Sorry, but this is all I can obtain from the fold.3 site.
David Shahan, 43, Private, Capt. Fitzhugh's Company (2d Service), Bell's Regiment Texas Mounted Vols. (Mexican War), Muster-in Roll at Elan Fork of Trinity, River, Texas, February 2, 1848, joined for duty and enrolled February 2, 1848 in Grayson, County for 12 months, valuation of horse $85, valuation of horse equioments $20, mustered out February 1, 1849, Company Stationed at Richland Creek, Tex., no further records
Compiled Service Records of Volunteer Soldiers Who Served During the Mexican War in Organizations from the State of Texas