TOP 10 GEOLOGICAL TIME PERIODS

 

 

We’re talking the big time – the geological time that was my first special mention for my Top 10 Ages. As I said there, geological time consists of time periods so vast that the ages of human history, indeed the entirety of human history or even human prehistory, are blinks of the eye in comparison – time measured in geological strata and based on events throughout the history of the planet itself, a time span of about 4.5 billion years or so.

But which of those geological time periods are the best or most interesting?

Technically speaking, not all of my geological time period entries are from the 22 periods formally defined as such. Most of them are but there’s one era and a couple of epochs. In order of length of time, there are eons, eras, periods, epochs, and ages – 4 formally defined eons divided into 10 formal eras divided into those 22 formal periods divided into 37 formal epochs divided into 96 formal ages. (There are also some sub-periods and sub-epochs, as well as a few informal epochs and ages).

However, the eons are simply too long – ranging from about half a billion to almost two billion years – to be distinctive enough to rank in my top ten. The eras are also too long to be distinctive. Apart from our own era, the Cenozoic Era, as the shortest at 66 million years, eras range from 185.9 million to 900 millon years. Interestingly, the two longest eras – the Mesoproterozoic (600 million years) and Paleoproterozoic (900 millon years) – are longer than the two shortest eons. The names of those eras demonstrates the recurring convention of being the new (“neo”), middle (“meso”) and old (“paleo”) parts of their era, compounding their lack of distinctiveness.

Epochs similarly tend not to be particularly distinctive – with the recurring convention of being the “upper”, “middle” and “lower” parts of their period, for example the Upper Jurassic, Middle Jurassic, and Lower Jurassic.

Hence, my top ten entries are almost all periods – that is, the formally defined periods in geological time, with the exception of my two top spots as epochs.

Spoiler alert – since I have my human and mammalian bias, no prizes for guessing which two epochs take the top spots, albeit with dinosaurs as close runners-up.

And yes – despite the relatively long introduction, this is one of shallow dip top tens.

 

(1) HOLOCENE EPOCH (11,700 YEARS AGO – PRESENT)

 

The Age of Humans, yeah!

Yes, it’s our present epoch. It rules

It followed the Last Glacial Period, with the titular Holocene glacial retreat.

It also coincided with the Neolithic Age – as well as all human prehistory and history after that.

Unfortunately, it also has the ongoing Holocene extinction, the sixth big extinction event in Earth’s geological history, because of – ahem – us.

 

 

(2) PLEISTOCENE EPOCH (2.6 MILLION – 11,700 YEARS AGO)

 

The Ice Age, yeah!

The preceding epoch to our own.

One of a number of Earth’s ice ages – but the most recent and popularized one, the one everyone thinks of when they say Ice Age, hence the capitalization (or being called the Great Ice Age).

Of course, its popularity is aided by its famed mammalian megafauna – mammoths and all – as well as our presence, the Pleistocene largely corresponding with the Paleolithic Age and evolution of modern humans, albeit our own species only evolved in the last 10% of it or so.

Also noted for the Late Pleistocene extinctions – not quite up there with the big extinction events but notable none the less for the extinction of megafauna, again probably due to, ahem, us. The Late Pleistocene also saw the extinction of all other human species but our own – as well as the spread of modern humans beyond Africa, including to the Americas and Australasia.

 

 

 

 

(3) CRETACEOUS PERIOD (143 – 66 MILLION YEARS AGO)

 

The Age of Dinosaurs, yeah!

Technically that’s non-avian dinosaurs since the avian dinosaurs continued as birds. That includes the most well-known and popular dinosaurs, ironically including most of those in Jurassic Park – which really should have been called Cretaceous Park but that doesn’t roll off the tongue the same way.

Ended with the most well-known of all extinction events, the one that killed the dinosaurs – the Cretaceous-Paleogene or K-Pg extinction event, although I prefer its former and catchier name of K-T or Cretaceous-Tertiary extinction event. Surprisingly not the biggest extinction event – even if it did kill 75% or more of all animal and marine species – but the most abrupt, brought about by the impact of a 10-15 km wide asteroid at Chicxulub in Mexico.

 

 

 

 

(4) JURASSIC PERIOD (201 – 143 MILLION YEARS AGO)

 

Dinosaurs again, hell yeah!

Probably the most famous period of geological time, thanks to its titular use in the Jurassic film franchise.

It kind of just blurs into the following Cretaceous Period, aptly enough given the use of Cretaceous dinosaurs in the Jurassic film franchise.

It did, however, have a definitive start – with the Triassic-Jurassic extinction event. Dinosaurs can’t complain too much about extinction events because that’s how they and the Jurassic Period got their start – albeit it was less of an asteroid bang and more of a whimper of ocean anoxia, ocean acidification, and elevated temperatures (probably volcanic in origin).

 

 

 

 

(5) TRIASSIC PERIOD (252 – 201 MILLION YEARS AGO)

 

The Age of Reptiles, yeah!

Well, the start of it, and not dinosaurs – or rather, not just dinosaurs. Sure, they were around, originating in this period but were not dominant until after the Triassic-Jurassic extinction event. Mammals also originated in this period but they needed a second extinction event for their dominance.

The Triassic not only ended but began with an extinction event, preceded as it was by the Permian-Triassic extinction event.

 

 

 

 

(6) PERMIAN PERIOD (299 – 252 MILLION YEARS AGO)

 

Amniotes, yeah!

Wait, what are amniotes? Essentially the origin of all non-amphibian terrestrial vertebrate animals.

It ended with the biggest extinction event in Earth’s history, the Permian-Triassic extinction event – also known as the Great Dying – estimated to have wiped out 90-95% of all marine species and 70% of those on land. Volcanic eruptions are hypothesized as the primary cause, with widespread climate change.

Sadly, the trilobites died out too.

 

 

 

(7) DEVONIAN PERIOD (419 – 359 MILLION YEARS AGO)

 

The Age of Fishes, yeah!

Also – coelacanths!

Don’t worry – the land also saw life, indeed the rapid evolutionary “colonization” by and diversification of life on land that continued from the preceding Silurian Period and is known as the Silurian-Devonian Terrestrial Revolution.

Plants evolved leaves, true roots and seeds in what is called the greening of the land of Devonian explosion.

The earliest land animals, predominantly arthropods, became well-established, having colonised land at least two periods before, while fish began flexing towards becoming the ancestors of all four-limbed vertebrates.

No, really – “their more robust and muscled pectoral and pelvic fins gradually evolved into forelimbs and hindlimbs, though they were not fully established for life on land until the Late Carboniferous”.

Sadly ended with – you guessed it – an extinction event (or two), in this case the Late Devonian extinction event, one of the big five in Earth’s history, with an estimated 40% of marine life going extinct. Happily, trilobites made it through to last until the Permian.

 

 

 

(8) SILURIAN PERIOD (443 – 419 MILLION YEARS AGO)

 

It would probably earn its place from being the other half of the Silurian-Devonian Terrestrial Revolution alone – also saw the first jawed and bony fish.

Loses points for spiders.

Also bounced back from the Ordovician-Silurian extinction event, another of the big five extinction events – which in the case of the Ordovician-Silurian extinction event caused up to 60% of all marine species wiped out.

Speaking of which…

 

 

 

 

 

(9) ORDOVICIAN PERIOD (488 – 443 MILLION YEARS AGO)

 

The Great Ordovician Biodiversity Event – the massive diversification of marine life, including jawless fish. While most life was marine, it also saw the emergence of land plants and possibly arthropods.

And then as usual, after life had crawled or swam its way up, the planet punched it in the guts – with one of the big five extinction events, the Ordovician-Silurian extinction event, which wiped out up to 60% of all marine species, as the great southern supercontinent Gondawana moved over the South Pole and the planet plunged into glaciation.

 

 

 

 

(10) CAMBRIAN PERIOD (542 – 488 MILLION YEARS AGO)

 

Life on Earth, yeah!

Well, not really. Life originated long before, perhaps as early as 3.7 billion years ago or so, about 800 million years after the formation of the Earth itself, but it only got interesting in the Cambrian Period, with the so-called Cambrian Explosion.

Arthropods, molluscs and chordates, oh my!

Mostly in water but still.

Also – trilobites, yeah!

That’s right, it took until the last 10% or so of all of Earth’s timespan for life to get interesting. Before that, the majority of living organisms were unicellular, although there was some multicellular life for about a billion or so years before the Cambrian Period. Life on Earth as we know it resembles nothing more than a painstakingly slow crawl before a mad sprint to the finish line in the last tenth of the track.

Don’t get too excited though – life on Earth was still almost entirely in water. There’s evidence of some microbial life on land, with some speculation as to land plants or even arthropods or molluscs venturing onto land but nothing solid until the subsequent period.

As usual with life, it was two steps forward, one step back – or more usually, almost two steps back). The later half of the Cambrian “was surprisingly barren” with declining biodiversity (until the subsequent Ordovician Period) and of course the Cambrian ended with – you guessed it – another extinction event, the Cambrian-Ordovician extinction event, albeit not one of the big five extinction events.