Basadromia longifrons, Artal, Pedro, Van Bakel, Barry W. M., Domínguez, José L. & Gómez, Guillermo, 2016

Artal, Pedro, Van Bakel, Barry W. M., Domínguez, José L. & Gómez, Guillermo, 2016, A new dromiid crab (Crustacea, Brachyura, Dromioidea) from the Upper Eocene of Huesca (Aragón, northern Spain), Zootaxa 4061 (4), pp. 438-446 : 440-445

Basadromia longifrons n. sp.

( Fig. 1–4 )

Diagnosis. As for genus.

Etymology. From the Latin longus, meaning long and frons, front, in allusion to the fairly projected front, with long subtriangular teeth.

Material examined. The holotype, MPZ 2011.185, and six paratypes, MPZ 2013.73– MPZ 2013.78, all from the Upper Eocene (Priabonian) in the Yebra de Basa area, Huesca. Maximum carapace width and length in the holotype are 14.1 and 16.4 mm, respectively.

Description. Carapace transversely elliptical in outline; slightly longer than wide, widest anteriorly, at level of epibranchial region; dorsal surface strongly convex in both directions. Front strongly projected, narrow, with noticeable V-shaped notch, deep groove; frontal margin defined by 4 long, subtriangular teeth, short rostral tooth in lower plane, not visible in dorsal view. Orbits small, somewhat rimmed, directed anteriorly, laterally; supraorbital margin nearly vertical in inner portion ( Fig. 1 ). Anterolateral margin short, arched, densely granular, with small, subtle teeth. Posterolateral margin longer than anterolateral, broadly arched. Posterior margin slightly concave, notably reduced, shorter than orbitofrontal margin. Dorsal carapace surface very well defined by numerous, strong swellings, fairly marked grooves. Epigastric regions small, slightly swollen, separated by frontal groove.

Protogastric regions large, elongated, posterior portion nearly circular, strongly swollen. Mesogastric region transversely subelliptical, swollen, with axial depression; anterior extension large, broadly subtriangular. Hepatic region slightly swollen, bounded by notable cervical, hepatogastric grooves. Urogastric region tranversely subrectangular, bounded by lateral grooves, divided into 2 portions by short axial depression. Epibranchial region notably large, divided into 2 portions by longitudinal groove; axial portion large, subelliptical, strongly swollen. Posterior branchial regions large, broadly swollen. Cardiac region large, gently swollen, subpentagonal in shape, with apex pointed downwards. Intestinal region small, flat. Cervical groove well marked, continuous from side to side, notching lateral margins. Branchial groove deep, oblique in lateral portions, converging abruptly towards rear in posterior portion of cardiac region. Dorsal carapace surface densely, uniformly covered by small granules. Chelipeds not completely preserved, short, robust, strongly granular; moveable finger thin, delicate, strongly curved ( Fig. 3 ).

Remarks. The dorsal carapace of Basadromia longifrons n. gen., n. sp. exhibits the main characters of the family Dromiidae (sensu Guinot and Tavares, 2003; Ng et al., 2008; Guinot et al., 2013), including general carapace outline, shape and distribution of dorsal regions, presence of well-defined cervical, branchial, and branchiocardiac grooves, a narrow and projected front with two inner orbital teeth, two marked median teeth, and a rostral tooth situated in a lower plane. Following recent revisions and general classifications ( Guinot and Tavares, 2003; Schweitzer and Feldmann, 2010; Karasawa et al., 2011; Schweitzer et al., 2012; Guinot et al., 2013), these main features, as well as the distribution and shape of dorsal regions in Basadromia n. gen., are also seen in members of various subfamilies. The epigastric, protogastric, mesogastric, urogastric, hepatic, epibranchial, and cardiac regions, either weak to strongly marked, are similar in general shape and distribution in extinct forms assigned to the Dromiinae, Basinotopinae, Sphaerodromiinae and Dynomenidae (compare Schweitzer et al., 2012).

With regard to dorsal carapace grooves (frontal, cervical, branchial, and branchiocardiac) are shared by representatives of the various subfamilies as well. The mesogastric region (of fairly constant shape amongst dromioids) in Basadromia n. gen. resembles that of other members of the superfamily. This region, which is either weakly to strongly divided, with two somewhat swollen portions separated by either a faint or deep longitudinal depression, usually with the anterior extension broadly triangular, shows rather constant features in extinct genera such as Dromilites, Ferricorda Schweitzer & Feldmann, 2010 , Pseudodromilites Beurlen, 1928, Basinotopus , and even Kromtitis Müller, 1984 (Schweitzer & Feldmann, 2010; Karasawa et al., 2011; Schweitzer et al., 2012). The new form yet exhibits a set of distinctive features that set it apart from all known fossil and extant dromioids, warranting the erection of a new genus. The front is distinctly projected, with four long teeth, the two median ones being longer, the two inner orbital ones shorter; the rostral tooth, situated in a lower plane, is short, not visible in dorsal view; the anterolateral margin is broadly arched, densely granular, bearing small teeth or protuberances; the posterolateral margins are broadly arched, the posterior margin is fairly narrow; the posterior portion of the protogastric region presents a near-circular, subhemispherical swelling; the epibranchial region is divided into two portions by a longitudinal groove, and the axial portion is strongly swollen and subelliptical in shape. This set of features differentiates the new taxon from all known extinct and extant forms.

Basadromia n. gen. appears to be close to the extant genus Petalomera Stimpson, 1858 , and the extinct Pseudodromilites. The former is characterised by a granular dorsal surface of the carapace, well-defined regions with numerous swellings and grooves (similar to the new taxon), granular anterolateral margins, usually without long spines or noticeable teeth, and a relatively projected front with a single rostral tooth in lower plane, two lateral median teeth and supraorbital (inner orbital) lobes ( McLay & Ng, 2007). Petalomera , however, presents a less projected front than Basadromia n. gen., with the rostral tooth clearly visible in dorsal view and the inner orbital teeth (supraorbital) are less salient, the outer orbital nodes less pronounced. For describing orbitofrontal construction different terms have been used in the literature to capture the same morphology. Here we use “frontal margin, front, rostral tooth, lateral median teeth, and inner orbital teeth”. Collins & Jakobsen (2004) also used the terms “front” and “inner orbital”; “produced front…slender and slightly diverging inner orbital spines” ( Collins & Jakobsen, 2004: 69). These authors considered dromioids to have usually a frontal margin with one rostral tooth and two lateral median teeth isolated (i.e., differentiated) from the inner orbital teeth or lobes, all the lobes from weakly to strongly marked in the different genera, but to some degree always present ( Collins & Jakobsen, 2004: 70). Guinot & Tavares (2003) usually described the front as “front with median rostrum and two pseudorostral teeth, one at each side of the rostrum” and “supraorbital teeth well developed” ( Guinot & Tavares, 2003: 49, 50). McLay (2007: 109) employed “rostrum”, rather than “front”, “lateral teeth” instead of “median frontal teeth” (or “pseudorostral teeth”) and “supraorbital teeth” instead of “inner orbital teeth”; “rostrum tridentate, median tooth deflexed, lateral teeth separated by a V-shaped sinus…well developed supraorbital tooth”.

In the absence of well-preserved ventral features, we tentatively assign the new taxon to the Dromiinae. Pseudodromilites, included in the Dromiidae by Schweitzer & Feldmann (2010), presents a dorsal surface densely covered by granules; the distribution and shape of dorsal regions is similar to that seen in Basadromia n. gen. Pseudodromilites, however, has a subpentagonal carapace shape, with near-straight portions of posterolateral margins and the posterior margin is longer; the dorsal regions are more uniformly swollen, less protuberant and subdivided, not clearly differentiated as in Basadromia n. gen. The frontal margin presents only two median teeth and nearly undifferentiated inner orbital lobes. The anterolateral margin in Pseudodromilites was described as having triangular teeth by Schweitzer et al. (2010: 422), due to the appearance conveyed in drawings of the original descriptions. A good illustration of a complete specimen ( Beschin et al., 2012: 32, fig. 25; pl. 4/2) confirms that the anterolateral margin is slightly notched, with two granular protuberances, but lacking triangular teeth.

Basadromia n. gen. appears to be morphologically close to some fossil species that have recently been assigned to the subfamily Sphaerodromiinae Guinot & Tavares, 2003 (see Schweitzer & Feldmann, 2010). Dromilites bucklandii H. Milne Edwards, 1837 , the type species of the genus, also presents a frontal margin with one rostral tooth, two notable median teeth and two supraorbital teeth, thus five frontal teeth in total. Dromilites nevertheless presents a less salient front, with smaller frontal teeth, the two inner orbital ones being fairly smaller, the lateral margins of the carapace have salient teeth, and the dorsal carapace surface shows different dorsal regions, being more individualised, nearly hemispherical. Dromilites simplex Quayle & Collins, 1981 presents similar characters. The frontal margin is less projecting, without pronounced teeth or spines, and in general the dorsal regions are weakly defined, less swollen and separate. This species exhibits a typical marked groove, subparallel, between the cervical and branchial grooves (Schweitzer et al., 2010: fig. 4).

Basadromia n. gen. shares with Dromilites vicensis Barnolas, 1973 , the two fairly long median teeth in the front, and the deep, broadly V-shaped notch ( Barnolas, 1973, figs. 2, 4). Despite that the dorsal regions and grooves are weakly pronounced in D. vicensis , some characters, such as the shape of the mesogastric, urogastric and cardiac regions, appear to be closey similar to those of the new species. The main differences in D. vicensis are the absence of marked supraorbital spines (teeth or spines in the orbitofrontal corner) and the presence of salient teeth along the anterolateral margins. In spite of sharing some diagnostic features such as the general carapace outline, the produced front and dorsal regions and grooves in D. pastoris Via, 1959 , differ from Basadromia n. gen. There are notable lobes along the lateral carapace margins, the front is axially produced, the rostral tooth being the more projected one, and the four adjacent few marked, notably small; dorsal regions are more uniformly swollen, less individualised; there is a subparallel dorsal groove between the cervical and branchial grooves, the post-cervical groove. This groove is very characteristic in some dromioids, being notably marked in genera such as Pseudodromilites, Basinotopus , and Ferricorda. A cast of the holotype of D. pastoris , illustrated by Schweitzer et al. (2010: fig. 3) is either broken or incomplete; the original specimen, housed in the MGSB collections, has recently been reexamined by the senior author and does exhibit a complete front. The holotype of D. pastoris actually possesses a very complete front, showing the rostral tooth to be the more produced ( Via, 1959; Via, 1969: fig. 10; pl. 4). The Italian specimen assigned to D. pastoris also exhibits the same features, as clearly indicated in the text and illustrations ( Beschin et al., 2012: 34, fig. 26; pl. 4). Quinquerugatus, assigned to Dromiidae (Schweitzer et al., 2010) , presents numerous characters that are also present in D. pastoris , as mentioned by Beschin et al. (2012: 34), despite the fact that the two genera were assigned to two different subfamilies, Dromiinae and Sphaerodromiinae, respectively (sensu Schweitzer & Feldmann, 2010). The carapace is subpentagonal in outline in these two species, the dorsal regions are weakly marked and the frontal margin has a median spine that in both species is the more projected in the front. Dromilites pastoris was assigned to Sphaerodromiinae despite the produced rostral tooth which is utterly different from all other assigned members, whereas Dromilites was diagnosed as having a clearly “bilobed rostrum” (Schweitzer et al., 2010: 418). All data mentioned above clearly indicate that much more work is needed to make assignment of the various dromiid species clearer.

The Basinotopinae is a recently erected subfamily that is characterised by a subtriangular carapace shape, being notably longer than wide, a triangular front with three lobes (spines), the rostral one being the longer, and lateral carapace margins with clearly marked spines ( Karasawa et al., 2011: 539). Basinotopus has the strongly swollen carapace regions and the produced front in common with Basadromia n. gen. The dorsal regions present a general aspect and similar distribution, are numerous and well divided by numerous grooves in both genera. Basinotopus lamarckii and B. tricornis Collins & Jakobsen, 2004 are easily distinguished from Basadromia longifrons n. gen., n. sp. in that both have a subtriangular front with a markedly long rostral spine, notably more projecting than the two inner orbital spines; extremely long spines along the anterolateral and posterolateral margins of carapace; a longer posterior carapace margin, at least of the same size of the orbitofrontal margin. Schweitzer & Feldmann (2010: 422) mentioned that Dromilites alpina Glaessner, 1929 , and Dromilites lothi Förster & Mundlos, 1982 should be assigned to Kromtitis . A close reading of the text and a careful examination of the images for D. alpina confirms that this species exhibits a subtriangular carapace, with typical strong, long lateral spines and cervical and branchial grooves characteristic of the configuration in Basinotopus . Collins & Jakobsen (2004:70) also mentioned that this species should be assigned to Basinotopus . Dromilites lothi presents a similar carapace outline, lateral spines along the lateral carapace margins, distribution of dorsal regions and shape of cervical, branchial and branchiocardiac grooves ( Förster & Mundlos, 1982: 155, figs. 5, 6). Despite the fact that the frontal margin appears to be different, we consider this species to belong to Basinotopus , but for now we retain it in Dromilites until better-preserved material becomes available. Kromtitis is characterised by a subquadrate carapace outline, a broad and near-straight frontal margin in dorsal view, dorsal regions that are subdivided into numerous portions, presenting a range of small swellings and absence of cervical and branchial grooves that are typical of genera such as Dromilites , Basinotopus , or Pseudodromilites ( Beschin et al., 2007: pl. 3; Schweitzer et al., 2012: fig. 21). Reasons to exclude B. alpina and D. lothi from Kromtitis were discussed by Van Bakel et al. (2009: 49, 50).

Figures

FIGURE 1. Basadromia longifrons n. gen., n. sp., MPZ 2011.185 (holotype, indeterminate sex). Dorsal view. Yebra de Basa (Huesca, northeastern Spain). Scale bars equal 5 mm.

FIGURE 3. Basadromia longifrons n. gen., n. sp., MPZ 2011.185. Lateral view.