Probable Plesiosaur Vertebra

Remarks

• Here's a cool vertebra.  Several vertebrate paleontologists have had a look at it, and it was tentatively identified as Cimoliasaurus sp., which is a historical "catch all" genus for any miscellaneous, plesiosaurian, post-cranial material.
• Since it was found about thirty years ago by the property owner in float from Hackberry Creek, Hill County, TX, all stratigraphic information is lost.  The formation at that locality is the upper Britton Formation (Eagle Ford Group), and it is possible that the Britton is the source of the bone, since both mosasaur and plesiosaur (Welles, 1949) material have been found from the upper (Turonian) part of the Britton.
• However, we've relegated it to the "Incertae sedis" category since a paleontologist commented on the lack of what mosasaur/plesiosaur experts call "insertion points" or the two prominent foramen on the ventral side of the vertebra.  These entry points are where the blood vessels supply the bone with blood, and you can't miss them!  Check out the Cimoliasaurus sp. page. 
• One rationalization is that this was the second cervical (neck) vertebra, or proatlas, of the plesiosaur.  
• However, there is a possibility that this is actually a dinosaur vertebra.  This idea really isn't that crazy since the upper Britton is a mudstone that was deposited in shallow waters very close to a shoreline just to the west.  There are examples of such dinosaur bones being found in marine formations very far away (hundreds of miles!) from the correlative (age-equivalent) shoreface deposits!  Presumably, these animals got bloated and floated out to sea before sinking to the bottom or being scavenged, or both (see Horner, 1979 and Weishampel, 1990 below).

General

For references on Eagle Ford Group Plesiosaurs:

• Welles, S. P.  1949.  A New Elasmosaur From the Eagle Ford Shale of Texas.  Southern Methodist University Fondren Science Series 1.
• Welles, S. P.  1952.  A Review of the North American Cretaceous Elasmosaurs.  University of California Publications in Geological Science 29, 46–144.
• Williston, S. W.  1903.  North American Plesiosaurs.  Field Columbian Museum Publication 73, Geological Series 2 (1), 1–79.

Weishampel has a comprehensive of all dinosaur reports around the world (up to 1990), including those found in marine formations.  See also Horner, 1979:

• Horner, J. R.  1979.  Upper Cretaceous Dinosaurs From the Bearpaw Shale (Marine) of South-Central Montana with a Checklist of Upper Cretaceous Dinosaur Remains from Marine Sediments in North America.  Journal of Paleontology 53, 566–577.
• Weishampel, D. B.  1990.  "Dinosaurian Distribution".  In:  Weishampel, D. B.; Dodson, P.; Osmólska, H., ed's.  1990.  The Dinosauria.  Berkeley:  University of California Press, Chapter 3.