South Africa hooker Marx out of World Cup with knee injury
Marx, 29, who was not included in coach Jacques Nienaber's match-day squad to face Romania in Bordeaux on Sunday, "went down at training on Wednesday after having his knee caught in contact".
"First and foremost, we are extremely disappointed for Malcolm, and we wish him all the best in his recovery," said Nienaber.
"He's been working very hard to be in the best form possible for the tournament and it's tragic that his campaign has ended this way."
Nienaber said that Marx would not be immediately replaced in the squad.
"We have enough cover at hooker with Bongi (Mbonambi) and Deon (Fourie) in the squad, while Marco (van Staden) has also been training there and has been named on all our team sheets so far as a hooker option," he said.
"We'll assess our needs for the rest of the tournament and confirm who we are calling up as an additional player later this week."
South Africa beat Scotland 18-3 in their opening Pool B match and take on Romania, beaten 82-8 by Ireland in their opener, on Sunday.
Marx's injury is a blow for the Boks, who only named two specialist hookers in their 33-man squad while Joseph Dweba of the Stormers was placed on standby when that was announced.
Marx, capped 63 times, is an integral part of the Bok pack and came off the bench as part of the so-called 'Bomb squad' in the 2019 final which South Africa won 32-12 over England.
Sunday's match against Romania sees Mbonambi starting at hooker, with Fourie providing cover from the bench.
South Africa already have lock Eben Etzebeth out for 10 days with a shoulder injury sustained in the win over the Scots, but there are hopes he will be fit for the crunch pool match against Ireland on September 23rd.
