An unbeaten 49-run fourth-wicket stand between Shane Watson (44) and Michael Clarke (21) took Australia to 132 for three at tea in the second Test against the West Indies at Queen's Park Oval on Sunday.
Australia are bidding to wrap up the three-game series after winning the opening match by three wickets in Barbados, and after winning the toss, they made steady progress.
He removed Cowan and Ricky Ponting (seven) before the tea interval.
A quick consultation with fellow umpire Ian Gould saw the decision overturned.
However, had Erasmus's decision been given for an lbw appeal, rather than a catch, then Warner would have been on his way.
Australia decided to play two spinners for the first time in nearly four years, with Michael Beer coming in to partner Nathan Lyon, while West Indies off-spinner Shane Shillingford came in for Devendra Bishoo, who had disappointed in Barbados.
Shillingford bowled the first over of spin in the match in the 15th over and had immediate success.
Warner did not quite get to the pitch of his fifth ball but continued through with a drive and the ball came off the outside edge straight to Sammy at slip.
Shillingford, back in the team after undergoing remedial work on his action, continued to bowl a tight spell and only conceded six runs from his first four overs as he found some slow turn from the pitch.
Roach, who had opened the bowling from the Media Centre End, was brought back by Sammy at the Brian Lara Pavilion End for a couple of overs before lunch.
The third ball of his new spell hit Cowan low on the front pad and, again, Erasmus gave the decision against the batsman.
Cowan decided to use the review but the replays showed that the ball had hit him marginally outside the line of the off stump but not enough to change the verdict.
Australia went to lunch on 74 for two, having lost both of their openers.
There was a light shower at the end of the lunch interval that meant the second session started five minutes late.
In the sixth over after play resumed, Roach took his second wicket.
Ponting was man of the match in his last appearance in a Test at Queen's Park Oval, having scored a double-century in the first innings.
It was not to be this time around.
He had scored just seven by the time he was squared up by a delivery outside off-stump that took the edge and flew to wicket-keeper Carlton Baugh at chest height.
Somehow the keeper spilt it but he had got enough glove on the ball that Sammy had time to step across from first slip to grab the rebound.
With the ball not coming off the pitch and with a lush outfield, scoring was always going to be slow.
So it proved during most of the session as Watson and Clarke dug in.
Two of Australia's pace attack pulled up sore after their efforts in Bridgetown.
Source: AFP Global Edition