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Gesomyrmex gallicus
| †Gesomyrmex gallicus Temporal range: Ypresian, Early Eocene Oise amber, France | |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Insecta |
| Order: | Hymenoptera |
| Family: | Formicidae |
| Subfamily: | Formicinae |
| Tribe: | Gesomyrmecini |
| Genus: | Gesomyrmex |
| Species: | †G. gallicus |
| Binomial name | |
| †Gesomyrmex gallicus Aria, Jouault, Perrichot & Nel, 2023 | |
A fossil species described from minor and major workers preserved in the early Eocene amber of Oise, France. This species provides direct evidence the genus radiated during the Palaeocene–Eocene Thermal Maximum (PETM) and, considered with the rest of the Gesomyrmex fossil record, occurred across Eurasia. As the extant species of the genus are confined to warm and humid forests, this provides evidence that constrains palaeoclimatic reconstructions of the Early Eocene by requiring the presence of an extensive, homogeneous and interconnected rainforest-like biome across palaeo-Eurasia. The phases of regression of warm and humid forests and the widening of dry biomes suggested as occuring during the PETM probably occurred later during the Eocene, in between hyperthermals, and were less extensive than some computer models predicted.
Photo Gallery
Gesomyrmex gallicus (a, b), Holotype MNHN.F.A32941, minor worker: (a) habitus in left lateral view; (b) habitus in ventral view. (c, d) Paratype MNHN.F.A32938, major worker: (c) habitus in dorsal view; (d) head in dorsal view. (e–g) Paratype MNHN.F.A32939, minor worker: (e) habitus in dorsal view; (f) habitus in ventral view; (g) head in frontal view. Scale bars: 1 mm (a–e, f), 0.25 mm (g). (Aria et al., 2023, Fig. 1)
Gesomyrmex gallicus (a, b), Holotype MNHN.F.A32941, minor worker: (a) habitus in left lateral view; (b) habitus in ventral view. (c, d) Paratype MNHN.F.A32938, major worker: (c) habitus in dorsal view; (d) head in dorsal view. (e–g) Paratype MNHN.F.A32939, minor worker: (e) habitus in dorsal view; (f) habitus in ventral view; (g) head in frontal view. Scale bars: 1 mm (a–e, f), 0.25 mm (g). (Aria et al., 2023, Fig. 1)
Comparative illustration of diagnostic characters for Gesomyrmex gallicus (a–f) Dorsal view of mandibles in minor workers of Gesomyrmex (black arrows in (b, c) point to the concave external margin): (a) Gesomyrmex luzonensis (specimen ANTWEB1008527); (b) Gesomyrmex hoernesi (specimen FANTWEB00075, JCW coll.); (c) G. hoernesi (specimen FANTWEB00009, NHMW coll.); (d–f) G. gallicus (d) paratype MNHN.F.A32938; (e) holotype MNHN.F.A32941; (f) paratype MNHN.F.A32939. (g–l), Dorsal and lateral views of petiole in minor workers of Gesomyrmex: (g, j) G. luzonensis (specimen ANTWEB1008527); (h, k) G. gallicus (holotype MNHN.F.A32941); (i, l) G. hoernesi (specimen FANTWEB00075, JCW coll.). Scale bars: 0.1 mm. (Aria et al., 2023, Fig. 3)
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Identification
This species is readily distinguishable from all extant and extinct species by its petiolar node with dorsal margin acute instead of rounded in profile view; and from all species except the extinct Gesomyrmex hoernesi by the masticatory margin of mandibles with nine teeth. It differs from the latter by its external mandibular margins (sensu Richter et al. (2019)) straight to feebly convex in full-face view (medially concave in G. hoernesi), and the petiolar node with dorsal margin medially concave (flat or weakly convex in G. hoernesi).
Workers of G. gallicus share a diagnostic nine-toothed mandible with Gesomyrmex hoernesi, contrasting with the five, six or eight teeth in workers of extant species (when known), but differs from G. hoernesi in its straight to slightly convex external mandibular margins (vs medially concave), which is more similar to extant species (Fig. 3). Gesomyrmex gallicus also differs from G. hoernesi and all extant species in its petiolar node with dorsal margin angled between anterior and posterior faces (vs rounded), and from G. hoernesi, Gesomyrmex chaperi, Gesomyrmex howardi or Gesomyrmex luzonensis in its petiolar node with dorsal margin emarginate medially (vs flat or slightly convex) (Fig. 3). Based on the findings by Peeters et al. (2017), we assign the new larger morph to a soldier cast. A ‘supersoldier’ would be characteristically defined by a sub-rectangular head and relatively smaller eyes. Additionally, this species differs from all other known Gesomyrmex species by its temporal range.
Distribution
This taxon was described from Oise amber, France (Ypresian, Early Eocene).
Castes
Described from major and minor specimens.
Nomenclature
The following information is derived from Barry Bolton's Online Catalogue of the Ants of the World.
- †gallicus. †Gesomyrmex gallicus Aria et al., 2023: 2, figs. 1-3, Supp. Figs. 1-2 (w.) FRANCE (Oligocene).
Type Material
- Holotype MNHN.F.A32941 (minor worker), Le Quesnoy farm, commune of Chevrière, near Creil, department of Oise, France; Lower Eocene (Ypresian), level MP7 of the mammal fauna of Dormaal.
- Paratypes MNHN.F.A32938 (major worker), MNHN.F.A32939 (minor worker), MNHN.F.A32940 (minor worker), same data as holotype.
Taxonomic Notes
Théobald (1937) described a series of specimens from Eocene–Oligocene rock-imprints of Kleinkems (Germany) he assigned to the genus Gesomyrmex, but this type material was subsequently lost and the description precludes their confident placement within this genus. The alate specimen illustrated by Théobald (1937: pl. XIV, fig. 22) has a wing venation strikingly different from that of the other representatives of the genus, inter alia because of the absence of the vein Rs distad pterostigma and of the small rhomboidal along RsþM. Therefore, the species is considered as incertae sedis in Formicidae (Dlussky et al. 2009). Species erected based on gynes from Eocene and Miocene compression deposits (Dlussky et al. 2009, 2015) also require a critical reappraisal, developed hereafter.
Unless otherwise noted the text for the remainder of this section is reported from the publication that includes the original description.
Description
Worker
Minor (holotype in brackets). BL 2.5 (2.95) HL 0.75–0.81 (0.92), HW 0.62–0.66 (0.72), EL 0.43–0.43 (0.45), MFC (0.36), SL 0.43 (0.40), ED 0.17–0.2 (0.18), AL 0.9 (0.95), PW 0.4–0.4 (0.52), PtL 0.17–0.18 (0.17), PtH 0.28 (0.30), FL 0.5 (0.53), CI 76–82 (78), FCI (50), SI (55), OI 21–222 (19), PtI 164 (176), FI 55 (55), AI 0.44 (54).
Body 2.95 mm long, head slightly longer than broad, trapezoid-shaped, with convex sides, round posterior corners and tenuously concave posterior margin. Large and elongate eyes with straight lateral borders, somewhat more than 0.5 times the size of the head, converging anteriorly. Malar area and cheeks short. Small but distinct ocelli. Frontal and clypeal areas smooth but finely laminated antero-dorsally. Frontal triangle neat. Frontal carinae poorly visible, slightly bulging, diverging and ending on indistinct, tenuous medial torular arches covering half of condylar bulbs. Clypeus slightly bulged, projecting in an anteriorly long, rounded lobe, covering almost half of the mandibles’ length; long, decussate mandibles with eight nearly homodont teeth and a slightly longer apical tooth, whose extremity strongly curves downward; masticatory margin with setae; in full-face view, mandibular external margins straight to feebly convex. Short, incrassate, antenna covered with setae, with eight antennomeres, ending in a proto-club formed by apical two flagellomeres, apicalmost flagellomere longest (twice as long as broad); scape approximately as long as eye; palp formula 2/4, labial and maxillary palps short, covered with minute setae.
Mesosoma rather elongate but small in comparison to head size, punctate. Promesonotal articulation distinct, assumed inflexible. Constriction in posterior half-part of mesonotum; prominent, round metathoracic spiracles located at posterodorsal corners of mesonotum, just anterior to mesopropodeal margin. Propodeum short, with rather straight dorsum and slightly concave declivity subequal in length. Metapleural gland large, oriented latero-posteriorly. Short, rather slender legs, with sparse setae on apical tibial part and tarsi. Fore tibia short. Hind leg with a tiny tibial spur. Pretarsal claws without pre-apical tooth but with distinct, medial seta; arolium massive.
Squamiform, unpedunculate petiole; in profile view, petiolar node with anterior and posterior faces convex, meeting at acute angle dorsally; in posterodorsal view, dorsal margin slightly concave medially; inserted at ventral base of helcium. Gaster elongate, elliptical, mostly glabrous except scarce setae ventrally and on pygidium. Four distinct tergo-sternal complexes. Acidopore well developed.
Major BL 4.7, HL 1.10, HW 0.98, EL 0.51, MFC 0.38, SL 0.50, ED 0.23, AL 1.45, PW 0.60, PtL 0.30, PtH 0.40, FL 0.6, CI 0.89, FCI 50, SI 51, OI 20, PtI 113, FI 41, AI 41.
Blackish body, 4.7 mm long, flattened dorso-ventrally, gaster elongate. Head subrectangular and glabrous, except mandibles and a few scarce and minute white setae on top; occipital margin rather straight, with round corners; dorsal and lateral head cuticular sculpture consisting of thin parallel lines of striation, eventually diverging on clypeus. Apical tooth of mandible as long as second one, so that masticatory margin ends in a double-toothed complex; remaining visible part of masticatory margin damaged; long setae on ventral surface of mandibles. Presence of mandibular lobes above clypeo-mandibular joints. Clypeus very large, smooth; its anterior margin straight, slightly wrinkled medially; posterior clypeal margin narrow, surmounted by a small frontal triangle. Frontal carinae almost flat, of convex shape and strongly diverging on either lateral margin of clypeus, ending in slight laterally oriented prominences. Medial torular arches assimilate with tip of frontal carinae, situated near angles formed by lateral and anterior clypeal margins; condylar bulbs of antennae covered in great part by frontal carinae/lobes. Eight antennomeres; scape short, not reaching posterior eye margin, less than half as long as head capsule (excluding mandibles); funiculus inversely incrassate, antepenultimate flagellomere as long as broad, apical flagellomere distinctly largest. Eyes very elongate, twice as long as wide, placed rather anteriorly, covering nearly one-third of head length. Ocelli absent. Palp formula: 4,2; palp segments scarcely pubescent, approximately of equal length.
Mesosoma glabrous, shagreened, globally flat. Promesotonal and mesopropodeal articulations visible but supposedly inflexible. Propodeal spiracle large, with thick border, placed on lateral top of a conspicuous and bulged metapleuron ending with metapleural gland opening as large as the spiracle and bordered with short setae; latero-ventral extremities of propodeum form prominent and large, circular, awning-like structures covering hind coxa with rather long, marginal setae. Slight constriction in posterior half-part of mesonotum. Legs rather short, almost glabrous; tarsomeres 1–4 of all legs with —two to three apicoventral setae inserted on each side; protibial spur ogive-shaped; metatibial spur tiny; heavy protarsal pubescence, initiated just above insertion of spur; pretarsal claws simple, with a medial distinct setae; arolium massive.
Petiole sub-nodiform, in lateral view with anterior face strongly convex, dorsal margin acutely angled, posterior face weakly convex; in posterodorsal view with dorsal margin slightly concave medially; petiole without tergosternal fusion. Gastral tergites extending latero-ventrally. Short and scattered setae on ventral face of tergosternites I, II and III. Pygidium and hypopygium covered with long, thick hairs. Acidopore distinct.
Type Material
From the Latin gallicus, meaning ‘Gallic’ (from Gaul).