A History Of Life On Earth: The Palaeozoic Era
The Palaeozoic Era
The term Palaeozoic literally means ‘ancient life’ and it’s the period that marks the first appearance of animals with hard parts, such as shells and carapaces in their bodies. Such hard parts fossilise remarkably well, thus it is from this period onwards that scientists have been able to chart the rise and fall of individual groups of animals and plants.
The Rise of Complex Life
The First Super-Predator
Cambrian Period: 543-490 Million Years Ago
In the Cambrian Period the fossils of many animals, such as the shelled trilobites become common and can be found at hundreds of locations right across the world. Most significantly, the Cambrian heralded the first appearance of the complex eye- an organ that some palaeontologists believe helped accelerate the process of evolution because it led to the development of active hunters, which in turn drove prey to develop better defences.
During the Cambrian, land was still a barren and hostile place, so all animal life lived in the shallow seas around the edge of the Earth’s continents. Occasionally, huge underwater landslides would engulf these communities, burying them under tonnes of mud. These landslides would preserve even the most delicate of soft bodied animals as fossils, allowing us an extraordinary glimpse of just how weird and strange the Cambrian really was.
From the rocks of the Canadian Burgess Shale (and other locations in China and Greenland), we know that bizarre animals such as the giant arthropod predator Anomalocaris, swam through an alien landscape dominated by sponges and primitive seaweeds.
The Cambrian seas contained representatives from most of the major animal groups including the arthropods (Anomalocaris and the trilobites), molluscs (sea shells) and echinoderms (sea urchins, starfish). More importantly though is the presence of a creature called Haikouichthys, a jawless fish that lived some 535 million years ago. Not only is it among the earliest forms of fish, but also one of the earliest vertebrates, thus making it one of the oldest known ancestors of all living vertebrates including us.
The First Ever Vertebrate
Spreading onto Land
Ordovician Period: 490-443 Million Years Ago
As the Cambrian gave way to the Ordovician, animal life remained in the seas, which were home to corals, sea urchins, starfish and sea shells, but the most numerous creatures were the arthropods.
The trilobites were by far the most common group of animals on the planet, but they had now been joined by the first chelicerates, the arthropod group that includes scorpions. One particular type of chelicerate, a sea scorpion known as Megalograptus grew to monstrous proportions and was even capable of crawling out onto land for short periods of time. Land at this time was still barren, apart from a few species of slime mould and lichen that lived alongside stream banks.
In the seas, the biggest predator of the age was a giant relative of modern squid known as the giant orthocone, it grew to around 33 feet in length and being the biggest predator of its time, presumably terrorised other sea creatures including our tiny back boned ancestors. Speaking of which, at this time our ancestors were represented by creatures that resembled jawless fish and probably lived on the seabed, searching for small food fragments.
A Monstrous Ancestor of Squid
Life in the Ordovician
A Pioneering Plant
Silurian Period: 443-417 Million Years Ago
The Silurian world saw life progress at a slow and steady pace. In the shallow tropical regions complex reef systems developed, built from corals, sponges and bryozoans. These reefs were home to smaller animals, such as jawless fish, sea lilies and brachiopod sea shells, but the arthropods still dominated life.
One such arthropod, a sea scorpion called Pterygotus reached a massive size, but there were also true scorpions such as Brontoscorpio, which was capable of making short visits on to land. The evolution of so many large predators saw some jawless fish develop armour plating and advanced senses.
It was towards the end of the Silurian that life first began to colonise the land in a meaningful way. The first recognisable plants such as Cooksonia, which was among the first of its kind to send shoots upwards to generate energy directly from the sun grew in clumps near to streams and rivers along with several species of fungi. But at this time plants were small, barely reaching more than 4 inches in height.
Among these pioneering plants were the first land animals, which included creatures that resembled millipedes and other small arthropods. The majority of these animals were plant eaters, but there were some predators as well.
Life in the Silurian Seas
An Armoured Fish
The Rise of a Natural Dynasty
Devonian Period: 417-354 Million Years Ago
The Devonian Period saw big changes both on land and in the seas. At the beginning of the Devonian life on land was still sparse, but within just a few million years, pioneering plants such as Cooksonia had transformed into the first true forests dominated by a tree like plant called Archaeopteris, which grew in vast numbers alongside rivers and estuaries.
Animal communities on land were dominated by millipedes and predatory animals such as the trigonotarbids, who were distant relatives of modern spiders. It was during the Devonian that the first fish crawled out of the water onto the land to transform into air breathing, four legged amphibians.
Back in the seas meanwhile, there were now two types of swift and terrifying predator. The fish had come of age; with the evolution of a powerful jaw armed with sharp teeth, which enabled them to tackle active prey; they very quickly increased in both variety and size. There were also the newly evolved sharks, which were represented by Stethacanthus, whose sleek shape and sharp teeth made them formidable hunters. However, the biggest and meanest fish in the Devonian seas was a giant placoderm fish, known as Dunkleosteus, which could reach lengths of more than 26 feet. These were joined by the first bony fish, such as Hyneria, some of which were the ancestors of the bony fish that swim in our oceans today.
How Fish Transformed into Amphibians
A Giant Dragonfly
The First Reptile
Life in the Carboniferous Period
Carboniferous Period: 354-290 Million Years Ago
The Carboniferous was a period where Earth sweltered under a balmy greenhouse climate that engulfed the entire planet including the Arctic and Antarctic. Lowland areas had been colonised by thickly forested swamps dominated by tree sized ferns and horsetails, and the gigantic, alien looking lycopsid trees, some of which grew up to 165 feet tall.
Oxygen levels were very high, and may help explain why these flooded forests were home to an abundance of life that included giant arthropods, such as Arthropleura, which resembled a giant millipedeand flying insects such as mayflies and the eagle sized dragonfly Meganeura.
The waterlogged conditions favoured the amphibians, such as Proterogyrinus, who could move and hunt in the streams and breed in the lakes. Although dominated by the amphibians, the Carboniferous also bore witness to the evolution of the first reptiles, which were mostly small, lizard like creatures such as Petrolacosaurus. These little reptiles laid eggs with hard shells, meaning that they could be laid away from water, something that would help lay the foundations of their future success.
The Carboniferous seas were also teeming with life. The sharks and bony fish dominated the oceans, while the seabed was home to complex coral reefs, some stretching for many miles along the ancient coasts.
The Carboniferous ended some 290 million years ago with the onset of a global ice age. Temperatures plummeted dramatically and as a result the great tropical forests shrank. At this time no organism had yet evolved that was capable of breaking down wood, resulting in millions of relatively intact trees becoming buried under the soil, eventually the wood transformed into something that helped fuel a human revolution, coal. In place of the trees came vast ice sheets and glaciers, which spread outwards from the North and South poles, scouring the landscape. Many species simply could not cope with the extreme change in climate, and in time became extinct.
More on the Permian Mass Extinction
- The Permian Mass Extinction: 250 Million Years Ago
A hub I wrote that covers the Permian Mass Extinction in much greater detail than I do here.
- The Day the Earth Nearly Died - YouTube
A BBC documentary that attempts to answer what exactly caused the Permian Mass Extinction- the greatest erasing of life known to science.
An Iconic Relative
Permian Period: 290-248 Million Years Ago
The global ice age that scoured the planet at the end of the Carboniferous period left the world a much drier and cooler place. In the early Permian the tropical forests and swamps shrank and were replaced by open plains populated with scattered pockets of ferns and the first coniferous trees.
Amphibians such as Seymouria, had previously dominated the Earth, but they needed to live close to water, so found the lack of tropical swamps very hard going indeed. As they declined, so dry adapted reptiles became more common. They quickly increased in number and size, producing animals such as the famous Dimetrodon with its iconic sail on its back and its close relative Edaphosaurus; they were Earth’s first truly large land animals. The cold climate led to innovation among the reptiles, which included most notably the aforementioned large, heat gathering sails found on creatures similar to Dimetrodon.
By the late Permian, the world’s continents had become joined together to form one gigantic landmass called Pangaea. In many parts of the world the climate became hot and dry with sparse rainfall, producing vast deserts. These vast arid wildernesses provided home to a group of reptiles that bore a strange resemblance to mammals called therapsids; among their number was the largest predator of the day, Gorgonops and a tiny burrowing plant eater called Diictodon. The therapsids dominated the landscape, but there were other large animals, such as the lumbering and armoured Scutosaurus, a possible ancestor of the turtles, and the giant amphibian Rhinesuchus, which never strayed too far from water holes that provided lifelines for all kinds of animals in the vast deserts.
Towards the end of the Permian, something truly terrible happened. It all started in Siberia, with an event known as a flood basalt eruption, essentially it involved the Earth’s crust literally splitting apart and releasing vast quantities of lava that possibly covered the entire continent and lasted for millions of years. The fallout from this monumental event was that the Earth’s atmosphere was coated in vast sums of dust and sulphur, which caused nuclear winters that lasted decades.
The consequence of this was that the Earth was wrapped in a warm blanket of carbon dioxide, which caused a greenhouse effect that makes the one occurring today seem minute. The Earth warmed by around five degrees, the shift in temperature caused the oceans to heat up, which killed off most life that dwelt there including the trilobites which had been present since the Cambrian Period. But that wasn't the end, the final death knell occurred when the superheated water released streams of methane gas into the atmosphere, which warmed the planet by a further five degrees. Now, ten degrees hotter than normal, a great dying occurred on land over a period of 80,000 years. The end result was the loss of around 95 per cent of all life, but from out of the ashes of Earth’s greatest tragedy, new and more terrifying varieties of life would soon emerge.
More to follow...
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