DOI: 10.5061/dryad.2fqz612z2
title: Segmentation in early Xiphosura and the evolution of the thoracetron

description: Xiphosuran chelicerates, also known as horseshoe crabs, are a long-lived
 clade characterized by a highly distinctive morphology and are a classic
 example of supposed evolutionary stasis. One key feature of horseshoe
 crabs is the fusion of the opisthosomal segments into a single sclerite
 referred to as a thoracetron. There has been historical uncertainty as to
 whether the thoracetron originated once or multiple times within the
 clade. Here, we review criteria for determining whether segments are fused
 and apply them to a broad census of taxa for which their ontogeny is known
 or the adult status of specimens can be reasonably asserted to explore the
 evolution of the thoracetron within a developmental framework. Our
 findings indicate that the thoracetron evolved once in the common ancestor
 to Xiphosura. However, subsequent independent loss of the thoracetron
 segment boundaries is identified and shown to be the result of
 heterochronic processes acting upon a shared developmental pathway. The
 multiple cases of effacement of the thoracetron within Limuloidea are
 cases of peramorphically-driven parallelism, while the effacement of the
 thoracetron in the paedomorphic Belinurina is a case of convergence.
 Xiphosurids therefore represent an interesting case study for recognizing
 parallelism and convergence on the same structure within closely related
 lineages. We also demonstrate that somite VII has been incorporated into
 the prosoma multiple times within the chelicerate lineage, which has
 implications for interpreting the ground pattern of the group.
description: # Segmentation in early Xiphosura and the evolution of the thoracetron
 \[Access this dataset on Dryad]
 ([http://doi.org/10.5061/dryad.2fqz612z2](http://doi.org/10.5061/dryad.2fqz612z2)) The dataset comprises the phylogenetic matrix and character list formatted in a single nexus file (**Lamsdell_Ocon_thoracetron_evolution.nex**). ## Description of the data and file structure A single nexus (.nex) file comprising the morphological character matrix and character list. The .nex file can be opened in standard text editor programs. Parsimony analysis can be performed using the file in in PAUP* v4.0a167, with all characters treated as unordered and of equal weight. The data matrix is composed of 259 characters coded for 156 taxa. Character states are represented by 0-9, with unknown states represented by ? and inapplicable character states represented by -. Polymorphic codings for a character are enclosed within square [] brackets. Character and state descriptions are listed under 'cnames'. ## Sharing/Access information Data was derived from primary observations on fossil material held in the Australian Museum (Sydney, New South Wales, Australia), Buffalo Museum of Science (Buffalo, New York, USA), Chernyshev Central Museum of Geological Exploration (Saint Petersburg, Russia), Field Museum of Natural History (Chicago, Illinois, USA), Silesian University Faculty of Earth Sciences (Sosnowiec, Poland), University of Kansas Biodiversity Institute (Lawrence, Kansas, USA), Manitoba Museum (Winnipeg, Manitoba, Canada), Museum National d’Histoire Naturelle (Paris, France), Natural History Museum (London, England, UK), New Mexico Museum of Natural History (Albuquerque, New Mexico, USA), National Museums of Scotland (Edinburgh, Scotland, UK), Museums Victoria (Carlton, Victoria, Australia), National Museum of Wales (Cardiff, Wales, UK), Nova Scotia Museum (Halifax, Nova Scotia, Canada), Oxford University Museum of Natural History (Oxford, England, UK), Royal Ontario Museum (Toronto, Canada), National Museum of Natural History (Washington, DC, USA), Rock Library and Geological Museum of the University of Tasmania (Hobart, Tasmania, Australia), Yale Peabody Museum (New Haven, Connecticut, USA).