A very brief history of Marine direct fire systems.

LVT(A)1

LVT(A)4/5

 LVTH6

 M4 Sherman

M103

LVTPX-12* Note I found this photo on the internet but am having trouble finding information.  The issue lies in its designation.  When the Marine Corps was developing an LVTP-5 replacement, it came up with two different sized vehicles in its evaluation phase.  One was a larger (some would say full sized APC) and the other is pictured here...more of an M-113 sized purpose built amphibian.  From what I've read they both fell under the LVTPX-12 designation but history only records information on the victory...the vehicle that would eventually become the AAV.  No worries, I'm still looking for more information.  See the update below.

The rest of the history everyone already knows.  The M-60 MBT, the LVTP-7/AAV and the M1 Abrams MBT.  What I wanted to show in this brief over view is the startling fact that the Marine Corps once insisted on Direct Fires to be amphibious.  Even if that requirement no longer applies, then certainly new constraints are appropriate.  Weight, logistics tail...being where the Infantry needs it, when the Infantry needs it.

These are things that the current MBT just can't do.

How do we know this?  Quite simply by the way that these vehicles are being utilized in Afghanistan.  They're not working with and protecting the Infantry...the guarding MSRs.  A properly equipped MRAP can do that ---mount the proper sensors, put a few designated Marksmen on it and you have your guard.

We need INFANTRY SUPPORT VEHICLES...not MBTs.  More to come.

UPDATE:
Got this from BB1984.

It's off topic but I believe the last pic you have is actually of the LVTHX4, an armed development of the earlier M59 based LVTPX2, so yes roughly the size of an M113. There was also a twin 40mm armed AA version that was sort of in between the two called the LVTAAX2. As the designations suggest, these were all developed before the LVTP5.

About the only reference I have seen for the LVTPX-12 says it was the designation given to LVTP-7 prototypes delivered in '67-'68 before the production run started in '70.

Just FYI, FMC did propose an AAV version of the M113 called the LVT(X). It looked like a smaller LVTP-7, carrying 13 troops, and had a fire support variant with a turreted low pressure 90mm gun and troop carrying cut to 6 to make room for the turret and ammunition. The Italian San Marco brigade also deploys a modified M113, which again looks like a mini-LVTP-7, for amphibious work.

I stand corrected.  This does bring up another point though.  Our armor history is being lost.  Alot of the sources for this type of material are withering away.  FAS and others are becoming pay sites...the only hope is that the US Army Armor Center and Marine Corps History stay in the fight to preserve our military history.