© B. Carbuccia et al., Published by EDP Sciences 2020

1 Introduction

Oise amber (Ypresian, reference level MP7, 55–53 Ma) is one of the oldest known Cenozoic fossil resins (Penney, 2010), and, with more than 20,000 identified inclusions (Brasero et al., 2009), is the best-known deposit from the sparsely recorded earliest Eocene. While insects have been extensively studied and described (see Brasero et al., 2009, for a comprehensive list), arachnids (spiders, scorpions, mites and their relatives) are not as well-known. Penney (2006, 2007a, 2007b) described four species in the spider families Oonopidae, Selenopidae, Anapidae and Pholcidae, and also recorded the presence of the spider families Archaeidae, Hersiliidae, Segestriidae, Uloboridae and questionable Agelenidae and Tetragnathidae. A more recent study (Carbuccia et al., in press) yielded 23 families, including the ones identified by Penney (2006, 2007a, 2007b), and a new species in the family Segestriidae. Spiders are thus quite diverse in Oise amber, although, with 296 registered specimens, they are not especially abundant, representing less than 2% of the inclusions (ibid.).

Except for the four aforementioned species, Oise amber spiders have mostly been identified at family level, and seldom at the species or genus level. The assemblage of Oise amber spiders, in its composition, is markedly different (Carbuccia et al., in press) from the geographically close, albeit more recent, Central European Palaeogene ambers (Baltic, Bitterfeld and Rovno ambers). Differences between these assemblages are not surprising, as the global climate underwent some quick and drastic climatic shifts during the terminal Paleocene and early Eocene (Röhl et al., 2007; Zachos et al., 2008). Oise amber is indeed situated in geological time at a thermal optimum, between the PETM (Paleocene-Eocene Thermal Maximum) and the EECO (Early Eocene Climatic Optimum), two short, but extreme (Röhl et al., 2007; Zachos et al., 2008) global warming events that allowed the existence of paratropical environments, with taxa that would later become restricted to intertropical zones (Saupe et al., 2019) occurring high latitudes. Meanwhile, the younger Baltic amber forests seem to have been a much cooler, mesothermal environment (Wolfe et al., 2015; Sadowski et al., 2017).

However, because of the geographic proximity of Oise and Baltic ambers, one would expect the taxa in the families shared by both of these sites to be somewhat closely related. This seems to be the case with the adult male Archaeidae from Oise amber that we describe here. The genus to which this species belongs, Myrmecarchaea, has thus far only been described from Baltic amber (Wunderlich, 2004), so this new species thereby extends the geographical range into western Europe, and extends its time range to basal Ypresian.

2 Material and methods

2.1 Material

The amber fragment containing the spider described herein has been excavated at the Le Quesnoy site (Oise), which is now permanently flooded and inaccessible. Oise amber occurs in clayey sands containing lignite and continental vertebrate remains, a formation known as “argiles à lignites du Soissonnais” (Nel et al., 1999). These strata correspond to a hypoxic freshwater palaeoenvironment. Transport is minimal, as attested by some fossil wood fragments still associated with amber (Nel et al., 1999). The vertebrate groups found in Le Quesnoy are the same as in the Dormaal reference locality (Nel et al., 1999), thus allowing unequivocal dating as lowermost Eocene (MP7).

3 Methods

The amber fragment went through a preliminary observation to evaluate the quality of the sample and to locate the position of the inclusion. Then, the fragment was faceted with a Crystal Master Pro 12 polisher; and scratches and marks on the facets were polished with diatomite. Preparation of the inclusion was kept to a minimum in order to preserve, as much as possible, the general shape and structure of the amber sample, and to minimize the loss of taphonomic information.

Observations were made with a Nikon SMZ25 stereomicroscope, with optic fibre auxiliary light. The fragment was immersed in tap water (as Oise amber seems slightly soluble in ethanol) during observation. High definition photographs were taken using a Nikon D800 camera attached to the microscope, activated from a distance with Helicon Remote^®, and then stacked with Adobe^® Photoshop CS6 and adjusted for contrast and sharpness. Drawings were made by hand using a drawing tube attached to a Wild M3C stereomicroscope. Because a different microscope was used for drawings and photographs, for the same illustrated body part, the angles may differ slightly.

4 Results

4.1 Family Archaeidae Koch and Berendt (1854)

Genus Myrmecarchaea Wunderlich (2004) (type species: Myrmecarchaea petiolus Wunderlich, 2004)

Wunderlich (2004) described the genus Myrmecarchaea based on the remarkably elongated petiolus, longer than the width of the carapace. He described two species, M. petiolus and Myrmecarchaea pediculus, both from Baltic amber, which was, until now, the only known occurrence of the genus. Nothing is known about their behaviour, but their peculiar morphology was interpreted as possible ant or wasp mimicry (Wunderlich, 2004).

Myrmecarchaea antecessor sp. nov.

Etymology: “antecessor”: Deverbal noun of the Latin verb “antecedo”, which means “to precede, to predate”. “Antecessor” literally means “the one which predates”, as M. antecessor sp. nov. is older than all the previously known Myrmecarchaea species.

Material: holotype MNHN.F.A71310 (PA 12262, adult male) in Oise amber, deposited in the Palaeontological collections of the Muséum National d’Histoire Naturelle (MNHN), Paris, France.

Type locality and horizon: Le Quesnoy (Oise), in the Sparnacian (lower Ypresian) “argiles à lignites du Soissonnais”.

Preservation and syninclusions: in a very clear, but broken, cylindrical amber fragment (diameter 6 mm) no syninclusions, except a very large, hairy spider leg on the left side of the specimen, partly hiding its ventral side. Very well-preserved but incomplete: posterior part of the abdomen, and parts of legs are missing.

Diagnosis: (male; female unknown) head region raised and strongly individualized, with a very short “neck”, that forms a marked angle with the thoracic part (Fig. 1). Coxae IV are separated from the coxae III by a noticeable diastema (space between coxae III and IV 2.8 × the length of the space between coxae II and III), and a clear thoracic constriction between coxae III and IV (Fig. 2). Male palp (Figs. 3 and 4) with long, thin setae on the tibia and cymbium. Tegulum large, bean-shaped, embolus thick and relatively long, almost straight, with two strongly recurved, relatively large embolic apophyses. Thoracic region is very long and narrow, and markedly elongated posteriorly, the part behind coxae II represents 3/5 of the total cephalothorax length, while it is only 1/3 in M. pediculus (Fig. 5). Petiolus relatively short (less than half the length of the cephalothorax) and thick, compared to the Baltic species (Figs. 5 and 6), 2.8 × longer than wide.

Description: body (chelicerae excluded) 5.0 mm long. Carapace 3.0 mm long, opisthosoma 2.0 mm long (note that the posterior part of the abdomen is broken off).

Chelicerae (Fig. 4): fairly long (1.0 mm) and robust, distal half slightly recurved, with 14 peg teeth on the promargin, the longest at the proximal end of the cheliceral furrow, and 12 true teeth on the retromargin. The fangs are long and slightly curved, about half as long as the basal segment.

Stridulatory apparatus (Fig. 3): same as in Archaea paradoxa Koch and Berendt (1854) (Fig. 7).

Mouthparts (Fig. 4), very long, labium long and narrow, fusiform, with an apical incision.

Cephalothorax (Figs. 1 and 2), very long, 2.5 × longer than wide, 1.7 × longer than wide in M. pediculus (Fig. 5). Cephalic region large, raised, protruding forward and separated from the thoracic region by a very distinct constriction forming a short and thick “neck”, unlike M. petiolus (Fig. 6) in which the cephalic part is not sharply raised. The “head” is globally rounded, slightly flattened at the top, without spines, and covered with short setae on the top and longer, plumose setae on the sides. Thoracic region long and narrow, markedly tapering posteriorly; coxae IV conspicuously separated from the other three (Fig. 2).

Eyes (Fig. 1): the eyes are relatively close to each other, with the anterior median eyes 2.5 × larger than the other eyes and anteriorly oriented. Lateral eyes contiguous and on a small hump.

Opisthosoma (Fig. 2): petiolus (characteristic of the genus Myrmecarchaea) distinctly elongated, but much shorter than in the Baltic amber specimens: petiolus length is less than half of the carapace in M. antecessor, while in the Baltic species M. pediculus (Fig. 5), it is almost as long as the carapace, and ¾ of the carapace length in M. petiolus (Fig. 6). Rear part of the opisthosoma broken off; remaining parts suggest that it was relatively small and elongated.

Legs very long and slender; femora I and II as long as the whole body. Femur I slightly curved, with a very slight hump. Leg III shorter, femur half as long as femur I. Length of legs IV unknown, as they are incomplete.

Remarks: Although the petiolus is shorter than in other Myrmecarchaea spp., it is still markedly more elongated (but hard to see due to its proximity to the edge of the fragment) compared to all other known archaeid species, fossil and extant. The carapace in Myrmecarchaea is posteriorly more elongated compared to other fossil and extant archaeids, and M. antecessor n. sp., compared to other Myrmecarchaea, has the greatest elongation. M. antecessor sheds light on many unknown morphological traits in this genus. As hypothesized by Wunderlich (2004), the cheliceral stridulating file is situated in the basal half of the chelicerae, very close to the base, as in Archaea (Fig. 7); the stridulatory apparatus is overall very similar to Archaea paradoxa (Fig. 7). The general structure of the male palp is also very similar to Archaea (Fig. 7), with the same general shape of the palpal bulb, the same orientation and shape, including a spiral of the embolus, and also with tegular apophyses in similar positions.

Fig. 1 Myrmecarchaea antecessor sp. nov., holotype MNHN.F.A71310, lateral view. (photograph and drawing: B. Carbuccia).

Fig. 2 Myrmecarchaea antecessor sp. nov., holotype MNHN.F.A71310, ventral view. C3: coxa 3; C4: coxa 4 (photograph: B. Carbuccia).

Fig. 3 Myrmecarchaea antecessor sp. nov., holotype MNHN.F.A71310, details of genitalia. Teg: tegulum; TA: tegular apophyses; Em: embolus; ST: stridulatory teeth; SF: stridulatory file; Ti: tibia; Cy: cymbium (photograph: B. Carbuccia).

Fig. 4 Myrmecarchaea antecessor sp. nov., holotype MNHN.F.A71310, mouthparts (photograph and drawing: B. Carbuccia).

Fig. 5 Myrmecarchaea pediculus Wunderlich (2004), possible adult female, holotype, Geologisch Paläontologisches Institute und Museum (GPIH) specimen No. S3907/4338. A: general habitus, ventral view; B: Prosoma, ventral view (photographs: H. Wood).

Fig. 6 Myrmecarchaea petiolus Wunderlich (2004), juvenile male, holotype, GPIH specimen No. S3999/4337. A: general habitus, dorsal view; B: detail of the carapace (photos: H. Wood).

Fig. 7 Archaea paradoxa Koch and Berendt (1854). A: female stridulatory apparatus, Staatliches Museum für Naturkunde, Stuttgart specimen No. BB-A4000; B: male copulatory palp, Senckenberg Museum, Frankfurt Section, specimen No. SMF Be 5080. SF: stridulatory file; SP: stridulatory picks; Teg: tegulum; TA: tegular apophyses; Em: embolus; Cy: cymbium (photographs: H. Wood).

5 Discussion

5.1 Taxonomic placement

The genus Myrmecarchaea Wunderlich (2004) is diagnosed over a remarkable elongation of the petiolus, longer than the width of the carapace, and is interpreted as possible ant/wasp mimicry (Wunderlich, 2004). Very long legs and probable Archaea-like stridulatory organs are also mentioned as characters for the genus. M. antecessor sp. nov. possesses such a petiolus, although it is stouter and shorter than in the Baltic amber Myrmecarchaea species.

On the other hand, M. antecessor sp. nov. sports several unique features among Myrmecarchaea species, for instance the extremely elongated, tapering posterior part of the cephalothorax, with a very conspicuous diastema between the coxae III and IV. Many Archaeidae fossil genera (Archaea, Baltarchaea, Burmesarchaea, Myrmecarchaea) possess an extended, tapering posterior carapace that is truncated in extant Archaeidae, but M. antecessor sp. nov. is the most extreme case. A posteriorly truncated carapace is a synapomorphy of the monophyletic extant Archaeidae, and the tapering carapace observed in fossil archaeids may be a plesiomorphic trait or may be unique to Archaeidae fossils (Wood et al., 2012).

M. antecessor sp. nov. also shares all the other typical Myrmecarchaea traits (Wunderlich, 2004), notably the extremely long legs: while incomplete, femora I are much longer than the whole body.

Besides M. antecessor, the genus Myrmecarchaea is comprised of two other species (Dunlop et al., 2018): M. petiolus (Fig. 6), which is known from only a juvenile male specimen, and M. pediculus (Fig. 5), which is known from only one, possibly adult female specimen (Wunderlich, 2004). Thus, our knowledge of the genus, and of the morphological disparity among its species, is highly fragmentary. Therefore, we choose a conservative approach, including the new species in the pre-existing genus rather than creating a new monospecific taxon, as, despite its peculiarities, it fits the Myrmecarchaea diagnosis.

However, as the morphology of adult males from the other species is unknown, some of the singularities of M. antecessor could actually be due to sexual dimorphism. This being said, another unidentified Myrmecarchaea sp., described as an immature male by Wunderlich (2004), as well as the juvenile male M. petiolus (Fig. 6), have a much longer petiolus. More specimens would thus be necessary for a better comprehension of this highly peculiar genus, and an improved validity of the taxonomic statements.

5.2 Archaeidae in Oise amber

The family Archaeidae is not abundant in the faunal assemblage, with only three specimens (and one moulted exoskeleton) currently registered in the MNHN collections, of which the holotype of M. antecessor is the most well-preserved. The two other specimens are very incomplete, with only the anterior portion of the prosoma preserved, and are likely females or juveniles due to their unmodified pedipalps. Both specimens are markedly different from the holotype of M. antecessor. They both seem to have smaller AME; eyes are all about the same size, and AME are widely separated. Peg teeth and true teeth also seem different, in number and repartition, from the holotype of M. antecessor. These differences seem to indicate Archaeidae are represented by at least two distinct taxa in Oise amber. An almost complete specimen from a private collection, that we unfortunately only saw a low-resolution picture of (de Ploëg, pers. comm.) clearly lacks the petiolus which characterizes the genus Myrmecarchaea. While we cannot really say much about this fossil, as we did not manage to physically observe it, it confirms the presence of several morphologically and taxonomically distinct Archaeidae in Oise amber.

Acknowledgements

We thank Professor Jean-Paul Saint Martin for inviting us to publish this article and Associate Professor Erin Saupe for the friendly reviews and advice she provided us. We thank Mr. Gael de Ploëg for the picture confirming that more Archaeidae are yet to be described in Oise amber, and the SAGA association for the huge amount of voluntary preparation work they conduct on Oise amber samples. We thank the Lafarge-Granulat company for help with the fossil sampling and the Langlois-Meurinne family for the authorization of working on its property in Oise.

References

All Figures

	Fig. 1 Myrmecarchaea antecessor sp. nov., holotype MNHN.F.A71310, lateral view. (photograph and drawing: B. Carbuccia).
In the text

	Fig. 2 Myrmecarchaea antecessor sp. nov., holotype MNHN.F.A71310, ventral view. C3: coxa 3; C4: coxa 4 (photograph: B. Carbuccia).
In the text

	Fig. 3 Myrmecarchaea antecessor sp. nov., holotype MNHN.F.A71310, details of genitalia. Teg: tegulum; TA: tegular apophyses; Em: embolus; ST: stridulatory teeth; SF: stridulatory file; Ti: tibia; Cy: cymbium (photograph: B. Carbuccia).
In the text

	Fig. 4 Myrmecarchaea antecessor sp. nov., holotype MNHN.F.A71310, mouthparts (photograph and drawing: B. Carbuccia).
In the text

	Fig. 5 Myrmecarchaea pediculus Wunderlich (2004), possible adult female, holotype, Geologisch Paläontologisches Institute und Museum (GPIH) specimen No. S3907/4338. A: general habitus, ventral view; B: Prosoma, ventral view (photographs: H. Wood).
In the text

	Fig. 6 Myrmecarchaea petiolus Wunderlich (2004), juvenile male, holotype, GPIH specimen No. S3999/4337. A: general habitus, dorsal view; B: detail of the carapace (photos: H. Wood).
In the text

	Fig. 7 Archaea paradoxa Koch and Berendt (1854). A: female stridulatory apparatus, Staatliches Museum für Naturkunde, Stuttgart specimen No. BB-A4000; B: male copulatory palp, Senckenberg Museum, Frankfurt Section, specimen No. SMF Be 5080. SF: stridulatory file; SP: stridulatory picks; Teg: tegulum; TA: tegular apophyses; Em: embolus; Cy: cymbium (photographs: H. Wood).
In the text