BRACHYURA Linnaeus, 1758

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 : 5-7

Infraorder BRACHYURA Linnaeus, 1758 Section PODOTREMATA Guinot, 1977 Subsection DYNOMENIFORMIA Guinot, Tavares, and Castro, 2013

Superfamily HOMOLODROMIOIDEA Alcock, 1900

Family PROSOPIDAE von Meyer, 1860

Included genera (see Figure 3 ). Acareprosopon Klompmaker, 2013 ; Bajoprosopon n. gen.; Europrosopon Klompmaker, Starzyk, Schweigert and Fraaije, 2020 ; Nipponopon Karasawa, Kato and Terabe, 2006 ; Prosopon von Meyer, 1840 ; Protuberosa Schweitzer and Feldmann, 2009 and Rathbunopon Stenzel, 1945 .

Diagnosis. Carapace elongated, longer than wide, suboval in outline, widest at posterior branchial region, may be constricted at epibranchial regions; tumid in longitudinal and transverse sections, lateral margins undefined, blunt; hepatic and epibranchial margin may be developed into spine or tubercle. Carapace tripartite by well-separated, complete, distinct cervical, and branchial grooves. Cervical groove wide, divided into three arcs; branchial grooves at steeper angle. Fronto-orbital margin wide, front wide, projected beyond orbits, subtrapezoidal, about one-third of maximum carapace width; orbital margin oblique to subhorizontal, with two coarse, shallow notches; orbital fossae large, subdivided by arched rim, orbits anteriorly and weakly anterolaterally directed. Anterior carapace regions usually inflated, mesogastric region completely delineated, with large, broadly triangular base; protogastric region not completely subdivided by oblique groove; cardiac region well defined, inverted triangular, completely delineated, urogastric region as distinct low rectangle. Posterior margin sinuous or W-shaped (or ‘biconvex’), occupying entire carapace width. Dorsal carapace surface densely granular, central and anterior regions may have low nodes or tubercles. Elements other than carapace are unknown.

Remarks. Three species previously assigned to Prosopon , namely P. abbreviatum Schweitzer and Feldmann, 2009 , P. verrucosum [= P. aculeatum ] and P. barbulescuae Schweitzer, Feldmann, Lazăr, Schweigert and Franţescu, 2018 , have recently been reassigned to a new genus, Europrosopon , by Klompmaker et al. (2020). These authors also argued that the orbital construction in Prosopon , Europrosopon , and Rathbunopon was less unique than previously thought by Schweitzer et al. (2012, 2018, pp. 326, 327); we concur. The diagnosis supplied by Schweitzer and Feldmann (2009, p. 65) is outdated and based upon a different composition; Fraaije et al. (2013, p. 252) subsequently remarked that the diagnoses of the families Prosopidae and Tanidromitidae were not sufficiently diagnostic. Klompmaker et al. (2020, p.19) did remark on the Prosopidae , but failed to provide a novel diagnosis. Thus, a new diagnosis is presented here (see above).

In recent years, Laeviprosopon has generally been included in the Prosopidae (Schweitzer and Feldmann, 2008, p. 274; Guinot, 2019, p. 774; Klompmaker et al., 2020, p. 19; Starzyk, 2020, p. 10). Here, we exclude it from that family, because it possesses a linea homolica (compare Patrulius, 1966, Collins and Wierzbowski, 1985; N. Starzyk and colleagues, work underway; BWMvB, pers. obs.), a weakly projecting trifurcate front, a nonsinuous, arched concave posterior margin, and undefined orbits (the cornea resting on the dorsal carapace surface). In addition, Laeviprosopon usually has a complete oblique groove that subdivides the protogastric region. The genus is here interpreted as an early representative of the superfamily Homoloidea , and thus excluded from the Homolodromioidea , adopting the views expressed by several previous authors (e.g., Patrulius, 1966; Collins and Wierzbowski, 1985). This fact is the subject of further study (N. Starzyk and colleagues, work under way).

Acareprosopon , from the upper Albian of Navarra (northern Spain), has a rostrum with wide, upturned extensions, and a strongly downturned tip, which is developed into a pointed triangular front. At present, it is unclear if other members of the Prosopidae also had such a downturned tip. The orbital fossae in Acareprosopon are large, anterolaterally directed, with a median arched rim; it thus documents the configuration that is typical of members of the Prosopidae .

Taxonomic placement of the ancient, monospecific genera Homolus Eudes-Deslongchamps, 1835 and Eoprosopon Förster, 1986 is difficult and currently not unambiguous. The available material of these taxa is severely limited and rather poorly preserved; in addition, coeval brachyurans are either missing or extremely rare. Eoprosopon was studied in detail by Haug and Haug (2014), who concluded that it belonged to the superfamily Homolodromioidea . At first sight, the carapace of Eoprosopon is quite similar to that of members of the Prosopidae . In general, Eoprosopon would fit our new diagnosis of the family, but there are some differences. For example, the cervical groove in Eoprosopon is not formed by three arcs, such as in members of the Prosopidae , but takes the form of a wide, continuous ‘V’ such as in homolodromiids. The basal part of the mesobranchial region is clearly outlined in prosopids, but weakly defined in Eoprosopon . The branchial groove in Eoprosopon is parallel to the cervical groove (as in homolodromiids) and unlike the more steeply angled branchial groove seen in prosopids. The front in Eoprosopon is not well preserved, but it does not seem to be prominent, projected and wide as it is in prosopids. Although there is no absolute certainty, we do agree with Haug and Haug (2014) in considering Eoprosopon to be best accommodated in the Homolodromiidae rather than in the Prosopidae , at least for the time being.

Homolus has been placed in the Homolodromiidae (rather than the Prosopidae ) by all recent authors (e.g., Guinot, 1995, pp. 164, 265; Schweitzer and Feldmann, 2010, p. 252; Krobicki and Zatoń, 2016, p. 705; Guinot, 2019, p. 766). Homolus has a bifurcate rostrum, which matches that of homolodromiids better than that of prosopids. More importantly, Homolus lacks well-defined orbital fossae, but has undefined orbits instead (see Schweitzer and Feldmann, 2010, figure 2B, C); the eyestalk is protected by the rostral and outer orbital spines, which is the normal condition in the Homolodromiidae . Thus, Homolus does not fit the new diagnosis of the Prosopidae , and we agree with placement in the Homolodromiidae , at least for the time being.

Figures

FIGURE 3. Composition of the family Prosopidae, as considered herein. A. Europrosopon (E. verrucosum), reconstructed after Klompmaker et al. (2020: figure. 11E). B. Rathbunopon (R. obesum), reconstructed after Klompmaker et al. (2011: figure 3C). C. Bajoprosopon n. gen. (B. piardi n. gen. and sp.). D. Protuberosa (P. protuberosa), reconstructed after Wehner (1988: Pl. 1, figure 3). E. Nipponopon (N. hasegawai), reconstructed after Karasawa et al. (2006: figure 1A). F. Acareprosopon (A. bouvieri). G. Prosopon (P. tuberosum, left drawing after von Meyer, 1840; P. mammillatum, right photograph). Scale bars equal 2 mm (A-F) or 5 mm (G).