Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders

Abstract

Arachnids are an important group of arthropods. They are: diverse and abundant; a major constituent of many terrestrial ecosystems; and possess a deep and extensive fossil record. In recent years a number of exceptionally preserved arachnid fossils have been investigated using tomography and associated techniques, providing valuable insights into their morphology. Here we use X-ray microtomography to reconstruct members of two extinct arachnid orders. In the Haptopoda, we demonstrate the presence of ‘clasp-knife’ chelicerae, and our novel redescription of a member of the Phalangiotarbida highlights leg details, but fails to resolve chelicerae in the group due to their small size. As a result of these reconstructions, tomographic studies of three-dimensionally preserved fossils now exist for three of the four extinct orders, and for fossil representatives of several extant ones. Such studies constitute a valuable source of high fidelity data for constructing phylogenies. To illustrate this, here we present a cladistic analysis of the chelicerates to accompany these reconstructions. This is based on a previously published matrix, expanded to include fossil taxa and relevant characters, and allows us to: cladistically place the extinct arachnid orders; explicitly test some earlier hypotheses from the literature; and demonstrate that the addition of fossils to phylogenetic analyses can have broad implications. Phylogenies based on chelicerate morphology—in contrast to molecular studies—have achieved elements of consensus in recent years. Our work suggests that these results are not robust to the addition of novel characters or fossil taxa. Hypotheses surrounding chelicerate phylogeny remain in a state of flux.

Ancestral reconstruction of tick lineages

“…Their monophyly is supported by morphological and molecular data (Garwood and Dunlop, 2014;Sharma et al 2014;Shultz, 2007Shultz, , 1990Wheat and Wahlberg, 2013).…”

“…Our confidence that ticks fall within this classification scheme (Eukaryotes, Metazoa, Bilateria, Nephrozoa, Protostomia, Ecdysozoa, Arthropoda, Chelicerata, Arachnida, Parasitiformes, Ixodida) is fairly robust based on morphological and molecular evidence (Dunlop, 2010;Edgecombe et al 2011;Edgecombe and Legg, 2014;Garwood and Dunlop, 2014;Giribet and Edgecombe, 2012;Legg et al 2013;Philippe et al 2009;Shultz, 2007Shultz, , 1990. This phylogenetic placement allows us to make some generalized statements with regard to ancestral reconstruction, with the understanding that there is a gradation of lineage specific conservation.…”

“…The sister group to the Acari (Acariformes and Parasitiformes) has been considered to be the Ricinulei (Shultz, 2007;Wheeler and Hayashi, 1998), unresolved (Edgecombe and Legg, 2014;Giribet and Edgecombe, 2012;Klompen et al 2007), or the Palpigradi and Pycnogonida (Giribet et al 2002). Additionally, a number of morphological and molecular studies have suggested that the Acari is not monophyletic (Borner et al 2014;Dunlop, 2010;Dunlop and Selden, 2009;Garwood and Dunlop, 2014;Ovchinnikov and Masta, 2012;Pepato et al 2010;Regier et al 2010;Sharma et al 2014;Shultz, 2007; van der Hammen, 1989). The relationship between Parasitiformes and other arachnids is also not clear, since it has been considered a sister group of the Solifugae (Borner et al other Chelicerata) (Sharma et al 2014).…”

Segmentation and tagmosis in Chelicerata

“…Braddy et al, 1999). Cladistic analyses usually recovered a monophyletic Arachnida (Weygoldt and Paulus, 1979;Shultz, 2007a;Pepato et al, 2010;Regier et al, 2010;Garwood and Dunlop, 2014a;Sharma et al, 2014b), although a stable phylogeny based on morphological and/or molecular data has not been forthcoming. Most arachnids are terrestrial and caution is needed to tease out potential autapomorphies for the group from homoplastic response to life on land.…”

“…They have been likened either to harvestmen or opilioacarid mites, but a recent attempt to resolve their phylogenetic position (Garwood and Dunlop, 2014a) was unable to recover a robust position for this group as a number of key characters, such as the nature of the mouthparts, remain equivocal. Accounts of their external morphology can be found in, e.g., Dunlop and Horrocks (1997) and Pollitt et al (2004).…”