Tyrannomecia inopinata, Jouault & Nel, 2021

Jouault, Corentin & Nel, André, 2021, The oldest Cenozoic ant fossil: † Tyrannomecia gen. nov. (Formicidae: Myrmeciinae) from the Palaeocene Menat Formation (France), Historical Biology 34 (11), pp. 2241-2248 : 4

Tyrannomecia inopinata sp. nov. ( Figures 1–2 )

urn:lsid:zoobank.org:act:DA568D60-0C56-4C59-8298- C2BAFEB72C98

Etymology

The specific name derived from the Latin word inopinatus, meaning ”unexpected”. The epithet refers to the rather unexpected nature of the discovery in Menat deposit.

Type material

Holotype MNHN.FA71374 (Oli 435, collection Philippe Olivier, both sides), deposited in the Muséum national d’Histoire naturelle in Paris. Type locality Historical quarry of ‘Noir d’Auvergne’ , Menat, Puy-de-Dôme, France .

Type horizon

Middle Palaeocene (ca. 60 Ma), Menat Basin.

Diagnosis

As for the genus (vide supra).

Description

Worker. Head slightly longer than wide (ca. 1.2 × longer than wide), 4.80 mm long and ca. 4.20 mm wide, sides rounded, occiput rounded; antennae not preserved; mandibles elongate, ca. 2.92 mm long, without conspicuous teeth, with parallel and touching masticatory margin when closed, broader basally, tapering towards apex, not broadly crossing apically; eyes conspicuous, ovoid, ca. 1.28 mm long and 0.94 mm wide, located slightly anterior to head mid-length.

Mesosoma. Quite robust, elongate, ca. 7.05 mm long; dorsal surface slightly convex; pronotum slightly convex in lateral view, with posterior margin convex; propleuron visible laterally (but difficult to describe); pro-mesonotal articulation slightly impressed, presence or absence of fusion uncertain; mesonotum indistinct (poorly preserved); mesopleuron distinct, ovoid, elongate towards insertion of third pair of legs; metanotal groove apparently well impressed; propodeum long, unarmed (without tooth), with dorsal surface convex, propodeal declivity without strong angle with dorsal surface, propodeal lobes apparently small. Legs not preserved.

Petiole. In lateral view, longer than high, bell-shaped, anterior surface short, slightly concave, dorsal surface broadly rounded, posterior surface dorsally with a blunt tooth located near midlength (not an artefact of preservation since recorded in part and counterpart and not corresponding with petiolar collar visible just behind), subpetiolar process, if present, not visible, helcium clearly visible and strongly constricted between petiole and AIII.

Gaster. Medium size, longer than mesosoma, ca. 10.80 mm long, distinctly constricted between AIII and AIV; separations between tergites and sternites visible; sternite of AIII without visible prora; first two segments the longest, then decreasing in length, second segment the widest, length of segments from base to apex (in mm): ca. 3.36, 3.55, 2.88, 2.00, 1.56; sting not visible.

Integument without visible sculpturing nor colouration pattern. Body dark brown to brown.

Remark. When we refer to the mandibles, teeth designate sharp, acute cuticular structures along the masticatory margin of the mandible while tubercles refer to small, blunt structures/teeth along the masticatory margin of the mandible. The large eyes recorded in the genus are sufficient to rule out affinities with the majority of ant genera (extant and extinct). Some formicine genera also possess similar large eyes ( Myrmoteras and Gigantiops ), but in the latters, they are always longer than half the head length (vs. 0.26 × head length in the new species). Representatives of the Pseudomyrmecinae also possess large eyes but they are rapidly distinguished from the new genus owing to their distinctly nodiform postpetiole. The genus Harpegnathos (Ponerinae) is superficially similar to the new genus but differs from the latter in having up-curved mandibles as long or longer than the head (vs. conspicuously shorter than the head and without apparent curvation in the new genus); presence of a huge gap between clypeus and beggining of the masticatory margin while the gap presents in the part and counterpart is only due to damages caused during the preparation process; head quadrate (vs. clearly rounded), and the petiole is dorsally slightly bulging posteriorly in workers or queen (vs. with a clear rounded dorsal surface, and a nearly straight posterior dorsal surface). The genus Aquilomyrmex (Haidomyrmecinae) also possesses large eyes but it is easily distinguished from the new genus, at least, owing to the presence of a cephalic horn (absent in the new genus). The sole extant Myrmeciinae genus possessing similar eyes is Myrmecia but it differs from Tyrannomecia at least owing to the presence of strong teeth along the masticatory margin (see additional comparisons in discussion).

Figures

Figure 1. Tyrannomecia inopinata gen. et sp. nov., holotype MNHN.FA71374. Photographs. (a) habitus on part; (b) habitus on counterpart. Scale bars = 2 mm.

Figure 2. Tyrannomecia inopinata gen. et sp. nov., holotype MNHN.FA71374. Photographs. (a) head; (b) petiolar region. Scale bars = 2 mm (a), 1 mm (b).