Paleoautecology of Avitelmessus grapsoideus
The morphological characteristics of Avitelmessus grapsoideus include an unusually large size, a carapace which is large, oval, and armored with spines along the front margin, long, flattened appendages with rows of sharp spines along the dorsal edges, equant and very spiny chelipedes with cutting teeth on occlusional surfaces deeply incised orbits with large eye stalks, divided orbits with trough between, compact antennules, and a broad mouthframe with broad spatulate third maxillipeds, large mandibles. Morphologically, Avitelmessus grapsoideus is especially unusual because it is significantly larger than any other North American Cretaceous crab and its cross-sectional shape is flat, suggesting these animals inhabited a unique niche, perhaps sheltering by tightly pressing their flattened bodies against the mud-sand bottoms on which they lived on the shallow shelf. In addition to a flattened carapace with spines on the dorsal shield margin, Avitelmessus also is patterned and counter-shaded. One specimen from North Carolina is encrusted with bryozoans on its legs, but not its carapace nor venter.
Avitelmessus is known from a few specimens in North Carolina (holotype plus 13 specimens) and great numbers (i.e. thousands) of specimens in the Mississippi Embayment and Eastern Gulf of Mexico. The pattern of distribution and local abundance of Avitelmessus grapsoideus suggests that this crab was opportunistic and capable of rapid fluctuations in population size in response to as yet undetermined ecological parameters. Avitelmessus grapsoideus is usually found in muddy lithologies, muddy sands of the Ripley Formation in the Eastern Gulf and Mississippi Embayment associated with abundant marine invertebrates and in papery shale at Graham, Mississippi, and from the muddy sandstones of the Peedee Formation of North Carolina where it is associated with marine vertebrates and invertebrates.
All of these attributes led to the conclusion that Avitelmessus was an opportunistic crab that lived epifaunally as a "Laufer" on muddy-sand bottoms, feeding omnivorously most of the time but capable of exploiting rapidly expanding local food resources. Locally, this crab probably inhabited the very nearshore and lagoonal habitats.
7.5 Paleoautecology of Tetracarcinus subquadratus
Tetracarcinus subquadratus is a diminutive crab with a small, highly sculptured and granulated carapace that is relatively rectangular and high standing. Orbits are deeply inset and rimmed by a turberculate ridge, eye stalks thin and fold back into outer orbital fossae. Pereiopods are still poorly known, preserved ischium? and merus? on one specimen are thin and round. Chelae are equant, carpus massive in dorsal aspect, quite long and stout, anterior surface granulate. Claws with enlarged bulbous propodi and thin fingers. Tetracarcinus subquadratus is unusual because of its diminutive size, seldom larger than ca. 13 mm in width, contrasting rather sharply with Dakoticancer and the giant Avitelmessus. The small size of this animal and the specialized claws, particularly the tweezer-like pincers, may indicate Tetracarcinus was adapted to a specialized mode of life, perhaps as an epizoan on plants or animals such as sponges. This hypothesis remains unsubstantiated by data at this time. These crabs are associated with a relatively normal marine fauna. Tetracarcinus is the common crab in the Merchantville Formation of the C & D Canal on the Delmarva Peninsula and is a secondary element in the Dakoticancer australis Assemblage from the Coon Creek Formation of the Mississippi Embayment in Union County, Mississippi. Only one specimen is known from the Western Interior Basin, that from the Lewis Shale of Wyoming. Tetracarcinus is unknown from the Gulf Coastal Plain west of the Mississippi Embayment.