Europrosopon, Klompmaker & Starzyk & Fraaije & Schweigert, 2020

Klompmaker, Adiël A., Starzyk, Natalia, Fraaije, René H. B. & Schweigert, Günter, 2020, Systematics and convergent evolution of multiple reef-associated Jurassic and Cretaceous crabs (Decapoda, Brachyura), Palaeontologia Electronica (a 32) 23 (2), pp. 1-54 : 19-21

Family PROSOPIDAE von Meyer, 1860

Included genera. Acareprosopon Klompmaker, 2013a ; Europrosopon gen. nov.; Laeviprosopon Glaessner, 1933 ; Nipponopon Karasawa, Kato, and Terabe, 2006 ; Prosopon von Meyer, 1835 ; Protuberosa Schweitzer and Feldmann, 2009d ; Rathbunopon Stenzel, 1945 .

Remarks. The crab Rathbunopon was originally placed in Prosopidae by Stenzel (1945), which was followed by others (Withers, 1951; Wright and Collins, 1972; Müller et al., 2000; Collins, 2002; Karasawa et al., 2006), while Bishop (1986) was unsure about placement of the genus in his list. However, Schweitzer et al. (2010) and Karasawa et al. (2011) moved Rathbunopon within Glaessneropsidae in an entirely different superfamily, Glaessneropsoidea. Karasawa et al. (2011, p. 538) briefly explained this change was based on “carapace proportions and the ornamentation and conformation of the orbits.” Subsequently, Klompmaker et al. (2011) included Rathbunopon in Prosopidae again based on its close similarity to what is now known as Europrosopon , a fit with the diagnosis of Prosopidae in Schweitzer and Feldmann (2009d), and the presence of an augenrest in Rathbunopon , a feature that would be characteristic of prosopids (but see some species of Laeviprosopon ). Subsequent papers by Guinot (2019) and Collins et al. (2020) kept Rathbunopon in Glaessneropsidae , but others favored placement in Prosopidae (González-León et al., 2016; Ossó et al., 2018). Schweitzer et al. (2012b, 2018, p. 326-327) maintained placement in Glaessneropsidae and provided a more detailed explanation providing only differences in orbital characters. Given the close morphological similarity of Rathbunopon and Europrosopon , placement in two different superfamilies would imply dramatic convergent evolution. Hence, further study is in order. We studied Rathbunopon obesum (Van Straelen, 1944) , Europrosopon aculeatum (von Meyer, 1857) , and E. abbreviatum (Schweitzer and Feldmann, 2009d) anew and affirm that nearly any character other than the orbital structure is similar, including the shape and delimitation of regions on the dorsal and lateral sides, a swollen subhepatic region, and the triangular shape and orientation of the rostrum ( Figure 10 ). Differences exist in the orbital structure, but, overall, they appear less prominent than proposed. The outer orbital angle possesses a laterally oriented spine in Europrosopon , while a forwardly oriented projection with a wider base is present in Rathbunopon on the lateral side of the orbital structure; as a consequence, the orbital structure may appear wider and slightly more anterolaterally directed in Europrosopon . A ridge is present within the orbital structure in Europrosopon , but this feature is less prominent in Rathbunopon . Similarities of the orbital structure include the presence of a lower orbital projection that is similar in shape (although the one in Rathbunopon may be more pronounced), and both taxa possess a tubercle on the upper orbital margin. Rather than extreme convergent evolution of the entire carapace except for some parts of orbital structure justifying placement in different superfamilies, a much simpler explanation is that Rathbunopon and Europrosopon are more closely related and best placed in the same family. Regarding differences in orbits, recent work has shown that remarkable variation can be found within genera in the expression of the margins and shape of the orbital structure (e.g., Starzyk, 2015b, 2016; herein), implying that considerable variation may also be expected within families. Guinot (2019: p. 764) found the groove just anterior to it to represent a striking feature, but also the ovoid carapace shape, the triangular rostrum, the well-defined and elevated regions, the metagastric and urogastric regions that are parallel and linear, and the deep and wide grooves. However, most or all these features can also be found in Prosopon and particularly in Europrosopon . The placement of genera currently classified within Glaessneropsidae resembling Rathbunopon , such as Ekalakia Bishop, 1976 , and Vectis , requires a restudy as well, both genera originally being placed in Prosopidae (Withers, 1945; Bishop, 1976). Much hinges on whether the orbital structure of Ekalakia and Glaessneropsis Patrulius, 1959 , are linked phylogenetically as proposed by Feldmann et al. (2008) or whether the similar-looking orbital structure of both taxa is an example of convergent evolution.

EUROPROSOPON gen. nov.

zoobank.org/ A7050696-60FF-4E8B-AAC7-6591CAEAE746

Type species. Prosopon aculeatum von Meyer, 1857 (= Prosopon verrucosum Reuss, 1858 ).

Included species. Europrosopon abbreviatum (Schweitzer and Feldmann, 2009d) comb. nov., as Prosopon ; E. aculeatum (von Meyer, 1857) comb. nov. (= Prosopon verrucosum Reuss, 1858 , and Prosopon barbulescuae Schweitzer, Feldmann, Lazăr, Schweigert, and Franţescu, 2018 ).

Diagnosis. Carapace excluding rostrum about as wide as long or slightly longer than wide (l/w ratios 1.0-1.1), ovate to subsquare. Widest point at mid-length or posteriorly. Outer orbital structure spine directed outward. Main grooves generally well-developed. Mesogastric region not strongly subdivided. Urogastric region separated from metagastric region. No deep groove separating epibranchial region from metagastric region. Small tubercles present across carapace.

Etymology. Contraction of euro and Prosopon , where euro refers to the fact that all known species thus far are found in Europe. Gender is neuter.

Remarks. This genus differs from other genera by a combination of a relatively low length-width ratio of the carapace compared to Acareprosopon , Laeviprosopon , Prosopon , and Protuberosa ; a less nodose anterior carapace than Nipponopon , Prosopon , Protuberosa , and Rathbunopon ; a lack of a strong groove separating the epibranchial region from metagastric region, which is the case for Prosopon , Protuberosa , and Rathbunopon ; and a urogastric region separated from the metagastric region, which is not clearly observed in Acareprosopon , Laeviprosopon , Nipponopon , Prosopon , and Protuberosa .

Figures

FIGURE 10. Comparisons between specimens of the mid-Cretaceous crab Rathbunopon obesum (Van Straelen, 1944) from Koskobilo, Spain, and the crab Europrosopon aculeatum (von Meyer, 1857) from Ernstbrunn, eastern Austria (Tithonian, NHMW numbers), and the Kotouc quarry (locality 3 of level 6), Czech Republic (Tithonian, MAB and UF numbers). A, F, H, J, dorsal, frontal, upper orbital, and orbital views of R. obesum (MAB k3600). B, G, dorsal and frontal views of E. aculeatum (NHMW/1990 0041/4059). C, K, dorsal and orbital view of E. aculeatum (NHMW 2007z0149/0005). D, right-lateral view of R. obesum (MAB k3141). E, right-lateral view of E. aculeatum (MAB k3605). I, upper orbital view of E. aculeatum (UF 288756). For approximate size of H and J, see A and F. Scale bar width equals 2.0 mm.