Yantaromyrmex

Individuals of Yantaromyrmex species have been found as inclusions in four different Middle Eocene to Early Oligocene amber deposits in Europe.[1] Baltic amber is approximately 46 million years old, having been deposited during the Lutetian stage of the Middle Eocene. There is debate on what plant family the amber was produced by, with evidence supporting them being relatives of either Agathis or Pseudolarix.[2] Rovno amber, recovered from deposits in the Rivne region of Ukraine, is slightly younger in age, being dated to the Bartonian to Priabonian of the Late Eocene.[3] Bitterfeld amber is recovered from coal deposits in the Saxony area of Germany and the dating of the deposits is uncertain. Bitterfeld represents a section of the Eocene Paratethys Sea, and the amber that is recovered from the region is thought to be redeposited from older sediments. The fossil record of Bitterfeld and Baltic amber insects is very similar with a number of shared species, and that similarity is noted in the suggestions of a single source for the paleoforest that produced the amber.[4] The amber deposits on the Danish coast, often referred to as Scandinavian amber, is of similar age to the other three European ambers, however a study of the ant fauna published in 2009 indicates Scandinavian amber has a fairly distinct ant assemblage.[5] Y. constricta and Y. geinitzi are both identified from all four European ambers, while Y. samlandicus has been described from Baltic, Bitterfeld and Rovno fossils. Y. intermedius and Y. mayrianum, are each known from only a few fossils, Y. intermedius from only a single Bitterfeld amber fossil while Y. mayrianum is known from a Baltic amber and a Rovno amber fossil.[1]

Larva and pupae of Y. geinitzi, illustrated by William Morton Wheeler

The type specimens of Y. geinitzi and Y. constrictus were collected in 1868, and when first described were part of the University of Königsberg's amber collection. The fossils were initially studied by Austrian entomologist Gustav Mayr who placed the two species in the genus Hypoclinea, now considered a junior synonym of the living genus Dolichoderus.[6] Mayr's 1868 type descriptions of the new species were published in the journal Beiträge zur Naturkunde Preussens.[7] All the Y. samlandicus syntypes were collected by 1915, and the first 73 described were part of the University of Königsberg amber collection. These fossils were first studied by American entomologist William Morton Wheeler, whose type description of "Iridomyrmex" samlandica was published in the journal Schriften der Physikalisch-Ökonomischen Gesellschaft zu Königsberg.[7] Both Y. geinitzi and Y. constrictus were both moved from Hypoclinea to the small related genus Bothriomyrmex in 1873 by Dalla Torre. In his 1915 paper The ants of Baltic Amber Wheeler suggested both species and the newly named "I." samlandica would be better placed in the genus Iridomyrmex and noted Y. geinitzi to be one of the most abundant ant species in the Baltic amber he had studied.[7]

Wheeler based the placement on the structuring of the labial and maxillary palpi. This placement was unchallenged until the genus was reviewed, redefined and split up in 1992 by Steven Shattuck. In his review, Iridomyrmex was split into a more restricted genus group while the other species which did not match the definition of that genus were moved to other genera.[8] At that time Shattuck provisionally kept Y. geinitzi in Iridomyrmex citing a lack of specimens for him to study, while both Y. constrictus and Y. samlandicus were transferred to the genus Anonychomyrma. The placements of Y. geinitzi was retained until 2011 when Shattuck and Brian Heterick again reviewed Iridomyrmex. With more fossils to study and based on a number of characters, Y. geinitzi was also moved to Anonychomyrma.[9] The three species were reviewed again, this time in 2013. The species were examined by Russian entomologists G.M. Dlussky and D.A. Dubovikoff, who they came to the conclusion that the three species, along with two unnamed species, were distinct from Anonychomyrma and Iridomyrmex. Based on the differences they noted, Dlussky and Dubovikoff erected the new genus Yantaromyrmex in 2013 for these ants and described two new species, Y. intermedius and Y. mayrinaum. The name is a combination of the Russian word янтарь, translated as yantar, meaning "amber" and the Greek μυρμήγκα which means "ant". They noted the new species name intermedius was derived from the Latin word of that same spelling, meaning "intermediate". They chose the name mayrianum to honor Gustav Mayr for his work as a myrmecologist.[1]

The genus is characterized by workers which have head-capsules that are trapezoidal in shape, narrowing at the front and widening towards the rear of the capsule. The oval compound eyes are generally placed slightly to the rear of the capsules midpoint, and lacking ocelli completely. The mandibles have a number of teeth along the masticatory margin (the middle area of the mandibles) and an overall triangular shape. The gaster has a flat first tergite that does not cover the petiole, a flat fourth abdominal sternite.[1]

Y. constrictus

Y. constrictus worker

Overall Y. constrictus can be distinguished from the congeneric Y. geinitzi in several ways. Y. geinitzi individuals are overall more gracile in form with a less constricted mesonotum and the mesosoma has a less convex appearance. Y. constrictus specimens have maxillary palps (sensory organs) which are six-jointed, labial palps which are four jointed, and an abundantly hairy body. The antennae have a scape (the first segment of the antenna) which just passes the back-edge of the head capsule on both female and ergatomorphic (male) workers. The ergatomorphs have larger and more rounded compound eyes than the workers, and the antennae are overall longer than in the workers. Both males and workers have a five segmented gaster and the males are distinguished by the slightly protruding stipites (the second segment of the maxilla) at the tip of segment five.[7]

Y. mayrianum

Y. samlandicus worker

Workers of Y. mayrianum range in length from approximately 4 to 5 mm (0.16 to 0.20 in) and look very similar to workers of Y. geinitzi. In both species the indent behind the first abdominal segment is broader and shallower than seen in Y. constrictus, Y. samlandicus and Y. intermedius. Also unlike the other three species the first abdominal segments surface is smooth and unsculptured. Y. mayrianum can be distinguished from Y. geinitzi by the amount of hairs found on the workers body. In Y. mayrianum there is abundant erect hair covering the entire body, on the underside head capsule and along the eye margins and leg undersides. In contrast Y. geinitzi workers have smooth eye margins and legs, and a sparse scattering of hairs on the mesosoma, last segments of the abdomen and along the upper side of the head.[1]