Contributions to Zoology, 75 (3/4) 121-132 (2006) Late Cretaceous dwarf decapods from Guerrero, southern Mexico and their migration patterns René H.B. Fraaije1 , Francisco J. Vega2, Barry W.M. van Bakel1 and Luis M. Garibay-Romero2,3 1 Oertijdmuseum De Groene Poort, Bosscheweg 80, NL-5283 WB Boxtel, The Netherlands, E-mail: info@ oertijdmuseum.nl; 2 Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510, México, D. F., México, E-mail: [email protected]; 3 Unidad Académica de Ciencias de la Tierra, Universidad Autónoma de Guerrero, Ex Hacienda de San Juan Bautista, 40200, Taxco el Viejo, Guer- rero, México Key words: Dwarfi sm, decapoda, Cretaceous, new species, Mexico Abstract Introduction Two new brachyuran species are described for the Upper Cre- The Mexcala Formation crops out in southern Mexi- taceous Mexcala Formation, Guerrero State, Mexico. Longu- co. It is reported from the states of Puebla, Morelos sorbis quadratus new species (Coniacian, Temalac region) is the oldest and southernmost record for the genus. Xanthosia and Guerrero. This lithostratigraphic unit is a com- zoquiapensis new species (Campanian, Zoquiapa region) is the plex sedimentary sequence, as it includes deep-wa- fi rst record for the genus in Mexico. In addition, the age for ter facies as well as very shallow marine sediments, Costacopluma bishopi Vega and Feldmann is discussed (Co- with lateral and vertical changes, overlying Lower niacian, Temalac region) and represents the oldest and south- Cretaceous limestones of the Morelos Formation. ernmost record for Cretaceous representatives of this genus in North America. All specimens are considerably smaller com- Defi ned by Fries (1960) as a 1,220 m fl ysh-like se- pared to other species of the same genera and are interpreted as quence of pelagic limestones and marls at its base, the fi rst example of brachyuran dwarfi sm in the geological the Mexcala Formation grades upward to shales, record. These species add new insight into possible migratory siltstones, sandstones, and conglomeratic lenses. An routes during the Late Cretaceous. Within Longusorbis, a north- early Coniacian age was proposed by Bohnenberg- western migratory route is documented from the Coniacian in Mexico to the Campanian - Maastrichtian of the west coast of Thomas (1955) for the Mexcala Formation. Cserna North America (Canada), whereas within the genus Xanthosia, (1965) named, described, and interpreted the litho- a western migratory route from the Albian of Europe to the logical units in the area. More recent stratigraphic Campanian of Mexico is indicated. Costacopluma migrated east reports have detailed facies distribution, sedimen- and north to the west coast of Africa, southeast North America tology, diagenesis, and subdivided the formation into and Greenland. members (Ontiveros-Tarango, 1973; Cserna et al., 1980; Ortega-Gutierrez, 1980; González-Pacheco, Contents 1988; Hernández-Romano et al., 1998; Hernández- Romano, 1999, among others). Most authors agree Introduction .................................................................................... 121 that deposition of the fi rst sediments of the Mexcala Systematic palaeontology ........................................................... 124 Formation occured in Turonian times, while the last Longusorbis quadratus n. sp. ............................................... 124 marine deposits are found in late Maastrichtian silt- Xanthosia zoquiapensis n. sp. .............................................. 125 stones and sandstones of the study area (Alencáster, Costacopluma bishopi Vega and Feldmann, 1992 ........... 128 1980; Perrilliat et al., 2000). Discussion ...................................................................................... 128 Acknowledgements ...................................................................... 129 Fossil content of the Mexcala Formation includes References ...................................................................................... 129 foraminifera (Hernández-Romano et al., 1997; Lang Downloaded from Brill.com10/07/2021 10:16:36PM via free access 122 Fraaije et al – Dwarf decapods from Mexico Fig. 1. Location map of the two studied localities in northeast Guerrero state, Southern Mexico. and Frerichs, 1998; Aguilera-Franco, 2000; 2003; tion in Guerrero. Locality IGM-2448 is found 5 Aguilera-Franco et al., 2001; Aguilera-Franco and km north of Temalac, a small village located 35 km Hernández-Romano, 2004; Aguilera-Franco and Al- southeast of Iguala, Guerrero. Locality IGM-3557 lison, 2005), gastropods and bivalves (Böse, 1923; is found near the town of Zoquiapa, northeast Alencáster, 1980; Alencáster et al., 1987; Perrilliat Guerrero (Fig. 1). and Vega, 1996; 2001; Garibay-Romero et al., 1998; Vega and Feldmann (1992) described Costaco- 2002; Kiel et al., 2000; 2002; Perrilliat et al., 2000; pluma bishopi based on very small specimens found Kiel and Perrilliat, 2001; 2004; Reyes-Prieto, 2004; on light-brown siltstones of locality IGM-2448. Vermeij et al., 2004); ammonoids (Burckhard, 1919; They proposed a Maastrichtian age for these sedi- González-Arreola, 1977); crustaceans (Vega and ments, but from recent studies on foraminifera and Feld mann, 1992); sea urchins and planktic crinoids rudists it is now clear that the age is Coniacian. Al- (Barrios-Matías, 1992; Sánchez-Rodríguez, 1997; though no complete section was measured due to Rosendo-Brito et al., 2002); fi shes (Garibay-Romero intense folding and covered outcrops, a composite and Alvarado-Ortega, 2004; Alvarado-Ortega et al., column of 25 m of limestones, siltstones and sand- 2004, and dinosaur ichnites (Ferrusquía-Villafranca stones has been constructed (Fig. 2). The base of et al., 1993). this sequence is represented by an alternation of The brachyuran decapods described herein were dark-brown limestones and light-brown siltstones collected at two localities of the Mexcala Forma- and mudstones, with a rich content of fossils (fo- Downloaded from Brill.com10/07/2021 10:16:36PM via free access Contributions to Zoology, 75 (3/4) – 2006 123 raminifera, rudists, ostreids, echinoids and plant remains). This locality is interpreted to be the tran- sitional contact between the Morelos and Mexcala formations, marking the fi rst 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 Touca- sia, of possible Turonian age. Based on foraminifera studies, Lang and Frerichs (1998) proposed a Coni- Fig. 2. Composite stratigraphic section at localities IGM-2448 acian age for the fi rst Laramide pulses in the study and IGM-3557, showing lithology and thickness of Morelos and area, marking as a change in the sedimentary regi- Mexcala formations. men from carbonates to fi ne siliciclastic sediments. Based on identifi cation of the Di ca rinella concavata cies, was found both in the uppermost limestone biozone, Lang and Frerichs (1998) placed the fi rst layers of the Morelos Formation, and in siltstones of strata of the Mexcala Formation in the Coniacian the lower part of the Mexcala Formation. It is im- just above what they called the Cuautla Formation portant to note that Lang and Frerichs (1998) pro- limestones. An interesting fact is that one of the spe- posed that the southern margin of the North cies here reported, Longusorbis quadratus new spe- American Plate in northern Guerrero was isolated Downloaded from Brill.com10/07/2021 10:16:36PM via free access 124 Fraaije et al – Dwarf decapods from Mexico from the Coniacian-early Santonian Pacifi c Ocean. and outwardly directed, massive outer orbital spine. However, occurrence of the other representative Including the outer orbital spine, the lateral margin species of Longusorbis, L. cuniculosus Richards, is armed with 4 equally outwardly directed spines 1975 in upper Campanian – lower Maastrichtian decreasing in size posteriorly. Regions are distinct sediments of the Shelter Point locality at Vancouver and demarcated by furrows. The anterior process of Island, Canada (Richards, 1975; Schweitzer et al. the relatively small mesogastric region extends al- 2003), clearly indicates a northwest migration of most to the frontal, the distinct top fading into the this genus during the Late Cretaceous. The upper- downturned sulcus of the rostrum. The most promi- most part of the Mexcala Formation is represented nent furrow is the cervical furrow extending sinu- by siltstones and sandstones of early to late Maas- ously from the base of the mesogastric lobe ending trichtian age in the Temalac – Mitepec area (Perril- below the outer orbital spine defi ning the posterior liat et al., 2000). margin of the swollen protogastric and hepatic re- The Mexcala Formation is represented by shal- gions. The forwardly-directed hepatic furrow sepa- low marine light brown siltstones nearby Zoquiapa rates the protogastric and hepatic regions. The width town, northeastern Guerrero, where a composite of the urogastric region is smaller than the maxi- section of 25 m (Fig. 2) contains several species of mum width of the mesogastric region. The broad gastropods and bivalves of Campanian age. shield shaped cardiac region is separated from the The material studied is deposited in the Colec- branchial regions by broad shallow furrows. The ción Nacional de Paleontología, Instituto de Ge- mesobranchial lobes bear three tubercles forming a ología, Universidad Nacional Autónoma de México. triangle. The posterolateral margins