Reconstructing extinct Arachnid orders in three-dimensions.

There are currently sixteen recognized orders of Arachnids, twelve of which are extant (have living relatives) and four extinct, the Trigonotarbids (Trigonotarbida), Phalangiotarbids (Phalangiotarbida), Haptopodids (Haptopoda) and Uraraneids (Uraraneida).  Members of two of these groups, the Triganotarbids and Uraraneids, have previously been reconstructed as three-dimensional computer models based upon computerised tomography (CT) scans.

In a paper published in the journal Peer J on 13 November 2014, RussellGarwood of the School of Earth, Atmospheric and Environmental Sciences and The Manchester X-ray Imaging Facility at The University of Manchester and JasonDunlop of the Museum für Naturkunde at the Leibniz Institute for Research onEvolution and Biodiversity at the Humboldt University Berlin present three-dimensional reconstructions of members of the two remaining extinct Arachnid orders, the Phalangiotarbids and Haptopodids.

Garwood and Dunlop obtained specimens of the single known Haptopodid species, Plesiosiro madeleyi, from the Natural History Museum in London, and of two Phalangiotarbid species, Goniotarbus tuberculatas and Goniotarbus angulatus from the Lapworth Museum in Birmingham and the Natural History Museum in London. All specimens originally came from the Coseley Lagerstätte, near Dudley in Staffordshire, England, making them about 315 million years old (Late Carboniferous). The Birmingham specimen (Goniotarbus tuberculatas) was found to be too poorly preserved for the project, so only the Natural History Museum specimens (Plesiosiro madeleyiand Goniotarbus angulatus) were used.

The reconstruction of the Haptopodid Plesiosiro madeleyi conformed largely to previous reconstructions made using conventional techniques. It was not possible to reconstruct the tips of the limbs, due to a crack that ran through the nodule in which it was preserved, and the specimen was slightly distorted due to lateral compression. Some joints could not be resolved due to ridges on the exoskeleton. The median (middle) eyes appeared to be set in a depression, which is unusual in Arachnids, where such eyes are normally raised, and Garwood and Dunlop suggest this may be an artefact of preservation.

Digital visualisations of the Haptopod Plesiosiro madeleyi. (A) Dorsal view showing opisthosomal segmentation and prosomal shield architecture. (B) Lateral view of the anterior ventral prosoma, nearest limbs and lateral prosoma removed, showing the nature of haptopod chelicerae. (C) Ventral view, showing ventral segmentation, and divided sternum. (D) Haptopod walking leg. Abbreviations: 1–10, opisthosomal segment number; as, anterior sclerite; ch, chelicerae; cx, coxa; fa, fang; fe, femur; L1–L4, walking legs 1–4; me, media eyes; mt, metatarsus; pa, paturon; pp, pedipalps; ps, pofsteriorsclerite; pt, patella; ta, tarsus; ti, tibia; tr, trochanter.Scale bars 3mm in (A) and (D), 1 mm in (B) and (C). Garwood & Dunlop (2014).

The Phalangiotarbid Goniotarbus angulatus was found to have a broad prosoma-opisthosoma boundary. The eye arrangement could not be fully resolved, which reflects earlier work on the group where between two and six eyes were reported. The chelicerae (mouthparts) cannot be resolved and are thought to be very small. The pedipalps (foremost limbs, not usually used for locomotion) are also small.

Digital visualisations of Phalangiotarbid Goniotarbus angulatus. (E) First left walking leg, showing typical segmentation. (F) Lateral view of the anterior ventral prosoma, showing the small pedipalps, median ridge, and possible chelicerae—below the resolution of the scan. (G) Fourth right walking leg. (H) Dorsal view showing median eyes and dorsal opisthosomal segmentation. (I) Ventral view showing opisthosomal segmentation and coxo-sternal region. Abbreviations: 1–10, opisthosomal segment number; as, anterior sclerite; ch, chelicerae; cx, coxa; fa, fang; fe, femur; L1–L4, walking legs 1–4; me, media eyes; mt, metatarsus; pa, paturon; pp, pedipalps; ps, pofsteriorsclerite; pt, patella; ta, tarsus; ti, tibia; tr, trochanter.Scale bars 3mm in (F - I), 1 mm in (E). Garwood & Dunlop (2014).

Garwood and Dunlop also hoped that these three dimensional reconstructions would help to resolve our understanding of phylogenic relationships between Arachnid groups. In recent years attempts to build a family tree for the group upon genetic analysis have proved problematic, while those based upon anatomy have proved consistent. However the addition of these formerly poorly known groups led to a severe rearrangement of computer generated family trees for the Arachnids, suggesting that these are not as robust as currently assumed.

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