Academic literature on the topic 'Rhynchothoracidae'

The internal and external relationships within the Class Pycnogonida are rationalized based on the entire history of whole-animal understanding, detailed morphometrics, the fossil record, larval structure, mathematical multivariate analyses, and molecular analyses (electrophoretic protein and DNA). The pre-Jurassic fossils are placed in separate orders, with the Lower Silurian Haliestes giving the clearest indications of the form of a hypothetical “protopycnogonid”. Living forms, together with some Lower Devonian and the Jurassic fossils, are retained in the Order Pantopoda. No relationships to other Classes of the Arthropoda are yet indicated, and the concept of the Pycnogonida as a sister group to the Euarthropoda is maintained. The Pantopoda are divided into two suborders, isolating the Austrodecidae. The remaining taxa are classified into six superfamilies, on a consensus of overall morphological trends, larval forms, and the findings of the only previous comprehensive morphological multivariate analysis, and recent molecular analyses. The Colossendeidae, Pycnogonidae and Rhynchothoracidae are isolated within their own superfamilies. The Ammotheidae sensu lato is subdivided (within one superfamily), unfortunately leaving a number of genera incertae sedis. The Nymphonidae, Callipallenidae and Pallenopsidae are associated within another superfamily. The Jurassic fossils are placed within the Endeidae within a superfamily together with the Phoxichilidiidae, while some Lower Devonian fossils remain incertae sedis. Diagnoses are given as appropriate where possible.

Sabroux, Romain, Hassanin, Alexandre, Corbari, Laure (2022): Sea spiders (Arthropoda: Pycnogonida) collected during the Madibenthos Expedition from Martinique shallow waters. European Journal of Taxonomy 851 (1): 1-141, DOI: 10.5852/ejt.2022.851.1999, URL: http://dx.doi.org/10.5852/ejt.2022.851.1999

Müller, Hans-Georg, Krapp, Franz (2009): The pycnogonid fauna (Pycnogonida, Arthropoda) of the Tayrona National Park and adjoining areas on the Caribbean coast of Colombia 2319. Zootaxa 2319 (1): 1-138, DOI: 10.11646/zootaxa.2319.1.1, URL: https://biotaxa.org/Zootaxa/article/view/zootaxa.2319.1.1

Abstract Despite significant advances in invertebrate phylogenomics over the past decade, the higher-level phylogeny of Pycnogonida (sea spiders) remains elusive. Due to the inaccessibility of some small-bodied lineages, few phylogenetic studies have sampled all sea spider families. Previous efforts based on a handful of genes have yielded unstable tree topologies. Here, we inferred the relationships of 89 sea spider species using targeted capture of the mitochondrial genome, 56 conserved exons, 101 ultraconserved elements, and 3 nuclear ribosomal genes. We inferred molecular divergence times by integrating morphological data for fossil species to calibrate 15 nodes in the arthropod tree of life. This integration of data classes resolved the basal topology of sea spiders with high support. The enigmatic family Austrodecidae was resolved as the sister group to the remaining Pycnogonida and the small-bodied family Rhynchothoracidae as the sister group of the robust-bodied family Pycnogonidae. Molecular divergence time estimation recovered a basal divergence of crown group sea spiders in the Ordovician. Comparison of diversification dynamics with other marine invertebrate taxa that originated in the Paleozoic suggests that sea spiders and some crustacean groups exhibit resilience to mass extinction episodes, relative to mollusk and echinoderm lineages.

Abstract Background Pycnogonida (sea spiders) is the sister group of all other extant chelicerates (spiders, scorpions and relatives) and thus represents an important taxon to inform early chelicerate evolution. Notably, phylogenetic analyses have challenged traditional hypotheses on the relationships of the major pycnogonid lineages (families), indicating external morphological traits previously used to deduce inter-familial affinities to be highly homoplastic. This erodes some of the support for phylogenetic information content in external morphology and calls for the study of additional data classes to test and underpin in-group relationships advocated in molecular analyses. In this regard, pycnogonid internal anatomy remains largely unexplored and taxon coverage in the studies available is limited. Results Based on micro-computed X-ray tomography and 3D reconstruction, we created a comprehensive atlas of in-situ representations of the central nervous system and midgut layout in all pycnogonid families. Beyond that, immunolabeling for tubulin and synapsin was used to reveal selected details of ganglionic architecture. The ventral nerve cord consistently features an array of separate ganglia, but some lineages exhibit extended composite ganglia, due to neuromere fusion. Further, inter-ganglionic distances and ganglion positions relative to segment borders vary, with an anterior shift in several families. Intersegmental nerves target longitudinal muscles and are lacking if the latter are reduced. Across families, the midgut displays linear leg diverticula. In Pycnogonidae, however, complex multi-branching diverticula occur, which may be evolutionarily correlated with a reduction of the heart. Conclusions Several gross neuroanatomical features are linked to external morphology, including intersegmental nerve reduction in concert with trunk segment fusion, or antero-posterior ganglion shifts in partial correlation to trunk elongation/compaction. Mapping on a recent phylogenomic phylogeny shows disjunct distributions of these traits. Other characters show no such dependency and help to underpin closer affinities in sub-branches of the pycnogonid tree, as exemplified by the tripartite subesophageal ganglion of Pycnogonidae and Rhynchothoracidae. Building on this gross anatomical atlas, future studies should now aim to leverage the full potential of neuroanatomy for phylogenetic interrogation by deciphering pycnogonid nervous system architecture in more detail, given that pioneering work on neuron subsets revealed complex character sets with unequivocal homologies across some families.