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Decapod crustaceans from the Puente formation (late middle to early late Miocene), California: a possible mass death.

Abstract.--Decapod crustaceans are reported for the first time from the late middle to early late Miocene Puente Formation, Riverside County, California. A single specimen of penaeid shrimp and numerous cancrid crabs, referred to Metacarcinus danai Nations, were collected in association with a mixed assemblage of terrestrial plants, a few bivalves, marine mammals, and numerous taxa of microfossils. Living species of Metacarcinus are found in temperate, normal-marine conditions in water depths up to about 100 m. Thus, an inner shelf, shallow-water environment of deposition is postulated for this part of the Puente Formation. The accumulation of many articulated fossils suggests rapid killing and burial of many individuals, suggestive of a mass death.

Thirteen samples of siltstone bearing fossil decapod crustaceans from the Miocene Puente Formation, Riverside County, California, form the basis for this study. The samples were examined in the context of the Eagle Glen paleontologic resource impact mitigation program conducted by Paleo Environmental Associates, Inc., and L & L Environmental, Inc., during grading associated with development of the Corona Country Club Estates in the city of Corona, California (Lander 2002). The fossils have been identified, and their biostratigraphic and paleoecologic significance has been evaluated. It appears that the fossils accumulated in a nearshore, marine environment, possibly as a result of a mass-mortality event.

Location and Stratigraphic Position

The studied fossils were collected from rocks assigned by Gray (1961) to the undifferentiated Puente Formation of Eldridge and Arnold (1907) at a locality situated on the north slope of Bedford Canyon, Riverside County, California, at latitude 33[degrees]48'21"N, longitude 117[degrees]32'17"W, in section 19, T4S, R6W, of the Corona South 7.5' Quadrangle (Figure 1). The stratigraphic section for the upper part of the rock sequence in this general area (Figure 2) spans an interval of over 700 feet (213 m) of Miocene and post-Miocene sedimentary rock. The lower 170 feet (52 m) of this section is composed of fossiliferous siltstone and claystone that are lithologically similar to those identified as the Puente Formation by Gray (1961) approximately 11 km (7 mi) to the north and northwest of the Bedford Canyon locality in the Corona North and Prado Dam 7.5' quadrangles. In the northern part of his study area, Gray (1961) recognized four members of the Puente Formation. In ascending order these are the La Vida, Soquel, Yorba, and Sycamore Canyon members. He established the age of the undifferentiated formation as middle to late Miocene, based upon benthic foraminiferans.

Some confusion about the naming of the Puente Formation has arisen recently by the assignment of the La Vida, Yorba, and Soquel members to the Monterey Formation and the elevation of the Sycamore Canyon Member to formational rank (Dibblee 1999a). However, Dibblee (1999b) and Dibblee and Ehrenspeck (2000) continued to use the name Puente Formation for subsurface rocks north of the Palos Verdes Hills, approximately 80 km (50 mi) west from Corona. Subsequently, Dibblee (2000) and Dibblee and Ehrenspeck (2001) applied the terms Monterey Formation and Sycamore Canyon Formation as far east as the Prado Dam Quadrangle. However, the terminology of Dibblee (1999a and b) has not been used in the area of the Corona South Quadrangle, and the rocks in that area have not been assigned to a specific member within the Puente Formation. Thus, the most prudent course of action is to retain the name Puente Formation in an undifferentiated sense for the purposes of this study.

Vertebrate and invertebrate megafossils as well as foraminiferans and diatoms previously have been reported from the Puente Formation. In describing the geology of the formation, Gray (1961, p. 35) listed 27 species of foraminiferans in nine genera and one species of diatom, based on the identifications of P. B. Smith. Gray made no mention of megafossils. Although Rigby and Albi (1996) described a new species of hexactinellid sponge from the Puente Formation in Orange County and noted that pelecypods as well as fish were present in the unit, Schoellhamer et al. (1981) do not show any Puente Formation in that area. Although this accounting is not exhaustive, I know of no previous references to decapods in the formation. Schweitzer and Feldmann (2002) described several new decapod fossils from southern California and summarized known occurrences of Cretaceous through Pliocene decapods in the state. None was noted in the Puente Formation. During the present paleontological resource evaluation, a wide range of micro-and mega-invertebrates, vertebrates, and plants were collected. Those relevant to the present study will be noted below and are listed in the Appendix.

Age of the Puente Formation

Based upon the enclosed benthic foraminiferans, Gray (1961, p. 35) placed a late Miocene (upper Mohnian) and possibly middle Miocene (Luisian) age on the Puente Formation. However, benthic foraminiferan ages in the Cenozoic rocks of the Pacific coast of North America have been shown to be unreliable because they are time transgressive, relative to ages derived from planktonic microfossils (Prothero 2001: 389). Foraminiferans, diatoms, and radiometric dates determined as part of the Eagle Glen paleontologic resource impact mitigation program confirm that age (Lander 2002). Specifically, [sup.40]Ar/[sup.40]K dates were determined on ash beds 3 and 4 (Figure 2) by Geochron Laboratories. The dates of 12.6[+ or -]0.4 MA

Fig. 1. Location map showing the position, south from Corona, Riverside County, California, from which fossil decapods were collected.

for ash bed 3 and 12.4[+ or -]0.4 MA for ash bed 4 are statistically indistinguishable. These radiometric ages place the ash deposits at a late middle Miocene age. However, the diatoms are assignable to early late Miocene Subzone D of the Denticulopsis hustedtii-D. lanta Zone (Lander 2002). Because both ash beds and diatoms occur within the part of the formation in which the crabs in the present study were collected, the age of the decapods is reasonably well defined as late middle to early late Miocene.

Metacarcinus spp. range from Oligocene to Recent (Schweitzer and Feldmann 2000) with certainty. A single chela from the Centinela Formation, Santa Cruz Province, Argentina, probably is referable to the genus (Schweitzer and Feldmann 2000), and this would extend the range back to the Eocene. The genus appeared in the Oligocene of Alaska and, by the Miocene, had dispersed along the west coast of North America and into Japan. That same geographic range is observed in the Pliocene. Recent occurrences of the genus are known from the northeast, southeast, and southwest Pacific Ocean basin and from the North Atlantic. Thus, the late middle to early late Miocene age based upon radiometric dating and the enclosed microfossils is well within the temporal and geographic range of the genus. The type locality of M. danai is in the middle Miocene Briones Formation, based upon a single specimen at a single locality (Nations 1975). The occurrence of this species in the Puente Formation reinforces that age.

Systematic Paleontology Order Decapoda Latreille, 1802 Suborder Dendrobranchiata Bate, 1888 Superfamily Penaeoidea Rafinesque, 1815 Family Penaeidae Rafinesque, 1815

Remarks.--The sole specimen under consideration, LACMIP 6945, bears most of the characteristics of the superfamily Penaeoidea and family Penaeidae. The carapace is preserved in lateral aspect, which suggests that it was originally laterally compressed and that the sternal region was very narrow. There is no evidence of a well developed groove pattern. The rostrum is well developed and is spinose, at least on the dorsal surface. The ventral surface of the rostrum is obscured. Although the pereiopods are not well-represented, one of the anteriormost, possibly the first, is relatively long. The abdomen is well-developed, curving ventrally and then anteriorly, and the telson and uropods are well developed. Unfortunately the detail of the pleura of the second abdominal somite is lacking. Penaeidea are characterized by having a second pleuron that does not overlap the pleuron of the first somite whereas the Caridea includes taxa in which the second pleuron overlaps the pleura of the first and third somite. One aspect of the morphology that confirms that the specimen is not an oddly preserved crab is that a segment of the antenna is preserved. That segment is longer than the carapace and is folded back over the top of the carapace. This is characteristic of the shrimps and quite unlike the brachyurans, or true crabs. The antennae on crabs tend to be very short. Thus, although the specimen is crushed and incompletely preserved, placement in the Penaeidea is likely; however, the Caridea cannot be entirely ruled out.

Genus and Species Indeterminate Figure 3

Discussion.--The single specimen, LACMIP 6945, appears to bear no resemblance to any of the other decapods collected in the Puente Formation, all of which are crabs. It is an elongate form with a smooth carapace and curved abdomen. A long, slender, chelate appendage extends downward from near the anterior end of the specimen. Although segmentation appears to be present on the abdomen, details necessary to confirm the generic placement are not discernable.

Although it might be anticipated that crabs and shrimps would be preserved together, it rarely happens, perhaps because the conditions for preservation of the benthic crabs is different from that of the pelagic shrimp. Glaessner (1969) noted that preservation of the fragile remains of shrimp is favored in acidic conditions. Such conditions would not enhance preservation potential of the more strongly calcified crabs. Both groups occupy normal marine habitats and both could be found in coastal areas. However, because the identification of this single specimen is tentative, no paleoecological conclusions will be based upon its occurrence.

Infraorder Brachyura Latreille, 1802 Superfamily Cancroidea Latreille, 1802 Family Cancridae Latreille, 1802 Genus Metacarcinus A. Milne Edwards, 1862

Diagnosis.--The diagnosis of the genus Metacarcinus, with reference to the carapace only, is, "Carapace ovate, about two-thirds wider than long. Front with five spines including inner-orbital spine, inner three spines closely spaced; front usually not produced beyond orbits. Fronto-orbital width about 0.26-0.34 maximum carapace width; orbits shallow, directed forward. Anterolateral margin with nine or ten spines; anterolateral spines variable in form; small, sharp, and separated to bases or small, sharp and fissured; spine margins simple, serrate, or granular. Posterolateral margins rimmed, sometimes with one spine; carapace regions poorly developed, smooth or ornamented with fine granules " (Schweitzer and Feldmann, 2000, p. 235).

Discussion.--Nations (1975) studied the genus Cancer and subdivided that genus into four subgenera: Cancer (Cancer)Linnaeus, 1758; C. (Gelbocarcinus) Nations, 1975; C. (Romaleon) Gistl, 1848; and C. (Metacarcinus) A. Milne Edwards, 1862. The genus Cancer subsequently has been demonstrated to be comprised of several clusters of species, and Schweitzer and Feldmann (2000) elevated the subgenera described by Nations to generic level, reevaluated the placement of species within those genera and, in so doing, revised the entire family Cancridae. Although none of the specimens at hand is preserved well enough to exhibit all the above-mentioned characteristics, each of the specimens exhibits some of the diagnostic features. Therefore, nearly all the characteristics can be recognized. Thus, the generic identity of the crabs from the Puente Formation is certain.

Metacarcinus danai Nations, 1975 Figure 4

Discussion.--Of the 24 specimens of decapods from the Puente Formation that can be identified with any confidence, all but the one referred to the Penaeidae are referable to Metacarcinus danai (Nations 1975). Cancer danai Nations, 1975, was referred to the subgenus Cancer (Metacarcinus) by Nations (1975, p. 53). He noted its close similarity to Cancer (Metacarcinus) magister Dana, 1852. Examination of the description and illustration of Cancer danai (Nations 1975, p. 53, fig. 34-4) confirms that the Puente Formation material can confidently be referred to that species. The frontal region on all the preserved carapace material is very poorly preserved, but the texture of the dorsal surface; the size, shape, and serration pattern on the teeth on the anterolateral margin; and the nature of the posterolateral margin conform closely to the type description. Nations (1975) based this taxon, known only from the middle Miocene Briones Formation, Contra Costa County, northern California, on a single specimen with a carapace width of 54.2 mm. This is slightly larger than the largest specimen that could be measured in the Puente Formation, which was over 40 mm wide. Most specimens are too fragmentary to measure. That difference in size suggests that the specimens at hand may be juvenile forms. Certainly, the size difference is not extraordinary.

Paleoecology

The occurrence of decapod crustaceans collected in the Puente Formation is atypical in that they were found in association with a wide variety of vertebrate and invertebrate megafossils and terrestrial plants (see Appendix). Microfossils were reported from one of the sites, LACMIP 17582; however, at the other two localities they were absent. This absence is unusual for the Puente Formation, which is dominated by microfossils elsewhere. For example, the microfossils of the formation were first detailed by Smith (1960), and subsequently Gray (1961) presented a list of 27 species of foraminiferans, identified by Smith, from the formation in the Corona South Quadrangle. Despite the fact that four of the species identified by Smith, Buliminella curta Cushman, 1925; B. subfusiformis Cushman, 1925; Epistominella subperuviana (Cushman, 1926); and Uvigerina subperegrina Cushman and Kleinpell, 1934, are considered indices of outer shelf and bathyal depths (Finger, 1990), Gray interpreted the Puente Formation in his study area as having been deposited in a nearshore, shallow water environment. He made no mention of a megafauna. Schoellhamer et al. (1981) also noted the dominance of microfossils in the formation, in an area west from the current site of interest, but noted the presence of bivalves and fish scales as well.

Interpretation of the paleoecological setting in which the crabs lived, and presumably died, will be drawn from three independent lines of evidence. First, the ecology of living representatives of Metacarcinus will be used to define the modern ecological and biogeographic setting of the genus. Second, the association of the crabs with the other elements of the biota will be discussed. Third, the nature of the occurrence of the crabs will be considered.

The ecological setting of four extant species of Metacarcinus was summarized by Rathbun (1930). The species presumably most closely related to M. danai, M. magister (Dana, 1852), has been collected from low water to 50 fathoms (100 m) and is known to live on a wide range of substrates from mud to gravel and bare rock. The species ranges from Alaska to Monterey Bay, California. Another species, M. anthonyi (Rathbun, 1897), ranges from 6-50 fathoms (12-100 m) in water temperatures from 13-19.5[degrees]C. This species is known from Monterey Bay to Baja California, Mexico. Metacarcinus gracilis (Dana, 1852) lives from low water to 56 fathoms (112 m) and has been collected from Alaska to Baja California, Mexico. In what is probably a good example of a disjunct, amphitropical generic distribution, M. edwardsii (Bell, 1853) ranges from Ecuador to Chile. Garth (1957) recorded a depth range for the species of 0-45 m. All species of the Cancridae living today are restricted to temperate and subpolar temperatures of 1.3-25[degrees]C (Williams and Wigley, 1977, in Williams, 1984). There are no strictly tropical or subtropical occurrences of cancrids except in very deep, offshore habitats where the animals live in cool water below the thermocline. No restricted, brackish water occurrences have been noted. Thus, presuming that the ecological requirements have not changed substantially since the Miocene, Metacarcinus danai probably lived in temperate waters at oceanic salinity, and in water depths ranging somewhere from low water to about 100 m: that is to say, it lived somewhere within the typical continental shelf depths in the temperate zone.

Consideration of the biotic associations of Metacarcinus danai may further help to define its ecological habitat. The associated fossils in the Puente Formation (Appendix) suggest mixing of marine and non-marine elements. Presence of several types of plant material, including algae, conifers, and dicotyledonous plants, indicates mixing of terrestrial and marine elements, a condition that would be anticipated in inshore habitats and would not be nearly as likely in offshore, outer shelf settings. Presence of bivalves and marine mammals is consistent with this interpretation, and, although the precise identity of the mammals and the algae is not known, they may suggest a kelp forest or other euphotic area in which attached plants flourished. The light-colored sediments enclosing the crab fossils suggests a well-aerated, oxidizing sedimentary environment.

Finally, the mode of occurrence of the fossil crabs is unusual and provides evidence suggesting that they may have been victims of a mass-kill event, conceivably a toxic algal bloom. Because of the fragmentary nature of the crab specimens, it is not possible to determine with confidence whether the specimens represent molted remains or corpses. The carapaces are extremely thin and, in some cases, folded and distorted. This suggests that the animals might have been in the molt condition. Typically, the carapaces of cancrid crabs are quite thick and strong. However, molted remains are often typified either by complete separation of the legs from the carapace or by partial separation and rotation of the legs (Glaessner 1969; Feldmann and Tshudy 1987). A sufficient number of specimens exhibit the legs in living position, relative to the carapace, to suggest that at least some individuals were corpses. Partial dissolution of the carapace would account for the fragile nature of the preserved remains.

Examination of the systematic literature and personal observations indicate that the "typical" mode of occurrence of fossil decapod crustaceans is as single individuals. The individuals can be relatively abundant in a rock unit and can be preserved in concretionary structures or on bedding plane surfaces. In either case, individuals generally are not confined to a single stratigraphic horizon but may be found throughout the sedimentary sequence. Examination of the specimens in the collection at hand indicates that at least two horizons within the Puente Formation were sites of accumulation of numerous individuals that died suddenly enough to come to rest on single bedding horizons and were buried very rapidly. This type of mass mortality was recently documented in a Turonian decapod assemblage from Colombia (Feldmann et al. 1999). The preservational style in the Colombian rocks is quite similar to that in the Puente Formation; both display a very large number of specimens arrayed on discrete bedding planes, and, in both cases, the cuticular material is extremely thin and delicate. It is possible that in both cases the calcareous material in the cuticle was leached out during diagenesis. Event beds such as this are relatively rare in the fossil record of crabs and, for this reason alone, the occurrence is noteworthy.

In summary, it is likely that the crabs were killed during a brief interval of time, possibly as victims of an algal bloom, and that they were buried and preserved very near their living site. The composite of information regarding paleoecology suggests a normal marine, inshore habitat that was biologically rich. Water depth was probably very shallow, much less than the projected maximum depth of 1000 meters, and water temperature was temperate.

Acknowledgments

This work was originally undertaken as part of the Eagle Glen Phase III paleontologic resource impact mitigation program. The paleontologic monitoring of grading, preparation of fossils, and final report on the Corona Country Club Estates were funded by Forecast homes under the direction of Dorian Johnson, Vice President of Land Development, and Mr. Gene Schutt, Project Manager. Ms. Leslie N. Irish of L & L Environmental provided environmental compliance management. Paleontologic monitors were David A. Alexander, Nina Jimerson, Michael H. Stevens, Kimberley M. Scott, and Jeffrey D. Cassidy, who collected the fossils and ash samples and recorded associated stratigraphic data. Kimberley Scott measured and drafted the stratigraphic section. The paleontologists were Dr. E. Bruce Lander and Mr. Mark A. Roeder of Paleo Environmental Associates, Inc., who prepared the final report for the mitigation program (Lander, 2002). Data regarding associated microfossils and radiometric ages were provided by Micropaleo Consultants, Inc. (Boettcher and Kling, 2001) and Geochron Laboratories. Dr. Carrie E. Schweitzer, Department of Geology, Kent State University Stark, and Dr. Frederick Schram, University of Amsterdam, read drafts of the manuscript and provided useful comments. Reviews by Dr. William Orr, University of Oregon, and an anonymous reviewer provided thoughtful comments.

Literature Cited

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Accepted for publication 11 December 2002.

Appendix

Stratigraphic Array of Samples Locality LACMIP 17582 Sample JDC 010208-3--part of left lateral margin with anterolateral teeth like those of Metacarcinus. Associated organisms: pollen, fungal spores, silicoflagellates, dinocysts, foraminiferans, diatoms, leaves, bivalves, fish

Locality LACMIP 17583

Sample MHS 010130.1--good claw specimen and possibly a telson

Sample MHS 010130.3--possible penaeid shrimp, LACMIP Hypotype 6945--antennae very long, curving abdomen exhibiting some metamerism, pleurae vague (Figure 3)

Sample MHS 010130.6--numerous specimens, two with anterolateral margins lobed with denticles posteriorly becoming simple anteriorly--typical of Metacarcinus danai

Sample MHS 010130.7-42 mm wide carapace with good anterolateral margin and claw with small, domed denticles

Sample MHS 010130.8--Large sample with fragments of carapace--small sample with claw or walking leg fragments

Associated organisms: ebredians, silicoflagellates, diatoms, whale bone

Locality LACMIP 17584

Sample KMS 010109. 1--part and counterpart of large sample with as many as eight complete specimens, some of which exhibit well preserved margins, LACMIP Hypotype 6946 (Figure 4)

Sample MHS 001228.1--venter with legs, possibly a sternum, possibly a male; good carapace wider than long, with orbits, sulcate front, and weakly developed axial regions

Sample NLJ*001227.1--crushed venter of? female]

Sample NLJ 001228.8--pair of claws; venter with buccal frame, antennal fragment

Sample NLJ 001228.9--fragment of dorsal carapace

Sample NLJ 001228.11--walking legs with lanceolate dactyl

Sample NLJ 001228.12--venter of male? with some pereiopods Associated organisms: plants, bivalves, fish

* Note that NLJ, as denoted here and on the specimens, appears to correspond to NHJ on the Eagle Glen Phase III--master list.
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