APPARENTLY, next Monday, January 17, is the saddest day of the year.

Presumably, in some past survey or other, the majority of people questioned rated their spirits lowest on this particular Monday morning with Christmas and New Year celebrations a fading memory, daylight still in short supply and the weather typically cold and wet.

At this time of year its certainly hard to imagine Ryedale as a warm and sunny oasis and the North Sea anything less than freezing.

Amazingly, in the past Yorkshire has not just been warmer than now, it has been properly tropical and we have rock-solid proof…literally.

The limestone which makes up the core of the Tabular Hills between Helmsley and Scarborough is actually, in the main, a huge fossilised coral reef. It was laid down on the bed of a shallow sea in Jurassic times and, then as now, coral can only live in tropical sea water.

It’s not that the North Sea was much warmer 150 million years ago (because it didn’t exist) but that Yorkshire, indeed the whole of what would become the British Isles, was at the same latitude as modern day Egypt and bathed by the balmy waters of what geologists call the Tethys Sea.

Since then, continental drift has been responsible for putting Ryedale where it is now, a chilly 54 degrees north of the equator.

Just before Christmas I was lucky enough to experience life on a modern-day coral reef and get an insight into what tropical Jurassic Ryedale must have been like.

Some of the animals that I swam with in the Red Sea, like turtles and sharks, would also have been around 150 million years ago, but the fossil record for bony fish is quite sparse.

Teleosts, to give them their Sunday scientific name, only really proliferated in species numbers relatively recently – a mere 50 million years its thought.

Today’s coral reefs are completely dominated by this branch of the fish family with very few other animals, like shrimps, crabs, worms and molluscs on show. Zoologists have described a staggering 1,000 different types of fish from the Red Sea alone and at times it seemed as if they were all visible simultaneously through my face-mask as I snorkelled.

Stripy sergeant major fish cruising in ranks, clouds of orange anthias hovering over the coral and shoals of green chromis darting into it. There were parrotfish pecking the coral, surgeonfish nibbling at the algae growing on it and cleaner wrasse nibbling at the surgeonfish.

Pufferfish hung motionless like underwater balloons as bird wrasse fluttered around, poking their long beaks into crevices and manoeuvring between drifting butterflyfish and angelfish – and these were just the ones that I managed to identify.

The range of colours on display was almost as overwhelming as the diversity; every shade you have ever seen on a paint chart was there (in the case of the Picasso fish, all on the same animal!) in blotches, dots, stripes, marbling, cross-hatching and combinations of all of the above.

The lion fish undoubtably had the most flamboyant body design while the colour intensity prize went to a tiny fish called a purple dottyback. Purple is an understatement – ultra ultra-violet would be a better description.

Our Yorkshire coral reefs were probably just as colourful even though far fewer fish were around. The dominant animals in Jurassic seas were members of the squid family and their fossilised shells turn up in huge numbers in our local rocks.

Ammonites, with an external snail-like shell, and belemnites, like cuttlefish with an internal shell, are all extinct now but they were the “fish” of their time. Fossilised remains give us no idea of the animals colours when they were alive but if their modern-day relatives, nautiloids and cuttlefish, are anything to go by they were quite spectacular. Not only are they gaudily decorated but they can even change colour instantly depending on their mood.

If you need cheering up on the 17th of the month and can’t manage a tropical snorkelling holiday then why not visit one of our local fossil reefs.

The old limestone quarries at Betton (near East Ayton), Spaunton and Ravenswick are the best places to see them.