Planobranchia, Schweitzer and Feldmann, 2010

Ferratges, Fernando A., Domínguez, Josep Lluis, Ossó, Àlex & Zamora, Samuel, 2023, Spider crabs (Decapoda: Brachyura: Majoidea) from the upper Eocene of south Pyrenees (Huesca, Spain), Palaeontologia Electronica (a 27) 26 (2), pp. 1-29 : 11-14

Genus PLANOBRANCHIA Schweitzer and Feldmann, 2010

Type species. Micromaia laevis Lőrenthey, 1909 , by original designation of Schweitzer and Feldmann (2010, p. 407).

Fossil included species.? Planobranchia egyptensis Feldmann, Schweitzer, Bennett, Franţescu, Resar, and Trudeau, 2011 ; P. elongata n. sp. (herein); P. laevis ( Lőrenthey, 1909) ; P. palmuelleri Artal, Van Bakel, and Onetti, 2014 ; P. simplex (Remy in Gorodiski and Remy, 1959).

Emended diagnosis. Carapace pyriform, widest at midlength of branchial region; moderately vaulted transversely and longitudinally, front produced, singular, sulcate longitudinally. Orbits small, laterally situated, with strong and subtriangular orbital spines. Gastric regions only weakly differentiated; defined laterally by prominent V-shaped Groove converging from anterior margin of orbits to urogastric region, the narrowest part of axial regions. Cardiac region nearly as wide as widest part of gastric regions, hexagonal to ovoid in outline; bearing two nodes on medial transverse ridge. Intestinal region well defined, long, approximately as wide as urogastric region. Epibranchial and mesobranchial regions strongly inflated, separated from one another by subtle arcuate attachment scar expressed on mold of the interior of the carapace; widest part of these regions converge as angular projections toward urogastric region. Metabranchial region extends from widest part of cardiac region posterolaterally around posterior margin of metabranchial region and clearly defined axially by posterior margin of cardiac region and intestinal region; depressed below other regions. Surface of carapace weakly ornamented by fine granules or pits; lacking strong tubercles, posterior margin convex, rimmed (new additions to the original diagnosis of Schweitzer and Feldmann, 2010).

Remarks. The studied specimen can be assigned to Planobranchia Schweitzer and Feldmann, 2010 , because it shares the diagnostic characteristics of the genus (see Schweitzer and Feldmann, 2010) like: 1) the moderately vaulted transversely and longitudinally pyriform carapace; 2) weakly differentiated gastric regions, defined laterally by prominent V-shaped groove converging from anterior margin of orbits to urogastric region; 3) hexagonal to ovoid cardiac region, bearing two nodes on medial transverse ridge; 4) strongly inflated epibranchial and mesobranchial regions, separated from one another by subtle arcuate attachment scar; 4) weakly ornamented surface of the carapace by fine pits, lacking strong tubercles.

Some authors assigned Planobranchia to the subfamily Majinae (Schweitzer and Feldmann, 2010; Feldmann et al., 2011; and Schweitzer et al., 2020), and justify this inclusion by the supraorbital margin with an “eave orbital” and a postorbital spine. Subsequently, Artal et al. (2014) proposed to include the genus Planobranchia in Inachidae , justifying its inclusion by similarities in the frontal and orbital construction. Nevertheless, due to the bad preservation, specifically of the pseudorostrum and part of the supraorbital margin, Artal et al. (2014) have misinterpreted the fronto-orbital conformation and the anterolateral spines. Due to the lack of the anterior part of the pseudorostrum spines, these authors have suggested that Planobranchia could have a short pseudorostrum like many inachids, placing the orbits laterally on the sides of the pseudorostrum. Also, the antorbital spine has been interpreted as the postorbital spine, and the two following spines (intercalated spine and postorbital spine) as anterolateral spines. Instead, Planobranchia has a rather elongated rostrum and an orbital construction consisting of three spines.

The characteristics of the new material and the reanalysis of previously known taxa suggest that this genus has more affinity with the subfamily Pisinae , so its inclusion in this group is suggested here. Placement in Pisinae is supported by the morphology of the carapace outline, the distribution and shape of the dorsal regions, supraorbital margin formed by a prominent antorbital spine, a small intercalated spine, and a well-developed postorbital spine; axial regions separated from the periphery by deep grooves; hepatic lobe marked by a lump or spine; highly developed branchial regions; thickened cardiac region; mesogastric region in which a prominent lump stands out (see Griffin and Tranter, 1986, Schweitzer et al., 2020).