Exucarcinus gonzagai 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 : 740-744

Exucarcinus gonzagai Prado and Luque sp. nov.

Fig. 3.

ZooBank LSID: urn:lsid:zoobank.org:act:B206279B-DB4B-4221-AB34-E8AE0D90E1E1

Etymology: In honour of Luiz Gonzaga do Nascimento (1912–1989) “the king of baião”, a singer, composer, and one of the most influential musicians of Brazilian popular music of the twentieth century, born in the Exu municipality, Pernambuco, Brazil.

Type material: Holotype (carapace): DGEO-CTG-UFPE-8114. Paratypes (carapaces): DGEO-CTG-UFPE-7745, 7748, 7749, 7751, 8103, 8105, 8107, 8109, 8115, 8116, 8133, 8106; all from the type locality.

Type locality: Zé Gomes , Santo Antônio , and Cedro sites, Araripe Basin, municipality of Exu, Pernambuco, Brazil .

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

Diagnosis.—Carapace sub-hexagonal, nearly equidimensional, weakly vaulted to flattened, widest at carapace mid-length at level of epibranchial spine, finely granulated, lacking tubercles, bosses, ridges, or reniform swellings. Fronto-orbital margin slightly wider than half the carapace maximum width; rostrum long, extending beyond outer-orbital spine, sulcate axially, bifid distally, with a short lateral spine on each side of the rostrum; post-rostral slits absent. Orbits wide, horizontal, upturned, with two relatively narrow orbital fissures; innermost fissure the shortest; intra-orbital lobe short, truncated, squarish; outer orbital spine well-developed, simple, triangular, shorter than rostrum, directed anteriorly. Anterolateral margin nearly as long as posterolateral margin, with 5–7 spines excluding outer orbital spine, acute, unequal, directed anterolaterally; epibranchial spine short. Posterolateral margin slightly convex to straight, lacking spines, denticles, or tubercles. Posterior margin concave, slightly narrower than fronto-orbital margin. Cervical and branchiocardiac grooves distinct, shallow, sub-parallel to each other; cervical groove reaching anterolateral margin; secondary groove not defined; post-cervical groove absent. Dorsal regions without tubercles, bosses, reniform swellings, or ridges; intestinal region wide, depressed laterally, delimited by swollen metabranchial regions; epibranchial region swollen proximally near contact with branchial groove; meso- and metabranchial regions not differentiated, lacking conspicuous spines or tubercles, but bearing an incipient oblique ridge.

Description.—Carapace sub-hexagonal, about as long as wide in smaller specimens and slightly wider in larger specimens; dorsal carapace weakly vaulted to flattened, widest at carapace mid-length at the level of epibranchial spine. Dorsal carapace smooth, lacking conspicuous tubercles, bosses, ridges, or reniform swellings, covered with fine granulations. Fronto-orbital margin broad, slightly wider than half the carapace maximum width, and wider than posterior margin. Rostrum well-developed, long, extending well beyond outer-orbital spine, nearly one-third of fronto-orbital margin width, wider at base, with straight lateral margins converging antero-medially, bearing a pair of short but well-developed lateral spines positioned at mid-length, and a pair of distal spines forming a weakly downturned bifid rostrum; rostrum sulcate axially, with posterolateral margins at higher level than orbits; post-rostral slits absent. Orbits wide, upturned, directed forward, each orbit about one-third of fronto-orbital margin; supra-orbital margin straight, horizontal, with two relatively narrow orbital fissures parallel to each other, innermost orbital fissure shorter than outermost fissure; inner orbital lobe short, moderately developed, grading from lower margin of rostrum; intra-orbital lobe short, truncated, squarish in outline, separated from inner and outer lobes by the orbital fissures; outer orbital spine well-developed, simple, triangular, shorter than rostrum, with outer margin slightly converging anteriorly; sub-orbital margin visible in dorsal view, with one short, narrow suborbital fissure.Anterolateral margin slightly convex in smaller specimens, less so in larger specimens, nearly as long as posterolateral margin, bearing five to seven acute spines excluding outer orbital spine, of different sizes, and directed anterolaterally; epibranchial spine short, weakly to moderately developed, directed laterally. Posterolateral margin slightly convex in smaller specimens, nearly straight in larger specimen, without spines, denticles, or tubercles. Posterior margin wide, concave, slightly rimmed, much shorter than half of carapace maximum width, and slightly shorter than fronto-orbital margin. Cervical groove distinct, shallow, sinuous, arcuate, with smooth deflections, axially interrupted, distally reaching anterolateral margin; secondary groove not defined; post-cervical groove absent; branchial grooves distinct, well-developed, crescent-shaped; branchiocardic groove present, well-developed, sub-parallel to cervical groove. Epigastric region narrow, barely defined laterally by faint sub-parallel grooves; protogastric and mesogastric regions faintly tumid; hepatic region wide, depressed, without transverse ridge; epigastric, protogastric, mesogastric, and hepatic regions poorly delimited, without tubercles or bosses; metagastric and urogastric lacking tubercles or longitudinal ridge, faintly separated by a shallow depression, both regions defined laterally by crescent-shaped branchial grooves; urogastric region narrow; cardiac region well-developed, tumid, 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, depressed laterally and delimited by swollen metabranchial regions; epibranchial region moderately defined,

spine. D. Paratype DGEO-CTG-UFPE-8115; D 1, dorsal carapace; D 2, counterpart; D 3, close-up of ventral view of right anterolateral margin bearing multiple anterolateral spines; D 4, close-up of posterolateral and posterior margins lacking spines; D 5, close-up of dorsal carapace cuticle with fine granulations. E. Paratype DGEO-CTG-UFPE-8116, SEM image of bifid rostrum. Abbreviations: as, anterolateral spines; ios, inner orbital spine; iof, innermost orbital fissure; its, intra-orbital spine; oof, outermost orbital fissure; oos, outer orbital spine; sof, sub-orbital fissure. All specimens photographed dry and uncoated, except B, C, which are coated with ammonium chloride.

limited by cervical and branchiocardic grooves, swollen proximally near contact with branchial groove; meso- and metabranchial regions not differentiated, lacking conspicuous spines or tubercles, but bearing an incipient oblique ridge extending towards the posterolateral corner.

Dimensions. —For a list of measurements see Table 3.

Remarks.— Exucarcinus Prado and Luque gen. nov. strongly differs from Camarocarcinidae Feldmann, Li, and Schweitzer, 2007 , Necrocarcinidae Förster, 1968 , and several Cenomanocarcinidae Guinot, Vega, and Van Bakel, 2008 , in its slightly vaulted to flattened dorsal carapace, the lack of posterolateral spines ( Fig. 3A 1 , A 2 , B 1, C 1, D 1 , D 4 ), and the absence of transverse or branchial ridges, contrasting with the strongly vaulted and ornamented carapaces seen in these families. Also, it lacks the deep cervical and branchiocardic grooves and short rostrum seen in Necrocarcinidae ( Fig. 3A 1 , A 2 , B, C, E). Among stem raninoidans, the presence of a well-defined longitudinal mesial ridge, absent in Exucarcinus , is mostly seen among Palaeocorystidae Lőrenthey in Lőrenthey and Beurlen, 1929, Cenomanocarcinidae , Necrocarcinidae in part (i.e., Elektrocarcinus Schweitzer, Karasawa, Luque, and Feldmann, 2016 , and Hadrocarcinus Schweitzer, Feldmann, and Lamanna, 2012 ), and most of the non-American Orithopsidae genera (see below). Exucarcinus Prado and Luque gen. nov. differs from Paranecrocarcinidae Fraaije, Van Bakel, Jagt, and Artal, 2008 , in its flattened mesogastric region, the spinose anterolateral margin (5–7 spines) Fig. 3A 1 , A 2 , D 1 –D 3), and the lack of post-rostral slits, so distinctive of paranecrocarcinids. The new genus also differs from Paranecrocarcinidae and several Orithopsidae in the lack of protogastric swellings, but shares with them the wide, somewhat spatulate bifid rostrum extending well beyond the outer-orbital spine, the overall sub-hexagonal carapace with wide fronto-orbital margins, and the lack of posterolateral spines.

Exucarcinus Prado and Luque gen. nov. differs from other orithopsids (e.g., Bellcarcinus , Cherpiocarcinus , Marycarcinus , Orithopsis , Paradoxilissopsa , Silvacarcinus , Texicancer ) in the lack of dorsal tubercles, reniform swellings, keels, or ridges ( Fig. 3A 1 , A 2 , B 1, C 1, D 1 , D 2 ). Other genera like Cristella and Shazella have pronounced epibranchial spines or pustulose dorsal carapaces with conspicuous dorsal grooves, none of which are apparent in the new genus. In fact, among orithopsids, Exucarcinus Prado and Luque gen. nov. seems to be more similar to some genera endemic to the Americas, like Aetocarcinus , Colombicarcinus , and Planocarcinus based on the lack of posterolateral spines, the smooth dorsal carapace lacking conspicuous dorsal ridges, keels, or tubercles, and the broad spatulate and bifid rostrum extending beyond the outer-orbital spine. However, these genera lack the numerous well-developed and acute anterolateral spines diagnostic of Exucarcinus Prado and Luque gen. nov. ( Fig. 3A 1 , A 2 , D 1 –D 3).

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

Figures

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.

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.