Romualdocarcinus salesi Prado and Luque, 2018

Prado, Ludmila A. C., Luque, Javier, Barreto, Alcina M. F. & Palmer, A. Richard, 2018, New brachyuran crabs from the Aptian-Albian Romualdo Formation, Santana Group of Brazil: Evidence for a Tethyan connection to the Araripe Basin, Acta Palaeontologica Polonica 63 (4), pp. 737-750 : 744-747

Romualdocarcinus salesi Prado and Luque sp. nov.

Fig. 4 .

ZooBank LSID: urn:lsid:zoobank.org:act:86A8591D-A6D8-436B-AD02-8A29B36E3CF6

Etymology: In honour of the late Professor Alexandre Magno Feitosa Sales (1969–2016), an important figure in the paleontological scenario of Brazilian Northeast, mainly the Araripe Basin.

Type material: Holotype (carapace): DGEO-CTG-UFPE-8122. Paratypes (carapaces): DGEO-CTG-UFPE-8110, 8111, 8119, 8123, 8124, 8125, 8126, 8127, 8128, 8129, 8131; all from the type locality.

Type locality: Cedro site, Araripe Basin, municipality of Exu, Pernambuco, Brazil .

Type horizon: Romualdo Formation (upper Aptian–lower Albian) .

Diagnosis.—Carapace sub-hexagonal to sub-square, nearly equidimensional, weakly vaulted transversely and longitudinally, widest anterior to carapace mid-length and to epibranchial spine, finely granulated, lacking ridges, bosses, tubercles, or reniform swellings. Fronto-orbital margin nearly as wide as carapace maximum width; rostrum well-developed, elongated, bifid, somewhat spatulate, axially sulcate, extending well beyond outer orbital spine, bearing a pair of short lateral spines at anterior third; post-rostral slits absent. Orbits wide, directed forward, faintly upturned; supra-orbital margin straight, sub-horizontal to slightly diverging postero-laterally, with two very short but wide orbital fissures somewhat oblique to each other; inner orbital lobe short; intra-orbital lobe short, roundish in outline; outer orbital spine well-developed, relatively long, simple, triangular, shorter than rostrum, with outer margin directed anteriorly or slightly diverging antero-laterally. Anterolateral margin shorter than posterolateral margin, with three short, broad, well-developed triangular spines excluding the outer orbital spine, first two spines the largest; epibranchial spine weakly developed, directed laterally. Posterolateral margin entire, slightly convex to nearly straight. Posterior margin slightly concave to straight, rimmed, narrower than front-orbital margin. Cervical groove moderately defined, shallow, sinuous, distally reaching anterolateral margins; secondary groove faint; postcervical groove absent. Branchiocardic groove shallower than cervical groove. Epigastric region narrow, protogastric and mesogastric regions slightly tumid; hepatic region wide, slightly tumid; intestinal region wide; epibranchial region with two swellings near contact with branchial groove; meso- and metabranchial regions not differentiated.

Description.—Carapace sub-hexagonal to sub-square, as long as wide; dorsal carapace weakly vaulted to flattened, widest anterior to carapace mid-length at level of second anterolateral spine; dorsal carapace smooth, finely granulated, with well-defined regions lacking ridges, tubercles, or reniform swellings. Fronto-orbital margin broad, nearly as wide as carapace maximum width and wider than posterior margin. Rostrum well-developed, broad, long, somewhat spatulate, extending well beyond outer orbital spine, nearly one-third of fronto-orbital margin width, wider at the base, with straight lateral margins converging antero-medially, bearing a pair of short lateral spines at anterior third, and a pair of distal spines forming a weakly downturned bifid rostrum; rostrum sulcate axially, with posterolateral margins at level of supraorbital margin; post-rostral slits absent. Orbits wide, faintly upturned, directed forward, each orbit slightly over one-third of fronto-orbital margin; supra-orbital margin straight, sub-horizontal to slightly diverging postero-laterally, with two very short but wide orbital fissures somewhat oblique to each other; innermost orbital fissure slightly wider than outermost fissure; inner orbital lobe short, moderately developed, grading from lower margin of rostrum; intra-orbital lobe short, roundish in outline, separated from inner and outer lobes by orbital fissures; outer orbital spine well-developed, simple, triangular, shorter than rostrum, with outer margin directed anteriorly or slightly diverging antero-laterally. Anterolateral margin slightly convex to straight, slightly shorter than posterolateral margin, bearing three triangular, short, well-developed spines excluding outer orbital spine; first two spines the largest, directed antero-laterally, second antero-lateral spine wider than adjacent spines; third antero-lateral spine representing the epibranchial spine, weakly developed, directed laterally. Posterolateral margin slightly convex to nearly straight, without spines, denticles, or tubercles. Posterior margin wide, slightly concave, rimmed, shorter than fronto-orbital margin and slightly wider than half of carapace maximum width. Cervical groove distinct but moderately defined, shallow, sinuous, arcuate, with smooth deflections, axially interrupted, distally reaching anterolateral margins; secondary groove present but faint; post-cervical groove absent; branchial grooves distinct, moderately developed, crescent-shaped; branchiocardic groove present, weakly developed, sub-parallel to cervical groove. Epigastric region narrow, well-defined laterally by sub-parallel grooves; protogastric and mesogastric regions slightly inflated; hepatic region wide, without transverse ridge but slightly tumid; epigastric, protogastric, mesogastric, and hepatic regions without tubercles or bosses; metagastric and urogastric regions lacking tubercles or longitudinal ridges, faintly separated by a shallow depression, both regions defined laterally by crescent-shaped branchial grooves; urogastric region narrow; cardiac region well-developed, inflated, broader than meta- and urogastric regions, wider anteriorly, lacking tubercles, without transverse or longitudinal ridges; intestinal region wide, depressed anteriorly by contact with posterior cardiac region; epibranchial region well-defined, limited by cervical and branchiocardic grooves, with two swellings near contact with branchial groove; meso- and metabranchial regions not differentiated, lacking spines, tubercles, or conspicuous ridges, but with posterior metagastric region inflated.

Dimensions.—For a list of measurements see Table 3. Remarks.—The systematic affinities of Romualdocarcinus Prado and Luque gen. nov. are puzzling, in part due to its incompleteness (only known from dorsal carapaces), and its superficial resemblance to brachyuran crabs from different groups. For instance, Romualdocarcinus shares a sub-hexagonal carapace outline with Paranecrocarcinidae , Cenomanocarcinidae , and Orithopsidae ,although Romualdocarcinus is more sub-squarish than most orithopsids, and lacks the posterolateral spines and weakly projected rostrum of Cenomanocarcinidae , or the postrostral slits seen in most Paranecrocarcinidae . Romualdocarcinus also shares with Orithopsidae , Necrocarcinidae , and Paranecrocarcinidae a fronto-orbital margin that is wider than half the carapace maximum width, but differs from Necrocarcinidae , Paranecrocarcinidae , or any other necrocarcinoid-like raninoidan, in its fronto-orbital margin being almost as wide as the carapace ( Fig. 4A 1 , B, C, D). The subspatulate bifid rostrum that extends beyond the outer-orbital spine seen in several Paranecrocarcinidae and Orithopsidae , and the smooth dorsal carapace lacking ridges or rows of tubercles distinctive of orithopsids like Planocarcinus , Colombicarcinus , and Aetocarcinus , would advocate for affinities with the group.

Romualdocarcinus Prado and Luque gen. nov. and the orithopsid Exucarcinus Prado and Luque gen. nov., are known from the same strata and localities in the Araripe basin, yet both taxa differ in several features. In Romualdocarcinus the carapace outline is squarish, its dorsal regions are more tumid, and the posterior margin nearly straight, while in Exucarcinus the carapace is more hexagonal, has less marked dorsal regions, and the posterior margin is concave. Also, Romualdocarcinus has a shorter anterolateral margin, only three anterolateral spines ( Fig. 4A 1 , B 2, C 2, D 2 ), and a maximum width anterior to its mid-length, whereas Exucarcinus has anterolateral and posterolateral margins about the same length, has six to seven anterolateral spines, and is wider at its mid-length. Furthermore, in Romualdocarcinus the supra-orbital margins are weakly upturned and slightly divergent postero-laterally, and bear two short and wide orbital fissures faintly oblique to each other ( Fig. 4A 2 , A 3, C 2, D 2 ), which differ from the sub-horizontal margins with two narrower and sub-parallel orbital fissures seen in Exucarcinus .

Despite the superficial similarities between Romualdocarcinus and several necrocarcinoid-like raninoidans, the new genus seems to fit better within Eubrachyura, in particular Eogeryonidae , known from the Cenomanian of Spain ( Ossó 2016). Both taxa share a sub-hexagonal to sub-square carapace outline, they have anterolateral margins with three spines (excluding the outer orbital spine), wide orbits with two open fissures, and a broad bifid rostrum with secondary lateral spines ( Ossó 2016). However, in Eogeryonidae the rostrum is short and four tipped, while in Romualdocarcinus it is elongated and bifid ( Fig. 4A, C ). Other early eubrachyurans like Marocarcinidae Guinot, De Angeli, and Garassino, 2008 , from the Cenomanian of Morocco, also share with Romualdocarcinus the bifid rostrum with secondary lateral spines, and the presence of three spines in the anterolateral margin, but it has smaller orbits than those of Romualdocarcinus , it lacks orbital fissures, and has cervical and branchiocardial grooves marked only towards the lateral margin. Another family from the Upper Cretaceous of the Americas, Longusorbiidae Karasawa, Schweitzer, and Feldmann, 2008 , shares with Romualdocarcinus a fronto-orbital margin nearly as wide as the carapace maximum width, the broad orbits, and a rostrum elongated and axially sulcate. However, Longusorbiidae has a considerably narrower downturned rostrum and well-defined dorsal carapace regions, both absent in Romualdocarcinus .

Although most eubrachyuran families have their earliest fossil representatives in Upper Cretaceous or Cenozoic rocks, only a handful of eubrachyuran-like families are currently known from Lower Cretaceous units, i.e., Telamonocarcinidae Larghi, 2014 , from the lower Albian of Colombia; Tepexicarcinidae Luque, 2015b , from the lower Albian of Mexico; and Componocancridae Feldmann, Schweitzer, and Green, 2008 , from the lower Albian of USA (see summary in Luque 2015b). A fourth family of eubrachyurans, Priscinachidae Breton, 2009 , is known from the upper Albian and Cenomanian of Spain and France ( Breton 2009; Klompmaker 2013). Although only two of these families, i.e., Telamonocarcinidae and Priscinachidae , can be placed within crown superfamilies such as Dorippoidea and Majoidea, respectively, all four taxa belong to basal eubrachyuran branches, whereas Romualdocarcinus is more reminiscent of modernlooking eubrachyurans like Eogeryonidae . If Romualdocarcinus is indeed an eogeryonid, it will extend the temporal distribution of the family into the Early Cretaceous, and represents one of the oldest “higher” true crab yet known.

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 4.?Eogeryonid brachyuran Romualdocarcinus salesi Prado and Luque gen. et sp. nov. from the upper Aptian–lower Albian Romualdo Formation of the Santana Group, Araripe Basin, Pernambuco, Brazil. A. Holotype DGEO-CTG-UFPE-8122; A1, dorsal view showing the two anterolateral spines, the epibranchial spine, and the outer orbital spine; A2, close-up of rostrum and left orbit showing the inner-, intra-, and outer- orbital spines; A3, inverted colour image of A2, showing the short and wide inner and outer orbital fissures. B. Paratype DGEO-CTG-UFPE-8119; B1, dorsal carapace; B2, inverted colour image of B1 showing the outer orbital spine and the two anterolateral spines. C. Paratype DGEO-CTG-UFPE-8137; C1, dorsal carapace; C2, inverted colour image of C1, showing the anterolateral spines and the short epibranchial spine. D. Paratype DGEO-CTG-UFPE- 8127; D1, SEM image of dorsal carapace; D2, inverted colour image of D1, showing the anterolateral spines and the short epibranchial spine. Abbreviations: as, anterolateral spines; ios, inner orbital spine; iof, innermost orbital fissure; its, intra-orbital spine; oof, outermost orbital fissure; oos, outer orbital spine. All specimens photographed dry and uncoated, except for C1, which was coated with ammonium chloride.

Fig. 1. A. Map showing the known occurrences (stars) of orithopsid and eubrachyuran genera in the Early Cretaceous of South America: upper Aptian– lower Albian San Gil Inferior Formation, Boyacá and upper Aptian Paja Formation, Santander, Colombia and upper Aptian–lower Albian Romualdo Formation, Araripe Basin, Brazil. B. Sedimentary basins of northeast Brazil; arrows show the three possible routes of Cretaceous marine ingression into the Araripe Basin. C. New fossiliferous localities (stars) with brachyuran crabs from upper Aptian–lower Albian Romualdo Formation, Araripe Basin, Pernambuco, Brazil (base map modified from Assine 2007).

Fig. 2. Stratigraphic sections (A1–D1) and field photographs (A2–D2) of the upper Lower Cretaceous Romualdo Formation, Pernambuco, Brazil, where the new brachyuran crabs were discovered: Arrojado site (A), municipality of Araripina, Zé Gomes (B), Cedro (C), and Santo Antônio (D) sites, municipality of Exu. Scale bars 25 cm.