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...
Mollisquama 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...
Evidence 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...
Carcharocles 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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