1. Introduction

During the 1980s, Professor Mario Ramírez (Facultad de Ciencias, Universidad de Guadalajara, Jalisco), and geologist Jaime Villarino-Guzmán, recovered fossiliferous Neogene rocks from the Santa Rosa Dam region, Municipality Amatitán, Jalisco (Figure 1). The fossil specimens were donated to the paleontology collection of the Instituto de Geología [currently Colección Nacional de Paleontología (CNP) of the Universidad Nacional Autónoma de México (UNAM), Mexico City]. Based on data provided by the collectors, the fossils were linked to two sites with incomplete field data, recorded in the Catalogue of Localities of the CNP under numbers IGM-loc 2449 and IGM-loc 2450. The fossil assemblage includes so far: pollen, diatoms, carbonized plants, ostracods, isopods, insects, fish remains, possible bird bone fragments, ichnofossils, and coprolites (^{Álvarez and Arriola-Longoria, 1972}; ^{Guzmán et al., 1998}). Among these fossils, the only properly identified species is the goodeid fish Tapatia occidentalis^{Álvarez and Arreola-Longoria, 1972}. The presence of complete or partially preserved isopods, laying together with some specimens of T. occidentalis was observed by Mario Ramírez through a handmade note provided together with his fossil donation, also mentioned in a geological study of the Amatitán region (^{Guzmán et al., 1998}), who analyzed the Neogene volcanic-sedimentary deposits present in the vicinity of the Santa Rosa Dam, calling these deposits the “Paleolago Amatitán”, where T. occidentalis is abundant. According to ^{Rosas-Elguera et al. (2000)}, these sediments represent a Pliocene sequence between 5.0 to 2.5 My old, deposited into the Plan de Barrancas Santa Rosa Graben. In this region, ^{Guzmán et al. (1998)} interpreted four stratigraphic units along the ravine of the Santiago River between the curtain dam and the Chome village, suggesting that 100 mm of thick laminar, parallel, and millimeter-tuffaceous silt horizons (corresponding to their section C) is the only fossiliferous portion of this sequence. During a 2014 fieldwork, Alvarado-Ortega and his students found that the fossiliferous portion of ^{Guzmán et al. (1998)} is more complex, because the millimeter-lamination tuffaceous silts are interspersed with less-compacted, thinner greenish-gray limolites, which together form a 20 m thick sequence, exposed 500 m alongside the El Amarillo and Chome road (Figure 1). The new exposures found in 2014 were labeled as IGM-loc 3954, registered at the CNP, UNAM.

Figure 1 Location map and stratigraphic section of locality IGM-loc 3954, north of Amatitlán, Jalisco, Mexico. The stratigraphic section shows the occurrence of plant remains, fishes and isopods, near the top of the exposed sequence. 

The aim of this study is to determine the taxonomical identity of the Neogene fossil isopods of Amatitán. Specimens are deposited at the Colección Nacional de Paleontología, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, Mexico City, 04510, Mexico.

2. Methodology

The fossil isopods were observed and photographed with a Zeiss Axiozoom v.16; the resulting images were digitized with a Kanvus Life 106 graphic tablet using the software Inkscape v0.92.3.

3. Systematic Palaeontology

Type species: Jaliscosphaera pliocenica n. gen. et sp., by monotipy.

Etymology: This name refers to the state of Jalisco, Mexico.

Diagnosis: Head partially inserted into pereonite 1; antennal peduncles robust. Pereonite 1 spaced from the rest. Coxal projections visible overlapped. Pleotelson with posterolateral margins with two indentations; uropods biramous finger-like, flattened, obtuse apex. Jaliscosphaera pliocenica n. gen. et sp. Figures 2-4.

Figure 2 Jaliscosphaera pliocenica n. gen. et sp. A, holotype IGM 13091; B, drawing of same specimen; C, paratype 13092; D, drawing of same specimen. Abbreviations: Ant = antenna, Antl = antennula, Cx = coxa, En = endopod, Ex = exopod, Fs = fish, Hd = head, Pln = pleon, Plt = pleotelson, Pr = pereonite, Prp = pereopod, Ur = uropod. 

Figure 3 Jaliscosphaera pliocenica n. gen. et sp. A, paratype IGM 13091; B, drawing of same specimen. C, paratype IGM 13092; D, paratype 13093; E, 13094; paratypes 13095 (left), and 13096 (upper right); G, paratype IGM 13097. 

Figure 4 Jaliscosphaera pliocenica n. gen et sp. A, paratype IGM 13098; B, close up to pleotelson, pereopods 4 - 7 and uropods. 

Etymology: The name refers to the Pliocene age of volcanic ash sediments of the Santa Rosa region, Amatitlán, Jalisco, Mexico.

Material: Holotype IGM 13091; paratypes IGM 13092 to IGM 13104.

Measurements: See Table 1.

Diagnosis: Body vaulted, convex margins without setae. Pereonite 1 separated from rest, pereionite 3 reduced in length. Coxae 1 separated from the rest posteriorly triangular, coxae 2-7 posteriorly triangular, slightly overlapped. Pleotelson posterolateral margins with two indentations.

Description: Body vaulted, convex, margins smooth, head inserted into pereonite 1. Antennae projecting laterally, extending posteriorly to anterior margin pereonite 2, with 5 peduncular segments, flagellum with 4 articles. Antennula projecting laterally, with 3 peduncular segments, flagellum with 6 articles. Pereon margins strongly convex, pereonite 1 separated from pereonites 2-7, pereonites 2-7 overlapped. Pereopod not prehensile, ambulatory; coxae length 2.25 width; basis triangular reduced as long as coxae; ischium with external margin concave, length 2.6 width; merus subtriangular length; carpus, propodus, dactylus not well differentiated. Pleon subrectangular, width 5.3 length, 5 pleonites visible fused indicated by lateral sutures. Pleotelson subsquare, length 0.95 width, presumptively posterolateral margins with two indentations, uropods inserting anterolaterally. Uropods smooth biramous, protopod trapezoidal broader than long; exopod length 0.7 endopod length, endopod lateral margin simple, smooth, apex obtuse.

Remarks: Jaliscosphaera pliocenica n. gen. et sp. is the first freshwater isopod recorded from the Pliocene from the western slope of Mexico, the rest of the fossil isopods belong to Cretaceous marine environments and terrestrial species as inclusions in early Miocene amber (^{Feldmann et al., 1998}; ^{Vega et al., 2005}, ²⁰⁰⁶, ²⁰¹⁸, ²⁰¹⁹, ²⁰²²; ^{Serrano-Sánchez et al., 2007}, ²⁰¹⁵, ²⁰¹⁶; ^{Broly et al., 2017}, ²⁰¹⁸; ^{Bruce et al. 2021}). Since most of the specimens of Jaliscosphaera pliocenica n. gen. et sp. are preserved in ventral view, the morphological description is limited, although sufficient to erect a new genus and species. Poorly preserved pereopod and no pleopods are preserved and the sex could not be assigned. Along with type specimens of the here described new taxon, other smaller isopods with apparently different morphological characteristics are found. Some these individuals present conglobation (Figures 3A, 3B and 3G), probably due to the sudden burial event occurred that may cause a mass mortality event, affecting not only isopods, but other organisms, such as fishes. Sphaeroma burkatii^{Bárcena, 1875} is another lacustrine fossil species recorded from Ameca, Jalisco (whose physical position is unknown), the morphological differences in body conformation such as the shape of the head, pereon, pleon, pleotelson, and uropods, allow us to achieve clear separation with Jaliscosphaera pliocenica n. gen. et sp.

4. Conclusion

To the moment, 27 fossil isopod species from Mexico have been reported, with an age range from the Lower Cretaceous to the Pliocene (see ^{Bruce et al., 2021, table 1}; plus ^{Vega et al., 2022} and data herein). Additions to this high diversity are waiting to be published from other Mexican localities under study. Jaliscosphaera pliocenica n. gen. et sp. represents the second freshwater sphaeromatid isopod reported from Mexico, since S. burkartii might also be inhabitant of the Neogene freshwater lacustrine environments of Jalisco. The main problem is that it seems the type specimen of S. burkartii is lost, so a detailed review of the many collections from Jalisco (and elsewhere) is needed to confirm if the name is still valid. Opposed to S. burkartii, Jaliscosphaera pliocenica n. gen. et sp. is represented by several specimens, most of them incomplete, due to the poor preservation, possibly linked to by mass mortality events, caused by volcanic ash fall that might increase not only the temperature, but also trigger anoxia events. The associated second species of isopod is relatively abundant, but its small size and poor preservation prevents a more detailed identification. Future collections at the same locality might produce more complete specimens of both species, but in the meantime, the report of a new fossil isopod is an important addition to the paleobiodiversity of Mexico.