Including a Spherical (Ball) Joint in Robot Model

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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...) 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:

...
    <link name="${leg_name}_psi_link">
        <visual><geometry><sphere radius=".00001"/></geometry></visual>
        <collision><geometry><sphere radius=".00001"/></geometry></collision>
        <origin xyz="0 0 0" rpy="0 0 0"/>
        <mass value=".00001"/>
        <inertia ixx="0.00001" ixy="0.0" ixz="0.0" iyy="0.00001" iyz="0.0" izz="0.00001" />
    </link>

    <link name="${leg_name}_theta_link">
        <visual><geometry><sphere radius=".00001"/></geometry></visual>
        <collision><geometry><sphere radius=".00001"/></geometry></collision>
        <origin xyz="0 0 0" rpy="0 0 0"/>
        <mass value=".00001"/>
        <inertia ixx="0.00001" ixy="0.0" ixz="0.0" iyy="0.00001" iyz="0.0" izz="0.00001" />
    </link>

    <link name="${leg_name}_foot_link">
        <visual><geometry><mesh filename="${meshes_path}/foot_link.stl" /></geometry></visual>
        <collision><geometry><mesh filename="${meshes_path}/foot_link_collision.stl" /></geometry></collision>
        <xacro:default_inertial mass="${foot_mass}"/>
    </link>

    <joint name="${leg_name}_psi_joint" type="continuous" >
        <parent link="${leg_name}_tibia_link" />
        <child link="${leg_name}_psi_link" />
        <axis xyz="0 0 1" />
        <dynamics damping="0.005" />
        <origin xyz="0.075 0 0" rpy="0 0 0" />
        <limit lower="${-pi/2}" upper="${-pi/2}" velocity="${joint_velocity_limit}" effort="${joint_effort_limit}" />
    </joint>

    <joint name="${leg_name}_theta_joint" type="continuous" >
        <parent link="${leg_name}_psi_link"/>
        <child link="${leg_name}_theta_link"/>
        <axis xyz="0 1 0" />
        <dynamics damping="0.005" />
        <origin xyz="0 0 0" rpy="0 0 0" />
        <limit lower="${-pi/2}" upper="${-pi/2}" velocity="${joint_velocity_limit}" effort="${joint_effort_limit}" />
    </joint>

    <joint name="${leg_name}_phi_joint" type="continuous" >
        <parent link="${leg_name}_theta_link"/>
        <child link="${leg_name}_foot_link"/>
        <axis xyz="1 0 0" />
        <dynamics damping="0.005" />
        <origin xyz="0 0 0" rpy="0 0 0" />
        <limit lower="${-pi/2}" upper="${-pi/2}" velocity="${joint_velocity_limit}" effort="${joint_effort_limit}" />
    </joint>
...

launch file (relevent parts)

  <!-- Load robot description -->
  <param name="robot_description" command="xacro --inorder 'path/to/robot.xacro'" />

  <!-- load the controllers -->
  <node name="controller_spawner" pkg="controller_manager" type="spawner" respawn="false"
        output="screen" ns="/syropod" args="joint_state_controller
              AL_coxa_joint
              AL_femur_joint
              AL_tibia_joint
              AR_coxa_joint
              AR_femur_joint
              AR_tibia_joint
              BL_coxa_joint
              BL_femur_joint
              BL_tibia_joint
              BR_coxa_joint
              BR_femur_joint
              BR_tibia_joint"/>

  <node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model"
        args="-urdf
        -model syropod
        -param robot_description
        -x 0.0
        -y 0.0
        -z 0.2
        -R 0.0
        -P 0.0
        -Y 0.0
        -J AL_coxa_joint 0.0
        -J AL_femur_joint 0.0
        -J AL_tibia_joint 0.0
        -J AL_psi_joint 0.0
        -J AL_theta_joint 0.0
        -J AL_phi_joint 0.0
        -J AR_coxa_joint 0.0
        -J AR_femur_joint 0.0
        -J AR_tibia_joint 0.0
        -J AR_psi_joint 0.0
        -J AR_theta_joint 0.0
        -J AR_phi_joint 0.0
        -J BL_coxa_joint 0.0
        -J BL_femur_joint 0 ...