Trudy Myers's Blog, page 21
June 5, 2021
Mesozoic Era
Okay, we're going to take a quick overview look at the Mesozoic Era, also known as the Age of Reptiles. It lasted from 252 to 66 million years ago. It has 3 Periods nestled within it; the Triassic, Jurassic and Cretaceous periods. It was characterized by dinosaurs, conifers and ferns, a hot greenhouse climate, and the tectonic break-up of Pangaea.
The Mesozoic began just after the largest well-documented mass extinction in Earth's history, and it ended with another extinction event.
During this era, the supercontinent Pangaea broke into separate landmasses that would eventually move into their current positions during the following era. There was not much mountain building during this era, but what little did occur took place around what is now known as the Arctic Ocean. In contrast, the supercontinent Pangaea gradually split into a northern continent, Laurasia, and a southern continent, Gondwana. (Seems like this name has been used before.) By the end of the era, the continents had split up and some had rejoined into their present forms, though not their present positions. Laurasia became North America and Eurasia, while Gondwana split into South America, Africa, Australia, Antarctica and the Indian subcontinent, which would eventually slam into Asia, giving rise to the Himalayas, but not during this Era.
The climate varied, alternating between warm and cool periods. Overall, the Earth was hotter than it is today. The Triassic (first) Period was generally dry and highly seasonal, especially in Pangaea's interior. Low sea levels would have exacerbated temperature extremes. Pangaea's interior probably included expansive deserts.
Sea levels began to rise during the Jurassic (second) Period, most likely caused by seafloor spreading. The sea levels could have risen as much as 656 ft (200 m) above today's sea level. This would have flooded coastal area. In addition, the breaking up of Gondwana into smaller continents created new shorelines. Temperatures continued to increase for a time, then began to stabilize. With the proximity of water, humidity also increased, and the deserts retreated.
The climate of the Cretaceous (third) Period is more widely disputed. Probably, higher levels of carbon dioxide in the atmosphere could have almost eliminated the north-south temperature gradient, meaning that temperatures were about the same across the planet, about 10 degrees Centigrade higher than today.
Dinosaurs first appeared mid-way through the first period, and became the dominant terrestrial vertebrates by early in the second period, then died out at the end of the third period. Archaic birds appeared during the 2nd period (Jurassic), evolving from a branch of dinosaurs. True birds appeared in the third period. Mammals also appeared during this era, but they remained small (less than 33 lb) until the third period. Flowering plants appeared early in the third period and rapidly diversified, replacing conifers and other gymnosperms as the dominant group of plants. But we'll take a closer look at flora and fauna as we get to those periods.
May 22, 2021
Permian Period Part 2
Mammals and reptiles could cope with the desert conditions better than amphibians, and so rose to dominate the Permian. Such dry conditions favored plants with seeds enclosed in a protective cover, rather than plants like ferns, that disperse spores. Modern trees like conifers, ginkgos and cycads appeared in the Permian.
On land, life included diverse plants and fungi. At first, a lot of the Carboniferous flora still flourished. But around the middle of the period, a massive desert covered the interior of Pangaea, and there are indications that the desert was devoid of vegetation. Even in areas where it wasn't such a stark desert, more advanced seed ferns and early conifers moved in. A number of types of plants and animals died out or became marginal elements. By the end of the Permian, swamps reminiscent of the Carboniferous survived only on a series of equatorial islands east of Pangaea that would later become South China.
The Permian saw the diversification of many important conifer groups, including ancestors of many present-day families. Rich forests were present in many area, with a diverse mix of plants. Southern Pangaea saw extensive seed fern forest, and oxygen levels were probably high there. The ginkgos and cycads also appeared.
Insects were very prevalent during the Permian, especially considering that 90% of the insects at the start of this period were cockroach-like insects. Primitive forms of dragonflies were the dominant aerial predators. Several insect groups appeared or flourished during this time, including beetles, true bugs and others.
Larger 'more advanced' types of animal life included reptiles and amphibians. A number of the reptiles seemed to be what I call the 'sail-backed' lizards, where they have bone ridges protruding from their spine and covered in skin. Some of these were herbivore and some were carnivore. I didn't find any indication of how large they might have gotten.
At some point, amphibians declined, and at least one family of the 'sail-backed' lizards was replaced with more advanced animals, including mammals and mammal-like animals. Towards the very end of the period, the first archosauriforms appeared. These were reptiles that possibly rose from the crocodile family and would go on in the next period to produce dinosaurs and pterosaurs. There were no flying vertebrates, although there was a family of gliding reptiles by the end of the period.
Also appearing late in the Permian were the first synodonts, which would evolve into mammals during the next period. Permian synapsids (the group that would later include mammals) thrived and included some large members, such as Dimetrodon. Their special adaptations enabled them to flourish in the drier climate, and they grew to dominate the vertebrates.
In the Permian sea, life was everywhere. You might have found mollusks, starfish, sea dollars, sea urchins and sea cucumbers if you went swimming, or animals very much like these modern versions. But as stated earlier, most of the marine life went extinct during the last days of this period. Nothing was said about the sharks or fish that were so prevalent in the previous period.
Could we live here? Plenty of plants are growing, at least, where it wasn't a desert, so maybe we could grow crops. Could we domesticate some of the herbivore sail-backed lizards? Would we want to? They're lizards, so they wouldn't give any milk. I have no idea how big their eggs might be, so that might be a reason to domesticate them. Okay, we'll leave some of us here to colonize the Permian, and the rest of us will go on to see what comes next.
https://en.wikipedia.org/wiki/Permian
May 15, 2021
Permian Period Part 1
The Permian Period is the last period of the Paleozoic Era, and it spans the 47 million years from 298.9 million years ago to 251.9 million years ago.
The Permian witnessed the diversification of two groups, the mammals (and all creatures more closely related to mammals than to reptiles and birds) and the reptiles. But I'm going to look at the conditions existing on the Earth at the time before I consider the animals that populated it.
At the time, the world was dominated by the supercontinent Pangaea, which formed when Euramerica and what was left of Gondwana collided during the Carboniferous. There was a smaller continent that lay to the northeast of Pangaea called Angara. There were also some small islands east of Pangaea called Cathaysia. Of course, with most of the land mass gathered into one supercontinent, the rest of the globe was ocean. The superocean of the time was the Panthalassa. Pangaea straddled the equator and reached for the poles (but didn't necessarily get there). This, of course, had an effect on the ocean currents of the time.
Early in this time period, Cimmeria—a string of microcontinental islands—tore off from the Gondwana area in the Southern Hemisphere and during the course of the period, moved up to join the Eurasian part of Pangaea in the Northern Hemisphere. Cimmeria included parts of today's Turkey, Iran, Afghanistan, Tibet and the Malay Peninsula. The Central Pangean Mountains, which began forming due to the collision of Laurasia and Gondwana during the Carboniferous Period, reached their maximum height shortly after the beginning of the Permian, and would have been comparable to the present Himalayas.
The Carboniferous rainforest collapse left behind vast regions of desert stretching over the continental interior. From approximately 262 million years ago to 259 million years ago, a series of volcanic erruptions in what is now China (was then part of Pangaea) led the way to an extinction event. This was compounded when a similar series of volcanic activity over a wide swath of Siberia (then located in Angara) continued to change the atmosphere. This mass extinction event ended the Permian Period, and started the next period. It was the largest mass extinction in Earth's history, with nearly 81% of marine species and 70% of terrestrial species dying. On land, it took 30 million years into the next time period for the ecosystems to recover.
Sea levels were mostly stable during the Permian, at several tens of meters (yards) above the present level. There was a sharp drop at one point, producing the lowest sea level of the entire Palaeozoic Era, roughly the same as today's level.
At the start of the Permian, the Earth was still in an icehouse. Around 323 million years ago, glaciers began to form around the South Pole, which would eventually cover a vast area of the southern Amazon Basin, Southern Africa, Australia and Antarctica. The coldest period was around 293 million years ago. By 285 million years ago, temperatures warmed, and a great deal of the ice retreated, although some glaciers remained. The Permian was cool compared to other time periods, at least until those volcanoes started pouring CO2 (a greenhouse gas) into the atmosphere.
It sounds like the Permian had a lot of diverse climates and ecosystems, right? So as long as we stayed away from that awful desert, we should be able to find a nice place to live. So let's take a look at the neighbors we'd have in the next blog.
https://en.wikipedia.org/wiki/Permian
May 8, 2021
Carboniferous Period
The Carboniferous Period spanned 358.9 million years ago to 298.9 million years ago. On a time table, it sits behind the Devonian Period and before the Permian Period.
In the Early Carboniferous Period, average global temperatures were approximately 68 F (20 C). During the Middle Carboniferous, they dropped to about 54 F (12 C). Carbon dioxide levels fell during the period from roughly 8 times today's level at the beginning to about the same as today's level at the end. Lack of growth rings of fossilized trees suggest a lack of seasons, or a tropical climate. Glaciations in Gondwana were triggered by its southward movement, and continued beyond this time period.
Also, there was mountain building as continents collided to form Pangaea. There was a minor extinction event at the end of the period, caused by climate change.
Now, the last we heard, pieces of Gondwana had broken off and were sitting at the equator or slightly north, while the rest of Gondwana drifted south to the south pole.
During the Carboniferous Period, the land was covered by vast forests, which would eventually become the coal beds characteristic of this period's stratigraphy. Some of the Early Carboniferous land plants were very similar to those of the Late Devonian, but new groups also appeared. The main plants of the early period were horse-tails, scrambling plants, club mosses, scale trees, and ferns. The club mosses of this period are cousins to today's tiny club-moss, but not ancestors; and they had trunks 30 meters high and up to 1.5 meter in diameter. Another type of large tree was ancestor of today's ferns. These continued throughout the period, but late in the period, they were joined by cycads (a new form of 'seed fern') and plants related to conifers.
At least one tall plant (6 to over 30 meters tall) was related to cycads (often mistaken for palm trees) and conifers and are thought to have lived in swamps. True conifer trees appeared later in the period and preferred higher, drier ground.
During this period, animals and bacteria had great difficulty processing the lignin and cellulose that made up the gigantic trees. After the trees died, they piled up on the ground, occasionally becoming part of long-running wildfires after a lightning strike. Others very slowly degraded into coal. White rot fungus was the first living creature that could process these trees and break them down.
Animal life was well established by now. Amphibians were diverse and common by the middle of the period, the dominant type of land vertebrates. Some were as long as 6 meters, but most were probably about 6 in (15 cm) in length. Some were aquatic and lived in rivers, while others may have been semi-aquatic. One branch of amphibians would eventually evolve into the first solely terrestrial vertebrates. The cooling climate slowed the evolution of amphibians, who could not survive as well in the new conditions.
Insects, spiders, crustaceans, and others were also very common, and many were much larger than those of today. The atmospheric content of oxygen reached its highest level in Earth's history—35% compared to today's 21%—which allowed the land invertebrates to get so large. There was a millipede-like creature that grew to 8.5 ft (2.6 m) long. Startling to come across, but if you could kill it, there should be some meat on all those legs. This was the largest known land invertebrate of all time. Among the insects, there were the griffinflies, which included a dragonfly-like insect with a wingspan of 30 in (75 cm). This was the largest flying insect ever to roam the planet. Many other insects flew and crawled about, including cockroach ancestors.
Reptiles, however, prospered due to specific adaptations, such as the amniote egg, which allowed the laying of eggs in a dry environment. Some of the small lizard-like animals gave rise to many descendants, including reptiles, birds and mammals.
I wanted to include a statement about the marine animals of the time, but that section of the article was mostly full of names and little else. I did gather than sponges were fairly prolific and diverse. Also sharks evolved into a multitude of shapes and sizes. Although sharks were mostly sea creatures, there was one type that would sometimes visit swamps. At least one marine fish explored river outlets, and fresh-water fish were common in rivers.
During the latter half of this period, there were glaciations, which meant low sea levels. The cooling and drying of the climate led to a minor extinction event at the end of the period. The tropical rainforest fragmented and then was devastated by climate change.
April 24, 2021
Long excerpt - Hank's Widow
“Who are you filling up with chips and cake, Ella?” Wanda asked as Zita hurried away.
“Lyle’s wife,” Ella answered.
Wanda knew a moment of abject fear and looked around nervously. “Lyle’s here?”
“No, he should be at work. Mom stopped by and picked up Gloria and Sammy and brought them here for the party.” The girl paused, took a deep breath and pasted a smile on her face. “Now, this isn’t the time to talk about the failures of some of my relatives. I have succeeded in getting through high school, and that’s a reason to celebrate.”
“Indeed it is. And it’s a lovely party, Ella. The cake is delicious. Did all the graduates’ parents get together to organize the party?”
“No, the only thing the parents have to do is provide a dessert for their child, and a picture for the wall. The party itself is organized by the Belgrade Fire Brigade.”
“The fire brigade?”
“Well, it’s more than a fire brigade, but they worked hard to raise the money to build this station and buy some used fire engines. They raise money every year, and some of it is used to educate members of the volunteer fire department, and some of it is used to have parties for important milestones, like kids who graduate from high school!”
“So it sounds like it started as an organization to get a fire brigade started, and now it does other things too, but it hasn’t changed its name.”
“That’s right. Oh! There’s Shirley. I should go tell her how great her mother’s banana walnut cake is.”
April 17, 2021
Devonian Period
The Devonian Period spanned about 60 million years of the Paleozoic, from 419.2 million years ago to 358.9 million years ago. It is sandwiched between the Silurian Period, which came earlier, and the Carboniferous Period, which is more recent.
The first significant adaptations of life on dry land occurred during this period, for life was well underway in colonizing the land. Moss forests and bacterial/algal mats of earlier were joined early in the period by primitive rooted plants that created the first stable soils and harbored such arthropods as mites and scorpions. By far the largest land organism at the beginning of this period was a poorly-understood plant which was possibly the fruiting body of an enormous fungus, a rolled liverwort mat, or another organism of uncertain affinities. This plant stood more than 8 meters (26 ft) tall, which means it towered over the low, carpet-like vegetation that covered the land. The first fossils of insects appeared around 416 million years ago, in the Early Devonian.
Many Early Devonian plants did not have true roots or leaves like modern plants. These were generally very short, growing hardly more than a few centimeters tall. By the middle of the period, forests of shrub-like plants existed, for plants like horsetails and ferns had evolved. These had true roots and leaves, and many were quite tall. Also, the earliest-known trees appeared in the Middle Devonian, although probably not any that we would recognize. Certainly I didn't recognize them from their descriptions, and they all seemed to have 'extinct' in their description. By the end of the Devonian, the first seed-forming plants had appeared. There was such a rapid appearance of so many plant groups and growth forms that it is called the "Devonian Explosion". Various terrestrial arthropods (which includes insects, spiders, and crustaceans) also became well-established.
Fish reached substantial diversity, so that Devonian is often called the Age of Fishes. Among the marine vertebrates, jawless armored fish declined in diversity, while jawed fish increased in both the sea and fresh water. Early cartilaginous and bony fishes also became diverse in the seas. The first abundant genus of shark appeared during this period. The ancestors of all four-limbed vertebrates began adapting to walking on land, as their pectoral and pelvic fins evolved into legs.
The first ammonites (an extinct specie of mollusc) appeared during the Devonian. Trilobites (which look something like armored roaches), brachiopods (similar to a mollusc) and great coral reefs were common in the seas. The Late Devonian extinction, which started about 375 million years ago, affected marine line severely, killing off—among other things—all trilobites save for a few species.
The Devonian was relatively warm, and probably lacked any glaciers. For this reason, the sea level was high. The temperature gradient from the equator to the poles was not as large as it is today. The weather was also very arid, particularly along the equator. Surface temperature of the tropical seas was probably 86°F in the Early Devonian, but CO2 levels dropped steeply throughout the period, because the newly evolved forests drew carbon out of the atmosphere. By Mid-Devonian, there was a cooling of about 9°F. However, there is evidence that the temperature rose again in the Late Devonian, which may have contributed to the extinction event.
The geography was dominated by the supercontinent Gondwana to the south, Siberia to the north, and the early formation of Euramerica in between. It was a time of great tectonic activity, as Euramerica and Gondwana drew closer together. In the early Devonian, Laurentia and Baltica collided, forming Euramerica, which rotated into the natural dry zone along the Tropic of Capricorn (appromately 23.3° South of the equator). Then the plate of Euramerica and Gondwana started to meet, beginning to form the supercontinent Pangaea. This raised the northern Appalachian Mountains and formed the Caledonian Mountains in Great Britain and Scandinavia. The west coast of Euramerica was low lying, with deep silty embayments, river deltas and estuaries (found today in Idaho and Nevada). However, a volcanic island arc approached the west coast in the Late Devonian, and began to uplift that coast in a prelude to mountain-building that happened later.
Hey, we're getting somewhere! If trees and tall bushes could grow during this time period, then the soil must be decent, right? So maybe we could grow crops, so long as we bring our own seeds. And maybe we could set up a farm with some barnyard animals, too, like chicken, ducks, cows and goats. There's insects for the fowl to eat. Do you think cows and goats would eat horsetails and liverworts? Because the article didn't say anything about grasses. And as long as we don't go swimming in shark-filled waters, I think we'd be relatively safe.
April 9, 2021
Silurian Period
After the Ordovician Period came the Silurian Period, lasting 24.6 million years, from 443.8 million years ago to 419.8 million years ago. It began approximately the same time as the major series of extinction events mentioned at the end of the Ordovician Period, when a large number of marine fauna were wiped out.
The Silurian saw wide-spread diversification of jawed fish as well as bony fish. Multi-cellular life also appeared on land as small plants similar to liverworts, hornworts, and mosses, which grew besides lakes, streams and coastlines. Invertebrate animals with an exoskeleton also found land during this time period. This includes insects, spiders and crustaceans. However, terrestrial life would not greatly diversify until later.
With the supercontinent Gondwana still covering much of the southern hemisphere, a large ocean occupied most of the northern half of the globe. High sea levels and relatively flat land (there were few significant mountain belts) produced a number of island chains, meaning there was a rich diversity of environmental settings.
What was left of Gondwana remained intact and continued to drift south, but there is evidence that the icecaps were less extensive than those of the late-Ordovician time period. The smaller continents of Avalonia, Baltica, and Laurentia drift together near the equator, starting the formation of a second supercontinent known as Euramerica.
When proto-Europe (Baltica) collided with proto-North America (Laurentia), the collision folded coastal areas from modern New York State through Europe and Greenland to Norway. At the end of the Silurian Period, sea levels dropped again, and the new mountain ranges were rapidly eroded.
The Silurian period enjoyed relatively stable and warm temperatures, in contrast with the extreme glaciations of the period before it, and the extreme heat that would follow it. Sea levels rose during the first half of this period, and fell during the second half.
Climate was warm because high CO2 levels and warm shallow seas produced a greenhouse phase. Glaciers at the South Pole nearly disappeared entirely. There is strong evidence of a climate dominated by violent storms generated by warm sea surfaces.
The Silurian was the first period to have megafossils in the form of moss-like miniature forests along lakes and streams. The first fossil records of vascular plants (land plants with tissues that carry water and food) appeared in the second half of the Silurian period.
Fish reached considerable diversity. A diverse fauna of sea scorpions (some of them several meters in length) prowled the shallow Silurian seas of North America. Leeches made their appearance.
About the middle of the Silurian, the earliest-known animals fully adapted to terrestrial conditions appeared, including a millipede. There is also some evidence of predatory spiders and millipedes and centipedes. Predatory invertebrates indicate that simple food webs were in place that included prey animals. These may have included those who grazed on micro-organisms.
Yes, we are getting closer and closer modern Earth. That is what happens when you start at the beginning and work your way towards today. I'm still not seeing any fauna that would be worth hunting. Although a millipede several meters in length might have enough meat to make a stew. Is the ground fertile enough to grow carrots and potatoes to put in that stew? And just how bad did those storms get? Maybe I'll keep going before I try to colonize.
March 10, 2021
Big Sale
March 5, 2021
Ordovician Period
When I first started reading the article on the Ordovician Period, I got to the mention of Gondwana, which made me stop and wonder, "Wait, haven't we been through this breakup before?" And the answer is yes, we have, in the blog I wrote about the Paleozoic Era, which the Ordovician Period is part of. I had to remind myself that this is not really déjà vue, that the Paleozoic Era has 6 periods to it, and my blog on the Era would have been like an overview, while the blogs on the periods would have more details. So, some of the big events, like the breakup of Gondwana, will be mentioned in both posts.
The Ordovician Period spans 41.6 million years, from the end on the Cambrian Period some 485.4 million years ago to 443.6 million years ago.
Life continued to flourish during the Ordovician Period, although there was an extinction event at the end of the period. The Ordovician Period is known for its biodiversification event, which considerably increased the diversity of life. Invertebrates, namely molluscs and arthropods, dominated the oceans, although fish, the world's first true vertebrates, continued to evolve, and fish with jaws may have first appeared late in the period. Life on land had yet to diversify.
However many meteorites strike the Earth in a year today, there were 100 times that many hitting the Earth per year during this period.
The southern continents were collected into Gondwana, which started the period in equatorial latitudes but then drifted toward the South Pole. Meanwhile, other continents, Laurentia (part of North American), Siberia, and Baltica (northern Europe), were drifting north, and Baltica started moving towards Laurentia later in the period. Another small continent, Avalonia, separated from Gondwana and began moving north towards Baltica and Laurentia.
Temperatures were mild in the early and middle Ordovician Period, but from 460-450 million years ago, volcanoes along one of the oceans spewed massive amounts of carbon dioxide (a greenhouse gas) into the atmosphere, turning the planet into a hothouse. These volcanic island arcs eventually collided with proto-North America and formed the Appalachian Mountains. [At last! I've been waiting for billions of years for the Appalachians to appear!]
Initially, sea levels were high, but as Gondwana moved south, ice accumulated into glaciers and the sea levels dropped. At first, low-lying sea beds increased diversity, but later glaciation led to mass extinctions as the seas drained and continental shelves became dry land. By the end of the period, the volcanic emissions had stopped. By then, Gondwana had neared the South Pole and was largely glaciated.
Reef-forming corals first appeared early in this period. Land plants probably evolved from green algae, first appearing in a form resembling liverworts. Fungi was also an early adopter of living on land, and facilitated the colonization of land by making mineral nutrients available to plant cells.
This period closed with a series of extinction events that are generally regarded as one major event, in which 49% of all fauna died. It is generally agreed that this event or series of events were caused by an ice age. That ice age had several pulses of increasing/decreasing glaciation. Each time the glaciation increased, the sea level dropped, killing many of the fauna that inhabited the shallow seas. When the sea levels rose during the next decrease of glaciation, there were entire families of fauna that had not survived to re-establish themselves in the shallow seas. This may have happened several times, producing a series of extinction events.
Well, now we're starting to get some place. Plants (of sorts) on land, starting to make the soil arable. Not sure about the carbon dioxide level in the atmosphere, but the meteor showers would be beautiful, as long as they didn't land too close. Hunting probably wouldn't do much good yet, but, hey, there's fish! Too bad I can't stand fish.
Maybe the next period will be even more amenable?
February 26, 2021
Cambrian Period
The Cambrian Period was the first geological period of the Paleozoic Era. It lasted 55.6 million years, from 541 million years ago to 485.4 million years ago. The Cambrian is known for sites of exceptional preservation where even 'soft' parts of organisms are preserved, so our understanding of the Cambrian biology surpasses that of some later periods.
A profound change in life on Earth happened during the Cambrian Period, in that mineralized multicellular organisms became common. The rapid diversification of life-forms in this period—known as the Cambrian Explosion—produced the first representatives of all modern animals, probably from a single common ancestor.
Although life prospered in the oceans, the land is thought to have been comparatively barren. Shallow seas flanked several continents and were relatively warm. Polar ice was absent for much of the period.
Large, high-velocity rotational movement of Gondwana appears to have occurred in the early Cambrian, and may have resulted in Laurentia (North America), Baltica and Siberia being 'tossed away' and forming isolated land masses. Most continental land was clustered in the Southern Hemisphere, but was drifting north.
With a lack of sea ice, the sea level was high, which led to large areas of the continents being flooded in warm shallow seas, which were ideal for sea life. The sea levels fluctuated, suggesting there were pulses of expansion and contraction of a south polar ice cap.
The article stated that the Earth was generally cold during the early Cambrian, and then said the average temperatures were 7 degrees Celsius higher than today. That doesn't seem very cold to me.
The Cambrian flora was little different from what had existed in the previous period. Primarily, there were marine macroalgae in the seas, and that was pretty much it. There were no land plants known from the Cambrian, although biofilms and microbial mats were well developed on tidal flats and beaches 500 million years ago. There were also microbes forming microbial Earth ecosystems, comparable with modern soil crust of desserts, which contributed to soil formation.
It was once thought that trilobites were the dominant life form of the time period. But it turns out that these had a heavy armor which fossilized far more easily than the bodies of other animals, so there were plenty of trilobite fossils, even though trilobites were only a minor part of the animal diversity.
Earth suffered a mass extinction at the start of the Cambrian Period. It is thought that animals that burrowed into the sea bed, destroyed the microbial mats covering the seabed, and many organisms dependent on the mats became extinct, while other species adapted to the changed environment.
Despite the 'Explosion' at the start of this period, the later half saw a sharp drop in biodiversity. 500 million years ago, oxygen levels in the oceans dropped dramatically, while the level of poisonous hydrogen sulfide increased, producing more extinction events, making the latter half of the period surprisingly barren.
However, some organisms did venture onto land, producing trace fossils of their movements. Some of these fossil trackways suggest a large, slug-like mollusc.
Just when you thought Earth pre-history was going to get interesting, it takes one step forward and two steps back. Dry land is still pretty barren, except for an occasional slug-mollusc looking for some tasty soil microbes.
Well, we're pretty sure humans arrive on the scene eventually, so we're just going to keep slogging forward until we find us.
