I know what you're thinking. How can this be true when magnificent cars like the Lamborghini Diablo and Ferrari 550 were instantly rendered automotive oxymorons the moment their roofs were lopped off? You have to look back nearly five years to find the answer.

Officially, Pagani's Roadster broke cover for the first time at this year's Geneva motor show, but it's a car that we've known about unofficially for more than four years. Back in 1999 we were the first magazine to drive the precious Zonda C12 coupe prototype and Horacio Pagani, father of the Zonda and founder of the Modena Design composite fabrication company from which Pagani Automobili was formed, told us then that a roofless version was on the drawing boards. In fact, he said, it had passed crash tests, was homologated and had gained European type approval at the same time as the coupe.

Despite this, Pagani decided not to let the Roadster become a distraction, and instead concentrated solely on development of the coupe. His focus was honing the Zonda coupe steadily and thoroughly; from the original 389bhp, 6-litre V12, five-speed C12, to the all-conquering 7.3-litre, 555bhp, six-speed Zonda C12S. Perhaps he knew coupe supercars are always taken more seriously than roadsters. Perhaps Pagani Automobili's modest resources couldn't stretch to developing the two models in parallel. Whatever the reason, the Zonda is now rightly regarded as the most significant supercar of the post-McLaren F1 era, a fact that endows the Roadster with an impeccable pedigree.

All of which makes the prospect of a Zonda Roadster both mouth-watering and nail-biting: mouth-watering because Horacio Pagani is an engineering genius with obsessive standards, nail-biting because to our knowledge there has never been a supercar that has benefited from decapitation, either in dynamic or stylistic terms. In short, the Zonda has nothing to gain and everything to lose in the transition from world-beating coupe to reputation-risking Roadster. High stakes indeed.

If there's one safe bet in all of this it's that this new Zonda, any Zonda for that matter, is a car you always look at as though for the first time. Sure enough, when Andy Morgan and I arrive at Pagani's Modena factory, we spend the first ten minutes of our visit inspecting the silver C12S in the marble-lined reception area, even though it's a car I've seen countless times before, and a similar car to the one Morgan has just spent three days with in Germany. So you can imagine how, when the bright pearlescent, flip-tastic yellow Roadster emerges into the sharp Modenese sun, we're instantly sucked into a swirling vortex of sparkling new details, glossy carbonfibre and breathtaking proportions. It looks like nothing else on earth.

Sounds like nothing on earth, too, and no doubt has the performance to match, as it weighs just 30kg more than the C12S coupe with which it shares its running gear. Pagani reckons the Roadster matches the coupe to 100mph (that's sub-8sec), and although the v-max is yet to be set there's nothing to indicate it'll be far behind the coupe, which we clocked last month at a whisker under 200mph.

Sadly, due to overwhelming commercial pressures, or rather the demands of a German customer who already has two Zonda coupes in his garage, this car is already sold and Pagani is unwilling to release it for a no-holds barred evo-style test. Fair enough, I reckon, as anyone with the nerve to specify tanned ostrich hide interior has to be a pretty scary dude. Either that or Bernie Clifton had a good pantomime season.

It's strange, but the sensations you get while travelling roof-down in the Roadster aren't hugely different from sitting in the coupe's transparent turret. You sit low and well forward, seemingly tucked beneath the crazy arc of windscreen like a superbike rider hunching behind the fairing. There's more noise, of course, like hearing a band live rather than on CD, but when a motor sounds as savage as the Zonda's AMG-fettled V12, that's a positive bonus. At low speed there's the gentlest of breezes to ruffle your hair, but as speed increases the airflow whips around the cockpit, both over the top of the screen and around the back of your head. It's no Merc SL in this respect, but then that's the idea.

Best news of all is that thanks to the Zonda's carbonfibre construction there's no perceptible deterioration in the way it goes to work on a stretch of road. The alertness, the keenness, the iron-fisted damping and supple ride are all there intact. It still feels like the strongest, most rigid car you've ever experienced, which in reality it probably is. Pagani paid close attention to the various key elements of the Zonda's chassis to achieve this. The central tub has been subtly strengthened, and the tubular chrome molybdenum rear subframe has also been tweaked to cope with the altered loadings. Finally, and most obviously, there are new carbon and molybdenum roll-hoops behind the seats. As a result the Roadster retains the torsional stiffness of the coupe, which in turn means it retains the same suspension settings. And, as you can imagine, this means the Zonda Roadster has the ability to take on just about any current supercar, chew it up and spit out the bits.

Looks-wise the Roadster is even more jaw-slackening than the coupe, which is hardly a shrinking violet. Without the forward located blister canopy to balance its extreme width at the rear, the Roadster's tail and engine deck assume even more exaggerated proportions. It works though; in fact from some angles the Zonda looks born to be a Roadster. What is slightly disappointing is the way in which the Roadster's dramatic scale makes occupants look like pinheads. Incredibly wealthy pinheads, but pinheads nonetheless.

Rulebook-rewriting super-roadsters don't come cheap. Pagani quotes an ex-factory price of some €481,500 plus taxes, which, depending on the exchange rate, equates to between ΂£350,000 and ΂£380,000 in proper money. While the traditionally roadster-hungry Arab market is understandably subdued at present, Pagani has already taken seven orders for the Roadster since Geneva; impressive given the production run will total just 40 cars. To put that more clearly in perspective, some 35 Zonda coupes have been delivered since 1999, with a further five in build and 15 on the order books. And that's before Pagani begins exporting to the States. When it does, don't expect the Roadster order book to be open for long.