These specimens are from Eagle Pass, TX. They were found on top of the ground. This area is Upper Cretaceous and a marine environment. I see many casts of the burrows of marine animals. Some have parallel lines as in these specimens, but I've never seen any parallel lines on flat casts. I think they might be drag marks. I see some voids in the ends of one of the specimens. The marks are more or less parallel. There are some deeper marks widely spaced and some shallower marks between the deep ones.
Specimen 1
Specimen 2