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What Came Before the Dinosaurs?

Updated: Jan 26, 2023

Whenever people think of prehistoric times, most often they'll think of 1 or all of 3 things: Dinosaurs, cave-men or the most recent ice age. However, life on Earth has a rich and slightly convoluted history, with an incredibly wide array of life and ecosystems both before and after those 3. Given this fascinating story, let's take a look at what came before the dinosaurs!

I'll go through the periods leading up to the Mesozoic (the time of the dinosaurs) in a similar fashion to the post going over a brief timeline of planet Earth, which will also serve as a useful read for this!


This is the most difficult one to answer. The Precambrian is a somewhat informal term referring to any before...well...the Cambrian. This particular portion of time spans around 4 billion years, so a lot can happen in that time. In terms of life, it was during this time that the first forms of cellular life began appearing. If organisms are what you're into, the Precambrian is probably the most 'modest' of times in terms of excitement.

Another potential issue arises when we look at preservational bias. You see, nothing really lived at the time with any ready to fossilise hard parts, even if they were multicellular towards the end. Not only that, nothing was really big enough to leave any direct fossil evidence of their existence, making the fossil record seem very scant at this time.

One structure seen often in Precambrian rocks, however, are stromatolites. Stromatolites are sedimentary structures that are built up as a result of microscopic photosynthesising organisms. These are the oldest pieces of evidence for life and are still found being built in certain areas today!

Modern stromatolites in Shark Bay, Australia
Modern stromatolites, Shark Bay, Australia. Image credit:

Other than stromatolites, other fossils found towards the end of the Precambrian are the first to start showing hard, shelly parts that can actually fossilise. These are so small and obscure though, that they are simply classified as 'small shelly fauna' or SSFs (creative, I know). Some are tiny full body fossils, but most are fragments of potentially larger organisms. Due to the obscurity, we'll never know!

Microscopic image of small shelly fauna
Microscopic SSFs, Paterson et al.


The Cambrian is the first period in time that people will start referring to as a period, especially when talking about life. This is a time long before the dinosaurs (around 316 million years before the first dinosaurs), in fact, it was long before any vertebrate whatsoever!

No life existed on the land just yet, be it animal, plant, microbe or otherwise. But the oceans were all of a sudden teeming with life in what's known as the 'Cambrian explosion', referring to how quickly life seemed to diversify. Things were really weird too...just take a look at some of the weird beasties swimming around:


We then go into the Ordovician period, where life had kind of slowed down in its experimental stage...before speeding right back up again and getting just as weird! This event is known as the Great Ordovician Biodiversification Event (GOBE for short). As weird as things got though, the fauna that started popping up started to look very familiar to what managed to stick around right up until the time of the dinosaurs.

The most common animals you'll see swimming around (or stationary) were the trilobites, brachiopods, echinoderms, crinoids, corals and cephalopods such as the Ordovician poster child, Orthoceras. In fact cephalopods were the dominant fauna at the time!

Artist's reconstruction of the Ordovician
Image Credit; John Sibbick

There was also a rather strange up and coming group. During the Cambrian, these little critters first found they had a strange rod of nerves running through their bodies and thought 'hey, why not put some bones around this thing?'. Come the Ordocivian, we had a newly rising group that we like to call 'vertebrates'. The first fish were making their mark!

Artist's reconstruction of Haikouichthys
Image Credit: Talifero

Another form of life that had blazed some trails were plants. Plants had begun leave the ocean and infiltrate the land. This marked an important event, as the newly added oxygen to the atmosphere caused vast planet cooling, causing an extinction event...


Despite the extinction event at the end of the Ordovician, around 219 million years before the first dinosaurs were around, life bounced back without looking all that different. The groups that were around were more or less the same as before, with slightly modified species.

Artist's reconstruction of the Silurian
Image credit: Image credit:

The main additions here though were the sea scorpions and those pesky fish starting to try out bony jaws. It was also during this time that some of those arthropods (invertebrates with hard exterior skeletal armour) such as the aforementioned sea scorpions began to test out short burst on land. So vertebrates were the last to conquer land, but, as we'll soon see, sometimes in life not being the first to do something doesn't mean you won't be the best...


Now the vertebrates were really cooking on gas! Commonly referred to as the age of the fish, the Devonian period began around 194 million years before the first dinosaurs came about. The sea was still rich in bivalves and brachiopods, bryozoa, corals and crinoids, as well as trilobites commonly being seen crawling around. Ammonites were also now a new addition to the seas too and were, as we know, here to stay.

It was the new fish that had really begun to change things up at this point though. Jawless fish, whilst still around, were dwindling due to competition from their jawed counter-parts (young whipper-snappers) and fish had now dipped their fins into the realm of hyper-predators. This is where we see placoderms, or armoured fish, such as the monstrous Dunkleosteus competing with equally terrifying cartilaginous fish, such as the newest kids on the block, sharks. Ultimately the Placoderms lost this competition by the end of the Devonian, with reasons of less speed being suggested.

Skull of a Dunkleosteus
Image credit:

Despite being the age of fish, the Devonian was actually a time of great change on the land, with the first rooted plants beginning to form large clumps, resembling large swamps and strange looking forests being unbothered by large herbivores that were yet to exist on land. The invertebrates were very much settled here too, with various terrestrial arthropods, such as insects and the first arachnids, making their mark on the land. Closely following them however, were various jawed fish, with fins that just so happened to help with flopping on land for short bursts. Amphibians were coming onto the scene...

3D reconstruction of Tiktaalik
Image credit:


Soon the Devonian extinction event took its toll, severely reducing many marine groups such as groups of jawed and jawless fish, trilobites, ammonites, brachiopods and most of the reef builders that had created the extensive great Devonian reefs. However, our amphibious ancestors, along with most of the terrestrial groups, remained largely unaffected and so were able to diversify into what we see in the Carboniferous.

If you don't like creepy-crawlies, look away now.

With the large influx of land plants completely taking over the landscape, being unencumbered by the lack of anything that ate them, oxygen was being pumped out like there was no tomorrow! In fact, the Carboniferous is named that for the high amounts of fossilised plant material we find from this time.

Beginning 134 million years before the start of the dinosaurs, these high oxygen levels and abundance of floral food meant an absolute buffet for the animals. Vertebrates decided they quite liked it here and so evolved a way of keeping vital water inside with the embryo without having to keep it in water by laying hard shelled eggs. These are the amniotes and took the form of the very first reptiles. The first reptiles split into two major groups: the synapsids and the sauropsids. The distinguishment between these two is important, but I'll get into that later. For now, just know they looked more or less the same, superficially at least, and that is the appearance of small to medium sized lizards running around in these lush forests. And boy would they have to be quick.

You see, all that oxygen means one thing for an arthropod: they can get big...real big. Throughout these thick forests and swamps you may have seen lizards approaching the size of dogs, but they would have looked like geckos next to the dragonflies the sizes of eagles, scorpions with tails that would have reached your waist and centipedes that were over 8 feet long!

Artist's reconstruction of Meganeaura with a a size comparison with a human
Art by Emily Willoughby

Artist's reconstruction of Arthropleura with a size comparison with a human
Art by Tim Bertelink

These massive bugs were what gave the Carboniferous its infamy with the general public. This didn't last forever though and the Permian soon began 60 million years later.


Following a rapid cooling of the Earth by the end of the Carboniferous, those big bugs suddenly weren't so clever. Amphibians also suffered here, since the atmosphere was becoming much more dry and arid. Thankfully for reptiles though, they were the bullied underdogs and those guys almost always win when there is an extinction event, mostly due to generalisation (something I'll go over in a later post).

Enter the age of the reptiles. By the time the Permian was in full swing, most groups had made a comeback. In the oceans, echinoderms and brachiopods were at an all time high, Trilobites were trudging along and fish in both marine and freshwater environments were doing well for themselves, even getting a little experimental with utter weirdos like Helicoprion.

Artist's reconstruction of Helicoprion.
Image credit:

On the land things were looking good too. Landscapes had begun to vary a lot more now, with Earth forming the supercontinent Pangea, meaning inland areas were more isolated from water, hence a lot more arid and dry than other areas on the outskirts. Arthropods had dropped their 'compensating for something' phase and, whilst modest in size by comparison, had really done well to diversify into niches they carved out for themselves. These along with various surviving amphibians were supported and many swamp and rich forest areas. The reptiles were the big daddies now though.

Remember those synapsids and sauropsids from earlier? Well, that important distinction lies in their skulls, specifically the fenestrae, or holes.

Diagram showing the difference between anapsid, diapsid, eurapsid and synapsid skulls.
Image credit: De Iuliis et al.

Synapsids have a single temporal fenestra, which is the hole in the temple, as well as differentiated teeth, whereas sauropsids will either have two or no temporal fenestrae. These are oversimplified differences of course, but these two groups of reptiles took very different paths during the the Permian. Synapsids that dominated included various species of gorgonopsids, Lystrosaurus and the famous Dimetrodon. These synapsids, whilst they appeared fairly reptilian, are actually the ancestors of us mammals! Not true mammals, but they have still been dubbed as the 'mammal-like reptiles'.

The sauropsids, on the other hand, gave rise to the true reptiles of today, in particular a group called Archosaurs (again, we'll get to that). Sauropsids during the Permian were made up mostly of small, lizard-like tree dwellers (some of which could glide), but they remained small and sparsely diverse during this time.

Artist's reconstruction of Gorgonopsids
Two fighting Gorgonopsids. Artwork by Julius Csotonyi.

Then, the great dying happened...

Artist's reconstruction of the Permian mass extinction
Art by Julio Lacerda

I won't go over this cataclysmic event in this post, since there are others on this blog that go through in extensive detail, but at the start of the Triassic, Earth was a derelict wasteland. The remaining synapsids were tiny generalists, soon to evolve into the very first mammals. The sauropsids, however, bounced back with a vengeance, diversifying into familiar groups such as lizards, snakes, marine reptiles, crocodilians, pterasaurs and, yes, dinosaurs. The first dinosaur we see in the fossil record was from the mid-late Triassic, around 20 million years later.

So, to answer the question of what came before the dinosaurs, it was quite a lot!

Until next time!

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