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.  

CERITHIUM NODULOSUM A "VISITOR FROM "THE GEOLOGIC PAST

Genus Cerithium is a member of a diverse group of shells characterized by having an elongate, high-spired shell ornamented by many spiral ribs and crossed by varices (= groups of swollen axial ribs). The aperture is smooth interiorly and has a distinctive frilled margin and a slightly reflected (strongly bent) anterior canal. 

The genus Cerithium has a geologic record of Late Cretaceous to present day. There are many Cenozoic species and most of these were shallow-water dwellers that lived mostly intertidally in tropical waters (Houbrick, 1992).

One of the extant (living) species of Cerithium is Cerithium nodulosum, which has a fossil record extending back to the late Miocene. Today, this species is commonly found immediately shoreward of reef edges, as well as on intertidal rocky shelves, having veneered sand, as well as shallow, sandy depressions. It is found today only in the Indo-Pacific region. For example, it occurs along tropical continental shorelines and among island groups throughout the Red Sea and Indian Ocean, including East Africa and Mozambique. It is found throughout SE Asia, Indonesia, and the tropical part of Australia. In the Pacific, it ranges from near Japan throughout Melanesia, Micronesia, and Samoa (Houbrick, 1992).

Apertural and abapertural views of a Recent specimen of Cerithium nodulosum, height 4 inches (= about 10 cm), collected in shallow water from Okinawa in the western Pacific Ocean. The white-circular area on the apertural view is a small-sized encrusting oyster.

Note: When I look at specimens of C. nodulosum, I am immediately reminded of the abundant cerithid shells that I have seen in certain Eocene beds in the Paris Basin region of France.

Reference Cited

Houbrick, R. S. 1992. Monograph of the genus Cerithium Bruguiere in the Indo-Pacific (Cerithiidae: Prosobranchia). Smithsonian Contributions to Zoology, number 510. 

CORNULINA, A FOSSIL GASTROPOD FROM ENGLAND AND ALABAMA

In 1987, during a sabbatical from my teaching, I travelled to England and France in order to collect Paleocene and Eocene fossil mollusks for my on-going research. I was invited by a local fossil-collecting club to accompany them to a rich-fossil locality at “Baron-on-Sea” in southern England. On that excursion, I met John Quayle, who was an avid collector of Eocene fossils, especially at the this particular locality. He was kind enough to donate some really nice fossil specimens to me. Two of these specimens were the Eocene gastropod Cornulina minax (Solander in Brander, 1766). One of these specimens is figured below.

Cornulina minax, upper middle Eocene (Bartonian Stage), “Barton-on-the Sea,” southern England. Specimen dimensions: length 2.75 inches [6 cm], width 1.5 inches [4 cm].

Cornulina, a carnivorous neogastropod that belongs to family Melongenidae. Its geologic time range is Paleocene to Eocene, and this gastropod lived in warm shallow seas in southern Europe, southeastern United States, and reportedly also in Africa, northeast Mexico, and Columbia.

For those who are interested in Cornulina, it should be noted that Dockery (1980:pl. 2, figs. 5A, 5B) reported a supbspecies, Cornulina minax compressa, in the Eocene fossil record of Mississippi. There are some minor differences between this subspecies and C. minax from England.

 

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Two years later, I made fossil-collecting trip to Alabama and Mississippi in order to collect Paleocene and Eocene mollusks for my on-going research. Based on some information from a geologist I encountered while exploring the area, I visited a rich-fossil locality in the Eocene Gosport Sand at Little Stave Creek in western Alabama. While collecting for a considerable length of time, I found a single specimen of the gastropod Cornulina armigera. This specimen is figured below.

Cornulina armigera, upper middle Eocene (Bartonian Stage), upper Gosport Sand, Little Stave Creek, Jackson, Alabama. Specimen dimensions: length 3 inches [7 cm], width 3 inches [7.25 cm, including spines].

Reference Cited:

Dockery, D.T. III. 1980. The invertebrate macropaleontology of the Clarke County, Mississippi area. Mississippi Department of Natural Resources Bureau of Geology, Bulletin 122, 392 pp.

TWO LIVING SPECIES OF TIVELA CLAMS

Tivela clams (bivalves) have a fossil record dating back to the Eocene. They have medium-sized to large, heavy shells that are strongly built to resist waves in the surf zone. They live today in moderately cool (i.e., but not polar!) to warm to marine waters where they burrow into nearshore sands. Their depth range is typically the surf zone to about 25 m depth (ranging from low tide to very shallow subtidal. Their two valves are symmetrical, sturdy (can be thick), trigonal (triangular), and smooth (so as to enable burrowing in sand). The three cardinal teeth on the hinge teeth of each valve are also sturdy and help hold the two valves securely together (it is tough living in the turbulent waters of the surf zone!). A thin shell layer (the periostracum) is a thin coating on the surface of their valves and, like a coating of varnish protects the outside of their valves from corrosion. 

The classification of Tivela is:

Class Bivalvia

Order Veneroida

Family Veneridae

Genus Tivela

There are about 30 species of this clam in the world today. Two of these are mentioned in this blog.

1) TIVELA STULTORUM [The Pismo Clam]

Tivela stultoram. Exterior and interior views of a Los Angeles County Museum of Malacology (LACM 186650) specimen that was actually collected from Pismo Beach. 

Image of an exhibit from the old Marine Hall exhibit at the Los Angeles County Museum (LACM) depicting the life position of a Pismo Clam. These images were kindly provided by Lindsey T. Groves, LACM Collections Manager of Malacology at LACM.

Tivela stultoram (Mawe, 1823) is well-known as the Pismo clam—a name  derived from the Indian word Pismu, “meaning tar”]

At Pismo Beach (about half-way between Los Angeles and San Francisco), there used to be a great abundance of “Pismo” clams. Over the years, their number was drastically reduced by *over-collecting; namely, hordes of people, digging into the beach sands in order to collect these clams. Nowadays, California law requires possession of a valid ocean license for most ocean fishing, clamming, lobstering and related activities in order to collect.

*An estimated 150,000 people once sought the clams on Pismo Beach during a single weekend! (see Shaw and Hassler, 1989:p. 1).

The geographic range of the Pismo clam is along the Pacific coast from Monterey, northern California to the Bay to Bahia Magdalena, Baja, California, Mexico.

Most Pismo clams reach a size of five inches within five years. The largest Pismo clam on record was 7 1/8 inches across, and was estimated to about 26 years old.

Shells of the Pismo clam vary in both color and pattern. The principal color is pale buckskin. Some individual shells are maker with lines radiating out to the margin. These stripes disappear as the clam ages (Fitch, 1950, p. 287).

2) TIVELA MACTROIDES (from the Caribbean Sea)

Tivela mactroides: Upper image is the exterior of one of the right valve; lower image shows the interiors of both valves of this  specimen.

Some useful references used in the preparation of this blog post:

Fitch, J.E. 1950. The Pismo clam. California Fish and Game v. 36, no. 3, 285-312.

Fitch, J.E. 1961. The Pismo clam. Marine Resources Leaflet No. 1. State of California. 23 pp.

Shaw, W.N. and T. J. Hassler. 1989. Species profiles: life histories and environmental requirements of coastal fishes and invertebrates (Pacific Southwest). U.S. Fish Wildl. Serv. Biol. Rep. 82(11.95). U.S. Army Corps of Engineers, TR EL-72-4, 12 pp.

FIMBRIA: A FRILLED CLAM

Frimbria clams have a very distinctive ornamentation with many closely spaced commarginal ribs on both valves that produce a  "frilled" look). Genus Fimbria originated about 180 million years ago during earliest Jurassic time in east Asia and became widespread in warm oceans thereafter; until late Cenozoic time when its geographic range slowly contracted. Today, they are confined to the Indo-Pacific area, where they live in close association with reef-coral habitats (Squires, 1990). Fimbria is currently classified (WoRMS, 2024) as belonging to the subfamily Fimbriidae, within the family Lucinidae.

A specimen of Fimbria fimbriata (Linne, 1758), (2 inches long, and almost 2 inches tall) collected in sand at low tide, in 10 feet of water on the Great Barrier Reef, off the coast of McKay, Queensland, Australia. The images in the above successive sequence are: left-valve exterior and interior; right-valve exterior and interior, and top view of both valves together (articulated). Notice how confusingly similar the two valves are exteriorly.  

References Cited:

Squires, R. L. 1990. New Paleogene Fimbria (Mollusca: Bivalvia) from the Pacific coast of southwestern North America. Journal of Paleontology 64(4): pp. 552-556, figs. 1-3.

WoRMS. 2024. https://marinespecies.org

The Allure of Cowrie Gastropod Shells

Some of the most beautiful seashells found in shallow-marine tropical waters today are those of cowries. There are about 200 living species of them in the world today (Abbott and Dance, 1992). Their morphology and vivid coloration can be very impressive. Two such species are highlighted here. 

Cowries had a Late Jurassic origin in Italy, and they have been widespread since Cretaceous time. Their subsequent fossil record is extensive, including California, Oregon, and Washington (Groves and Squires, 2023).

The higher classification of cowries is:

Class Gastropoda

Order Littorinimorpha

Superfamily Cypraeoidea

Family Cypraeidae

Today, as in the geologic past, the greatest abundance and diversity today of cowries is in tropical to subtropical seas. In this blog, I focused on two very “eye-catching”  species of cowries.

The first of these two species is Mauritia mauritiana (Linnaeus, 1758), which is a common cowrie. Its shell, which can be up to 11 cm in length, has an ovate-humped shell, with a slightly concave base bearing coarse teeth in the aperture. The dorsum (upper surface) of its shell has a brownish color with scattered light spots. The shell margins are dark brown. The lower surface [= the base] is black to brown. The shell surface is glossy and strongly reflects light. Photography of specimens is difficult because of reflections off this very glossy shell. This cowrie is common and lives in the tropical Indo-Pacific region (e.g., east Africa, Indian Ocean, western Pacific, western and northern Australia, and the Philippines, and well as Hawaii. It prefers the low-intertidal zone (approximately 1 to 8 m depth) along wave-washed basalt cliffs where it hides under stones and in crevices.

Mauritia mauritiana, abapertural and apertural views, length 9 cm, width 6.5 cm, on lava rocks at the south end of Lahaina and Maui, Hawaii.

The other species highlighted in this blog is Umbilia hesitata (Iredale, 1916), an uncommon species restricted to eastern and south-eastern Australia. Its shell, which can be up to 12 cm in length, has a noticeably inflated spire area that is noticeably sunken. The dorsum has a quite variable color, namely, brownish mottled with scattered white patches and blotches. The lower shell margins are mostly white. The lower surface [= the base] is white and has numerous small teeth. This species is uncommon and is found offshore. 

Umbilia hesitata (Iredale, 1916), abapertural and apertural views, length 10.2 cm, width 5.5 cm, Australia.

Acknowledgment: I want to thank Lindsey T. Groves, Collections Manager of Marine Mollusks at the Natural History Museum of Los Angeles County, California, for providing me with the latest information concerning the generic names of the cowries mentioned in this blog.

References Used:

Abbott, R.T. and S.P. Dance. 1992. Compendium of seashells. A full color guide to more than 4,200 of the world’s marine shells. E.P. Dutton, New York, 410 pp.

Groves, L.T. and R.L. Squires. 2023. Revison of northeast Pacific Paleogene cypraeoidan gastropods, including recognition of new species: implications for paleobiogeographic distribution and faunal turnover. PaleoBios 40(10):1-52. [pdf is free online]