Travel back in time, millions of year ago, when dinosaurs roamed the land, fearsome marine reptiles ruled the seas. 

Icthyosaurs, Plesiosaurs and Mosasaurs had some pretty unique adaptations to help them survive and thrive in prehistoric ocean environments, and you can learn all about these in our current exhibition Sea Monsters – Prehistoric Ocean Predators.

Suitable for ages:
2 years and up
Time required:
30 mins
2 out of 5
Related Exhibition: Sea Monsters

For this make-at-home-craft activity we’ve taken inspiration from one of their more unusual hobbys - eating rocks! 

Many plesiosaur fossils have been found with large collections of stones known as gastroliths in their stomachs. Some fossils show plesiosaurs with as much as 6kg of stones in their guts!

Make your own goofy gastroliths - prehistoric pet rocks 

Experts don’t know for sure but they think that these stones were eaten to help with digesting food or as a kind of ballast weight to help them maintain buoyancy. It’s also possible that these creatures needed the minerals in the stones (who would have thought there is nutritious content in rocks! Gravel-eating toddlers might be onto something there.) Or maybe they just ate them by mistake when scooping up other bottom-dwelling prey (like how you just 'accidentally' ate some colourful foil wrapping when swallowing that delicious chocolate egg...) 

A lot of birds and crocodiles eat stones for these same reasons today – penguins use stones for ballast weight, and crocodiles swallow them to crush up food in their stomachs to help with digestion. A serving of sedimentary, igneous or metamorphic stones may not be on the lunch menu for you any time soon but they sure do make for a great craft project! In these isolation times, brightly painted prehistoric pet rocks might just be the amusing new friends we all need to brighten up the day. 

How to make you own goofy gastroliths - prehistoric pet rocks 

Make your own goofy gastroliths - prehistoric pet rocks 

What You'll Need:
  • Some smooth stones (we used path pebbles available at hardware/garden stores or you may already have some in your garden you could use)
  • Some posca paint markers (available from stationary stores and newsagencies, you could also use acrylic paint if you need)
  • Glue or stickytape (optional)
  • Some cardboard shapes to add legs and arms to your rock (optional)

To Make:

Step 1. 
Sketch out your design. Like any painting, it helps to plan ahead what colours you will use and what patterns, shapes, eyes you want to draw, before you apply paint markers to the rock. 

Make your own goofy gastroliths - prehistoric pet rocks

We decided to make 2 abstract monsters with playful patterns and expressions just for fun and one that was inspired by a KRONOSAURUS QUEENSLANDICUS. Kronosaurus was a pliosaur (a short-necked plesiosaur) and one of the fiercest predators in Australia's inland seas. We gave him a mean reptilian eye, some scaly patterns and some jaw-some jagged-teeth.

Step 2. 
Start painting. 

To get the paints to flow well shake your pen with the lid on. Take the lid off and gently pump your posca by pressing down the tip onto your sketching sheet/scrap paper a few times. Repeat these steps until you see a good flow of paint. 

Hint: Don’t shake with the lid off as the paint marker may splatter everywhere and it can be tricky to clean out of your clothing, walls and floors. Make your own goofy gastroliths - prehistoric pet rocks 

Use one colour at a time and WAIT for each layer to dry before you add the next layer. We started with body/skin colours first, then patterns (allowing each colour to dry before the next colour was applied will ensure you don’t get muddy mixed colours), then the whites of the eyes and teeth, then the black outlines last. 

If you make a mistake, don’t worry, it’s easy to wash or wipe the paint when it’s wet, or you can just wait for it to dry and add another layer of colour to correct that area.

Step 3. 
Once you have completed painting (and your rock is dry) you can add fun arms, legs and mixed materials with tape or PVA glue to give your pet rock even more personality. If you want your design to never scratch or wash off, you could add some protective coating like varnish or mod-podge. Make your own goofy gastroliths - prehistoric pet rocks 

There you have it - your prehistoric pet rock-ta-saurus! 
Make your own goofy gastroliths - prehistoric pet rocks

You might want to go ahead and give your rocks some names – we went with rocktopus, stonosaurus, and sketchasaurus. 

There’s a great prehistoric name making game also in our sea monsters activity book HERE if you need some inspiration. 

Make your own goofy gastroliths - prehistoric pet rocks 

Fun ideas if you’ve got some more time:

• You can make your rock a crafty handmade home to live in
• Write a story about their life, or a profile about them - what creature they were inspired by, what are their superpowers, what do they like to do for fun?  
• Perhaps they could even star in your next stop-motion video or a comic strip!

Make your own goofy gastroliths - prehistoric pet rocks 

We’d love to see your creations! 
Please send us photos either by email to: or tag us on Instagram - just tag the photo with our username

Be inspired to create your own Prehistoric Pet Rock by watching this video about Queensland's very own Plesiosaur!


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