AutoCad 3D :: How To Take 2D Section Of 3D Mesh

Here is the problem: I have a 3D model in the format of 123D (Freeware by Autocad). I converted this to .DXF then imported into AutoCad 2012 and saved it in .DWG format. The 3D model is of a crocodile. I wish to take cross-sections of this model so to generate width traverse sections from the longitudinal head to the tail of the animal. This is so I can form a model of the animal out of wire.

So how to best do this? Apparently the model in .DXF/.DWG became a 2D "mesh" so I must first convert to a solid? (I say 'became' because originally it was in .123c format, but my version of Autocad 2012 does not recognize this format, so I had to convert to .DXF, which I believe rendered the 3D into a 2D mesh) Or can AutoCad take a 2D "mesh" and generate cross-sections?

Top-down general advice is good but even better is a detailed step-by-step explanation of how to do this. I did "select" the entire animal and tried to make a "solid" out of it using a button icon on the ribbon panel (as opposed to a "surface") but I think I stopped prematurely--there was no progress bar so hard to tell. So then I guess you generate a cross-section once you generate the solid by doing what? is there a dialog box that ask for your end points (of the plane, defined by a line?)