Tuesday, 20 September 2016

Megachasma alisonae: A Megamouth Shark from the Late Eocene of Denmark.

The living Megamouth Shark, Megachasma pelagios, is a large (up to 5.5 m), suspension feeding Shark that extracts small Shrimps, Copepods and other plankton from the water column using its gill-rakers. Remarkably for such a large animal it was not discovered until the early 1980s, largely due to its cryptic habits; it lives in open marine waters away from shores and spends the daytime in deep water below the photic zone, migrating upwards at night along with the zooplankton on which it feeds. Genetic and morphological studies have suggested that it forms part of a unique lineage, the Megachasmidae, which diverged from its closest relatives at least as early as the Cretaceous, and the discovery in 2007 of a Cretaceous Megamouth from the Colorado, Megachasma comanchensis appeared to confirm this, though this species has subsequently been shown to be an unrelated Odontaspidid Shark and renamed Pseudomegachasma comanchensis. This leaves only a single known fossil species of Megamouth Shark, the Late Oligocene-Early Miocene Megachasma applegatei from the western United States.

In a paper published in the journal Acta Palaeontologica Polonica on 30 June 2016, Kenshu Shimada of the Department of Environmental Science and Studies and Department of Biological Sciences at DePaulUniversity and David Ward of the Department of Earth Sciences at The Natural History Museum describe a new species of Megamouth Shark from the Late Eocene Søvind Marl of Moesgård Strand in Denmark.
  
The new species is named Megachasma alisonae, in honour of Alison Ward; the species being described from a single lateral tooth from a bulk sediment sample collected by David and Alison Ward at Moesgård Strand in 1988. This tooth is small, only 4.0 mm in height and 4.5 mm in width, although if the living Shark had a similar tooth-to-bodysize ration to a modern Megamouth Shark then this would indicate an animal over 3 m in length.

Tooth of megamouth shark Megachasma alisonae from the Pyt Member (mid-Priabonian) of the upper Eocene Søvind Marl Formation of Moesgård Strand, Denmark. (A) Photograph in labial (A1), lingual (A2) and basal (A3) views. Shimada & Ward 2016.

Modern Megamouth Sharks are almost exclusively filter-feeders, with use of teeth to capture prey thought to be rare and incidental, and the species is found only in deep waters where it migrates between the photic and aphotic zomes each day, thogugh the appear to be tolerant of a range of water temperatures, being found throughout the tropical and temperate seas,. The extinct Oligocne-Miocene Megachasma applegatei is interpreted as having been less specialized, with teeth more similar to those of Odontaspidid Sharks, which feed on small Fish and Invertebrates, it is thought to have had a mixed feeding strategy, with some filter-feeding but also preying on shoaling Fish or Squid. It also appears to have lived in a wider range of habitats than the Modern Megamouth, with teeth found in both deepwater and shallow, inshore environments, though it had a more restricted geographical range, being found only in warm tropical seas.. The Søvind Marl is interpreted as having been laid down in a cool well oxygenated, fully marine environment, with a water depth of 200-600 m. This indicates that Megachasma alisonae could live in cool-temperate deep-water environments, though it does not preclude the species aldo being found in other environments. While it is impossible to evaluate the dietary preferences of a species from a single tooth, the specimen shows signs of chisel fracturing, something associated with bting down on a hard surface, such as a shell or hard bone, potentially indicating that Megachasma alisonae was also capable of tackling non-planktonic prey.

See also...

http://sciencythoughts.blogspot.co.uk/2016/01/mollisquama-sp-pocket-shark-from-gulf.htmlMollisquama sp.: A Pocket Shark from the Gulf of Mexico.                                                In 1984 the Russian ichthyologist Vladamir Dolganov described an unusual Dalatid Shark (the group which also includes Cookie Cutter Sharks, Kitefin Sharks and Taillight Sharks) from the Professor...
http://sciencythoughts.blogspot.co.uk/2015/11/evidence-of-shark-predation-on-whale.htmlEvidence of Shark predation on Whale bones from the Early Pliocene of South Africa. Despite extending for almost 2000 miles, the west coast of South Africa is home to relatively few fossil-producing marine deposits. Two sites that are particularly productive are the Miocene-Pliocene deposits...
http://sciencythoughts.blogspot.co.uk/2015/07/carcharocles-megalodon-did-megashark.htmlCarcharocles megalodon: Did the Megashark get bigger over time?                                                The largest Shark ever to live was Carcharocles megalodon, which reached sizes of about eighteen meters and survived from the Middle Miocene until the end of the Pliocene. This was formerly thought to be closely related to...
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