Greetings arthropod lovers of all stripes! Today Mike Skvarla and I sit down with Ray Fisher of the University of Arkansas to discuss Chelicerata. Now you may be saying to yourself, "What is a Chelicerata? It sounds like an exotic sports car." and you would be wrong. Chelicerates are a diverse group of animals that we interact with on a regular basis and includes things like spiders, ticks, mites, scorpions and many more.  In this episode, we will unpack this large and diverse group by discussing their appendages, evolution, and interactions with humans. Ray is an expert in his field and has provided us extensive show notes if you care to check them out! (Please note that some of the links are to papers posted on Research Gate. If you wish to access these papers, you will need to create a profile, which is free). Here are few sources for chelicerate phylogeny highlighting Lamsdell's work (the first one has a nice summary tree). Seldin et al. (2015) - An unusual euchelicerate linking horseshoe crabs and eurypteridsLamsdell et al (2015) - A new Ordovician arthropod from the Winneshiek Lagerstätte of Iowa (USA) reveals the ground plan of eurypterids and chasmataspidids Here are two major works that discuss the new system for arthropods and chelicerates. The first highlights some review material from another titan in this field, Javier Ortega-Hernández, and discusses some interesting terminological problems with words like "Arthropoda". He introduces a new term, Deuteropoda, which emphasizes the shift from stem euarthropods to those that have the first appendage being the deuterosomal one. The second paper is Lamsdell's beast where he demoloshes Xiphosura, and in turn, Merostomata.Ortega-Hernández (2014) - Making sense of ‘lower’ and ‘upper’ stem-groupEuarthropodaLamsdell (2013) - Revised systematics of Palaeozoic ‘horseshoe crabs’ and the myth of monophyletic Xiphosura I had to pass this gem along. I didn't mention it, but it's cool to see such ecological diversity in horseshoe crabs, a mere relic group today. Check it out: Lamsdell (2015) – Horseshoe crab phylogeny and independent colonizations of fresh water I mentioned that the timing of terrestrialization for most major terrestrial lineages (insects, arachnids, myriapods) has been supported by both molecular and fossil data. So, both lines of evidence supports an Ordovician or Silurian terrestrialization event for Arachnids and an Ordovician terrestrialization event for Hexapods. However, a recent study found that millipedes are different. For them, molecular data is in stark contrast to fossil data; in other words, millipedes likely colonized land long before there were even trace fossils of them... so we're looking at a terrestrialization event at the mid-to-late Cambrian! So Myriapoda likely colonized land twice! Apparently this is supported by millipede biogeography and also by differing structure of the tracheal systems between millipedes and centipedes.Lozano-Fernandez et al. (2016) - A molecular palaeobiological exploration of arthropod terrestrialization Here is the paper I mentioned that supports, albeit very weakly, the idea that arachnids colonized via the interstitial as small-bodied creatures, vs. the popular conception of larger-bodied scorpions.Schaefer et al 2010 - Arthropod colonization of land – Linking molecules and fossils in oribatid mites It's hard to find a compelling figure illustrating the evolution of the chelicera, but Figure 3 of the below article describing Yohoia seems to be best. The point is that chelicerae (Chelicerates) evolved from raptorial deuterosomal limbs used for prey-capture, whereas mandibles evolved from 4th-segment biramous legs with endites that were already acting as gnathobases, but then by reducing the telopodite and simultaneously enhancing the protopodite. There's a great figure for this in the below dissertation (Fig. 1.3.)Haug et al 2012 – Function morphology, ontogeny and evolution of mantis shrimp-lik