The demosponge Leptomitus
cf. L. lineatus, first occurrence from the Middle Cambrian of Spain
(Murero Formation, Iberian Chains)
GARCÍA-BELLIDO CAPDEVILA, D.1
The locality of Murero
in the Iberian Chains (SW of Zaragoza, Spain), has been known for more than a
century for the quantity, and quality, of its trilobites. But in the last
decades these Cambrian outcrops have also produced a small collection of
soft-bodied fossils (palaeoscolecid worms, onychophorans, algae), as well as
numerous brachipods, trace fossils and echinoderms. Here we describe the only
specimen known so far from Spain of the demosponge Leptomitus lineatus (Walcott, 1920). It was found in the talus
slope of the Rambla de Valdemiedes, on rocks from the Murero Formation
assigned to Middle-Upper Caesaraugustinan Stage (Middle Cambrian). The
sponge fossil is 13 cm long and has the peculiarity of a specimen of the
brachiopod Micromitra sp. attached
to its side, gaining hight above the sea floor and thus in a better position
for filtering food particles in suspension. This makes it a case of
commensalism by the brachiopod over the sponge. However, this is not the first
example of this type of interspecific relationship, and such cases are known
between sponges and brachipods from the Burgess Shale. The taphonomical study
suggests that there was little or no disarticulation of the sponge spicules
and that a quite complex series of alteration processes have occurred since
these organisms were buried.