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Stratigraphic SettingDMNH 11843 was collected by L. and M. Duchouquette, and K. Bridges of the Dallas-Ft. Worth area (Duchouquette, personal commun., 2009). The locality that produced DMNH 11843 is now covered by a housing development, which is unfortunate given that the specimen was recovered near the contact between the underlying Grayson Marl (Late Cretaceous: early Cenomanian) and the overlying Woodbine Formation (Late Cretaceous: early middle Cenomanian). However, the fossil preservation exhibited by DMNH 11843 is very similar in appearance to other isolated ichthyosaur elements that are definitively from the Grayson Marl (e.g., SMU 76508) and unlike that of vertebrate fossil remains from the Woodbine Formation. Given this similarity, we are inclined to consider the lithologic source of DMNH 11843 as the Grayson Marl. The Grayson Marl is a Cretaceous unit from the Comanchean series of north central Texas that overlies the Paw Paw Shale and Main Street Limestone (Kennedy et al. 2005). The section thins westward across Tarrant County by progressive truncation at the top (Foster 1965). The Grayson Marl is a calcareous, gray, blocky claystone interbedded with thin-bedded siltstone (Mancini 1982). The biozonation of the Grayson Marl is based primarily upon ammonites and foraminfers (Mancini 1979). Kennedy et al. (2005) pick the Late Albian – Early Cenomanian boundary at the bottom of the Graysonites adkinsi zone near the top of the Main Street Limestone in Tarrant County. This boundary coincides with the boundary of the Mariella (Plesioturrilites) brazoensis and Graysonites adkinsi Interval Zones (Mancini 1979). The rarity of keeled rotliporids in the Grayson Marl supports the paleoenvironmental interpretation of the Grayson as a normal inner to middle neritic depositional system (Mancini 1979). |
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