Genus Alahexapus n. gen.

urn:lsid:zoobank.org:act: C230B4E9-59C4-4322-9057-5513F4722A93

TYPE SPECIES. — Stevea martini Feldmann, Schweitzer & Portell, 2014, by present designation.

ETYMOLOGY. — Ala - refers to Alabama and - hexapus refers to the type genus of Hexapodidae . Gender: masculine.

DIAGNOSIS. — As for the type species.

REMARKS

Stevea martini was ascribed to Stevea based on a limited number of specimens then available without male and female pleons. Two additional specimens collected since 2014 with male and female pleons preserved suggest assignment to this genus is not tenable. Specifically, the female pleon from the only specimen of Stevea williamsi, the type and sole extant species of Stevea, was demonstrated to be narrow and have somites 2-6 fused (Guinot et al. 2010), whereas the female individual of Alahexapus martini n. comb. has proportionally wider pleonal somites 4-6 and these somites are unfused. Additional differences include the anterolateral borders passing below the outer orbital angle rather than joining the outer orbital angle as in A. martini n. comb., the rostrum has a concave border rather than a straight border as in A. martini n. comb., and the rostrum is widening distally rather than being straight laterally (UF 254042). The degree of fusion of pleonal somites and the pleonal width are considered important characters distinguishing between genera among extant hexapodids (Rahayu & Ng 2014). Other characters frequently used in that paper for genus diagnoses are carapace length-width ratios, cuticle ornamentation, groove development on the carapace, anterolateral margin shape, eye size, maxilliped morphology, ornamentation including stridulatory striae on the pterygostome, cheliped morphology, the degree of sternite and somite fusion, the shape of the pleon, and the shape and reach of the sternopleonal cavity. Many of the same characters are used to define genera present in the fossil record when preserved, but also other characters related to the carapace have been used including the widest point of the carapace, the frontal margin morphology and relative dimensions, and the orbital cavity size and shape (e.g., De Angeli et al. 2010; Schweitzer et al. 2022).

Key differences between Alahexapus n. gen. and exclusively fossil genera exist. Bellhexapus is widest at the posterior margin, male somite 6 is longer, and male somites 3-5 are not fused (De Angeli et al. 2010: figs 2, 3). In Eohexapus, the orbits are circular rather than oval and the dorsal carapace is smooth rather than showing some grooves (De Angeli et al. 2010: figs 4-6). Eurohexapus exhibits a carapace about as long as wide rather than wider than long, its fronto-orbital width is greater (c. 65% of maximum width vs 50% for Alahexapus n. gen.), and male somites 2-6 are fused (De Angeli et al. 2010: figs 7-9). Goniocypoda has a relatively wide fronto-orbital width (c. 65% of maximum width vs c. 50% for Alahexapus n. gen.) (Schweitzer & Feldmann 2001: 335). For Headonipus, the “grooves curving from the base of the cardiac region to the middle of the coxigeal incisions isolate elongated intestinal lobes” (Quayle & Collins 2012: 40), are not observed in Alahexapus n. gen. Moreover, the posteriormost portion of the carapace of Headonipus appears much more depressed and the orbits are subcircular rather than oval (Quayle & Collins 2012: pl. 3.9-3.10; Schweitzer et al. 2022: fig. 7.5). For Holthuisea, carapace grooves are absent and male somites 3-6 are fused rather than somites 3-5 only (Guinot et al. 2010: figs 2-4). Lucahexapus has a proportionally greater fronto-orbital width (c. 67% of maximum carapace width vs c. 50%) and the cervical groove is much wider (De Angeli & Caporiondo 2022: figs 2.5, 3.5). For Palaeopinnixa, the carapace is widest just anterior to posterolateral reentrants (e.g., Hyžný & Artal 2018; Schweitzer et al. 2022; Gustafson 2023), but Alahexapus n. gen. is widest about mid-length and does not widen toward the posterior carapace. Moreover, somites 5-6 in females are fused for the type species P. rathbunae and this species does not exhibit stridulating apparatus/striae on the pterygostome (Schweitzer et al. 2000: 57). Rodneyellus lacks a U-shaped cervical groove and has a lower ratio of fronto-orbital width of maximum carapace width (0.37 vs 0.50).

Key differences also exist with extant genera other than Stevea as discussed above, primarily using diagnoses in Manning & Holthuis (1981), Guinot (2006), and Rahayu & Ng (2014). Hexalaughlia exhibits a pterygostome without a row of stridulatory striae, and male somite 6 is much longer. Hexapinus has a carapace widening posteriorly, the carapace regions are not demarcated except for a poorly defined cardiac region, the orbits are much smaller, and male somite 6 is longer. For Hexaplax, the carapace regions are more indistinct, the orbits are larger and are subcircular rather than oval. For Hexapus, the carapace regions are less distinct, and the orbits are much smaller. Lambdophallus bears much smaller orbits and exhibits a male transverse sternal groove extending laterally from the sternopleonal cavity not seen in Alahexapus n. gen. Latohexapus has much more distinct carapace regions and its carapace widens posteriorly. Mariaplax has a carapace widening posteriorly, smaller orbits, and a much shorter male telson. Paeduma exhibits a much longer male somite 6 and carapace grooves are absent to very faint. Parahexapus bears a much longer male somite 6 that is narrower than male somites 3-5, and has orbits that are smaller. For Pseudohexapus, the pterygostome lacks a row of oblique striae. Rayapinus has a carapace without clear grooves, bears a relatively narrow female pleon, has female somites 1-5 fused, and has a male telson almost as long as somite 6. Spiroplax exhibits a carapace that is widening posteriorly, and has a much broader male pleon with a triangular telson. For Thaumastoplax, the pterygostome lacks a row of oblique striae, the orbits are smaller and circular, and the carapace regions are indistinct. Theoxapus has much smaller orbits and a proportionally longer male somite 6. Tritoplax has a male telson with a triangular tip, a male pleonal somite 6 that is divided longitudinally, and apparently smaller orbits.

As none of the diagnoses of fossil and extant genera match the species under study, we erect Alahexapus n. gen. It is possible additional fossil species may belong to the new genus. For example, Palaeopinnixa rocaensis (Feldmann, Casadío, Chirino-Gálvez & Aguirre-Urreta, 1995), from the Danian of southern Argentina does not widen toward to posterior carapace as for the genus diagnosis of Palaeopinnixa (Schweitzer & Feldmann 2001) . The fronto-orbital width is similar (45% of maximum width vs c. 50% for Alahexapus n. gen.), the outline is similar, the flanks for both taxa are straight, the length-width ratio is similar (0.71 vs c. 0.67 for Alahexapus n. gen.) (Feldmann et al. 1995: figs 14, 15). Unfortunately, no cuticle is preserved nor any ventral characters, which hinders further evaluation.