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Differentiation of Tehuacana and Hyphalocarcinus new genus from similar forms in Palaeoxanthopsidae (Decapoda, Brachyura, Eubrachyura, Carpilioidea)

Published online by Cambridge University Press:  22 July 2025

Carrie E. Schweitzer Open the ORCID record for Carrie E. Schweitzer [Opens in a new window] ,
Adiël Klompmaker Open the ORCID record for Adiël Klompmaker [Opens in a new window]  and
Javier Luque
Carrie E. Schweitzer*
Affiliation:
Department of Geology, https://ror.org/049pfb863Kent State University Stark Campus, 6000 Frank Ave. NW, North Canton, OH 44720, USA
Adiël Klompmaker
Affiliation:
Department of Museum Research and Collections and Alabama Museum of Natural History, https://ror.org/03xrrjk67University of Alabama, 313 Mary Harmon Bryant Hall, Box 870340, Tuscaloosa, Alabama 35487, USA
Javier Luque
Affiliation:
Department of Zoology, Museum of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK
*
Corresponding author: Carrie E. Schweitzer; Email: cschweit@kent.edu
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Abstract

The brachyurans Tehuacana tehuacana Stenzel, 1944 and Dromilites americana Rathbun, 1935 have historically been difficult to place in families. A reevaluation of type and referred material from several institutions suggests that the two species are referrable to separate genera in Palaeoxanthopsidae. Hyphalocarcinus new genus is erected to accommodate H. americanus new combination, and Tehuacana remains a distinct genus. Palaeoxanthopsidae evolved and radiated in the Atlantic Ocean, ranging from Late Cretaceous (Maastrichtian) to early Eocene (Ypresian) in age. This work adds to the known diversity of Palaeoxanthopsidae and demonstrates that the family survived and thrived in the Atlantic Ocean in the wake of the end-Cretaceous extinction. Differential preservation of specimens must be evaluated carefully when placing superficially similar taxa at the family, genus, and species level.

UUID: http://zoobank.org/73da26c7-aff7-4abe-8aa5-768944bbf6ee

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Journal of Paleontology , First View , pp. 1 - 12
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Non-technical Summary

The true crabs Tehuacana tehuacana and Dromilites americana have historically been difficult to place in families. A reevaluation of type and referred material from several institutions suggests that the two species are referrable to separate genera within the family Palaeoxanthopsidae. Hyphalocarcinus is a new genus erected to accommodate H. americanus, and Tehuacana remains a distinct genus. Palaeoxanthopsidae, the family to which these genera belong, evolved and radiated in the Atlantic Ocean, ranging from Late Cretaceous (Maastrichtian) to Eocene in age. This work adds to the known diversity of Palaeoxanthopsidae and demonstrates that the family survived and thrived in the Atlantic Ocean after the end-Cretaceous extinction. Differing preservation styles, including broken margins, loss of cuticle, and abrasion of specimens, must be evaluated carefully when placing superficially similar taxa within a family, genus, and species.

Introduction

Investigation of the impact of the end-Cretaceous extinction on the origins, evolution, diversity, and radiation of brachyuran crabs during the Cenozoic is pivotal to expanding our understanding of the fossil record of true crabs during the Paleocene, their post-Mesozoic radiation, and their phylogenetic relationships (Klompmaker et al., Reference Klompmaker, Jakobsen and Lauridsen2016; Luque et al., Reference Luque, Feldmann, Vernygora, Schweitzer and Cameron2019, Reference Luque, Brecken-Grissom, Ortega-Hernández and Wolfe2024; Schweitzer and Feldmann, Reference Schweitzer and Feldmann2023; Van Bakel et al., Reference Van Bakel, Ossó and Téodori2023).

When decapod fossils are incomplete or differentially preserved, it can be difficult to distinguish between and among related taxa. The presence or absence of cuticle or some cuticular layers can render specimen appearance very different, even those of the same species (Waugh et al., Reference Waugh, Feldmann and Schweitzer2009; Klompmaker et al., Reference Klompmaker, Hyžný and Jakobsen2015; Schweitzer et al., Reference Schweitzer, Feldmann, Audo, Charbonnier and Fraaije2024). Such complications have affected interpretation of various taxa, including the Paleocene forms Dromilites americana Rathbun, Reference Rathbun1935 and Tehuacana tehuacana Stenzel, Reference Stenzel1944. Generic and suprageneric placement of D. americana has been previously questioned, noting that it was likely not a member of Dromiidae (Guinot, Reference Guinot2008; Schweitzer and Feldmann, Reference Schweitzer and Feldmann2010). Tehuacana had been placed in Goneplacidae and Mathildellidae previously (Stenzel, Reference Stenzel1944; Karasawa and Kato, Reference Karasawa and Kato2003). Furthermore, the two species had been synonymized, with Tehuacana americana becoming the senior synonym (Armstrong et al., Reference Armstrong, Nyborg, Bishop, Ossó-Morales and Vega2009). Although the two taxa have many similarities, we investigated their proposed synonymy and their generic and suprageneric placement by statistical and morphological analyses, considering differential preservation.

Geological settings

Specimens in Alabama were collected from the upper Danian Porters Creek Formation localities (Mancini and Tew, Reference Mancini and Tew1989; Pietsch et al., Reference Pietsch, Harrison and Allmon2016). These include the Black Bluff (massive claystones), Tombigbee River, Sumter County, Alabama, of the Porters Creek Formation, upper Danian (ALMNH loc. 5). The UF specimen was collected from the upper Danian Porters Creek Formation in Furman, Wilcox County, Alabama. The USNM specimens were collected from Pine Barren Creek + Prairie Creek and Allenton + Prairie Creek and Pine Barren section, Wilcox County, Alabama, of the Porters Creek Formation, upper Danian (Rathbun, Reference Rathbun1935).

Specimens in Texas were collected from localities of the upper Danian–Selandian Wills Point Formation (Armstrong et al., Reference Armstrong, Nyborg, Bishop, Ossó-Morales and Vega2009). The MMNS and non-type NPL specimens were collected from Hansen Quarry, Limestone County, Texas, of the Wills Point Formation, Mexia Clay Member, upper Danian (Armstrong et al., Reference Armstrong, Nyborg, Bishop, Ossó-Morales and Vega2009). The type specimen of Tehuacana tehuacana was collected from Tehuacana Creek, Limestone County, Texas, of the Wills Point Formation (no member specified), upper Danian–Selandian (Stenzel, Reference Stenzel1944).

Materials and methods

Materials and preparation

Some specimens were whitened with ammonium chloride before photography with a Nikon D7200 camera with an AF Nikkor 28-105 mm lens. Measurements were taken with Mitutoyo analog calipers to the nearest tenth of a millimeter or derived from photos with a ruler placed at the same height as the specimen. Incomplete specimens were measured as half of the measurement where possible and then doubled. T-tests and Mann–Whitney tests were performed in Past 4.08 (Hammer et al., Reference Hammer, Harper and Ryan2001), all at p = 0.05, to compare several carapace ratios (carapace length/carapace width, fronto-orbital width/carapace width, intestinal length/carapace length) of specimens of Dromilites americana versus Tehuacana tehuacana. Images were toned in Adobe Photoshop 23.1.0 before making figures in Adobe Illustrator 26.0.2. Photos of ALMNH and MMNS specimens were taken using a Canon EOS 90D camera with a 60 mm lens in an Ortech Photo-e-Box Plus lightbox.

Materials

Dromilites americana, holotype, USNM PAL 371688; paratypes USNM 335993A–D, USNM PAL 795622–795626; UF 17090; ALMNH:Paleo:21474–21481, MMNS IP-7402; UT NPL 31165. Tehuacana tehuacana, holotype UT BEG 21289 (cast and photos); USNM PAL 794470, 795627–795629; MMNS IP-7399 (four specimens).

Repositories and institutional abbreviations

Types, figured, and other examined specimens in this study are deposited in Alabama Museum of Natural History Paleontology Collection, University of Alabama, Tuscaloosa, Alabama, USA (ALMNH:Paleo:); Cátedra de Geología Histórica, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa, Santa Rosa, Argentina (GHUNLPam); Georgia Southern Museum, Georgia Southern University, Statesboro, Georgia, USA (GSCM); Department of Earth Sciences, Kent State University (KSU D); Geologisk Museum, University of Copenhagen, Copenhagen, Denmark (MGUH); Mississippi Museum of Natural Science, Jackson, Mississippi, USA (MMNS IP); Muséum National d’histoire naturelle, Paris, Collection de Paléontologie, France (MNHN); Institut für Palaöntologie, Universität Bonn, Germany (UBIP); Florida Museum, University of Florida, Gainesville, Florida, USA (UF); United States National Museum of Natural History, Smithsonian Institution, Washington, DC (USNM PAL); Jackson School Museum of Earth History, Non-Vertebrate Paleontology, University of Texas, Austin, Texas, USA (UT). Other institutional abbreviations are: Colección de Paleontología, Museo del Desierto, Saltillo, Coahuila, Mexico (CPC); Museo de Paleontologia, Instituto de Geologia, Universidad Nacional Autonoma de Mexico (GM).

Systematic paleontology

Infraorder Brachyura Linnaeus, Reference Linnaeus1758

Section Eubrachyura de Saint Laurent, Reference de Saint Laurent1980

Superfamily Carpilioidea Ortmann, Reference Ortmann1893

Family Palaeoxanthopsidae Schweitzer, Reference Schweitzer2003

Included genera

Hyphalocarcinus new genus; Jakobsenius Schweitzer, Reference Schweitzer2005; Lobulata Schweitzer, Feldmann, and Gingerich, Reference Schweitzer, Feldmann and Gingerich2004; Palaeoxantho Bishop, Reference Bishop1986; Palaeoxanthopsis Beurlen, Reference Beurlen1958; Paraverrucoides Schweitzer, Reference Schweitzer2003; Remia Schweitzer, Reference Schweitzer2003; Rocacarcinus Schweitzer, Reference Schweitzer2005; Tehuacana Stenzel, Reference Stenzel1944; Verrucoides Vega et al., Reference Vega, Cosma, Coutiño, Feldmann, Nyborg, Schweitzer and Waugh2001.

Diagnosis

Carapace usually wider than long but can be about as long as wide, widest about 65% of the distance posteriorly, strongly vaulted longitudinally, moderately vaulted transversely; carapace regions moderately to strongly inflated, often ornamented with large swellings, strongly delineated; front triangular and axially notched, with spines on either side of notch and with inner-orbital spines; frontal width about 20% maximum carapace width; orbits square, with two fissures; fronto-orbital width about half maximum carapace width; anterolateral margins long, convex, with straight segment followed by three spines, the last spine longest, stout, directed posterolaterally; posterolateral margins short, sinuous; swellings on epibranchial and branchial regions forming transverse ridges; carapace usually about as long as wide or wider than long; female gonopores very large, ovate, positioned centrally on sternite 5 just lateral to the axis (adapted from Schweitzer et al., Reference Schweitzer, Feldmann and Karasawa2018, p. 10).

Remarks

Palaeoxanthopsidae embraces a somewhat superficially heterogeneous group of genera that are united by several diagnostic characters. The carapace is usually wider than long or about as wide as long, and ornamentation is somewhat variable, with carapace regions ranging from moderately inflated to bearing very large, spherical swellings (Schweitzer et al., Reference Schweitzer, Feldmann and Karasawa2018).

However, the family is united by (1) the presence of well-defined regions, (2) the presence of a transverse ridge or swelling on the branchial regions, (3) orbits bearing two fissures, (4) anterolateral margins with discrete, well-separated spines, separated from the outer-orbital angle by a straight segment, (5) a quadrilobed front with two medial lobes sometimes projected beyond the orbits, (6) a moderately wide sternum with a Y-shaped groove pattern on sternites 3 and 4, and (7) strong lateral swellings on sternite 4 (Fig. 1). The two genera, Tehuacana and Hyphalocarcinus new genus, possess all of these features. They differ from Palaeoxanthopsidae as diagnosed by Schweitzer et al. (Reference Schweitzer, Feldmann and Karasawa2018) in being about as wide as long rather than wider than long and hexagonal. The carapace of Hyphalocarcinus new genus is much more strongly domed than other genera in the family. These differences are distinctive; but in all other ways, these two genera possess the diagnostic characters of the family.

Figure 1. Palaeoxanthopsidae. (1, 2) Lobulata lobulata (Feldmann et al., Reference Feldmann, Casadío, Chirino-Galvez and Aguirre-Urreta1995), holotype GHUNLPam 7001: (1) dorsal view; (2) ventral view (Feldmann et al., Reference Feldmann, Casadío, Chirino-Galvez and Aguirre-Urreta1995, fig. 7.1, 7.2). (3) Jakobsenius cretaceus (Segerberg, Reference Segerberg1900), KSU D 37, cast of holotype MGUH 2483. (4, 5) Paraverrucoides alabamensis (Rathbun, Reference Rathbun1935): (4) dorsal view of holotype USNM PAL 371718; (5) ventral view of paratype USNM PAL 371707. (6) Palaeoxantho libertiensis Bishop, Reference Bishop1986, holotype GSCM 1692. (7, 8) Rocacarcinus gerthi (Glaessner, Reference Glaessner1930), holotype Glaessner 1, UBIP: (7) dorsal view; (8) ventral view (Feldmann et al., Reference Feldmann, Casadío, Chirino-Galvez and Aguirre-Urreta1995, fig. 9.3, 9.4). (9) Palaeoxanthopsis cretacea Rathbun, Reference Rathbun1902, syntype USNM PAL 73709. (10, 11) Hyphalocarcinus americanus (Rathbun, Reference Rathbun1935) n. comb., paratype USNM PAL 335993B: (10) dorsal view; (11) ventral view. (12) Remia africana (Remy in Remy and Tessier, Reference Remy and Tessier1954), KSU D 1100, cast of holotype MNHN R03885. (13) Tehuacana tehuacana Stenzel, Reference Stenzel1944, holotype BEG021289 (photo by S. Skwarcan). (14) Verrucoides verrucoides (Collins and Rasmussen, Reference Collins and Rasmussen1992), KSU D 1803, cast of holotype MGUH 21.612. (1–12, 14) Scale bars = 1.0 cm; (13) scale bar = 5.0 mm. All except (10, 11, 13) appear in Schweitzer et al. (Reference Schweitzer, Feldmann and Karasawa2018).

Hyphalocarcinus new genus is erected to accommodate Dromilites americana (Rathbun, Reference Rathbun1935), originally considered as a member of the podotrematous Dromiidae De Haan, Reference de Haan1833. Dromilites americana cannot be referred to Dromilites Milne Edwards, Reference Milne Edwards1837 for several reasons, the most important of which is that D. americana is not a dromiid or even a podotrematous crab, as revealed by its thoracic sternum, which exhibits typical eubrachyuran sternal features, including the size, the width, and the shape of the sternites, the interrupted sternal sutures 4/5 and 5/6, and a median line on sternites 7 and 8 (Schweitzer et al., Reference Schweitzer, Feldmann, Audo, Charbonnier and Fraaije2024) (Fig. 1.11).

Tehuacana was originally placed within Goneplacidae on the basis of its flattened rectangular shape (Stenzel, Reference Stenzel1944). Later, the genus was moved to Mathildellidae Karasawa and Kato, Reference Karasawa and Kato2003, where it has remained. Similarities with other genera of Mathildellidae include a flattened carapace, dorsal regions weakly to moderately defined, an ovate sternum, and posterolateral reentrants. The holotype of Tehuacana tehuacana is not well preserved, so placement of the taxon is difficult. Because of the superficial similarities with Dromilites americana and the carapace swellings and morphology that are diagnostic for Palaeoxanthopsidae, the two genera are referred together to the same family, noting that each diverges somewhat from other palaeoxanthopsids.

Dromilites americana has many superficial similarities with genera within Aethridae Dana, Reference Dana1851. It has a well-developed sternal locking mechanism on sternite 5, sternal sutures 4/5 and 5/6 are incomplete, and sternal sutures 6/7 and 7/8 are complete, seen in Aethridae but also many other eubrachyurans (Guinot, Reference Guinot1979). The dorsal carapace of Dromilites americana has affinities with some taxa within Aethridae. Among extinct and extant aethrids, several genera have strongly inflated carapace regions (Schweitzer and Feldmann, Reference Schweitzer and Feldmann2019; Poore and Ahyong, Reference Poore and Ahyong2023). However, most extant and many extinct aethrids have very strongly convex anterolateral margins, not seen in D. americana. Thus, we consider Palaeoxanthopsidae as the best placement for the new genus for D. americana at this time. Members of the family are already known from the Late Cretaceous through Eocene of the Gulf Coastal Plain, so neither the geologic nor geographic range is extended by these referrals.

Hyphalocarcinus new genus

Reference Rathbun1935 Dromilites Milne Edwards, Reference Milne Edwards1837 (partim); Rathbun, p. 79.

Reference Davidson1966 Dromilites Milne Edwards (partim); Davidson, p. 211.

Reference Bishop, Whitmore, Harris, Zulle and Otte1986 Dromilites Milne Edwards (partim); Bishop and Whitmore, fig. 1D.

Reference Števčić2005 Marydromilites Števčić, Reference Števčić2005; nomen nudum (ICZN, 1999, Article 13.1).

Reference Guinot2008 Dromilites Milne Edwards (partim); Guinot, p. 21.

Reference Armstrong, Nyborg, Bishop, Ossó-Morales and Vega2009 Tehuacana Stenzel, Reference Stenzel1944 (partim); Armstrong et al., p. 753.

Reference Schweitzer, Feldmann, Garassino, Karasawa and Schweigert2010 Dromilites Milne Edwards (partim). Schweitzer et al., p. 64.

Reference Schweitzer and Feldmann2010 Dromilites Milne Edwards (partim); Schweitzer and Feldmann, p. 422.

Reference Franţescu, Feldmann and Schweitzer2010 Dromilites Milne Edwards (partim); Franţescu et al., p. 264.

Reference Klompmaker, Jakobsen and Lauridsen2016 Dromilites Milne Edwards (partim); Klompmaker et al., table S9.

Reference Luque, Schweitzer, Santana, Portell, Vega and Klompmaker2017 Tehuacana Stenzel, Reference Stenzel1944 (partim); Luque et al., p. 46.

Reference Martínez-Díaz, Aguillón-Martínez, Luque and Vega2017 Tehuacana Stenzel, Reference Stenzel1944 (partim); Martinez-Díaz et al., p. 77.

Reference Vega, Ahyong, Espinosa, Flores-Ventura, Luna and Gonález-González2018 Tehuacana Stenzel, Reference Stenzel1944 (partim); Vega et al., fig. 5.

Type species

Dromilites americana Rathbun, Reference Rathbun1935, by original designation.

Diagnosis

Carapace longer than wide; protogastric, metagastric, epibranchial, cardiac, and mesobranchial regions with strongly inflated, spherical, densely granular swellings; anterolateral margins with four conical blunt spines with densely granular tips; posterior margin with triangular projections on each end; thoracic sternum longer than wide, pleonal locking mechanism on sternite 5; sternal sutures 4/5 and 5/6 incomplete, suture 6/7 and 7/8 complete, median line on axis of sternites 7 and 8.

Etymology

The genus name is a combination of the Greek words hyphalo, meaning under the sea, secret, and karkinos, a common stem for crab, in reference to the difficulty in placing this taxon within a family. The gender is masculine. The name suggested by Števčić (Reference Števčić2005), Marydromilites, is a nomen nudum under ICZN Article 13.1 (ICZN, 1999; see also Franţescu et al., Reference Franţescu, Feldmann and Schweitzer2010).

Remarks

Dromilites was considered a member of Dromiidae by Rathbun (Reference Rathbun1935), but since then it has been referred to another dromiacean family, Sphaerodromiidae Guinot and Tavares, Reference Guinot and Tavares2003. Notably, both families include crabs exhibiting the podotrematous condition, with gonopores of both males and females on the pereiopod coxae, and they have distinctive sterna consisting of narrow sternites with reduced episternal projections (Schweitzer et al., Reference Schweitzer, Feldmann, Audo, Charbonnier and Fraaije2024, fig. 122). Schweitzer and Feldmann (Reference Schweitzer and Feldmann2010) restricted Dromilites to just four species, and they noted the affinities of D. americana with eubrachyuran crabs.

Dromilites americana cannot be referred to Dromilites for several reasons, the most important of which is that it is not a podotrematous crab. Its sternum exhibits features of a eubrachyuran, including the size, width, and shape of the sternites and the interrupted sternal sutures with a median line on sternites 7 and 8 (Figs 1.11, 2.6). Sternite 4 is longer than it is wide in D. americana, as in most eubrachyuran crabs, whereas Dromilites sensu stricto have a long anterior projection on sternite 4. No podotrematous crabs have a sternum like that of D. americana. Other differences between Dromilites americana and Dromilites sensu stricto are the presence of strong cervical, post-cervical, and branchiocardiac grooves in Dromilites, all of which are absent in D. americana. Dromilites has crispate lateral margins, lacking in D. americana. The carapace swellings of D. americana are large and spherical, whereas those of Dromilites are more subdued and not granular.

A new genus, Hyphalocarcinus, is erected to accommodate D. americana and is placed in Palaeoxanthopsidae as discussed. It differs from all other genera in the family in its longer than wide carapace and very large, inflated swellings on the carapace regions. Detailed differences from Tehuacana are discussed in the following. At this time, it is monotypic, with occurrences on the Gulf Coastal Plain of North America, late Danian to Selandian in age.

Armstrong et al. (Reference Armstrong, Nyborg, Bishop, Ossó-Morales and Vega2009) synonymized Tehuacana tehuacana Stenzel, Reference Stenzel1944, with Dromilites americana, resulting in Tehuacana americana (Rathbun, Reference Rathbun1935) as the species name and T. tehuacana as the junior synonym. They attributed the differences in dorsal carapace morphology between and among specimens of T. tehuacana and D. americana as being intraspecific variation but not sexual dimorphism. This synonymy was maintained by later workers (Luque et al., Reference Luque, Schweitzer, Santana, Portell, Vega and Klompmaker2017; Martínez-Díaz et al., Reference Martínez-Díaz, Aguillón-Martínez, Luque and Vega2017; Vega et al., Reference Vega, Ahyong, Espinosa, Flores-Ventura, Luna and Gonález-González2018).

To test the validity of this synonymy, we analyzed images and a cast of the holotype of Tehuacana tehuacana, and the holotype and paratype specimens of Dromilites americana, and other specimens assigned to both species (see Materials and methods). For this study, specimens were attributed to the Dromilites americana morphotype if they had a strongly domed carapace; large, spherical, granular carapace swellings; a rostrum with strongly produced axial spines; and/or posterior margins with blunt spines on each end (Fig. 2.2). Specimens were attributed to the Tehuacana morphology if they had a flattened carapace and flattened carapace regions without large spherical swellings; blunt short axial rostral spines; and/or lack of blunt spines on the posterior margin (Fig. 2.1).

Figure 2. (1, 3, 5, 7) Tehuacana tehuacana Stenzel, Reference Stenzel1944, holotype BEG021289: (1) dorsal, (3) anterior, (5) ventral, and (7) lateral views, unwhitened. (2, 4, 6, 8) Hyphalocarcinus americanus (Rathbun, Reference Rathbun1935) n. comb., paratype USNM PAL 335993B: (2) dorsal, (4) anterior, (6) ventral, and (8) left lateral views, whitened with ammonium chloride. Photos of Tehuacana by Stacie Skwarcan (UT). Scale bars = 5.0 mm.

Statistical analysis of carapace measurement ratios of specimens separated into these two morphologies showed significant differences with both t-test and Mann–Whitney analyses. The carapace length to carapace width ratio, the fronto-orbital width to carapace width ratio, and the length of the intestinal region to carapace length ratio were all significantly different at the p = 0.05 level between the two groups in both tests (Table 1; Supplementary Data Tables 1–5). Additional differences include the nature of the anterolateral spines. In the D. americana morphotype, the spines are discrete, conical, blunt structures with circular cross sections. In the Tehuacana morphotype, the spines are flattened, short, and sharp. The regions of the carapace in Tehuacana are granular, as are the large domed inflations of D. americana, but this is true of most genera within Palaeoxanthopsidae. Tehuacana has large posterolateral reentrants to house the bases of the fifth pereiopods, which are lacking in D. americana. In lateral view, the shape of the linea brachyura separating the branchiostegite from the pterygostomial region is sinuous in T. tehuacana (Fig. 2.7) and strongly arcuate in D. americana (Fig. 2.8) (Table 1).

Table 1. Similarities and differences between Tehuacana tehuacana and Hyphalocarcinus americanus n. comb.

We tested whether the two morphotypes were due to sexual dimorphism or ontogeny. The only specimens of either species for which sex is known are male (N = 1 each), so no evidence of sexual dimorphism is currently available. Moreover, the T. tehuacana morphology is not found in Alabama thus far, whereas both morphologies are present in Texas and Coahuila (Mexico). The various features of the carapace discussed here that separate the two groups are present regardless of size; thus, ontogeny does not explain the differences. Although there are superficial similarities between and among specimens referred to these two taxa, enough differences, some of which are statistically significant, exist to retain the two species as separate and in different genera. Dromilites americana is placed in a new genus herein, Hyphalocarcinus, resulting in Hyphalocarcinus americanus (Rathbun, Reference Rathbun1935) new combination. Tehuacana tehuacana remains the type species of that genus.

The specimens of Tehuacana schweitzerae Vega et al, Reference Vega, Nyborg, Coutiño and Hernández-Monzón2008 should be reevaluated to determine to which taxon they belong; the specimens are incomplete as illustrated. It should be noted that when the margins of specimens are broken or if the carapace lacks some or all layers of cuticle, the superficial similarities between the two morphologies may be magnified. The discussions of Dromilites americana, T. tehuacana, and T. schweitzerae in many recent publications should be used with caution as they are based either on misidentified specimens as constrained herein or on a combination of specimens referrable to multiple taxa.

Hyphalocarcinus americanus (Rathbun, Reference Rathbun1935) new combination

Figs 1.10. 1.11, 2.2, 2.4, 2.6, 2.8, 3

Figure 3. Hyphalocarcinus americanus (Rathbun, Reference Rathbun1935) n. comb., dorsal carapace except (5, 6). (1) ALMNH:Paleo:21474. (2) ALMNH:Paleo:21475. (3) ALMNH:Paleo:21476. (4) Holotype USNM PAL 371688 (image from https://collections.nmnh.si.edu/search/paleo/). (5) ALMNH:Paleo:21475, ventral view. (6) ALMNH:Paleo:21476, frontal view. (7) Paratype USNM PAL 335993B. (8) MMNS IP-7402. (9) USNM PAL 795622. (1–3, 5–9) Scale bars = 5.0 mm; (4) scale bar = 1.0 cm.

Reference Rathbun1935 Dromilites americana Rathbun, p. 79.

Reference Davidson1966 Dromilites americana; Davidson, p. 211.

Reference Bishop, Whitmore, Harris, Zulle and Otte1986 Dromilites americana; Bishop and Whitmore, fig. 1D.

Reference Guinot2008 Dromilites americana; Guinot, p. 21.

Reference Armstrong, Nyborg, Bishop, Ossó-Morales and Vega2009 Tehuacana americana (Rathbun, Reference Rathbun1935); Armstrong et al., p. 753.

Reference Schweitzer and Feldmann2010 Dromilites americana; Schweitzer et al., p. 64.

Reference Schweitzer and Feldmann2010 Dromilites americana; Schweitzer and Feldmann, p. 422.

Reference Schweitzer, Feldmann, Garassino, Karasawa and Schweigert2010 Dromilites americana; Franţescu et al., p. 264.

Reference Klompmaker, Jakobsen and Lauridsen2016 Dromilites americana; Klompmaker et al., table S9.

Reference Luque, Schweitzer, Santana, Portell, Vega and Klompmaker2017 Tehuacana americana (Rathbun, Reference Rathbun1935); Luque et al., p. 46.

Reference Martínez-Díaz, Aguillón-Martínez, Luque and Vega2017 Tehuacana americana (Rathbun, Reference Rathbun1935); Martinez-Díaz et al., p. 77, partim.

Reference Vega, Ahyong, Espinosa, Flores-Ventura, Luna and Gonález-González2018 Tehuacana americana (Rathbun, Reference Rathbun1935); Vega et al., fig. 5.13, 5.15.

Holotype

Originally identified as Dromilites americana, holotype, USNM PAL 371688.

Diagnosis

Diagnosis: As for genus.

Occurrence

Occurrences range in age from the late Danian to Selandian (Paleocene), in Alabama and Texas, USA, and Coahuila, Mexico (Table 2).

Table 2. Occurrences for Hyphalocarcinus americanus (Rathbun, Reference Rathbun1935) n. comb. Those with asterisks were studied from specimens or high-resolution images provided by museums. Non-asterisked entries are compiled from examination of published images only.

Description

Carapace longer than wide, length about 110% width; widest at about 40% distance posteriorly on carapace at position of fourth anterolateral spine (including outer-orbital projection); carapace strongly vaulted longitudinally, moderately vaulted transversely so that it is dome-like; entire surface covered with widely and evenly spaced setal pits. Front projected beyond orbits, notched axially and deeply axially sulcate, with wide granular rim, sides on either side of notch sloping distally, with slight projection at inner orbital angle; orbits broad, with very weak, blunt intraorbital projection bounded on each side by short fissures; suborbital margin projecting beyond upper orbital margin, with sub-inner orbital spine; fronto-orbital width about 60% carapace width. Outer-orbital spine small, anterolateral spines increasing in size posteriorly, projected increasingly anterolaterally, last spine directed laterally, spines with circular cross sections, blunt, ornamented with densely spaced granules on tips. Posterolateral margin initially straight, parallel, then converging posteriorly; posterior margin short, with strong, blunt spines at posterior corners, spines with dense granules on tips; posterior margin weakly concave between spines; posterior width about 32% carapace width.

Epigastric regions small, granular, weakly inflated; protogastric regions moderately inflated, densely granular; hepatic regions much lower than protogastric regions, forming a weakly to moderately inflated ovate swelling along anterolateral margin. Mesogastric region with long, narrow anterior process, greatly inflated posteriorly into spherical region, densely granular on surface; urogastric region depressed well below level of other regions, with concave lateral margins; cardiac region most inflated region of all carapace regions, transversely ovate, densely granular on surface; intestinal region long, depressed well below level of cardiac region, length from posterior edge of cardiac region to posterior margin about 26% carapace length. Epibranchial region arcuate, composed of two swellings, lateral swelling smaller, separated from axial swelling by deep groove, axial swelling ovate and positioned with long axis parallel to axis of carapace; both swellings densely granular on surface; mesobranchial region flattened anteriorly, with very strong transverse ridge at level of cardiac region, ridge densely granular on surface; metabranchial region depressed to level of intestinal region.

Branchiostegite and pterygostomial region high, linea brachyura wide, very clearly developed; coxae of pereiopods broadly spaced between pterygostome and sternum. Orbital area with pair of axial openings, orbital depression lateral to these openings; second pair of openings directly posterior to/under first pair; epistome a triangular projection between ventralmost pair of openings; endostome with oblique transverse ridges positioned just posterior to suborbital margin.

Sternites 1–2 fused. Sternite 3 long separated from 1–2 by shallow groove. Sternite 4 high, with two rounded swellings along lateral margin and on episternal projection; sternite 5 with sternal locking button on posterior margin; sternites 5, 6, and 7 with transverse ridges; sternites 7 and 8 directed strongly posterolaterally. Sternal sutures 4/5 and 5/6 incomplete; sutures 6/7 and 7/8 complete; sternal median line on sternites 7 and 8.

Materials

Dromilites americana, holotype, USNM PAL 371688; paratypes USNM 335993A–D, USNM PAL 795622–795626; UF 17090; ALMNH:Paleo:21474–21481, MMNS IP-7402; UT NPL 31165. Measurements (in mm) on specimens of Hyphalocarcinus americanus n. comb. are presented in Table 3 and Supplementary Data Table 1.

Table 3. Carapace measurements (in mm) taken on specimens of Hyphalocarcinus americanus (Rathbun, Reference Rathbun1935) n. comb. FOW = fronto-orbital width; PW = posterior carapace width; length = maximum length of carapace, including rostrum/front; width = maximum width of carapace taken at the bases of the last anterolateral spines.

Remarks

The morphology of specimens of Hyphalocarcinus americanus n. comb. is remarkably uniform across time, space, and size of individuals (Fig. 3). The smallest specimen has a proportionally wider fronto-orbital margin compared with larger specimens (Table 3). Ranges of variation in size of the anterolateral spines and granulation on the carapace swellings are present but not broad. The differences between and among specimens are related mostly to the state of preservation. If cuticle is preserved, ornamentation is observable, and spines appear more distinct than if cuticle is absent or poorly preserved.

Genus Tehuacana Stenzel, Reference Stenzel1944

Type species

Tehuacana tehuacana Stenzel, Reference Stenzel1944, by original designation.

Other species

Tehuacana schweitzerae Vega et al, Reference Vega, Nyborg, Coutiño and Hernández-Monzón2008.

Diagnosis

Carapace wider than long, flattened; anterolateral margins with four wide, triangular, flattened projections; regions well defined, with dense granular ornamentation centrally on each; posterolateral reentrants large; sternum wider than long, sternal median line absent on sternites 7 and 8.

Tehuacana tehuacana Stenzel, Reference Stenzel1944

Figs 1.13, 2.1, 2.3, 2.5, 2.7, 4

Figure 4. Tehuacana tehuacana Stenzel, Reference Stenzel1944, dorsal carapace. (1) Holotype. (2) MMNS IP-7399.1. (3) MMNS IP-7399.2. (4) USNM PAL 794470. Scale bars = 5.0 mm.

Reference Stenzel1944 Tehuacana tehuacana Stenzel, p. 546, figs. 1–3, pl. 93, figs. 6–9.

Reference Glaessner and Moore1969 Tehuacana tehuacana; Glaessner, p. R527.

Reference Karasawa and Kato2003 Tehuacana tehuacana; Karasawa and Kato, p. 138.

Reference Armstrong, Nyborg, Bishop, Ossó-Morales and Vega2009 Tehuacana americana (Rathbun, Reference Rathbun1935); Armstrong et al., p. 753.

Reference Karasawa, Schweitzer and Feldmann2008 Tehuacana tehuacana; Karasawa et al., p. 97.

Reference Schweitzer, Feldmann, Garassino, Karasawa and Schweigert2010 Tehuacana tehuacana; Schweitzer et al., p. 137.

Reference Martínez-Díaz, Aguillón-Martínez, Luque and Vega2017 Tehuacana americana (Rathbun, Reference Rathbun1935); Martínez-Díaz et al., p. 77, partim.

Reference Vega, Ahyong, Espinosa, Flores-Ventura, Luna and Gonález-González2018 Tehuacana americana (Rathbun, Reference Rathbun1935); Vega et al., fig. 5.17.

Holotype

Tehuacana tehuacana, holotype UT BEG 21289.

Diagnosis

As for genus.

Description

Carapace rectangular, slightly wider than long, length about 95% width, widest at about half the distance posteriorly on carapace; moderately vaulted transversely and longitudinally.

Frontal margin with shallow axial notch; frontal margins slope posteriorly to either side of notch. Orbits shallow, with short, blunt intraorbital projection, outer-orbital angle weakly projecting; fronto-orbital width about 67% carapace width. Anterolateral margins convex, with four projections, short with narrowed tips, projections increasing in size posteriorly, not really forming spines, with granular tips. Posterolateral margin weakly convex, with strong posterolateral reentrants; posterior margin including reentrants about 73% carapace width, excluding reentrants about 27% carapace width; posterior margin including reentrants with granular rim, central part of posterior margin straight.

Mesogastric region with long anterior process, widened posteriorly into pentagonal area; protogastric regions wider than long, weakly inflated; hepatic regions triangular, lower than protogastric regions; urogastric region long, depressed; cardiac region wide anteriorly, narrowing posteriorly, pentagonal; intestinal region short, flattened, about 15% carapace length. Epibranchial regions composed of two elongate segments, one directed from last anterolateral projection anteriorly and axially; second directed obliquely posteriorly toward axis and terminating at edge of urogastric region. Remainder of branchial regions undifferentiated, inflated centrally at level of cardiac region and flattened posteriorly. All regions densely granular on highest points.

Buccal frame rectangular, slightly wider than long; branchiostegite granular. Sternites 1 and 2 narrow. Sternite 3 wide, anteriorly with transverse ridge; sternite 4 wide, long, with short episternal projections. Sternites 5 and 6 shorter and wider than sternite 4, directed laterally. No axial line on sternites 7 and 8.

Materials

Tehuacana tehuacana, holotype UT BEG 21289 (cast and photos); USNM PAL 794470, 795627–795629; MMNS IP-7399 (four specimens). Measurements (in mm) taken on Tehuacana tehuacana are presented in Table 4 and Supplementary Data Table 2.

Table 4. Carapace measurements (in mm) taken on Tehuacana tehuacana Stenzel, Reference Stenzel1944. FOW = fronto-orbital width; PW = posterior carapace width; length = maximum length of carapace, including rostrum/front; width = maximum width of carapace taken at the bases of the last anterolateral spines.

Occurrence

Occurrences range in age from late Danian to Selandian (Paleocene), in Texas and Coahuila, Mexico (Table 5).

Table 5. Occurrences of Tehuacana tehuacana Stenzel, Reference Stenzel1944. Those with asterisks were studied from specimens, casts, or high- resolution images provided by museums. Non-asterisked entries are compiled from examination of published images only.

Remarks

Specimens of Tehuacana tehuacana look quite different with cuticle as compared with the mold of the interior of the holotype (Fig. 4). Whereas the holotype appears smoother (Fig. 4.1), the cuticle present in other specimens is granular, especially on the margins and raised areas of the carapace (Fig. 4.2–4.4). Specimens of Tehuacana tehuacana co-occur with Hyphalocarcinus americanus n. comb. in Texas and Mexico but appear to be much less common in Texas, on the basis of the collections studied herein.

Discussion

Palaeoxanthopsidae is already well known from the Late Cretaceous and early Cenozoic of Atlantic Ocean localities (Schweitzer, Reference Schweitzer2003; Luque et al., Reference Luque, Schweitzer, Santana, Portell, Vega and Klompmaker2017; Schweitzer et al., Reference Schweitzer, Feldmann and Karasawa2018) (Supplementary Data Table 6). The radiation of modern heterotrematous crabs, of which Paleoxanthopsidae is one, began in the Paleocene, diversifying in both reef and siliciclastic environments (Schweitzer and Feldmann, Reference Schweitzer and Feldmann2023). The new referrals to Palaeoxanthopsidae reinforce this pattern. The oldest palaeoxanthopsid genera are known from the Late Cretaceous, i.e., Palaeoxanthopsis from Brazil, Mexico, and Puerto Rico (Rathbun, Reference Rathbun1902; Vega et al., Reference Vega, Cosma, Coutiño, Feldmann, Nyborg, Schweitzer and Waugh2001; Schweitzer et al., Reference Schweitzer, Velez-Juarbe, Martinez, Collmar Hull, Feldmann and Santos2008, Reference Schweitzer, Feldmann and Karasawa2018; Luque et al., Reference Luque, Schweitzer, Santana, Portell, Vega and Klompmaker2017); the monotypic genera Palaeoxantho from the United States (Bishop, Reference Bishop1986) and Remia from Senegal (Remy and Tessier, Reference Remy and Tessier1954); Palaeoxanthopsidae sp. from Colombia (Luque et al., Reference Luque, Schweitzer, Santana, Portell, Vega and Klompmaker2017) and Jamaica (Morris, Reference Morris1993; Luque et al., Reference Luque, Schweitzer, Santana, Portell, Vega and Klompmaker2017); and the monotypic genera Lobulata and Rocacarcinus from the late Maastrichtian and early Danian of Argentina (Glaessner, Reference Glaessner1930; Feldmann et al., Reference Feldmann, Casadío, Chirino-Galvez and Aguirre-Urreta1995; Schweitzer et al., Reference Schweitzer, Feldmann and Gingerich2004, Reference Schweitzer, Feldmann and Karasawa2018; Schweitzer, Reference Schweitzer2005) (Fig. 5). After the end-Cretaceous mass extinction, the family seems to have persisted during the Paleocene and Eocene across the northern Atlantic into Alabama and Texas in the United States (Rathbun, Reference Rathbun1935; Armstrong et al., Reference Armstrong, Nyborg, Bishop, Ossó-Morales and Vega2009), Mexico (Vega et al., Reference Vega, Cosma, Coutiño, Feldmann, Nyborg, Schweitzer and Waugh2001, Reference Vega, Nyborg, Coutiño and Hernández-Monzón2008), Europe (Segerberg, Reference Segerberg1900; Robin et al., Reference Robin, Van Bakel, Pacaud and Charbonnier2017; Miller et al., Reference Miller, Schweitzer and Feldmann2023), and Greenland (Collins and Rasmussen, Reference Collins and Rasmussen1992), possibly filling niches vacated by the end-Cretaceous extinction there, during which well over half of the Gulf Coastal genera became extinct (Schweitzer and Feldmann, Reference Schweitzer and Feldmann2023). The family disappeared during the Eocene, as indicated by the last known occurrences of palaeoxanthopsids, i.e., Verrucoides stenohedra Vega et al., Reference Vega, Cosma, Coutiño, Feldmann, Nyborg, Schweitzer and Waugh2001 and Tehuacana schweitzerae from the Ypresian of Mexico (Vega et al., Reference Vega, Cosma, Coutiño, Feldmann, Nyborg, Schweitzer and Waugh2001, Reference Vega, Nyborg, Coutiño and Hernández-Monzón2008) (Fig. 5; Supplementary Data Table 6).

Figure 5. Stratigraphic ranges and paleobiogeographic distribution of palaeoxanthopsid crabs through the Late Cretaceous (Maastrichtian) and early Paleogene (Paleocene to early/middle Eocene). Base map after Scotese (Reference Scotese2016).

Acknowledgements

L. Boucher and S. Skwarcan, Jackson School Museum of Earth History, University of Texas at Austin, USA, provided images of the holotype of Tehuacana tehuacana as well as images of other specimens referred to that genus. N. Drew and J. Nakano facilitated the loan and study of specimens from the collections in the Department of Paleobiology, USNM. R. Portell, Florida Museum, University of Florida at Gainesville, provided images of specimens from that collection. C. LaBonte (KSU) drew renewed attention to the issues attendant with the generic placement of Dromilites americana. We thank P. Atkinson for collecting one of the figured specimens (ALMNH:Paleo:21476) from Alabama, A. Armstrong for collecting the MMNS specimens from Texas, and J. Shaw for site access in Alabama. Most of the images of Palaeoxanthopsidae herein were taken by R.M. Feldmann (late of KSU). The manuscript benefitted from detailed reviews by H. Karasawa (Mizunami Fossil Museum, Japan), B.W.M. Van Bakel (Oertijdmuseum and Utrecht University, the Netherlands), and two anonymous reviewers. Thanks to all who assisted us in this effort.

Competing interests

The authors declare that they have no known competing interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability statement

Supplementary Data Tables 1–6 may be found at https://doi.org/10.5281/zenodo.15148347.

Footnotes

Guest Editor: Ovidiu Franţescu

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Figure 0

Figure 1. Palaeoxanthopsidae. (1, 2) Lobulata lobulata (Feldmann et al., 1995), holotype GHUNLPam 7001: (1) dorsal view; (2) ventral view (Feldmann et al., 1995, fig. 7.1, 7.2). (3) Jakobsenius cretaceus (Segerberg, 1900), KSU D 37, cast of holotype MGUH 2483. (4, 5) Paraverrucoides alabamensis (Rathbun, 1935): (4) dorsal view of holotype USNM PAL 371718; (5) ventral view of paratype USNM PAL 371707. (6) Palaeoxantho libertiensis Bishop, 1986, holotype GSCM 1692. (7, 8) Rocacarcinus gerthi (Glaessner, 1930), holotype Glaessner 1, UBIP: (7) dorsal view; (8) ventral view (Feldmann et al., 1995, fig. 9.3, 9.4). (9) Palaeoxanthopsis cretacea Rathbun, 1902, syntype USNM PAL 73709. (10, 11) Hyphalocarcinus americanus (Rathbun, 1935) n. comb., paratype USNM PAL 335993B: (10) dorsal view; (11) ventral view. (12) Remia africana (Remy in Remy and Tessier, 1954), KSU D 1100, cast of holotype MNHN R03885. (13) Tehuacana tehuacana Stenzel, 1944, holotype BEG021289 (photo by S. Skwarcan). (14) Verrucoides verrucoides (Collins and Rasmussen, 1992), KSU D 1803, cast of holotype MGUH 21.612. (1–12, 14) Scale bars = 1.0 cm; (13) scale bar = 5.0 mm. All except (10, 11, 13) appear in Schweitzer et al. (2018).

Figure 1

Figure 2. (1, 3, 5, 7) Tehuacana tehuacana Stenzel, 1944, holotype BEG021289: (1) dorsal, (3) anterior, (5) ventral, and (7) lateral views, unwhitened. (2, 4, 6, 8) Hyphalocarcinus americanus (Rathbun, 1935) n. comb., paratype USNM PAL 335993B: (2) dorsal, (4) anterior, (6) ventral, and (8) left lateral views, whitened with ammonium chloride. Photos of Tehuacana by Stacie Skwarcan (UT). Scale bars = 5.0 mm.

Figure 2

Table 1. Similarities and differences between Tehuacana tehuacana and Hyphalocarcinus americanus n. comb.

Figure 3

Figure 3. Hyphalocarcinus americanus (Rathbun, 1935) n. comb., dorsal carapace except (5, 6). (1) ALMNH:Paleo:21474. (2) ALMNH:Paleo:21475. (3) ALMNH:Paleo:21476. (4) Holotype USNM PAL 371688 (image from https://collections.nmnh.si.edu/search/paleo/). (5) ALMNH:Paleo:21475, ventral view. (6) ALMNH:Paleo:21476, frontal view. (7) Paratype USNM PAL 335993B. (8) MMNS IP-7402. (9) USNM PAL 795622. (1–3, 5–9) Scale bars = 5.0 mm; (4) scale bar = 1.0 cm.

Figure 4

Table 2. Occurrences for Hyphalocarcinus americanus (Rathbun, 1935) n. comb. Those with asterisks were studied from specimens or high-resolution images provided by museums. Non-asterisked entries are compiled from examination of published images only.

Figure 5

Table 3. Carapace measurements (in mm) taken on specimens of Hyphalocarcinus americanus (Rathbun, 1935) n. comb. FOW = fronto-orbital width; PW = posterior carapace width; length = maximum length of carapace, including rostrum/front; width = maximum width of carapace taken at the bases of the last anterolateral spines.

Figure 6

Figure 4. Tehuacana tehuacana Stenzel, 1944, dorsal carapace. (1) Holotype. (2) MMNS IP-7399.1. (3) MMNS IP-7399.2. (4) USNM PAL 794470. Scale bars = 5.0 mm.

Figure 7

Table 4. Carapace measurements (in mm) taken on Tehuacana tehuacana Stenzel, 1944. FOW = fronto-orbital width; PW = posterior carapace width; length = maximum length of carapace, including rostrum/front; width = maximum width of carapace taken at the bases of the last anterolateral spines.

Figure 8

Table 5. Occurrences of Tehuacana tehuacana Stenzel, 1944. Those with asterisks were studied from specimens, casts, or high- resolution images provided by museums. Non-asterisked entries are compiled from examination of published images only.

Figure 9

Figure 5. Stratigraphic ranges and paleobiogeographic distribution of palaeoxanthopsid crabs through the Late Cretaceous (Maastrichtian) and early Paleogene (Paleocene to early/middle Eocene). Base map after Scotese (2016).