The Mexcala Formation crops out in southern Mexico. It is reported from the states of Puebla, Morelos and Guerrero. This lithostratigraphic unit is a complex sedimentary sequence, as it includes deep-water facies as well as very shallow marine sediments, with lateral and vertical changes, overlying Lower Cretaceous limestones of the Morelos Formation. Defined by Fries (1960) as a 1,220 m flysh-like sequence of pelagic limestones and marls at its base, the Mexcala Formation grades upward to shales, siltstones, sandstones, and conglomeratic lenses. An early Coniacian age was proposed by Bohnenberg-Thomas (1955) for the Mexcala Formation. Cserna (1965) named, described, and interpreted the lithological units in the area. More recent stratigraphic reports have detailed facies distribution, sedimentology, diagenesis, and subdivided the formation into members (Ontiveros-Tarango, 1973; Cserna et al., 1980; Ortega-Gutierrez, 1980; González-Pacheco, 1988; Hernández-Romano et al., 1998; Hernández-Romano, 1999, among others). Most authors agree that deposition of the first sediments of the Mexcala Formation occured in Turonian times, while the last marine deposits are found in late Maastrichtian siltstones and sandstones of the study area (Alencáster, 1980; Perrilliat et al., 2000).
Fossil content of the Mexcala Formation includes foraminifera (Hernández-Romano et al., 1997; Lang and Frerichs, 1998; Aguilera-Franco, 2000; 2003; Aguilera-Franco et al., 2001; Aguilera-Franco and Hernández-Romano, 2004; Aguilera-Franco and Allison, 2005), gastropods and bivalves (Böse, 1923; Alencáster, 1980; Alencáster et al., 1987; Perrilliat and Vega, 1996; 2001; Garibay-Romero et al., 1998; 2002; Kiel et al., 2000; 2002; Perrilliat et al., 2000; Kiel and Perrilliat, 2001; 2004; Reyes-Prieto, 2004; Vermeij et al., 2004); ammonoids (Burckhard, 1919; González-Arreola, 1977); crustaceans (Vega and Feldmann, 1992); sea urchins and planktic crinoids (Barrios-Matías, 1992; Sánchez-Rodríguez, 1997; Rosendo-Brito et al., 2002); fishes (Garibay-Romero and Alvarado-Ortega, 2004; Alvarado-Ortega et al., 2004, and dinosaur ichnites (Ferrusquía-Villafranca et al., 1993).
The brachyuran decapods described herein were collected at two localities of the Mexcala Formation in Guerrero. Locality IGM-2448 is found 5 km north of Temalac, a small village located 35 km southeast of Iguala, Guerrero. Locality IGM-3557 is found near the town of Zoquiapa, northeast Guerrero (Fig. 1).
Vega and Feldmann (1992) described Costaco-pluma bishopi based on very small specimens found on light-brown siltstones of locality IGM-2448. They proposed a Maastrichtian age for these sediments, but from recent studies on foraminifera and rudists it is now clear that the age is Coniacian. Although no complete section was measured due to intense folding and covered outcrops, a composite column of 25 m of limestones, siltstones and sandstones has been constructed (Fig. 2). The base of this sequence is represented by an alternation of dark-brown limestones and light-brown siltstones and mudstones, with a rich content of fossils (foraminifera, rudists, ostreids, echinoids and plant remains). This locality is interpreted to be the transitional contact between the Morelos and Mexcala formations, marking the first pulses of terrigenous sediments. Some well-preserved rudists are found on limestone layers of the Morelos Formation at the base of the section, and belong to the genus Toucasia , of possible Turonian age. Based on foraminifera studies, Lang and Frerichs (1998) proposed a Coniacian age for the first Laramide pulses in the study area, marking as a change in the sedimentary regimen from carbonates to fine siliciclastic sediments. Based on identification of the Dicarinella concavata biozone, Lang and Frerichs (1998) placed the first strata of the Mexcala Formation in the Coniacian just above what they called the Cuautla Formation limestones. An interesting fact is that one of the species here reported, Longusorbis quadratus new species, was found both in the uppermost limestone layers of the Morelos Formation, and in siltstones of the lower part of the Mexcala Formation. It is important to note that Lang and Frerichs (1998) proposed that the southern margin of the North American Plate in northern Guerrero was isolated from the Coniacian-early Santonian Pacific Ocean. However, occurrence of the other representative species of Longusorbis, L. cuniculosus Richards, 1975 in upper Campanian – lower Maastrichtian sediments of the Shelter Point locality at Vancouver Island, Canada (Richards, 1975; Schweitzer et al. 2003), clearly indicates a northwest migration of this genus during the Late Cretaceous. The uppermost part of the Mexcala Formation is represented by siltstones and sandstones of early to late Maastrichtian age in the Temalac – Mitepec area (Perrilliat et al., 2000).
The Mexcala Formation is represented by shallow marine light brown siltstones nearby Zoquiapa town, northeastern Guerrero, where a composite section of 25 m (Fig. 2) contains several species of gastropods and bivalves of Campanian age.
Fig. 2. Composite stratigraphic section at localities IGM-2448 and IGM-3557, showing lithology and thickness of Morelos and Mexcala formations.
The material studied is deposited in the Colección Nacional de Paleontología, Instituto de Geología, Universidad Nacional Autónoma de México. Types are included in the Type Collection and classified under the acronym IGM.