Megamonodontium mccluskyi: A new species of Brush-footed Trapdoor Spider from the Miocene of New South Wales.

Mygalomorph Spiders (Tarantulas and related species) are considered to be one of the most ancient groups of Spiders. They have two pairs of book lungs (many other Spiders have lost a pair) and downward pointing, rather than opposable fangs, again considered to be a primitive state in Spiders. Many species of Mygalomorph attain large sizes, all have flattened, disk-shaped bodies (rather than the more globular bodies of most other Spiders), and most are ambush predators.

These Spiders are known to have been present in Australia since before the break-up of Gondwana, with twelve families of Mygalomorphs found there today. Unfortunately, only a single fossil Mygalomorph has been described from Australia to date, Edwa maryae from the Triassic of Queensland, probably due to a combination of these Spiders both having fragile bodies unlikely to be preserved as fossils, and living in an environment unlikely to favour preservation (burrows, mostly in dry areas). Because of this, our understanding of the evolutionary history of Mygalomorph Spiders in Australia is based entirely upon phylogenetic analysis of extant species. By combining these phylogenetic studies with molecular clock analyses, it has been suggested that the group went through a significant evolutionary radiation during the Miocene, which coincides with a period of rapidly changing climate in Australia, with the rainforests which had covered much of the continent being replaced by the arid environment we see today. 

Within the Mygalomorphae as a whole, Brush-footed Trapdoor Spiders, Barychelidae, are considered to be the sister group to the True Tarantulas, Theraphosidae, differing from them in being smaller, having shorter spinnerets, and possessing tufts of setae on their feet which enable them to climb smooth surfaces. Brush-footed Trapdoor Spiders are found across all the major landmasses of the former Gondwana, with the exception of Antarctica and New Zealand, but are at their most diverse in Australia, which has led to the suggestion that they might have originated there. However, the most basal genus within the group, Monodontium, is not found in Australia, being found in Singapore, Borneo, and Papua New Guinea, where they inhabit Diptocarp rainforest environments. Members of the genus Monodontium are small for Mygalomorphs, and can be distinguished by the possession of biserial dentition of the paired claws in females, something only present in males in other genera.

In a paper published in the Zoological Journal of the Linnean Society on 15 September 2023, Matthew McCurry of the Australian Museum Research Institute, the Earth & Sustainability Science Research Centre at the University of New South Wales, and the Smithsonian National Museum of Natural History, Michael Frese, also of the Australian Museum Research Institute, and of the Commonwealth Scientific and Industrial Research Organisation, and the Faculty of Science and Technology at the University of Canberra, and Robert Raven of the Biodiversity and Geosciences Program at the Queensland Museum, describe a new species of Brush-footed Trapdoor Spider from the Miocene (16-11 million years old) McGraths Flat fossil sitet located  about 25 km north-east of Gulgong in New South Wales.

The new species is described from a single compressed specimen, AM F.145559, preserved as part and counterpart in a finely bedded goethite matrix. It is named Megamonodontium mccluskyi, where 'Megamonodontium' is a combination of the modern genus Monodontium, which it closely resembles, plus 'Mega', meaning 'very big', a reference to the specimen being five times the size of the largest known member of the genus Monodontium, while 'mccluskyi' honours Simon McClusky, who found the specimen.

Part (A) and counterpart (B) of Megamonodontium mccluskyi (AM F.145559). McCurry et al. (2023).

The specimen is split through the  carapace and abdomen, so that the chelicerae, eyes, fovea, sternum and spinnerets are unknown, while the e distal ends of the first, third and fourth legs are present only in it the part. This means that it was only realistic to examine the legs of the specimen when attempting to classify it, although this was certainly sufficient to place the specimen within the Barychelidae, based upon the legs being of similar thickness, an absence if strong spines on the first two pairs of legs, a reduced third, or middle, claw on all legs, and the presence of pads around the claws. The specimen lacks claw tufts, a distinguishing feature of the Brush-footed Trapdoor Spiders, but this has only ever been observed in fossil specimens preserved in amber, and therefore cannot be considered significant. 

While Megamonodontium mccluskyi superficially resembles some modern Australian Brush-footed Trapdoor Spiders, it retains a number of primitive features not seen in any of these, such as a long patella and teeth on the tarsal claw, making it most likely to be a close relative of the genus Monodontium. However, unlike the tiny Monodontium, Megamonodontium mccluskyi is a moderately large Spider with a carapace length of about 10 mm, making it the second largest fossil Spider ever discovered (behind the 24.6 mm Mongolarachne jurassica from the Jurassic of China).

Neither Megamonodontium nor Monodontium is found in Australia today, suggesting that if they do forma single clade, that clade has since become extinct in Australia. Moreover, the modern Monodontium lives in rainforest environments similar to those which would have been found around McGraths Flat when the fossil-bearing strata there were deposited. Since that time the Australian climate has changed, and the environment become dominated by dryland species such as Casuarina and Eucalyptus. The Miocene is known to have been a major interval for species turnover among Vertebrates, largely as a response to this changing climate, and molecular clock evidence has suggested that the same was true for Mygalomorph Spiders, but the absence of Spider fossils has made it impossible to determine if this was true from direct evidence until now.

See also...

Online courses in Palaeontology. 

Follow Sciency Thoughts on Facebook.

Follow Sciency Thoughts on Twitter.

Stick Insects of the Mascarene Islands.

The Mascarene Islands, Mauritius, Réunion and Rodrigues, lie in the western Indian Ocean, to the east of Madagascar and to the south of the Seychelles. They form part of a chain of biodiversity hotspots across the eastern Indian Ocean, where many islands and island groups have developed unique faunas and floras seen nowhere else. However, while Madagascar and the Seychelles are continental fragments left over from the breakup of Gondwana and started as islands with some unique fauna, the Mascarene Islands are of volcanic origin, formed by hotspot volcanism in a similar way to Hawaii or the Canary Islands, and all of the flora and fauna there must have reached the islands since they first rose above the waves, about 8-10 million years ago in the case of Mauritius, the oldest and largest of the islands.

Much of the fauna of the Mascarenes, shows a close affinity with that of Madagascar, the largest landmass in the southeast Indian Ocean and only 700 km from Mauritius. Mascarene Groups such as Day Geckos, Slit Eared Skinks, Giant Tortoises, Land Snails and Orb Spiders have all been shown to have originated from Madagascar. Other animals have been shown to be of Indo-Pacific origin, having apparently descended from animals that crossed the 5600 km of open-ocean from Australia or 7000 km from New Guinea. This group includes the famous but extinct Dodos and Solitaires, Skinks of the genus Leilopisma and Geckos of the genus Nactus.

The Stick Insects, Phasmatodea, of the Mascarenes present a unique problem to biogeographers, as they include members of four different subfamilies and one species of uncertain origin, apparently implying repeated colonization of the islands by members of different Stick Insects from different areas. Given the remoteness of the Mascarene Islands, this would be implausible for a group of flying, wide-ranging Insects, and is highly unlikely for Stick Insects, which have only very weak flying abilities, being able to slow their descent when falling from a tree, but not to make long, trans-oceanic migrations.

In a paper published in the journal BMC Evolutionary Biology on 16 September 2015, Sven Bradler of the Johann-Friedrich-Blumenbach-Institute of Zoology andAnthropology at the Georg-August-University Göttingen, Nicolas Cliquennois of the Collège français in Antsirabe, Madagascar and Thomas Buckley of Landcare Research, the School of BiologicalSciences at The University of Auckland and the Allan Wilson Centre, describe the results of a genetic study of the origin of Mascarene Stick Insects, which included data from 120 species of Stick Insects from around the globe, including seven species from Mauritius and three from Réunion (the single described Stick Insect species from Rodrigues, Xenomaches incommodus, is thought to be extinct, and no material was available for inclusion in the study).

Mascarene stick insects. (a) Couple of Apterograeffea reunionensis (Platycraninae) from Réunion. (b) Male of Epicharmus marchali (Xeroderinae) from Mauritius. (c) Female of Rhaphiderus spiniger (Tropidoderinae) from Réunion. Bradler et al. (2015).

Bradler et al. found that, contrary to expectations, all of the Mascarene Stick Insects belonged to a single lineage within the Lanceocercata, an Australasian group, having split from their closest Australian relatives about 27.15 million years ago, and with all the Mascarene Island Stick Insects having shared a last common ancestor that lived about 22.03 million years ago. The group has subsequently diversified into a number of forms that closely resemble members of other groups through convergent evolution, i.e. evolving the same traits to deal with similar ecological problems. For example, the Mascarene genus Apterograeffea has been considered to be a member of the Platycraninae (Coconut Stick Insects), closely resembling members of this group due to also having greatly enlarged cheeks, which support extra muscles which support the mandibles when chewing on tough foliage, while the Mauritian Epicharmus has been assigned to the Xeroderinae, resembling several members of this group with which it shares the habit of living on tree bark.

Chronogram of the sampled stick insect specimens with taxa distributed across the Indian Ocean highlighted in tones of red. Numbers at nodes indicate bootstrap values (left) and clade posterior probabilities (right); grey bars show 95 % highest probability density. Circled numbers refer to fossil calibration points: (1) Renphasma, (2) Eophasma, (3) fossil Malacomorpha, (4) fossil Clonistria. Abbreviations: Pl, Pliocene; Qu, Quaternary. Bradler et al. (2015).

This timescale is slightly problematic for the colonisation of the Mascarene Islands, as it requires the Stick Insects to have separated from their Australian relatives and begun diversifying into newly available niches considerably before the origin of the islands.

However, the Mascarene islands are of volcanic origin, the latest in a chain of islands formed by the movement of a volcanic hotspot, the Réunion Hot Spot, moving southwards across the Indian Ocean. Volcanic hot spots are formed where deep plumes of hot magma rising up from the Earth's interior intersect with the tectonic plates on the surface of the planet. Since these plumes originate deep within the mantle, their movement at the surface is independent of the movement of the tectonic plates. This means that while the currant Mascarene Islands, Mauritius, Réunion and Rodrigues, are quite young, a chain of previous islands, now sunken beneath the seas, extends northwards from them along the line of the volcanic hotspot.

Bradler et al. reason that the Mascarene Island Stick Insects could have originally colonised one of these sunken islands, most probably Siant Brandon, which lies 385 km to the northeast of Mauritius and which first emerged from the sea about 31 million years ago, or Nazareth Bank, slightly to the north of Saint Brandon, which first emerged about 35 million years ago, and subsequently colonised the current Mascarene Islands from these now sunken 'stepping stones', possibly via the smaller uprisings of Baissac Bank and Soudan Bank.

Dispersal scenario of Mascarene stick insects superimposed on a map of the Indian Ocean. (a) Current map of the Indian Ocean. (b) Enlarged view of Mascarene plateau. Red arrows indicate postulated colonisation events: The ancestral Mascarene stick insect arrived on currently submerged islands located to the North of Mauritius followed by a radiation and at least three (maximal six) independent dispersals to Mauritius and another three dispersals to Réunion, most likely facilitated by ocean currents. Bradler et al. (2015).

See also...

Eruptions on Piton de la Fournaise.
The Observatoire Volcanologique du Piton de la Fournaise reported a sharp rise in sulphur dioxide emissions from Piton de la Fournaise, a shield volcano which forms much of the eastern part of Réunion...

Syzygium pyneei: A new species of Myrtle from Mauritius. 
The genus Syzygium is the largest within the Myrtle family, Myrtaceae, with over 1200 described species from across the tropical and subtropical regions of the Old World, including fifteen previously described species from Mauritius.

Eruptions on Piton de la Fournaise, Réunion Island.                                          On Wednesday 4 February 2014 the Observatoire Volcanologique du Piton de la Fournaise recorded 180 Earthquakes between 4.00 and 9.00 am, on Piton de la Fournaise, a shield volcano which forms much...

Follow Sciency Thoughts on Facebook.