Orithopsis siouxensis ( Feldmann, Awotua & Welshenbaugh, 1976 )

Van Bakel, Barry W. M., Guinot, Danièle, Artal, Pedro, Fraaije, René H. B. & Jagt, John W. M., 2012, A revision of the Palaeocorystoidea and the phylogeny of raninoidian crabs (Crustacea, Decapoda, Brachyura, Podotremata) 3215, Zootaxa 3215 (1), pp. 1-216 : 68

Orithopsis siouxensis ( Feldmann, Awotua & Welshenbaugh, 1976)

’Necrocarcinus’ siouxensis Feldmann, Awotua & Welshenbaugh, 1976: 986 .

Remarks. Necrocarcinus siouxensis (see Feldmann et al. 1976: 986, pl. 1, figs. 1–5), from the Maastrichtian of North Dakota ( U.S.A.), was considered either to be of uncertain position ( Fraaye 1994: 264, fig. 1; Guinot et al. 2008: 698), or a member of Cenomanocarcinus ( Bishop & Williams 1991: 452; Schweitzer et al. 2003a: 36; Crawford et al. 2006: 5; Schweitzer et al. 2010: 71). Feldmann et al. (2008: 1743) retained the species in Necrocarcinus and used it as reference for Necrocarcinidae in their comparison with Cretacocarcinus , which they assigned to Camarocarcinidae . Based on ventral characters, they stated ( Feldmann et al. 2008: 1744) that, ‘ N. siouxensis and N. labeschii … are more similar to one another than they are to Cretacocarcinus smithi .’

The ventral surface of ‘ Necrocarcinus ’ siouxensis was interpreted as ‘to be a composite’ by Guinot et al. (2008: 698), comprising portions of the abdomen and thoracic sternum. In addition, it must be noted that female palaeocorystoids may have a sternum that is flatter than in males; this may cause misinterpretation in cases where only few specimens are examined. In a photograph of ‘ N.’ siouxensis ( Feldmann et al. 2008: fig. 4.2), sternite 4 is partially overlapped by what appears to be the telson and partially by sediment particles; it is unclear if a deep axial gutter, which O. tricarinata and S. laurae reveal, is present. Additional preparation of the holotype of ‘ N.’ siouxensis is necessary to illustrate the ventral details more precisely.

The ventral surface of ‘ N.’ siouxensis does not appear very useful, having suffered from taphonomic bias, and in view of the numerous plesiomorphic features displayed by palaeocorystoid ventral surfaces. The carapace may effectively be used in this case, however. This species has a carapace in which the anterolateral margin has three teeth, which rules out both Paranecrocarcininae (with a tubercular anterolateral margin) and Camarocarcinidae (with the anterolateral margin with a single strong spine). Necrocarcinines have convex carapaces, the carapace of ‘ N.’ siouxensis being rather flat ( Feldmann et al. 1976: fig. 3). In addition to the rather low-vaulted carapace, the tricarinate dorsal surface is not seen in any necrocarcinids but is characteristic of Cenomanocarcinidae and Orithopsidae . The branchial carinae in ‘ N.’ siouxensis are concave ( Feldmann et al. 1976: figs. 1, 4) and the orbital cavities are distinctly large ( Feldmann et al. 1976: fig. 3): this makes it an orithopsid rather than a cenomanocarcinid, which have convex to straight branchial carinae and small, narrow orbits (see also Guinot et al. 2008). The orbitofrontal margin is poorly preserved in the holotype. Until better-preserved material is available all characters of ‘ N.’ siouxensis match those of Orithopsis .