 Complete the following phrase:  The gun-armed amtank LVTA-1 and LVTA-4 were developed to provide fire support. Armed with a 75mm howitzer, the latter was introduced in 1944 just before the Marianas campaign and was especially effective in this role as it was capable of destroying Japanese fortifications as it came ashore. Its howitzer complemented the 75mm gun of the Sherman tanks used by the Marines. However, the LVTA-4 had an open-topped turret which left the crew vulnerable to artillery and infantry attack, especially to the latter, as it lacked any sort of machine gun armament. The lack of machine gun armament was eventually rectified, though the open-topped turret remained in order to save weight. Although usually used in a direct role during landings only once inland the amtanks were assigned to artillery formations to augment their firepower, in the 