Autrigoniscus resinicola, Sánchez-García & Peñalver & Delclòs & Engel, 2021

Sánchez-García, Alba, Peñalver, Enrique, Delclòs, Xavier & Engel, Michael S., 2021, Terrestrial Isopods from Spanish Amber (Crustacea: Oniscidea): Insights into the Cretaceous Soil Biota, American Museum Novitates 2021 (3974), pp. 1-32 : 13-19

Autrigoniscus resinicola , new species

Figures 6–8

DIAGNOSIS: As for the genus (above).

DESCRIPTION: Body (figs 6, 7A–B) more or less oblong, of “clinger” habitus; small sized, total length 1.62 mm, nearly twice as long as broad (1.78×); dorsal surface only moderately convex, of a relatively smooth appearance, covered with small semicircular scales and sparsely distributed small triangular-shaped scale setae arranged in transverse rows (fig. 8B); gland pores not visible as preserved.

Cephalon transversely semioval, maximum width 0.49 mm, with front evenly rounded, and lateral lobes small but distinct; suprantennal line nearly straight medially (in holotype) to V-shaped (in specimen MCNA 12617 ); antennal lobes absent. Eyes small but distinct, consisting of at least three ommatidia .

Pereon poorly visible due to blackening of cuticle; pereonites overlapping, lamellar; lateral parts of pereonites (i.e., epimeral plates) only moderately prominent, those of two posterior pereonites with posterior corners acutely triangular, all edged with small appressed spikes and some small triangular-shaped scale setae (as a continuity to those of dorsal surface). Pleon distinctly narrower than pereon (abruptly contracted); pleotelson short, subtriangular, with rounded apex.

Mouthparts not visible as preserved.

Antennule (fig. 8A) with three articles; first article obscured by head, second article partly obscured by head, 0.04 mm long as preserved, without visible setae; third article shorter than second, 0.02 mm long, with a tuft of several short and relatively thick apical aesthetascs.

Antenna not very spinose in appearance, rather stout, with five peduncular articles and a multiarticulate flagellum, reaching posteriorly to middle pereonite 1, elbowed between fourth and fifth articles, with flagellum much shorter than peduncle; first article partly obscured by head, short and stout, 0.08 mm long as preserved, slightly broader than long, with no visible spines; second article short and stout, 0.09 mm long, slightly broader than long, with one visible thick spine distally; third article short and stout, 0.10 mm long, about as long as broad, with up to two visible thick spines distally; fourth article elongate, stout, 0.20 mm long, 2.0× length of preceding article, 2.5× longer than broad, with two visible thick spines distally; fifth article elongate and much slender than fourth article, 0.25 mm long, 1.2× length of preceding article, 6.2× longer than broad; flagellum with six articles, 0.20 mm long, 0.8× length of fifth article of peduncle, flagellar articles gradually tapering distally, distal article without visible aesthetascs.

Pereopods (figs 7C, 8C–D) of moderate size, increasing in length and thickness posteriorly; basis and ischium mostly obscured by body; merus poorly visible, with elongate, thick, nearly straight spines on sternal margin; carpus with elongate, thick, nearly straight spines more or less arranged in a longitudinal row on sternal margin giving it a rippled appearance, spines becoming longer distally; at least carpus of pereopods 4–7 having a large distal process on tergal margin with a row of scales (probably male sexually dimorphic characters, but females unknown) (fig. 8C); propodus longer than carpus, with elongate, thick, nearly straight spines on sternal margin, less numerous than on carpus; dactylus simple, with enlarged outer claw (fig. 8D); dactylar setae unbranched, glabrous, long and slender; ungual setae not visible as preserved.

Pleopods poorly visible; pleopodal exopods without respiratory structures; endopod of first pair of pleopods modified, biarticulate; proximal article very short; distal article long and slen- der, styliform (fig. 8F).

Uropod (fig. 8E) moderately elongate, about 0.27× of body length, not surpassing length of pleon, freely projecting caudally; protopod produced inside pleotelson, rather short and stout, 0.15 mm long, about as long as broad, with endopod and exopod inserted at same level; exopod conical, maximum width at base 0.07 mm and strongly tapered distally, 0.29 mm long (excluding terminal tuft of setae), distinctly longer than endopod, with a tuft of short setae apically (0.07 mm long); endopod conical, maximum width at base 0.06 mm and strongly tapered distally, 0.20 mm long (excluding terminal tuft of setae), with a tuft of long setae apically (0.11 mm long), lengths of endopod and endopodal tuft of setae combined reaching length of exopod (excluding terminal tuft of setae).

HOLOTYPE: MCNA 12522 , male, virtually complete, exposed dorsally, ventrally, and laterally. Preserved in darkened orange-colored amber, trimmed to 2.5×1.5×1.0 mm (in an epoxy trapezoid 22.5×14.5× 1.1 mm), surrounded by numerous pseudoinclusions ( Lozano et al., 2020) and particles of detritus; no other major inclusions. The specimen shows indirect evidence of the digestive tract from fossilized fecal matter (termed “cololite”).

ADDITIONAL MATERIAL: Specimen MCNA 12678, presumed male, partially complete, exposed ventrally, ventrolaterally, and dorsolaterally. Preserved in darkened orange-colored amber, trimmed to 6.0×2.1× 1.2 mm (in an epoxy trapezoid 23.0×13.0× 1.2 mm), surrounded by numerous pseudoinclusions ( Lozano et al., 2020), particles of detritus and fungal hyphae; no other major inclusions. Specimen MCNA 12617, presumed male, poorly preserved, exposed dorsally, ventrally, and laterally. It shows indirect evidence of the digestive tract from fossilized fecal matter. Preserved in darkened yellow-colored amber, trimmed to 6.9×4.0×1.0 mm (in an epoxy trapezoid 23.1×14.9× 1.1 mm), surrounded by numerous pseudoinclusions and particles of detritus; no other major inclusions. Specimen MCNA 13823.1, presumed male, partially complete. Preserved in two darkened orange-colored amber pieces, one trimmed to 10.5×6.0×3.0 and preserving the major part of the body, and the other trimmed to 4.5×4.5×1.0 and consisting of an impression of the cuticle. Syninclusions include one possible rove beetle ( Staphylinidae ?, MCNA 13823.2).

Specimens MCNA 12678, MCNA 12617, and MCNA 13823.1 match the diagnosis of Autrigoniscus resinicola for some characters, but other features remain unclear and we cannot attribute them to this species with complete confidence (refer to Remarks, below).

OCCURRENCE: Peñacerrada I amber site [Peñacerrada I = Moraza], eastern margin of the Basque-Cantabrian Basin, Burgos, northern Spain; Early Cretaceous (Late Albian).

ETYMOLOGY: The specific epithet refers to the occurrence of the species in plant resins (Latin, resina, meaning “resin,” and the suffix - cola, meaning “dweller”), which in turn become amber when fossilized.

REMARKS: Specimen MCNA 12678 (fig. 9A) closely resembles the holotype of A. resinicola , sharing with it the relative head, uropod, and body measurements (total body length is 1.64 mm), the overall morphology of the body (more or less oblong, moderately convex dorsally, with the pleon distinctly narrower than pereon, and the epimeral plates only moderately prominent), the very spinous pereopods (with spines arranged as described for the holotype, and a modified carpus apparently visible in one of the posterior pereopods), the dactylus morphology (with one claw and a flagellar dactylar seta), and a similarly shaped uropodal endopod (conical, distinctly shorter and narrower than exopod, and with an apical tuft of long setae). Despite the similar endopod, the poor preservation of the exopod prevents its comparison with that of the holotype. Moreover, the eyes, antennae, pleopods, and cuticle ornamentation are not visible in MCNA 12678 , and the antennulae are poorly preserved due to shrinkage and desiccation during preservation .

Specimen MCNA 12617 (fig. 8B–D) shares with the holotype the overall morphology of the body and the cuticle ornamentation (i.e., dorsal surface with small semicircular scales and small triangular-shaped scale setae, and lateral parts of the pereonites edged with small appressed spikes). The total length is 1.26 mm, nearly twice as long as broad (1.94×), and thus the specimen is slightly smaller than the holotype. However, the cephalon differs in having a V-shaped supraantennal line, and the lateral lobes are broadly rounded and larger than in the holotype. The pereopods seems less spinous than the holotype (probably an artifact of preservation), and the pattern of the spines cannot be assessed because of the hidden position of the pereopods. However, the distinctive modified carpus of A. resinicola is clearly visible in a posterior pereopod, and the dactylus is identical to those described for the holotype (fig. 8D). Unfortunately, the ommatidia, antennae, antennulae, pleopods, and uropods are not visible in MCNA 12617 , preventing comparison of these structures with the holotype .

Specimen MCNA 13823.1 has few characters visible. The cuticle shows the ornamentation as described for A. resinicola (although more pronounced than in the holotype), and a modified carpus is visible on some posterior pereopods .

All the shared features listed lead us to consider MCNA 12678 , MCNA 12617 , and MCNA 13823.1 as conspecific, but this remains somewhat unresolved, and so we have not designated them as a part of the type series. Hopefully, more complete material will eventually be recovered to permit a more thorough characterization of the species and a critical test as to whether these three specimens are correctly placed within A. resinicola .

Section Crinocheta Legrand, 1946

Family Detonidae ? Budde-Lund, 1906

Figures

FIGURE 6. Microphotographs of Autrigoniscus resinicola, new genus and species, family Trichoniscidae (holotype, MCNA 12522, male). A. Lateral habitus. B. Dorsal habitus. C. Ventral habitus. Figures made with consecutive photographs taken at successive focal planes. Scale bar = 1 mm (all panels to the same scale).

FIGURE 7. Camera lucida drawings of Autrigoniscus resinicola, new genus and species, family Trichoniscidae (holotype, MCNA 12522, male). A. Lateral habitus, with magnified area showing details of endopod of first pair of pleopods. B. Dorsal habitus, with magnified area showing details of antennule. C. Fifth pereopod in ventral view, with magnified area showing details of modified carpus. Scale bar = 0.5 mm (all panels to the same scale).

FIGURE 8. Detail microphotographs of Autrigoniscus resinicola, new genus and species, family Trichoniscidae (holotype, MCNA 12522, male). A. Head in dorsal view, showing the antennule (arrowhead). B. Semicircular scales and triangular-shaped scale setae in ventral view. C. Carpus of pereopods 5 and 6 in ventral view, showing the distal process (arrowheads). D. Simple dactylus of pereopods 2 and 3 in lateral view. E. Uropods in dorsal view. F. Endopod of first pair of pleopods in lateral view. Figures made with consecutive photographs taken at successive focal planes. Scale bars: A–D, F = 0.05 mm; E = 0.1 mm.