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1. Introduction
The middle Eocene Castle Hayne Formation (North Carolina) has produced a rich decapod fauna including 13 families (excluding incertae sedis), 19 genera, and 23 species. Though Rathbun (1935) reported two decapods from this formation, there has been little interest in fossil crabs until recently. Bishop and Whitmore (1986) figured several fossils from the Castle Hayne Formation, providing preliminary assignments without descriptions. Later, Salva et al. (1995) introduced the rich decapod fauna and its palaeogeographic implications and Blow and Manning (1996) reported three new genera and eight new species from this formation. Feldmann et al. (1998) gave a new impulse to the knowledge of the decapod fauna of the Castle Hayne Formation, describing four new species and reporting one species previously recorded from the Santee Limestone of South Carolina by Blow and Manning (1996). The most recent contributions to the decapod fauna of the Castle Hayne Formation have been provided by Ossó and Clements (2016) and Davis et al. (2020) who described a new genus and a new species respectively, the latter representing the first report of a decapod crab from the Spring Garden Member of the Castle Hayne Formation (see Table 1).
Table 1 Updated list of decapod crustaceans from the Castle Hayne Formation (after Blow and Manning, 1996; Feldmann et al., 1998; Clements, 2014; Ossó and Clements, 2016; Davis et al., 2020).
The purpose of this paper is the description of a new spider crab, Eoinachoides bretoni n. sp. from the Comfort Member of the Castle Hayne Formation from which all decapod crabs known to date have been collected, except Matutites collinsiDavis, Garassino and Weaver, 2020, recovered from the Spring Garden Member of the same formation. It represents the second spider crab reported from the Comfort Member.
2. Geological setting
Eoinachoides bretoni n. sp. was collected from the Comfort Member of Ward et al. (1978) from Bartonian aged rocks of the Castle Hayne Formation in the north quarry at approximately 34° 50’ 34.4”N x 77° 32’ 45.3”W (Figure 1). Harris and Zullo (1991) reported five sequences within the Castle Hayne ranging in age from Lutetian to Priabonian. Temporal placement of Eoinachoides bretoni n. sp is possible based upon Kier’s echinoid biozones (Kier, 1980). Indeed, E. bretoni n. sp was found associated with Middle Biozone echinoids such as Linthia harmatuckiKier, 1980 and Eurodia rugosa depressaKier, 1980. Kier’s Middle Biozone correlates to sequence 2 and 3 of Harris and Zullo (1991) which is Bartonian. An excellent graphic correlating the lithosomes of Ward et al. (1978), the sequence stratigraphy of Harris and Zullo (1991) and Kier’s echinoid biozones can be found in Ciampaglio et al. (2007).
Figure 1 Map of the Onslow County (North Carolina, USA), reporting the coordinates of the fossiliferous locality.
Simms (2012: 42) reported nearly 100 ft. (ca. 30 m.) of Castle Hayne Formation at the Onslow Quarry. The overburden varies from 2 to 57 ft. (from ca. 0.6 m. ca. 17 m.) with much of the variable thickness a factor of the Castle Hayne formation’s dissolution and sediment filled stream erosion features, as the modern landscape has almost no relief. The section where E. bretoni n. sp was discovered is in the upper product limestone at the new north quarry. In other areas with less overburden in nearby quarries, there have been younger Castle Hayne formation rocks that include Kier’s late echinoid biozone above a sequence break that is Priabonian. The provenance of E. bretoni n. sp. does not include rocks of this age.
3. Systematic paleontology
Superfamily Majoidea Samouelle, 1819
Family Epialtidae MacLeay, 1838
Subfamily Epialtinae MacLeay, 1838
Genus EoinachoidesVan Straelen, 1933
Type species: Eoinachoides senniVan Straelen, 1933, by original designation [late Eocene (Priabonian) - Venezuela (South America)].
Other fossil species: Eoinachoides latispinosaCarriol, de Muizon and Secrétan, 1987, Miocene - Argentina, Peru, Venezuela (South America); E. bretoni n. sp., middle Eocene (Bartonian) - North Carolina (USA) (herein).
Figure 2 Eoinachoides bretoni n. sp., NCSM 12629. A) Dorsal view. B) Lateral views. C) Frontal view. D) Close-up of the frontal and gastric regions. Scale bar equals 1 cm. Photographs: R. Chandler.
Diagnosis: Pyriform carapace, slightly longer than wide; front slightly protruded forward, axially depressed, with a pair of lateral tuberculate ridges; protogastric region with one median tubercle; triangular-shaped mesogastric region with one distal tubercle; subpentagonal-shaped cardiac region with a pair of median tubercles; smooth depressed intestinal region; smooth orbital and hepatic regions; inflated subhepatic region with a transverse ridge of three aligned small tubercles; epibranchial region with one lateral tubercle; mesobranchial region with a median tubercle; gastric regions slightly tuberculate; hepatic, cardiac, and branchial regions covered with small tubercles uniformly arranged.
Etymology: after Gérard Breton (1944-2019) for his important contribution to the knowledge of the Cenozoic fossil crabs.
Type material: Holotype, NCSM 12629 (carapace length: 26 mm; maximum carapace width at level of mesobranchial region: 23 mm). (NCSM = NC Museum of Natural Sciences, Raleigh, North Carolina, USA).
Type locality: Onslow quarry (= Richlands quarry), Onslow County, NC.
Stratigraphy: Eocene (Bartonian), Comfort Member of the Castle Hayne Formation.
Description: Pyriform carapace, slightly longer than wide, widest at level of mesobranchial regions; carapace regions with swellings; apparently short bifid rostrum slightly protruded forward, axially depressed; very narrow orbits; supraorbital eave poorly preserved, apparently without intercalated spine; slightly spiny anterolateral margin almost straight; slightly spiny posterolateral margin strongly convex; smooth, short posterior margin weakly convex medially; smooth epigastric region; protogastric region with one median large tubercle and a pair of small median tubercles, aligned to form a square-shaped structure; triangular-shaped mesogastric region with one distal tubercle; smooth metagastric region with two deep gastric pits; smooth depressed urogastric region; pentagonal-shaped cardiac region with a pair of median tubercles; smooth depressed intestinal region; smooth orbital and hepatic regions; inflated subhepatic region with a transverse ridge of three aligned small tubercles; branchial regions strongly inflated; epibranchial region with one lateral tubercle; mesobranchial region with a median tubercle; smooth metabranchial region; deep, sinuous cervical groove; gastric regions slightly tuberculate; hepatic, cardiac, and branchial regions covered with small tubercles uniformly arranged. Cephalic and thoracic appendages and ventral parts not preserved.
Discussion: Although the supraorbital eave is poorly preserved, based upon Schweitzer et al. (2020), the studied specimen has been tentatively assigned to EoinachoidesVan Straelen, 1933 in having carapace ovate, narrowed anteriorly and widened posteriorly, axial regions well defined, and cardiac region with a pair of tubercles. Based upon Schweitzer et al. (2010) this genus includes just two species, E. senniVan Straelen, 1933 from the Eocene of Venezuela and E. latispinosaCarriol, de Muzon and Secrétan, 1987, from the Miocene of Argentina, Peru, and Venezuela (Feldmann and Schweitzer, 2004; Collins et al., 2009; Aguilera et al., 2010; Luque et al., 2017).
Eoinachoides senni was described by Van Straelen (1933: 6, 7) from one complete carapace. The smooth protogastric regions distinguish this species from E. bretoni n. sp. having protogastric regions with one median tubercle.
Eoinachoides latispinosa was described by Carriol, de Muizon and Secrétan (1987: 8, 9) from one incomplete carapace. The smooth protogastric regions, the mesogastric region with three aligned tubercles, and the mesobranchial regions with a semicircle row of four tubercles distinguish this species from E. bretoni n. sp. having protogastric regions with one median tubercle, mesobranchial region with one distal tubercle, and mesobranchial region with one median tubercle.
In conclusion, although the new species does not enlarge the stratigraphical range for the genus which is currently restricted from the Eocene to the Miocene, it represents the first report in North America, extending the palaeogeographic distribution of Eoinachoides, limited currently in the fossil record of South America.
