While the Mahantango is mostly a deltaic environment there were periods of clearer, calmer and deeper waters where branching corals could thrive. The most common of these is Thamnoptychia, which used to be called Trachypora in older references. Certain horizons of the Mahantango like the Centerfield member and the Tully limestone are full of their preserved skeletons.
In this picture you can see a view of a quarry wall with horizontal beds. There are a number of coral pieces sticking out.
Just above the layer with the coral sticking out are many hollow spaces. Those are areas where the calcite that replaced the coral skeletons has dissolved away. Another example of this is below:
In this rock you can see a half dissolved piece of coral. The upper portions have dissolved away and you can see the molds of the individual corallites preserved as small pin like projections on the walls of the rock.