Giant planktivorous fish once ruled seas

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Long before there were whales and planktivorous sharks, their role on the Mesozoic seas was fulfilled by a group of giant planktivorous bony fishes, according to a study published in the most recent issue of the journal Science.

In the paper, Matt Friedman and coauthors present the results of their study on new fossil material of a group of extinct bony fishes known as the Pachycormidae.

The pachycormids were capable of reaching great sizes (up to 9 m in length for Leedsichthys, considered the largest bony fish of all time), and while a number of them were pelagic predators shaped very much like modern-day billfishes and tunas, one lineage consisted of gigantic members that were toothless and fed exclusively on plankton (Asthenocormus, Bonnerichthys, Leedsichthys, Martillichthys and Rhinconichthys).

Prior to this study, giant pachycormids were thought to be an inconsequential and short-lived radiation of suspension-feeders at massive body sizes because of the paucity of their remains in the fossil record.  

Friedman and coauthors have uncovered fossil material from the UK, Japan and the USA that suggests that the group was much longer-lived and more successful than previously imagined.  

With the exception of one recently discovered specimen, these fossils were housed in museum collections in which they were either unstudied or had been described but incorrectly identified in the 19th century.

The results of this study greatly extend the fossil record of the giant pachycormids to cover much of the Mesozoic era (from the middle Jurassic to the upper Cretaceous, a period of about 100 million years).  

The oceans during much of this interval were previously thought to be devoid of large-bodied suspension feeders, but this study now recognizes that pachycormids occupied this ecological role for much of the Mesozoic.

The authors conclude, “he radiation of large-bodied suspension-feeding chondrichthyans and whales in the Paleogene follows the disappearance of Bonnerichthys and many other large-bodied marine teleosts during the end-Cretaceous extinction, suggesting that familiar modern groups of planktivores diversified into the ecospace vacated by giant pachycormids.”

For more information, see the paper: Friedman, M, K Shimada, LD Martin, MJ Everhart, J Liston, A Maltese and M Triebold (2010) 100-million-year dynasty of giant planktivorous bony fishes in the Mesozoic seas. Science 327, pp. 990–993.