One of the less common sights in the Rhynie chert is a dense cluster of dozens of mm-size oval objects superficially looking like as many sheep (Fig.1). These objects have been recognized only recently as crustaceans [1] and tentatively put into the cladoceran clade, which means they are related to the common water fleas. Unlike the latter, which seem to be in perpetual motion, Ebullitiocaris led a virtually sessile life, clinging to plant debris and possibly grazing on algal mats, a behaviour also known from various extant water flea relatives.

The fossils do not show much detail except for a segmented tube inside (Fig.2). The authors in [1] favour a tentative interpretation of this tube as the gut, which requires the assumption that “the apparently segmented appearance of the tube is actually caused by the line of section cutting through the three-dimensional spiralling structure of the gut.” The alleged spiral structure serves as an argument in favour of the water flea connection.

						Fig.1: Flock of the crustacean Ebullitiocaris in Rhynie chert.
						Fig.2: Ebullitiocaris with a segmented tube seen inside.
						Fig.3: Cross-section of Ebullitiocaris with concentric rings.

However, the pictures in [1] do rather not suggest a spiral structure, neither do the specimens in our sample. The tube, or at least part of it, seems to be really composed of short segments like plumbers’ fittings (Fig.3), with a ribbon-like cord inside (Fig.2 & Fig.3), which excludes its interpretation as the gut. Apparently in anticipation of evidence contradicting the spiral assumption, the authors [1] discussed the implication of a segmented structure, too: It would place Ebullitiocaris halfway between conchostracans and cladocerans, as a basal link, and thus give it greater importance than originally thought.

text & photographs: H.-J. WEISS/Rabenau