The Pachycormiform Fish were large Teleost (Ray-finned) fish from the Mesozoic. One group of these fish appears to have taken up filter-feeding, and reached very large sizes, at least 8m, with estimates for one species (Leedsicthys) of up to 27 m (bigger than an average, though not the largest, Blue Whale). The non-filter-feeding members of the group were pelagic predators superficially resembling modern Barracuda.
Issue 279 of the Proceedings of the Royal Society B contains a paper by Matt Friedmann of the Department of Earth Sciences at the University of Oxford, reexamines a Pachycormiform Fish from the Lower Jurassic Posidonia Shale of Southern Germany. This fish, Ohmdenia multidentata, is known from a single fragmentary skeleton held at the Institut für Geowissenschaften at Eberhard Karls Universität Tübingen. It is thought to be the sister species to the filter-feeding Pachycormiform Fish, that is to say the closest relative of the group that is not actually a member of it.
Ohmdenia multidentata. (a) Specimen photograph. (b) Interpretive drawing. (c) Reconstruction. Belemnites associated with abdominal region are shaded in grey and marked with an asterisk (‘*’) in (b). a.f, anal fin; ang, angular; c.f, caudal fin; cle, cleithrum; den, dentary; ?epb, possible epibranchials; hym, hyomandibular; ipb, infrapharyngobranchial; max, maxilla; op, opercle; p.f, pectoral fin; pop, preopercle; qu, quadrate; rad, pectoral radial; sang, surangular; scl, supracleithra; sclr, sclerotic ring. From Friedmann (2012).
Ohmdenia lived at the same time as the earliest filter-feeding Pachycormiforms, but as there closest relative still has the potential to shed light on how their ancestors lived. Unlike most non-filter-feeding Pachycormiforms, which had needle or blade-shaped teeth, Ohmdenia had blunted teeth, of a type usually associated with a diet of soft-bodied cephalopods (squid and octopus), suggesting that switching to such a diet may have been a stage on the way to filter-feeding.
Friedmann also suggests that such a change in diet may have occurred in other groups that have switched from a pelagic predatory diet to a filter feeding one, such as whales and sharks.