The 'explosion' of early life in the "Cambrian explosion" (542 to 488 mya).
For more than 2 billion years of its existence, life on earth changed very little. The evolution of animal diversity in the ocean, and eventually on land, began only about 600 million years ago. Multi-celled animals had been around for nearly 600 million years before the Cambrian period, and, as we shall see and have seen, they certainly got going in a big way during the Ediacaran. Why then did so many groups of animals and innovative body plans evolve so suddenly? The fossil record can show us what happened, but can’t tell us conclusively why it happened.
Or, conversely, why the delay? This is also difficult to answer, in part because rocks that formed immediately before the Cambrian period appear to have vanished: Cambrian rocks often sit on much older layers, creating what is known as The Great Unconformity. It is now suggested that this crucial rock record may have dissolved in the ocean, leaving it rich in mineral that would have turned them alkaline. To survive, animals developed ways of pumping calcium out of their bodies, which would have formed outer crusts –the first shells, which would later turn into bones[1].
Many scientists suggest that environmental changes in the Earth played a major role in accelerating the diversity of complex multicellular organisms that climaxed during the Cambrian explosion. Some have suggested that the melting of “snowball earth”, the period after the Earth’s surface had become entirely or nearly entirely frozen, was a trigger. Others refer to the rapid increase in oxygen availability and the advent of plate tectonics and the consequent mixing of surface and ocean chemistry. Tectonics work as a global thermostat, recycling chemicals that help regulate the levels of carbon dioxide and keep the global temperature stable. Without it, surface water would not have remained liquid for billions of years and life, especially complex life, would have faced insurmountable obstacles. Another possible cause may have been the interactions among the increasingly complex animals. Competition and predation often spark innovation, but whatever its causes, the Cambrian explosion was unique. Never again have so many dramatically different body plans evolved so quickly.
The evolution of hard parts in the Cambrian period was a revolutionary advance. Shells offered protection, and hard parts, eventually including bones, also made possible greater size, more varied shapes and new ways to move. Mineralised, readily fossilised, organisms become common.
However, the uniqueness of the Cambrian experience has recently be called into question following the discovery of fossils in Gabon West Africa that suggest that multicellular life on Earth may in fact have commenced at least 1.5 billion years earlier. According to the fossil evidence, the organisms appear to have lived in colonies and were shaped like cookies with ragged edges and a lumpy interior. More than 250 specimens have been found so far. They are said to have different body shapes, and vary in size from one to 12 centimeters 0.4 to 5.0 inches.
These newly discovered fossilised creatures may also have crossed another threshold of evolution far earlier than any other known organism. Unlike simple bacteria, their cells appear to have membrane-bound nucleus housing protecting its chromosomes, the genetic blueprints for life. Geochemical analysis shows that the organisms lived in slightly-oxygenated ocean waters, leading the researchers to speculate that oxygen may have been an essential catalyst for the leap from single- to multi-cell life forms, as it was in the case of the so-called Cambrian explosion. Both events occurred during the Proterozoic era.
As Bill Bryson points out, we now know that complex organisms existed at least a hundred million years before the Cambrian. However, the Cambrian fossils were initially unearthed (by one Charles Walcott in 1909 in the so-called Burgess Shale in British Columbia) and subsequently reinterpreted to accord with the idea of a sudden explosion of life during the middle Cambrian period[2]. It wasn't until the 1970s and 1980s that the Ediacaran fossils were discovered (see /the-proterozoic-era.html) attesting to complex life during a much earlier period of time. Other thoughts are that many of these so called new species were "not so different after all", "just interesting elaborations of well-established designs", "not quite so explosive as all that", and "there all along, but too tiny to be preserved", leaving no fossil record behind them [3]. So perhaps the Cambrian period may attest to something more like a blossoming forth rather than an explosion, although the proliferation of life during this period certainly did happen rather quickly!
The post-Cambrian period
Commencing with the Cambrian Period, a summary of the more recent geologic periods and the major geological developments and occurrences which took place within them is as follows:[3.1]
Period Duration Development
Cambrian 542–488 mya Explosion of animal life
Ordovician 488-444 First coral reefs
Silurian 444-416 Major extinction 1 - 60% of marine species wiped out; first land plants
Devonian 416-359 First fish evolved and started to walk on land as tetrapods around 397 mya; first seed bearing plants spread across dry land
giving rise to huge forests; first ammonite molluscs appeared;
first mollusc like brachiopods and great coral reefs; first four-legged vertebrates; palaeogeography dominated by
super-continent of Gondwana to the south, Siberia to the north,
and Euroamerica in between; 373 mya - later Devonian extinction
Mass extinction 2.
Carboniferous 359-299, incl. First reptiles and four-legged vertebrates begin to diversify;
Mississippian (359-318), and abundant coal-forming swamps; four-legged vertebrates begin to
Pennsylvanian (318–299) diversify (Miss); first reptiles (Penn)
named after the regions in which
the various fossils discovered
Permian 299-252 Formation of Pengaea (supercontinent encompassing all the earth's present land masses); epoch ending event–largest mass
extinction known to science; Mass extinction 3 - last period of the Paleozoic era
Triassic 251-200 First flying vertebrates, the pterosaurs, evolved; first dinosaurs[1];
first mammals; 201 mya - End of Triassic extinction Mass extinction 4.
Jurassic 200-146 Age of the reptiles; Atlantic Ocean begins to form; first birds.
Cretaceous 146-65.5 Age of the dinosaurs; first flowering plants; Gondwana divides
into the land masses of Africa, Madagascar, Antarctica, Australia
and South America; mass extinction of the dinosaurs and nearly
half the Earth’s species,including nearly 75% of ocean species.
Mass extinction 5. End of the Mesozoic.
Commencing with the Cambrian Period, a summary of the more recent geologic periods and the major geological developments and occurrences which took place within them is as follows:[3.1]
Period Duration Development
Cambrian 542–488 mya Explosion of animal life
Ordovician 488-444 First coral reefs
Silurian 444-416 Major extinction 1 - 60% of marine species wiped out; first land plants
Devonian 416-359 First fish evolved and started to walk on land as tetrapods around 397 mya; first seed bearing plants spread across dry land
giving rise to huge forests; first ammonite molluscs appeared;
first mollusc like brachiopods and great coral reefs; first four-legged vertebrates; palaeogeography dominated by
super-continent of Gondwana to the south, Siberia to the north,
and Euroamerica in between; 373 mya - later Devonian extinction
Mass extinction 2.
Carboniferous 359-299, incl. First reptiles and four-legged vertebrates begin to diversify;
Mississippian (359-318), and abundant coal-forming swamps; four-legged vertebrates begin to
Pennsylvanian (318–299) diversify (Miss); first reptiles (Penn)
named after the regions in which
the various fossils discovered
Permian 299-252 Formation of Pengaea (supercontinent encompassing all the earth's present land masses); epoch ending event–largest mass
extinction known to science; Mass extinction 3 - last period of the Paleozoic era
Triassic 251-200 First flying vertebrates, the pterosaurs, evolved; first dinosaurs[1];
first mammals; 201 mya - End of Triassic extinction Mass extinction 4.
Jurassic 200-146 Age of the reptiles; Atlantic Ocean begins to form; first birds.
Cretaceous 146-65.5 Age of the dinosaurs; first flowering plants; Gondwana divides
into the land masses of Africa, Madagascar, Antarctica, Australia
and South America; mass extinction of the dinosaurs and nearly
half the Earth’s species,including nearly 75% of ocean species.
Mass extinction 5. End of the Mesozoic.
Mass extinctions
The Big History site https://www.bighistoryproject.com/chapters/3#extinctions also describes five main extinctions, slightly different in nuance as regards dating to those mentioned above, but in substance the same:
What caused these extinctions?[4] Over the years may causes have been put forward, such as volcanic eruptions and asteroid impacts but recently a new cause has been suggested. Recent evidence now points to cataclysmic eruptions (so-called large igneous province eruptions or LEPs) as the real culprits behind four of the five major extinctions, the sole exception being the Ordovician dying some 444 billion years ago. The Ordovician dying of 444 mya is the only one for which evidence is thus far lacking.
What are LIPs? Large igneous province eruptions are characterised by huge magma bursts into the air from deep within the Earth’s interior in mile high ‘incandescent fountains’, ‘long rivers of yellow-hot lava’ and a scorching sulphurous haze. “But it is not the lava or the ash that makes the extinctions which follow them truly “mass”. It is the sulphur dioxide and carbon dioxide gases emanating from these phenomena which do so.
The process: Baked sediments from the eruptions release huge amounts of climate changing carbon dioxide and sulphur dioxide in to the air. When the sulphur dioxide reaches the stratosphere, wind currents carry it around the globe. Sunlight is deflected away from the Earth. The planet cools, and rains pours down as sulphuric acid rain which can be likened to battery acid. Carbon dioxide from eruptions builds in the air, causing global warming that last millennia. The gas also dissolves in seawater, causing acidification. The warming oceans become oxygen starved dead zones. Halocarbons from eruptions damage the ozone layer, exposing land life to harmful UV radiation. The combination is lethal for most land and marine life, and cataclysmic extinction is the result. Shades of the legacy of human made climate change perhaps, though on a grander scale!
The technique: Using the Permian extinction as a starting point, scientists collected tiny crystals of zircon and perovskite from erupted rocks. When the crystals were formed, they contained uranium which converts to lead at a steady rate as the crystals cool on the Earth’s surface. The ratio of uranium to lead reveals how long ago the eruption occurred. There was also evidence of huge bursts of carbon dioxide into the air, a pattern repeated for the extinctions at the end of the Devonian, Triassic and Cretaceous periods.
Today: The remnants of the LIPs today form vast swathes of hardened lava in remote areas of Asia and elsewhere.
The evolution of the dinosaurs - genesis and development
As Stephen Brusatte has pointed out,[5], the age of the dinosaurs began and ended with extinctions. The first dinosaurs were born of the Permian extinction around 252 mya when up to 95 percent of Permian species went extinct. Among those which managed to survive were various small amphibians and reptiles, which later diverged into today's frogs, salamanders, turtles, lizards and mammals.
The first dinosaurs were small and insignificant, representative among them being a small catlike creature called the Prorotodactylus dating to about 250 million years ago, and identified by fossil tracks in the Polish mountains about one or two million years after the volcanic eruptions that brought the Permian to a close. They belonged to a specialized group of reptiles called archosaurs that emerged after the Permian extinction with a newly evolved upright posture that helped them run faster, cover longer distances and track down prey with greater ease.
Almost as soon as the archosaurs originated, they branched into two major lineages: the pseudosuchians, leading to today's crocodiles, and the avemetatarsalians, which developed into dinosaurs. Prorotodactylus was not a dinosaur per se but a primitive member of the avemetatarsalian subgroup that includes dinosaurs and their very closest cousins. Over the next 10 million to 15 million years these dinosauromorphs continued to diversify. Then, at some point between 240 million and 230 million years ago, one of these primitive dinosauromorph lineages evolved into true dinosaurs.
By the middle part of the Triassic, around 230 million to 220 million years ago, the three main dinosaur subgroups, representing all three of the main branches of the dinosaur family: the meat-eating theropods; the long-necked, plant-eating sauropodomorphs and the beaked, plant-eating ornithischians roamed the single supercontinent Pangea which stretched from pole to pole. During these formative years of their evolution, dinosaurs were slowly diversifying in the humid temperate regions but were seemingly unable to colonize the deserts. Then, the dominant large herbivores of the time went into decline, disappearing entirely in some areas for reasons still unknown. Also - around 215 million years ago - dinosaurs finally broke into the deserts of the Northern Hemisphere. However, these deserts were unstable environments of fluctuating temperatures and rainfall, which meant that plant-eating dinosaurs did not have a steady source of food, leaving the species somewhat marginalised for much of the Triassic and overshadowed by the likes of their crocodile relatives. However, toward the end of the Triassic, enormous volcanic eruptions caused the supercontinent Pangea to fracture. As had occurred some 30 million years earlier, this triggered another mass extinction. The crocodile-line archosaurs were decimated, with only a few species—the ancestors of today's crocodiles and alligators—able to endure. All the major dinosaur subgroups—the theropods, sauropodomorphs and ornithischians—survived into the Jurassic Period. “It had taken more than 30 million years, but they had, at long last, arrived”.
Fossil finds
In 2013, paleontologists analysed some bones excavated during the 1930s near Lake Nyasa, also known as Lake Malawi in Tanzania, one of the great rift lakes of Africa. They believe them to be representative of the oldest species of dinosaur, Nyasasaurus parringtoni, which lived more than 240 million years ago, that is, between 10 to 15 million years earlier than the hitherto oldest known dinosaurs. The species weighed between 20 and 60 kgs and stood about a metre tall. These findings place the early evolution of dinosaurs and dinosaur-like reptiles firmly in the southern continents at a time when Africa was joined to South America, Antarctica and Australia in a giant landmass called Pangaea. The find “established that dinosaurs evolved earlier than previously expected and refutes the idea that dinosaur diversity burst on to the scene in the later Triassic”. Instead the researchers believe that dinosaurs diversified from a group called archosaurs, which were the dominant creatures living during the Triassic period, 250 – 200 mya[6].
Fossil finds have also discovered that for most of their history, the tyrannosaurs were mainly marginal, human sized carnivores, achieving huge size and ecological dominance only during the final 20 million years of the Age of the Dinosaurs, which began about 250 mya and spanned the Triassic, Jurassic and Cretaceous periods[7]. The giant Tyrannosaurus rex evolved was merely the last survivor of a startling variety of the species that lived across the globe until the asteroid impact 66 mya which brought the dinosaur era to a close. There was in fact a greater separation in time between the ancestral tyrannosaurs and T rex (at least 100 million years) than between T rex and humans (66 million years).
The turning point seems to have been sometime between 85 and 100 mya during the middle part of the Creataceous, when dinosaurs ecosystems underwent a radical reconstruction and the tyrannosaurs assumed the top predator role on the northern continents. It is unclear why this happened, but a mass extinction about 94 mya when temperatures increased and sea levels fluctuated, may be the cause. It meant the end of species such as the allosaurs and ceratosaurs that had previously been at the apex of the food pyramid allowing the tyrannosaurs to become dominant[8]. And after living in the shadows for some 80 million years until environmental changes gave them their opportunity, they were “at the peak of their game” when an asteroid fell from the sky and they disappeared from view[9].
As a footnote, in 2019 a new species and the only one of its type named Fostoria dhimbangunmal, a genus of iguanodontian ornithopod dinosaur, a herbivore from the Griman Creek Formation of New South Wales, Australia was announced as having been discovered in the Lightning Ridge area of New South Wales, about 750 km west of Sydney. The actual discovery dated back to 1984, and were discovered in an old opal mine. To discover dozens of bones from the same dinosaur skeleton is a first, scientists claim. It is said to be evidence of the first dinosaur herd discovered in Australia. The remains are said to be 100 million years old.
The Big History site https://www.bighistoryproject.com/chapters/3#extinctions also describes five main extinctions, slightly different in nuance as regards dating to those mentioned above, but in substance the same:
- 450 mya the Ordivician, when all known life at the time existed in the seas and oceans, and marine invertebrates suffered the heaviest losses during this extinction - 50% of species lost.
- 330 mya the late Devonian - most marine groups impacted, with reef ecosystems ceasing to exist completely - estimated 30% of species lost.
- 250 mya the end-Permian, "the Great Dying" - believed to be the most intense extinction event, with marine and land animals greatly impacted - estimated 60% of species lost.
- 200 mya the end-Triassic - thriving mammal-like reptiles and large amphibians disappeared, as well as many dinosaur groups - estimated 35% of species lost. Described in more detail on the next page: /the-permian-triassic-extinction.html
- 65.5 mya the K-T extinction - occurring at the boundary between the Cretaceous (K) and Tertiary (T) time periods (also known as the Cretaceous-Paleogene (K-P or C-P) extinction); end of the dinosaurs - estimated 75% of species lost. Described in more detail on a subsequent page: /the-cretaceous-paleogene-boundary.html
What caused these extinctions?[4] Over the years may causes have been put forward, such as volcanic eruptions and asteroid impacts but recently a new cause has been suggested. Recent evidence now points to cataclysmic eruptions (so-called large igneous province eruptions or LEPs) as the real culprits behind four of the five major extinctions, the sole exception being the Ordovician dying some 444 billion years ago. The Ordovician dying of 444 mya is the only one for which evidence is thus far lacking.
What are LIPs? Large igneous province eruptions are characterised by huge magma bursts into the air from deep within the Earth’s interior in mile high ‘incandescent fountains’, ‘long rivers of yellow-hot lava’ and a scorching sulphurous haze. “But it is not the lava or the ash that makes the extinctions which follow them truly “mass”. It is the sulphur dioxide and carbon dioxide gases emanating from these phenomena which do so.
The process: Baked sediments from the eruptions release huge amounts of climate changing carbon dioxide and sulphur dioxide in to the air. When the sulphur dioxide reaches the stratosphere, wind currents carry it around the globe. Sunlight is deflected away from the Earth. The planet cools, and rains pours down as sulphuric acid rain which can be likened to battery acid. Carbon dioxide from eruptions builds in the air, causing global warming that last millennia. The gas also dissolves in seawater, causing acidification. The warming oceans become oxygen starved dead zones. Halocarbons from eruptions damage the ozone layer, exposing land life to harmful UV radiation. The combination is lethal for most land and marine life, and cataclysmic extinction is the result. Shades of the legacy of human made climate change perhaps, though on a grander scale!
The technique: Using the Permian extinction as a starting point, scientists collected tiny crystals of zircon and perovskite from erupted rocks. When the crystals were formed, they contained uranium which converts to lead at a steady rate as the crystals cool on the Earth’s surface. The ratio of uranium to lead reveals how long ago the eruption occurred. There was also evidence of huge bursts of carbon dioxide into the air, a pattern repeated for the extinctions at the end of the Devonian, Triassic and Cretaceous periods.
Today: The remnants of the LIPs today form vast swathes of hardened lava in remote areas of Asia and elsewhere.
The evolution of the dinosaurs - genesis and development
As Stephen Brusatte has pointed out,[5], the age of the dinosaurs began and ended with extinctions. The first dinosaurs were born of the Permian extinction around 252 mya when up to 95 percent of Permian species went extinct. Among those which managed to survive were various small amphibians and reptiles, which later diverged into today's frogs, salamanders, turtles, lizards and mammals.
The first dinosaurs were small and insignificant, representative among them being a small catlike creature called the Prorotodactylus dating to about 250 million years ago, and identified by fossil tracks in the Polish mountains about one or two million years after the volcanic eruptions that brought the Permian to a close. They belonged to a specialized group of reptiles called archosaurs that emerged after the Permian extinction with a newly evolved upright posture that helped them run faster, cover longer distances and track down prey with greater ease.
Almost as soon as the archosaurs originated, they branched into two major lineages: the pseudosuchians, leading to today's crocodiles, and the avemetatarsalians, which developed into dinosaurs. Prorotodactylus was not a dinosaur per se but a primitive member of the avemetatarsalian subgroup that includes dinosaurs and their very closest cousins. Over the next 10 million to 15 million years these dinosauromorphs continued to diversify. Then, at some point between 240 million and 230 million years ago, one of these primitive dinosauromorph lineages evolved into true dinosaurs.
By the middle part of the Triassic, around 230 million to 220 million years ago, the three main dinosaur subgroups, representing all three of the main branches of the dinosaur family: the meat-eating theropods; the long-necked, plant-eating sauropodomorphs and the beaked, plant-eating ornithischians roamed the single supercontinent Pangea which stretched from pole to pole. During these formative years of their evolution, dinosaurs were slowly diversifying in the humid temperate regions but were seemingly unable to colonize the deserts. Then, the dominant large herbivores of the time went into decline, disappearing entirely in some areas for reasons still unknown. Also - around 215 million years ago - dinosaurs finally broke into the deserts of the Northern Hemisphere. However, these deserts were unstable environments of fluctuating temperatures and rainfall, which meant that plant-eating dinosaurs did not have a steady source of food, leaving the species somewhat marginalised for much of the Triassic and overshadowed by the likes of their crocodile relatives. However, toward the end of the Triassic, enormous volcanic eruptions caused the supercontinent Pangea to fracture. As had occurred some 30 million years earlier, this triggered another mass extinction. The crocodile-line archosaurs were decimated, with only a few species—the ancestors of today's crocodiles and alligators—able to endure. All the major dinosaur subgroups—the theropods, sauropodomorphs and ornithischians—survived into the Jurassic Period. “It had taken more than 30 million years, but they had, at long last, arrived”.
Fossil finds
In 2013, paleontologists analysed some bones excavated during the 1930s near Lake Nyasa, also known as Lake Malawi in Tanzania, one of the great rift lakes of Africa. They believe them to be representative of the oldest species of dinosaur, Nyasasaurus parringtoni, which lived more than 240 million years ago, that is, between 10 to 15 million years earlier than the hitherto oldest known dinosaurs. The species weighed between 20 and 60 kgs and stood about a metre tall. These findings place the early evolution of dinosaurs and dinosaur-like reptiles firmly in the southern continents at a time when Africa was joined to South America, Antarctica and Australia in a giant landmass called Pangaea. The find “established that dinosaurs evolved earlier than previously expected and refutes the idea that dinosaur diversity burst on to the scene in the later Triassic”. Instead the researchers believe that dinosaurs diversified from a group called archosaurs, which were the dominant creatures living during the Triassic period, 250 – 200 mya[6].
Fossil finds have also discovered that for most of their history, the tyrannosaurs were mainly marginal, human sized carnivores, achieving huge size and ecological dominance only during the final 20 million years of the Age of the Dinosaurs, which began about 250 mya and spanned the Triassic, Jurassic and Cretaceous periods[7]. The giant Tyrannosaurus rex evolved was merely the last survivor of a startling variety of the species that lived across the globe until the asteroid impact 66 mya which brought the dinosaur era to a close. There was in fact a greater separation in time between the ancestral tyrannosaurs and T rex (at least 100 million years) than between T rex and humans (66 million years).
The turning point seems to have been sometime between 85 and 100 mya during the middle part of the Creataceous, when dinosaurs ecosystems underwent a radical reconstruction and the tyrannosaurs assumed the top predator role on the northern continents. It is unclear why this happened, but a mass extinction about 94 mya when temperatures increased and sea levels fluctuated, may be the cause. It meant the end of species such as the allosaurs and ceratosaurs that had previously been at the apex of the food pyramid allowing the tyrannosaurs to become dominant[8]. And after living in the shadows for some 80 million years until environmental changes gave them their opportunity, they were “at the peak of their game” when an asteroid fell from the sky and they disappeared from view[9].
As a footnote, in 2019 a new species and the only one of its type named Fostoria dhimbangunmal, a genus of iguanodontian ornithopod dinosaur, a herbivore from the Griman Creek Formation of New South Wales, Australia was announced as having been discovered in the Lightning Ridge area of New South Wales, about 750 km west of Sydney. The actual discovery dated back to 1984, and were discovered in an old opal mine. To discover dozens of bones from the same dinosaur skeleton is a first, scientists claim. It is said to be evidence of the first dinosaur herd discovered in Australia. The remains are said to be 100 million years old.
[1] “Ocean’s toxic alkalization made animal life flower”, New Scientist, 21 April 2012, 16.
[2] Bill Bryson, A Short History of Nearly Everything, Broadway Books, 2003, 328. See also http://www.burgess-shale.bc.ca/discover-burgess-shale/burgess-shale-fossils-and-their-importance
[3] Bryson, op cit, 333-334.
[3.1] In the table below, the Devonian period owes its name to the "Old Red Sandstone" of the 'beautiful county of Devon', which also appears in various other parts of the British Isles, in Germany, Greenland, North America and elsewhere. Devonian rocks are recognisable as Devonian wherever they are found, partially because of the internal evidence of the fossils they contain. The terms Pennsylvanian and Mississippian are alternative terms to the Carboniferous period used by American geologists: Richard Dawkins and Yan Wong, The Ancestor's Tale - A Pilgrimage to the Dawn of Life, Weidenfeld and Nicolson, London, 2004, 2nd ed, 14-15.
[4] What follows is an edited summary of the article "Anatomy of a Mass Murderer" by Howard Lee which appeared in the March 2016 edition of the Scientific American, pages 56-57.
[5] “The unlikely Triumph of Dinosaurs”, Scientific American, My 2018.
[6] “Fossil find pushes back dawn of dinosaurs by 15 million years”’, SMH, 6.12.2013. Dinosaur photo link: home.alphalink.com.au/~dannj/waprints.htm Further photos appear in Appendix 9.
[7] For the story here, see Stephen Brusatte, “Rise of the Tyrannosaurs – New fossils put T. rex in its place”, Scientific American, May 2015, 24-31.
[8] Ibid, 30.
[9] Ibid, 31. As to which, see the page on the Cretaceous-Paleogene boundary at /the-cretaceous-paleogene-boundary.html
The evolution of early marine sea creatures
Trilobite evolution 542 – 252 million years ago (duration: Cambrian – Permian periods)
Trilobites are related to modern lobsters and scorpions in that they carry their skeletons on the outside of their bodies. They were among the earliest arthropods with segmented, flattened bodies with three sections, a head, middle and tail, with long antennae and jointed legs. The name trilobite (three lobed) does not refer to these three sections. It describes the three lobes running lengthwise from head to tail, distinguishing trilobites from all other arthropods (invertebrates having an external skeleton, a segmented body, and jointed appendages). Trilobites ranged from a few millimetres (less than half an inch) to 70 millimetres (28 inches). Most lived in sediment, eating worms and other mud-dwelling invertebrates.
[2] Bill Bryson, A Short History of Nearly Everything, Broadway Books, 2003, 328. See also http://www.burgess-shale.bc.ca/discover-burgess-shale/burgess-shale-fossils-and-their-importance
[3] Bryson, op cit, 333-334.
[3.1] In the table below, the Devonian period owes its name to the "Old Red Sandstone" of the 'beautiful county of Devon', which also appears in various other parts of the British Isles, in Germany, Greenland, North America and elsewhere. Devonian rocks are recognisable as Devonian wherever they are found, partially because of the internal evidence of the fossils they contain. The terms Pennsylvanian and Mississippian are alternative terms to the Carboniferous period used by American geologists: Richard Dawkins and Yan Wong, The Ancestor's Tale - A Pilgrimage to the Dawn of Life, Weidenfeld and Nicolson, London, 2004, 2nd ed, 14-15.
[4] What follows is an edited summary of the article "Anatomy of a Mass Murderer" by Howard Lee which appeared in the March 2016 edition of the Scientific American, pages 56-57.
[5] “The unlikely Triumph of Dinosaurs”, Scientific American, My 2018.
[6] “Fossil find pushes back dawn of dinosaurs by 15 million years”’, SMH, 6.12.2013. Dinosaur photo link: home.alphalink.com.au/~dannj/waprints.htm Further photos appear in Appendix 9.
[7] For the story here, see Stephen Brusatte, “Rise of the Tyrannosaurs – New fossils put T. rex in its place”, Scientific American, May 2015, 24-31.
[8] Ibid, 30.
[9] Ibid, 31. As to which, see the page on the Cretaceous-Paleogene boundary at /the-cretaceous-paleogene-boundary.html
The evolution of early marine sea creatures
Trilobite evolution 542 – 252 million years ago (duration: Cambrian – Permian periods)
Trilobites are related to modern lobsters and scorpions in that they carry their skeletons on the outside of their bodies. They were among the earliest arthropods with segmented, flattened bodies with three sections, a head, middle and tail, with long antennae and jointed legs. The name trilobite (three lobed) does not refer to these three sections. It describes the three lobes running lengthwise from head to tail, distinguishing trilobites from all other arthropods (invertebrates having an external skeleton, a segmented body, and jointed appendages). Trilobites ranged from a few millimetres (less than half an inch) to 70 millimetres (28 inches). Most lived in sediment, eating worms and other mud-dwelling invertebrates.
Morphologic (structural) features Asapherits sp.
of a trilobite showing its major Trilobite
anatomical characteristics Ordovician, Draa valley Morocco
Illustrations from the Smithsonian Exhibition, Life and the Oceans (2010)
of a trilobite showing its major Trilobite
anatomical characteristics Ordovician, Draa valley Morocco
Illustrations from the Smithsonian Exhibition, Life and the Oceans (2010)
Like all arthropods, trilobites shed their shells to grow. Typically, seams along the shell’s head segment split. The location of the seams varies from species to species, which assists paleontologists in classifying species. Trilobites are among the most diverse groups of extinct organisms. More than 15,000 species evolved during their 300 million year history, and as trilobites spread to new environments, they changed both physically and behaviourally.
As trilobites spread and diversified, their most significant adaptations included:
Complex eyes Living on the bottom, trilobites needed only a limited range of vision. Upon moving into the water column, they entered a three-dimensional world where it was necessary to see above, below and all around. Trilobites evolved some of the earliest complex eyes (compound eyes with many lenses). Eyes varied with lifestyle: swimming trilobites often had big eyes; some living in sediment had eyes on stalks to peek above the muck; many on the deep bottom had small eyes, or were blind.
Rolling up The ability to roll up in a ball – like modern pill bugs – helped protect trilobites from increasingly powerful predators. Rolled up, the trilobites could watch and wait until the coast was clear.
Spines Rolling up in a ball offered some protection. Sharp spines, which stuck out from the rolled up trilobite, provided an extra layer of defence while the animal waited for danger to pass.
Below: Trilobite diversity as shown in the Smithsonian Exhibition, Life and the Oceans (2010). Some trilobites reacted to change by becoming swimming predators: middle illustration.
As trilobites spread and diversified, their most significant adaptations included:
Complex eyes Living on the bottom, trilobites needed only a limited range of vision. Upon moving into the water column, they entered a three-dimensional world where it was necessary to see above, below and all around. Trilobites evolved some of the earliest complex eyes (compound eyes with many lenses). Eyes varied with lifestyle: swimming trilobites often had big eyes; some living in sediment had eyes on stalks to peek above the muck; many on the deep bottom had small eyes, or were blind.
Rolling up The ability to roll up in a ball – like modern pill bugs – helped protect trilobites from increasingly powerful predators. Rolled up, the trilobites could watch and wait until the coast was clear.
Spines Rolling up in a ball offered some protection. Sharp spines, which stuck out from the rolled up trilobite, provided an extra layer of defence while the animal waited for danger to pass.
Below: Trilobite diversity as shown in the Smithsonian Exhibition, Life and the Oceans (2010). Some trilobites reacted to change by becoming swimming predators: middle illustration.
Trilobite diversity never recovered from the major extinction at the end of the Devonian period (359 million years ago). New predators may also have contributed to their decline. A few phillipsid trilobite species remained in the Permian period (299-251 mya), but these vanished before the catastrophic extinction at the end of that period.
The basic body plans of all modern animals were set during the Cambrian period (542 to 488 million years ago). “Your friend, family and pet turtle may not look like the creatures here, but we and our fellow animals are heirs of these ancient ocean dwellers. Not every Cambrian body plan was successful, but those that did succeed set the pattern for every animal that followed – in the water and on land”[1].
The evolution of insects[2]
The first insects appeared on Earth about 479 million years ago at the start of the Ordovician period, and were probably a close relative of today’s wingless silverfish. Insects are essential to human health and the environment, recycling plant and animal material into nutrients and pollinating food crops but they also spread diseases. They first took to the air about 479 million years ago.
The first winged bugs were probably a variety of primitive butterfly, whose facility for flight evolved about the same time plants started to grow tall. They were able to crawl up on to plants and probably began their quest to fly by jumping, then gliding and gradually evolved the flapping wings we see today, thereby helping them escape predators and get to new food. They next evolved sucking mouth parts to feed on plants. Then some species began to metamorphose from squashy larvae to pupa to adult as seen in moths and butterflies today.
An evolutionary road map for the insect has been devised using large genetic data sets, integrated with fossil evidence in order to reconstruct relationships between species far back in time. The project required specialists in molecular biology, insect morphology, computing, palaentology and insect taxonomy and the use of super-powered computers to crunch through the data sets.
Other evolutionary movements during this period
The discovery of fossil spores from the Ordovician era suggest that plants were the first multicelled organisms to leave the sea and colonise the dry land. The first animals to move on land were probably arthropods (carrying their skeletons on the outside), a bit like giant insects. We know of their presence during the Silurian period. The Ordovician period also saw the first arthropods (vertebrates) appear on Earth after their evolution in the sea from wormlike ancestors. They included early forms of fish and sharks. The first mass extinction marked the end of the Ordovician, when an ice age began about 440mya.
The Silurian Period saw the first comeback in life on Earth. Recovering from the first mass extinction, plants once again colonised land. These were soon followed by terrestrial animals, such as spiders and centipedes. This quick colonisation of land was greatly helped by a stabilisation of Earth's climate. With the earlier glaciers melting, new habitat in the form of freshwater appeared, fish soon moving into this new area. Coral appeared, slowly growing higher as the melting glaciers added to the ocean level. With the newer habitats of deep and open oceans jawed fish soon dominated in oceans, growing larger with more room to move[3].
Amphibians in the late Devonian, early Carboniferous period
Fossils from the Devonian era (416 to 359 mya) are interesting in that they capture the transition between water-dwelling fish and the first vertebrates to live on land[4]. However, there is a gap in the record from about 360 million years ago at the end of the Devonian period to about 340 million years ago in the early part of the Carboniferous era. This is known as Romer’s gap, named after the person who identified it. After that we find unequivocal evidence of amphibians crawling through the swamps, and salamander-like animals, some as big as crocodiles whom they superficially resembled. This period of the Carboniferous era from about 340 million years ago was the amphibian equivalent of the age of the dinosaurs. Before that Romer could only see fish, lobe-finned fish living in water. But where were the so-called ‘missing links’, a description which Dawkins and most other scientists worthy of the name deplore, or intermediates?
Romer hypothesised that annual droughts caused lakes and ponds and streams to dry up only to flood again the following year. Fishes that made their living in water could benefit from a temporary ability to survive on land, while they dragged themselves from a shallow lake or pond that was threatened with imminent desiccation to a deeper one in which they could survive until next season, the dry land being a temporary bridge to escape back into the water.
Fossil traces of animals that went some way towards bridging the gap between the lobe-finned fishes so abundant in Devonian seas (Eusthenopteron), and the amphibians that later slithered through the Carboniferous swamps do appear in the late Devonian, the period immediately preceding the Carboniferous. Some 20 million years later, at the border between the Devonian and Carboniferous Periods, appears the amphibian Ichthyostega, whose fossil remains were discovered in 1881. It lived mostly in water with occasional forays onto the land and had a flat head characteristic of amphibians. We will revisit some more amphibious phenomena later when reviewing the Paleocene - Eocene epochs.
Crinoids - Mississippian habitat (359-318 million years ago)
Crinoids ruled the earth 350 million years ago, so what was the secret of their success? Crinoids (echinoderms related to sea stars and sea urchins) varied in size and shape. They evolved a wide range of adaptations for feeding and sharing space, allowing a great number to flourish side by side. Other organisms adapted to life amid ‘crinoid’ forests. Vast accumulations of dead crinoids produced most of the period’s limestone. All crinoids got food from the same source: organic particles floating in the current.
The factors which enabled so many crinoids to share the same ecosystem, and the same food supply included: Their stems: Differences in stem length meant less competition. Tall crinoids captured floating food from one level, while medium height and shorter crinoids (and other filter feeders) fed at lower levels. Their arms: Crinoids filter food from the water with feathery “arms”. Because the number and characteristics of the “arms” varied, different crinoids were suited to different current strengths. Their roots: The base of crinoid stalks anchored the animals to the bottom. This adaptation allowed crinoids to live in a variety of different sediments and to withstand different currents.
[1] Smithsonian exhibition on Life and the Oceans.
[2] Nicky Phillips, “Bugs wing way on to mighty ‘tree of life’”, SMH, 7 November 2014. The isformation is drawn from the journal Science.
[3] http://www.starsandseas.com/SAS%20Evolution/SAS%20geoltime/geotime_silurian.htm
[4] Dawkins, Greatest Show, 164.
How did trilobites and echinoderms adapt - or not adapt - to new conditions?[1]
As explained in the Smithsonian Exhibition, Life and the Oceans[2], life seldom holds still. Species rise and fall in bursts of evolution and extinction, and diversity increases, though not necessarily steadily, ecosystems change and organisms respond to those changes. Predators and prey develop new forms of attack and defence, and occasional mass extinctions eliminate whole groups, creating new opportunities for the survivors. Some animals adapt, evolving new features or behaviours and may become anew species, others don’t and may become extinct.
Some notable interruptions along the evolutionary pathway[3]
The rearrangement of the continents caused by plate tectonics has shaped and reshaped the earth’s surface and climatic patterns, recycling chemicals that help regulate the levels of carbon dioxide, acting as a global thermostat and generally keeping the global temperature stable. This may explain why in the past 500 million years, there have been at least five period of sharply declining biodiversity – periods in which perhaps 75% or more of all species may have vanished. Of these, the most catastrophic occurred in the late Permian period as the supercontinent of Pangaea formed[4].
[1] Smithsonian exhibition, Life and the Oceans.
[2] October 2010.
[3] Source: Smithsonian exhibition, Life and the Oceans.
[4] Christian, Maps of Time,130-131.
The basic body plans of all modern animals were set during the Cambrian period (542 to 488 million years ago). “Your friend, family and pet turtle may not look like the creatures here, but we and our fellow animals are heirs of these ancient ocean dwellers. Not every Cambrian body plan was successful, but those that did succeed set the pattern for every animal that followed – in the water and on land”[1].
The evolution of insects[2]
The first insects appeared on Earth about 479 million years ago at the start of the Ordovician period, and were probably a close relative of today’s wingless silverfish. Insects are essential to human health and the environment, recycling plant and animal material into nutrients and pollinating food crops but they also spread diseases. They first took to the air about 479 million years ago.
The first winged bugs were probably a variety of primitive butterfly, whose facility for flight evolved about the same time plants started to grow tall. They were able to crawl up on to plants and probably began their quest to fly by jumping, then gliding and gradually evolved the flapping wings we see today, thereby helping them escape predators and get to new food. They next evolved sucking mouth parts to feed on plants. Then some species began to metamorphose from squashy larvae to pupa to adult as seen in moths and butterflies today.
An evolutionary road map for the insect has been devised using large genetic data sets, integrated with fossil evidence in order to reconstruct relationships between species far back in time. The project required specialists in molecular biology, insect morphology, computing, palaentology and insect taxonomy and the use of super-powered computers to crunch through the data sets.
Other evolutionary movements during this period
The discovery of fossil spores from the Ordovician era suggest that plants were the first multicelled organisms to leave the sea and colonise the dry land. The first animals to move on land were probably arthropods (carrying their skeletons on the outside), a bit like giant insects. We know of their presence during the Silurian period. The Ordovician period also saw the first arthropods (vertebrates) appear on Earth after their evolution in the sea from wormlike ancestors. They included early forms of fish and sharks. The first mass extinction marked the end of the Ordovician, when an ice age began about 440mya.
The Silurian Period saw the first comeback in life on Earth. Recovering from the first mass extinction, plants once again colonised land. These were soon followed by terrestrial animals, such as spiders and centipedes. This quick colonisation of land was greatly helped by a stabilisation of Earth's climate. With the earlier glaciers melting, new habitat in the form of freshwater appeared, fish soon moving into this new area. Coral appeared, slowly growing higher as the melting glaciers added to the ocean level. With the newer habitats of deep and open oceans jawed fish soon dominated in oceans, growing larger with more room to move[3].
Amphibians in the late Devonian, early Carboniferous period
Fossils from the Devonian era (416 to 359 mya) are interesting in that they capture the transition between water-dwelling fish and the first vertebrates to live on land[4]. However, there is a gap in the record from about 360 million years ago at the end of the Devonian period to about 340 million years ago in the early part of the Carboniferous era. This is known as Romer’s gap, named after the person who identified it. After that we find unequivocal evidence of amphibians crawling through the swamps, and salamander-like animals, some as big as crocodiles whom they superficially resembled. This period of the Carboniferous era from about 340 million years ago was the amphibian equivalent of the age of the dinosaurs. Before that Romer could only see fish, lobe-finned fish living in water. But where were the so-called ‘missing links’, a description which Dawkins and most other scientists worthy of the name deplore, or intermediates?
Romer hypothesised that annual droughts caused lakes and ponds and streams to dry up only to flood again the following year. Fishes that made their living in water could benefit from a temporary ability to survive on land, while they dragged themselves from a shallow lake or pond that was threatened with imminent desiccation to a deeper one in which they could survive until next season, the dry land being a temporary bridge to escape back into the water.
Fossil traces of animals that went some way towards bridging the gap between the lobe-finned fishes so abundant in Devonian seas (Eusthenopteron), and the amphibians that later slithered through the Carboniferous swamps do appear in the late Devonian, the period immediately preceding the Carboniferous. Some 20 million years later, at the border between the Devonian and Carboniferous Periods, appears the amphibian Ichthyostega, whose fossil remains were discovered in 1881. It lived mostly in water with occasional forays onto the land and had a flat head characteristic of amphibians. We will revisit some more amphibious phenomena later when reviewing the Paleocene - Eocene epochs.
Crinoids - Mississippian habitat (359-318 million years ago)
Crinoids ruled the earth 350 million years ago, so what was the secret of their success? Crinoids (echinoderms related to sea stars and sea urchins) varied in size and shape. They evolved a wide range of adaptations for feeding and sharing space, allowing a great number to flourish side by side. Other organisms adapted to life amid ‘crinoid’ forests. Vast accumulations of dead crinoids produced most of the period’s limestone. All crinoids got food from the same source: organic particles floating in the current.
The factors which enabled so many crinoids to share the same ecosystem, and the same food supply included: Their stems: Differences in stem length meant less competition. Tall crinoids captured floating food from one level, while medium height and shorter crinoids (and other filter feeders) fed at lower levels. Their arms: Crinoids filter food from the water with feathery “arms”. Because the number and characteristics of the “arms” varied, different crinoids were suited to different current strengths. Their roots: The base of crinoid stalks anchored the animals to the bottom. This adaptation allowed crinoids to live in a variety of different sediments and to withstand different currents.
[1] Smithsonian exhibition on Life and the Oceans.
[2] Nicky Phillips, “Bugs wing way on to mighty ‘tree of life’”, SMH, 7 November 2014. The isformation is drawn from the journal Science.
[3] http://www.starsandseas.com/SAS%20Evolution/SAS%20geoltime/geotime_silurian.htm
[4] Dawkins, Greatest Show, 164.
How did trilobites and echinoderms adapt - or not adapt - to new conditions?[1]
As explained in the Smithsonian Exhibition, Life and the Oceans[2], life seldom holds still. Species rise and fall in bursts of evolution and extinction, and diversity increases, though not necessarily steadily, ecosystems change and organisms respond to those changes. Predators and prey develop new forms of attack and defence, and occasional mass extinctions eliminate whole groups, creating new opportunities for the survivors. Some animals adapt, evolving new features or behaviours and may become anew species, others don’t and may become extinct.
Some notable interruptions along the evolutionary pathway[3]
The rearrangement of the continents caused by plate tectonics has shaped and reshaped the earth’s surface and climatic patterns, recycling chemicals that help regulate the levels of carbon dioxide, acting as a global thermostat and generally keeping the global temperature stable. This may explain why in the past 500 million years, there have been at least five period of sharply declining biodiversity – periods in which perhaps 75% or more of all species may have vanished. Of these, the most catastrophic occurred in the late Permian period as the supercontinent of Pangaea formed[4].
[1] Smithsonian exhibition, Life and the Oceans.
[2] October 2010.
[3] Source: Smithsonian exhibition, Life and the Oceans.
[4] Christian, Maps of Time,130-131.