Archaeochiapasa mardoqueoi, Guinot, Carbot-Chanona & Vega, 2019

Guinot, Danièle, Carbot-Chanona, Gerardo & Vega, Francisco J., 2019, Archaeochiapasidae n. fam., a new early Cenomanian brachyuran family from Chiapas, Mexico, new hypothesis on Lecythocaridae Schweitzer & Feldmann, 2009, and phylogenetic implications (Crustacea, Decapoda, Brachyura, Eubrachyura), Geodiversitas 41 (7), pp. 285-322 : 293-294

Archaeochiapasa mardoqueoi n. gen, n. sp. ( Figs 5-12 )

urn:lsid:zoobank.org:act:A95A8701-7B0F-4790-AB45-5070B93F8E07

TYPE MATERIAL. — Holotype and single specimen, male, collected by Gerardo Carbot-Chanona March 14, 2018 (IHNFG-5616 part and counterpart); carapace length: 10.3 mm; width: 11.1 mm; carapace + chelipeds length: 16.2 mm; P2 length: 22.5 mm; P3 length: 26.5 mm; P4 or P5 length: 13.5 mm.

ETYMOLOGY. — In honour of Don Mardoqueo Carbot-Trujillo, father of Gerardo Carbot-Chanona and well-known natural sciences enthusiast from Chiapas, Mexico.

TYPE LOCALITY. — El Chango, 17 km southwest of Tuxtla Gutiérrez, Chiapas, southeastern Mexico.

HORIZON AND AGE. — El Chango quarry, Cintalapa Member, Sierra Madre Formation, early Cenomanian ( Fig. 4 ) near the top of the section, along with diagnostic ammonites.

DESCRIPTION

Counterpart

Cuticle very thin, not heavily calcified. A depression well marked along carapace postero- and posterolateral border; posterior margin deeply concave. A few small, regularly dis-

tributed spiniform tubercles, only visible on the counterpart. Rostrum produced, bifurcate.

Carapace (internal cast)

Carapace nearly devoid of cuticle, small, nearly as long as wide, semiquadrate to subpentagonal, with moderate anterior narrowing. Anteriorly, an area enclosing the whole lateroanterior region, including front. A strong tooth at level of each mesobranchial region at the junction of both antero- and posterolateral margins. Lateral margin with globose swellings. Dorsal surface marked by bosses. Mesogastric region prolonging into elongate process, on each part of protogastric regions; hepatic regions large, as two lateral conspicuous areas, externally delimitated by deep groove forming right angle; subhepatic region represented by markedly swollen area on each side; gastric region developed; cardiac region inflated. Branchial regions expanded laterally, forming swellings. Several grooves well defined, deep, mainly cervical, postcervical and branchiocardiac; deep branchiocardiac grooves forming two marked median depressions. Rostrum bifurcate, medially depressed, arising well below carapace margin, seemingly integrated into carapace outline. Antero-lateral carapace margins rounded, unarmed, except for a strong tooth at level of each mesobranchial region about at the junction of both antero- and posterolateral margins. Posterior margin long, deeply concave. Carapace posterolateral and posterior margins strongly thickened and raised, expanded to form all around a narrow, markedly depressed area, continuous and undivided (here called ‘flange’). Eyes, cephalic appendages not preserved.

Thoracopods

Mxp3 complete, broad, almost entirely covering buccal cavern; exopodite stout, reaching about distal margin of merus. Chelipeds extended forwards, elongated, not longer than twice carapace width, stout, equal, homomorph, probably unarmed; merus, carpus robust; propodus thicker; fingers rather long, not bent at an angle to palm. P2, P3 extended, very long, much longer than P1 (P3: 26.5 mm, the longest), very slen- der, cylindrical throughout length, unarmed. Only one last leg (P4 or P5) preserved, shorter than P2, P3, straight, only reduced by about half compared to P3, the only one clearly recognisabe measuring 13.5 mm.

Thoracic sternum

Thoracic sternum preserved in greater part, extremely wide and extended laterally, mainly sternite 5; sternite 4 proportionally narrower; sternite 1 seemingly small, triangular; no apparent suture between sternites 2-3; sternites 5-8 laterally extended; sternites 6 to 8 gradually diminishing backwards; sternite 8 small, obliquely oriented. Sutures 5/6-7/8 reaching margins of pleon, complete on exposed portion of sternum, sutures underneath the folded pleon being not visible.

Male pleon

Pleon preserved in most part, triangular, of six three somites; first three somites not folded ventrally, in continuity with carapace, dorsally exposed; last somite (somite 6) enlarged, with two lateral prominences, probably at location of sockets for pleonal-locking system, seemingly corresponding to an effective mechanism; telson triangular, moderately long, reaching the level of suture 4/5.

Male gonopores

Not visible (see Discussion).

REMARKS

The most characteristic character of the carapace of the new family is the depressed, undivided area, delimitated by a strongly rimmed and raised margin, here called ‘flange’, which is all along the concave posterior margin of the carapace and extends along most part of posterolateral margin ( Figs 7 ; 9B ). The flange is well visible in the counterpart ( Fig. 6 ).

Figures

FIG. 5. — Archaeochiapasa mardoqueoi n. gen., n. sp., holotype, male Chiapas, El Chango quarry, lower Cenomanian Cintalapa Member of the Sierra Madre Formation (IHNFG-5616): A, specimen whitened with ammonium chloride; B, specimen submerged in water. See first three dorsally positioned pleonal somites, extremely long P2, P3, and a reduced last pereiopod, P4 or P5. Scale bar: A, B, 0.5 cm.

FIG. 6. — Archaeochiapasa mardoqueoi n. gen., n. sp. holotype, male (IHNFG-5616). A, specimen photographed in situ at El Chango quarry; B-D, steinkern (counterpart):B, illuminated with oblique yellow light; C, illuminated with incident light and coated with ammonium chloride;D, illuminated with oblique, blue light and submerged in water. Scale bars: A, 1 cm; B-D, 5 mm.

FIG. 7. — Archaeochiapasa mardoqueoi n. gen.,n. sp., Chiapas,El Chango quarry,lower Cenomanian Cintalapa Member of Sierra Madre Formation,G.Carbot-Chanona coll., holotype, male, 10.3 × 11.1 mm (IHNFG-5616): dorsal view, with proximal part of pleon (actually a1-a3) dorsally visible. Abbreviations: a1, pleonal somite 1; fl, flange; p.c., concave posterior border of carapace; r, thickened carapace border. Scale bar: 5 mm.

FIG. 8. — Archaeochiapasa mardoqueoi n. gen., n. sp., male 10.3 × 11.1 mm, holotype (IHNFG-5616): superposition of the decorticated crab carapace and the counterpart with cuticle to show ornamentation of dorsal surface. Abbreviation: fl, flange. Scale bar: 5 mm.

FIG. 9. — Archaeochiapasa mardoqueoi n. gen.,n. sp.,male 10.3 × 11.1 mm,holotype (IHNFG-5616):A, close-up of front and rostrum;B, close-up of posterior portion of carapace.Abbreviations:a1-a3, pleonal somites 1-3; p.c., concave posterior border of carapace; P, reduced P4 or P5; ro, rostrum.Scale bars:A,2 mm; B, 2.5 mm.

FIG. 10. — Archaeochiapasa mardoqueoi n. gen., n. sp., male 10.3 × 11.1 mm, holotype (IHNFG-5616): A, close-up of mxp3, with developed exopodite; B, C, left and right chelipeds. Abbreviations: ex, exopodite; i, endopodite ischion; m, endopodite merus; t.s., thoracic sternum. Scale bars: A, 1 mm; B, C, 2 mm.

FIG. 11. — Archaeochiapasa mardoqueoi n. gen.,n. sp., male 10.3 × 11.1 mm, holotype (IHNFG-5616): thoracic sternum and pleon. Abbreviations:a4-a6, pleonal somites 4-6; t, telson; 3-8, thoracic sternites 3-8; 4/5, 5/6, 7/8, sternal sutures 4/5, 5/6, 7/8. Scale bar: 3 mm.

FIG. 12. — Archaeochiapasa mardoqueoi n. gen., n. sp., male 10.3 × 11.1 mm, holotype (IHNFG-5616), close-up of pleon: A, first three somites (a1-a3) in dorsal position; B, last somites plus telson; C, close-up of posterior portion of carapace, with first three pleonal somites and reduced P4 or P5. Abbreviations: a1- a3, pleonal somites a1-a3 in dorsal position; a4-a6, pleonal somites 4-6, ventrally folded; b.p., location of pleonal socket covering sternal press-button;fl, flange; P, P4 or P5; r, thickened carapace border; t, telson. Scale bars: A, 2 mm; B, 1 mm; C, 3 mm.

FIG. 4. — Stratigraphic section of the Sierra Madre Formation, showing relative position of deposits that crop out at El Chango, sedimentological structures and fossil distribution.