A large brush-footed trapdoor spider (Mygalomorphae: Barychelidae) from the Miocene of Australia (2024)

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Abstract

INTRODUCTION

Mygalomorph spiders have been present on the Australian landmass since before the break-up of Gondwana (Raven et al. 2015). Today, mygalomorphs are represented on the continent by a diverse fauna of 12 families. Despite this extensive evolutionary history, only one fossil mygalomorph spider has been found in Australia: Edwa maryae from the Triassic of Queensland (Raven et al. 2015, Dunlop et al. 2023). Presumably, this dismal fossil record is the result of a combination of the fragility of the animals (Selden and Penney 2010) and the tendency of females to stay within burrows, both of which prevent many spiders from entering the fossil record. The lack of fossils from Australia has made it difficult to decipher the evolutionary history of the group.

Without fossils, researchers could only rely on phylogenetic analyses of modern species to infer the evolutionary history of mygalomorph spiders. These analyses suggest that the Miocene was a time of particularly high divergence in this group (Rix and Harvey 2012, Rix et al. 2017). During that time, Australia was experiencing widespread aridification that caused the contraction of rainforest environments and the establishment of the current arid biome (Christophel 1989). But inferences based on phylogenies constructed from extant species lack information from clades that went extinct and, as a result, do not show the complete picture of how diversity has changed over time.

Brush-footed trapdoor spiders (Araneae: Barychelidae) are a diverse clade of mygalomorph spiders that have evolved several features, including tufts of setae that allow them to climb smooth surfaces (Raven 1994). Furthermore, their spinnerets are fairly short, which distinguishes them readily from the larger sympatric tarantulas (Theraphosidae). Australia is currently home to 10 genera of brush-footed trapdoor spiders, a centre of biodiversity, if not also their origin. Three subfamilies are reported: the intertidal Idioctidinae; the tree-dwelling Indopacific ‘tramps’, Sasoninae; and the brush-footed trapdoor spiders that also occur on New Caledonia and Hawaii. The most basal taxon of the brush-footed trapdoor spiders is the genus Monodontium Kulczyński, 1908, which is restricted to Singapore, Indonesia and Papua New Guinea (Raven 2008). Monodontium species are tiny rainforest dwellers with biserial dentition of the paired claws in females, a condition that is normally found only in male barychelids and most nemesiids (s.l.) (Raven 2008).

A newly documented Australian fossil site, named McGraths Flat, provides a rare glimpse into the rainforests that were once prevalent across the continent. The site has been dated to the Miocene and is 11–16 Myr old. It preserves insects, fishes, leaves and flowers with high fidelity (Cantrill et al. 2022, McCurry et al. 2022, Moulds et al. 2022). Here, we describe a new genus and species of mygalomorph spider from McGraths Flat.

MATERIALS AND METHODS

Specimen and imaging

The specimen was collected at McGraths Flat in June 2020 and has been accessioned into the Australian Museum palaeontology collection (Fig. 1). After collection, the fossil was prepared using an ME-9100 airscribe (Paleotools, Brigham City, UT, USA). Light photographs were taken with a Canon EOS 7D Mark II camera mounted on a BK Lab Plus imaging system (Dun, Charlottesville, VA, USA). Scanning electron micrographs were taken with a FEI Quanta 650F variable-pressure field-emission scanning electron microscope (SEM; FEI Company, Hillsboro, OR, USA). The SEM was operated at 15 kV, with a spot size of 2.45, and working distances of ~12–15 mm, without coating the specimen. Measurements were taken directly from the specimen using Absolute Digimatic digital callipers (Mitutoyo, Kawasaki, Kanagawa, Japan).

SYSTEMATIC PALAEONTOLOGY

Order Araneae Clerck, 1757
Suborder Opisthothelae Poco*ck, 1892
Infraorder Mygalomorphae Poco*ck, 1892
Family Barychelidae Simon, 1889
Genus Megamonodontium gen. nov.

Remarks:

The similarity in the length of the legs (Fig. 2) suggest that the genus belongs to the Tuberculotae sensuRaven (1985) and amongst them, shows the strongest affinities with the Barychelidae, which also have truncated-looking tarsi (Fig. 3A; see Raven 1994: Tarsal Extremity, p. 313 and fig. 12A–Q). The fluted ridges on setae on the legs are also consistent with those in the Barychelidae (see Raven 1994: figs 21F, 22A).

The parts studied have cleaved through the carapace and abdomen, meaning that, as noted, the chelicerae, eyes, fovea, sternum and spinnerets are unknown, as are the distal ends of the first, third and fourth legs on the counterpart. Much of the specimen shows the body wall from the inside of the specimen; only in some areas are external details of the body evident. The specimen is compressed but shows some three-dimensional relief. Almost all of what we know about this spider derives from the morphology of the legs. However, of the 185 characters used by Raven (1985), 42 are based upon the legs including the claws and spines, excluding the male palp and coupling spurs. The fossil surface visible seems to be that immediately below the cuticle, with only setal scars evident (Fig. 3F, G).

Despite the limited number of characters, we can still place the specimen reliably within the brush-footed trapdoor spiders. Raven (1985) posited that the mygalomorphs form two groups, based upon the relative sizes of their legs. The Atypoidea and Rastelloidina (Fornicephale) all have noticeably weaker front (I and II) legs than rear (III and IV) legs (Fig. 4A–C); also, in Atypoidea and Rastelloidina, the spination of the distal segments (tibia to tarsi) of legs I and II is very strong; the spines on the third leg are the strongest and those on the fourth leg are the weakest ventrally. Although a number of other characters, such as the presence of digging spines (rastellum) on the chelicerae, the reduction of the eye tubercle, the absence of dense hair (scopula) on the legs, and the steeply elevated head region or caput are not discernible on this specimen, we assume that they are present, because they are strongly correlated with the spination characters (Raven 1985).

Raven’s second group are the Tuberculotae, which are characterized by eyes raised on a common tubercle, a lower head region, and often hirsute distal leg segments. The Tuberculotae have legs of similar thickness (Fig. 4D–F) and weak, if any spines on the legs apart from the third and fourth leg, and it is to this group that this species clearly belongs. Further correlated with those legs are the reduction in size of the third or middle claw on the legs and the development of a pair of pads around the claws, claw tufts, as found in the tarantulas and the brush-footed trapdoor spiders, Barychelidae (Raven 1994).

The inclusion of this species in the brush-footed trapdoor spiders is challenged by the apparent absence of claw tufts on the tarsi. The only fossil relative of brush-footed trapdoor spiders (Psalistops Simon, 1889, now in the Theraphosidae; Mori and Bertani 2020) had its affinities recognized because a preservation in amber (Wunderlich 1988) made it possible to observe the tufts.

Within the Tuberculotae, the Crassitarsae were so named for those with hirsute matts (scopula) of setae on the ventral metatarsi and tarsi, at least of legs I and II (Raven 1985). The best developed of those scopulae are found in the tarantulas and the brush-footed trapdoor spiders, Barychelidae (Raven 1985, 1994). In those families, the relative lengths of the leg segments are a taxonomically useful character: as in most other spiders, the patella of the first and second leg of females is usually the shortest segment, but in many brush-footed trapdoor spiders, the patellae of legs I and II are longer than their respective tibiae (Raven 1994). This contrasts with the usually short patellae in females of their putative sister group, the tarantulas (Raven 1985: 311; 2005: 311).

Holotype:

AM F.145559, part (Fig. 1A) and counterpart (Fig. 1B) of a single compressed specimen, registered and housed in the palaeontology collection of the Australian Museum, Sydney, NSW, Australia (LSID Urn:lsid:zoobank.org:act: 51339D5C-590D-424B-B3FD-0CCF02126C44).

Etymology:

The genus name alludes to the nearest living relatives, tiny litter-dwelling brush-footed trapdoor spiders of the genus Monodontium.

Locality:

The holotype derives from McGraths Flat, a fossil site that is located ~25 km north-east of Gulgong, MSW, Australia. The fossil is preserved in a finely bedded goethite matrix with low levels of silica present, which has been dated to the Miocene (11–16 Mya), based on pollen and spores (McCurry et al. 2022).

Diagnosis:

Unlike most brush-footed trapdoor spiders, Megamonodontium has females with patella I shorter than tibia I (Fig. 1) and teeth on both the palpal claw (Fig. 3A, B) and the outer face of the paired tarsal claws of females. Megamonodontium differs from Tungari kenwayaeRaven, 1994 in that the first leg is longer than the second and from all species of Zophorame in that its legs are robust, that it lacks well-developed scopulae, and that its head and abdomen are less slender. Megamonodontium differs from its putative sister genus, Monodontium, in having the carapace shape asymmetrical around the midpoint, with the widest point in the posterior third; Monodontium has the carapace asymmetrical around the midpoint (Raven 2008: fig. 8A, C); it is also about five times the size of known Monodontium (10 vs. 2 mm).

Remarks:

Megamonodontium does not have visible scopulae, a condition that is considered one of the key features of the Crassitarsae. Putatively, the most plesiomorphic genus in the Crassitarsae is the barychelid genus Monodontium Kulczyski, 1908 revised by Raven (2008). Although a brush-footed trapdoor spider, Monodontium has several highly plesiomorphic characters, including hardly developed scopulae and claw tufts. Amongst its species, most females have relatively long patellae I and II, but Monodontium sarawakRaven, 2008 has a female with short patellae I and II (Raven 2008). Equally, in Monodontium, not only the males, but also the females have biserially dentate, paired claws. Megamonodontium shares this trait, having teeth on the outer face of the paired tarsal claws of females (Fig. 3A, B). In most species of TungariRaven, 1994 and the New Caledonian Barycheloides alluviophilusRaven, 1994 and Barycheloides chiropterusRaven, 1994, females have teeth on the paired claws of the first leg but none on the palpal claw. In species from the Cape York Peninsular in Northern Australia, Tungari kenayaeRaven, 1994 and all species of Zophorame Raven, 1990, females also have teeth on the palpal claw and on the paired claws of the first leg, but Megamonodontium differs from these species in many other ways, including the length and proportions of the legs, the shape of the cephalothorax and the shape of the abdomen. In other barychelids, the paired claws of females are either edentate or have only few teeth and then medially, not in a row on each face of the claw. Raven (1985) posited that this condition derived from the sister group of the Theraphosidae and Barychelidae (plus Paratropididae), the Nemesiidae (now split into Anamidae, Bemmeridae and Nemesiidae). In the Nemesiidae s.l., both males and females of all but one genus (Spiroctenus) have the paired claws biserially dentate. In the tarantulas, males of the plesiomorphic Ischnocolinae are like those of most brush-footed trapdoor spiders and have biserially dentate paired claws, whereas females have little or no teeth.

The most recent phylogeny of spiders that includes both Barychelidae and Theraphosidae is that of Goloboff-Szumik and Ríos-Tamayo (2022), based upon the work of Mori and Bertani (2020). This study places the brush-footed trapdoor spiders as the sister group of the tarantulas but widely disperses other taxa with biserial dentition of the paired tarsal claws, intermixing them with small-bodied or monoserially dentate taxa. Older studies have yielded similar results (Wheeler et al. 2017, Opatova et al. 2020). Within the Tuberculotae (Fig. 3D–F), the Idiopidae and the Nemesiidae s.l. can be eliminated by the highly reduced leg spination in Megamonodontium, in addition to the dentition of the palpal claw. Hence, by a process of elimination, using only the leg characters in the species, we conclude that this species is a close relative of the brush-footed trapdoor spider Monodontium, a tropical genus of dipterocarp rainforest dwellers.

Type species

Megamonodontium mccluskyi sp. nov.

Etymology:

The species is named after Simon McClusky, who found the specimen.

Diagnosis:

The diagnosis is as for the genus.

Description:

Almost complete part and counterpart of an adult female spider. Moderately large spider (carapace length, ~10 mm), with stout legs. The femora of legs I and II are thicker than those of III and IV. Palpal claw with three teeth distally on the claw (Fig. 3A, B); pedal claws (at least on leg III) with at least three teeth on one face of paired tarsal claws, presumably on both faces. Several spines on the prolateral face of the palpal tarsus (Fig. 3A, B) and at least one on prolateral tibia III; few spines present on legs; no spines on pedal tarsi. Setae with anastomosed ridges (Fig. 3C, D). Abdomen ovate, with pustules (Fig. 3E, F) across the dorsal and posterior surface; a pair of book lungs evident. Chelicerae, eyes, fovea, sternum, spinnerets and male unknown. Additional measurements are presented in Table 1. A paired set of book lungs can be observed within the anterior portion of the abdomen (Figs 1, 2). There is a light-yellow-coloured structure around the midline of the anterior portion of the cephalothorax. This structure appears in the approximate position of the epipharyngeal ganglion, but there are no morphological features present to confirm its identity (Figs 1, 2). However, another fossil spider has been found at McGraths Flat that provides further evidence for the preservation of neuronal tissues. Richardson et al. (2023) report a jumping spider (Aranaeae: Salticidae) that contains ‘a putative neuropile’ composed of ‘semi-parallel bundles of mostly hollow tubes with a diameter of 0.5–1 μm that have the general appearance of axons’.

DISCUSSION

In this paper, we describe Megamonodontium mccluskyi. The species superficially resembles brush-footed trapdoor spider species that are currently present in western New South Wales, such as Idiommata, in that it has robust body and leg proportions and a truncated pedipalp (Raven 1994). However, the lack of a dense scopula, the presence of a long patella and the presence of teeth on the tarsal claw suggest that the species is more basal than those species currently found in Australia. We propose that this species is closely related to Monodontium from Southeast Asia and Papua New Guinea.

In terms of size, Megamonodontium mccluskyi is the second largest fossil spider worldwide, being rivalled only by Mongolarachne jurassicaSeldon, Shih and Ren, 2013 from the Jurassic of China, which has a slightly larger body length of 24.6 mm (Selden et al. 2013). Megamonodontium mccluskyi is approximately five times the size of extant Monodontium.

The type specimen of Megamonodontium mccluskyi AM F.145559 is preserved with extremely high fidelity, as has been found in other fossils from McGraths Flat (McCurry et al. 2022, Moulds et al. 2022). Many setae, including the setae bases, are well preserved in AM F.145559 (Fig. 3G, H). In comparison to their hom*ologues in insects, the external sense organs in spiders are understudied, and the analysis of evolutionary trends relies largely on extant material. Setae can be chemosensory, mechanosensory, defensive (urticating) and adhesive. In some mygalomorph spiders, setae can also produce sound (e.g. those that have been imaged by Lima and Guadanucci 2018). We did not detect any stridulating setae on Megamonodontium mccluskyi, most probably because this species did not possess them, given that the detailed preservation of cuticular structures at McGraths Flat would almost certainly allow us to detect them if they had been present. It will be interesting to study the setae in other spiders, but we have only started to investigate the rich diversity of fossil spiders at McGraths Flat. So far, more than a dozen specimens have been found (McCurry et al. 2022). The rich material from this deposit provides an exciting new opportunity to study the preservation and evolution of setae along with other sophisticated cuticular structures.

The absence of both Monodontium and Megamonodontium in present-day Australia suggests that the clade has gone locally extinct within the last 15 Myr. Outside Australia, Monodontium still occurs in tropical rainforests with similar characteristics to the Miocene forest surrounding McGraths Flat (McCurry et al. 2022). As dryland-adapted plant species evolved and became dominant (e.g. trees such as Casuarina and Eucalyptus), other species went extinct (Christophel 1989, Martin 2006). Based on fossil sites such as Alcoota, Bullock Creek and Riversleigh, we know that during the late Miocene there was also a turnover in the vertebrate fauna (Archer et al. 1989, Murray and Megirian 1992). However, without a fossil record for spiders it has been difficult to predict which lineages went extinct during this time. Based on this one species, it is evident that the aridification during the Miocene also caused the extinction of at least some spider lineages.

AUTHOR CONTRIBUTIONS

All authors contributed to study conception and design, data collection, interpretation of results, constructing figures, drafting the manuscript and editing the manuscript. All authors reviewed the results and approved the final version of the manuscript.

ACKNOWLEDGEMENTS

We thank Nigel McGrath and Lyn and Geoff Gale for their generosity and help in accessing the site, in addition to Matt J. Hensley for assistance in excavating the site. We thank Cameron Slatyer, Tara Djokic, Patrick Smith and Viktor Baranov for useful discussions on the topic of preservation and spider evolution. We thank the two anonymous reviewers for thier comments on the manuscript. Finally, we thank all who have participated in fieldwork at McGraths Flat, especially Simon McClusky, who found Megamonodontium. We also wish to acknowledge the Wiradjuri people, the traditional owners of the land where the fossil site is located.

FUNDING

We thank the Etheridge descendants, whose generous donation funded this work along with the Australian Research Council (Linkage Grant LP210301049).

CONFLICTS OF INTEREST

The authors declare no conflicts of interest.

DATA AVAILABILITY STATEMENT

This published work and the nomenclatural acts it contains have been registered in ZooBank at: http://zoobank.org/References/AB55C84B-655E-4188-9241-C94323F1488A

REFERENCES

© 2023 The Linnean Society of London.

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