The Frenchman was the lone DNF on the results sheet while Lotto Soudal's Tim Wellens was suffering from a cold and sore throat and did not take the start in order to recover for the opening Belgian race, Omloop Het Nieuwsblad.
Calmejane suffered contusions to his right shoulder and road rash in addition to his head injury.
Calmejane had a strong start to the four-stage race, entering into the early breakaway that lasted for over 100km. He claimed the first mountains classification jersey of the race and was still wearing the jersey when he crashed out.
It is unclear how long he will be out of competition.
"His unavailability will be communicated later," said team doctor Laurent Duperron, who was present at the race.
The UCI strengthened its protocols with specific signs to watch for in riders after last season's crash of Romain Bardet during the Tour de France that saw the Frenchman staggering and struggling to stand up before being put back on his bike to finish the stage with a serious concussion.
The protocol specifies complete rest for 24-48 hours for even the mildest concussions, with stepwise assessments and testing before riders are allowed to return to training and racing.
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