Chelicerates - Subphylum: Chelicerata - The Northern Rivers Project

Chelicerates - Subphylum: Chelicerata

The Phylum: Arthropoda is divided into four subphyla of which the Chelicerata is the first dealt with here. Like all Arthropods the Chelicerates have a segmented body, they have jointed appendages on each segment and they have an exoskeleton. There are however some differences that sets the Chelicerates apart. They have a fused* head and thorax so there are two rather than three body segments. And, unlike other Arthropods, Chelicerates do not possess mandibles or antennae.

From the fore segment (the prosoma or cephalothorax) a total of six pairs of appendages emerge. Two pairs that form mouthparts which are the 'pedipalps' and the 'chelicerae' (= fangs), and four pairs of legs used for either walking or swimming. The lack of mandibles, as well as a very narrow digestive tract, means that Chelicerates are not able to eat solid food. Instead, they expel digestive enzymes onto or into their prey, which liquifies it so they can then eat and digest it.
Note: 'prosoma' is preferred over 'cephalothorax'. See note below.

Image 1. By Dunlop, Jason A. and Garwood, Russell (2012). ;Tomographic reconstruction of the exceptionally well preserved trigonotarbid arachnid Eophrynus prestvicii. Acta Palaeontologica Polonica. DOI:10.4202/app.2012.0032. https://creativecommons.org/licenses/by-sa/4.0/

The oldest known fossil of a land arachnid, a Trigonotarbid, has been dated at 414 My old. Trigonotarbids were animals that were very spider-like in appearance but which differed mainly in that they lacked spinnerets. The currently living Opilionids, or Harvestmen, are probably the closest to the extinct Trigonotarbids in that they too, are mostly ground-dwelling arachnids without the ability to spin silk and, possess rather large and well developed fangs with which they kill their prey.

Chelicerates, like all Arthropods originated from a marine environment. The oldest certain chelicerate currently known is Sanctacaris from the Middle Cambrian Burgess Shale of British Columbia, Canada. Although it lacks the chelae, or claws, which give the group its name, it does show the same pattern of tagmosis (clustering and specialization of segments) that characterizes all chelicerates. The Burgess Shale itself is dated at around 508 My old.
By the Silurian era (about 444 to 420 Mya) true scorpions had evolved although nearly all of them were aquatic species; Sea Scorpions aka Eurypterids, the largest of which (Jaekalopterus)
reached a length of about 2.5 meters. Fossils of these remarkable creatures have been found in th U.S.A., Germany and the Czech Republic.
From the start of the Devonian (about 420 Mya) more and more fossils of mites, spiders and other are starting to appear and spread out at about the same time as insects.

The Subphylum: Chelicerata is divided into three Classes the largest of which is the Class: Arachnida with upto 110,000 described species out of an estimated total of 1 Million species. Spiders, Mites, Ticks, Scorpions, Harvestmen and others are all included in this Class.
The second largest is the Class: Pycnogonida or Sea-Spiders with about 1,300 described extant species and, oddly enough, only 13 extinct species fossils of which have so far been found. These animals are so named because of their physical resemblance to land spiders and not because they are so closely related to them.
And then there is the Class: Merostomata or Horse Shoe Crabs of which a mere four species remain. They are regarded by many as 'living fossils' as their appearance does not seem tho have changed much since they arrived onto the scene some 445 Mya. Horse Shoe Crabs are not found in Australia and so are not further dealt with here.

Image 2. Horse Shoe Crab. By Shubham Chatterjee. Published here under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Arachnids - Class: Arachnida

Sea Spiders - Class: Pycnogonida

* Note: No fossil (or living) arachnid of any type has ever been discovered in which the head and thorax were separable sections of the animal. So saying 'the head and the thorax are fused together' does not really make much sense. Likewise, embryological studies have found no evidence of the two parts being separate during any stage of the embryo's development. Again, 'fusion' of the two never happened. The term 'cephalothorax' therefore should be phased out in favour of 'prosoma'. As for the term 'abdomen'; in arachnids the abdomen contains the heart as well as the breathing organs the presence of which, sensu stricto, makes the use of the term 'abdomen' incorrect. The term 'opisthosoma' is preferred instead.

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Header photo:
Fossil Eurypterid. Image by Richard Droker; Creative Commons-Attribution-NonCommercial-NoDerivs 2.0 Generic license.

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