A reappraisal of feeding current systems inferred for spire-bearing brachiopods

Abstract

International audienceSpire-bearing brachiopods formally comprise four different rhynchonelliform orders. A calcified spiral brachidium (presumably supporting a spirolophe when alive) and variable median fold and sulcus (probably aiding separation of incurrent from excurrent flows) are peculiar characteristics they all share. Inferences regarding feeding current systems for these extinct taxa have long remained controversial. Two rival models (the Williams–Ager model and the Rudwick–Vogel model) have been developed, each of which has gained supporters as well as critics over the years. In this present paper they are both contrasted and reassessed on the basis of available evidence, together with a new approach that combines: (a) a morpho-functional analysis applying the plankton net as a suitable seston-collecting paradigm; (b) a review of actualistic data showing that all extant spirolophes are functionally inhalant (irrespective of water entering the valves laterally or not); (c) an evaluation of known outcomes from flume experiments yielding consistent empirical results where gaping shells are oriented transversally and dorsally upcurrent; and (d) a reappraisal of the distributions of certain epizoobionts and endosymbionts revealing compatible patterns. The evidence thus accumulated supports the main conclusion that, in most groups (with laterally tapering spiralia), the inhalant current was located medially with the exhalant currents on either side; only in atrypides (with centrally to dorsally tapering spiralia) does the reverse situation appear to have occurred

Demise of the last two spire-bearing brachiopod orders (Spiriferinida and Athyridida) at the Toarcian (Early Jurassic) extinction event

“…The flow tests demonstrated that the continuous stream of the surrounding water generated a medial inflow current into the gaping shell models and wide zones of outflows along the lateral sides. Besides proving the previous model (Manceñido and Gourvennec, 2008), Shiino's (2010) experiments brought forward an important new element as they revealed the presence of an invariable spiral flow system inside the spiriferide models. This gyrating flow closely followed the laterally oriented spiral brachidium of the model specimens.…”

“…The selective extinction of spiriferinids, together with the other spire-bearing group, the athyridids, is best explained by their internal features: the spiral brachidia and the firmly attached lophophore. Manceñido and Gourvennec (2008) gave an exhaustive review and evaluation of the decades-long research and debates by a great number of authors on the feeding current system of spire-bearing brachiopods, including the results of both early flume experiments and observations on fossil interactions with epi-and endobionts. Their tentative conclusion is that the extinct spiriferids and spiriferinids used their laterally tapering spiralia and the attached spirolophs as a kind of plankton net and took advantage of a passive flow system with a median inhalant and two lateral exhalant sectors.…”

“…The Athyridida and Spiriferinida were the last spire-bearing brachiopods and their decline and Early Jurassic extinction has long been thought to be related to the properties of the spiral brachidium, which supported a less flexible, therefore less effective lophophore (Ager, 1987). The inferred feeding mechanism of spirebearing brachiopods was widely discussed by several authors (Rudwick, 1970;Vogel, 1975) and was recently studied by Manceñido and Gourvennec (2008), Shiino et al (2009), andShiino (2010).…”

Theoretical approach to the functional optimisation of spiriferide brachiopod shell: Optimum morphology of sulcus

“…The results indicate that a deeper sulcus could generate a higher pressure difference, thereby creating a more vigorous inflow through the sulcus gape and resulting in stronger spiral flows with a higher number of revolutions (Shiino and Kuwazuru, 2010b). Using a different line of reasoning, a deeper sulcus may result in a more vigorous inflow because the water flowing into the shell through the sulcus gape would be available for the basal spire to capture food particles (Manceñ ido and Gourvennec, 2008).…”

“…This has also been suggested for extinct productids (Shiino andSuzuki, 2007, 2010). An allometric relationship along ontogeny has been described, relating both the progressive broadening of the sulcus and concomitant widening distance between the bases of each spiralium (Manceñ ido and Gourvennec, 2008). To understand the differences due to ontogeny more clearly, the relationship between allometric scaling and functional performance should be examined in the future.…”