That defeat for the Ospreys meant the Scarlets were home and dry regardless of their result at home to Treviso, and it was a good job as the Italians ran riot in a 41-17 win.
That proved a sharp contrast to the emotions at the Zebre, who fell agonisingly short of a first win as they pushed Heineken Cup semi-finalists Munster all the way before slipping to a 27-25 defeat.
Glasgow Warriors knew they would need help from elsewhere to get a home semi-final but they fulfilled their part of the bargain with a 20-3 win in Galway against Connacht.
It was not enough though as Ulster sealed top spot by beating Cardiff Blues 37-13 to ensure they will host the Scarlets in the last four, with the Warriors off to Dublin.
In the final game Edinburgh and the Dragons had little to play for but served up a treat with the Scots emerging 31-24 winners in six-try thriller.
TRY OF THE WEEKEND
Munster went into their final game desperate to avoid being the Zebre's first victims but in the end they left it late to steal the win.
While BJ Botha's try proved decisive, it was Johne Murphy's early score that was the pick of the game.
From first phase possession Munster spread the ball wide in their own half with Denis Hurley feeding Casey Laulala inside him before the former All Black released Murphy to sprint over from 30 metres out.
BREAK OF THE WEEKEND
The Scarlets needed a win to be sure of their place in the RaboDirect PRO12 play-offs but Treviso left them relying on other results with a tremendous display.
Italy second row Antonio Pavanello pounced on a mix-up in the lineout to power through a gap and towards the line but rather than going for glory himself, he showed the presence of mind to put Robert Barbieri over for the try.
If Pavanello's break was opportunistic, the same was not true of Luca Morisi, who spotted a gap in the Scarlets' defence to sneak through before showing enough pace and balance to race over for a try.
PLAY OF THE WEEKEND
Toby Faletau was named in Warren Gatland's British & Irish Lions squad this week and he used all his experience to set up the Dragons' first try at Murrayfield.
With his scrum going backwards, Faletau kept his eye on the ball, and as soon as it popped out of the back of the Edinburgh eight he pounced before beautifully releasing Steffan Jones to score.
PLAYER OF THE WEEKEND
Andrew Conway will be joining Munster from Leinster next season and after his display against the Ospreys at the weekend Rob Penney will be licking his lips.
The young winger showed his pace chasing kicks for two of his tries, while he also demonstrated his awareness as he popped up to collect the ball from Jack McGrath after the scrum-half made a darting break.
LIONS WATCH
Two of the three Scottish Lions were in action for Glasgow Warriors this weekend, making a major impact off the bench for the RaboDirect PRO12 semi-finalists.
Included for his ability to fill in at fly-half, Hogg impressed replacing Ruaridh Jackson off the bench, slotting one long-range penalty, while Maitland produced a mazy run to score a great individual try.
Dan Lydiate's injury-disrupted season did not cost him a Lions spot and the Dragons flanker celebrated with the try that got his side to within a score at Edinburgh.
In the end the Scots held out for the win with Lydiate calling time on his Dragons career with a defeat before a summer move.
Leinster matched Leicester as the side with largest Lions contingent and three of their six call-ups played leading roles in the defeat of the Ospreys. Sean O'Brien was named RaboDirect PRO12 man of the match for a storming display out of position at No.8 while Cian Healy showed his mobility with a fine try that sealed the win.
Jonny Sexton meanwhile was accurate from the kicking tee as the Irish side warmed up for this week's semi-finals in style.
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