Sociable trigonotarbids
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The most sociable creature hitherto discovered in the Lower Devonian Rhynie chert, the small crustacean Ebullitiocaris, comes seldom alone but seems to feel more comfortable in the crowd, grazing on alga layers [1]. A smaller number of mites have been found huddled together within an empty sporangium of Aglaophyton, apparently feeding on the inner wall [2]. Quite differently, trigonotarbids are assumed to prowl alone through thickets of the humble vegetation of that time [3] or to sit in the empty sporangium watching the flock of mites as they feed and grow unawares of the predator lurking behind them, and eat one of them every now and then [2]. Also there is evidence that they prefer sheltered places for moulting as, for example, a hollow straw of Aglaophyton (Fig.1), where not rarely more than one moult is seen [4]. trigonotarbid moult, Rhynie
Fig.1: Moult parts of trigonotarbid, probably Palaeocharinus rhyniensis within a hollow straw of Aglaophyton
(photograph by H. Sahm).
body cross-section, 2mm wide, and legs (above left). Part of the inclined section of the poorly preserved tube, width about 5mm, is also seen.

In view of their solitary way of life, one would hardly expect to see two trigonotarbids incidentally at the same place in the chert. Hence, the below pictures suggest that they came to the place on purpose, perhaps for moulting or mating.
trigonotarbid moults, Rhynie trigonotarbids, Rhynie
Figs. 2,3: Rare examples of trigonotarbid sections
arranged unexpectedly close to each other.

Apparently the aspect of the sections does not allow easy conclusions to be drawn as to whether they represent the moult or the creature entombed as a whole. The incomplete longitudinal body section with separate leg sections in Fig.2 suggest that at least this one is a mere moult.
Since the cut faces are usually more or less tilted and displaced with respect to the symmetry plane of the body, the actual size and shape of the cross-section and the longitudinal section cannot be derived from sections like those in the above images. However, with the available knowledge of trigonotarbids it can be stated that the upper section in Fig.2 and the lower section in Fig.3 come close to cross-sections. (For a whole trigonotarbid, bigger species than this one, cut well along the symmetry plane, see
Rhynie Chert News 9 .)  
Although the upper section in Fig.3 is apparently more tilted and thus cannot immediately be compared with the section below, it seems to belong to a specimen bigger than the one below. This could possibly be an indication of sexual dimorphism.
2 trigonotarbids

Fig.4: Two trigonotarbids in Rhynie chert: top view (width 1.75mm) and inclined cross-section (width 2mm, corrected for inclination) on the raw surface of a chert layer fragment.

The trigonotarbid in Fig.4 (below) is not a section. It is preserved in its true 3D-shape owing to the crack which had separated it from the embedding rock, except for the end on the right where part of the body got lost by spalling fracture. The segmentation of the body is exceptionally clearly seen.

The above images seem to indicate that if there are two trigontarbids in one small sample, they are more likely seen close together than far apart. It has to be checked whether this is a rule or merely incidental. Hence, one should look for more paired trigonotarbid sections with the aim to find out some indication concerning the behavior of these early terrestrial predators.

H.-J. Weiss      2011        complemented 2014

[1]  L. Anderson et al.: A new univalve crustacean from the Early Devonian Rhynie chert hot-spring complex.
        Trans. Roy. Soc. Edinburgh, Earth Sciences 94(2004 for 2003), 355-369.
[2]  H. Kerp, H. Hass: De Onder-Devonische Rhynie chert. Grondboor & Hamer 58(2004), 33-50.

P.G. Kevan, W.G. Chaloner, D.B.O. Savile: Interrelationships of early terrestrial arthropods and plants.
        Palaeontology 18 Part 2 (1975), 391-417, Plates 54-56.
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