Unusual Eocene Invertebrate Fossils Found in Oceanside
Discoveries include previously unknown sand dollar and comb-clawed shrimp
August 5, 2001
During late winter and early spring of 2001, the staff from the Paleontology Department monitored the construction activities of Ocean Ranch, an industrial park in eastern Oceanside being developed by Stirling Homes. Initial digging exposed terrestrial deposits of Eocene age about 42 to 45 million years old. The remains of land mammals were found, including the skull and other bones of Metarhinus pater, a brontothere, or large rhinoceros-like animal. Also found were molds and casts of land snails.
Lower down in the sedimentary rock sequence, marine strata were unearthed representing a wide range of marine environments, from a muddy, oyster dominated estuary to a sandy ocean bottom inhabited by many kinds of sea creatures including crabs, shrimps, sand dollars, mollusks, fish, sharks, and rays. These deposits are from the Santiago Formation of the Eocene and are about 45 to 47 million years old.
Among the most interesting fossils found were specimens collected by paleontologists Brad Riney and Gino Calvano, at lots 2 and 5 of the project (San Diego Natural History Museum localities 4573 and 4572 respectively). Locality 4573 primarily consisted of a consolidated siltstone matrix and locality 4572 was predominately sandstone, but both locations sampled different parts of the same stratum, generally referred to as the "shrimp bed", because of the prevalence of the remains of shrimp and other decapod crustaceans. This prevalence of crustaceans is itself unusual, because fossils of crustaceans are not often found in our local Eocene deposits, and those found are generally fragmentary or poorly preserved. At Ocean Ranch however, crustaceans were common, and between the sites, no less than 7 different species of crustaceans in several different families were recovered. The state of preservation is very good, with claws, tails and carapaces and nearly whole bodies of some species preserved.
Most of the invertebrates found at Ocean Ranch are representatives of families that are familiar to us today. A very large species of ghost shrimp, Callianassa sp. (up to18 cm. in length, with its tail extended) was found at locality 4572. A couple of species of ghost shrimp are commonly encountered in bays along the California coast today. Swimmer crabs, Family Portunidae, are common at both localities, and are presently common in bays from southern California south to Baja, Mexico. A single specimen of the serrated segment of the second thoracic leg of a mantis shrimp was found. Collected at both localities, but far more common at locality 4573 was a strange and fascinating species of shrimp with comb like claws. This shrimp is so unusual it may be a new species, possibly a new family.
This shrimp has long, thin, and sharply pointed "teeth" projecting from the inner edge of the larger of two front claws. Behind this pair of claws there appears to be a second appendage that is either forked or clawed. I am not familiar with any other crustacean that has two pairs of claws at the front of the body. Crabs, lobster, and shrimp generally only possess one pair of claws in front, except for the American Lobster and its close relative, the crayfish, in which every leg segment is biramous, or clawed. Spiny lobsters on the other hand have no claws. It is tempting to speculate that the strange Eocene shrimp of Ocean Ranch used its spiny claw for catching small fish, an adaptation to which this unusual design would be perfectly suited. This claw somewhat resembles the mouth of the barracuda, which is adapted for catching fast moving fish, and that of the Gavial, the relative of crocodiles and alligators whose long, toothy snout is adapted for feeding on fish, as well the mouth of the European dinosaur Baronyx walkeri (related to the Spinosaur (Suchomimus sp.)) which is believed to have lived on a diet of fish. The shrimp cannot be officially be called the "fishing shrimp" because we have no direct evidence of it catching fish, such as a fossil fish trapped in its claw, or fish remains in its gut. However, there are a number of burrows in the collection filled with scales and other disarticulated fish parts, as well as one nearly complete fish. If this shrimp did in fact catch fish, the prey may very likely have tended to get stuck in the fine "teeth" of the claw. If there was a second pair of claws or forked appendage behind the combed claw, it would have been useful in freeing food from the claw, and also would have been able to chop the food into smaller bite size pieces and bring these pieces to the shrimp's mouth. The "teeth" look delicate and may have easily broken, but they would have been replaced after the shrimp molted and grew all new parts.
The best specimens of the fossil crustacean material have been sent to Doctors Carrie Schweitcher and Rod Feldman, experts on fossil decapods, at Kent State University, Ohio. We will look forward to learning more about these fascinating crustaceans.
The recent discovery of fossil crustaceans at Ocean Ranch has contributed to our knowledge of the marine environment in San Diego County during the Eocene. As other collections of marine Eocene fossils from our area have demonstrated, the climate was tropical. What is most exciting about the recent discoveries of fossil invertebrates in Oceanside is the possibility that we have found a number of new species of previously unknown marine organisms.
Text by Scott Rugh,
Collections Manager, Invertebrate Fossils; photos by Jim Melli