San Diego Natural History Museum--Your Nature Connection[BRCC San Diego Natural History Museum: Paleontology]

The Elusive Pliocene Rocky Shore

Sandy beach, photo by Jim Melli Salt marsh Rocky beach, photo by Erick Bolton

Fossil discoveries from the San Diego Formation have shown us that a high diversity of Pliocene marine environments (2-4 million years before present) have been preserved in San Diego County. At this time, a very large area--from Mission Valley south to Rosarita Beach and extending as far east as College Grove--was under the ocean. The bottom of the sea ranged from cobbley, with barnacles and oysters anchoring to the rounded rocks, to beds of clean, fine sand, often with many species of large clams burrowing under the surface. Although the appearance of these fine sand beds would generally be barren, the scattered bone remains of large sea mammals such as walruses, sea cows and whales would have provided a place for barnacles, oysters, and other attaching organisms to settle, where they would have formed small "island communities" of life at the depths of the sea.

Shells from mudflats
Salt Marsh/Mud Flat Species
1a. Modern California Bubble (Bulla gouldiana); 1b. Fossil Bulla sp.; 2a. Modern Irus Macoma (Macoma inquinata); 2b. Fossil Irus Macoma (Macoma inquinata); 3a. Modern Crucibulum spinosum; 3b. Fossil Crucibulum sp.

A vast, shallow lagoon occurred east of the area now known as Chula Vista. Along the edge of the lagoon were muddy and silty intertidal flats supporting mollusk species similar to those that can be observed along the shore of the San Diego Bay today, including Cup-and-Saucer snails (Crucibulum sp.), and Bubble snails, Bulla sp. In one place, horses ran across the surface of a shallow mud flat, which lay over the surface of a more sandy, fossil clam bed. West of the lagoon was a sand bar, and surf crashed on the surface of a sandy beach, where Pismo Clams and other sandy shore invertebrates lived, from the water's edge to the depths of the ocean.

Shells and fossil shells from sandy shoreline Sandy Shore Species
1. Pismo clam (Tivela stultorum); 2. Moon snail (Polinices sp. cf. P. lewisii); 3. Razor clam (Solen sicarius); 4. Bodega Tellin (Tellina bodegensis)

But among the many Pliocene shore localities the Paleontology department has uncovered over the years, there is no evidence of a rocky intertidal environment. There are places in the south San Diego Bay area where the ancestors of todays rocky intertidal species may be found. Most notable are the two predatory fossil snail species Acanthina emersoni and Thais trancosana which can be abundant in some areas. Species of these two genera are today among the most common snails in the rocky intertidal from Alaska to Baja California. But the fossil evidence found in the field indicates that the extinct Pliocene species were actually found offshore on the sandy and cobbly ocean bottom. This evidence includes the frequent recovery of large clams which are known to occur subtidally, and the bones of large marine mammals. A very large mussel shell, Mytilus coalingensis is sometimes encountered. This species bears some resemblance to the California Mussel, Mytilus californianus, although it generally lacks the distinct ribbing of the modern species. The fossil mussel commonly grew to 8 inches (20 cm) in size. This is larger than the typical California Mussel you might find on the beach, but it is interesting to note that our modern species can reach impressive sizes when it lives in shallow water below the low tide zone.

Pliocene shells and fossils from rocky shore
Rocky Shore Species
1. Abalone (Haliotis corrugata); 2a. Modern Acanthina spirata; 2b. Fossil Acanthina emersoni; 3a. Modern Thais emarginata; 3b. Fossil Thais trancosana

Other intertidal invertebrates that would indicate a prehistoric rocky shore include chitons and limpets, but these mollusks are extremely rare in fossil deposits of Pliocene age. Occasionally, fossil abalones, Haliotis sp., and Turban snails, Astraea sp. are found. But where they occur, they are associated with other species of invertebrates and some vertebrates, such as the bat ray, Myliobatis sp. that are unquestionably subtidal species. A number of marine deposits of later Pliocene age, found from around Mount Soledad and the metropolitan area of San Diego, contain the white limpet Acmaea mitra, and the predatory snail Thais lamellosa, species that are today common along rocky shores of northern California and the Pacific Northwest. However, again, these mollusk species are found associated with the remains of marine organisms that live offshore.

For comparison, learn about modern marine habitats along the San Diego shore.

All fossils found in Chula Vista.
Text and shell photos by Scott Rugh, Collections Manager, Invertebrate Fossils
Sandy beach photo by Jim Melli; Rocky beach photo by Erik Bolton