Late Cretaceous dwarf decapods from Guerrero, southern Mexico and their migration patterns

Two new brachyuran species are described for the Upper Cretaceous Mexcala Formation, Guerrero State, Mexico. Longusorbis quadratus new species (Coniacian, Temalac region) is the oldest and southernmost record for the genus. Xanthosia zoquiapensis new species (Campanian, Zoquiapa region) is the ﬁ rst record for the genus in Mexico. In addition, the age for Costacopluma bishopi Vega and Feldmann is discussed (Co-niacian, Temalac region) and represents the oldest and southernmost record for Cretaceous representatives of this genus in North America. All specimens are considerably smaller com-pared to other species of the same genera and are interpreted as the ﬁ rst example of brachyuran dwarﬁ sm in the geological record. These species add new insight into possible migratory routes during the Late Cretaceous. Within Longusorbis , a north-western migratory route is documented from the Coniacian in Mexico to the Campanian - Maastrichtian of the west coast of North America (Canada), whereas within the genus Xanthosia , a western migratory route from


Introduction
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. Defi ned by Fries (1960) as a 1,220 m fl ysh-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 fi rst 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).
The brachyuran decapods described herein were collected at two localities of the Mexcala Forma-tion 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 Costacopluma 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 (fo- cies, 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 raminifera, rudists, ostreids, echinoids and plant remains). This locality is interpreted to be the transitional 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 Toucasia, of possible Turonian age. Based on foraminifera studies, Lang and Frerichs (1998) proposed a Coniacian age for the fi rst Laramide pulses in the study area, marking as a change in the sedimentary regimen from carbonates to fi ne siliciclastic sediments. Based on identifi cation of the Di ca rinella concavata biozone, Lang and Frerichs (1998) placed the fi rst 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 spe-from the Coniacian-early Santonian Pacifi c 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.
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 classifi ed under the acronym IGM.

Longusorbis quadratus new species
Description. Relatively small carapace, quadrate in outline, widest across the anterior third located at the outer orbital spines, weakly convex transversely and longitudinally. Rostrum medially sulcate and downturned, broadly rimmed and bicornate base. Wide orbits cut by two fi ssures, inner third concave, outer two-third fi rst sinuous ending in a forwardly and outwardly directed, massive outer orbital spine. Including the outer orbital spine, the lateral margin is armed with 4 equally outwardly directed spines decreasing in size posteriorly. Regions are distinct and demarcated by furrows. The anterior process of the relatively small mesogastric region extends almost to the frontal, the distinct top fading into the downturned sulcus of the rostrum. The most prominent furrow is the cervical furrow extending sinuously from the base of the mesogastric lobe ending below the outer orbital spine defi ning the posterior margin of the swollen protogastric and hepatic regions. The forwardly-directed hepatic furrow separates the protogastric and hepatic regions. The width of the urogastric region is smaller than the maximum width of the mesogastric region. The broad shield shaped cardiac region is separated from the branchial regions by broad shallow furrows. The mesobranchial lobes bear three tubercles forming a triangle. The posterolateral margins are long and converging posteriorly, slightly concave at the probable point of attachment of the fi fth pereiopods.
Etymology. Refers to carapace outline.
Remarks. This new species differs from L. cuniculosus in having a quadrate outline, a spinose postero-lateral margin, a signifi cantly narrower urogastric region and a more robust front.
This genus migrated at the end of the Cretaceous to the north where, in the late Campanian-early Maas trichtian, the closely related L. cuniculosus reaches up to 5 cm in carapace width (4 times larger than L. quadratus n. sp.) in the Shelter Point locality at Vancouver Island, Canada (Richards, 1975;Schweitzer et al. 2003).
An eastward migration has also to be taken in account considering the close evolutionary relationship with the genus Carcineretes known from the Maastrichtian of SE Mexico, Belize and Jamaica (Vega et al. 1997;2001). Longusorbis and Carcineretes have a very similar quadratic outline, bicornate and downturned rostrum, orbital incisions, carapace groovearrangement and posterior margin morphology.
The genera Withersella, Torynomma and Binkhorstia, all belonging to the Torynommidae (van Bakel et al., 2003), also have very similar to almost identical dorsal carapace morphologies as seen in the carcineretids. Distinction often can only be made on the ventral characters. Torynommidae differ only from the Carcineretidae in having a dorsally orientated, strongly reduced fourth and/or fi fth pair of pereiopods, and being much smaller in overall size. Either the Carcineretidae and the Torynommidae are evolutionary very closely related or the dorsal similarities are a matter of convergence. In the fi rst case the Torynommidae should also be placed in the Superfamily Portunoidea. To solve this matter more study and material is needed.
The carcineretids and torynommids, although relatively successful during the Cretaceous, fi nally didn't cross the K/T boundary. It seems that they couldn't cope with the thinner, more spinose and hexagonal carapaces (better swimming morphology) and much larger orbits (better predatory morphology) of the Late Cretaceous radiating genera like Xanthosia and Cretachlorodius (Fraaye, 1996). Distribution of the two known species of Longusorbis is given in Figure 5.

Xanthosia zoquiapensis new species
Description. The carapace is subhexagonal in outline, almost fl at longitudinally and gently inclined at the margins transversely, length about two-thirds the width. The frontal area is bilobed and slightly extended beyond the orbits; a deep median sulcus divides around the anterior part of the mesogastric process. The orbits are extremely large, elliptical and outward facing. The rimmed orbital margin bears two short notches. Orbitofrontal margin covers about 70% the total carapace width. The anterolateral margin is straight starting at the large, blunt outer orbital spine, and regularly divided by four notches. The widest part of the carapace is at the epibranchial angle. The posterolateral margin is clearly convex and indented by a relatively long, frontally directed mesobranchial notch. Carapace separated into distinct regions by shallow groove system. Cervical furrow strongly sinuous. Posterior margin concave, bordered with distinct ridge and about half the total carapace width.
3 2 X. buchi towards the Campanian species from the American Western Interior and Atlantic Coastal Plain. The occurrence of X. zoquiapensis n. sp. in the Campanian of Mexico perfectly fi ts into this paleomigratory model. Figure 5 illustrates distribution of species of Xanthosia during the Late Cretaceous.
Superfamily Retroplumoidea Gill, 1894Family Retroplumidae Gill, 1894Genus Costacopluma Collins and Morris, 1975 Type species. Costacopluma concava Collins and Morris, 1975 by original designation. Vega and Feldmann, 1992 Figures 3.5, 3.6, 4.3 Description. Carapace small, ovate in transverse section, widest at level of mesobranchial areas, with three prominent, rounded transverse ridges. Anterolateral margins straight, with a small spine at level of cervical groove; posterolateral margins rounded; posterior margin straight; anterior margin nearly straight, slightly concave above orbits, bordered by two prominent, forward directed spines. Orbits large, rounded. Rostrum prominent, bilobulate. Fused protogastric and mesogastric lobes form anterior transverse ridge, slightly inclined backwards in central part to ovate mesogastric lobe. Cervical groove deep, parallel to protogastric lobes, curved at level of mesobranchial lobes. Second ridge formed by fusion of epibranchial and mesobranchial lobes, of nearly uniform width, inclined posteriorly to base of mesogastric lobe where they become narrower. Posterior ridge formed by metabranchial lobes and cardiac/intestinal regions. Metabranchial lobes perpendicular to carapace length, half as long as epibranchial/mesobranchial lobes. Cardiac region subtrapezoidal, intestinal region narrow at base of cardiac region.

Remarks. In documenting Costacopluma concava
from the Upper Cretaceous of Nigeria, Collins and Morris (1975) mentioned presence of two paratypes from the Coniacian of the Awgu Limestone, Abakaliki Province. However, none of these paratypes is illustrated and although their morphology may resemble Costacopluma, it is not clear if these specimens do belong to this genus. Thus, C. bishopi is the fi rst well-documented report for Costacopluma in Coniacian beds and may represent the oldest occurrence of a widely distributed genus during Late Cretaceous -Northeastern Mexico, Greenland, Nigeria, Northern India -(Collins and Morris, 1975;Gaetani et al., 1983;Vega and Perrilliat, 1989;Collins and Rasmussen, 1992;Vega and Feldmann, 1992;), and the Paleocene -California, Venezuela, Senegal, Brazil, Argentina -(Collins et al., 1994;Feldmann and Martins-Neto, 1995;De Araújo-Távora and Da Cruz-Miranda, 2004;Nyborg et al. 2003) (Fig. 5).
C. binodosa from the upper Campanian of Greenland was described by Collins and Rasmussen, 1992 on the basis of one incomplete specimen. It is larger than C. bishopi and bears straight lateral margins.
If the genus raised in America, it must have migrated eastwards to Africa, and northwards to Greenland. African retroplumids also migrated to the east, to reach the north part of India, and by Paleocene times, Costacopluma prevailed in the Paleocene seas of Senegal (Fig. 5). Although abundant in northeastern Mexico during Maastrichtian times, the genus vanished in this area by Paleocene times, and dispersed west to California and south to the north and central parts of South America (Fig. 5).
During Eocene times in Europe, Costacopluma may have given rise to the extant genus Retropluma Gill, 1894 (see Via, 1982), known from deep-water settings of the modern Indopacifi c sea (Saint Laurent, 1989). Vega and Feldmann (1992) described Costacopluma bishopi on the basis of eight carapaces. Because of the extremely small size (average width: 4.9 mm) of their material, these authors interpreted them as juveniles, yet, at the same time, they also noted overall carapace morphology to be closely similar to adults of the related taxa, Costacopluma mexicana and C. concava. Comparing the mean carapace width of C. bishopi (ten specimens and four fragmentary carapaces known to date), with that of C. mexicana (numerous specimens known) and C. concava (four known specimens) yields ratios of 1:3.5 and ca. 1:4, respectively.

Discussion
A comparison of average carapace width in Longusorbis quadratus n. sp. and Xanthosia zoquiapensis n. sp. with their closest relatives reveals a striking similarity; that of the former species (two specimens known) is approximately 3.5 times less than of L. cuniculosus (numerous specimens available) and that of the latter (two specimens at hand) equals around 30% of that of X. buchi, around 30% of that of X. socialis (a single specimen known) and around 25% of that of X. semiornata (six specimens).
The above-mentioned consistency in width ratios in all three species described, i.e. all being about 3.5 times smaller than their closest relatives, and overall adult carapace morphology (Fig. 6) suggests interpretation of this decapod assemblage as the fi rst example of brachyuran dwarfi sm in the geological record. The environmental selection to favour this type of dwarfi sm in this tectonically active basin are still unknown, and further study is needed. Factors such as limited food supply, size-correlated predators and/or strong fl uviatile input/admixture inducing brackish waters might be worthwhile pursuing in detail to understand the observed size abnormality.