Bajoprosopon, van Bakel & Maerten & Jagt & Fraaije, 2021

van Bakel, Barry W. M., Maerten, Lionel, Jagt, John W. M. & Fraaije, René H. B., 2021, Bajoprosopon piardi n. gen. and sp. from the Middle Jurassic of France, with a revised diagnosis of the family Prosopidae von Meyer, 1860 (Brachyura, Podotremata) and notes on the availability of names introduced by Hermann von Meyer (1835, 1857), Palaeontologia Electronica (a 26) 24 (2), pp. 1-15 : 7-8

Genus BAJOPROSOPON nov.

zoobank.org/ 0F4A9BFA-57EC-4CD0-B6E0-AAE8A75C1DB2

Type species. Bajoprosopon piardi n. sp., the sole species known to date.

Derivation of name. A combination of the Bajocian Stage and the generic name Prosopon , the nominative genus of the Prosopidae .

Diagnosis. Carapace convex in both directions, suboval in outline, longer than wide (L/W ratio 1.4 [excluding rostrum]). Carapace widest in posterior third. Fronto-orbital margin occupying entire anterior carapace margin; orbits conspicuously large, anteriorly directed, with distinct inner orbital rim. Lateral carapace margins rounded, not defined. Carapace subequally tripartite by well-developed, subparallel, widely V-shaped cervical and branchial grooves, both complete and continuing onto lateral carapace flanks. Mesogastric region large, well delineated, with narrow anterior process and large subtriangular posterior portion. Cardiac region relatively small, less than one third of maximum carapace width, diamond shaped. Carapace anterior of branchial groove with coarse, non-central tubercles; posterior branchial region granular.

Remarks. Bajoprosopon n. gen. can be assigned to the Prosopidae with confidence, in displaying all diagnostic features (see above). Of other members of the family, it can be differentiated as follows:

Bajoprosopon n. gen. differs from Acareprosopon (type: Pithonoton bouvieri Van Straelen, 1944 ; upper Albian, Navarra, northern Spain) in having a weaker branchial condensation (i.e., cervical and branchial grooves further apart), a branchial groove that is more divergent and subparallel to the cervical groove, regions anterior to branchial groove with low tubercles, a less elongate cardiac region, and lateral margins more equally divided into three portions (vs posterior portion much longer than anterior two portions in Acareprosopon ).

Bajoprosopon n. gen. differs from Europrosopon (type: Prosopon verrucosum Reuss, 1858 [= Prosopon aculeatum von Meyer, 1860 ]) in having a more elongate carapace (L/ W 1.4 for Bajoprosopon n. gen. vs 1.24 for Europrosopon ; both measured without rostrum), regions anterior of branchial groove with low tubercles, and orbits more anteriorly directed (vs anterolateral in Europrosopon ).

Bajoprosopon n. gen. differs from Nipponopon (type: N. hasegawai Karasawa, Kato and Terabe, 2006 ; Barremian, Gunma Prefecture, Japan) in having a more elongate carapace (vs pyriform) with subparallel lateral margins, a weaker branchial condensation, a branchial groove that is more divergent, and subparallel to the cervical groove. Nipponopon and Rathbunopon in particular are morphologically close. Karasawa et al. (2006, p. 345) described the differences as follows, “... the carapace is slightly wider than long and triangular in outline, the protogastric region bears three pointed tubercles, the cervical groove is complete and deep, and the urogastric region is weakly bilobed and is not ornamented with two transverse ridges.” The original material appears to be slightly compressed and additional specimens are needed for a more reliable reconstruction and comparison with Rathbunopon and other prosopids.

Bajoprosopon n. gen. differs from Prosopon (type: P. tuberosum von Meyer, 1840 ) in having a less pyriform carapace (i.e., sides more subparallel), in lacking inflated epibranchial and hepatic bulges on lateral flanks, in having anteriorly, rather than anterolaterally, directed orbits, in possessing tubercular anterior regions and a weaker branchial condensation (i.e., cervical and branchial grooves further apart).

Bajoprosopon n. gen. differs from Protuberosa (type: Prosopon protuberosum Wehner, 1988 ) by the same characters as listed for Prosopon . In addition, Bajoprosopon n. gen. can be differentiated from Protuberosa in having coarse tubercles on regions anterior to branchial groove, a subhorizontal orbital margin (rather than an oblique one), and in having substraight lateral margins instead of clearly constricted ones in epibranchial regions.

Bajoprosopon n. gen. differs from Rathbunopon (type: R. polyakron ) in having a more elongate carapace, not pyriform in outline (i.e., having subparallel lateral margins instead of convergent ones) and in having a weaker branchial condensation.

Of other primitive and coeval forms, Bajoprosopon n. gen. differs from Laeviprosopon (type: Prosopon laeve von Meyer, 1860 , by original designation) in lacking a linea homolica (as far as can be observed in the sole known specimen; a linea homolica is not present in all specimens of Laeviprosopon , as N. Starzyk and colleagues will document shortly [work underway]), in having a projected, wide, and normal front (non-projecting, trifurcate in Laeviprosopon ) and in having well-delineated, deep orbital fossae. The posterior margin (simple and concave in Laeviprosopon ) is unfortunately not preserved in Bajoprosopon n. gen., it could be another distinguishing character. The new genus differs from Homolus (type: H. auduini Eudes-Deslongchamps, 1835 , Bathonian, Ranville, Calvados, France) in having a much weaker branchial condensation (i.e., branchial groove well distant from cervical groove), branchial groove subparallel to cervical groove (branchial groove more V-shaped, under steeper angles than cervical groove, in Homolus ), as well as by having well-delineated orbital fossae (undefined, protected by coarse spines, in Homolus ). Unfortunately, the front is damaged in the single available specimen of Bajoprosopon piardi n. gen. and sp. In view of the fact that the orbits are of a morphologically similar construction to that of prosopids, the front may have been closer to that of prosopids than to the bifurcate one in Homolus and homolodromiids.

Eoprosopon (type and sole known species: E. klugi Förster, 1986 ) has a different groove pattern: the cervical groove is not formed of three arcs, but rather takes the form of a wide, continuous ‘V’; in addition, the branchial groove is parallel to the cervical groove. For further details, see above.