Siagonodon exiguum, Martins & Folly & Ferreira & Garcia da Silva & Koch & Fouquet & Machado & Lopes & Pinto & Rodrigues & Passos, 2023

Martins, Angele, Folly, Manuella, Ferreira, Guilherme Nunes, Garcia da Silva, Antonio Samuel, Koch, Claudia, Fouquet, Antoine, Machado, Alessandra, Lopes, Ricardo Tadeu, Pinto, Roberta, Rodrigues, Miguel Trefaut & Passos, Paulo, 2023, An evolutionary paradox on threadsnakes: Phenotypic and molecular evidence reveal a new and remarkably polymorphic species of Siagonodon (Serpentes: Leptotyphlopidae: Epictinae) from Amazonia, Vertebrate Zoology 73, pp. 345-366 : 345

Siagonodon exiguum sp. nov.

Figs 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11

Holotype (Figs 3-5).

Adult female, MNRJ 27562 collected on the Monte Branco Plateau located in the Saracá-Taquera National Forest, Pará, Brazil (01°37 ’56.99” S, 56°33 ’29.03” W, 195 meters above sea level (m a.s.l).

Paratypes (Figs 3-5).

Six specimens, two females (MNRJ 27557 and 27561) and four males (MNHN-RA-2023.0001, MNRJ 27558-60). MNRJ 27557-61, unknown collector, from the type locality (01°37'S, 56°33'W); and MNHN-RA-2023.001, collected by T. Decaens and E. Lapied on March 2015, from Mitaraka (02°14 ’8.77” N, 54°26 ’57.41” W; 328 m a.s.l), French Guiana.

Diagnosis.

The putative autapomorphy of Siagonodon exiguum sp. nov. may be represented by lateral pointed projections on the snout or with inconspicuous lateral projection by presenting a slight concavity on the mid-rostral portion of snout (see below in comparison the opposite states for snout condition).

Definition.

Siagonodon exiguum sp. nov. can be distinguished from all congeners by the following combination of external characters: midbody scale rows 14; midtail scale rows 14; supralabials two (1+1); infralabials four; subcaudals 15-18; middorsal scale rows 255-289; total number of precloacal vertebrae 239-263; supraocular scales absent; frontal scale distinct, not fused with rostral; body dorsum and vent uniformly beige; eye totally covered by ocular scale, eye spot unconspicuous and reduced; caudal scales not fused; terminal spine absent; cloacal vertebrae 3-5, caudal vertebrae 18-20, the last one composed of 3 fused vertebrae; snout concave with conspicuous lateral tapered projections, without tapered projections, and slightly concave anteriorly or without projections and truncate anteriorly.

Comparisons with other congeners (conditions for other species in brackets; Table 1).

Siagonodon exiguum is distinguished from all congeners by the presence (in most specimens) of lateral pointed projections on the snout, or by a slightly concave snout (when present; vs. acuminate in S. borrichianus ; truncate in S. cupinensis and S. septemstriatus ); by having 15-18 subcaudal scales (vs. 10-13 in S. borrichianus , 12-16 in S. cupinensis , and 10-12 in S. septemstriatus ); by having a pair of optic nerve foramina (vs. single foramen in S. septemstriatus ); from S. cupinensis and S. septemstriatus by the fusion of the prootic+otooccipital bones (vs. distinct bones); from S. borrichianus and S. septemstriatus by having 255-289 middorsal scales (vs. 163-285 in S. borrichianus and 215-244 in S. septemstriatus ); from S. borrichianus and S. septemstriatus by having 230-269 midventral scales (vs. 245-272 in S. borrichianus and 196-224 in S. septemstriatus ); from S. borrichianus and S. septemstriatus by having 14 scales around midtail (vs. 10 in S. borrichianus and 12 in S. septemstriatus ); from S. septemstriatus and S. borrichianus by its uniform beige dorsum and vent color (vs. striped pattern in S. septemstriatus , and uniformly dark brown in S. borrichianus ).

Description of the holotype (Figs 4 and 5).

Adult female, Total length (TL) 186 mm, Tail length (TAL) of 8.0 mm; midbody diameter (MB) 3.2 mm; midtail diameter 2.7 mm; TL/TAL 23.2; TL/MB 5.8; rostral width 1.2 mm; inconspicuous eye spot covered by ocular scale; head length 4.9 mm, head width 2.9 mm; head subcylindrical, markedly truncate anteriorly; body subcylindrical, slightly tapered caudally near the tail; head not enlarged, indistinguishable from neck. Snout truncate in lateral and ventral views; rostral hexagonal in frontal views, and trapezoidal in ventral view; dorsal apex concave, not reaching the anterior limit of oculars; rostral contacting supranasal and infranasal laterally, and frontal dorsally; nasal completely divided horizontally by oblique suture crossing nostril; nostril roughly semicircular, positioned mostly dorsal of nasal suture; supranasal twice as high as long, contacting rostral anteriorly, infranasal ventrally, first supralabial ventrally, ocular posteriorly, and frontal dorsally; supranasal ventral boundary slightly longer than upper border of infranasal scale; infranasal twice as high as long; upper lip border formed by rostral, infranasal, anterior supralabial, ocular, and posterior supralabial scales; temporal scale not distinct in size from dorsal scales of lateral rows; two supralabials (1+1) entirely separated from each other by ocular; first supralabial about 1.5 times higher than long, not reaching the nostril level or the eye spot; second supralabial roughly semicircular, slightly longer than high, twice as high as first supralabial, its posterior margin in broad contact with temporal; ocular high, dorsal apex acuminate, anterior border straight and vertical, about 3 times higher than long, contacting posterior margins of supranasal and first supralabial anteriorly, parietal posteriorly, frontal and postfrontal dorsally; eye indistinct, covered by ocular scale; eye spot positioned in central area of expanded upper part of ocular, displaced far above nostril level; frontal subcircular, slightly wider than the other middorsal head plates (postfrontal, interparietal, and interoccipital); middorsal head plates (postfrontal, interparietal, and interoccipital) subequal in size, trapezoidal in dorsal view, weakly imbricate; frontal enlarged, about twice as wide as long, contacting rostral, supranasals, oculars, and postfrontal; postfrontal about twice as wide as long, contacting frontal, oculars, parietals, and interparietal; interparietal twice as wide as long, contacting postfrontal, parietals, occipitals, and interoccipital; interoccipital about as wide as long, contacting interparietal, occipitals, and first dorsal scale of vertebral row; parietal hexagonal, about as wide as long; lower margin contacting upper border of second supralabial, posterior margin contacting temporal, occipital, and interparietal, anterior margin in broad contact with ocular and postfrontal; occipital almost twice wider than long, its lower limit attaining level of upper margin of second supralabial, although separated from the latter by temporal; symphysial subcircular, anterior and posterior borders respectively straight and slightly convex, about twice wider than long; four infralabials; first infralabial small, subtriangular; second and third infralabials subequal, wide, somewhat higher than long, not pigmented; fourth infralabial twice longer than high high, distinctively longer than others, as high as second and third supralabial, not pigmented; dorsal scales homogeneous, cycloid, smooth, slightly imbricate, arranged in 14 scale rows around midbody and in the middle of the tail; middorsal scales 289; midventral scales 266; cloacal shield short and semicircular, almost twice as wide as long; 16 subcaudals; fused caudals absent; terminal spine indistinct or absent.

Coloration of holotype relatively faded after preservation; dorsal, paraventral and ventral scales uniformly beige; dorsal and ventral head shields creamish white; cloacal shield creamish white.

Osteology.

Skull (Fig. 6 ; n = 3; holotype condition indicated with asterisk). Premaxilla wide, expanded anterolaterally, roughly rectangular (n = 2*) or trapezoidal (n = 1) in anterior view; and triangular in ventral view, edentulous, pierced by seven (n = 2) or eight (n = 1*) foramina; nasal process of premaxilla reduced and tapered toward apex, transverse process absent and vomerian process tapered and single; premaxilla with internal septum composed of two laminae that support the septum nasi dorsally, expanding posteriorly and contacting septomaxilla ventrally; nasals paired, elongated, expanded anterolaterally, with convex anterior limit, and undulated dorsoposterior suture with frontals; being pierced by pair of foramina in posterolateral border of contact with prefrontals and frontals (foramen for the apicalis nasi); additional pair of foramina pierce on the medial contact within both nasals; nasal septum descending as double medial vertical flanges that contact premaxilla and septomaxilla ventrally (internally); prefrontals paired, irregular in shape, in contact with septomaxilla midventrally, and separated by short gap (n = 2*) from maxilla ventrolaterally or in contact with it (n = 1); each prefrontal perforated by two (n = 2*) or three (n = 1*) foramina; septomaxillae paired, complex in shape, expanding dorsoanteriorly and comprising posterior limit of naris; conchal invagination absent; ascending process of premaxilla pierced by single large foramen (n = 2*) or by two foramina (n = 1); internally, dorsal surface of each septomaxilla pierced by single foramen (n = 3 sides*) or two foramina (n = 3 sides*), and with medial inconspicuous sulcus (n = 2*), or sulcus absent (n = 1); vomers paired, located midventral to vomeronasal cupola, transversal arms absent, with posterior arms in contact with each other posteriorly (n = 2*) or not (n = 1); pair of foramina pierce the ventral lamina of each vomer; frontals paired, elongated, approximately twice longer than wide, both elements bearing short anterolateral projections to attach to prefrontals; frontal pillars absent; optic nerve foramen paired (n = 2*) or single (n = 1), restricted to lateral descending surface of frontals; maxilla edentulous, irregular in shape, pierced by four (n = 1*) or six (n = 2) foramina; posterior process of maxilla reaching (n = 1) or not reaching (n = 2*) level of anteriormost optic nerve foramen in lateral view; posterior orbital element absent; parietal single, wide, representing largest bone of braincase; parietal internal pillars (sensu Martins et al. 2021a) present; parabasisphenoid arrow-like, with tapered anterior tip lying dorsally to palatine, and fitting in medial line of vomeronasal cupola; posterior limit of parabasisphenoid convex medially; internal (dorsal) face of parabasisphenoid with inconspicuous lateral sulci; anterior opening for palatine artery indistinct or absent, internal carotid artery foramen and abducens nerve foramen present; opening for palatine ramus of facial nerve formed by lateral edge of parabasisphenoid and ventral edge of parietal; basioccipital single and approximately triangular in ventral view, lateral process for attachment of tendons for nuchal muscles ( Martins et al. 2019b) absent; basioccipital does not participate in formation of foramen magnum; supraoccipitals fused into single unit, approximately twice wider than long; prootics fused to otooccipitals; prootics+otooccipitals forming trigeminal nerve foramen together with parietal and parabasisphenoid (on one side of one specimen parabasisphenoid does not participate in it); prootics pierced medially (internally) by single acoustic nerve foramen; two statolythic masses present in cavum vestibuli (n = 2*) or statolythic mass absent; stapedial footplate apparently not co-ossified with prootic; otooccipitals dorsally forming short but distinct atlantal process; descend to ventral midline and contact each branch ventrally to exclude basioccipital in formation of foramen magnum; medial surface (internal) of otooccipitals+prootics pierced by internal opening for recessus scalae tympani, perilymphatic foramen, posterior vagus nerve foramen, and reduced foramen that might represent hypoglossal nerve passage; palatines paired and triradiate; anterior margin of maxillary process flexing ventrally and contacting prefrontal (n = 2*) or not (n = 1); each palatine pierced by a foramen in its ventral surface; pterygoids slender and rod-like, not contacting quadrate posteriorly, and not extending beyond anterior margin of basioccipital; ectopterygoid indistinct.

Suspensorium (Fig. 7; n = 3; holotype condition indicated with asterisk).

Dentary supports series of five long teeth ankylosed to inner surface of medial margin of dental concha; mental foramen nearly under fifth tooth; dorsoposterior process of dentary with rounded enlargement (n = 1), possibly to provide attachment for Musculus ceratomandibularis and/or to Musculus cervicomandibularis ( Martins et al. 2019b) or not as above (n = 2*); splenial conical, visible in lateral view, representing smallest bone in lower jaw, extending from level of third tooth to contact angular posteriorly; anterior mylohyoid foramen absent on splenial; posterior mylohyoid foramen on ventral surface of angular; angular conical, extending posteriorly to level of posterior surangular foramen; compound bone pierced by two foramina on surangular lamina, posterior elliptical and wide, anterior small and rounded; anterior surangular foramen located slightly anterior to coronoid; foramen for chorda tympani of hyomandibular ramus of facial nerve (VII) present (n = 2*) or indistinct (n = 1); foramen of retroarticular process absent; prearticular lamina of compound bone presenting dorsal process to support coronoid (n = 1) or not (n = 2*); coronoid wide, approximately triangular in shape, about as wide as tall, pierced by foramen in its medial side (n = 2*) or not (n = 1); quadrate long and slender, about 50% of skull length, posterior process absent (sensu Martins 2016); dorsal foramen in anterior half of quadrate absent; dorsoanterior lamina of quadrate with shallow sulcus that extends throughout half its extension (n = 2*) or not (n = 1).

Cervical vertebrae (n = 3; holotype condition indicated with asterisk; Fig. 8).

Atlas composed of neural arches, not fused dorsally or ventrally; ventral element (intercentrum I sensu Holman 2000) reduced and not fused to neural arches; lateroposterior process of atlas present; axis with inconspicuous spinal process that projects both dorsally and posteriorly; lateral foramina of axis (centrum) indistinct or absent; short lateral processes present (n = 2) or absent (n = 1*). Odontoid process of axis osseous and sutured to axis, approximately lozenge shaped in anterior view, with an anterior tapered process; intercentra II and III (sensu Holman 2000) fused into a single unit, compressed laterally.

Pelvic girdle (n = 3; holotype condition indicated with asterisk; Fig. 9).

Composed of ilium, ischium, femur, and pubis. Ilium and pubis rod-like; ischium approximately rectangular and fused to pubis (n = 1) or ischium rod-like and not fused to pubis (n = 2*); ilium represents longest bone of pelvic girdle, being fused to pubis (n = 2*) or not (n = 1); femur approximately rectangular and curved with dorsolateral claw-like process (n = 2*) or without such process (n = 1).

Hyoid (n = 3).

Hyoid Y-shaped, extending from 8-11th (MNHN-RA-2023.0001), 10-14th (Holotype), or 11-14th vertebrae. Lingual process distinctively shorter than cornua, with the former being approximately one vertebra long.

Hemipenis (paratype MNRJ 27560, n = 1) (Fig. 10).

Fully everted and almost maximally expanded hemipenis (examined ex-situ) renders a unilobed organ, about 2 mm long, slightly broadened at base, followed by a short tapered area that posteriorly expands towards apex; basal portion not ornamented on proximal half; hemipenial body with no ornamentation on either sulcate or asulcate sides; sulcus spermaticus single, entering organ on basal surface and extending toward apex of organ; sulcal folds raised and not ornamented; distal half of hemipenial body covered by three inconspicuous transverse flounces; organ apex convex and not ornamented; single and tapered projection develops from apex of asulcate side.

Meristic and morphometric variation.

Middorsal scales 285-289 (287 ± 2.8; n = 2) in females and 255-280 (269 ± 12.5; n = 4) in males; midventral scales 266-267 (266.5 ± 0.7; n = 2) in females and 230-269 (253 ± 16.4; n = 4) in males; subcaudals 15-16 (15.5 ± 0.7; n = 2) in females and 16-18 (17 ± 0.8; n = 4) in males; TL 184-204.2 (195.1 ± 12.9; n = 2) in females and 171.1-200.3 (187.1 ± 12; n = 4) in males; TL/TAL ratio 20.0-23.2 (21.6 ± 2.2; n = 2) in females and 19.4-22.5 (20.6 ± 1.5; n = 4) in males; TAL 4.3-4.9% of TL (4.6 ± 0.5; n = 2) in females and 4.4-5.1% (4.9 ± 0.3; n = 4) in males; TL/midbody diameter 2.4-3.1 (2.7 ± 0.5; n = 2) in females and 2.3-2.8 (2.6 ± 0.6; n = 4) in males; TAL/midtail diameter 2.8-3.5 (3.1 ± 0.4; n = 2) in females and 2.5-3.4 (3 ± 0.3; n = 4) in males.

Color pattern variation.

Color pattern of paratypes mostly resembles that of holotype except for the following: dorsum and vent uniformly light brown in three paratypes (MNRJ 27559-61).

Postcranial quantitative variation.

Precloacal vertebrae 248-260 (254 ± 8.5; n = 2) in females and 239-263 (253 ± 10; n = 4) in males; cloacal vertebrae 4-5 (4.5 ± 0.7; n = 2) in females and 3-4 (3.5 ± 0.6; n = 4) in males; caudal vertebrae 18 in females (n = 2) and 18-20 (19.2 ± 0.9; n = 4) in males.

Etymology.

The specific epithet exiguum is a Latin word meaning little, short, scanty and/or poor. Although readers might think that the name was chosen in allusion to the small size of the species, we dedicate the species to all minorities in science including gender, race/color, ethnicity, and sexual orientation that are considerably underrepresented in the academia throughout the world, especially in higher positions. Although several worldwide initiatives have been implemented in the past few years, the scientific environment is still biased, discouraging and, sometimes noxious, largely because of persisting systemic social, economic and cultural aspects.

Distribution and Natural History (Fig. 11).

Siagonodon exiguum is currently known from the type locality in Pará State (Brazil) and French Guiana (Mitaraka, Nouragues - A. Fouquet pers. obs and Angouleme - Dewynter et al. 2020). In Brazil, the species was found during plant suppression in "Terra Firme" (non-flooded forest) forests on the Monte Branco Plateau. The plateau is part of the Saracá-Taquera National Forest, a Sustainable Use Conservation Unit where bauxite mining activities take place, in the municipality of Oriximiná, at 190-200 m altitude. The known occurrences lie within the Guiana Region ( Vacher et al. 2020) which correspond to the lowland of the Eastern part of the Guiana Shield. The specimen from Mitaraka was found when digging in search for earthworms.

The X-ray images of two specimens (MNRJ 27558 and 27562) revealed the presence of dense and granular digestive content over half of the specimens’ snout-vent length. In MNRJ 27562, however, X-ray images most likely reveal the presence of at least six elongated eggs, with both anterior and posterior margins hardly delimited. Images also seem to reveal the presence of what seems to represent an embryo in the anteriormost egg (see Supplementary Material 3, X-ray images).

New combination.

Pinto and Curcio (2011) described Siagonodon acutirostris based on a combination of morphological characters, including a new character for the diagnosis of the genus. Martins (2021a) proposed the following osteologically diagnostic characters for the genus Trilepida based on a high sample of species: (i) the basioccipital participating in the formation of the foramen magnum, (ii) paired nasals and (iii) fused supraoccipitals. Based on the combination of morphological characters proposed by Adalsteisson et al. (2009), Pinto and Curcio (2011), and Martins (2016, 2021a), and the molecular data provided herein, we propose a new combination and new diagnosis for S. acutirostris and regard the presence of fused caudals as a homoplastic character.

Figures

Figure 3. Dorsal (A, D), lateral (B, E), and ventral (C, F) views of the holotype (MNRJ 27562; Figs A - C) and paratype (MNHN-RA- 2023.0001; D - F) of Siagonodon exiguum sp. nov.

Figure 4. Comparisons of the head in dorsal (left column), lateral (middle column) and ventral (right column) views of the type series of Siagonodon exiguum sp. nov. A MNHN-RA- 2023.0001; B MNRJ 27557; C MNRJ 27558; D MNRJ 27559; E MNRJ 27560; F MNRJ 27561; G MNRJ 27562 (holotype).

Figure 5. Comparisons of the tail in dorsal (left column), lateral (middle column) and ventral (right column) views of the type series of Siagonodon exiguum sp. nov. A MNRJ 27558; B MNRJ 27559; C MNRJ 27560; D MNRJ 27561; E MNRJ 27562 (holotype); F MNHN-RA- 2023.0001.

Figure 6. Three-dimensional reconstruction of the skull of Siagonodon exigumm sp. nov. based on µ-CT data in dorsal (A, D, G), lateral (b, e, h), and ventral (c, f, i) views. MNRJ 27562 (a - c), MNHN-RA- 2023.0001 (d - f), MNRJ 27558 (g - i). Abbreviations: bo basioccipital; fr frontal; ma maxilla; na nasal; ot otooccipital; pa parietal; pal palatine; pbs parabasisphenoid; pf prefrontal; pm premaxilla; pr prootic; pt pterygoid; sm septomaxilla; so supraoccipital; vo vomer.

Figure 7. Three-dimensional reconstruction of the lower jaw of Siagonodon exigumm sp. nov. based on µ-CT data in lateral (a, c, e) and medial (b, d, f) views. MNRJ 27562 (a - b), MNHN-RA- 2023.0001 (c - d), MNRJ 27558 (e - f). Abbreviations: an angular; asf anterior surangular foramen; co coronoid; de dentary; dppd dorsoposterior process of dentary; mf mental foramen; par prearticular lamina of compound bone; qd quadrate; qdf quadrate foramen; rp retroarticular process; sp splenial; spd symphyseal process of dentary.

Figure 8. Three-dimensional reconstruction of the atlas (first to third column) and axis (fourth to sixth column) of Siagonodon exigumm sp. nov. in anterior, lateral and posterior views based on µ-CT data. MNRJ 27562 (a), MNHN-RA- 2023.0001 (b), MNRJ 27558 (c). Abbreviations: af articular facet; con condile; iI intercentrum I: iII intercentrum II; iIII intercentrum III; na neural arch; od odontoid; odp odontoid process; sp spinal process.

Figure 9. Three-dimensional reconstruction of the pelvic rudiments of Siagonodon exigumm sp. nov. based on µ-CT data. MNRJ 27562 (a), MNHN-RA- 2023.0001 (b), MNRJ 27558 (c). Abbreviations: fe femur; il ilium; is ischium; pu pubis.

Figure 10. Schematic illustrations (upper line) and photos (lower line) of the asulcate (left) and sulcate (right) sides of the hemipenis of Siagonodon exigumm sp. nov. (MNRJ 27560).

Figure 11. Distribution map of Siagonodon exigumm sp. nov. based on the type series (7 specimens).