EARLY EOCENE OSTREIDS FROM THE ADJUNTAS FORMATION, DIFUNTA GROUP, NORTHEASTERN MEXICO MARIA DEL CARMEN PERRILLIAT and FRANCISCO J. VEGA INSTITUTO DE GEOLOGiA UNIVERSIDAD NACION AL AUTONOMA DE MEXICO MEXICO , D.F. , MEXICO I. ABSTRACT Eocene beds of the Mesogean region a nd Four ostreid species from the Lower Eo­ the southern United States (Raulin and cene Adjuntas Formation are reported, Delbos, 1855; Gardner, 1927; Vyalov, 1936, two of them described herein. The stratig­ 1948). A wide distribution of ostreid taxa is raphic range and paleoenvironmental in­ proposed to be the result of island-hop­ terpretation of the material reinforces pre­ ping, when explaining interoceanic distri­ vious conclusions on deltaic plain deposits butions (Stenzel, 1971). Ostreid beds of the of Lower Eocene age for the Adjuntas For­ Adjuntas Formation are included in a del­ mation. taic plain sequence, chiefly of red and green beds. The Adjuntas comprises II. INTRODUCTION mudstones, siltstones, and sandstones, with a thickness of 260 meters. Some gray The Adjuntas Formation has proved to beds at the base and the top of the forma­ be the youngest and one of the most fos­ tion include Venericardia (Venericor) siliferous formations of the Difunta Group. planicosta and Turritella mortoni postmor­ Previous works have been devoted to the toni, indicative of a Lower E ocene age. description of new molluscan taxa, found These gray beds have been in terpreted as in the deltaic plain and inner shelf deposits inner shelf deposits (Vega and Perrilliat, of this formation (Vega and Perrilliat, 1989b). The green beds of this formation 1989a; 1989b; 1992). The first formal stratig­ contain a great abundance of freshwater raphic report on the Difunta Group consi­ gastropods, interpreted as inhabitants of dered the Adjuntas Formation to be of deltaic plain deposits, below the phreatic Maastrichtian age, based on the presence level (Vega and Perrilliat, 1992). of Exogyra costata Say (McBride et al., The lithology of the Adjuntas Formation 197 4). Subsequently, Vega and Perrilliat varies laterally and between the synclines (1989a) reported in the Adjuntas Forma­ where it outcrops. L ithology at localities tion the presence of Venericardia (Ven­ IGM 1973 and IGM 2645 is more sandy than ericor) planicosta Lamarck and Turritella at the type section (McBride et al., 197 4), mortoni postmortoni Conrad, index fossils with almost no green beds, and the os­ for the Ypresian of the Gulf Coastal Plain treids are distributed in well-defined strata (Shimer and Shrock, 1944). Without de­ of 1 to 2 meters thickness. Some sandstone tailed morphological examination, the os­ beds between the ostreid banks contain treid species here reported superficially tree-trunks in vertical position, replaced resemble Exogyra costata. It is probable by light brown sandstone. Some red beds that previous authors confused this Creta­ include leaf remains. The ostreid banks ceous grypheid with the Tertiary ostreids seem to be monospecifi c and the valves, al­ present in the Adjuntas Formation out­ though very near one to each other, rarely crops (McBride et al., 1974; Wolleben, are found to be cemented. A coarse sand­ 1977). stone matrix of brown color includes all the The Adjuntas Formation has its most ex­ ostreid remains. In other parts of the locali­ tensive outcrop areas in the synclines of ty 2645, the left valves of the ostreids were the La Popa basin (Text-figure 1). The os­ found still attached to bored limestone treids here described were collected at fragments. This could represent an es­ three localities in the Escondida and Del­ tuary , near the distrib~tary channel~, at gado synclines (Text-figure 2). whose ed ges the ostre1d banks prolifer­ ated. A subsidence period in the La Popa III. STRATIGRAPHY AND basin generated conditions for the deposi­ PALEO ENVIRONMENT tation of the Adjuntas Formation sedi­ The ostreid subgenus Turkostrea has ments, forming marshes, swamps, and es­ been reported in the Lower and Middle tuaries, in which a diverse molluscan 15 16 Tulane Studies in Geology and Paleontology Vol. 26 fauna developed during Lower Eocene Suborder OSTREINA Ferussac, 1822 time. Superfamily OSTREACEA Rafinesque, 1815 IV. ACKNOWLEDGMENTS Family OSTREIDAE Rafinesque, 1815 We are grateful to M. en C. Zoila Graciela Castillo R. of the Instituto de Genus OSTREA Linnaeus, 1758 Ciencias del Mary Limnologia, UN AM, for Ostrea Linnaeus, 1758, Systema naturae per tria her kind help in comparison of the fossil regna naturae, p. 696. material with collections of Recent ostreids Type species: Ostrea edulis Linnaeus; Re­ at her laboratory. Mr. Luis Burgos P. cent, England; by subsequent designation, Chil­ cheerfully helped in doing the figures. Dr. dren, 1823. E.H. Vokes (Tulane University) made val­ uable comments and critically reviewed Subgenus TURKOSTREA Vyalov, 1936 the manuscript. Turkostrea Vyalov, 1936, Acad. Sci. USSR, Comptes Rendus (Doklady), new ser., v. 4 V. SYSTEMATIC DESCRIPTIONS (13), no. 1, p. 18. Order PTERIOIDA Newell, 1965 Type species: Ostrea turkestanensis Roma- 0 30Km TEXAS San Antonio 0 I •• Eagle Pass CHIHUAHUA/ Piedras Negras za•N . \. COAHUILA \. Monclovao . -·\ .,r.. ./ .) Z6°N ._,..._rownsvi Ile Torreon .! PARRAs ~ f B4s1N ~ \ ../., ___. .Saltillrr '· DURANGO \... -v· {\\ "----... j' . ~ / ·-~ . ../ . /'. TAMAULIPAS ./. ./ ZACATECAS ./ \ ~ / ./ I02°W /·..JS.L.P. \ 100°w\ Cd. Victoria Text-figure 1. Generalized location map of Difunta Group in northeastern Mexico, showing distribution of Parras and La Popa basins in Coahuila and Nuevo Leon states. Enclosed area in center is shown in text-figure 2. No. 1 Adjuntas Formation Ostreids 17 novsky Ostrea strictiplicata Raulin and De­ gauche ornee d'environ 60 plis fins; surface lbos; Central Asia (USSR, Uzbekistan); Mon­ d'adherence petite; crochet, 116 ; canal assez tagne-Noire (Aude), France; Middle Eocene; by profond, 1/2; bourrelets saillants; sillons pro­ original designation. fonds se continuant clans la valve par des points; expansions bien developpees. Valve droite un peu convexe. Impression musculaire grande au 0STREA (TURKOSTREA) STRICTIPLICATA centre de la moitie posterieure." (Raulin a nd Raulin and Delbos, 1855 Delbos, 1855) Plate 1, figures 2, 5; Type locality: Terrain a Nummulites de la Plate 2, figures 2, 3 Montagne Noire (Aude), France. Ostrea strictiplicata RAULIN and DELBOS, Occurrence: Lutetian, Paris Basin; Lower Eo­ 1855, Bull. Soc. Geol. France, ser. 2, v. 12, p. 1158. cene, between Moussolens and Montolieu (Aude); Lutetian, Egypt, Transylvania Basin, Ostrea (Ostrea) moussoulensis ASTRE, 1922, Perse and Turkestan. Bull. Soc. Hist. Nat. Toulouse , v. 50, p. 161 , Figured specimens: Plate 1, figs. 2, 5, IGM pls. 1-6. 6312 ; length 24.9 mm, height 38.4 mm, diameter Ostrea multicostata Deshayes. KIEH, 1930, Bull. (both valves) 22.8 mm. Plate 2, figs. 2, 3, IGM Soc. Geol. France, ser. 4, v. 30, p. 89. 6311; length 75.4 mm, height 91.2 mm, diameter "Test epais. Coquille arrondie-ovalaire. Valve (both valves) 60. 7 mm; locality IGM 1973. 0 5 10 Km t::=::=::::t:=:===~ 1 26° 15 FOSSIL LOCAL ITIES ··t- IGM -1973 ( 1 ) +IGM-2645 ( 2 ) -tlGM-2647 Text-figure 2. Location ma p of the three ostreid localities in the Escondida and Del­ gado synclines, La Popa basin, Nuevo Leon , Mexico. Tulcrne Studies in Geology and Paleontology Vol. 26 Other occin-rences. IGM locality nos. 1973, right valve finely pitted, the pitting persistent in 2G47. some individuals around the entire inner mar­ Discnssicm: The complete synonymy gm. Adductor scars rather small. crescentic, not deeply excavated. posterior and below the me­ of this specir·s 1s in Kieh (1930, p. 89), who dian horizontal." (Gardner, 1927) made a revision of the subspecies and var­ ieties that have been assigned to this spe­ Description: Shell of large size, inequivalve cies. and which need not to be repeated and inequilateral, ovate-trigonal. The left valve has 20 lo 25 radial ribs, some of them bifurcated here. Within the Mexican specimens from in the ventral region. In the attachment area the locality IGiv1 1973, we have smaller speci­ surface of the shell is smooth. The ligament area mens than those called by Kieh as ex­ is broad, the catachomata are present in almost ogyroidals. We also have the gryphaeiform all the margin of the shell. The adductor muscle type; the Asiatic specimens show this fea­ is reniform. The right valve has an ornamenta­ ture also. while those from the meridional tion of concentric lamellae. The anachomata are region of France and northern Africa have present all around the margin of the shell. The this feature only partially. Kieh (1930, p. adductor muscle is big and deep. The Quenstedt 87) mentioned that this species lived in lit­ muscle is present. toral zones or shores during the Middle Eo­ Type material. USNM 369239. Right valve; al­ cene, having numerous varieties, following titude 10.6 mm, latitude 6.1 mm, semi-diameter various living conditions. 2.4 mm. Left valve of another individual; al­ titude 10.5 mm, latitude 6.95 mm, semi-diameter :3.45 mm. OsTRr:A (TuHKOSTH.I<:A) DUVALI Type locality: Austin-Elgin Ferry road, 1 mile Gardner, 1927 north of Austin-Bastrop Highway, Bastrop Plate I, figures 1, 6; County, Texas. Plate 2, figures 1, 6 Occurrence: Wilcox Group, probably the Ostren d11l•al1 GARDNER. 1927. Jour. Washing­ Indio Formation, Eocene. ton Acad. Sci.. v. 17, no. 14, µ. 366. rigs. 1-4. Figured specimens: Plate 1, figs. 1, 6, IGM 6314; length 80.1 mm, height 78.3 mm, diameter "Shell of moderate dimensions for the genus, (left valve) 34.0 mm. Plate 2, figs. 1, 6, IGM 6315; ineqwv<ilve. ovate-trigonal in outline, fre­ length 78.4 mm, height 93. 7 mm, diameter (right quently with <1 large attachment area and rel8- valve) 24.6 mm; locality IGM 2645. tively broad in consequence. Surface layer de­ corticated. right valve built uµ of overlapping concentric lamellae so that the shell thins to­ Discussion: The Mexican specimens are ward the ventral margrn, and would be heaviest broader than the one described and fi­ in the umbona1 area were it not for the en­ gured from Texas, possibly due to the croachment of the ligament area; a subcutane­ polymorphism of the oysters.