Taxonomic update on Astacodes sp. (Crustacea: Palinuridae) from the Lower Cretaceous of Guerrero, Mexico
Actualización taxonómica en Astacodes sp. (Crustacea: Palinuridae) del Cretácico Inferior de Guerrero, México
Oscar González-León1,*, Francisco J. Vega2
1 Facultad de Estudios Superiores Iztacala, Universidad Nacional Autónoma de México, Tlalnepantla, 54090, Estado de México, México.
2 Instituto de Geología, Universidad Nacional Autónoma de México. Ciudad Universitaria, Coyoacán, 04510, CDMX, México.
* Corresponding author: (O. González-León) This email address is being protected from spambots. You need JavaScript enabled to view it.
How to cite this article:
González-León, O., & Vega, F. J. (2026). Taxonomic update on Astacodes sp. (Crustacea: Palinuridae) from the Lower Cretaceous of Guerrero, Mexico. Boletín de la Sociedad Geológica Mexicana, 78(1), A230925. https://doi.org/10.18268/BSGM2026v78n1A230925
Manuscript received: August 29, 2025. Corrected manuscript received: September 18, 2025. Manuscript accepted: September 23, 2025.
ABSTRACT
A reexamination of the specimen originally described as Astacodes sp. cf. maxwelli (Stenzel, 1945) from the San Lucas Formation in southwest Mexico, allows us to consider it as Astacodes cf. falcifer based on morphologic criteria closer to Astacodes falcifer from Speeton, England and Neuquén, Argentina. Recognizing the reproductive structures known as gonopores and poorly preserved gonopods allowed a reinterpretation of the Mexican specimen as a male. Sexual dimorphism criteria in this taxon could help resolve taxonomic issues to differentiate between males and females, using sex-specific morphological characteristics, (ornamentation, size, sternum shape, among others). These characteristics can also help compare and differentiate it from other species.
Keywords: Astacodes cf. falcifer, San Lucas Formation, Lower Cretaceous, sexual dimorphism.
RESUMEN
Una reevaluación del espécimen originalmente descrito como Astacodes sp. cf. A. maxwelli (Stenzel, 1945) procedente de la Formación San Lucas, al suroeste de México, nos permite considerarlo como Astacodes cf. falcifer con base en criterios morfológicos más cercanos a los de Astacodes falcifer de Speeton, Inglaterra, y Neuquén, Argentina. El reconocimiento de las estructuras reproductivas conocidas como gonóporos y gonópodos mal preservados permitió hacer una reinterpretación del espécimen mexicano como un macho. Los criterios de dimorfismo sexual en este taxón podrían ayudar a resolver problemas taxonómicos para diferenciar entre machos y hembras, utilizando características morfológicas específicas del sexo (ornamentación, tamaño, forma del esternón, entre otras). Estas características también pueden ayudar a compararla y diferenciarla de otras especies.
Palabras clave: Astacodes cf. falcifer, Formación San Lucas, Cretácico Inferior, dimorfismo sexual.
1. Introduction
The record of Astacodes (Bell, 1863) is known from Late Jurassic to Late Cretaceous and is restricted to seven species in Europe and North America (Fӧrster, 1973) and South America (Aguirre-Urreta et al., 2012). This genus is represented by A. kamptneri and A. strambergensis (Bachmayer, 1959) from the Upper Jurassic of southern Germany, A. falcifer (Bell, 1863) from the Hauterivian in the United Kingdom and Upper Hauterivian-Lower Barremian, Neuquén Basin (Aguirre-Urreta et al., 2012), A. wenoensis (Rathbun, 1935) from the Albian of United States, A. woodwardi (Fritsch and Kafka, 1887) of the Turonian of southern Germany, A. davisi (Stenzel, 1945) from the Turonian and A. maxwelli (Stenzel, 1945) from the Santonian of the United States. However, other Cretaceous records are considered in open nomenclature (e.g., Woods, 1957; Mertin, 1941). Astacus sp. cf. A. maxwelli was described from the Hauterivian-Aptian of Guerrero, Mexico (Alencáster, 1977). Later, the same specimen was refigured (Vega et al., 2006) as Astacodes sp. In their genera classification (De Grave et al., 2009 p. 23) recognizes 6 species for the genus. In 2010 (Schweitzer et al. p. 45) published their systematic list, which included Astacodes with only six species and excluded A. wenoensis (Rathbun, 1935) because this species was originally described in the genus Enoploclytia, where it currently remains. In contrast with this, according to WoRMS and DecaNet [AphiaID 1616014 (urn:lsid:marinespecies. org:taxname:1616014)] only three species are recognized for Astacodes (Bell, 1863) as follows: falcifer (Bell, 1863), A. davisi (Stenzel, 1945) and maxwelli (Stenzel, 1945).
The specimen of Astacodes sp. reported from Mexico was found in deposits of the San Lucas Formation, Lower Cretaceous, Guerrero, Mexico. The poor preservation mainly in the dorsal part of the cephalothorax did not allow assignment of the specimen to the specie level. However, reexamination of the specimen revealed it preserves gonopores in the fifth pair of pereiopods, and poorly preserved evidence associated with gonopods. Aditionally, a 3D model of the specimen to complement the information related to this species was added. The re-description of the Mexican specimen joined to a line drawing of dorsal, ventral and lateral views, represents an addendum that allows us to consider this specimen as Astacodes cf. falcifer, following a better comprehension of this taxon from the Lower Cretaceous of Mexico.
2. Material and methods
The specimen is housed at the Colección Nacional de Paleontología at Museo María del Carmen Perrilliat, Instituto de Geología, UNAM, under the acronym IGM-2701. The specimen was photographed in dorsal, ventral and lateral views, coated with ammonium chloride for better details in photographs. Images of all views were also inverted, and line-drawings of the different views were prepared with the Photoshop CC® software. Line-drawings show the main morphological features. A 3D model was obtained with the EinScan-SP scanner at the Laboratorio de Invertebrados, Instituto de Geología, UNAM. The model in this work is found as Appendix 1. Upon request, the high-resolution model is available in pdf or digital format (.obj or .stl file formats) for easier viewing or 3D printing.
3. Geological setting
Astacodes sp. was collected from a locality southwest of Ciudad Altamirano (Michoacán), two kilometers southeast of Coyuca de Catalán, and comes from a sequence of fine to calcareous clastic rocks, covered by limestones in which Albian age caprinids and Chondrodonta munsoni (Hill, 1893) were recognized (Alencáster, 1977, fig.1). This area is considered part of the tectonostratigraphic Guerrero Terrain (Campa and Coney, 1983). The type locality is in the municipality of San Lucas, 10 kilometers southeast of Huetamo. The San Lucas Formation crops out at the eastern part of the western region of Huetamo and was considered by Pantoja-Alor (1959) as a marine clastic sequence with an age range from Hauterivian to Aptian. The depositional sequence is associated with marine turbidites and flysch facies (Alencáster and Pantoja-Alor, 1995). The formation is divided into two members: the Terrero Prieto and Las Fraguas (Pantoja-Alor, 1990). The age of the lower Terrero Prieto member has been considered to be Upper Valanginian due to the presence of the ammonite Taraisites bosei (Cantú-Chapa, 1966) and the apthychus Lammelaptychus seranonis (Coquand, 1841); the middle and the upper part of Terrero Prieto member represent a Barremian age due to the presence of the Subsaynella and Karsteniceras ammonites (Gómez-Luna et al., 1991 in Alencáster and Pantoja-Alor, 1995). Las Fraguas member is interpreted as a distal deltaic fan deposits with some turbiditic facies and is composed of massive bedded volcanic, feldspathic and lithic sandstone with interbedded siltstone and finely grained conglomerate (Pantoja-Alor, 1990). In relation to the fossil content (Guerrero-Suastegui, 1997) it extends the range of the San Lucas Formation from the Late Valanginian to late Aptian age (Morales-Gámez, 2005). More recently, some authors based on ammonites and planktic foraminifera, proposed that the lower part of the Tiringueo section and other sections studied in the San Lucas Formation represent an Early-Late Barremian age (Omaña-Pulido et al., 2005; Ramírez-Garza, 2007). As mentioned previously by Guerrero-Suastegui (1997), considering that diverse studies have been done in the Huetamo area, the nomenclature and age are still controversial. Environmental conditions are related to deposits close to a volcanic arc that includes coastal and open marine platforms (Gómez-Luna et al., 1993; Guerrero-Suastegui, 1997; Omaña-Pulido et al., 2005).
4. Systematic paleontology
Order Decapoda Latreille, 1802
Infraorder Achelata Scholtz and Richter, 1995
Family Palinuridae Latreille, 1802
Genus Astacodes Bell, 1863
Type species: Astacodes falcifer Bell, 1863, by original designation (Bell, 1863, p. 30) from the Lower Cretaceous (Hauterivian) of eastern England.
Included species: A. falcifer Bell, 1863, A. davisi Stenzel, 1945 and A. maxwelli Stenzel, 1945.
Astacodes cf. falcifer Bell, 1863
Figures 1–5
1863 Astacodes falcifer Bell, p. 30, pl. 9, figs 1–2 (non-figs. 3–6).
1925 Astacodes falcifer Bell; H. Woods, p. 34, pl. 8, figs 3–4; pl. 9, figs 1–5.
1977 Astacodes sp. cf. A. maxwelli Stenzel; Alencáster, p. 75, figs 2–3.
2006 Astacodes sp. Vega, Nyborg and Perrilliat, p. 83, fig. 1.5.
2012 Astacodes falcifer Bell; Aguirre-Urreta, Lazo and Rawson, p. 1096, fig. 5 A–O, fig. 6 A–B
Description. Specimen comprises incomplete cephalothorax with dorsal side partially missing; maximum length 12.68 cm; maximum width of 6.96 cm. Cephalothorax rounded and dorsoventrally compressed; maximum height approximately 3.25 cm approximately. Cervical groove (c) deep, divided into three parts, central part 2.45 cm wide and straight, the two lateral parts forming an obtuse angle approximately 128˚ pointed to the antennal keels(?). Branchiocardiac groove (a) moderately deep, forms a very wide, open curve, concave toward the marginal groove (m). Dorsal and lateral surface covered with small, rounded granules moderately visible, slightly larger in the branchiocardiac region and smaller in the lateral and anterior parts of the carapace. Anterior ventral part with third pair of thin and elongated maxillipeds partially present, second maxilliped and in the central part a short mandible. Sternum large 5.25 cm in length and 2.89 cm at its widest part, forming an isosceles triangle; two granules slightly visible in 2-7 thoracic sternum (ts). Five pairs of pereiopods (P1-P5) moderately preserved; three segments coxa (c), basis (b) and ischium (i) are present; gonopores (gon) broken are present at the base of the coxa in P5; separate of the marginal grove in both sides, a fragment of pleonal terga (ta) is preserved; poorly preserved gonopods are slightly visible in the central part of the thoracic sternum.
5. Discussion
The original assignment of the Mexican specimen to Astacodes cf. A. maxwelli (Stenzel, 1945) was based on the shape of carapace, dorsal grooves, the aspect of the antennal keels and the ornamentation (Alencáster, 1977). However, the author considered for comparison specimens that Woods (1925 p. 34) exclude from the figures presented by Bell (1863, pl. IX fig. 1–6). In this sense, figures 1 and 2 were referred to as types, and figures 3–6 would not correspond to A. falcifer. The specimens of A. falcifer figured by (Woods, 1925 pl. VIII figs. 3, 4 and pl. IX, figs. 1-5) present the lateral parts of the cervical groove approximately to 130˚ forming an obtuse angle as a Astacodes sp. from Mexico (Figure 1). Specimens described as A. falcifer illustrated by (Aguirre-Urreta et al., 2012, fig. 5H, J, N) show similar angular value between the central and lateral parts of the cervical groove. In fact, we consider the Mexican specimen to be very similar in dimensions, ornamentation and morphology of the ventral region to A. falcifer. The ornamentation in the Mexican Astacodes sp. seems to be more related to A. falcifer from the Speeton Clay of Speeton, eastern England and from the Agrio Formation (Agua de Mula member) from Argentina. The Mexican specimen has small, rounded cuticle granules on a large part of the carapace, more details of the dorsal and lateral views of the carapace can be appreciated (Figures 1–3). Granules are slightly bigger in the branchiocardiac region than in the lateral branchial region and smaller near of the anterior (pterygostomial region; Figure 3A2). Specimens of A. maxwelli and A. davisi (Stenzel, 1945) present a smaller number of granules per area unit, being larger in the branchiocardiac region and smaller in other areas. A. falcifer from Argentina has a more uniform granule size than those observed on the specimens from Mexico and England (Figure 1A). The shape of cervical groove in Astacodes maxwelli (Stenzel, 1945, pl. 35, fig. 3) appears to be slightly rounded, being angular in specimens of A. falcifer (Woods, 1925, pl. VIII fig. 3a; Aguirre-Urreta et al., 2012, fig. 5H, J, N and Alencáster, 1977, figs. 2–3).
The ventral region in Astacodes sp. from Mexico (Figure 1B), as well as the segments of the pereiopods are very similar to those of A. falcifer from Neuquén basin, although in the Mexican specimen they are moderately best preserved, represented by coxa, basis and ischium (Figure 1B2). The triangular shape of the sternum is similar to the Argentinian specimen, with the difference that in the Mexican specimen, a pair of rounded ornamentations can be distinguished on segments 2 to 7 of the sternum (Figure 1B2). The fossil preserves segments of the second and third maxillipeds as well as part of the mandibles (Figure 1B).
Although the Mexican specimen lacks important diagnostic characters in the rostrum, from our perspective, it should be conferred to Astacodes falcifer, as it has more resemblance to the English and Argentinian specimens that also belong to the Lower Cretaceous. Astacodes cf. falcifer from Mexico may be a good link between the records of Astacodes falcifer in both hemispheres (northern England and southern Argentina), with records from Speeton and Neuquén, as previously indicated by Aguirre-Urreta et al. (2012). To confirm this possible relationship, age calibration is required in the sedimentary deposits where this decapod was found in Mexico. Regarding its stratigraphic position, we infer that this record could be related to the Terrero Prieto member proposed by Pantoja-Alor (1990). However, it is not possible to precisely establish its location in the sequence of this member, and consequently, its association with a specific age.
6. Sexual dimorphism
Primary sexually dimorphic features such as gonopores are rarely fossilized. Secondary sexually dimorphic characters (e.g., shape of pleon, and the size of the carapace) are frequently used in fossils (Jones et al., 2022). A detailed observation of the ventral region in the Mexican specimen showed that on the base of the coxae of the last pair of pereiopods, there are elements that were recognized as gonopores (Figures 4A–4D). The structure is located just below the last thoracic sternum (ts-7). The chitinous structure is broken, but an oval shape is observed; only the edges are visible, with an apparent gap. This feature is much more evident in the right gonopore and less in the left one (Figures 4C–4D). A line-drawing (Figures 4C1–4D1) indicates the position and shape of the base in this reproductive structure. This is significant because we can consider the fossil studied as a male. In addition, the sternum seems to have unclear evidence of the gonopods. These structures are marked in the sternum as small rough lines preserved from the thoracic sternum 6 to 4 (Figure 5).
The fossil record of sexual dimorphism is well known in brachyurans (e.g., Bishop, 1974, 1983; Bishop et al., 1998; Schweitzer and Feldmann, 2011; Guinot et al., 2013; Mclay and Becker, 2015; Jones et al., 2022). An exceptional example of the presence of gonopores and gonopods in the fossil record was presented by Charbonnier et al. (2024, fig. 3i–j) for the lobster Eryma ventrosum (Von Meyer, 1835), observed in X-ray synchrotron tomography. It allowed recognition of different systems, among which different structures are associated with the reproductive system. Gonopores at the base of P5 and P3 for male and female, as well as gonopods and spermatheca, are figured respectively. The gonopores in current Palinura have a semicircular male gonopore that occurs ventrally on the coxal segment of pereiopod 5, in females, these structures occur as a crescent shaped aperture on P3 (Lavalli and Spanier, 2010). The ovaries are connected to the genital opening, located in the coxa of P3, and the gonopores in male are connected to a long and convoluted vasa deferentia to the testis (Phillips et al., 1980; MacDiarmid and Sainte-Marie, 2006; Lavalli and Spanier, 2010). The oval form of the gonopores observed in our specimen may have been deformed and fractured during the dorsoventral compression that apparently affected its original shape. Future revision of specimens that preserve the ventral morphology of P3 and P5 will allow recognition between females and males. With this, it will be possible to compare morphological characters associated with other structures such as ornamentation, size, shape of the appendages, sternum, among others that may be associated with sexual dimorphism, or on the contrary, help differentiate it from other species.
7. Conclusions
Based on similarities such as body size, ornamental features, the angular shape of the cervical groove, and the similar stratigraphic range, we consider that Astacodes falcifer records from England and Argentina show a greater affinity with Astacodes sp. from the San Lucas Formation, Guerrero state, Mexico. For this reason, in this work we propose to consider Astacodes sp. as Astacodes cf. falcifer, due to the incompleteness of the Mexican record. This is considered one of the oldest crustacean records from Mexico.
Recognizing sexual dimorphism may help us understand in the future whether changes in size, ornamentation, and body shape could help differentiate between palinurid males and females in the fossil record, along with features that distinguish it from other species.
Contributions of authors
Conceptualization: OGL; (2) Original manuscript development: OGL; (3) Development of corrected and edited manuscript: FJV, OGL; (4) Graphic design: OGL; (5) Interpretation: OGL, FJV.
Acknowledgments
The authors would like to thank editor-in-chief Antoni Camprubí Cano for his comments and support, as well as the comments and suggestions made by two anonymous reviewers, whose detailed observations provided excellent and crucial corrections to improve this work. We appreciate the technical support provided by Diana Ramírez Álvarez. We thank PhD Josep A. Moreno-Bedmar for allowing access to and use of the Laboratorio de Paleontología de Invertebrados facilities at the Instituto de Geología, UNAM. We thank PhD Jesús Alvarado-Ortega and MSc. Violeta Romero-Mayén for the facilities for the study of the specimen. We also thank Diego Andrés Beltrán-López for coating and photographing the fossil material, and PhD Jose Roberto Ovando-Figueroa for his support in creating the 3D model.
Conflict of interest
The authors declare no conflict of interest.
Handling editor
Alessandro Garassino.
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