Typilobus Stoliczka, 1871

Artal, Pedro & Hyžný, Matúš, 2016, An appraisal of Typilobus Stoliczka, 1871 (Crustacea, Brachyura, Leucosioidea), with description of a new family and genus, Zootaxa 4117 (3), pp. 387-398 : 390-392

Typilobus Stoliczka, 1871

Type species: Typilobus granulosus Stoliczka, 1871

Emended diagnosis. Carapace transversely oval, moderately convex; front very narrow, bilobed, scarcely projected; orbits small, subcircular, directed forward; anterolateral margins arched, with small indentation at level of the cervical groove; posterolateral margins nearly straight, converging posteriorly, bearing small protuberance at level of cardiac region; posterior margin narrow, straight; main dorsal regions not subdivided; gastric region large, swollen, bounded by conspicuous grooves; hepatic region small, flat, subtriangular; branchial region large, swollen, bearing small protuberance in posterior portion; cardiac region transversely ovate, fairly swollen; intestinal region small, depressed; gastrohepatic, cervical, cardiac grooves well marked; sternum fairly broad, sternites 1–4 fused, sternites 5–6 subparallel, short, broad, sternite 7 somewhat inclined, sternite 8 inclined, reduced, small portion of it not totally covered by male pleon; male pleon narrow, long, reaching sternite 3, pleonal segments 3–5 fused, telson long, narrow, with rounded tip; female abdomen narrow, long, nearly reaching sternite 2, pleonal segments 3–6 fused, bearing 3 conspicuous longitudinal rows of subtle protuberances. Exopodite, endopodite of Mxp3 long, equally broad, both completely covering buccal cavity; epistome as wide as frontal margin.

Remarks. Schweitzer et al. (2010: 91) listed sixteen species assigned to Typilobus , nine of them questionably indicating a problematic assignment. The rather broadly defined genus has been considered as embracing a wide range of morphotypes, as already noted by Vía Boada (1969: 158), Müller (1993: 10), and Feldmann et al. (2011: 327). Therefore, the emended generic diagnosis provided herein is based solely on the type species, Typilobus granulosus ( Fig. 1 ). The diagnosis of Feldmann et al. (2011: 327) stated that the male pleonal segments are free. This observation was apparently based on the inaccurate drawings in Stoliczka (1871: pl. 3, figs. 3, 5, see Fig. 1 C, D), which indicates all male and female abdominal segments free. According to the original description of Stoliczka (1871: 16), however, the male pleonal segments 3–5 are fused, whereas female pleonal segments appear “to consist of seven separate joints, but they are not very well defined in the single specimen”.

The outline of the carapace varies among the species of Typilobus . Typilobus sadeki Withers, 1925 ( Fig. 2 B, C) and T. kishimotoi Karasawa, 1998 , have a carapace with a transversely oval outline, similar to the type species; all three species are from Miocene strata and exhibit a similar distribution of dorsal regions and grooves. Other species assigned to the genus present a subcircular or longitudinally subelliptical carapace outline, similar grooves, and similar distribution of dorsal regions, e.g., T. semseyanus Lőrenthey, 1898 , T. corrodatus Noetling, 1885 ( Fig. 2 F), or T. trispinosus Lőrenthey, 1907 , all reported from Paleogene strata. Observation that the general outline of the Paleogene species assigned to Typilobus differs from that of species reported from the Miocene strata were presented by Müller (1993: 10, 11). Moreover, in his view, the type species, Typilobus granulosus Stoliczka, 1871 , is probably identical with Nucia baripadensis Bachmayer & Mohanti, 1973 , from the Miocene of India, and that the relatively narrow female pleon of T. granulosus reflects the immature stage of the type specimens. Müller (1993) suggested that Nucia Dana, 1852 , seemed to be very close to the Miocene Typilobus species, whereas the Paleogene species of Typilobus might belong to an undescribed new genus. One of them, Typilobus boscoi , is recognized herein as representing a separate genus (see below). We agree with Müller (1993) that Typilobus granulosus , is close to Nucia spp., but we refrain from synonymising Typilobus with Nucia , considering Typilobus as a distinct taxon.

There is a high degree of morphological distinction in the distribution and shape of dorsal regions and grooves, and identifying gastric, cardiac, and intestinal regions is often difficult. Except for Vía Boada (1969: 156), most authors consider the cardiac region to be the swollen region situated at the rear of the carapace; the gastric region is described as equidimensionally hexagonal in some species, whereas the region is very much elongated in others, which is confusing. On the basis of the posterior placement, this region is recognized as intestinal (e.g., Galil 2003a). The gastric region is described as nearly equidimensionally hexagonal in T. belli Quayle & Collins, 1981 ( Fig. 2 D), T. alponensis Beschin, De Angeli & Zorzin, 2009 , and T. modregoi Vía Boada, 1959 , whereas it is very much elongated in T. trispinosus ( Feldmann et al. 2011: 328) .

Karasawa (1998: fig. 3) presented a table with a summary of stratigraphic and geographic distribution of the genus. Based on his conclusion, the genus originated in the Western Tethys during the Eocene and subsequently migrated eastward to the Indo-West Pacific region in the Miocene. Although this scenario partly applies to other decapod taxa (e.g., Hyžný 2011; Hyžný & Müller 2012) and parallels the “Hopping Biodiversity Hotspots” concept of Renema et al. (2008), it is difficult to accept it uncritically for such a heterogeneous taxon as Typilobus .

Figures

FIGURE 1. Typilobus granulosus Stoliczka, 1871, the type species of Typilobus Stoliczka, 1871, from the Miocene of Pakistan. A, NHMUK In 28638 in dorsal (A 1), ventral (A 2), and posterior (A 3) views; B, NHMUK In 22990 (figured by Withers 1925: pl. 3, figs. 1 – 2) in lateral (B 1), dorsal (B 2), ventral (B 3), posterior (B 4), and frontal (B 5) views; C – D, digital images of Stoliczka (1871: pl. 3, figs. 3, 5) showing male (C) and female (D) specimens. Note that schematic drawings of pleon (C 2, D 2) do not fully correspond to the figures of entire specimens (C 1, D 1). Specimens in A and B are to scale. Scale bar equals 5 mm.

FIGURE 2. Morphological variation shown in several species assigned to Typilobus. A, Typilobus marginatus Morris & Collins, 1991, holotype NHMUK In 62163 in dorsal (A 1) and ventral view (A 2); B – C, Typilobus sadeki Withers, 1925, holotype NHMUK In 22991 (B) and In 22992 (C) in dorsal (B 1, C 1) and ventral views (B 2, C 2); D, Typilobus belli Quayle & Collins, 1981, holotype NHMUK In 61707; E, Typilobus moralejai Müller, 1993, holotype MGSB 51918; F, Typilobus cf. corrodatus Noetling, 1885, BSPG 1981 XI 15. Specimens in E and F were coated with ammonium chloride prior the photography. Scale bar equals 5 mm.