the Marder (I,II,II), Nashorn/Hornisse and the Elephant/Ferdinand were true panzer jagers designed specifically just to hunt out tanks from a distance (which explains the thin skin surrounding the gun crew) until they came out with the jagdpanzers (Hetzer, jagdpz IV, jagdpanther, jagdtiger). These being more successful due to low Silhouette and better crew protection.
"The Panzerjäger (abbreviated to PzJg in German) were anti-tank vehicles produced by taking an existing anti-tank gun complete with gun shield from its carriage and mounting it on a tracked chassis to give a mobile anti-tank gun.
Development of the Panzerjäger idea started before the start of the war and continued until about 1943 when the better protected Jagdpanzer ("Hunting tanks") designs took over. Panzerjäger continued to serve until the end of the war."
Panzerjäger - Wikipedia, the free encyclopedia