Including a Spherical (Ball) Joint in Robot Model

I'm having trouble including a spherical/ball joint in my robot model. I have tried using the 'ball' joint type defined by sdf (http://sdformat.org/spec?ver=1.6&elem=joint#joint_type) but the urdf will no longer compile, throwing the error: 'Joint [joint_name] has no known type [ball]'. I have also tried building a spherical joint using three orthogonal universal joints. The model does not throw any errors, and in rviz everything is displayed as expected and without issue. In gazebo however, neither the three universal joints composing the spherical joint nor the child link show up, or are listed as part of the model. .xacro: ... ... launch file (relevent parts) ***Why do none of the joints or links appear in the robot model?****What am I missing/doing wrong? Is there a better way to do this?* Notes 1. the three orthogonal joints represent the roll pitch yaw of the spherical joint they compose. their mass and inertia matrices are close to zero intentionally. they are not zero because I understand that this can cause issues in gazebo. 2. ${leg_name}_foot_link is the end effector link Thanks