Fossil Groups

and Evolution of Chelicerata


Chelicerate groups known only from fossils

Onychopterella augusti, c.7cm long (from Braddy et al., 1995)

 Rhinocarcinosoma dosonensis, c.20cm long (from Braddy et al., 2002)

 

Eurypterida:

Sea "scorpions"

Eurypterids are a diverse group that lived from the Ordovician to the Permian (approximately 450 to 250 million years ago). They are characterised by a differentiation of the opisthosoma into pre- and post-abdomen (meso- and metasoma), and the presence of genital appendages [although these are not defining characteristics of Eurypterida - Dunlop, 2002]. Eurypterids could reach over 2 m in length and therefore represent some of the biggest arthropods ever known.

Eurypterids were aquatic predators. During their reign they moved from being fully marine to inhabiting freshwater, and some groups even developed amphibious habits, being able to move across land, and respiring using book lungs similar to those of scorpions. They may have resembled modern xiphosurans in exhibiting 'mass-moult-mate' behaviour: coming ashore to moult and breed (as suggested by beds showing multiple parallel fossil trackways).They may also have resembled modern scorpions in having complex mating behaviour, involving terrestrial sperm transfer in the form of spermatophores ('sperm sacs') (as suggested by the anatomy of the genital appendages).

Read much more about eurypterids here.

 

Forfarella mitchelli, c.12mm long (adapted from Dunlop et al., 1999)

Chasmataspida:

Chasmataspids

Chasmataspids are a rare group of aquatic Palaeozoic arthropods, known only from the Ordovician of North America (around 450 million years ago) and Devonian of Europe (around 400 million years ago) (Dunlop, 2002). They resemble xiphosurans and eurypterids, but are distinguished on the basis of their segmentation. Further work may establish them as part of the eurypterid group (Dunlop, 2002). They were probably predators, in a manner similar to xiphosurans and eurypterids.

 

 Palaeotarbus jerami, c.2mm long (from Dunlop, 1996)

Trigonotarbida:

Trigonotarbid arachnids

Trigonotarbids were a diverse and important order of Palaeozoic arachnids. They are characterised by a laterally-divided opisthosomal plates. Trigonotarbids are most commonly found in Carboniferous coal forest deposits (around 300 million years old) but are known from as early as the Late Silurian (around 415 million years ago). This makes them some of the earliest terrestrial animals, and their importance in early terrestrial ecosystems is known from deposits such as the Rhynie Chert of Scotland.

Haptopoda: an extinct arachnid order known only from one Carboniferous species, possibly close to trigonotarbids (Dunlop, 2002).

 

 Generalised phalangiotarbid, c.1.5cm long (redrawn from Beall, 1991) (only half the appendages illustrated)

Phalangiotarbida:

Phalangiotarbid arachnids

Phalangiotarbids are an unusual order of Palaeozoic arachnids, most commonly found in Carboniferous coal forest deposits (around 300 million years ago). They had highly reduced chelicerae and pedipalps (it is not known how they fed), and respiratory spiracles suggesting that they were terrestrial. They may be related to harvestmen (Order Opiliones - see Modern Groups).

 

Chelicerate Evolution and Classification

The relationships between the different chelicerate groups, and between chelicerates and other groups, are difficult to resolve. However, it is useful to consider the different groups in a wider context. The following diagram shows a cladogram (tree of relationships based on analysis of morphological characters) superimposed on the fossil record of various groups (redrawn from Dunlop and Selden, 1997). This view of chelicerate phylogeny must be viewed as 'work in progress'.

Click on the small image below to see the full size diagram:

Heavy lines indicate known fossil ranges of each group, dotted lined represent 'ghost ranges' (inferred existence but no fossil record) and vertical lines represent relationships between the groups. True chelicerates (as defined by Dunlop and Selden, 1997) are distinguished from relatives and ancestors.

The timeline runs through the Palaeozoic, from the Cambrian (starting 545 million years ago) to the Permian (finishing 250 million years ago). Arrow on the 'Xiphosurida', 'Scorpiones' and 'other arachnids' lines indicates that they survive to the present day. Chasmataspids and eurypterids are now extinct, as are several other fossil groups which have been placed as 'sister taxa' to Chelicerata. Chelicerata can be seen to be set within a wider 'Arachnomorph' grouping, which includes Cambrian forms such as the aglaspids.

Future work may be expected to modify some of these relationships, and confirm or deny hypotheses such as a eurypterid ancestry for terrestrial scorpions.


Return to: Major subgroups of Chelicerata

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Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2003-4