CRASSATELLA: A LATE CRETACEOUS THROUGH EOCENE BIVALVE “GUIDE FOSSIL”

A “guide fossil” [or “index fossil”] is one that was common, with a distinctive shape and is restricted to a relatively narrow geologic time range. An excellent example is the shallow-marine bivalve Crassatella, now extinct. It has sturdy valves, with a thick shell strong-hinge teeth. This fossil bivalve  (clam) is commonly found in Upper Cretaceous through upper Eocene rocks in Baja California, California, Oregon, Washington, and Canada (British Columbia.

Figure 1. Crassatella uvasana Conrad, 1855, left-valve exterior and  right-valve interior (full outline of the interior not complete) of adult specimens, middle Eocene age, Llajas Formation, north side of Simi Valley, Ventura County, southern California. Specimens collected, identified, and photographed by R. Squires.

The word Crassatella means “thick,” based on its stout shell with a “strong,” thick shell and strong teeth holding its two valves together in shallow-marine, somewhat agitated waters. Crassatella was a shallow burrower. However, it might have been a recliner on the ocean floor, rather than a vertical burrower, as commonly assumed.

Crassatsella lived in warm waters, and the global cooling of the oceans at the end of Eocene time caused the decline and eventual extinction of this genus.

This genus has also been reported (Wingard, 1993) from the on east coast of the United States in Upper Cretaceous and lower Tertiary rocks.

Some bivalves found in post-Eocene to Recent deposits resemble Crassatella, but these species have been assigned to other genera, such as Eucrassatella, Hybolophus, Kalophus, etc. (see the online site: WoRMS). 

Some of these listed bivalves live today in warm-shallow seas (e.g., Australia, New Zealand, tropical American waters, etc.). 

Classification of Crassatella:

Class: Bivalvia

Order: Caritida

Superfamily: Crassatelloidea 

Family Crassatellida

Genus Crassatella

 

Recognition of the various species of fossil Crassatella requires skilled and careful cleaning of their hinges by means of micro-drills.  Most paleontologists, however, lack the enthusiasm, patience, and/or considerable time necessary to learn how to carefully clean these hinges and not destroy critically important parts of their hinges. The late Louella. R. Saul was one of the first west-coast molluscan paleontologists to to carefully clean some representative hinges and also provide some detailed photographs of these bivalves (see Saul, 1981, pls. 1 and 2). Without such detailed knowledge of these hinges, proper identification as to species and the recognition of the morphologic trends within the numerous species of west coast Cretaceous-age Crassatella would have been overlooked. 

REFERENCES CITED

DeVries, T.J. 2016. Fossil Cenozoic crassatelline bivalves from Peru: New species and generic insights. Acta Polonica 61(3):661–688.

Wingard, G.L. 1993. A detailed taxonomy of Upper Cretaceous and Lower Tertiary crassatellidae in the eastern United States–An example of the nature of extinction at the boundary. United States Geological Survey Professional Paper 1535, 131 pp., 22 pls.

Saul, L.R. and J. M. Alderson. 1981. Late Cretaceous Mollusca of the Simi Hills, an introduction).  Link, M.H., Squires, R.L. and Colburn, I.P., eds., In Simi Hills Cretaceous Turbidites, Southern California, Pacific Section, Society of Economic Paleontologists and Mineralogists. Fall Field Guidebook, pp. 29–42.