This week's Taxon of the Week post had to be delayed a couple of days because I thought the Monday slot was better devoted to the proposed new electronic publication rules for the ICZN. It now being Wednesday, I am perfectly seated to introduce the delayed post. Today's highlight taxon is something not too far from my own research stomping grounds - the harvestman (Opiliones) superfamily Sironoidea.
Sironoidea was one of the three superfamilies of Cyphophthalmi or mite-like harvestmen recognised by Shear (1980). Largely thanks to the work of Gonzalo Giribet and associates, cyphophthalmids have in recent years become the most thoroughly studied of the major groups of harvestmen. I have previously posted on them here, and I'd recommend reading through that post before this one. As originally defined by Shear, Sironoidea included two families, the Holarctic Sironidae and the temperate Gondwanan Pettalidae (the subject of the previous post). Later, Shear (1993) added the New Caledonian Troglosiro to the superfamily. Sironoidea were united by Shear (1980) on the basis of their having a single series of dorsal setae (three groups of setae in other cyphophthalmids), coxae II free from coxae III (vs. fused), and the presence of anal glands and a modified anal region. Sironoidea was the only superfamily in the infraorder Temperophthalmi, the temperate cyphophthalmids, while the other two superfamilies, Stylocelloidea (containing the single south-east Asian family Stylocellidae) and Ogoveoidea (Ogoveidae and Neogoveidae of tropical America and west Africa) in the infraorder Tropicophthalmi, both had tropical distributions. One tropical species, the Kenyan Marwe coarctata, has since been supported by Bivort & Giribet (2004) as forming a clade in the Sironidae with the genera Paramiopsalis and Iberosiro.
More recent studies have broken down Shear's classification. Both of Shear's infraorders are likely to be polyphyletic, but relationships between the various families are difficult to pin down. The molecular analysis of Boyer et al. (2007) identified two possible arrangements - one with Pettalidae sister to the remaining cyphophthalmids in the arrangement Sironidae + (Stylocellidae + (Troglosiro + Neogoveidae)), while the other had Stylocellidae as the basalmost branch and Pettalidae sister to the clade formed by Sironidae and (Troglosiro + Neogoveidae). In contrast, the morphological analysis of Bivort & Giribet (2004) supported a basal position for Stylocellidae with the Ogoveoidea forming a paraphyletic series to a monophyletic Sironoidea. Until recently, the belief that Stylocellidae were the only family of cyphophthalmids to retain eyes was thought to support a basal position for them, but eyes were identified in a species of Pettalidae for the first time by Sharma & Giribet (2006), and have since been shown to be widespread in the family (Boyer & Giribet, 2007). Cyphophthalmid eyes (in those species that have them) are minute, integrated into the ozophore, of doubtful functionality and lack lenses in many Pettalidae, which is how it was possible for them to have gone unnoticed.
Even the family Sironidae, established by Shear (1980) mainly on the basis of fused anal sclerites, is not necessarily monophyletic. As with Pettalidae (see the earlier post), each of the genera recognised within Sironidae shows a distinct geographical distribution, demonstrating that vicariance has been a significant factor - in fact, the primary factor - in the diversification of these spectacularly poorly-dispersing soil animals. Interestingly, there appears to be a closer evolutionary connection for cyphophthalmids between western Europe and North America (both inhabited by members of the genus Siro) than between western Europe and eastern Europe (the latter inhabited in the Balkan peninsula by the genus Cyphophthalmus - Boyer et al., 2005), which might be regarded as a case of life imitating politics. The Iberian peninsula has been the site of a notable mini-radiation of sironids, being the only home of the monotypic genera Odontosiro lusitanicus, Paramiopsalis ramulosus and Iberosiro distylos while the genus Parasiro is found in Spain, Corsica and Italy. Other sironid genera are the Bulgarian Tranteeva paradoxa and the Japanese Suzukielus sauteri. Boyer et al. (2007) supported the monophyly of a core clade for Sironidae including Siro, Cyphophthalmus and Paramiopsalis, but Suzukielus and Parasiro were not closely associated with this clade - Suzukielus only joined the core Sironidae in the Stylocellidae-rooted tree, while Parasiro never did.
It's worth my adding a bit more about cyphophthalmid eyes. One things arachnids are known for by the general public is their possession of multiple pairs of eyes. Harvestmen, however, stand out in this regard by never having more than a single pair of eyes (the one supposed exception, the "cyphophthalmid" Gibocellum sudeticum, seems never to have existed outside the overactive imagination of its author - Sørensen, 1906). In the Phalangida (the sister clade to Cyphophthalmi containing the majority of harvestmen), the eyes are usually placed on either side of a raised structure called the ocularium or eyemound positioned on the midline of the prosoma (visible in the photo here), but there are a few examples in which the ocularium has disassociated and the eyes are positioned closer to either side of the animal (some examples are visible among the images on this page). The eyes of cyphophthalmids may be homologous to the median eyes of Phalangida. Alternatively, they could correspond to the lateral eyes of other arachnids. Shultz (2007) identified the sister group of Opiliones as scorpions, which have both median and lateral eyes, so no help there. A paper in the most recent Journal of Arachnology (Alberti et al., 2008) apparently identifies the ultrastructure of cyphophthalmid eyes as more like the median eyes of other arachnids, but this paper is so new that I haven't even seen the issue in question as yet.
Alberti, G., E. Lipke & G. Giribet. 2008. On the ultrastructure and identity of the eyes of Cyphophthalmi based on a study of Stylocellus sp. (Opiliones, Stylocellidae). Journal of Arachnology 36 (2): 379-387.
Bivort, B. L. de, & G. Giribet. 2004. A new genus of cyphophthalmid from the Iberian Peninsula with a phylogenetic analysis of the Sironidae (Arachnida: Opiliones: Cyphophthalmi) and a SEM database of external morphology. Invertebrate Systematics 18: 7-52.
Boyer, S. L., R. M. Clouse, L. R. Benavides, P. Sharma, P. J. Schwendinger, I. Karunarathna & G. Giribet. 2007. Biogeography of the world: a case study from cyphophthalmid Opiliones, a globally distributed group of arachnids. Journal of Biogeography 34 (12): 2070-2085.
Boyer, S. L., & G. Giribet. 2007. A new model Gondwanan taxon: systematics and biogeography of the harvestman family Pettalidae (Arachnida, Opiliones, Cyphophthalmi), with a taxonomic revision of genera from Australia and New Zealand. Cladistics 23: 337-361.
Boyer, S. L., I. Karaman & G. Giribet. 2005. The genus Cyphophthalmus (Arachnida, Opiliones, Cyphophthalmi) in Europe: a phylogenetic approach to Balkan Peninsula biogeography. Molecular Phylogenetics and Evolution 36 (3): 554-567.
Sharma, P., & G. Giribet. 2006. A new Pettalus species (Opiliones, Cyphophthalmi, Pettalidae) from Sri Lanka with a discussion on the evolution of eyes in Cyphophthalmi. Journal of Arachnology 34 (2): 331-341.
Shear, W. A. 1980. A review of the Cyphophthalmi of the United States and Mexico, with a proposed reclassification of the suborder (Arachnida, Opiliones). American Museum novitates 2705: 1-34.
Shear, W. A. 1993. The genus Troglosiro and the new family Troglosironidae (Opiliones, Cyphophthalmi). Journal of Arachnology 21: 81-90.
Shultz, J. W. 2007. A phylogenetic analysis of the arachnid orders based on morphological characters. Zoological Journal of the Linnean Society 150 (2): 221-265.
Sørensen, W. 1906. Un animal fabuleux des temps modernes. Analyse critique. Oversigt over det Konigelige Danske Videnskabernes Selskabs Forhandlinger 4: 197-232.