GIANT COELACANTHS AS THE MISSING PLANKTIVORES IN SOUTHEASTERN LATE CRETACEOUS COASTAL SEAS

Remains of fossil fishes are common in Late Cretaceous marine deposits from Mississippi to North Carolina; however, none recognized to date are evidently from large planktivorous species. In modern seas and bays mammals and fish from diverse clades, ranging from whales to paddlefish, feed on macroplankton such as crustaceans, fish larvae, ctenophores, medusae and salps. All of these planktonic clades, with the addition of numerous cephalopods, were present in the Late Cretaceous, but their planktivores are previously unidentified. Planktivorous fish exploiting megascopic fauna generally exhibit some form of filtering apparatus and the capacity to engulf and expel large quantities of water. Here I show that a very large, locally common coelacanth species, Megalocoelacanthus dobiei, incorporated typical planktivore characters. It was heavy-bodied, and, like modern coelacanths probably slow-moving, with a large buccal cavity lacking marginal teeth. The filtering apparatus was the large pharyngeal branchial basket, featuring denticles embedded in branchial bones serving as gill rakers.

Megalocoelacanthus dobiei was first identified in 1994, and subsequently more than a dozen additional specimens have been collected in the Southeast, principally Alabama. A skull in excellent preservation has also been collected in the Smoky Hill Chalk in Kansas, and a single coronoid fragment is known from the Navesink Formation in New Jersey; however, these solitary external occurrences in well-explored deposits suggests that it was primarily a Southeastern species. The large size and abundance of M. dobiei fossils, especially in both open marine chalks and nearshore detrital facies in Alabama and vicinity, suggests it opportunistically filled the planktivore niche in the absence of competitors.