Corals, molluscs, and paleogeography of late Jurassic strata of the Cerro Pozo Serna, Sonora, Mexico

Palaeogeography, Palaeoclimatology, Palaeoecology, 19(1976): 275--301 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

CORALS, MOLLUSCS, AND PALEOGEOGRAPHY OF LATE JURASSIC STRATA OF THE CERRO POZO SERNA, SONORA, MEXICO

LOUISE BEAUVAIS and TOM E. STUMP

Laboratoire de Paldontologie, Universit~ de Paris, Paris (France) Department of Geology, University of California at Davis, Davis, Calif. (UoSoAo) (Received May 7, 1975; revised version accepted December 4, 1975)

ABSTRACT Beauvais, Lo and Stump, T0 E., 1976. Corals, molluscs, and paleogeography of late Jurassic strata of the Cerro Pozo Serna, Sonora, Mexico. Palaeogeogr., Palaeoclimatol., Palaeoecol., 19: 275--301. Jurassic strata of late Oxfordian to early Kimmeridgian age are reported from the Cerro Pozo Serna, northwest-central Sonora, Mexico° The presence of these marine strata greatly alters previous paleogeographic reconstructions of the Tethyan embayment in this area. Approximately 60% of the Sonorian molluscan fauna has been previously reported from Jurassic horizons in the Gulf of Mexico and west Texas regions. The remainder of the molluscs have previously been reported from either the western interior United States or Canada° The Pozo Serna faunas seem to be zoogeographically transitional between communities present in the southern North American Tethyan realm and regions within the central North American Tethyan realm. Additionally, the Pozo Serna fauna greatly resembles contemporaneous communities reported from the Jurassic of Morocco° Corals from the Sonorian Jurassic prove to be, in large part, conspecific with well-known central European and Algerian reef-building species. The scleractinian Lepidophylliopsis gen. nov. and Macgeopsis sonorensis spo nov. are described from central Sonora°

INTRODUCTION

Strata of Jurassic age crop out extensively in northwest-central Sonora. Despite these widespread occurrences, few authors have documented the presence of non-ammonoid megafossils in this region. Middle Mesozoic faunas from Sonora are also of great zoogeographic importance. These communities lie geographically intermediate between Jurassic faunas of the western interior United States to the north, the Gulf Coastal Plain to the east and Jurassic communities of Mexico and the southern Americas to the south. During the middle and late Jurassic faunal differentiation produced faunas of varying dissimilarities in these areas. Furthermore, faunal provinces attained definition for the first time in the Mesozoic, during the Jurassic. North America contained portions of these provinces (realms) by mid-Jurassic.

276

Traditionally these provinces have been recognized by the presence of certain characteristic a m m o n o i d faunas. Despite the many works on ammonites, however, relatively little is k n o w n a b o u t the temporal and spatial distributions of North American Jurassic Bivalvia and Gastropoda. Likewise, Jurassic corals are well known from Europe, but only scattered occurrences have been reported from North America. Therefore, our paper serves as further documentation concerning the faunal relationships between the Jurassic of western Mexico and other areas within North America, Europe, and North Africa. LOCATION

The Cerro Pozo Serna is located approximately 60 km south of Caborca, northwest-central Sonora, Mexico (Figs. 1 and 2). The prevailing geologic I

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Fig.1. Index map of the State of Sonora, Mexico.

structure of the region is north- to northwest-trending mountain ranges, composed of very thick Precambrian to early Cambrian limestones and fine-grained clastics. Neogene (?) volcanics also occur extensively in areas surrounding the Pozo Serna.

277 PREVIOUS WORK

The initial geologic reports on central Sonora were, of necessity, of a reconnaissance nature (see Aguilera in Dumble, 1900). Dumble (1900) set forth the Tertiary and Mesozoic stratigraphic framework of Sonora and adjacent states. In a series of monographs, Burckhardt (1927, 1930) established the biostratigraphic correlations between principal middle and late Mesozoic formations. Burckhardt's work was chiefly confined to regions far south of Pozo Serna, but his ammonite successions have been recognized in areas to the north. Imlay (1940--1965), in a series of excellent papers, further refined Burckhardt's ammonite zonations, documented faunas in northern Mexico and discussed the relationships of the Jurassic faunas from Pacific coast North America to these associations. 112 ° 3 0 '

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The first systematic geologic study of Sonora was made by King (1939). King worked principally in areas east of Hermosillo (Fig.l). His work provided additional stratigraphic and geologic information and summarized earlier work in this region. White and Guiza (1948) (see Fig.2) and De Cserna (1961) also discuss the Jurassic geology and faunas of Sonora. Merriam (1972) has summarized previous work in northern Sonora and his paper gives references to more recent unpublished geologic explorations in this area. Hardy (1973),

278 Eells (1972) and Dowlen and Stump (1973) have also briefly discussed the general geology or paleontology in the Pozo Serna or surrounding areas. GEOLOGY OF THE CERRO POZO SERNA The Cerro Pozo Serna consists of t w o low, northeast-trending mountain ranges. These ranges are separated one from the other b y Tertiary (?) northwest~striking normal faults. These faults juxtapose Jurassic strata against several hundred meters of Precambrian and Paleozoic (?) limestones and dolomites. The thickest and most stratigraphicaUy continuous of the Jurassic exposures in the Pozo Serna occurs in the southern portion of the range. These outcrops are covered on their southernmost exposures b y Tertiary dacites and andesites. The Jurassic section studied in this paper was not extensively faulted or folded. Therefore, a complete section was measured and fossils located with good stratigraphic control. The greatest concentrations of fossils were found in the lower half of the section (Fig.3). Unfortunately, fossils were not f o u n d continuously to the base of the strata. Likewise, the upper portions of the Pozo Serna section were barren of fossils, except for stellate crinoid columnals. STRATIGRAPHIC RELATIONSHIPS OF THE POZO SERNA TO OTHER JURASSIC STRATA IN CENTRAL SONORA The stratigraphic and facies relationships of Sonorian Mesozoic. strata are complex and poorly understood. In several areas Jurassic sections occur in fault contact with Precambrian and Paleozoic carbonates. In other areas, Mesozoic sections have been extensively folded and thrust-faulted, severely obscuring their stratigraphic relationships. Also, many of the Triassic and Jurassic formations of this region display marked facies changes over relatively short distances. These strata, while extensively exposed, are also separated b y greater distances along faults initiated during the Cordilleran Orogeny. Therefore tracing one formation from one mountain range to another is very difficult in the absence of detailed biostratigraphy and geologic mapping. King (1939) recognized these difficulties and included the presumed upper Triassic and lower Jurassic rocks in the Barranca Formation of Dumble (1900). King separated his formations on the presence of unconformities occurring between formations. By this m e t h o d he h o p e d to define a regional " c y c l e " of sedimentation. It has been our experience, however, that in extensively folded and overturned sections, thrust faults and unconformable contacts are sometimes difficult to distinguish from one another with accuracy. As a consequence of these many structural and facies-related problems with the previous stratigraphic subdivisions, De Cserna (1961) elevated the Barranca Formation to group status. She also separated the upper Triassic brackish-water and marine strata of the Santa Clara Formation from the continental Arrayenes

279 1795

Brittle, non-fossiliferous, non-calcareous, tan and brown shale.

Predominantly grey, brown or tan fissile tuffaceous shale. These nonfossiliferous shales interbedded with relatively thin, medium-grained, calcareous sandstone and minor litharenites. Contorted bedding and slump structures sometimes associated with sandstone contacts with shale beds.

1150

2306-Black, grey and brown, sandy and tuffaceous shales, locally very fossiliferous and with interbadded coarse-grained, feldspathic litharenita interbeds. Minor oolitic and massive limestones near the top of section alternate with light brown calcareous shale.

2307-2308-2309-" 2310--

Tan, thinly-bedded, fine-grained, very fossiliferous sublithic arenite with prominent tuffaceous shale interbeds.

2313-650

2317--

__

Massive, tan, very fossiliferous sublithic arenita.

Interbedded grey and black, highly fissile shale and pebbly, subarkosic, poorly sorted, well-indurated sandstone interbeds.

400

150

Predominantly grey to brown, fissile, non-fossiliferous sandy shale with thin, relatively uncommon, interbeds of well-indurated, fine-to medium-grained sandstone.

Interbedded pebble-bearing, lithic arenite and intraformational breccia composed of shale fragments. Base of section composed of grey, well-consolidated, slightly fissile shale.

Fault contact on mottled dolomites. 200 m I 150 m 100 m Vertical scale in meters above section base.

50 m Orn

Fig.3. The generalized stratigraphic section at Cerro Pozo Serna. and C o y o t e s Formations. These formations, as originally defined, seem confined to the "San Marcial basin" east o f Hermosillo. However, their unnamed correlative formations are u n d o u b t e d l y present in the lower portions of many Mesozoic sections exposed in central Sonora. It is clear from previous studies, and the present paper, that at times during the later Jurassic the San Marcial Basin was also c o n n e c t e d to areas north of Hermosillo. The basin, if it was such, probably was also c o n t i n u o u s with the middle and later Jurassic Carmel

280 seas of the western interior United States. On most late Jurassic paleogeographic reconstructions of northern Mexico, marine conditions are generally unrecognized. The presence of positive areas are often inferred based on the lack of d o c u m e n t e d late Jurassic strata in this region. The presence of Oxfordian through Kimmeridgian strata in the Pozo Serna throws considerable d o u b t on these past reconstructions. Within the basal deposits of the Pozo Serna there seem to be no unconformi.ties; but, because of the absence of fossils, the presence of disconformities cannot be excluded. Judging from descriptions of the Barranca group, portions of the lowest Pozo Serna Mesozoic sections m a y be lithologically correlative with the uppermost formations of this group. This last point remains highly tentative in the absence of an accurate age for these basal strata. The nearest Jurassic deposits to the Pozo Serna that have been studied in detail occur in the Sierra de Santa Rosa district (Fig.2). Hardy (1973), when mapping this area, did n o t correlate the Jurassic strata with any previously described formation. Nevertheless, the sections exposed in the Santa Rosa district appear to be biostratigraphically equivalent, at least in part, to the Pozo Serna (Hardy, 1973, p. 29). The strata of the Santa Rosa area, however, are severely thrust-faulted and represent quite a different lithological facies. Hardy f o u n d larger quantities of conglomerates, ruffs and volcanics than were present in the Pozo Serna. The base of the Pozo Serna section is in fault contact on Paleozoic (?) mottled dolomites. The t o p of the exposed strata occurs 1795 m above its base and is c o m p o s e d of tan tuffaceous shale (Fig. 3). Thus, from the age relationships, the Pozo Serna strata probably represent at least portions of the distal, fine-grained facies of the Santa Rosa Jurassic section. Deposits that may be correlative to the Pozo Serna occur in areas surrounding Caborca (Fig.2). These strata also contrast lithologically with b o t h the Pozo Serna and Santa Rosa sections. The difficulties of correlation between these areas only serves to underscore the complex and poorly understood facies and age relationships of this area. AGE OF THE POZO SERNA DEPOSITS The lower half of the exposed Mesozoic section in the Pozo Serna was locally very fossiliferous. Unfortunately, the majority of the molluscs were preserved as either internal molds, casts, or impressions. Occasionally individual Trigonia and Nerinea preserved some original material which aided in their identification. Most molluscan fossils from the Pozo Serna were identified b y making latex molds from impressions. All fossil material and molds are deposited at San Diego State University and the Laboratoire de Pal~ontologie des Invert~br~s, Universit~ de Paris. A duplicate collection was presented to the Government of Mexico and is housed in the Geological Institute of Mexico, Mexico City.

281 The most age-diagnostic taxa present in the P•zo Serna were the ammonoid cephalopods. This group is abundantly represented at San Diego State University (SDSU) localities 2313, 2306, 2307, 2308 and 2309. Most specimens were likewise preserved as molds or impressions. Fortunately, the most age-diagnostic taxa of this group can be identified from external morphology (R. W. Imlay, 1974, personal communication). Less helpful in dating the Pozo Serna deposits were the bivalves and gastropods. The corals furnished additional evidence concerning the age of this deposit. Due to the significance of this group, the corals will be treated systematically. The reported geologic ranges of the Pozo Serna taxa are summarized in Table I. Several of the bivalves and gastropods have range extensions in the Pozo Serna section. Such range extensions were not surprising. Little monographic work has been published concerning these groups within North American Jurassic sections and their total ranges are imperfectly known. As stated above ammonoids are abundant in the Pozo Serna deposits. The most abundant taxon, however, was so poorly preserved that it was indeterminate even as to family. However, three additional ammonoid species were identifiable to at least genus. Two of these taxa, Amoeboceras and Pseudocadoceras, have been widely reported from European and North American rocks of Oxfordian (early-late Jurassic) age. Likewise, Idoceras spp. is widely distributed in North America from Alaska to Mexico and reported in rocks ranging from late Oxfordian to late Kimmeridgian in age. The species to which the Pozo Serna specimens were tentatively assigned has been reported from the Logtown Formation of the Sierra Nevada, California (Imlay, 1961). In this formation, I. planula (Heyl) was associated with a fauna of latest Oxfordian or earliest Kimmeridgian age. Imlay (1961, p. D-6) likewise reports this species, or closely related taxa, from several European Jurassic sections of late Oxfordian age. The species, however, has been reported in Mexico only from strata of Kimmeridgian age. Furthermore, a closely related species, I. durangense Burckhardt, biostratigraphically succeeds the former taxon in the latest Jurassic of Mexico. The specimens from the Pozo Serna, however, appear closer t o / . planula (Heyl in Imlay, 1961) than t o / . durangense Burckhardt. This latter point also supports a late Oxfordian to early Kimmeridgian age. The ammonite genus Amoeboceras is reportedly restricted to rocks of late Oxfordian age (Moore, 1957). Imlay (1964a), however, reports a species associated with the late Oxfordian to early Kimmeridgian bivalve Buchia constricta (Sowerby) from the Naknek Formation of the Cook Inlet region, Alaska. While species of Buchia were not found in the Pozo Serna, A m o e b o ceras was judged to be associated with a similar-aged fauna. In this context, Imlay (1961) also reports A. dubium (Hyatt) from the Mariposa Formation (Late Jurassic) of the Sierra Nevada. Both A. dubium and a species occurring in Greenland, A. elegans Spath, appear to differ from the Pozo Serna material in a few characters and the latter may be a new species. A. dubium, in comparison

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Mytilus ( Faicimytilus) aft. stricapillatus Hayami Ostrea (Liostrea) strigilecula

(Quenstedt)

"Corbula" sp. "Lucina" pseudospherica

Imlay

Aulacomyella neogeae

(Castillo and Aguilera)

Arctica cf. A. coteroi

BIVALVIA

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Amoeboceras sp. Idoceras sp. aft. planula (Heyl) Pseudocadoceras sp.

CEPHALOPODA

The fauna of Pozo Serna

TABLE I

tO O0 t~

Allocoeniopsis bernensis (Koby) Brachyseris chellalensis Beauvais Latiastraea variabilis (Etallon)

SCLERACTINIA

DentaUium sp. indet.

SCAPHOPODA

Zygopleura aft. Z. mandawaensis Cox Gen. and spp. indet.

Sohl

Pleurotomaria sp. indet. Promathildia opalini (Quenstedt) Pseudomelania sp. Purpuroidea acatlana Cserna and Buitron Rhabdocolphus viriosus

Imlay

Nerinea goodelli (Cragin) N. nov. sp.? Nerinoides cf. N. stantoni

GASTROPODA

X

"Tapes" ef. " T . " cuneovatus Cragin Trigonia (T.) inexpectata of Jaworski, 1929 Gen. and spp. indeto X

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San Diego S t a t e University localities are d e s c r i b e d o n pp. 2 9 8 - - 2 9 9 . L o c a l i t y 2 3 1 6 is s t r a t i g r a p h i c a l l y t h e l o w e s t fossiliferous l o c a l i t y ; S D S U 2 3 0 6 was t h e highest.

Stellate c r i n o i d s c o l u m nals gem a n d sp. i n d e t .

CRINOIDEA

Montlivaltia sp. i n d e t . Stylina lobata ( G o l d f u s s )

nov. sp.

Lepidophylliopsis longimana ( Q u e n s t e d t ) Macgeopsis sonorensis

T A B L E I (continued)

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285

with the Pozo Serna material, has denser and more regularly spaced ribbing near the umbilicus, has fewer secondary ribs and has a stronger series of tubercules on the outer whorls. The genus Amoeboceras, while rare in the Mariposa and lower portions of the Naknek Formation, was c o m m o n in the Pozo Serna. Another similar, b u t distinct, species, A. alternans (Von Buch), has been reported from approximately 4 km south of San Pedros del Gallo, central Mexico (Burckhardt, 1930, p. 66). This r e p o r t seems to be the southernmost occurrence of the genus in North America, and perhaps the world (Imlay, 1961, p. D-12). The last identifiable ammonite in the present collection was Pseudocadoceras sp. This genus is also restricted to strata of Oxfordian age (Moore, 1957). Imlay (1961, p. D-6), however, reports this genus from rocks as old as middle Callovian. The youngest occurrence though seems to be late Oxfordian. Pseudocadoceras was rare in the Pozo Serna, only t w o individuals were found. The bivalve genus Trigonia is also widely distributed in rocks of middle Mesozoic age and its species are frequently used successfully as stage--age indicators. Trigonia (T.) inexpectata Jaworski (in Jaworski, 1929), also occurs abundantly in the Pozo Serna. This species was reported by Jaworski (1929, p. 7) from deposits near the Pozo Serna. At that time he regarded the age of the unnamed formation from which his material came as Lias, or early Jurassic. Jaworski assigned this age on the then known trigonid successions and apparently not on the established ammonite biostratigraphy of European workers. Subsequent studies by several authors on early and middle Jurassic trigonoid assemblages has revealed that the p h e n o t y p e of T (T.) inexpectata is more probably late Oxfordian to early Kimmeridgian in age. Similar species of this genus have been widely reported from the Jurassic of Mexico and East Africa (see Cox, 1965). Many of these nominal taxa are probably the same species, or are closely related to the species occurring at Pozo Serna. De Cserna (1963) also reports trigonid taxa which are conspecific with T (T.) inexpectata from the late Jurassic of Oaxaca and northeast Guerrero, Mexico. "Lucina" pseudospherica (Quenstedt), which also occurs in the Pozo Serna, has been widely reported from the Kimmeridgian of Mexico (De Cserna, 1963, p. 46). Imlay (1963, p. 35), however, noted that species of this form-genus were rare in the Carmel Formation and correlatives of western interior United States. Another species more abundant in the Pozo Serna deposits was Aulacomyella neogeae Imlay. This taxon was reported from northern Mexico in middle and late Jurassic horizons. The species was regarded by Imlay (1940) as an off-shore water-depth indicator. This taxon has subsequently been discovered in association with late Jurassic faunas which have some affinities to the middle Jurassic Malone Formation of southwest Texas. Aulacornyella neogeae Imlay was originally described from the La Casita Formation in the State of Zacatecas. Ammonites from this formation place its age as Oxfordian through Kimmeridgian. Because the genus Aulacomyella is similar to the Triassic genus Halobia, several of the younger reports of the latter m a y be referable to Aulacomyella.

286 The remainder of the Pozo Serna molluscan fauna was also compatible with a late Oxfordian to early Kimmeridgian age. The species R h a b d o c o l p h u s viriosus Sohl, Ostrea strigilecula (White) and Artica coteroi (Castillo and Aguilera) have all been reported from the middle Jurassic Carmel Formation of Utah and surrounding states. Likewise, Purpuroidea acatlana De Cserna and Buitron and " T a p e s " cuneovatum have been reported from the late Jurassic Mapache Formation of southern Mexico. " T a p e s " cuneovatus Cragin and Nerinoides stantoni Imlay have also been reported from the Malone Formation of Texas. Nerinea goodelli (Cragin) is likewise reported from the Malone Formation and has several related species in the late Jurassic of southern Mexico (Cragin, 1905; Imlay, 1940). In addition to the species occurring in the western interior United States, more southern faunal elements were common in the Pozo Serna. M y tilus aff. M. stricapillatus Hayami, for example, was reported by De Cserna (1963, p. 20) from the Taberna Formation of Yucatan. In Yucatan this species was associated with late-middle and late Jurassic faunas. A similar species, M. tifoensis Cox, was reported from Kimmeridgian strata of South Africa (Cox, 1965). The corals from the Pozo Serna were also indicative of a late Oxfordian to early Kimmeridgian age. Most of the species from the Pozo Serna proved to be conspecific with well-known European species (Table II). TABLE II Stratigraphic distribution of described coral species Oxfordian

AUocoeniopsis bernensis

Middle

Upper

Swiss

Jura

Kimmeridgian

Tithonian

Algeria

Stramberk (Czechoslovakia)

(Koby)

Stramberk

Lepidophylliopsis -longimana

(Quenstedt) Stylina lobata

eastern

France

(Goldfuss) Brachyseris cheUalensis

Nattheim (SW Germany} Algeria

Stramberk

France, Poland, Algeria

Stramberk

Beauvais La tiastraea variabilis

(EtaUon)

~

France, Poland

287 ZOOGEOGRAPHIC RELATIONSHIPS OF THE POZO SERNA FAUNAS Since the initial work on Mesozoic molluscs began authors have shown that early Mesozoic ammonoid stocks were largely cosmopolitan in distribution. Marked provinciality, on the other hand, became evident in rocks of post-early Jurassic age.. It is well known that certain ammonite stocks of middle and late Jurassic age have been found almost exclusively in areas bordering the Arctic Ocean. Other taxa appear to replace one another gradually northward or southward across central and northwestern Europe. Likewise, faunal replacements occurred in California and Oregon along a north--south gradient. Distributions of certain characteristic ammonoid faunas led Arkell {1956) to divide the Jurassic world into the Boreal, Tethyan and Pacific faunal realms. According to Arkell the Boreal realm differs from that of earlier authors by not including the Pacific coast of North America. Accordingly, the Pacific realm of Arkell encompassed all these Jurassic faunas bordering the Pacific Ocean. The significance, and even the presence, of this realm has been debated by authors and has not gained wide acceptance. The Tethyan realm, in ArkeU's meaning, existed from the Early Jurassic to the end of the Early Bajocian. Likewise, its faunas were cosmopolitan in areas of marine deposition in middle latitudes (Imlay, 1965). The Boreal realm, however, became distinct in more northerly latitudes from the Tethyan realm during a regression that was initiated in middle or late Bajocian time. In North America this regressive phase coincides approximately with the beginning of the Zuni sequence of Sloss (1963). Such seemingly clinal world-wide faunal changes are reminiscent of modem latitudinal gradients. But several arguments militate against a simple climatic control for the definition of these provinces. Most authors do, however, recognize a slight latitudinal gradient at these times. The distribution of corals during the Middle and Late Jurassic also suggests such a gradienL According to Arkell (1947, p. 83; 1956, p. 615) the main belt of coral reefs during the Jurassic extended across south-central Europe through Switzerland and south Germany between latitudes 46°N and 49°N. The distribution of corals expanded during the Late Oxfordian with "reefs" as far north as Scotland (Arkell, 1935, p. 100; 1956, p. 614). Reef corals are reported as far north as Japan (Tamura, 1961, p. 268) and Salchkin (Beauvais, 1973) duringthe Oxfordian and Kimmeridgian. The data which Arkell accumulated delimiting the distributions of Jurassic corals encompassed many definitions of the "reef" concept. It is equally apparent that some of these genera were probably not entirely indicative of tropical conditions. However, the bulk of these genera have been reviewed for the European Late Jurassic (see Beauvais, 1973, p. 318) and there seems to be a general latitudinal diversity gradient roughly paralleling the ammonite realms of Arkell (Beauvais, 1973, see her Table II, p. 324--325). The many problems inherent in molluscan biogeography are accentuated in the study of groups such as the anthozoans. The Scleractinia are in many respects extremely sensitive to local hydrographic

288 and sedimentological regimes. Many reefs, especially the larger ones, constitute facies in themselves and therefore substantially modify the local sedimentological setting. All these features, together with the fact that few workers have studied coral distributions in the detail which has been accumulated with respect to ammonites, serve to frustrate precise coral biogeography. The corals of the Mexican Late Jurassic occur in thin beds with few taxa represented by many individuals. The specimens however have well-expanded calices and most are globular to subcolumnar in growth form, indicative of favorable conditions for growth. The horizons in which these corals were found also were of limited vertical extent, further indicating intermittent favorable growing conditions. In this regard they are similar to the coral beds of Liesberg and Fringeli in Switzerland and of Auc-sur-Cocon in the French Jura (Beauvais, 1973, p. 320). Furthermore, over much of the Americas, the Jurassic "reefs" seem to have been similar accumulations of isolated individuals seldom forming masses of any great size. These reefs stand in contrast to many of the larger structures of the European Late Jurassic. The causes of Mesozoic provinciality have been extensively discussed. But whatever its origin, Jurassic provinciality was seldom complete. During the Late Jurassic a mixture of Boreal and Tethyan faunas occurred in the middle latitudes. In a similar manner, the Pozo Serna fauna contains mixtures of northern Tethyan and even some Boreal faunal elements. Characteristic ammonoids of the Boreal and Tethyan realms intermingled in California and Oregon at times during the Jurassic. It is also clear that Tethyan faunas of the western interior, Gulf Coastal Plain North America and southern Mexico were, in varying degrees, distinct from one another. Some of these areas even have endemic ammonoid genera. Imlay (1965) has discussed the details of the shifting Boreal Tethyan provincial b o u n d a r y during the Middle and Late Jurassic. But from the present study it is clear that some ammonoid genera (for example Amoeboceras sp. ) characteristic o f - b u t not confined to -- the Boreal realm, extended into northern Mexico. Other genera likewise characteristic of the Boreal realm are known to reach central California during the early to middle Callovian and early Kimmeridgian times. Likewise, ammonites of Tethyan affinities ranged as far north as central California during the Late Callovian and into southwest Oregon during late Oxfordian. Paralleling these shifts in the Boreal realm were the bivalves Gryphaea, Buchia and Inoceramus. Among North American gastropods marked faunal differentiation at high taxonomic levels also occurred. Many of these apparent faunal differences can be shown to be facies related. For example, in the Carmel seas lateral changes in faunal composition have been shown to be a function of water depth and salinity differences (Imlay, 1964a; Sohl, 1965). Likewise, nearshore littoral gastropods, which are abundant in west Texas, are also present in rocks of middle and late Jurassic age in Sonora. Also some apparent faunal differences between areas of Tethyan North America result from comparisons of faunas of differing ages. Faunas described from the Gulf of Mexico region

289

are of late Oxfordian to early Kimmeridgian age; those faunas of the western interior are of late Bajocian to early Oxfordian age. But as we have seen, the late Oxfordian to early Kimmeridgian strata of the Pozo Serna contained elements c o m m o n to b o t h the western interior and eastern Mexico. It seems therefore, that the Sonorian region :represented a faunal transition area between these two distinct zoogeographic and paleogeograpbic areas. The Pozo Serna fauna also shares remarkable affinities with the Oxfordian through Kimmeridgian communities reported from Morocco (Ambroggi, 1963). Similarly, it has been pointed out earlier in this paper (Table II) that the coral fauna of this age from Algeria also displays marked similarities with the Pozo Serna Scleractinia. In this c o n t e x t there seem to be no major differences between Jurassic North American and European coral faunas. Unfortunately, no Jurassic coral-reef structures of any size have been reported from North America. But, potentially reef-building corals have been reported from several scattered localities in North America. Isolated individuals of both solitary and colonial phenotypes have been reported from Oxfordian-aged beds in the Gulf of Mexico region (Burkhardt, 1930, p. 51, 61, 65, 86; Wells, 1942b, pp. 126, 127). Solitary Jurassic corals also occur in the Tylorsville area, California, in beds of Toarcian~ Bajocian and Callovian ages (Hyatt, 1892, pp. 401, 407; Crickmay, 1933, pp. 899, 903). Likewise, hermatypic and ahermatypic corals are known from beds of Bajocian and Callovian age from the western interior United States (Wells, 1942a; Imlay, 1964a, p. C-4). Corals also occur in British Columbia in strata of Bajocian age (Crickmay, 1930, pp. 34, 40). Their northernmost occurrence in North America is in the Cook inlet region, Alaska, in beds of early Jurassic and early Bajocian ages. From the Carmel Formation, Imlay (1964a, p. C-10) reports Astrocoenia at several localities in the middle and lower parts of the unit. Astrocoenia, a colonial, hermatypic genus, has also been found in Montana and Wyoming, but in strata of Bajocian and Kimmeridgian age only (Wells, 1942a; Imlay, 1956, pp. 577, 583, 584). The other reported occurrences of corals in the Jurassic of the western interior United States are in southeastern Idaho in the Wolverine Canyon Limestone Member of the Preuss sandstone (Imlay, 1952, pp. 1740--1742) and in southeast Montana as reworked clasts in the basal part of the Stump sandstone exposed in the Centennial Range (Imlay, 1964a, p. C-4). Few of the authors reporting these occurrences attempted identification and even fewer illustrated the corals found. Therefore, the zoogeographic similarities between these more nortlherly occurrences and the Pozo Serna coral faunas is difficult to evaluate. Because of the lack of monographic work on this group in North America it is often uncertain even if these occurrences represent hermatypic or ahermatypic genera. There does seem to be a slight latitudinal diversity gradient and the more northerly faunas appear to be distinct from the Mexican coral faunas of middle and late Jurassic age. Imlay (1964a, p. C-17) concluded that the Carmel Formation Bivalvia and Gastropoda of Utah were similar to taxa in beds of middle Jurassic age from southeast Idaho, southeast Wyoming and southern Montana. But as Sohl

290

(1965) pointed out, the water during Carmel time was warm and hypersaline. This served to lower the diversity of the Carmel faunas. The molluscan faunas of the Malone Formation and its lateral equivalents in northern Mexico appear to be more normal salinity faunas. Likewise, they have a higher diversity than the Carmel Formation. Also, the Malone Formation (Kimmeridgian--Portlandian) has a better representation of moderate to deep-burrowing bivalves unlike faunas to the north. SUMMARY

In summary, the Pozo Serna coral fauna is in large part conspecific with taxa of comparable age from Tethyan Europe and North Africa. The molluscs were also predominantly Tethyan in affinity, but with more northerly representatives present. About 60% of the taxa comprising the Sonorian molluscan fauna have been reported from Jurassic horizons in the Gulf of Mexico and west Texas regions. The remainder of the bivalves and gastropods have previously been reported from either the westerrl interior United States or Canada. The gastropods, bivalves and corals also suggest water depths and salinities comparable with those of the Malone Formation of west Texas. The lack of substantial data concerning the detailed spatial and temporal distributions of no.n-ammonoid molluscs and corals makes substantive zoogeographic interpretation very difficult. But it is clear that the late Oxfordian to early Kimmeridgian faunas of the Pozo Serna represent a transition. This transition was between faunas of the Gulf of Mexico region and areas south in the Tethyan realm and the molluscan faunas of the western interior Tethyan realm of the United States. Only when a substantial amount of monographic work has been completed on Jurassic Gastropoda and Bivalvia can the zoogeographic affinities of the Pozo Serna be completely analysed. SYSTEMATIC DESCRIPTIONS OF THE POZO SERNA CORAL SPECIES

Suborder ARCHEOCAENIIDAE Alloiteau, 1952 Family A C T I N A S T R A E I D A E Alloiteau, 1952 Genus Allocoeniopsis Alloiteau, 1958 Type-species: Stephanocoenia oolitica Koby, 1906, Koby collection, Museum nationale d'Histoire naturelle de Paris. Allocoeniopsis bernensis (Koby) (Plate I, 2) v. 1885 Astrocoenia bernensis Koby, p. 291, pl. 89, figs. 9, 10. 1897 A. bernensis Koby; Ogilvie, p. 150, Tab. XVI, fig.7. v. 1969 Allocoeniopsis bernensis (Koby); Beauvais, p. 20, pl. 1, fig. 1

291

PLATE I

1. Stylina lobata (Goldfuss), calicinal surface, x 1 2. Allocoeniopsis bernensis ( K o b y ) , calicinal surface, x 1. 3. Latiastraea variabilis ( E t a l l o n ) , calicinal surface, × 1.

292

Holotype: Astrocoenia bernensis Koby, 1885, Koby collection, Museum d'Histoire naturelle de B~le (SwitzerlandL Material: One specimen, Stump collection, University of California, Davis. Dimensions: D = 1 -- 2.5 mm; c to c = 0.5 -- 2.5 mm; Ns = 20 -- 24*. Description: Cerioid corallum with small prismatic corallites directly united to each other by their walls; but because of poor preservation, the colony seems, in places, plocoid. The colony has the form of a lamina and its upper surfaces are plane. The calices are very small and superficial. Costosepta thin, not confluent or only subconfluent. Their upper margin possesses large elliptic granules; they are arranged in radial symmetry including 6 S1 and 6 $2 of equal size, 12 $3 and perhaps some $4 very much smaller than the others. Parathecal wall present. A styliform or elliptic columella present in calicial center. The columella is surrounded by a crown of 12 generally lamellar pali which lie in front of the inner edge of the first two cycles of septa. Endotheca consists of thin and few dissepiments. Stratigraphic and geographic distribution: Upper Argovian and Sequanian of Switzerland; lower Kimmeridgian of Algeria; Tithonian of Czechoslovakia. Family S T Y L O P H Y L L I D A E Volz, 1896 emend. Alloiteau, 1952 Genus Lepidophylliopsis gen. nov. Type-species: Lepidophylliopsis longimana (Quenstedt) Lepidophylliopsis cf. longimana (Quenstedt) (Plate II, 1; Plate III, 1)

? 1858 Lithodendron laeve Quenstedt, p. 711,:Tab. 86, fig.12 (non Michelin 1843). 1882 Lithodendron longimanitm Quenstedt, p. 698, Tab. 170, fig.17. ? 1882 Lithodendron cylindratum Quenstedt, p. 699, Tab. 170, fig.18. ? 1882 Lithodendron laeve Quenstedt, p. 699, Tab. 170, fig.19. 1897 Thecosmilia longimana Quenstedt in Ogilvie, p. 203, Tab. XIV, figs.2--3. Holotype: Lithodendron longimana Quenstedt, 1882, Quenstedt collection, Institut und Museum ft~r Geologie und Pal~ontologie der Universit~t, Tt~bingen, West Germany. Material: One specimen, Stump collection, University of California, Davis. Dimensions: D = 7--15.5 mm; Ns = 60--70. Description: Phacelloid corallum. Cylindroid corallites arranged in bundles, new corallites formed by extracalicinal budding rising up becoming parallel, or nearly parallel, to the parent corallite. External surface covered with a thick and wrinkled epitheca. The corallites not straight but have a wavy and nodulus aspect. Septa (not costo-septa) almost laminar with some perforations * D i m e n s i o n s m e a s u r e d in t h e f o l l o w i n g m a n n e r : D = g r e a t e r d i a m e t e r o f calix; d = s m a l l e r calix d i a m e t e r ; c t o c = d i s t a n c e b e t w e e n t h e calices; h = h e i g h t o f s p e c i m e n ; N s = n u m b e r o f s e p t a ; N c = n u m b e r o f c o s t a e ; d s = d e n s i t y o f s e p t a ; dc = d e n s i t y o f c o s t a e .

293

P L A T E II

1. Lepidophylliopsis longimana ( Q u e n s t e d t ) , s i d e f a c e , x 1. 2, 3. Macgeopsis sonorensis Spo nov°, s i d e f a c e s o f t w o s p e c i m e n s , x 1. 4, 5. Montlivaltia s p . i n d e t . : 4 a , c a l i c e , x 1 ; 4 b , s i d e f a c e o f t h e s a m e s p e c i m e n , x 1 ; 5a, s i d e f a c e o f a n o t h e r s p e c i m e n , x 1 ; 5 b a p e x o f t h e s a m e s p e c i m e n , x 1.

294 localized at the inner edge of the youngest septa (= discontinuous septa); inner edge of the oldest septa are thickened with a spatulate appearance. Endotheca are few; columella fascicular. Septothecal wall formed by the incurving of the external edge of the septa. Remarks: This species does not belong to the genus Thecosmilia because it possesses no costo-septa and because its septa are discontinuous; these features are characteristic of the family Stylophylliidae. In this family the most similar genus is Lepidophyllia Duncan, 1867. LepidophyUia is a plocoid form and our species is a phaceloid one. "Lithodendron loeve Mitchelin" must be referred to the genus Dermoseris, because it possesses costo-septa and synapticula. Consequently we believe it necessary to propose a new genus for the Sonoran material which possesses the characters of the Stylophylliidae and a phaceloid form. The dimensions of the Mexican coral are the same as those of L. longimana Quenstedt, but we have not examined the type-specimen in t h e Quenstedt collection. Stratigraphic and geographic distribution: This species is reported from the Tithonian limestone of Stramberg (Czechoslovakia). Genus Macgeopsis Alloiteau, 1958 Type-species: Epismilia grandis de Fromentel, 1873, Fischer collection Museum nationale d'Histoire naturelle de Paris, France. Macgeopsis sonorensis sp. nov. (Plate II, 2, 3) Holotype: Macgeopsis sonorensis sp. nov., Stump collection, University of California, Davis. Materials: two specimens, Stump collection. Dimensions: D = 13--15 mm; Ns = about 50. Description: Ceratoid corallum. External surface covered by a thick and strongly wrinkled epitheca. Costae only visible by means of wearing away of the epitheca. Septa almost lameUar, the youngest showing some perforations near the inner edge. In some places the septa may be united by their inner edge. Septa arranged in radial symmetry, apparently in unequal and irregular systems. Septal planes are granulated but are not easily observed. Wall is septothecal and covered with the epithecm Columella fasc~cular. Endotheca is present but weakly developed. Remarks: The specimens at hand are poorly preserved but they possess characters which place them in the genus Macgeopsis Alloiteau. Until now this genus was unknown in late Jurassic strata. The four previously known species -- M. grandidieri (de Fromentel), M. subcylindrica Alloiteau, M. tubinata Alloiteau, and M. complanata Alloiteau -- were reported from the Upper Bathonian of Madagascar. All these species have 92--96 septa. The Sonoran material, having the characters of the genus Macgeopsis but possessing only 50 septa, seemingly justify, its description as a new species.

...X

c~

×

0

~

296

Suborder STYLINIDA Alloiteau, 1952 Family STYLINIDAE d'Orbigny, 1851 Genus Stylina Lamark, 1816 Type-species: Stylina echinulata M. Edwards and Haime in Lamark 1816, Milne-Edwards Collection, Museum nationale d'Histoire naturelle de Paris, France. Stylina lobata (Goldfuss) (Plate I, 1; Plate III, 2) Explanaria lobata Mtinster in Goldfuss, p. 110, Tab. 38, fig.8 Astraea tubulifera Phillip0 pl. 3, fig.6. Stylina lobata Goldfuss in Becker et Milaschewitsch, p. 146, Tab. XXXVII, fig.3. 1954 Stylina tubulifera Phillips in Geyer, p. 134, Tab. 9, fig.8. v. 1964 Stylina tubulifera (Phillip) in Beauvais, p. 140, pl. IX, figs. 2--3 and pl. X, fig. 5. 1826 ? 1829 1874

Holotype: Explanaria lobata Mi]nster in Goldfuss 1826, M/lnster collection, Geologisches Pal~iontologisches Institut der Rhein, Fr.-Wilhelms Universit~it, Bonn {West Germany). Material: One specimen, Stump collection, University of California, Davis. Dimensions: D = 2--3 mm; c to c = 3--7 mm; Ns = 10 S1 + 10 $2 + 10 $2 + 20 $3; Nc = 40. Description: Massive plocoid corallum with a strongly convex and irregular upper surface. The corallites are irregularly spaced. Cylindrical and very slightly prominent calices, which are straight, closely set, alternately strong and thin; no confluent costae are surrounding the calices. Intracalicinal and active budding present. Lamellar costo-septa with granular upper edge and with rhopaloid inner edge. Styliform, sometimes slightly elliptical and welldeveloped columella; the longer axis of columella situated in the prolongation of one or two of the S1. The inner edge of the septa are anastomosed with the columella. The wall is parathecal. Remarks: Stylina lobata and S. tubulifera are very similar, they are only distinguished by the size of their calices. Becker and Milaschewitsch stated that the holotype of Stylina lobata Goldfuss had corallites no larger than 3 m m and the diameter of the specimens t h e y described was only 2 or 3 mm; but De Fromentel and Milne-Edwards and Haime gave a diameter of 3--4 m m for this species. In fact S. tubulifera Phillips possesses a diameter of 3--4 mm. The two species appear, therefore, conspecific; S. lobata merely has smaller calices and S. tubulifera has larger ones. Stratigraphic and geographic distributions: The form with smaller calices is found in the Kimmeridgian of Nattheim and of Arnegg (Wiirttemberg); also in the Argovian and Sequanian of eastern France.

297 Suborder A S T R A E O I D A Alloiteau, 1952 Family M O N T L I V A L T I I D A E Dietrich, 1926 emend. Alloiteau, 1952 Genus Montlivaltia Lamouroux, 1821 Type-species: Montlivaltia caryophyllata Lamouroux, 1821 (disappeared) Neotype: Montlivaltia caryophyllata Lamouroux, collection Portes Universit~ de Caen, Calvados, France. Montlivaltia sp. indet. (Plate II, 4, 5) Material: Two specimens, Stump collection, University of California, Davis. Dimensions: D = 18.5 mm; d = 16.5 mm; h = 21 mm; Ns about 60; dc = 8--12 by 5 mm. Description: The specimens seem close to M. caryophyllata, but it has fewer septa. Suborder F U N G I I D A Duncan, 1884 Family L A T O M E A N D R I I D A E Alloiteau, 1952 Genus Brachyseris Alloiteau, 1947 Type-species: Latomeandra morchella Reuss, 1854, Reuss collection, NaturHistorisches Museum, Wien (Austria}. Brachyseris chellalensis Beauvais, 1969 ? 1897 Isastraea gourdani de Fromentel in Ogilvie, p. 192, pl. 15, fig.5. v. 1969 Brachyseris chellalensis Beauvais, p. 28, pl. 3, fig.2. Holotype: Brachyseris chellalensis Beauvais, 1969, Caratini collection, Laboratoire de Pal~ontologie des Invert~br~s, Universit~ Paris VI (France). Material: One specimen, Stump collection, University of California, Davis. Dimensions: D of corallum -- 40 mm; d of corallum = 30 mm; thickness of the corallum = 7--8 mm; D = 3--7 mm; ds = 8 by 20 mm. Description: Small, serio-meandroid corallum. The upper surface is almost flat. Very short calicular series with numerous isolated calices. The isolated calices are irregularly polygonal and directly united by their walls. Both intraand intercalicular budding present. Irregularly perforated septa present. The perforations are more or less regular and they are chiefly localized near the internal edge. Few and thin dissepiments present. Some synapticulae are present. Incomplete synapticulothecal wall, lacking in some places. Thin fascicular columella present. Remarks: This species seems to be conspecific with B. contorta K o b y from the Kimmeridgian of Valfin. But, its septal density is less and the size of the calices is also smaller. Stratigraphic and geographic distribution: Portlandian of Stramberg (Czechoslovakia}; lower Kimmeridgian of Algeria.

298 Genus Latiastraea Beauvais, 1964 Type-species: Latiastraea foulassensis Beauvais, 1964, L. Beauvais collection, Laboratoire de Pa]~ontologie des Invert~br~s, Universit~Paris VI (France) Latiastraea uariabilis (Etallon) (Plate I, 3; Plate Ill, 3) 1858 Microphyllia variabilis Etallon, p. 110. 1861 Latimeandra uariabilis Eta]Ion in De Fromentel, p. 161. v. 1885 Latimeandra variabilis Eta]Ion in Koby, p. 234, pl. 69, figs. 4--5. v. 1964 Latiastraea variabilis (Etallon) in Beauvais, p. 257, pl. 37, fig.4. v. 1969 Latiastraea uariabilis (Etallon) in Beauvais, p. 29, pl. 4, fig.1. Holotype: Microphyllia variabilis EtaUon, 1858 (disappeared). Neotype: Latimeandra variabilis Eta]Ion in Koby, 1885, Koby collection,

Naturhistorisches Museum, Basel (Switzerland). Material: One specimen, Stump collection, Universityof California, Davis. Dimensions: D = 4--9 ram; d = 2.5--6 mm; Ns = 45--57 (70 in the wall); ds = 7--8 by 2 mm. Description: Massive, cerio-meandroidcorallum with short series containing only one or two calicina] centers. Convex and irregularupper surfaces. Thin costae axe visible on the lower surface which is conical and without epitheca. The ca]ices with almost lamellar costo-septa with some perforations localized at their inner edges; these costo-septa are not confluent. Thin, fascicular columella. Endotheca and synapticulae are not abundant. The wall is a stereozone built with some synapticulae,some dissepimentspresent and with the thick external edges of the septa. Remarks: The specimens from the Pozo Serna are remarkably similarto specimens of this species from Algeriaand Va]fin. Stratigraphic and geographic distribution: Sequanian and Kimmeridgian of the French Jura and from Poland; Kimmeridgian of Algeria; Portlandian of Stramberg (Czechoslovakia). LOCALITY DESCRIPTIONS The section measured was located on the southern portion of the Cerro Pozo Serna and east of the road passing through the range. The line of section was up a prominent hill east of this road and continued over the crest of the Pozo Serna. The last exposures were west of a small rancho located on the flank of the Pozo Serna. Distances are given as meters above the section's base. The lithologic terminology used follows Folk (1968). SDSU (San Diego State University) 2 3 1 6 : 5 3 8 m, poorly sorted, calcareous, highly fossiliferous, sublithic arenite. SDSU 2 3 1 7 : 6 0 0 m, u p p e r m o s t portion of massive, tan, fine-grained sublithic arenite. SDSU 2 3 1 3 : 6 6 0 m, thin, tan, fine-grained, very fossiliferous, calcareous, sublithic arenite located immediately above a prominent tuffaceous shale bed. SDSU 2 3 1 0 : 7 5 0 m, locality at base of very coarse-grained, feldspathic litharenite.

299

SDSU 2309:833 careous shale. SDSU 2308:863 SDSU 2307:886 SDSU 2306:923

m, thin sandstone interbedded within thinly laminated calm, locally very fossiliferous, sandy, and tuffaceous tan shale. m, thin, bioclastic, fine-grained quartz arenite. m, thin, bioclastic, sandy limestone.

ACKNOWLEDGEMENTS

Mr. R. Dowlen of San Diego State University assisted the junior author in the collection of fossils from the Cerro Pozo Serna. We also express our appreciation to Dr. R. Imlay of the United States Geological Survey for his remarks on the first draft of this paper and for his continued interest in the project. Helpful discussions concerning the stratigraphy and paleontology of this region were conducted with Drs. R. G. Gastil of San Diego State University and Lopez Ramos, regional geologist for Pemex, Marine Nacional, Mexico City. Ms. Ellen Bailey executed the figures; for her aid we are grateful. REFERENCES Alloiteau, J., 1952. Piveteau, TraitA de PalAontologie. 1, 782 pp. Alloiteau, J., 1958. Monographie des Madreporaires fossiles de Madagascar. Ann. G~ol. Madagascar, XXV: 218 pp. Ambroggi, R. P., 1963. Etude gAologique du versant meridional du Haut Atlas occidental et de la plaine du Souss, Notes M~m. Serv. Carte GAol. Maroc, 1 5 7 : 3 2 1 pp. Anderson, T. H. and Silvers, L. T., 1971. Preliminary history for Pre-Cambrian rocks, Bamori region, Sonora, Mexico (abstr.). Geol. Soc. Am., Abstr. Progr., 3 (2): 72--73. Arkell, W. J., 1935. On the nature, origin, and climatic significance of the coral reefs in the vicinity of Oxford. Q. J. Geol. Soc. Lond°, 9 1 : 7 7 - - 1 1 0 (pl. 6). Arkell, W. J., 1947. The Geology of Oxford° Clarendon Press, Oxford, 267 pp. Arkell, W. J., 1956. Jurassic Geology of the World. Oliver and Boyd, London, 806 pp. Beauvais, L., 1964. Etude stratigraphique et pal~ontologique des formations ~ Madreporaires du Jurassique sup~rieur du Jura et de l'Est du Bassin de Paris. M~m. Soc. G~ol. Ft., N, l l e S~r., XLIII, m~m. no. 1 0 0 : 2 8 8 pp. Beauvais, L., 1973. Upper Jurassic hermatypic corals. In: A. Hallam (Editor) Atlas of Palaeobiogeography. Elsevier, Amsterdam, pp. 317--328 (pls. 1--2). Becket, E. and Milaschewitsch, C., 1874. Die Korallen der Nattheimer Schichten. Palaeontographica, 2 1 : 2 4 3 pp. (tab. XXXVI--LI). Burckhardt, C., 1927. Cephalopodes del Jurassico medio de Oaxaca y Guerrero. Inst. Geol. Mexico Bol., 4 7 : 1 0 8 pp. Burckhardt, C., 1930. Etude synth~tique sur le mAsozoique mexicain. Schweiz. Pal~ontol. Ges. Ahh., 4 9 - - 5 0 : 2 8 0 pp. Caratini, C. and Beauvais, L., 1969~ Les Polypiers du Kimmeridgien infArieur de ChellalaReibell. (D~partement du M~dAa-AlgArie). Publ. Serv. GAol. Alg~rie, N, 11e S~r., Bull., 39 : 19--39. Cooper, G. A. and Arellano, A. R. V., 1946. Stratigraphy near Caborca, northwest Sonora, Mexico. Am. Assoc. Pet. Geol. Bull., 30: 606--611. Cox, L. R., 1965. Jurassic Bivalvia and Gastropoda from Tanganyika and Kenya. Bull. Br. Mus. Nat. Hist., Suppl. 1, 213 pp. Cragin, F. W., 1905. Paleontology of the Malone Formation of Texas. Bull. U.S. Geol. Surv., 2 6 6 : 1 7 2 pp.

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