A coupler intended to be mounted at the end of an arm of a heavy-construction machine and to cooperate with a connection device fixed on a tool, to allow the connection of the tool to the arm of the machine, the coupler comprising a locking member intended to cooperate with a functional face of the connection device, the locking member having: • a transverse axis about which it can pivot, in operation, between an engaged position and a disengaged position, according to an engagement direction or according to an opposite disengagement direction, the locking member being urged towards its engaged position, in the engagement direction, by return means; • and a bearing face intended to cooperate by contact with the functional face of the connection device, in the engaged position, to ensure the retention of the tool, the bearing face including a main portion which is convex and such that the distance between the transverse axis and a point of this main portion increases from rear to front - that is to say when this point is displaced along the main portion from the rear edge to the front edge; characterized in that the bearing face further includes an additional portion situated as an extension of the main portion, towards the rear, the additional portion having, in a section plane orthogonal to the transverse axis, a shape • which is comprised in a zone delimited by: - a first convex circle arc which extends from a point B located on the rear edge of the main portion to a point C, said circle arc being centered at a point O located on the transverse axis, and which has a radius R1 equal to the distance OB, the point C being defined so that the length of the segment BC is comprised between (1/6 x R1) and (1/3 x R1), - and a second concave circle arc which extends from B to C and whose center is a point I located in the middle of the segment BC; • which extends from point B to a point C' situated in said zone, the point C' being such that the distance I'C' is comprised between (1/12 x R1) and (1/6 x R1), where I' is the intersection between the segment BC' and the line orthogonal to the segment BC' passing through O; • and which is such that the normal component of the resultant of the forces exerted by said functional face on the additional portion of the bearing face, applied to the contact point, creates a moment relative to the point O: - which is zero over a first position or a first angular range of the rotation of the locking member, - and which tends to pivot the locking member towards its engaged position over at least a second angular range when said locking member exceeds the first position or first angular range towards its disengaged position. • a transverse axis about which it can pivot, in operation, between an engaged position and a disengaged position, according to an engagement direction or according to an opposite disengagement direction, the locking member being urged towards its engaged position, in the engagement direction, by return means; • and a bearing face intended to cooperate by contact with the functional face of the connection device, in the engaged position, to ensure the retention of the tool, the bearing face including a main portion which is convex and such that the distance between the transverse axis and a point of this main portion increases from rear to front - that is to say when this point is displaced along the main portion from the rear edge to the front edge; • which is comprised in a zone delimited by: - a first convex circle arc which extends from a point B located on the rear edge of the main portion to a point C, said circle arc being centered at a point O located on the transverse axis, and which has a radius R1 equal to the distance OB, the point C being defined so that the length of the segment BC is comprised between (1/6 x R1) and (1/3 x R1), - and a second concave circle arc which extends from B to C and whose center is a point I located in the middle of the segment BC; - a first convex circle arc which extends from a point B located on the rear edge of the main portion to a point C, said circle arc being centered at a point O located on the transverse axis, and which has a radius R1 equal to the distance OB, the point C being defined so that the length of the segment BC is comprised between (1/6 x R1) and (1/3 x R1), - and a second concave circle arc which extends from B to C and whose center is a point I located in the middle of the segment BC; • which extends from point B to a point C' situated in said zone, the point C' being such that the distance I'C' is comprised between (1/12 x R1) and (1/6 x R1), where I' is the intersection between the segment BC' and the line orthogonal to the segment BC' passing through O; • and which is such that the normal component of the resultant of the forces exerted by said functional face on the additional portion of the bearing face, applied to the contact point, creates a moment relative to the point O: - which is zero over a first position or a first angular range of the rotation of the locking member, - and which tends to pivot the locking member towards its engaged position over at least a second angular range when said locking member exceeds the first position or first angular range towards its disengaged position. - which is zero over a first position or a first angular range of the rotation of the locking member, - and which tends to pivot the locking member towards its engaged position over at least a second angular range when said locking member exceeds the first position or first angular range towards its disengaged position.