Trigonotarbid arachnids

Although sharing a similar body plan with other arachnids, trigonotarbids are not spiders as such. They belong within a group of animals called the chelicerates. Other chelicerates include the horseshoe crabs (such as modern Limulus), scorpions, the extinct eurypterids and all other arachnids including mites. As the earliest recognised terrestrial animal fossil is an Upper Silurian trigonotarbid, understanding of the group obviously plays an important role in determining the timing and mechanism by which animals invaded the land.

There are a number of significant differences between trigonotarbids (informally called "trigs") and spiders which are tabulated below:

Hirst (1923) originally described five species of trigonotarbids from the Rhynie cherts under the generic names Palaeocharinoides and Palaeocharinus. These are Palaeocharinoides hornei, Palaeocharinus rhyniensis, P. scourfieldi, P.calmani and P. kidstoni.

The two genera were distinguished respectively by an acute vs. a straight posterior margin of the sternum (a central plate located on the ventral side of the head region). Shear et al. (1987) rejected this as a diagnostic generic character and synonimised Palaeocharinoides with Palaeocharinus.

The number of species should also be treated with caution as many were diagnosed as different from one another on very minor differences. Some of these apparent differences may have been introduced during the fossilisation process or could be accounted for by intra-specific variation. Of Hirst's trigonotarbid specimens, it would appear only two valid species are probably present: Palaeocharinus rhyniensis and Palaeocharinus hornei.

Recently a new species, Palaeocharinus tuberculatus, has been described from the Windyfield chert (Fayers et al. 2004), differing from Hirst's specimens in its larger size and distinctive tuberculate ornament (see inset below).

The following section gives the general morphology of the trigonotarbids known form the cherts at Rhynie.

The Rhynie trigonotarbid arachnids described to date all belong to the family Palaeocharinidae, and possess a distinctive and characteristic morphology by which they can be identified. Overall trigonotarbids consist of a prosoma (head region) and an opisthosoma (body region) (see inset below).

Each of these areas is explored in more detail below. The Rhynie 'trigs' are generally quite small in size. The most commonly encountered species (Palaeocharinus rhyniensis and P. hornei) have a body length of up to 4 mm. Palaeocharinus tuberculatus is slightly larger having a maximum body length of at least 6mm. However, other trigonotarbid specimens have occasionally been found in the cherts with body lengths ranging between 14 mm and 20 mm. These larger forms (currently being described) appear to belong to a new species quite distinct from Palaeocharinus and possibly belong to a separate family.

One of the most diagnostic features of trigonotarbid arachnids is the microscopic scale-like sculpture seen variably developed on the cuticle or skin of the animal (see inset right). This can allow identification of fragments of cuticle where no whole body fossil exists, and is particularly useful in studies where material is limited (e.g. in thin sections of chert and fragments in organic concentrates extracted from sedimentary rocks.

Head Region

The head region or prosoma consists of a highly domed carapace with a distinctive pointed 'beak' towards the front (anterior). In the Palaeocharinidae this 'beak' shows two  sub-parallel tubercled ridges that extend from the level of the median eyes to the anterior where they form a pair of acute spine-like processes. Lateral eye tubercles are present on either side of the carapace. In the Rhynie palaeocharinids each lateral eye tubercle possesses a series of major and minor eye lenses, comprising a staggered row of up to 10 minor lenses separating a larger lower lens and 2 larger upper lenses (see inset right).

Underneath the domed carapace hangs a pair of down and backwardly directed poison fangs or chelicerae; the morphological character from which the group derives its name (see inset below). Towards the posterior of the carapace are the leg attachment points (coxae). The position of the leg appendages, associated with the prosoma is a further diagnostic characteristic of the Chelicerates.

Body Region

The body region of trigonotarbids is visibly segmented and also divided longitudinally into a medial and two lateral regions. This pattern of segmentation is quite unlike modern spiders in which the abdomen consists of a single fused area. Within the abdomen are housed the opisthosomal appendages. In the case of trigonotarbid arachnids these appendages take the form of lamellate book lungs (see inset right) used in respiration. The presence of book lungs indicates that these animals were unequivocally terrestrial organisms. The abdomen of trigonotarbids is terminated by a small, circular anal tubercle.

Prosomal Appendages

Just as in extant spiders, trigonotarbids possessed a pair of chelicerae (fangs), a pair of pedipalps and four pairs of walking legs. The fangs were used in prey capture and the pedipalps may have had a sensory function, although in extant male spiders they are used in mating. Each of the individual leg segments or podomeres can be named. Starting from the insertion point they are as follows: coxa, trochanter, femur, patella, tibia, basitarsus and telotarsus (see reconstruction above and inset below). In trigonotarbids the most distal of the leg podomeres, the telotarsus, bears a pair of tarsal claws that were an adaptation for walking in the terrestrial environment. The more distal podomeres of the walking legs bear and increasing number of fine setae or hairs. These were used by the animal to augment its sensory array.

The legs of Palaeocharinus tuberculatus differ from the other Palaeocharinus species in that they possess distinctive longitudinal rows of tubercles. Many of these tubercles appear to flank and guard socketed sensory hairs.

Trigonotarbid arachnids such as the Rhynie form Palaeocharinus did not possess the morphological structures termed spinnerets. Extant spiders have spinnerets and these are used to produce silk, one of the most important substances in terms of spider ecology.

Silk threads spun from the spinnerets are used to build food-capturing webs, silk lines the burrows of trap door spiders, is used to wrap prey items and also to provide protective cocoons for the eggs.

Some species even use the silk as a mode of transport. By letting out a long thread of silk into the prevailing wind small spiders 'balloon' and can travel great distances. This can aid widespread dispersal of the species. As trigonotarbids lack this innovation, their possible palaeoecological interpretation must be somewhat limited.

Another group of extant arachnids which provide a more useful analogue to trigonotarbids are the opilionids, or 'harvestmen'. These ground living forms lack silk but instead immobilise their prey with  a bite from poisonous fangs (chelicerae). It is conceivable the Rhynie trigonotarbids behaved in a similar manner since they possessed particularly robust and well-developed fangs.

The robust walking legs and downwardly directed fangs suggest that Palaeocharinus may have been a ground predator stalking the micro-habitat provided by the stands of Rhynie plants. It may well have sat in wait and ambushed its prey from the cover of fallen plant stems. Indeed some trigonotarbid fossils have been discovered within the cherts within the hollow stems and sporangia of the Rhynie plants (Kevan et al. 1975) (see inset above). They may have used these as cover from which to ambush their prey. Alternatively such locations may have provided ideal conditions for ecdysis or the moulting process to take place in.