. Precambrian, ~ 1BY. Side view 15 cm.

4: Spriggina floundersi. Early annelid? (Ediacara, S. Australia) Ediacaran ~ 550 MY. ~30 mm exposed.

5: Mawsonites spriggi. Likely Cnidarian (Ediacara, S. Australia) Ediacaran ~ 550 MY. ~ 70 mm.

6: Dickinsonia costata. Likely early annelid (flatworm) (Ediacara, S. Australia) Ediacaran ~ 550 MY. ~ 15 cm.

7: Paradoxides sp. Trilobite (Morocco) M. Cambrian. ~ 515 MY. ~ 22 cm.

8: Anomalocaris sp. Lobopod? (Burgess Shale, BC). M. Cambrian. ~ 505 MY. ~ 75 cm (model).

9: Didymograptus murchisoni Graptolite (Abereiddy Bay, Wales) L. Ordovician. ~ 475 MY. ~ 10 cm.

10: Echinodermata (Crinoid) sp. unknown. (Dudley, Worcs, UK.) Silurian ~ 427 MY. ~ 20 cm.

11: Acidaspis deflexa Lake. Trilobite (Dudley, Worcs. UK) Silurian ~ 427 MY. ~ 25 mm.

12: Cephalopoda (Goniatite) - sp. unknown; early ammonoid. (Morocco). Devonian ~ 400 MY. ~ 28 cm.

13: Mucrospirifer arkonensis Brachiopod (Arkona, ON) Devonian ~ 395 MY. ~ 45 mm.

14: Fern frond (sp. unknown). (PA, USA) U. Carboniferous. (Pennsylvanian), ~ 310 MY. ~ 20 cm.

15: Eryops sp. Amphibian. (Loc. unknown, TX, USA) Permian. ~ 275 MY. ~ 3 m.

16: Kueichousaurus sp. Reptile. (Loc. unknown, China) Triassic. ~ 247 MY. ~ 2 m.

17: Araucorioxylon sp. Fossil log. (Petrified Forest region, AZ, USA) Triassic. ~ 225 MY. ~ 80 cm.

18: Morganucodon sp. Early mammal jaw and teeth. (S. Glam, Wales.) Triassic. ~ 203 MY. ~ 4 mm.

19: Stenopterygius sp. Marine reptile, Ichthyosaur. (Germany) Jurassic. ~195 MY. ~ 5 m.

20: Asteroceras obtusum (J. Sowerby) Ammonite. (Lyme Regis, UK) Jurassic. ~ 190 MY. ~30 mm.

21: Acrocoelites sp. (Belemnite). - sp. unknown. (loc. unknown, UK) Jurassic ~185 MY. ~12 cm.

22: Quenstedticeras sp. Ammonite. (Saratov, Volga R. Russia) Jurassic ~ 160 MY. ~ 40 mm.

23: Micraster coranguinum (Leske). Sea-urchin. (Nr. Rochester, Kent, UK) Cretaceous. ~85 MY. ~ 25 mm.

24: Leaf impression. sp. unknown. (loc. unknown; AB, Canada) Cretaceous. ~ 67 MY. ~ 60 mm.

25: Tyrannosaurus rex. Terrestrial reptile; Dinosaur. (Dinosaur Prov Park, AB) Cretaceous ~ 66 MY. ~ 2 m.

26: Brontothere. sp. unknown. Mammal. (Cypress Hills, SK) Oligocene ~ 30 MY. ~ 2 m.

27: Pecten sp. (Mollusc - pelecypod - clam). (C. Florida) Pleistocene ~ 120,000 years. ~ 12 cm (wide).

28: Echinodermata (sea urchin) 80 Mile Beach, W. Australia. Modern specimen. ~ 5 cm.

Some potential exercises with students

Using the centrefold chart, ask if they can identify any of the fossils present. Why can they do so? What do they know about them? How old do they think they are?

What fossils do they think are related to one another? (i.e.11 and 8? 12, 20, and 21? 23 and 28?). Why?

Are there any fossils represented that they think are alive today? (i.e.14, 24, 27). Why?

Using a corridor in the school, perhaps reproduce and place specimens according to the geological time scale for the last ~600 million years (Specimens 4 to 28). Corridor should be measured and a suitable scale found to place the images.

Perhaps some of these fossils can be found locally. If so, ask the students what the ages of local rock sequences might be.