Planobranchia, Schweitzer & Feldmann, 2010

Schweitzer, Carrie E. & Feldmann, Rodney M., 2010, New fossil decapod crustaceans from the Remy Collection, Muséum national d’Histoire naturelle, Paris, Geodiversitas 32 (3), pp. 399-415 : 407-408

Genus Planobranchia n. gen.

TYPE SPECIES. — Micromaja laevis Lőrenthey, 1909: 119 , pl. 1, fig. 2a, b, by present designation.

INCLUDED SPECIES. — Planobranchia laevis ( Lőrenthey, 1909) ; P. simplex (Remy in Gorodiski & Remy, 1959).

ETYMOLOGY. — Th e generic name alludes to the depressed and flattened metabranchial region which serves to readily distinguish the genus from closely related forms. The gender is feminine.

DIAGNOSIS. — Carapace pyriform, widest at midlength of branchial region; moderately vaulted transversely and longitudinally. 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.

DISCUSSION

Planobranchia laevis n. comb. and P.simplex n. comb. were both originally assigned to Micromaia based upon overall similarities in outline and, in the absence of other majid genera that the species more closely resembled,the assignments seemed reasonable.However, the two species exhibit several morphological features that readily distinguish them from other species assigned to Micromaia . Bittner (1875) originally defined Micromaia tuberculata , both the genus and species, simultaneously. Thus, species referable to Micromaia must conform to the concept of the genus as defined by Bittner (1875: 76) within his specieslevel description. Th e elements of his description of Micromaia tuberculata that we take as characterizing the genus Micromaia include a pyriform outline; very poorly defined regions, sometimes defined only by the pattern of tubercles; axial regions that are more or less uniform in width, and with the intestinal region being very short or not differentiated; branchial regions that are undifferentiated and uniformly inflated; and a uniform, dense array of coarse, often truncated-conical tubercles over the entire carapace. The orbits are within the range of typical majids: directed anterolaterally and arising posterior to the bifid, weakly downturned rostrum.

The two species herein removed from Micromaia referred to Planobranchia n. gen. have regions that are weakly defined by grooves, rather than tubercle patterns; axial regions that are quite variable in width, and with the cardiac region being nearly the widest region; a distinct, long, narrow intestinal region; a depressed, flattened metagastric region that is clearly differentiated from the epibranchial and mesobranchial regions;and an overall absence of strong ornamentation.Although the fronts of both type specimens of P.laevis n. comb. and P.simplex n. comb. are absent, the orbital margin appears to be quite similar to other majids and, at least on P. simplex n. comb., there appears to be a slight indication of a bifid rostrum. Thus, representatives of the two genera are readily distinguishable, based upon characters that are taxonomically important at the generic level in majids.